New Magni Enclosed Gyro........ [Archive]

gyrogreg

02-25-2007, 08:16 AM

I've been reluctant to speak up on this new Magni gyro - the Magni factory did not intend the release of some pictures to its dealers to be a public announcment - and it is not! The factory emphasized to its dealers that this is a prototype. They do say that this prototype has undergone all their testing and the configuration is now approved for production. There is a lot to do to get it into production though. The Magni factory has been trying to satisfy the strong demand for an "enclosed" Magni gyro. They have had difficulty simply enclosing the tandem configuration- probably mostly for the destabilizing effects of such a long, smooth, wing-like enclosure - forward of the CG.

Obviously, flight stability is a major requirement of the Magni factory. (I understand the other enclosed tandem prototypes probably met most standard's requirements. But, they apparently did not meet all of Luca's, Pietro's and Vittorio's handling requirements! The tandem enclosed are not ready for production!) The factory has told me this s-b-s design flies exactly like the M16. I do believe them, but, of course, I have not performed any flight tests myself. (They do assure me that I will actually fit in this gyro though!)

HTL?: But, before these conversations get too locked in on the "red herring" of HTL, I thought I ought to speak up.

First, looks can be decieving! I did a Photo Shop excercise of scaled side views of a standard M16 overlaid on one of the released pics of the M23 prototype - attached - I am curious too! From these images, it appears to me (not factory data!):

- The propeller appears to be positioned vertically exactly the same as the M-16. Probably the same 68 inch prop, the rotor head looks to be exactly the same height, and the keel is the same.

- Occupant location is lower - seats are lower. The fuel must be behind the seats, rather than part of the seats. This probably has some affect on exactly where the VCG might be located - but that is not really an issue with Magni designs - see below.

- The tail is positioned about 6-8 inches further aft. Looks to be the same tail as the M16, but more powerful due to it's longer moment arm from the CG.

- Occupant seating appears to be right on the normal RTV. The factory has said no ballast is required, in pitch or roll, with one or two occupants.

- Wheel base is the same as the M16, but the nose is shorter. The windscreen is more tapered and closer to the CG - this would be expected to minimize pitch and yaw stability - and the requirements of the tail.

- All in all, there is very little vertical and longitudinal configuration deviation from the M16.

HTL / stability:: I'm sure most of you know my views on HTL - this is a much over-blown "red herring" perception that stability is not compatible with a HTL. HTL is certainly not a good thing if the designer does not employ other configuration attributes such as a good HS and good airframe aerodynamics. (I have often maintained that probably even a RAF can be made stable if the right HS is employed - as many RAF people are now finding out with the big AC "Tripple Tail" mounted on an extended keel.)

I'm not saying this new M23 prop thrustline is any higher than the M16. But in my opinion, the challenge with stabilizing a HTL is doing it with an efficient HS so as not to affect efficiency. Vittorio Magni is a pro at this - with the sleek Magni styling! Notice that the enclosure provides for smooth airflow to the prop and tail - probably an efficiency necessity with a wider cabin.

Many people still like to critique Magni designs as "HTL, can't be stable"! Once they fly it or flight test it, those opinions change radically! The safety record also verifies stability - there are no known stability related accidents with a Magni! - worldwide - no buntovers, no PIOs, no complaints in pitch, yaw or roll stability, handling or control! This alone validates my contention that the HTL/CLT impressions by many or just simply technically deficient and unnecessarily restrictive. There are good engineering, aerodynamic reasons why a somewhat HTL can actually be more stable, much less affected by turbulence, easier to fly and learn to fly, and much more pleasurable than almost any other gyro - the proof is in flying it and the proof is in its accident record!

I'll hold final judgement on this new model until I get to fly and test it myself. But, I hold the Magni word and professionalism in the highest regard. If they tell me it is stable, I know it is. The Magni factory demands it - apparently much more so than do even many gyro pilots!

- Thanks, Greg Gremminger

rtfm

02-25-2007, 10:02 AM

Greg Mitchell

02-25-2007, 12:50 PM

Thanks for the response Greg.

Three years is not long in this sport BUT Man have I seen some people doing double backflips on their views and theories.
As I've said, I'm not sure we have all the facts on this thrustline yet.

With respect to your photoshop layover. It looks a little off to me. Your M16 pilot seems 'appears' larger than the other pilot, in the M23. The prop may still be in the same posi as M16 and if the mast is the same hieght and the blades are same weight, then clearly, most of the weight is under the thrusline, given the seating dropped........?????

I'm with you, I will wait and see how it pans out. Again thanks for chimming in. I always appreciate your detailed posts.

I flew with you at Mentone and it was awesome! I think the Magni M16 is a great gyro.

Cheers,

Mitch.

gyrogreg

02-25-2007, 03:09 PM

Greg,
Hi. Thanks for the full and informative post. However, I have one question: You have drawn in the RTV @ 18 degrees from vertical. Why 18 degrees? I would have expected an RTV of 9 degrees offset. Regards, Duncan

Duncan, I did not really try to draw the RTV at any specific angle. I drew it essentially perpendicular to the rotor head (cheek plate bolts). I didn't take particular pains to draw it exactly, or bother to actually measure it either. And, a big part of the 18 degrees you measure is the nose-up gyro deck attitude of about 5.5 degrees in the picture. There might be a bit of perspective distortion because the photos are not exactly straight side on. But:

Blowing up the image, leveling the deck angle in the image, and taking care to rederaw the lines as square as possible to the rotor head mounting bolts, it looks like the actual RTV in the photo is about 10 degrees. Again, I don't know how or if the photo perspective might distort this angle either - but 10 degrees is about right for around 60 mph for the Magni. New image attached.

- Greg

rtfm

02-25-2007, 03:49 PM

Greg,
Thanks for the follow-up post, and for going to all that trouble. You're a scholar and a gentleman. For a moment, I thought I had my calks very wrong indeed... :lol:

For interest sake, I thought I'd really clutter up the gyro montage by superimposing a line drawing of my project. Just to see how rthings line up. Quite well, actually. Slightly longer tail moment arm, identical rotor/tail clearance, and very similar RTV.

Thanks again,
Duncan

gyrogreg

02-25-2007, 04:00 PM

------ With respect to your photoshop layover. It looks a little off to me. Your M16 pilot seems 'appears' larger than the other pilot, in the M23. The prop may still be in the same posi as M16 and if the mast is the same hieght and the blades are same weight, then clearly, most of the weight is under the thrusline, given the seating dropped........?????
Cheers, Mitch.

Mitch,

I sized the M16 layer to match the rudder/Vstabs, HS tip plates, nose and main wheels - and the rotor head cheek plates and bolts. I guess they could have changed all of these, but those were my sizing references. Laying the two pilot's helmets over each other, they also appear to match in size. If you wanted to play with this in Photo Shop, attached are the two raw photos I used.

With the occupant's seats dropped, the CG would be lower. The fuel tank would probably need to be larger at the top than before, so a little fuel weight might be higher. But, it does seem reasonable to me that the actual CG would be lower than the M16. But, again, that does not matter if the HS is strong enough to hold the airframe level against whatever a HTL moment might be doing. Note, the HS is quite a bit further aft, so this would help balance any increased HTL moment - probably without increasing the download on the HS and rotor. The HTL moment only affects the CG/RTV relationship if the HTL is allowed to push the nose lower - and the CG more aft. That, I believe is one of the "secrets" of the Magni's legendary stability - the HS simply overpowers all other airframe moments and holds the keel essentially level and the CG forward of the RTV at all times - regardless of HTL, power/thrust, airframe aerodyanmic moments, etc. But, I think we would be surprised at just what the HTL offset really is. I'm sure the Magni factory doesn't care much - HTL is a popular, but misleading "red herring" - the Magni design isn't as affected by a moderate HTL, in fact it is probably an asset to their way of achieving stability. If I were the Magni factory, I certainly would not want to be responding to such "red herring" HTL concerns, since the Magni's powerful HS makes this basically a mute issue for them. I appreciate that the Magni team is a bit beyond incomplete and restrictive layman engineering.

Incidentlally, nobody is doing a "backflip" on this HTL concept. In fact, I've often argued that some amount of HTL, when properly "balanced" by an appropriate HS, actually makes the gyro more statically stable because the down-loaded HS holds the CG even further forward of the RTV. The Magni factory has certainly never ascribed to the CLT is best belief. And, the Magni gryos and safety records are the best argument that they are right! I'm not saying that CLT isn't a good thing to do, or one way to help skin the cat, I'm just saying it may not be the only basket you should put all your eggs in - and it certainly is not, by itself, a good indicator of the overall stability margins. But, any determination of stability from configuration measurements only, is probably not accurate anyway - that is why any gyro needs to be flight tested to find out. If the other Magnis are any indication of the Magni standard in stability, from the flight testing I have done, I would not expect the M23 to be any different! I do trust the Magni factory on the score of stability - especially more so than I trust conclusions based the sole perception of HTL/CLT - conclusions drawn from visual assessment rather than flight testing.

- Thanks, Greg

Aussie_Paul

02-25-2007, 06:40 PM

Greg G said, "Incidentlally, nobody is doing a "backflip" on this HTL concept. In fact, I've often argued that some amount of HTL, when properly "balanced" by an appropriate HS, actually makes the gyro more statically stable because the down-loaded HS holds the CG even further forward of the RTV. The Magni factory has certainly never ascribed to the CLT is best belief. And, the Magni gryos and safety records are the best argument that they are right! I'm not saying that CLT isn't a good thing to do, or one way to help skin the cat, I'm just saying it may not be the only basket you should put all your eggs in - and it certainly is not, by itself, a good indicator of the overall stability margins. But, any determination of stability from configuration measurements only, is probably not accurate anyway - that is why any gyro needs to be flight tested to find out. If the other Magnis are any indication of the Magni standard in stability, from the flight testing I have done, I would not expect the M23 to be any different! I do trust the Magni factory on the score of stability - especially more so than I trust conclusions based the sole perception of HTL/CLT - conclusions drawn from visual assessment rather than flight testing.

Greg, great section of your post. I found this with my Hybrid experiments. At that time I was trying for an average of CLT depending on gross weight. I almost succeeded BUT not quite. I added heavier wheels towads the end and had Hybrid the most stable of all the tests.

After I trained Michael, the Oz Magni agent, I was so surprisedto find that my Hybrid with the slightly HTL, approx 2", and rather smallish h/stab behaved like the Magni except it was over a smaller airspeed range. The RAF cabin was overpowering the stabs at speed.

I am no expert but I have actually conducted the tests, to prove in my mind, that the practical matched the theory.

Firebird is the next evolutionary step fo me. I hope it is a successfull as my Hybrid experiement, both aerodynamically and financially.:lol:

Aussie Paul. :)

A few pics from my enjoyable time spent training Michael in his wonderful Magni. Not my cup of tea BUT none the less a truly wonderful machine.

1.jpg
2.jpg
3.jpg
Michael and his Magni
4.jpg
5.jpg

gyrogreg

02-26-2007, 05:35 AM

Greg G said, ----------------some amount of HTL, when properly "balanced" by an appropriate HS, actually makes the gyro more statically stable because the down-loaded HS holds the CG even further forward of the RTV.

--------------------- I found this with my Hybrid experiments. At that time I was trying for an average of CLT depending on gross weight. I almost succeeded BUT not quite. I added heavier wheels towads the end and had Hybrid the most stable of all the tests.--------------------- I am no expert but I have actually conducted the tests, to prove in my mind, that the practical matched the theory. Aussie Paul. :)

Paul, thanks for the testimonial. Your comments are certainly respected and have valuable influence in the gyro community. I am finding more and more gyro people who actually do the filght tests and learn from them. Not only are they identifying the limits of their particular gyro and learning how to improve those limits, as you have, they are learning great insights into what really makes a gyro stable and safe and easy to fly and easy to learn to fly and able to fly safely in turbulence with confidence and ease! Some day, the real "secret" of stable gyros will be understood and accepted and applied - and someday all the really unnecessary hand wringing about thrustlines and HSs will a memory - as will PIO, Buntovers and PPO! That is the main reason I represent MAgni Gyro - the proof of the pudding is flying it!

- Thanks, Greg

bpearson

02-26-2007, 09:22 AM

I would take more notice of who is behind the gyro rather than eyeballing photos. The Magni safety record speaks for itself.

Greg Mitchell

02-28-2007, 12:32 AM

Udi

02-28-2007, 07:34 AM

Heron

02-28-2007, 08:24 AM

Five years ago I was asking all kinds of crazy questions!
I can see it but I can not explain what I see!
Today most gyros can do what I wanted five years ago and was told impossible!
We need a Moderator Group of academic savy people to analyze and perform final edit on our findings. Manufacurers and Reps should be part of a group, designers and builders other.
Stop beating around the stability bush, this should be over by now.
As Mr. Mayfield says: It is not an opinion!
write the rule down and those able will follow it . . .
thanks
Heron

bpearson

02-28-2007, 09:04 AM

Stop beating around the stability bush, this should be over by now.
As Mr. Mayfield says: It is not an opinion!
write the rule down and those able will follow it . . .
thanks
Heron

Don't like rules !

Heron

02-28-2007, 10:44 AM

People that don't like rules are the best to make them!
And you probabily don't need them anyway, you follow common sense . . .
The bigger the group the more it needs rules, too many minds to keep in check!
The Burg needes rules!
Is your statement a rule? If it is . . .is there an exception?
Heron

TomCarlisle

02-28-2007, 12:34 PM

This machine kind of looks like that Air Command one-off that never went into production. I think they called it the 447.

mikeconcannon

02-28-2007, 12:41 PM

Im sure that we all know deep down, that if it's a Magni, and it's relaesed for sale, then its going to be just fine.

bpearson

02-28-2007, 12:49 PM

I like rules that help keep aircraft from flying into each other. These affect others. If I hired a Cessna I would want it to be maintained to the rule book.

Don't like rules that insist I fly only certain configorations of gyroplane. Educate people, but when the only risk is to themselves I have a problem with restrictions.

Udi

02-28-2007, 05:34 PM

Im sure that we all know deep down, that if it's a Magni, and it's relaesed for sale, then its going to be just fine.
I would call this sentiment "irrational complacency". All the Magnis so far, seems to me, have followed the basic design of the Jukka Tervamaki JT-5 gyroplane. This new bird, however, looks completely out of character with Tervamaki's designs. I am not suggesting that Magni don't know what they are doing, but I wouldn't trust a new design blindly, just because "it's a Magni". You shouldn't either.

I hope Greg will find an opportunity to test this new gyro.

Udi
p.s. below is a typical Tervamaki design. note prop thrust line in relation to pilot.
http://www.icon.fi/~jtki/jt5kuvat/jt5kesa.jpg

gyrogreg

02-28-2007, 09:41 PM

The reasoning that a moderate HTL is actually "good" because it allows one to have a down-loading stab is flawed. I used to subscribe to this reasoning until Raghu straightened me out. You all can go back to this (http://www.rotaryforum.com/forum/showthread.php?t=10434) thread and learn why this reasoning is flawed.

A moderate HTL may not be too harmful when it is compensated by a proportionally down-loading stab, yes, but it does not give you any advantage, from an aerodynamic point of view, over a perfect CLT with a similarly sized stab. Go back and read that old thread and if you are not convinced we can go into this discussion here. -------------------
Udi

Udi, (and Raghu), since you brought it up, here is a little food for thought: That "Safest Gyro" thread was one of the most interesting technical discussions I think I have seen. Most of that discussion focuses on "AOA stability” and the effect of an up or down loaded HS on AOA stability. Pardon me, but I am still not convinced that AOA stability and/or other stabilities are not enhanced by a down-loaded HS, more so than an up-loaded HS. The AOA stability mechanism that Raghu describes (for a FW) is the result of the upward movement of the aircraft [due to increased airspeed and lift of both "wings" - producing a reduced AOA (and lift) on (forward) wing, and a corresponding AOA change on the HS.]

But, consider three cases of the AOI of the HS:

A) The HS AOI set at the same equivalent AOI as the main wing - both "wings" see the same AOA in the airstream: Upon increased airspeed, both "wings" experience increased lift - in the same direction and in the same proportion. Even though the aircraft starts to rise from the increased airspeed, both do so in the same proportion - so the "balance" of the moments remains the same and the aircraft does not change in pitch. - no AOA stability restorative pitch change.

(Note, that in this condition of positive lift generated by both "wings", the CG is necessarily positioned somewhere in-between the two lift vectors of the two "wings". - not forward of the front wing. With no other static moments on the airframe considered - prop thrust moment or airframe aerodynamic moments - the CG is actually exactly on the resultant lift vector of both "wings")

B) The HS is set at a higher positive AOI than the main wing: Upon increased airspeed, both "wings" experience increased lift - in the same direction and in the same proportion. But, when the aircraft starts to rise from the increased airspeed, the lower AOI wing approaches zero AOA before the HS does. So, the main wing starts reducing its lift significantly approaching zero AOA (and stops lifting) before the HS approaches it's zero AOA. So, the HS continues to provide up-lift even when the main wing stops lifting - this is an unbalance of moments pitching the nose of the aircraft downward and in the divergent direction for airspeed - airspeed instability!

(Note, that in this condition of positive lift generated by both "wings", the CG is necessarily positioned somewhere in-between the two lift vectors of the two "wings". - not forward of the front wing. With no other static moments on the airframe considered - prop thrust moment or airframe aerodynamic moments - the CG is actually exactly on the resultant lift vector of both "wings")

C) The HS set to a lower AOI than the main wing: – not necessarily down-lifting, but lower AOI than the main “wing”. Upon increased airspeed, both "wings experience increased lift, in the same proportion, but in different directions. The differential between the two "wings" immediately pitches the aircraft nose-up - in the airspeed restorative direction. In this case, the pitch up does not have to wait for an upward movement to start its pitch effect. And, once the aircraft starts to rise, the forward wing starts seeing reduced AOA (reduced lift), but the rear "wing" sees an even increasing negative AOA - more tail down, more nose-up pitch. Both of these nose-up pitch mechanisms slow the aircraft until the original trimmed airspeed is attained by the nose-up, climbing attitude - "Airspeed" stability!

(Note that in this condition of opposite lift directions generated by the two "wings", the CG is necessarily positioned somewhere forward of the lift vector of the forward wing. - not in between the two "wings". However, even with the CG forward of the front wing, with no other static moments on the airframe considered - prop thrust moment or airframe aerodynamic moments - the CG is actually exactly on the resultant lift vector of both "wings" - which is also forward of the front wing!)

This is all FW analogy that may not fully apply to a gyroplane rotor because the "front wing" of the gyro (the rotor) actually changes its lift vector upon changes in airspeed and eventually changes it’s RRPM as a result of increased load, which then changes the lift vector back some! Gyroplanes may be hard to apply the FW analogy to anyway!

But, two other things are pointed out by the mind games above:

1) Since the normal AOA of a gyroplane rotor disk is fairly high, higher than most positive AOA HSs, it stands to reason from above, that some degree of upward lifting HS (less AOA than the equivalent AOA of the rotor) will still provide the first AOA stabilizing mechanism that Raghu presents. This is essentially the higher lift curve of the rotor than of the HS providing the AOA stability in this case. But, with upward lift of the HS, the second, more immediate pitch mechanism of differential lift changes between the two is not present until the rise of the aircraft actually causes a downward AOA (and lift) of the HS. And, at higher airspeeds, where stability is more important due to sensitivity of rotor disk AOA changes, a highly up angled HS may then no longer be less than the rotor disk equivalent AOA – changes from condition “C” above (Airspeed stable) to condition “B” above (Airspeed unstable). Becoming Airspeed unstable at higher airspeeds – faster getting faster, steeper getting steeper!!

2) In all cases above, and not considering the RTV changes ("flapping"), the CG is located exactly on the resultant lift vector of both "wings" - in the case of up-lifting HS, it is aft of the main wing; and in the case of down-lifting HS, it is forward of the main wing. In either case the CG is on the resultant lift of both "wings". So, the concept of the CG being forward of the TV for G-Load stability is not so simple. A change in G-Load, with the CG being exactly on the resultant Lift Vector, creates no pitching moment - at least if the change in load is without a corresponding change in the AOI of either "wing". But, in the case of gyros, a change in G-Load, either from a change in relative wind or a pilot input, does change the RTV and does create a momentary moment arm from the resultant lift of the rotor and HS to the CG. And, this resultant momentary moment is in the G-Load restorative direction in all cases.

What this all says is probably that, without other static moments affecting the flight attitude and CG location - absolute CLT and absolute CLD - the gyro probably exhibits some positive airspeed (AOA) stability and NEUTRAL G-Load stability. This also means that with no other static moments from airframe aerodynamic moments and prop thrust offsets, this gyro described above, always flies with zero AOA on the HS - pitch of the aircraft is controlled and set by the AOI of the HS. That means an effective keel-level HS will maintain the keel level. A an effective positive AOI HS will maintain a nose-down keel attitude, and an effective negative AOI HS will maintain a nose-up keel attitude. In all cases, with no other airframe static moments, the CG will always be exactly aligned with the RTV - since the essentially zero lift HS is not contributing to the "resultant" lift of the two "wings".

But now, consider the effects on all this if you introduce other airframe moments such as an offset prop thrustline and/or an offset airframe drag line. Now, the HS, if there is one, is called on to "balance" these other two static moments:

1) The (probable) nose-down airframe aerodynamic moments (LG, windscreens, etc.) require a down-lifting HS to keep the keel reasonably level - at all airspeeds. If it doesn't, as airspeed increases the airframe nose-own moments, the airspeed continues to increase and the nose is forced lower - "Airspeed instability!)

2) A HTL static nose-down moment requires a down-lifting HS to keep the keel reasonably level - at all power settings. If it doesn't, airspeed will be highly a function of power - "Power instability!"

3) A LTL static nose-up moment requires an up-lifting HS to keep the keel reasonably level - at all power settings. If it doesn't, nose-high pitch attitude will keep increasing and airspeed will also be highly a function of power.

Notice, you will almost assuredly have the #1 condition above - requiring a down-lifting HS. This condition cannot be satisfied at the same time as trying to satisfy condition #3 above - the HS cannot be angled two different directions at once!

When these other moments, that are not necessarily closely related to actual lift changes, are introduced into the equation, now the CG position starts to not be on the "resultant" lift of the two wings. When this is the case, Chuck Beattie's classic visualization of the CG forward or aft of the RTV starts to warrant consideration. But, if there were no offset prop thrust and no airframe pitching aerodynamic moments, there would actually be no lift/CG moment arm and therefore no restorative convergent, or divergent pitch reactions to G-Load disturbances! But, in the real gyro world, there are these other moments, and how the HS "balances" those moments will determine whether the CG/RTV moment is stabilizing or destabilizing. Of course, if there is no HS at all, then these other static moments, prop offset and drag moments, completely determine the stable or unstable condition of the RTV and CG. And, if there is no HS, the airframe drag pitching moments would run amuck with airspeed changes, and the airspeed would radically change with offset thrust changes.

I have a basic question, and perhaps this shows my ignorance on all of this. People refer to "AOA stability". This may not be exactly what is referred to as "Airspeed stability", but are not these two essentially related - when an aircraft is AOA stable, it is "airspeed stable"? Rahgu's discussion on the other thread talks about “AOA stability”.

I may have been the one who introduced "G-Load stability" as the necessary ingredient to prevent buntovers. This is essentially a concept from Chuck's CG/RTV offset concepts. I think Raghu's concepts ignore such a thing as G-Load stability - lumping it all into AOA stability! But, this concept of AOA stability seems to me to be really the "airspeed" stabilizing mechanisms of changes in wing/rotor AOA correcting for airspeed changes. I don't feel the mechanism of a buntover can be totally AOA or "airspeed" instability – an airspeed unstable aircraft will simply continue to increase airspeed uncontrollably into a steep glide – not suddenly flip forward!.

I believe the mechanism for buntover (or PPO) is that the suddenly decreasing G-Load creates more nose-down pitching which decreases the G-Load further, and on and on! This would be Chuck's often described mechanism of a buntover from the CG aft of the RTV – pitch responding nose-down to a reducing G-Load..

Sorry for the length of this - maybe I should move this to another thread. Maybe this is just late night rambling. But, I am certainly not clear on how all these mechanisms interact - especially dynamically. And I am in no way convinced that the use of an up-lifting HS is totally compatible with other static stability issues such as Power and G-Load stability.

- Greg Gremminger

gyrogreg

02-28-2007, 10:54 PM

Greg,

I never accused you of doing a backflip BUT seeing as you worded it the way you did. Let me just say this. You might have often argued some amount of high thrust line >>>>>>> but not always. So we all change, evolve and sometimes change our beliefs.

My intent is to find what the 'magic' is that Magni will employ in this HTL craft with nose down moment slopy nose pod.....I'm not looking for a sale pitch. Nor to be chided with terms of reference, like...."Magni team is a bit beyond incomplete and restrictive layman engineering."

Read your post to me, then read your post to Paul Bruty and see and feel the difference in the tone and underlying message...."Paul, thanks for the testimonial. Your comments are certainly respected and have valuable influence in the gyro community."

That's just your point of view Greg, some of us dont feel that way.

You have indeed changed some of your ideas and Bruty has done the double backflip. And the way you talk about near CLT type craft now is certainly different to how you used to come across.

Anyway, I was just asking the question and I'm glad you found time to re-state you sales pitch. So unless and until this aircraft is flying I not sure I can accept the 'cookbook' throwbacks to the Magni Tandem being applied to this Magni.

Thanks. I wont ask again.

Mitch.

Sorry to upset you so Greg M.

I do believe I have posted numbers of times maintaining that even an RAF might be "stabilized" with an adequately effective HS. In saying so, I always couch that by saying that "balancing" such a HTL may affect efficiency since it may require significant HS download and more rotor loading. Even some of my earliest stability articles (see my 5 part series on
"Our Personal Safety Envelope" (http://www.magnigyro.com/USA/features.htm) published in 1999-2000, where the "Sum of Moments" analysis identifies no limit to what HTL can be compensated as long as an effective enough HS can be applied. I do admit my perspective, terminology and presentation of these principles has evolved since those years, but I don't think my concept that large of HTL offsets could actually be "balanced" to achieve stability - if not best efficiency!

I also say we or I could not confidently say this new configuration meets all the stability criteria until I actually flight test it. But, I do get a bit perturbed when people continue to assume a Magni is a HTL and is therefore unstable - I hear that a lot about the M16, which does probably have about a 3 inch HTL and still certainly manages to be safely stable. The M23 is probably higher HTL, and therefore could be a stability factor - but looks can be deceiving. But, I do have a lot of faith in the Magni factory - their standards for and attention to stability has always been higher than almost anyone elses!

As far as the "nose down moment slopy nose pod", That might be an issue. The fully enclosed and wider sloped windscreen may be more destabilizing than the M16 pod - I don't know, there are things you can do to minimize some of this - the whole enclosure is shorter than the M16, the windscreen starts further aft - closer to the CG, and the rounded, rather than flay top profile might spill some of the destabilizing down-load. Be aware, side loads on this enclosure might likely require stronger V stab power too. Could these be the reasons the tail feathers are considerably further aft - more moment for reduced HS loads on the rotor?

Greg, all this is speculation on my part -from pictorial analysis. People are asking, so I'm answering s best I can. Also, the factory has told us it flies excactly like an M16. Since stability is one of their primary requiremnents, and probably the reason they have not been able to get the M21 (fully enclosed tandem) to acceptable performance after over two years of trying, I have little doubt when they say this design meets their standards that it does. I just won't say so positively until I, and others, can actually fly it and test it too.

I hope we all find out what the "magic" is, if there is any. I would be more inclined to say it's not anything magical, just good attention to details. And maybe, just maybe, a little higher HTL has been effectively compensated or minimized. If higher loads on the HS or larger enclosure are an efficeincy impact, this configuration also seems to pay very close attention to enclosure aerodynamics to get some of that efficeincy loss back - cleaner air to the prop and tail and more overall efficient airfoil shape of the larger enclosure? - I dunno all the answers, and none of this is sales pitch, just speculation as to what may or may not be. My main intent, even on this thread, is to use the Magni example and even the questions that arrise, to help people better undersand the principles and issues involved. I do happen to believe that way more weight is given to prop thrustline than what it deserves - if you do other things right too! It will take a while, but we'll find out if that applies to this model too!

Thanks, Greg

bpearson

03-01-2007, 03:03 AM

Im sure that we all know deep down, that if it's a Magni, and it's relaesed for sale, then its going to be just fine.

I know deep down that I probably won't be able to afford one.

Greg Mitchell

03-01-2007, 03:26 AM

G'Day Greg G,

You did not upset me. Not at all.

IMHO, Udi's referrence to Jukka is on the money and this is a major deviation from anything like what I would have thought Jukka may have produced. I had included in a post on the OZ forum a similar referrence but removed same as I was unsure as to Jukka's involvement or not.

Greg I dont hear people saying the magni has a high thrustline and is therefore unstable. The opposite is surely the case. You paved the way with the testing and publishing your results. I have no problem with you or tandem magnis. So hopefully we are clear on that.

Now by your own admission in the abscence of the aircraft it has a higher offset, than the tandem. Tell us if you would, why this aircraft is not a candidate for a PPO?

"I do happen to believe that way more weight is given to prop thrustline than what it deserves - if you do other things right too! It will take a while, but we'll find out if that applies to this model too!"

Hmm, I'm not calling that statement a back flip but surely Dr Houstons (sp?) report and much of the consensus forum dealt with thrustline offset as well as other stuff, so to say it deserves less attention, now, puzzles me Greg.

Mitch

mikeconcannon

03-01-2007, 03:27 AM

I totaly agree with what you say, you should not trust the design until the expeirienced pilots have established that it is safe and also that you have yourself satisfied yourself that it is safe. What I was more trying to say is that Magni would not release it for sale unless it was 100% stable.
It's the manufacturer and their attitude towards stability and safety that is the known trustworthy quantity and thats the reassuring thing about dealing with Magni.

gyrogreg

03-01-2007, 06:37 AM

G'Day Greg G, You did not upset me. Not at all. Thanks! I'm very glad to hear that! The thought that I had bothered me last night.

IMHO, Udi's referrence to Jukka is on the money and this is a major deviation from anything like what I would have thought Jukka may have produced. I had included in a post on the OZ forum a similar referrence but removed same as I was unsure as to Jukka's involvement or not. As far as I know, Jukka was not involved with the M23 design - except as original inspiration maybe. But, Vittorio has certainly evolved the Magni concepts considerably beyond the original JTs. I do know that Jukka has flown in the M16 and was very pleased. Notice that the MT05 does apparently have a large sloping (destabilizing?) windscreen - but it does also "appear"to have a moderate prop thrustline offset.

----Tell us if you would, why this aircraft is not a candidate for a PPO? I don't think I can say that for sure. At this point I can only speculate that because I so trust Vittorio, Pietro and Luca to be sure it isn't. This family resurrected the gyro world from outlaw status in Italy and other countries by demonstrating that not all gyros are ready to kill you. They are proud of that. It is not only flight stability that resurrected gyros in Europe - there are other factors Magni contrubuted such as training practices, ruggedness and reliability. But I'm quite certain they would not jeopardize that achievement by taking chances with stability. Otherwise, because they have been struggling with adequate tail power for enclosed prototypes for several years now, I would suspect they found the right combination of enclosure aerodynamics coupled with iappropriate tail power by moving the tail feathers even further back than on any other model!

------------------- Hmm, I'm not calling that statement a back flip but surely Dr Houstons (sp?) report and much of the consensus forum dealt with thrustline offset as well as other stuff, so to say it deserves less attention, now, puzzles me Greg. Mitch
Here's where I'm probably going to get some real criticism. I am not enamored by the Houston report, or any subsequent presentations of his on this subject. I know a lot of work went into his report, but I don't accept how he draws some of his conclusions - never have and said so when the report first came out. I should have been more supportive - maybe - the project actually used real instrumented flight testing and reached the conclusion that the Magni it tested was stable! I'm not sure I am smart enough to know what conclusions to draw from this rather technically involved report either!

The project attempts to develop a computer model for gyros - and to verify that computer model with real flight testing of the Magni VPM 16. I am an Engineer from Missouri, so maybe I have more DNA reason to be questioning the logic of some of the conclusions:

- To validate the computer model, the model needed to match the real world of the flight test. It did this in several areas. But, I do question whether the focus on HS volume, placement, AOA, lift parameters, etc. is adequately analyzed in either the model or the flight testing - they did not vary the HS for any flight testing – much less airframe aerodynamics - and the report does not indicate any stability effects or even consideration of HS or other airframe aerodynamic factors.

- Although the report recognizes and emphasizes the additional "degrees of freedom" of RRPM in a gyro, it does not analyze the effects of RRPM, rotor inertia, etc. on the ultimate stability conclusions of the model? For instance, Magni will probably tell you the heavy Magni rotor inertia is part of the "harmony" of the design. I would think the report, after making so much to do about the variable of RRPM, does not even suggest it is a factor in the ultimate conclusion. The prominant conclusion focusses on the HTL thing. why would other supposedly important parameters and variables be so ignored in the final analysis and assessment?

- The report does indicate measurements found the prop thrustline to be about .02M (about 3/4 inch) LTL. There is a picture of the VPM 16 in the report, and it is not much different apparent thrustline than the current M16. In fact, the VPM and early models of the M16 had a lower rear seat - lower CG than currently. I have to suspect the CG determination in the report! It is generally accepted that the M16 / VPM16 has a thrustline about 2 -3 inches HTL fully loaded. Measuring VCG is notoriously difficult - especially on a tandem where fuel and occupants need to be held in positions difficult to hold in severe pitch hang attitudes required for accuracy. To suggest that the CG as been located so precisely, without identifying even the load considerations or variables for that measurement – heavy pilots, light pilots, no pilots – raises some questions? CG changes with fuel and occupant weights. Was this .02 M figure measured without occupants? – with or without fuel? And, to top it off, British Section “T” sets the HTL limit at 2 inches! Where did this 2 inches come from – I do not even see in the report where they varied this on even the computer model? “2 inches” would sound reasonable to me for a loaded VPM16 HTL! Coincidence?, intuitive guess? I don’t see anything in the study that even varies this parameter to possibly gauge what a regulatory limit ought to be!

- In several ways, the report seems to dismiss or ignore the contribution and stability factor of the HS. It even goes so far as to “suggest that the tailplane on this autogyro is somewhat ineffective, despite its relatively large size.” Now, I’m pretty sure the beneficial contribution of the HS stabilizer on Mw (Pitching moment rate - dampening) is an accepted effective mechanism. Why would this report essential dismiss the dampening effect of the HS?

- As further concern that the stability contribution of the HS was not so fully considered in this study, the lift component of the HS was not considered in the “Assessment of Autogyro Longitudinal Flight Dynamics”. This explanation of longitudinal stability is essentially the CG/RTV moment that Chuck and myself and others have always described to show how the CG must be forward of the RTV in flight. As I noted in an earlier post here, this analysis is not so simple! When the HS lift component is actually considered as part of the resultant lift vector, this CG relationship to the resultant lift vector is what can cause pitching moment when the two are not aligned. Although this report explanation addresses the effect of prop thrustline on the CG/RTV relationship, it apparently ignores any corrective or complicating effects of HS lift or airframe lift/drag. I would not think such a technical report – not limited to explaining all this to non-engineers - should just ignore other important factors in its “Assessment of Autogyro Longitudinal Flight Dynamics”.

- The extremely important longitudinal damping characteristics, Mq, between the model and real flight testing, are not identified or compared in the report. Apparently, when some of the model parameters compared favorably with real life testing, the report switches to just a report on the model – with little continued comparison to real life. Pitch damping is a function of friction, drag and other aerodynamic mechanisms – not of the static relationship of CG to any thrustlines! If there is dampening at all, it is a function of the airframe and HS aerodynamics – maybe of the rotor dynamic “degrees of freedom” too. The report does not even show where the computer model investigated these dampening variables to assess their effect on damping characteristics – I don’t see this as scientifically valid to conclude that Mq is good or bad, better or worse, based on two parameters (rotor and/or prop thrustline moments) that are not even dampening mechanisms. The aerodynamic dampening mechanisms should be studied if the mechanism(s) of the all-important dynamic damping characteristics are going to be concluded and reported. Certainly, a thorough engineering design and analysis of a computer model, or of a real model, ought to have considered all of the factors. If it did, why are they not included in the “Assessment of Autogyro Longitudinal Flight Dynamics”? Or otherwise detailed in the report?

So, as to my confidence that the Houston reports are complete, conclusive and valid, and that they validly can place such prominent weight on HTL only, I don’t think that can be determined from what has been presented to us. Perhaps the actual computer model ought to be presented so we can see just how diligently ALL stability mechanisms have been considered. In summary, I take the Houston report with a grain of salt – especially as to the benefits and contribution of a HS – which, IMHO, is not thoroughly investigated in the report. And as such, I cannot totally accept the conclusions on HTL either. In my opinion, the record of safety and pilot reports on handling and control are the ultimate validation of these theories. I do not see that the Houston analysis, conclusion and suggested parameters, are validated in real life. They are certainly important factors, but the flight record of at least the (HTL) Magni does not validate that the report explains everything!

- Greg Gremminger

PS: Presumably, this report had a strong impact on the British Section “T” requirements. Somehow, with the reports validation, Section “T” inflexibly requires no higher HTL than 2 inches! This is a “prescriptive” requirement, one that is not based on flight test results that I can see – testing on which a standard should be based. To set a “prescriptive” limitation of a 2 inch HTL, restricts other perfectly valid ways to improve stability/safety – such as a HS. The British regulations do not even allow the addition of a HS to existing gyros! Something is wrong here! It does not take a rocket scientist to appreciate that it is hard to hurt stability with installation of a HS! Yet, the Houston report, originally contracted by the CAA for the purpose of setting a safer requirement for gyros, IMHO, ought to at least address the contribution from a HS if it is so intended to improve gyroplane safety! All of this just contributes to my lack of confidence in the popular conclusions of this report.

bones

03-01-2007, 11:51 AM

Heron

03-01-2007, 12:05 PM

Gee . . .someone found a bone to pick, uh?
I was thinking about leftists and right wingers . . .
My guess is that Magni is kind of left of the center, more of a cousin to RAF than to other machines beyond and closer to CLT and all that good stuff.
If you tell someone he is right he will take it as acusation and go a little closer to center, and vice versa.
Now the Standards should be the line drawn in the stone, you are either a duck or an eagle.
It kawcks like a duck to me! (wtf is that?) quack!
Heron

Udi

03-01-2007, 12:08 PM

Hi Greg,

...But, two other things are pointed out by the mind games above:
These are not mind games - you wrote a good summary of AOA stability.

1) Since the normal AOA of a gyroplane rotor disk is fairly high, higher than most positive AOA HSs, it stands to reason from above, that some degree of upward lifting HS (less AOA than the equivalent AOA of the rotor) will still provide the first AOA stabilizing mechanism that Raghu presents. This is essentially the higher lift curve of the rotor than of the HS providing the AOA stability in this case.
I am not proposing having a lifting stab but essentially, yes, this are correct.

But, with upward lift of the HS, the second, more immediate pitch mechanism of differential lift changes between the two is not present until the rise of the aircraft actually causes a downward AOA (and lift) of the HS. And, at higher airspeeds, where stability is more important due to sensitivity of rotor disk AOA changes, a highly up angled HS may then no longer be less than the rotor disk equivalent AOA – changes from condition “C” above (Airspeed stable) to condition “B” above (Airspeed unstable). Becoming Airspeed unstable at higher airspeeds – faster getting faster, steeper getting steeper!!
I don't think I follow this logic. What is the "first" mechanism of AOA stability? And why must there be an uploading stab? But even if we assume that there is an uplifting stab - lets assume with a 2 degree positive AOA. The nose-down attitude of the gyro can never be greater than 2 degrees. It will not be "faster and faster - steeper and steeper" as you say.

But I think you may be referring to an uplifting stab with regard to a LTL gyro that has a parallel stab. Due to the LTL nose-up pitching moment, the gyro will fly a little nose high (with the CG fwd of the RTV), and thus the stab will have a positive AOA. What's the problem with that? This gyro will not be AOA unstable, or airspeed unstable for this matter, at any airspeed.

...A change in G-Load, with the CG being exactly on the resultant Lift Vector, creates no pitching moment - at least if the change in load is without a corresponding change in the AOI of either "wing".
There cannot be a change in G-load without a corresponding change in AOA. By definition, a change in G-load corresponds to a change in AOA because the balance between G and lift has changed. G-load stability is a product of AOA stability. There is no G-load stability that is independent of AOA stability - if the gyro (or the plane) was balanced in pitch before the G-load has changed, it isn't going to become unbalanced just be cause the load has changed. The pitching moments about the CG won't change until lift from the rotor or stab or both have changed. The lift changes as a result of airspeed change or AOA change.

What this all says is probably that, without other static moments affecting the flight attitude and CG location - absolute CLT and absolute CLD - the gyro probably exhibits some positive airspeed (AOA) stability and NEUTRAL G-Load stability.
If there is AOA stability, there is G-load stability.

This also means that with no other static moments from airframe aerodynamic moments and prop thrust offsets, this gyro described above, always flies with zero AOA on the HS - pitch of the aircraft is controlled and set by the AOI of the HS. That means an effective keel-level HS will maintain the keel level. A an effective positive AOI HS will maintain a nose-down keel attitude, and an effective negative AOI HS will maintain a nose-up keel attitude. In all cases, with no other airframe static moments, the CG will always be exactly aligned with the RTV - since the essentially zero lift HS is not contributing to the "resultant" lift of the two "wings".
I completely agree. Yet - this completely balanced gyro with the CG aligned with the RTV WILL exhibit positive static stability due to the stab, in addition to other contributing factors such as the rotor itself and the rotor offset gimbal head.

But now, consider the effects on all this if you introduce other airframe moments such as an offset prop thrustline and/or an offset airframe drag line. Now, the HS, if there is one, is called on to "balance" these other two static moments:

1) The (probable) nose-down airframe aerodynamic moments (LG, windscreens, etc.) require a down-lifting HS to keep the keel reasonably level - at all airspeeds. If it doesn't, as airspeed increases the airframe nose-own moments, the airspeed continues to increase and the nose is forced lower - "Airspeed instability!)
True. In this case the stab has to have sufficient negative AOA and power to cancel out these aerodynamic moments. The stab force is a square function of airspeed, just like the opposing force of the windshield, gear, etc.

2) A HTL static nose-down moment requires a down-lifting HS to keep the keel reasonably level - at all power settings. If it doesn't, airspeed will be highly a function of power - "Power instability!"
Also true. The best fix for that is not to have HTL at all. The second best fix is to have a down-loaded stab in the propwash to completely and exactly cancel out the HTL pitching moment.

3) A LTL static nose-up moment requires an up-lifting HS to keep the keel reasonably level - at all power settings. If it doesn't, nose-high pitch attitude will keep increasing and airspeed will also be highly a function of power.
I don't see the LTL gyros fly with excessively high-nose attitude. I agree that too much LTL is not good - just as to much HTL is not good. But there is no need for a lifting stab and I don't see anyone suggesting that as a design philosophy!

Notice, you will almost assuredly have the #1 condition above - requiring a down-lifting HS. This condition cannot be satisfied at the same time as trying to satisfy condition #3 above - the HS cannot be angled two different directions at once!
You are making this over-complicated, Greg. The stab is tailored to the gyro. CLT or moderately LTL gyros don't need a lifting stab. If you have aerodynamic nose-down pitching moments from windshieds, etc., then you certainly need a down-loading stab to compensate for that. This will be a CLT gyro with a down-loading stab - what's the problem with that? Why do you need to have a HTL in order to justify a down-loading stab?

I have a basic question, and perhaps this shows my ignorance on all of this. People refer to "AOA stability". This may not be exactly what is referred to as "Airspeed stability", but are not these two essentially related - when an aircraft is AOA stable, it is "airspeed stable"? Rahgu's discussion on the other thread talks about “AOA stability”.
Exactly! Airspeed stability is a by product of AOA stability. G-load stability is a by product of AOA stability. In gyros, there are secondary factor affecting both, like the offset gimbal rotor head and the stability of the rotor itself, but to the most part it's all about AOA stability.

I may have been the one who introduced "G-Load stability" as the necessary ingredient to prevent buntovers. This is essentially a concept from Chuck's CG/RTV offset concepts. I think Raghu's concepts ignore such a thing as G-Load stability - lumping it all into AOA stability! But, this concept of AOA stability seems to me to be really the "airspeed" stabilizing mechanisms of changes in wing/rotor AOA correcting for airspeed changes. I don't feel the mechanism of a buntover can be totally AOA or "airspeed" instability – an airspeed unstable aircraft will simply continue to increase airspeed uncontrollably into a steep glide – not suddenly flip forward!.

I believe the mechanism for buntover (or PPO) is that the suddenly decreasing G-Load creates more nose-down pitching which decreases the G-Load further, and on and on! This would be Chuck's often described mechanism of a buntover from the CG aft of the RTV – pitch responding nose-down to a reducing G-Load..
The highlighted line above is simply the result of AOA instability. There is no need to make the problem more complicated than it really is.

Udi

bpearson

03-01-2007, 12:23 PM

Glad to see that I'm not alone in doubting the Houston report Greg. Ask him how the v c of g was determined and he threatens to sue you. He refuses to enter into debate on how he measured variables. At least with peasants like me.

I may be wrong but I find it difficult to believe you can accurately determine v c of g by computer modelling.

The conclusions that he came to that now restrict us to 2" offset were arrived at by a 'distilation of the results' so he told me.

We are now told in the UK that HS's do nothing to help stability and fitting them is not a route to remove restrictions.

It would have been interesting if they had tested the Magni without the stab !

Doug Riley

03-01-2007, 12:34 PM

I'd go further and label "G-load stability" as simply a metaphor for AOA stability. The gyro doesn't change its weight. Any perceived "G" change is simply an acceleration produced by a (commanded) change in rotor lift.

There's nothing wrong with the term G-load stability. It does divert attention away from what is actually going on, however, which is that we are changing rotor thrust by (rapidly) changing its AOA.

gyrogreg

03-01-2007, 12:36 PM

Greg G , i dont know you from a bar of soap, but damn i got to give you a full score of 10 pionts for this double back flip with twist pike over this subject of CLT and prop thrust line, and alll the other crap that you have spewed over my puter screen in the preceeding couple of pages of this thread, wouldnt have any thing to do with the fact that you are the Magni dealer in the US would it, frig you people are truely unbelievable, you and PB are tarred with the one brush, i can only immagine the fall out if this machine had a RAF or Air Command badge on the side instead of the Magni name.
I asked this question on the Oz forum, still to get an answer thou.
Why are the Magni people SO against putting step in the keel, to do two things one lower the thrust line and to be able to swing a bigger prop, as the M22 i flew was a pig of a thing as far as performance went, and it had a 914 on it,,, and yet Birdy's RAF flys alot better with more weight on the same motor ????

Sorry "Bones", I really don't have time to keep up with more than one forum - usually can't keep up with one! You Aussies have such colorful language! I guess the Magni people have not put a step keel on their designs because, so far they have not had to - to maintain an excellent safety record - and especially from the PIO, buntover and rollover issues that seem to plague a lot of designs. But, I think they also like the stylish low profiile, the more stable ground handling, the easier entry, etc. But, those may be customer preferences to like or dislike.

I'm not aware Birdy has an RAF with a 914! Shows how much I keep up with forums! That would be interesting to know the RAF flies that well with 2/3 the normal HP. That's encouraging! We might expect similar performance from the Magni M23 then? As to whether they both share the same stability attributes, I guess we'll have to wait to see - 'cause I really don't know for sure. But, I have more than just a little bit of faith in that huge tail, in the harmony they have always achieved between the rotor and airframe and controls, and in the Magni passion for stability/safety. I am pretty sure they would not callously endanger that reputation!

I'm not sure what you think I've got against Air Command - I'm the very first person that ever modified an Air Command to a high seater - my High Command, back in the early '90's. I have often commended Air Command for improving their stability/safety. The stepped keel / better prop thrustline is another tool to "skin the cat" of stability.

As far as an "RAF sticker" - I have often said and advised people that a very large HS, such as maybe the new Air Command Tripple Tail, placed on an extended keel RAF is likely to be an excellent improvement. And, a bit like the M23, I am still interested in seeing some actual flight test results - but the subjective assessment of a few pilots with this RAF mod seem to attest to much improved stability/safety with this mod - similar to the M23? - I don't know! If the M23 gets into production, maybe we will find out!

Please post this answer on the Aussie forum if you like.

Udi

03-01-2007, 12:43 PM

...We are now told in the UK that HS's do nothing to help stability and fitting them is not a route to remove restrictions.

It would have been interesting if they had tested the Magni without the stab !
I feel sorry for you guys. A properly designed gyro with a 3" HTL may be far more stable than an improperly designed gyro with 1" HTL. I agree with Greg that prescribing a 2" limit on thrust line offset without any regard to other important parameters is ludicrous.

It seems odd to me that your regulators have chosen to base their laws on one man's study and recommendations. It appears that ignorance is running deep with everything gyroplane.

Udi

Aussie_Paul

03-01-2007, 12:50 PM

you and PB are tarred with the one brush, i can only immagine the fall out if this machine had a RAF or Air Command badge on the side instead of the Magni name.

.....;Being tarred with the "one brush" that wants less gyro fatalities and stable gyroplanes is indeed a great compliment.

Bones. The early Air Commands and the RAF with no h/stab and HTL made no attempt to produce a stable gyroplanes. Magni, and other manufactures do.

Aussie Paul. :)

Greg Mitchell

03-01-2007, 01:38 PM

Originally Posted by Greg Mitchell View Post
G'Day Greg G, You did not upset me. Not at all.
Thanks! I'm very glad to hear that! The thought that I had bothered me last night.

That's funny Greg! We go back three plus years on PM's and having met you and flown with you, I got a good laugh outta this. Cheers Mate!

Greg you know I have read and re-read you papers. I read every damn page of notes and PMs to and fro on the consensus form when you gave me sideline access to it way back at the beginning. Whilst I dont purport to being the academic that many are here on this forum, I do get the 'drift'. And as such, have simply asked a valid question or two.

As always I appreciate you detailed responses, however, I too feel you make the problem more complicated than it already is....and this happens to go hand in hand with supportive arguements for explaining away high thrustlines. Again let me say, Magni does indeed have a great safety records with their tandem BUT this craft looks like it could PPO in a heartbeat given the right conditions.....now if it's your contention that a considerable LTL craft can 'Bunt' (drag under/over, call it what you will) , then surely this aircraft should be talked/written about with the same vigor, that you do on other aspects of design configurations.

I'm starting to think that many of your arguments now are simply a 'base' covering excersise.

Perhaps after I do a course in higher level Math and Physics, I might be able to engage you rigorously with respect to near CLT (low) and having an inline stab or one which is parrallel to the thrustline, which even though immersed presents no turning moment when power is added or taken away....alas, until such time I will have to continue to follow the debate that Doug, Udi, Chuck, Rahgu and others are so adept at.

Certainly after being mad keen on gyros for 25 years and involved for the last 4 years, I have some info on board. Nearly 40 hrs in a variety of two seaters, 16 hrs of solo in Butterfly (1) about to take to the air again after a rebuild, it's plain to see I dont have a lot of flight time. This should not preclude me from asking questions or making comment, as your ole mate Bruty would have it.
If you dont fly you dont talk. PB you're a classic!

Butterfly, got a 'hammering' from the 'pundits' downunder when we first started out. Still cops flak from some quarters....dont care, proof is in the future sales and you should not forget to mention Larry Neal is largely responsible for the change in the aircommands, even if you think he uses cookbook fixes and incomplete and restrictive laymans engineering.
Infact, I dont believe you have any more or less of an engineering background. So much of your comment can be discounted as sales blurb can it not. Like you say Greg the proof is in the products history, so lets wait and see.

You side stepped the PPO question. Trust is not enough!

Mitch.

I guess we will have to wait and see if indeed it goes into production or not.

gyrogreg

03-01-2007, 01:50 PM

Hi Greg,--------------- Udi

Hi Udi, really enjoy the banter. We can all learn! I'm not even going to attempt to get into all the things in your last post right now - I'm kinda out of time for now. But, you make good points, and I'm not sure I sometimes don't just over explain more than I should.

Since we are on the subject, I do have a couple other basic questions though. If G-Laod and Airspeed stability are just manifestations of AOA stability, why do professional test pilots regularly evaluate both Airspeed and Maneuvering (G-Load) stability. Are these just ways to wring out AOA stability more completely? And, for the classic Chuck Beattie diagrams on the tailless HTL CG/RTV relationship indicting the propensity for a divergent nose-down pitching as the result of a reduced G-load - turbulence - how does that really relate in an understandable way to the mechanics of AOA stability?

I do agree that it is hard to imagine a pure disturbance to G-Load - although I have seen parachuters jump out of the back seat of a Magni a few times, and once out of the back seat of a Parson's. But that is not quite normal. Actual G-Load disturbances - that we are concerned with that might instigate a buntover or PIO - are two mechanisms that I can think of:

1) Pilot rapid cyclic input. This does two things when we talk about airframe pitch reaction. The rotor disk AOA changes - immediate change in rotor thrust. The RTV moment arm to CG changes too - at the same time. A delayed reaction is the RRPM adjusting to the new load and the rotor "Blowback" then re-adjust back closer to the original disk angle when the RRPM steadies out at new RPM!

2) Sudden up or down wind gust - down is the critical direction as it induces a reduced G condition where you don't want a divergent nose-down pitch reaction to that G-Load disturbance. If there is a HS, presumably that wind gust - relative wind change - affects the rotor disk and the HS at the same time. The lift reaction of these two lifting elements may be different (lift curves of the two) and the disturbance will change the RTV position as well. If this difference is in the right direction, and the RTV/CG moment arm doesn't go too far forward of the CG, the airframe probably responds in the corrective, stablizing direction to the gust.

I think it is traditionally presumed that a buntover in an unstable gyro (G-Load unstable?) can be initiated either by wind gust or pilot cyclic input. Those are probably the only two G-Load changes that can be quick enough to instigate a divergent reaction that the pilot could not readily stop - in an unstable gyro. So, in the traditional Chuck Beattie diagram of RTV forward of the CG (typically a HTL with no HS), there really is no clean sudden pure reduction of the Rotor Thrust that would initiate the buntover envisioned in those diagrams. I think both possible Rotor Thrust disturbances - in the reducing G-Load direction - will also change the rotor disk angle to less AOA and move the RTV rearward - relative to the CG. If the RTV is not initially too far forward of the CG, that rearward re-alignment of the RTV would likely cross over to aft of the CG to the stable condition.

I think this says that the RTV can be even slightly forward of the CG since the problem G-Load change will have to also move the RTV rearward toward or into the more stable condition.

Perhaps the gyros that are able to buntover actually have their steady state RTV way too far forward of the CG. In that case, a suddenly reduced G-Load, also accompanied with a rearward movement of the RTV, just doesn't move the RTV far enough to get into stable territory aft of the CG. In this case, upon sudden reduction of rotor thrust (due to either commanded forward cyclic or a down gust), after the RTV readjusts, it is still in the Negative stability (negative G-Load stability) condition - with the nose still dropping and the rotor thrust still reducing divergently. I can envision this resulting in a continued sustained nose-drop and G-load dropping, buntover.

Sorry, I just get going! As you are aware, Udi, I am interested in some simple flight test that reliably would indicate if the gyro is buntoverable or not! This is certainly why I continue to focus on G-Load stability - because verifying it - properly - can indicate just where the RTV is in relation to the CG. If we can verify that the RTV is not forward of the CG by testing the airframe reaction to a G-Load change, we can assure that the a sudden reduced G-Load along with the improving RTV movement will not end up with the RTV forward of the CG where divergent G-load reduction can continue. And that, seems to me, would assure that gyro would not buntover in that power/airspeed condition!

I have always been interested in this G-Load subject in an attempt to help the gyro community understand the stability/safety (buntoverability) of their gyro. With the HTL focus and PPO debate within this Magni M23 thread, I am taking the opportunity to re-visit this G-Load subject see if there are questionable areas of that particular test. As you know, we have recently surfaced the concern that cyclic stick POSITION, rather than PRESSURE, might be the more reliable and indicative parameter in that particular test. Wringing out the subject again and maybe more, might help validate or improve flight evaluations - for everyone.

- Thanks, Greg

Udi

03-01-2007, 07:52 PM

...and I'm not sure I sometimes don't just over explain more than I should.
:sad: :boink:

Since we are on the subject, I do have a couple other basic questions though. If G-Laod and Airspeed stability are just manifestations of AOA stability, why do professional test pilots regularly evaluate both Airspeed and Maneuvering (G-Load) stability. Are these just ways to wring out AOA stability more completely?
I am sure this is not the whole story, but the aircraft response to airspeed changes and G-load changes is different for different CG locations and loadings, airspeeds, and G-loads. The flight tests should define CG limits, loading limits (that is partially structural and partially aerodynamic), and define a safe flight envelope for all these conditions. Also, there are always additional factors that make the aircraft response maybe less predictable. In gyros, the use of an offset gimbal rotor head completely muddies the water. Also, the rotor itself is a player in the whole AOA stability issue and like you always like to say - there is nothing like flight testing to find out how the gyro really behaves.

And, for the classic Chuck Beattie diagrams on the tailless HTL CG/RTV relationship indicting the propensity for a divergent nose-down pitching as the result of a reduced G-load - turbulence - how does that really relate in an understandable way to the mechanics of AOA stability?
Without a stab, the only thing that gives a HTL gyro a small measure of AOA stability is the offset gimbal rotor head. Lock the stick (to cancel the pseudo-stability provided by the rotor head), and Chuck's diagrams of the RTV passing fwd of the CG will play themselves out very quickly. The explanation is simple (as you have explained many times, Greg). For an aircraft to have positive AOA stability the CG of the aircraft must be located fwd of the aerodynamic center of the aircraft. If you have only one lifting device, such as a fighter jet delta wing or a gyroplane rotor with no HS, the CG must be fwd of the aerodynamic center of this lifting device. Adding a stab is moving the aerodynamic center of the gyro back, allowing the CG to move back too. That is why a stabbed gyro may have positive AOA stability EVEN with the CG located aft of the RTV.

I seem to remember some posts by Raghu where he came up with an equation that determines whether a gyro is stable or not, simply based on stab volume and thrust line offset. I can't find it now.

I think it is traditionally presumed that a buntover in an unstable gyro (G-Load unstable?) can be initiated either by wind gust or pilot cyclic input. Those are probably the only two G-Load changes that can be quick enough to instigate a divergent reaction that the pilot could not readily stop - in an unstable gyro.
The more common one, I think, is the PIO -- this is the case where the pilot with his own actions is interfering with the only mechanism that can stop the oscillations - the offset gimbal rotor head. As the oscillations get larger and larger, the gyro is getting closer to the point of no return, where the prop nose-down pitching moment is becoming stronger than the <1G RTV nose-up pitching moment - and the gyro goes over.

So, in the traditional Chuck Beattie diagram of RTV forward of the CG (typically a HTL with no HS), there really is no clean sudden pure reduction of the Rotor Thrust that would initiate the buntover envisioned in those diagrams. I think both possible Rotor Thrust disturbances - in the reducing G-Load direction - will also change the rotor disk angle to less AOA and move the RTV rearward - relative to the CG. If the RTV is not initially too far forward of the CG, that rearward re-alignment of the RTV would likely cross over to aft of the CG to the stable condition.
One case this may not be true is a push at the top of a zoom. There was a famous crash in the UK in one of Commander Wallises gyros (the pilot was Pee Wee Judge?) where this is exactly what happened. In one push he crossed the point of no return. This cannot be done in a AOA stable gyro.

I think the important point that many people fail to see is that a properly sized and placed stab can dramatically improve the stability of a gyro, so much so, that the CG/RTV relationship becomes a secondary consideration, as long as it is not too out of whack. The Gyrobee is a good example.

Udi

Aussie_Paul

03-02-2007, 03:15 AM

If you dont fly you dont talk. PB you're a classic!

Mitch.

It's not the questions Mitch, it's the attitude. :rolleyes: :lol:

Aussie Paul. :)

Doug Riley

03-02-2007, 05:35 AM

Greg, I should clarify that I think AOA stability and G-stability are the same thing. I don't think airspeed stability and AOA stability are the same thing.

For example, (leaving out airframe moments) a gyro with a simple (non-offset) spindle head is airspeed stable thanks to rotor blowback. It is specifically UNstable with respect to AOA -- and, since you create increased G by increasing AOA -- it's G-unstable, too.

Heron

03-02-2007, 07:52 AM

Question if I may . . .
two machines, same situation and speed, and all that
They loose the stab for some reason . . .
what now?
Pick any two and compare.
thanks
Heron
PS situation means: at the edge of the problem

gyrogreg

03-02-2007, 08:01 AM

---------------------
I think the important point that many people fail to see is that a properly sized and placed stab can dramatically improve the stability of a gyro, so much so, that the CG/RTV relationship becomes a secondary consideration, as long as it is not too out of whack. The Gyrobee is a good example. Udi

Udi, thanks, good discussion. And, I agree especially with your final paragraph.

I keep pondering the idea to present all the variable and conditions in Sum of Moments diagrams - starting simple like a CLT, no drag, tailless gyro, then to HTL and LTL, then to the same configurations with a HS, then adding some airframe drag and drag offset, etc. With all of these put in a step function change, plus and minus for airspeed and cyclic input, and power - to see the resultant initial sum of moments imbalance and the final steady state condition after the disturbance settles out. I'd really like to do this with diagrams and equations - probably in an Excel spreadsheet. - But, I can barely find time to keep up with PRA business, some occasional training, and some occasional Magni business. But, this is a dream I'd like to work through some day - would help solidify a lot of concepts and engineering principles.

You said these are not "mind games". But, to me they are until we can put them on paper like I'm describing above.

However my "mind game" analysis tells me what I am pretty sure what the paper would show. Let me propose some key elements here:

1) "Dynamic Stability" is perhaps a misnomer. Dynamic response may be the all-important element in gyroplane safety. The real dynamic parameter is the damping factor. The Damping factor regulates the rate at which the gyro can pitch. It also determines how quickly the natural pitch oscillation can be "damped" to zero - how much overshoot there is to a pitch change, how many times pitch oscillates about level. These are important, especially in short-coupled, low MOI airframes where the response to a pitch moment might be quick and the natural oscillation rate is fast. Test Pilots tell me that the ideal aircraft is one that has a very high or rapid natural oscillation frequency and fast pitch response. Such an aircraft is very responsive to commanded pitch inputs - the thing many people like about gyros! But, such pitching rates are beyond what a human pilot can regulate or "dampen" themselves. So, such aircraft need an inherent pitch dampener - the easiest one of those is probably a good HS. The test pilots tell me that that "dampener", for safest and most pleasing control with quick response, needs to dampen any overshoot (or natural pitch oscillation) within 1/2 cycle. That way, the pitch responds quickly to a commanded pitch input, but does not overshoot much past steady state condition, and does not continue to oscillate. If the aircraft does “overshoot” far and/or does not tend to continue oscillations, the pilot is not inspired (or excited) to command stabilizing inputs his/herself - which could lead to PIO because the human pilot really cannot stabilize such quick natural oscillation rates! So, an essential element to avoid PIO and the normal resultant buntover, is to have a good Dynamic "Dampener" - good enough to dampen any natural pitch oscillation within 1/2 cycle ideally! We have often discussed how a HS can be optimized to be a very good pitch dampener - large, far aft, good aerodynamic lift characteristics, etc.

So, when we say we need a gyro to be "Dynamically Stable", we really mean that its quick natural pitch rates and oscillation frequency need to be critically "damped", enough so as to avoid PIO or pilot over reaction to strong overshoots or continued natural pitch oscillations. If this requirement is achieved by a HS, this alone probably resolves any STATIC pitch issues that could lead to a buntover anyway! Essentially too, if there is no HS, there is no Dynamic Pitch dampener - and PIO will always be the demon lurking to spring on the unaware pilot!

Static stuff:

2) A tailless gyro with CLT and CLD (Centerline Drag) will always fly with the CG located exactly on the RTV. This is neutral AOA stability - as well as neutral G-Load stability and Airspeed stability - in as much as disturbances of those two elements cause no reaction in pitch.

3) The critical pitch disturbances to any gyro are pilot commanded input, and wind disturbances. The most critical of these is a forward cyclic movement and a down gust of wind. This is because this is the direction of a buntover that might be initiated if the CG / "Resultant Lift Vector" moment is in the destabilizing direction to sustain a divergent nose-down pitching.

3a) Let's take pilot commanded input first: On the tailless, CLT, CLD gyro where the CG is exactly aligned on the RTV, a forward stick movement will immediately decrease loading on the rotor, but it will also move the RTV aft of the CG - creating a positively stable CG/RTV moment. So, in this case a buntover cannot be initiated because the CG/RTV moment is in the restorative direction after the cyclic is moved forward. The actual pitch response is a resisting, nose-up tendency because the RTV is now aft of the CG!

3b) Now consider a sudden wind downgust on the same tailless, CLT, CLD gyro: The downgust reduces the rotor load - reduces G-Load immediately. But, because the CG is exactly on the RTV, nothing happens in pitch. Very quickly though, the rotor "disk AOA flattens, reduced AOA, RTV moves aft - due to dissymmetry of lift created reduced "blowback". In this case, as in the case of pilot commanded forward cyclic, the RTV moves aft of the CG and a buntover cannot be initiated because the CG/RTV restorative moment is in the nose-up direction. The actual pitch response is a resisting, nose-up tendency because the RTV is now aft of the CG!

4) Now let’s consider this tailless gyro, but with a slightly high prop thrustline: In this case, the in-flight Sum of Moments forces the RTV forward of the CG in flight.

4a) For a pilot commanded forward cyclic, the rotor lift or G-Load is immediately reduced. But, also immediately the RTV is moved aft – maybe aft of the CG, maybe not! But, since the original CG/RTV offset was so small, the CG/RTV moment may actually be neutral or restorative/stable, or, if the RTV is still on the unstable side of the CG, the moment arm is not inducing such a bad sustaining nose-down pitching that the pilot might be able to stop the pitching. Actually, whether the pilot can stop the pitching or not, depends on the dynamic pitch response – fast, or slow. In the case of a tailless gyro, there is no real “damping”, so the MOI (short coupled body or long-coupled body) is the element that may determine whether the pilot allows the gyro to continue to a buntover or is able to stop it.

4b) For a sudden down gust, essentially the same thing happens with rotor load and RTV position – except that it may be a surprise for the pilot and therefore inspire a bit more over-reaction. Again, there is not help in softening the disturbance pitch reaction from a pitch dynamic dampener. But, for this minimally HTL gyro, it would likely not buntover if the pilot has any proficiency in that gyro at all! In this gyro configuration, the resistant nose-up pitch tendency might be somewhat weaker or non-existent from what it was for pure CLT in 3 above.

(Indulge me and let me keep going as long as my mind is rolling!)

5) Now let’s consider a severe HTL tailless, CLD gyro: In this case, the in-flight Sum of Moments forces the RTV well forward of the CG in flight.

5a) For a pilot commanded forward cyclic, the rotor lift or G-Load is immediately reduced and the RTV immediately moves aft. But, depending on the forward cyclic input, the RTV is likely not going to move to the stable condition aft of the CG. The reduced RTV moment initiates a nose-down pitching motion – how rapid depends on the MOI of the aircraft! With the nose-pitching down, the rotor load or G-Load is continuously reducing – more nose down moment because the RTV has remained forward of the RTV – a buntover is initiating!

5b) For the wind down gust – the same thing essentially happens – it may be likely that the RTV remains forward of the CG and a buntover may be initiated. So,

One conclusion we might draw from this is that if we can verify that the CG is on or forward of the RTV in the steady state flight condition, a buntover is assuredly prevented. That is what I want to the G-Load stability flight test to assure, and the ASTM criteria of this to require – the reason to emphasize that particular test – buntoverability or not!

Note also, that a commanded aft cyclic input or an up-gust might cause the RTV to move to the unstable position forward of the CG, but that might only result in a divergent nose-up pitching which cannot be sustained because of climb slowing the airspeed, etc. – it is not likely that it is going to “bunt up”! Essentially, we can probably ignore increased G disturbances – especially for stability type accidents!

A similar analysis can be done for a LTL tailless CLD gyro, but for buntover concerns this is not really necessary because the Sum of Moments with the LTL assures the RTV is in the stable condition to start with – aft of the CG.

6) Now, let’s add a HS to balance the HTL. The gyro is still CLD – not worrying right now about airframe drag and/or lift moments affecting pitch attitude or CG/RTV relationship. A smaller HS may not be adequate to well balance the HTL – in that case, we probably have a similar situation to 5 above – slightly less severe instability, but still possibly able to buntover depending on airframe MOI, how bad the HTL is and how much the HS compensates for the HTL.

But, if the HS is adequate to well balance the HTL, we have a little different situation with the standard concept of the CG/RTV relationship. Now, the HS is also providing some lift – downward in the case of this HTL, but the total “Composite lift” of the aircraft is now a composite of both the rotor lift and the HS lift. In this HTL case, this actual “Composite Thrust Vector” (CTV?) is a little forward of the actual Rotor Lift Vector. And, the important CG relationship for G-Load stability is now the CG relationship to this new CTV that can create the stable or unstable moment arm for airframe reaction to pilot commanded or wind gust initiated G-Load changes. Since the CTV is now even further forward, the HS must “balance” the prop offset enough to hold the nose up enough to hold the CG forward to closely match the new CTV. If the HS is adequate to do this, we do essentially have the conditions of the original CLT gyro in 3 above!

6a) Pilot Commanded cyclic input – forward stick: Forward stick movement immediately reduces rotor thrust and immediately reduces the total composite amplitude of the CTV. Also immediately, the CTV moves aft of the CG and the system is then G-Load stable - buntover is prevented. (Buntover potential is further reduced because now we have a dynamic dampener as well – regulating the pitch reaction - HS doing double duty!

6b) Wind down gust: This is a bit different than the situation in 3a above, but not much. The down-gust actually affects the rotor in both rotor thrust and CTV position, but the wind gust also affects the HS in the same direction. The down gust also increasing the down lift on the HS, prevents the airframe from pitching nose-down – at least not as much as if there were no HS! But, the CTV is still moved aft of the CG, so there is no nose-down moment that could initiate a buntover – buntover proof! And, the restorative CG/CTV moment provides the resistant nose-up tendency that further as in the CLT situation of 3 above.

(hang in there, I’m almost ready to quit!)

This exercise can be extended to the LTL condition and what the HS contribution is to CTV/RG relationship and reaction to commanded and/or wind down gusts, etc. – ends up to be even more buntover proof.

Where it all starts getting complicated – more interrelated variables to consider – is when we add some airframe drag and lift and check what happens to pitch as a reaction to power changes – especially abrupt power changes - airspeed as a result of power changes, etc. The nose-down airframe lift and drag moments are destabilizing – move the RTV forward – requiring the HS to do triple duty now (dynamic dampener, balance the prop thrustline, and balance the airframe drag/lift moments). It’s when we start considering all of these combinations that some of the “harmony” I often mention gets important – how well the HS is able to handle its “triple duty”, but especially its douple duty of balancing the prop thrustline and the airframe aerodynamic moments at the same time. This is where Power stability compromises with Airspeed stability compromises start getting involved.

Unless I were to continue adding more and more configuration variables to this list, I guess I’m ready to stop here. In summary, my main point is that there are many ways to skin the stability cat, and when all moments on a gyroplane are considered, things get more complicated - so complicated, that the only really valid way to make sure things end up proper is to do the flight tests – at least the Power, Airspeed and G-Load flight tests.

My main intent here is to assure that our G-Load flight testing is truly a way to verify (or not) that the gyro cannot buntover! Whether we are talking about CG/RTV moments, CG/CTV moments, AOA, AS or G-Load stability, I do believe that if the flight test can show that the airframe pitch or AS response to G-Load changes would be in the corrective direction, we can assure the that gyro cannot buntover. I do think this means the CTV (or RTV when there is no HS) is on or aft of the CG. When this is the case, either pilot forward cyclic movement or wind down-gust will not result in a destabilizing condition where the CTV (or RTV) ends up forward of the CG while the nose is pitching down sustaining further decreasing rotor lift and forward nose bunting.)

A very valuable by-product of this stable condition, if it is achieved by or accompanied with an adequate HS, that gyro is not only buntover proof, but PIO resistant because of the dynamic dampening that HS provides. That pitch dampening from that same HS, will however be further enhanced by positioning the HS well aft – even if the HS size is decreased when it is moved back. (Just moving it back, makes it more effective dynamically, and both the static and the dynamic value is improved by the longer moment arm!)

It might also be said that, employing as effective of a HS as you can and placing it far aft, the other Static stability and “balancing” jobs the HS does are either fully accomplished or maybe not even be as necessary any more. So, attending to the Static requirements from a HS, probably accomplishes the dynamic dampening requirements, and vice-versa!

This is all the same thing you said above, Udi – in far fewer words! I’m just trying to burn the principle in through some explanation of why all this is! Our challenge really isn’t showing technically just how it all works! The challenge is improving the understanding and attention and culture that still prevents full application of what we know works! And, I suggest the Magni is a good example also! (I’m hoping the M23 turns out to be just as good of an example too!)

- Thanks, Greg Gremminger

gyrogreg

03-02-2007, 08:05 AM

Udi

03-02-2007, 08:28 AM

Here is a cute little web site that talks about pitch stability, AOA stability, and the contribution of a stab to an otherwise unstable configuration... This is a FW discussion but if you replace the wing aerodynamic center with our RTV, it becomes a gyro :)

http://selair.selkirk.bc.ca/aerodynamics1/Stability/Page9.html

Udi

Doug Riley

03-02-2007, 09:43 AM

Oh dear.

Greg, I hope I'm failing to understand what you mean.

It's true that, in equilibrium flight, the stable position of the rotor thrustline is aft of CG (=CM). "Stable" means, of course, "tending to return to a prior state when disturbed."

Once you're in the middle of the "disturbance process," however -- once you're out of equilibrium -- that rule doesn't apply any more.

Look again at the simple CLT/no HS/centered drag gyro. The rotor thrustline comes right down through the CG during equilibrium flight.

Then pilot tosses the stick forward and holds it there. Rotor AOA decreases; rotor thrust decreases in proportion. If nothing else happend, the machine would sink in a level stance, increasing AOA again and thereby reacting quite stably.

But, darn it, the rotor thrustline also swings aft of CG. This new, aft-of-CG position of the rotor thrustline is, under the circumstances, NOT stabilizing at all, but DE-stabilizing.

Why? The pilot already has decreased rotor AOA. The last thing he wants is for it to decrease some more on its own. Yet, by moving the rotor thrustline aft of CG, he has upset the balance of moments on the frame. The airframe now has a net nose-down moment, caused by the rotor thrust pulling up, aft of CG. The gyro pitches nose-down which, if the pilot holds the stick still, will FURTHER decrease rotor AOA. With a massless rotor and airframe, this situation would clearly be statically unstable.*

Fortunately, the rotor isn't massless. It lags behind any cyclic input it gets. The lag can, if it's long enough, keep the rotor thrustline's aft movement slow enough that Sir Pilot takes out his stick input before much overshoot occurs. This is rotor damping (nope, not dampening).

You can walk through the other scenarios the same way. The point is simply this: that, once we create disequilibrium, the rule that RTV aft of CG is stable goes out the window. RTV in fact moves in an UNhelpful direction every time we change rotor AOA. It moves so as to cause a frame pitching rotation that amplifies the control input, whether fore or aft.

Movement of the RTV relative to the CG is an unfortunate byproduct of our method of controlling rotor AOA. Much of what we do to make gyros fly properly -- from jab-and-return control techniques to big H-stabs -- is targeted at reducing this unwanted secondary effect.
__________________________
*Remember Igor Bensen's admonition to jab-and-return the stick? As an engineering test pilot, he knew lousy damping when he saw it, though of course he'd never use those words.

Greg Mitchell

03-02-2007, 01:46 PM

I dont see you Talking the Talk here Bruty! Why not........???
Perhaps the subject is a " bit beyond incomplete and restrictive layman engineering." And just maybe, your in the same boat as me....not academic enough to post at this level. Or is it that FIREBIRD has a far higher thrustline than the figures you have released and this is all a headache for you. So lets hope some of this thread, sorts the s*it from the clay.

At least my questions have generated some valuable discussion, most of your posts are self promoting....so with all due respect and in the nicest possible way, go stick my attitude up your....:boom: :painkiller:

See you at Lameroo with your new Firebird;)

Greg, Udi and Doug, I very much appreciate you guys taking the time to hash this out and answering my earlier questions.
Greg if I you feel I have been at all rude to you, then I apolagise.:sorry: That certainly was not my intention. Vigorous debate is.:focus:

I think Bones makes a very good point.
If this craft had Raf or early AC (lowrider) Badges, then the discussion may well have been approached entirely differently.

My question as to this S X S Magni stands. If it has a thrustline higher than the current tandems, then given the right set of conditions, is this not at PPO risk? I understand fully that the body of the cabin and tail on a longer moment arm ect are all factors which may diminish this possibility, however, when their ability to do their jobs is overpowered by the other moments then surely, such a high thrustline craft represents risks.

Greg I might be reading between the lines too much but I get the feeling the thrustline in this case is something you would rather not have had to deal with. Would you have preferred to see a prototype with less of an offset, given the current generally accepted views? Now if this question is construed as bad attitude, or if you feel it's a loaded question, dont feel you need respond. That' how Bruty opperates, he just goes quiet or disappears to this forum. :boink: :D

Mitch.

gyrogreg

03-02-2007, 03:50 PM

My question as to this S X S Magni stands. If it has a thrustline higher than the current tandems, then given the right set of conditions, is this not at PPO risk? I understand fully that the body of the cabin and tail on a longer moment arm ect are all factors which may diminish this possibility, however, when their ability to do their jobs is overpowered by the other moments then surely, such a high thrustline craft represents risks.

Greg I might be reading between the lines too much but I get the feeling the thrustline in this case is something you would rather not have had to deal with. Would you have preferred to see a prototype with less of an offset, given the current generally accepted views? Now if this question is construed as bad attitude, or if you feel it's a loaded question, dont feel you need respond. That' how Bruty opperates, he just goes quiet or disappears to this forum. Mitch.

Greg M., I'm not afraid of this "HTL". I have no doubt that that tail is very powerful, and I doubt it would be "overpowered" by other moments. That's a BIG and efficient tail, and back further on the tail really multiplies its effectiveness - especially its dynamic effectiveness. If I did not have so much confidence in Vittorio and Luca and Pietro, I might be concerned the HS might not be adequate for the full static stability margins we are used to from Magni. You would really need to meet this family to fully appreciate their gyro talent and passion for anything that might affect safety.

A bit of a story on the M19 - their first effort to enclose the tandem configuration. At least this is the way I understand the story - maybe lost something in the translation! They were having trouble, as might be expected, getting the yaw stability they require with the extra "wing" area forward of the CG. They were working enclosure contours, stab configurations, etc. Vittorio was ready to declare that OK - a lot of gyros have yaw problems and might require a little footwork - after all Vittorio had flown a lot of gyros that had a lot more problems than that. Pietro and Luca balked - would not approve the configuration. In the end, they continued and did find the right configuration. Two prototypes of the M19 (one in South America, one in France) have been flying for probably 7-8 years now. The M19 never went into production because the cabin was too small for Yanks, South Africans (and probably Aussies!). They worked on a new larger cabin tandem, the M21, for several years - compensating for the forward larger tandem enclosure was apparently the same problem! They worked for several years on this. Apparently, they concluded they could not compensate for that long enclosed tandem and switched gears for the sbs. If they switched to a sbs configuration to solve the stability problems associated with the tandem cabin, I strongly doubt they would not have the same requirements for the sbs!

I don't have any real concerns for possible buntovers with this. I do believe that a well damped airframe, and that big HS will certainly be a strong dynamic dampener, just isn't a buntover risk because the dampener eliminates the short-period reaction enough for the pilot to never get into PIO or rapid nose-down pitching - at least not enough to sustain a buntover. (It would be very hard to draw conclusions about this M23 thrustline based on the RAF thrustline and history - an obvious part of the difference is that tail!)

I'll fess up! Knowing the Magni internal factory standards, my biggest concern might be that they could show up the ASTM criteria to be inadequate, especially the G-Load static criteria - by actually not passing it! It would then take time and a lot of flight time - accident data - to either show the M23 doesn't buntover even though it didn't pass the standard's criteria for G-Load static stability. Or, if long term accident data didn't show a propensity for buntovers, then something may be off base with the criteria in the ASTM standard on buntovers. My confidence in the Magni team makes me more afraid that the ASTM standard might be shown to be inadequate in this area - and that is BIG problems for me - the M23 could never be declared compliant with the standard - which is a goal of ours! And, my ASTM team might have to go back to the drawing board if reality doesn't match paper! But, I actually have confidence that the M23 will pass the tests, so I'm not that worried about this much!

I am curious though, how they do manage to avoid buntover potential with this - my theories are all speculation at this point. I may never know the technical specifics, moments, etc. The Magni factory usually doesn't rely on paper or computer design that shows exactly how a Sum of Moments "balance", for instance, is accomplished. That is so complicated it probably cannot be done anyway! I think Magni reaches it's goals like many other aircraft designers, by applying as many principles they have learned and well understand, then making adjustments on a flying model, iteration by iteration, until they either meet their standard, or reach a dead end. If you think some of the give and take between Udi and Doug and Raghu, etc. are complicated, all that stuff is probably childs' play for all the things that are really going on in any gyro configuration. That is why we are all saying, you have to test it, flight test results, to really confirm the design goals - you will never prove a gyro is totally stable or unbuntable or unPIOable on paper - the final proof is in the actual flight performance. When you try to hang your hat of specific thrustlines, draglines, rotor reactions, HS moments, etc., then throw in all the variables of dynamic interactions and aerodynamics of varying AOAs on airframes and enclosures, etc. - you just can't break down all the parts - you can only speculate on the final reasons it might actually meet the flight test critera. After all, the ultimate goal is results, and results need to be verified by results - can't do that on paper!

- Greg G.

Greg Mitchell

03-02-2007, 08:58 PM

Hey Now!:usa2:

I salute you Greg.

:hail: Greg G, Thankyou for that post. I appreciate your candour.

Can I put this together Greg :) :) ... "Greg M., I'm not afraid of this "HTL".......I don't have any real concerns for possible buntovers with this.............. I am curious though, how they do manage to avoid buntover potential with this." Bom! Bom! :drum:

If it does have buntover potential Greg G, which is what I am surmising, then the 'managing' of this is something I am greatly interested in. It's why I started the thread. I thought as it was about to go into production then you would have the details.

I hope you are right Greg G, I certainly enjoyed my flight with you and appreciate the educative/promotional role you play in getting the message out. I wish Magni , You and Steph every success for the future. Again I appreciate your promotion of the sport worldwide.

Thankyou,

Mitch.

gyrogreg

03-03-2007, 05:07 AM

Oh dear. Greg, I hope I'm failing to understand what you mean.
Doug, me too, or vice versa! I’m sure we’re on the same page here, I hope it’s just a “failure to communicate” the concepts clearly.

It's true that, in equilibrium flight, the stable position of the rotor thrustline is aft of CG (=CM). "Stable" means, of course, "tending to return to a prior state when disturbed." Once you're in the middle of the "disturbance process," however -- once you're out of equilibrium -- that rule doesn't apply any more.

Afraid I don’t see it this way! It is precisely the non-equilibrium, out-of-balance of the static moments and forces that causes the statically stable system to return TOWARD the equilibrium state. For a statically UNSTABLE system, it is precisely the unbalance of static moments and forces that starts and keeps it on its divergent course away from the equilibrium point. This is sort of the definition of static stability. For both a stable and unstable system, if it is “balanced” on its equilibrium point, and nothing disturbs it from that point, it stays in that equilibrium. It is when something disturbs that system from its “balance” or equilibrium of forces and moments that the stable system starts to return toward the equilibrium BECAUSE of the imbalance, and the unstable system starts to diverge from equilibrium BECAUSE of the imbalance causing continued and worsening divergence.

Once you are in the middle of the “disturbance process,” the dynamic effects of inertia and dampening enter the picture – but that just determines how the statically stable system will return to equilibrium – will it do so quickly, will it overshoot the “balance” point, will it oscillate around the” balance” point, and how long will it oscillate! For the statically UNSTABLE system, dynamic effects will simply determine how quickly it diverges to the extremes where something else has to happen – a buntover, for instance. I’m not saying either, that the overshoot or dynamic instability oscillations might not take the system to a static extreme where static stability goes negative, and the system begins to statically diverge – such as maybe an overshoot in pitch to an extreme where the static moments at that time are no longer stable and start to diverge. An example of this might be a buntover at the extreme end of a PIO experience.

I’m not going into this explanation to possibly patronize you, Doug, or anyone else. I see this as an opportunity for instructive perspective possibly for others. It may also be, with such detail, helpful for you to identify to me where I might be going wrong!

Look again at the simple CLT/no HS/centered drag gyro. The rotor thrustline comes right down through the CG during equilibrium flight. Then pilot tosses the stick forward and holds it there. Rotor AOA decreases; rotor thrust decreases in proportion. If nothing else happend, the machine would sink in a level stance, increasing AOA again and thereby reacting quite stably.

But, darn it, the rotor thrustline also swings aft of CG. This new, aft-of-CG position of the rotor thrustline is, under the circumstances, NOT stabilizing at all, but DE-stabilizing. Why? -------

Now I don't think I understand what you are saying. True, when the pilot moves the cyclic forward, the rotor AOA changes AND the RTV moves aft. At this point is where you are losing me. I think what you are describing infers that ANY gyro would continue to drop it's nose when the pilot puts the stick forward - even if the CG is well forward of the RTV (or CTV) – requiring the pilot to bring the stick back to the original position to stop the nose drop - maybe I'm confused!:

Perhaps my little sequence should’ve carried the sequence a bit further. At the point where the RTV is aft of the CG, and the rotor thrust is less than original S&L flight, there are static imbalances that will cause the system to proceed along the path their statically stable or unstable imbalances will take it. In this first scenario at least, let’s proceed from this point.

First, let me say that we certainly know that holding the stick forward in steady state is not going to initiate a buntover on a stable system. I don’t think anyone recognizes a pure CLT, even without a HS and with a moderate CLD, to be a statically unstable system. (A buntover might occur if the stick is too rapidly forced forward on an unstable system). The statically stable system will tend to return to a stable condition. In the case where the stick ends up being held more forward, the statically stable equilibrium point is with the aircraft at a higher airspeed – stick forward means higher airspeed, for the statically AIRSPEED stable aircraft, the slope of the stick/airspeed curve is positive! So, we do know where the steady state equilibrium point of this particular gyro should be – or at least I hope it will end up there if my conclusions are correct! The gyro will end up at a higher airspeed in a nose-down descent.

Let’s walk through how it gets there on the first tailless CLT /CLD example – starting from the out-of-equilibrium point where the RTV is suddenly aft of the CG and the rotor thrust is suddenly less because of its lower disk AOA:

There are several imbalances present at this instance. As you point out, there is a sum of moments imbalance that is going to try to lower the nose. But there is also a prop thrust and airframe and rotor drag imbalance that exists. The sudden loss of rotor drag, without an immediate increase in the other drag component (parasitic drag of the airframe), means that the excess of prop thrust is going to accelerate the aircraft to higher airspeeds. (Also, at the nose-down attitude, gravity adds some to the forward “thrust” supplementing prop thrust.) As airspeed grows, two things start to happen, the parasitic drag of the airframe starts to increase, and actually the induced drag of the rotor starts to increase as “blowback” from dissymmetry of lift starts to restore some of the rotor drag. Ignoring that the undamped dynamic response might cause overshoot, eventually the total parasitic drag of the airframe, and the induced drag of the rotor will match or balance the propeller thrust and gravity forward component – airspeed will stabilize at some new higher airspeed. Makes sense, because the stick is forward which should translate into a higher airspeed in a descent with the nose appropriately down.

At this point, the prop thrust (and gravity component in the descent) is balanced by the drag components (airframe and rotor). But, what happened to the “sum of moments” imbalance. As the airspeed was increasing to balance prop thrust (and gravity component), the share of aircraft drag between the rotor and the airframe was adjusting – the airframe was assuming more of the drag (parasitic), and the rotor was settling out with a bit less of the drag (induced). This means that the airframe drag moved the airframe back a bit, and the reduced rotor drag allowed the rotor hub to move a bit further forward. This means that ultimately the airframe is flying at a bit less forward cant angle relative to the relative wind (no longer the initial 9 degrees) – actually, when this settles out, the airframe will have an angle to the rotor closer to about 6 degrees at the higher airspeed. This all makes sense because that would be the exact angle of the resultant RTV at this higher airspeed – stick (spindle) forward – shallower rotor disk AOA - and a bit more rotor “blowback” rotor disk AOA – totaling a somewhat now lower rotor disk AOA – for the higher airspeed! Now, the RTV is realigned with the CG – back at equilibrium.

Movement of the RTV relative to the CG is an unfortunate byproduct of our method of controlling rotor AOA. Much of what we do to make gyros fly properly -- from jab-and-return control techniques to big H-stabs -- is targeted at reducing this unwanted secondary effect.

I don’t see this as an unfortunate byproduct. I see this as an essential component of why the right things happen and why the gyro is so maneuverable. But, it is precisely quick pitch response to this highly powerful pitch control mechanism, that presents the opportunity for extreme overshoot, pilot over control, PIO and buntover, that makes a good DYNAMIC dampener essential!. If that good dynamic dampener is a very proficient pilot, then that may be OK. But, if the pilot’s proficiency – to be super human in some cases – is in doubt, I would prefer to have the inherent and superbly simple and passive HS dynamic dampener to do that part of the flying for me!

Carrying this forward to the other gyro configuration examples with real airframe lift and drag offsets, prop thrust offsets, and horizontal stabilizers (hopefully), the pictures get really complicated. I think in general though it would be found that, if the system is inherently stable from the CG/RTV(CTV) relationship, then similar or better response to disturbances will result. But, if that CG/RTV(CTV) relationship ends up on the unstable side, the static divergence from this same starting point will end up in a buntover.

Precession Stall:
Another thing to keep in mind is that rapid cyclic changes can exceed the teeter limits of the rotor. This really only needs a spindle angle quick or instant change of something in excess of 10 degrees or so where at least one rotor blade might too fully stall! If reaching the rotor teeter limits is a commanded input from the pilot, the pilot will feel the hard knocking through the cyclic stick and will most likely “back off” at that strong tactile signal! But, if the teeter limits are struck, a rotor blade too fully stalled, because of airframe sudden pitching (or rolling), the pilot may not be able to “back off” – or do so in the right direction! This is essentially what happens in a buntover – probably far before the gyro actually “tumbles” completely. This just means there are limits in how far and quickly you could actually put in the commanded stick jab described above.
This is a different issue than just static or dynamic stability. But, a dynamic stabilizer may certainly temper the rate of any airframe pitching, amount of overshoot, or amplitude of oscillations that could otherwise cause this Precession Stall limit to be exceeded.

(A short story, related to quick forward cyclic movements – and to teeter bumping - then I’m finished):

Doing some dynamic testing in my M16 – not recommended for real safety reasons, especially if your gyro might be capable of PIO or buntover – I was jamming the stick forward to a hard fixed stop. I did this in increasing “step function” amplitudes to explore the dynamic response. For shorter stick “jabs”, the nose dropped and eventually recovered to a new airspeed according to its dynamic response (about a 14 second phugoid period for the M16). But, at some longer stick forward “jab”, the stick started bumping – a short series of bumps until the rotor either caught up or I backed of the stick. Ironically, this is sort of the “stick shaker” some people are suggesting we install on gyros to warn of buntovers!? This teeter limit “stick shaker” is inherent to every gyro already. Of course, I did not go any further with this – pretty hard to do anyway when the stick is bumping and scaring the s**t out of you anyway – I tended to back off of the stick pretty subconsciously and quickly! Just pointing out I have some experience with the “mind game” in my previous post, and there are teeter limits to just how much you can do this!

- Greg Gremminger

*Remember Igor Bensen's admonition to jab-and-return the stick? As an engineering test pilot, he knew lousy damping when he saw it, though of course he'd never use those words.
The reason for the “jab and counter jab” admonition – actually standard necessary proficiency for unstable gyros - is that that gyro is statically unstable – requiring a “jab” one way to start a motion, and an immediate “counter jab” to stop the static divergence and return somewhat closer to the equilibrium point where the “jabs and counter jabs” are not required to be so apparent (but still necessary if subconscious). This is much like balancing a yard stick vertically in the palm of your hand – “jab and counter jab”. The need for the “jab and counter jab” goes away in a statically stable aircraft because now the pilot is no longer required to be the active stabilizer! Good dynamic dampening helps too – in the case of the yard stick a large flat and light paddle on the top of the yard stick. Good MOI also helps by slowing the rate of reaction to an imbalance – such as a heavier yardstick.

Heron

03-03-2007, 05:55 AM

The very thing to be dampened is the fulcrum of this thread and also its relation with ASTM.
Will the ASTM say that all gyros should not have to use dampening devices by been designed properly or a little dampening is ok?
Latitude . . . show me both ends of this spectrum and I will find my parameter when buying a gyroplane. Lets pick RAF (excuse me RAF friends) as one end, which machine will be at the other end?
Lets dial them in and show public what is out there and wich one make ASTM guide lines.
After 5 years listening some of the talk here I still can't find a way to tell general public about the situation with stability.
My guess is that it will be more gray than black and white.
thanks
Heron

gyrogreg

03-03-2007, 06:14 AM

Will the ASTM say that all gyros should not have to use dampening devices by been designed properly or a little dampening is ok?
Latitude . . . Heron

The Dynamic Stability criteria in the ASTM requires that there are no sustained oscillations faster than a 5 second period. This would mean that the gyro either has a natural pitch oscillation with a period of 5 seconds or longer - that would be a real dog! - or a dyanamic dampener (HS, auto pilot?) effective enough to stop an oscillation before it is detectable (by the pilot who might otherwise over react to it and agravate it into a buntover or PIO).

Requiring a specific device (HS, autopilot,e tc.) would be a "prescriptive" requirement - how to do it rather than what the results should be. Such "prescriptive" requirements are not appropriate in a standard because it may unnecessarily restrict how the design might arrive at acceptable results. The standard requirement is "results" based.

It would be very difficult to "prescriptively" identify a requirement anyway! What about a heavier gyro, a longer gyro, a HTL or LTL or CLT, what about cabin drag or no cabin? All these things would have to be factors in some equations as to how to "prescribe" the required design - or else all gyros would have to look the same. All aircraft standards avoid "presicriptive" requirements.

Keep in mind that flight testing for Dynamic Stability can be dangerous (what if you find out in the flight test that it will PIO or buntover!?) Dynamic flight testing should be left to professionals, and gyro manufacturers who want to state that they do "comply" will have to have a professional do that part of the standards testing. Not for amateurs. That is why:

We present some guidelines for assuring your gyro is dynamically safe - without you having to do actual dynamic flight testing:

- First, make sure it meets the static flight test criteria - or at least you flight test to identify the operating limits in which your gyro is statically stable - then don't exceed those limits. Static stability flight tests are safe to do!

- Second, if you achieve static stability with the use of a HS, and if that HS is placed well aft to further enhance its dynamic dampening mechanics, you can be fairly confident that that gyro will have no dangerous dynamic characteristics - those that might promote over control or PIO.

- Thanks, Greg

JEFF TIPTON

03-03-2007, 07:17 AM

Heron

I dont think the standards will dictate how they design the gyro only that it fly with certain characteristics. The good part is all machines would have similar flight charactgeristics. It standard does care if you use a stabilizer or not. Any method is acceptable as long as it has the proper end result.

I agree that a listing of which machines are in complance with ASTM standards would be a great asset to all concerned. But I wonder if any manufacturers have as yet volunterily tested their machines to the standard. They are not required to unless they are going to build S-LSA.

On the other side of the fence would be the totally home built from plans. Even though the plans built machine meets the ASTM standard the one just built might not. Looking at other threads even a change in rotorblade manufactures effects the characteeristics of the machine.

We might even consider that the manufacturer might say the machine meets ASTM standards but we are left to wonder if it really does. Don't get me wrong I am not claiming any would. Just a thought on some sales people that I have met, sell the product no matter what. After all that is how they make a living.

An outside source to verify that the machines meet compliance would be nice. On the down side, this leads to more paperwork and more expense. Some could argue what is the cost of a life. I hope no ever does a value on life. To me it is pricless.

These are inteeresting times. Alot of people are shaking the tree, the bransches are swaying. Lets see what falls out.

Heron

03-03-2007, 08:10 AM

Thanks Jeff and Greg
In practical experiences we have to pick and position different gyros, the ones considered not to be on the safe zoned will have its owners and enthusiasts to come out and fight for them.
It is ok, but in time the discussion will turn sour and personal, this is when the organizing entities step in with official positions.
I am firm in believing that in short time we will advance a lot on this field and all out of boundaries discussion will become just that . . . out of bounds!
thanks
Heron

Douglas Riley

03-03-2007, 09:41 AM

Greg, I don't think we disagree on most of this, either. I pointed out a few exchanges ago that a rotor can be AOA unstable but airspeed stable. In the (relative) long run, a gyro may rescue itself from its own AOA instability because it speeds up. As it speeds up, rotor blowback returns the RTV partway forward, a stable reaction that helps to snub off the frame's pitching rotation. Rotor damping helps, too, by slowing down the rate at which the RTV swings aft in the first place.

However, I stick to my guns with respect to the notion that RTV movements themselves are an unhelpful byproduct of our system of controlling AOA. If we could somehow adjust rotor AOA WITHOUT any resulting movement of the RTV, we'd be better off. We could still use AOA changes to speed up or slow down, climb or descend, but without the frame pitching. Of course, a FW plane gets along very nicely without movements of its lifting surface's thrustline back and forth relative to the CG. In fact, airfoils are designed specifically to minimize such movements.

Who cares? It's an semi-interesting discussion it itself, but, more important, it offers a way to look at PIO and at the vital function of the H-stab.

Udi

03-03-2007, 12:28 PM

I believe the conclusion from all this discussion about the RTV moving back and fwd of the RTV as we move the stick, and how it all relates to static pitch stability, must be that -- keeping the CG fwd of the RTV must not, and cannot, be the main stability driving force in gyroplanes. Even when a gyro is carefully designed such that the sum of all moments are arranged to end up with the GC fwd of the RTV, still -- the distance of the CG from RTV will change across the flight envelope - not allowing for a solid, robust, mechanism for G-load or AOA stability.

This predicament in not unsimilar to one FW designers are faced with. Although most FW aircraft have a relatively fixed center of lift (AC), they must allow for significant variations in the location of the CG vs. the center of lift -- in between flights, and during the flight. That is why the main contributor to AOA stability in most FW planes is the horizontal stabilizer, not the location of the CG relative to the center of lift. Aircraft designers know that they must not rely on the pitch stability of the wing alone to take care of whole-aircraft stability. This is why the small wing at the back of the plane is called "stabilizer" - because it is the main stabilizing force.

Therefore - without any doubt - if we want to have gyros that are solidly pitch stable across the entire flight envelope, at all loadings, we must employ horizontal stabilizers. Placing an effective stabilizer far aft of the CG assures positive static stability in all conditions - even when the CG is not fwd of the RTV. The stab is also providing a few bonus benefits - faster control response and improved dynamic stability.

I believe we should stop preaching to the people that "CG must be fwd of the RTV" in order to have a safe and stable gyroplane. Saying this is just as wrong as saying that you "must have CLT" in order to have a safe and stable gyroplanes. Both claims are shallow and misleading. A 2" HTL gyro may be designed to be far more stable and safe than a perfect CLT or a LTL gyro. I also believe the Magni gyro is a proof of that. I don't believe the Magni (any Magni) is flying with the CG fwd of the RTV most of the time (if at all). I believe it's the huge stabilizer on the back that is making the gyro very stable. And no - I don't agree that the 2" HTL is helping the Magni be AOA or airspeed stable - it just doesn't hurt much thanks to that huge stab.

Udi

Heron

03-03-2007, 12:51 PM

So Udi
can we place the Magni's on the other end of the spectrum where the RAF is one end? Will most machine fall in between those two?
thanks
Heron

Chuck_Ellsworth

03-03-2007, 03:08 PM

A perfect example of what a HS accomplishes stabalizing a mass moving through the air was used thousands of years ago.

An arrow, remove the stabalizing vanes and the picture will become clear.

Udi

03-03-2007, 03:41 PM

So Udi
can we place the Magni's on the other end of the spectrum where the RAF is one end? Will most machine fall in between those two?
thanks
Heron
I don't know Heron. Only flight tests can quantify where each machine is on the scale. I have flown the Magni and it feels very stable. I have not flown a stock RAF and I don't intend to, but I don't have to fly it to know that it is not a Magni.

Udi

Douglas Riley

03-03-2007, 06:41 PM

Nice summation, Udi. The various possible moments that the RTV can create in its travels are one measure of the HS power you need.

chuter

03-03-2007, 06:59 PM

Damn, does this mean I need to re-make that stability video?

I’ve understood for a while that the HS helps turn the nose into the wind, but I’m just now getting it in Udi’s terms and how that’s static stability.

Duh.

This is really good stuff; thanks Udi, Doug, Greg and all.:hail:

JEFF TIPTON

03-04-2007, 06:16 AM

I would think the vertical stabilizer helps to turn the nose into the wind, the yaw axis. The horizontal stabilizer helps to control the nose movement up and down. IE pitch or Longitudinal stability.

chuter

03-04-2007, 06:42 AM

Right Jeff, that's what I meant. The HS turns the nose into the the wind in the pitch axis.

Heron

03-04-2007, 09:01 AM

Both stabs are wind orienting devices, natural wind and relative wind.
It is quite clear to me that by the history and testimonials we can arrange our gyros in a scale, if you dont like the Magni reference Udi I will throw the Golden Butterfly at one end, RAF on the opposite corner.
Anything passing middle point needs to be revisited and dealt with, it is not ok to fly those machines. The choices should be limited and time given to add estabilising devices or ground the machine.
AS time goes by (nice song) we are getting closer and closer on knowledge and having it more available thanks to all here and our Forums.
Thank you all
Heron

C. Beaty

03-04-2007, 09:40 AM

Marks’ Mechanical Engineers Handbook description of aircraft stability is the best concise explanation I’ve run across.

The top drawing of Fig. 25 is a good illustration of the typical Buntamatic gyro where the nosedown moment of the cambered wing is replaced by the nosedown moment of the high propeller thrust line.

All that is required for static stability is that the sum of all upward forces trails the CG. It is irrelevant whether the horizontal stabilizer operates unloaded, uploaded or downloaded.

As the CG is moved aft, the rear wing or tail must become progressively larger and as it takes on more of the load, the front wing can become smaller. Eventually we end up with a VariEze.

But don’t think of a VariEze as a tail first airplane. It is not.

Sorry about the size of this attachment; I compressed it as much as I could while still leaving it legible.

bpearson

03-04-2007, 09:50 AM

Anything passing middle point needs to be revisited and dealt with, it is not ok to fly those machines. The choices should be limited and time given to add estabilising devices or ground the machine.

Heron

Land of the free eh !

Aussie_Paul

03-05-2007, 02:09 AM

...I have no chance of keeping up with this discussion.:o I only have my worldly, and gyro practical testing experiences to call on.

I prefer to have slightly high, 2 or 3" with a stab that is always downloaded.
The figures I asked for in another thread seem to show what I prefer.

I liken it to a car with sloppy steering on an extremely cambered road. It is easier to drive on one side of the camber where the pressure on the steering wheel is only one direction.

There is a "sluggish" for want of a better word, stab response with the AoA of the stab changing from positive AoA to negative AoA.

F190 gave the figures of 24 lbs difference between -2 and +2 degrees AoA, and 52 lbs different from -2 to -6 degrees AoA. 4 degrees AoA change in each case.

It appears to me from those figures that there is approx double the effect of a 4 degree AoA change when the AoA does not have to pass throughthe zero degree AoA.

Does that make sense?

aussie Paul. :)

C. Beaty

03-05-2007, 03:17 AM

...Does that make sense?
aussie Paul. :)Not a lick.

Lift vs. angle of attack is a straight line just like the moment plots from Marks’: if airspeed and size is such as to give 1 lb. of lift at 1 degree; then you’ll have 2 lb. at 2 degrees and so on.

Fixed wing airplanes, over the range of allowable CG operate from slightly downloaded to slightly uploaded horizontal stabilizers and it’s true that angle of attack stiffness is lower with an aft CG limit, i.e., with stab producing an upload. But that’s because the CG is nearer the lift center. If the stab area was increased proportionately as CG moved aft, there would be no change of stiffness in pitch.

This stuff is easy to demonstrate with a balsa stick glider. If you begin with a tandem wing glider, lift center is about midway between ¼ chords of the two wings and for stability, nose weight must added to locate the CG slightly forward of this point. Both wings are uplifting.

As you alter the sizes of the of the two wings relative to one another, center of lift shifts so as keep the product of areas and moment arms equal. CG must always be forward of lift center for angle of attack stability.

Aussie_Paul

03-05-2007, 04:09 AM

Not a lick.

Ok Chuck, so are the figures I used, posted by F190, wrong?

Aussie Paul. :)

C. Beaty

03-05-2007, 05:05 AM

Ok Chuck, so are the figures I used, posted by F190, wrong?

Aussie Paul. :)Yes. Except near stall, lift vs. angle of attack is pretty much a straight line. Low aspect ratio airfoils have somewhat of a curved lift line because air spills around the ends but the curvature is not in your favor; as angle of attack is increased, the lift per degree decreases.

I don’t think it is possible with rigid airfoils to have rate of lift increase with angle of attack. Maybe with sailwings.

Aussie_Paul

03-05-2007, 12:31 PM

C. Beaty

03-05-2007, 01:03 PM

That’s called mind over matter, Paul.

The same reason my car drives better right after it’s been washed.

Fl90

03-05-2007, 01:06 PM

Aussie_Paul

03-05-2007, 01:27 PM

Pual, I was finnishing the answer from the other poster. The numbers were transposed, as the other post stated. As for the validity of the numbers given in the first place, I have no idea. Perhaps Cuck or Doug could answer your question here.

Cheers, Phil.

I appreciated the help Phil. As I always seem to be struggling for time I grabbed the figures to use instead of working them out from Dougs formulae. :o

Aussie Paul. :)

gyrogreg

03-05-2007, 08:12 PM

Thanks Chuck. Back to thinking!! No that seems like a wasted energy with me. :):) Can you think of any reason that would give me the impression I get with a continously loaded, in one direction, stab verses one varying either side of zero AoA? Aussie Paul. :)

Paul and Chuck - I'm probably all wet on this, and I'm sure some of you will hop all over this. But, please hear me out - I am still not convinced that the stronger AOA stability doesn't happen when the HS is downloaded.

There are two mechanisms for aircraft with both front and rear wings to respond to a change in airspeed (or AOA?) - consider a sudden increased airspeed due to a nose-on wind gust:

The first mechanism happens when both "wings" are at a positive AOA - both up-lifting - this is the mechanism Raghu has emphasized before in his arguments that it does not matter whether the HS is up or down lifting! When the airflow over the up-lifting wings increases, both "wings" increase their up-lift in the same proportion at the same time and the aircraft starts to rise. This rising, upward movement, creates a reducing AOA on the up-lifting wings - eventually reducing the AOA enough on both wings that the rising steadies out into the original trimmed airspeed. In this mechanism,the aircraft is rising more or less level because both wings are changing their up-lift at about the same rate at the same time.

The second mechanism happens when the front "wing" is up-lifting, but the rear "wing" is down-lifting: With a sudden increase in airspeed from a nose-on gust of wind, the front wing begins to rise with the increased up-lift - as in the example above. So the aircraft starts to rise as in the example above. However, the rear "wing", with the negative AOA responds to the increased airflow over it by increasing its down-lift. Now, with the front "wing" lifting up, and the rear wing lifting more down, an immediate nose-up pitch happens from the increased differential in lift between the two "wings".

What I am suggesting is possibly what Paul is describing as a "stronger" reaction when the HS AOA is negative rather than positive! With the HS in a positive AOA arrangement, AOA stability results with less actual nose pitch up (or down in the reverse). With the HS in a negative AOA arrangement, the increased differential lift between the two "wings" creates an immediate nose pitch reaction - raising the nose quicker to the point where the climbing airspeed and AOA on both "wings" is back at the original trimmed condition quickly - and the pilot gets the pitcha ttitude cue that it is climbing.

I can agree that AOA stability is (or can be) achieved with the HS lifting either up or down. I do understand that the difference in lift curves between a rotor and the HS "wing" are different and can contribute some to the AOA stability. And, for certain, the cyclic or "blowback" moving the RTV back and forth really confuses what happens during such transients. But, I do not see how an up-lifting HS can provide as strong static stability or the strong pitch response to changes in AOA or airspeed - as a down-lifting HS does!

My perceptions of this were probably formed when I was a kid building a line control P51 model. The plans said to balance forward of the 1/4 chord of the wing. I thought this is silly - why do you want the tail to push down, that's not efficient - that just makes the main wing have to lift harder! So I balanced it out with the CG right on the 1/4 chord - in fact I did this twice and wrecked it each time! Before the first circle was halfway through, I over-controlled, went right over my head and straight into the ground on the other side - twice! So, on the third P51 rebuild, I tried balancing it like the plans said! Had fun with that model the rest of the summer! Since then, I always bought into the reasoning that the forward CG was requiring the HS to push down so it would be stable and easier (or possible) to fly it!

If my reasoning is not correct, I sure would like someone to take more than one line to explain to me - and to Paul - why it is not correct! I'm just not comfortable with throwing away the common appreciation that the CG should be forward of the main "wings" thrust vector in order to be properly, or at least more strongly, statically (AOA or Airspeed!?) stable! If the CG is in-between the two "wings", the rear "wing" must also be up-lifting to balance it! That up-lifting Hs is what I cannot reason is as good as a down-lifting one!

- Thanks, Greg

Aussie_Paul

03-06-2007, 12:46 AM

That’s called mind over matter, Paul.

The same reason my car drives better right after it’s been washed.

Geez, your car does that too Chuck?:lol: My red car is faster though.:lol:

Aussie Paul. :)

JEFF TIPTON

03-06-2007, 05:23 AM

In the fixed wing aircraft the balance point will generally be in the 25% to 35% aerodynamic chord of the wing. This point is in front of the Center of Pressure that is generally around the 50% aerodynamic chord. As the center of pressure is always pushing up and gravity is pulling down in the CG, the nose of the aircraft will go down. The horizontal stabilizer applies a down force to counteract the Center of Pressure. As airspeed increases, the wing produces more lift, which causes the nose to rise. A decrease in speed reduces the lift of the wing and nose drops. The stability is a function of cubic feet not necessarily square feet, which many reference. A twenty square foot stabilizer at ten feet from the Center of Pressure would be just as effective as Ten square foot stabilizer at twenty feet from Center of Pressure. A canard configuration is interesting. It works because the front wing has more weight per square foot that the rear wing, IE a higher wing loading. As the airspeed reduces the front wing cannot support as much weight hence the nose drops, the airspeed increases the nose rises and it will oscillated a few times up and down before all is in balance.

In the end the configuration and design elements used by the engineer is a trade off. What they might want to use might not be possible due to weight, aerodynamics, cost, etc. Certified aircraft must meet a standard whether it is Cessna, Piper or even the new LSA's. Stability is just one of many considerations. The important thing, when it is all over, It works. :plane:

C. Beaty

03-06-2007, 05:30 AM

Both wings are uplifting on a VariEze, Greg. It is claimed to have excellent angle of attack stability.

gyrogreg

03-06-2007, 07:10 AM

Both wings are uplifting on a VariEze, Greg. It is claimed to have excellent angle of attack stability.

Hi Chuck. I also understand that the VariEze type canards rely on the wash from the canard on the main wing to achieve both this AOA and stall characteristics. I'm not totally familiar with this, I remember reading and listening to Burt Rutan back when he designed the first versions, explaining how they paid careful attention to this and relied on the canard interference with the main wing to achieve these flight characteristics. As Jeff mentions, the canard is more heavily loaded and flies at a higher AOA to stall before the main wing so the nose drops before the main wing stalls. I believe the downwash from the canard over the main wing is part of the AOA stability he achieved. I'm not sure we can count on these type of mechanisms in a gyro.

- Thanks, Greg

JEFF TIPTON

03-06-2007, 07:46 AM

It would be an interesting machine. One side note the earlyVarieze had a problem when going close to a thunderstorm. The nos would rise and put the aircraft in climb and some conditions there was not enough control to overcome this. Later models used a diffrent airfoil to correct this.

Chuck_Ellsworth

03-06-2007, 09:14 AM

The only close up time I ever had with a VariEze was a few years ago in Narsarsuaq Greenland on one of our ferry flights.

Some guy landed with a mechanical problem and we spent a couple days helping him get it resolved.

We departed Narsarsuaq about a half hour ahead of him and a few hours later we heard him frantically trying to contact Gander Oceaniac Control....

...we were able to relay for him and advise Gander that he was having engine problems about half way between Greenland and Canada......

...it was very stressful thinking about what he was going through and only be able to help by relaying the messages.

Fortunately it kept running until he reached a Canadian airport.

They are a neat airplane, but I wouldn't want to fly one across the North Atlantic route.

Udi

03-06-2007, 09:29 AM

...I'm not sure we can count on these type of mechanisms in a gyro.

- Thanks, Greg
We ARE counting on this kind of mechanism in a gyro, Greg. The rotor is significantly more heavily loaded than the typical stab. That is part - not all - of the AOA stability mechanism.

Udi

gyrogreg

03-06-2007, 10:26 AM

We ARE counting on this kind of mechanism in a gyro, Greg. The rotor is significantly more heavily loaded than the typical stab. That is part - not all - of the AOA stability mechanism.

Udi

Udi, agreed! I was really refering to the front "wing" wash depended on to affect the lift and AOA of the rear "wing". It might be hard to imagine that deflected air form the rotor has much affect on the air the HS is flying in..

Other things can affect this "wash" on the HS though too! - air deflection around a cabin and into the prop, etc. On some gyros, including both the Magni M23 and the new Xenon, I am suspecting that some of the aerodynamics, perhaps the center of pressure of the enclosure and the airflow aft of the engine cowling, airflow at the HS, etc, may actually be integral parts of the final stability performance. It is interesting to note that, at least the aft portion of the enclosure (around the engine) is similar on both the Magni M23 and the Xenon. Not that either designer might be able to tell us specifically how that works, or that that works, I suspect both designers arrived at their final configurations via flight test iterations. But, it would be very interesting to see some wind tunnel testing comparisons on a lot of gyro enclosures.

- Greg

Al_Hammer

03-07-2007, 10:18 AM

I made up a diagram showing the 9 basic combinations of CLT, HTL and LTL with RTV ahead of, behind, or On the CG.
To understand the graphs, all you need to know is that the Blue line is total pitching moment and it must have a negative slope for static stability. Angle of attack goes up, moment gets "less positive", or" more negative."
The steepness of the slope of the pitching moment also tells us something. It indicates the PITCH STIFFNESS.
It tells us how aggressively the gyro will try to oppose a pitch disturbance from equilibrium.

You can compare the different configs and see that some have strongly sloping blue lines and some have more anemic slopes. All configs can be made stable if the stab is made large enough to completely counteract other moments.
A downloaded stab loads the rotor and is less efficient than a non loaded stab.

The thing to notice is that there is no inherent advantage to a moderate HTL with a downloaded stab vs a CLT machine with no load, or a LTL with moderate upload, etc. If the pitch moment lines have equal slopes than the configs have equal stability.
I made no attempt to get the curves precisely drawn and I didn't label the AoA and pitch moment axes, etc. Also, I may have mistakes in the graphs, so hopefully someone will check my work and let me know.
(EDIT: updated graph with some minor fixes.)

gyrogreg

03-07-2007, 12:30 PM

I made up a diagram showing the 9 basic combinations of CLT, HTL and LTL with RTV ahead of, behind, or On the CG.
-------
Al, Very interesting. You did a lot of work here. Summing the moments to get the total moment is very instructive in these diagrams! I hope people will try to digest what these diagrams are telling us.

Do you mind if I raise some thoughts though? These might be a bit of more basic. I might be blind, but these diagrams do seem to support my suggestion that a down-loaded HS is at least better. Please tell me where I am wrong!:

1) In the CLT column: This seems to show a steeper (stronger) total pitching moment for the downloaded stab. Is this correct? If this is truly CLT, we would not want or need a lot of HS load in either direction reacting to propwash because that would make the gyro pitch sensitive to power setting - flying nose higher or nose lower with a non-neutral HS. With CLT, at least for "balancing" the non-existant prop thrust moment, any HS moment flies the nose higher or lower - depending on both propwash on the HS and ambient airspeed on the HS. But also, if the gyro has any airframe drag/lift moments (usually in the nose-down direction) you would need some down-loaded HS to balance that. My conculsion from all above, and from the stronger pitch moment curve in your top diagram in the first column is that a down-loaded HS will both have stronger pitch response and balance the airframe drag/lift moments so it would not lower the nose at higher airspeeds (to be more unstable at those higher airspeeds). But, with CLT, there are no prop thrust moments to balance, so with any HS AOI to the keel, the nose pitch is going to change with power changes because of propwash on the HS!

2) In the HTL column: Again, the higher HS download has the stronger total pitch moment. But, for any HTL, the gyro is going to fly nose-down a bit, and that airframe attitude will automatically provide a download on the HS mounted at least level on the keel. But, you don't want the gyro flying nose-low, so the HS down-lift does need to be the stronger version as in the top middle diagram. This down lift should not be left to result from nose-down flight of the gyro - it is much better to create the download from either its mounted AOI or taking advantage of any propwash created AOA on the HS. But, essentially, if the gyro has a HTL, the HS needs to be powerful enough and arranged such to effectively "balance" the prop thrust offset. My conculsion from this, for several reasons in this paragraph, is that the bottom two examples of HTL are not adequate design of the HS - unless it's not really HTL!

Just a note, you are correct, any downloaded HS is less efficient - it creates more load on the rotor. But, good design of the HS can achieve significant necessary nose-up moments without presenting a lot of actual download on the HS - that the rotor has to add to it's lift! A clean airfoil, placed far aft for a long moment arm, and other techniques can be employed to minimize the "down" side of a down-loaded HS (pun intended!)

3) In the LTL column: Again, this seems to show the less up-loaded HS provides the stronger total pitch moment. Unless I'm really missing something, this seems to support my argument that the downloaded HS is more pitch stable! Where am I going wrong? As in the HTL example, a stronger HS lift is required to "balance" a stronger prop thrust offset. The situations where little HS moment is required is the situation where there is little prop thrust offset, closer to CLT anyway - neglecting any airframe drag/lift moments. And, as in the HTL case, the airframe flight attitude - nose-up in the LTL example, will automatically create the up-lift on a keel level HS due to the flight attitude created by the LTL! And, a LTL cannot really handle a down-loaded HS because that causes the nose to raise even higher than it does with the LTL! (Same reason you don't want an up-lifting stab on a HTL!)

But, we should not neglect the effects of airframe drag/lift either - it gets much stronger at higher airspeeds where we really need the HS to "balance" those moments so the gyro doesn't pitch to a less stable attitude at higher airspeeds - probably a contributing root cause to stability accidents at higher airspeeds anyway! So, since most gyro airframe drag/lift moments are nose-down, the same HS that is holding the LTL nose down, is not helping to hold the nose up with increasing airframe drag/lift moments at higher airspeeds.

Therefore, taking all of the above into account, and your drawings that seem to verify that a stronger down-lifting HS does provide stronger stabilizing pitch moments, where am I going wrong when I suggest that the down-loaded HS is more stable? If you want to establish this stronger down-loaded HS, it requires a HTL to do so. And that same down-loaded HS will also be able to "balance" the nose-down airframe drag/lift moments AT THE SAME TIME!

Looking at the lower right, LTL with large up-loaded HS, how would that balance any additional nose-down moments from the enclosure, landing gear, etc. I still submit, an up-lifting HS, as required with a LTL (if you want to reasonably balance the prop thrustline so the aircraft won't pitch so bad with power changes), is not able to "balance" the airframe moments - it just isn't lifitng in the right direction to do so!

If the up lifting HS, with increasing airspeed, is adding to the increasing nose-down pitch moment from the airframe drag/lift moment, it seems to me that increasing airspeed lowers the nose more and increases the airspeed, which further lowers the nose, etc., etc., etc. Is this Airspeed stability, or is speed just going to get faster and faster - airspeed unstable?

Have at me - where am I wrong.

Thanks again for the diagrams. Now you wouldn't want to expand your matrix of diagrams to add in the effects of airframe drag/lift moments, would you? I don't think the real story is over until ALL the moments on the aircraft are considered!

- Greg

Al_Hammer

03-07-2007, 12:45 PM

Greg,

Thanks. I hope to have time later to elaborate on the diagrams and respond to your comments.

Udi

03-07-2007, 02:00 PM

...Also, I may have mistakes in the graphs, so hopefully someone will check my work and let me know.
Al, to check your work we need to know what assumptions you are making and some numbers to chew on... I am working on my own spreadsheet so hopefully soon we can compare results.

Greg, a stronger pitching moment doesn't mean better stability. The **slope** of the line that represents the sum of moments is an indication of static AOA stability.

Udi-

gyrogreg

03-07-2007, 02:02 PM

Al, another clear (to me) conclusion that I think can be drawn from your diagrams is that the more stable pitch moment - steeper negative slope - is when the CG is forward of the RTV.

You have always argued (with me at least), that the CG aft of the RTV can be stable - your diagrams support that - just not as stable! I have always argued that a gyro with the RTV forward of the CG is "G-Load" unstable - able to pitch nose down and agravate a reduced G transient - buntover progression!

Chuck has said several times recently, that the CG needs to be foward of the total thrust (rotor and HS), in order to be stable. It might be very helpful to see where the "Composite Thrust Vector" (CTV) is relative to the CG when a HS is installed. (Originally, Chuck's traditional argument that a gyro can buntover is for HTL gyros without a HS). But, for gyros with a HS, up or down lifting, where does the CG end up relative to the CTV? I think we will find, that considering all the moments on the airframe, the properly and adequately down-loaded HS can put the CG forward of the CTV.

(Maybe an uploaded HS (LTL) can also position the CTV aft of the CG. But, I would think that it is difficult to get an uploaded HS (LTL) to handle the other airframe drag/lift moments at the same time - for positive Airspeed stability!)

I am puzzled by this thing I have been calling G-Load stability - and that that relates to whether a gyro is capable of bunting over or not. This is the extention of Chuck's traditional scenario of the CG aft of the RTV being set up for a buntover if a reduced G-Load transient is encountered. I think you and others say this CG/RTV response to a G-Load transient is simply AOA stability. No matter what the name is, I think the essential issue is, when a gyro is confronted by a reduced G load transient, does the nose pitch in the down direction that agravates and can progress a nose-down progression buntover, or will this CG/CTV moment pitch the nose in the upward direction to counter the reduced G-Load transient and prevent a buntover?

To me, this is the essential static stability issue with gyros - is it capable of bunting over, and can this be identified by a flight test that increases the G-Load to see if the airframe response is in the stable direction. I believe it can, and I also believe that indicates whether the CG is fore or aft of the CTV - buntoverable or not! And, I believe that is what the "G-Load static stability flight test can determine!

I would just like to better understand if, why and how this is so. We do know that some HTL gyros without a HS require the stick to be pushed forward in a bank - to maintain airspeed - under the increased G-Load in the bank. This indicates the CG is aft of the RTV pulling the nose up! This also infers that a reduced loading on this gyro would push the nose down - progressive and in the buntover direction! According to Chuck's original CT/RTV argument, this is what can cause a bunvover!

As you are aware, we have been having very technical discussions elsewhere as to whether this G-Load flight test should be conducted fixed stick with airspeed as the indicator, or constant airspeed and floating stick, with stick pressure being the indicator. I think we have resolved that the indicator in the bank should be airspeed with a fixed stick so that the offset gimbal does not confuse the stick pressure. I am satisfied that this is correct, and that that will show whether a gyro will be G-Load stable and unable to buntover. Whether we need to change the terminology somehow doesn't matter to me. That we can identify the condition Chuck says can lead to a buntover, is important to me. Your thoughts?

- Greg

PS: The sad thing about all this, is it is all probably just academic. We all know, that if you install an adequate pitch dampener (HS), the pitch rates probably would not be so bad that buntovers in even a statically unstable gyro might be stopped by the pilot. And, the reverse is true too - if an adequate HS is employed to achieve good strong static pitch stability, it is probably dynamically dampened enough to avoid PIO too!

Al_Hammer

03-07-2007, 03:20 PM

Greg, in response to your number :

1) The CLT configuration with RTV behind the cg and download on the stab does have a strong negative slope to the pitching moment line and therefore it is more pitch stable than a CLT with no loading.
This does not say anything about its response to having the stab in the propwash, or any other interactions.
It merely shows that you'll be AOA stable when trimmed up at a given airspeed.

2.) You said "My conculsion from this, for several reasons in this paragraph, is that the bottom two examples of HTL are not adequate design of the HS - unless it's not really HTL! "
The best pitch response of the HTL gyro comes from having a sufficiently loaded stab to force the RTV to be in front of the cg.
The other designs are less positively stable, all else being equal, although they are still stable.

3.) "In the LTL column: Again, this seems to show the less up-loaded HS provides the stronger total pitch moment. Unless I'm really missing something, this seems to support my argument that the downloaded HS is more pitch stable! Where am I going wrong? "

The 3 configs are equal in pitch stiffness: LTL with no download , HTL with large download, or CLT with download and RTV behind the cg.
Actually, the diagrams are arranged such that the top row has the best pitch stiffness.
Note that all these configs have the RTV behind the cg
The next row has medium pitch stiffness, with RTV always on the cg
and the weakest pitch response is when the gyro has RTV ahead of the cg, even if it can be made stable.
=========================
Al, another clear (to me) conclusion that I think can be drawn from your diagrams is that the more stable pitch moment - steeper negative slope - is when the CG is forward of the RTV.

That's certainly true.

Udi,
I might be able to work up some numbers at some point, but I used good old fashioned graphical analysis. I'll, of course, be interested to see your spreadsheet figures and hope that they don't make a liar out of me.!

I know that the rotor lift slope is less than the slope of the stab(reasons given in other threads) and it will slope down or up depending on whether RTV is behind, or ahead of the cg , but I didn't try to portray an exact value, but I did try to be consistent in how I drew the lines from diagram to diagram.
The stab has a steeper slope(green line) and it always slopes down with increasing AOA.
If you add two lines, which is what we're doing graphically, the slopes will add. Two lines sloping the same direction will have a resultant line with twice the slope. If the lines are sloping in opposite directions, the slopes tends to cancel.
A line plus a horizontal line is sloping at the same angle, but shifted up or down as in thre ones where I showed prop thrust as a constant moment that doesn't change with AOA.
Using this basic reasoning it is pretty easy plot the curves. The results are only approximate, but they should give an accurate indication of the trend.

======================
Re; G-load stability. I always tend to think in terms of AOA stability so I don't confuse myself.
If a gust ,(or anything)increases the g-load, it must raise the AOA of the rotor. Even if the the rotor thust is on the cg and there is no moment, the stab will pitch up from the same gust and so the AOA stability (negative sope to pitch moment) provides g load stability I think.

Some other terms still confuse me. Raghu insists that the front wing be more heavily loaded than the rear lifting surface and I do not understand that rule other than to recall that it is essential in a FW design to have the front wing stall first.

Thanks again for the diagrams. Now you wouldn't want to expand your matrix of diagrams to add in the effects of airframe drag/lift moments, would you? I don't think the real story is over until ALL the moments on the aircraft are considered!

The next step would be to create a little javascript program that would run on a webpage that would generate the graphs on the fly. You'd just select the config you wanted from a pull down menu, or maybe type in some parameters and it would plot the lines and run the numbers. No big deal to do. I thought it would be better to start with static diagrams before getting too fancy with the graphics.
It would be nice to look at dynamic response by having some animation too. Chuck emphasized the fact that the spring constant of pitch stiffness helps static stability, but adds to dynamic instability becaue of overshoot.
Dynamic response could be animated but things tend to get out out of hand quickly if you try to cram too musch clever stuff into a web page script.
You guys that want to work with spreadsheets are nuts. I hate those things and am more comfortable with my other tools.

StanFoster

03-07-2007, 03:25 PM

I have to add this intellectual tidbit to this fine discussion....

I feel so stupid.......:hail:

You guys are in a class all by yourselves....

Stan

Al_Hammer

03-07-2007, 03:35 PM

Stan, you are a refreshing addition to this discussion. Unfortunately I am not at home and so cannot whip out my grinning skeleton for you. :D

Aussie_Paul

03-07-2007, 04:18 PM

I have to add this intellectual tidbit to this fine discussion....

I feel so stupid.......:hail:

You guys are in a class all by yourselves....

Stan
I know how you feel Stan.:lol:

Aussie Paul. :o :o

C. Beaty

03-07-2007, 04:34 PM

The negative slope of the restoring moment represents the spring coefficient tending to return an aircraft to trim following a perturbation. It is this spring that causes dynamic instability so we don’t want it to be too steep without a corresponding increase in damping.

It is also true that high drag rotors will have a shallower positive lift slope than low drag rotors, resulting in a stronger restoring moment with a given tail volume. But I don’t think this is really the way we ought to go.

The slope of the restoring moment can be anything we wish it to be with appropriate adjustment of tail volume.

Al_Hammer

03-07-2007, 05:49 PM

Chuck, that's better than Haiku, or whatever that poetry is called . Not a word wasted and my understanding is suddenly improved.

gyrogreg

03-07-2007, 06:14 PM

Al, I think one thing that is bothering me about the matrix of diagrams. I illuded to it in my earlier response. I'm not sure all these diagrams are consistant with static equilibrium. For equilibrium, and ignoring the other airframe (airspeed related moments such as airframe drag/lift) the prop thrustline essentially determines what the HS AOA needs to be..

This is hard to explain: If a goal is to avoid strong pitch changes with changes in power, it appears to me that only one condition in each column really represents the necessary static equilibrium point - the center diagram in each column where the RTV is exactly aligned on the CG. Essentially the prop offset is "balanced" by the HS moment.

At this point, because we are ignoring any other airframe aerodynamic airspeed related (drag/lift) moments, the ambient airspeed moment on the HS must also be ignored. (This won't be true when we complicate our model to include the effects of airframe drag/lift moments, and the necessary airspeed related HS moments to balance those!)

For instance then, in the CLT column, to avoid airframe pitch attitude (and trimmed airspeed) from varying with power (propwash on the HS), only the middle, no HS load condition, will avoid airframe pitch changes with power. This is assuming that the HS is affected by propwash - which it would be to some extent on almost any gyro - even a tractor gyro. If the HS moment is going to proportionately balance the prop ofset, it has to be affected by, and a function of propwash (power). In both the top case and the bottom case, where propwash must be providing either the download or the upload, the CG/RTV moment must change to reach a new equilibrium - CG and RTV aligned - if power changes.

Same thing happens in both the HTL and LTL columns. The middle condition, where the propwash effect moment on the HS exactly "balances" the high or low prop thrustline, will be the only condition where nose attitude and trimmed airspeed are not a function of power.

This is saying that the "Power Stability" ideal condition is whatever balance between the prop thrustline and the HS lift as a function of propwash balances the CG directly on the RTV - the only equilibrium condition that will not be pitch and airspeed trim sensitive to power level. This is interesting because, if we achieve a balance of prop thrustline to HS load, where pitch and trimmed airspeed are constant at all power levels, the RTV will be exactly on the CG - not aft of it! The middle square in each of these columns is the ideal "Power Stability" condition.

Just to be sure all diagrams are graphically consistent so we can have confidence in what these diagrams are showing us: On the diagrams, I'm a bit confused by the "prop" moment line on some diagrams. the "prop and stab" moment line on other diagrams, and the "stab" moment line on other diagrams. The top two rows of diagrams show, I think correctly, that the "stab" moment slopes are the same regardles of the AOA of the HS. The slopes of the stab lines in the lower row of diagrams don't look to be consistent with this - they have different slopes between the uploaded stabs and the more neutral stab. Just mentioning this to assure ourselves all is consistent for confidence in the results - I think the stab slopes in all diagrams should be the same.

Notice in all conditions of prop thrustline (CLT, HTL or LTL), the steepest total moment (strongest return "spring") occurs when the RTV is somehow held aft of the CG by the sum of moments (top row). In the HTL and CLT case this is by a download on the HS. In the LTL case, this is by the LTL.

Notice in all conditions of prop thrustline, in the ideal Power Stability middle row (RTV aligned with CG), the pitch moment is less than the top row, but the same for all prop thrustline conditions. This center row, where the CG and RTV are aligned in all conditions of prop thrustline, is the only condition for ideal "Power Stability.

But, since Power stability does not need to be "ideal" or exactly zero, we can fudge the HS balance of the prop thrustline a bit toward the steeper total moment slope:

- For the CLT, this means a bit of a download on the HS.

- For the HTL, this means a bit more of a download, or a large download on the HS.

- For the LTL, this means a bit more upload on the HS.

If I have any point here besides just trying to clarify what these diagrams are telling us, it is just to still point out that to achieve the same stabilizing negative slope total pitch moment on the LTL requires an upliftng HS that does not to me seem to be consistent with the downloaded HS required to balance the airframe drag/lift probable nose-down moments when our model starts to include those unavoidable moments.

- Greg Gremminger

C. Beaty

03-07-2007, 07:16 PM

Some other terms still confuse me. Raghu insists that the front wing be more heavily loaded than the rear lifting surface and I do not understand that rule other than to recall that it is essential in a FW design to have the front wing stall first.I don’t know whether to call this one a chicken or an egg. Consider a tandem wing aircraft with a pair of identical wings. The lift center will be equidistant between aerodynamic centers of the two wings. For AOA stability, the CG must be forward of that point and the front wing is inherently more heavily loaded. And of course, that situation prevails no matter what the relative sizes of fore and hind wings.

Designers of FW aircraft must always build in benign stall behavior; if the hind end stalls first and the machine falls that way, the pilot is as good as dead. No chance of regaining flying speed.

But there are many ways of managing stall other than with loading. Some airplanes have stall strips on inboard wing sections to ensure the outer sections with ailerons stall last. Stall strips make the air think the inner wing is pointy.

But that’s something we don’t have to fret over.

gyrogreg

03-07-2007, 07:38 PM

My post above may be a bit of a revelation (to me at least):

My ramblings above say that equal pitch (AOA) restoring moments can be achieved with either CLT, HTL or LTL - as long as the HS is doing the right "balance" job in each case - and still achieve ideal or zero Power Stability.

It also says that a better and equal AOA stability can be achieved in all prop thrustline cases if you are willing to have some amount of pitch (trimmed airspeed) change with power - a bit of Power instability (The ASTM standard currently allows 10%). In all cases, this "cheating" would amount to a bit of a nose drop upon power reduction, and a bit of a nose rise upon power increase.

Is this surprising to anyone? The same AOA pitch stability can be achieved with either CLT, HTL or LTL - for the same degree of power instability. In all cases, this is controlled by how far forward of the RTV the sum of moments holds the CG. For equal CG/RTV moments, the same AOA stability for the same Power Stability penalty - no matter CLT, HTL or LTL - as long as you do the HS right!

The penalty for HTL is a little less efficiency due to the extra load on the rotor the downloaded HS creates - reason to keep the HS aerodynamically clean and on a long keel! The advantage of the HTL is that it will likely be able to be configured to "balance" the airframe nose-down drag/lift moments - it is at least lifting in the right direction!

The advantage of a LTL is that it will be more efficient because it is helping the rotor with some of the lift. The disadvantage of the LTL is that the HS is lifting in the wrong direction to balance the nose-down airframe drag/lift moments as a result of ambient airflow - so increased airspeed will result in more nose-down pitch and increasing airspeed - Airspeed instability as airframe nose-down moments start increasing with airspeed.

Al, it will be interesting when your diagrams include the pitching moment AOA curves for the airframe aerodynamic moments. Those moments, likely downward and forward of the CG, will be in the destabilizing direction - positive slope to the lift moment lines! And this is just the pitch moments as a function of AOA. I'm not sure you can factor in the aerodynamic pitch moments - as a function of airspeed instead of AOA - on these same diagrams (incompatible x-axis parameters - AOA and airspeed!).

In all cases, even pure CLT, the very important dynamic dampening requires an adequate HS anyway. A larger prop thrustline offset - for either HTL or LTL - requires a significant HS moment. In the case of a HTL, there are efficiency advantages to placing a smaller HS farther aft. Even as the size of the HS is reduced propotionately to how far it is moved aft from the CG, that smaller HS does the same job for static stability, while it does an even proportionately stronger job for dynamic dampening - but that's another story often told on these pages too! Just understand, the HS is necessary for dynamic dampening (PIO), as well as for all the static stabilities - and for G-Load or AOA buntover immunity.

I hope some others find this as interesting and enlightening as I do! The technical details are getting deep and starting to look a lot like the complex "harmony" from all components that Vittorio Magni stressed to me is the real "secret" to the stability and handling characteristics of the Magni gyro - nearly a decade ago now! (I'm afraid these are not all of his secrets though, at least that is what he tells me - but he won't tell me the rest of his secrets. I think those secret secrets involve some inertial "harmony" between the rotor MOI and the airframe MOI. That's way out of my pay grade!)

- Greg

Al_Hammer

03-07-2007, 07:56 PM

Chuck, I had no idea that's what Raghu was babbling about. Thanks!
======================
Greg,

good, you've caught a slight error. I wasn't careful to draw all the stab pitch slopes at exactly the same angle- and you're right to observe that they should be the same (no matter the loading). This is showing the benefit of graphical aids to help us sort things out..
The slopes are still accurate enough to be valid and useable , but I can clean that up to make it more consistent.
(Edit: I have replaced the diagram with a new version.)

The prop thrust line(black line) is shown on some diagrams and not others you've rightly noted. I combined it with stab thrust where I could, merely to simplify the diagram. In some diagrams it was clearer to show prop thrust moment as a separate steady line. A constant moment, which may not be strictly true, but basically we assume thrust stays the same no matter the pitch angle. I had hoped to make these graphics informative without too much clutter, but I may have made it just intimidating enough to turn people off, when actually the concepts are fairly simple to understand. As Chuck said, its just springs and dampers.

As for changes due to airspeed and power, It may in fact be possible to tune the model by adding drag below or above the cg as needed to allow the stab moment to track airspeed changes. As I alluded to (in my edit) I think the extra parameters that you mentioned, plus the damping thats so important to understand from a dynamic stability standpoint, could be better illustrated with an interactive program that would create the graphs on the fly and it might also show the dynamic effects by animating over several frames. That is something to put on the to-do list...

chuter

03-08-2007, 03:15 AM

I hope some others find this as interesting and enlightening as I do!

My pea-brain is barely keepin' up, but this is really good stuff!

Heron

03-08-2007, 04:49 AM

Al and all
Is it possible to match machines with those graphics? (if not too controvertial, off course) Just like a name-to-faces propostion?
But keep it going by all means!! :)
thanks
Heron

Udi

03-08-2007, 08:13 AM

gyrogreg

03-08-2007, 11:54 AM

Glad to see you are coming around, Greg! :first:

There are still some assumption you are making that you shouldn't be. You are assuming that LTL gyros must have a lifting stab. That is no necessarily true. There is no reason a LTL gyro can't have a neutral or even a down-loading stab. The LTL nose up pitching moment is moving the CG fwd of the RTV as a function of power, but the location and incidence of the stab is independent of that. The stab does not have to be lifting. Having the CG fwd of the RTV is good and there is no reason to have a stab that is countering this moment.

Udi, I am coming around a bit, but not as much as you might be thinking! I'm actually not following your logic here. If the LTL is moving the CG forward (desireable from an AOA or G-Load stability standpoint), it is not very power stable unless something is holding it down a bit. That "something" would be an up-loaded HS reacting to the propwash as a function of power (proportional to the LTL thrust). That is the only way to make it reasonably "power stable" - reasonable affect on pitch and trimmed airspeed with power changes! Without that, since it is the LTL thrust holding the nose up, the nose must necessarily drop when the LTL thrust reduces or disappears! Are you dismissing Power Stability as a requirement? That may be our difference of opinion! I am saying that achieving a reasonable static power stability is essential and that does dictate whether the stab needs to be uplifting or down lifting. If we do not consider Power stability, I agree, for AOA stability only, there may be one less reason an up-loaded stab is undesireable.

The second reason I maintain the up-loaded stab is not desireable, is simply that an up-loaded stab cannot "balance" the unavoidable aerodyanmic drag/lift moments on the airframe. This requires a down-loaded stab - unless the designer manages to design the airframe, enclosure, windscreen, landing gear, etc. to present a high drag line and upward pitch moment from drag/lift of on the airframe. The typical drag/lift aerodynamic moment on the aiframe, windscreen, LG, etc. is in the nose-down direction. This moment is proportional to the square of the airspeed. To "balance" this, requires a down loaded HS - and it's moment will also be a function of the square of the airspeed, so it does "balance" the airframe moments throughout the speed range. But, I maintain, it just can't do this if it is not angled in the right direction. An up loaded HS is not angled in the right direction for most, if not all gyros!

With regard to power instability - again, there are more than one way to slice the cake. For example - a LTL gyro may have a stab in the prop wash that is countering the HTL moment, which would make the gyro power-stable. Now -- the stab, and the engine, may be installed at an angle that in flight will make the stab down-loaded -- to counter any aerodynamic nose-down pitching moments. This kind of configuration would solve both your power instability and airspeed instability scenarios. This is not the ideal design, in my opinion, just an example of how different designs might meet the same stability goals. Udi

I'm assuming that is a typo in the second sentence: "HTL" must be "LTL"? If that is the case, I agree with that sentence, and the stab to do this must be up loaded! 3rd sentence - I guess this would work tilting the engine - Bensen did it for HTL correction (engine alone), and Pitcairn/Cierva did it on some tractor autogyros for the same purpose. But, I don't see many pusher gyros with the engine mounted on a tail-down tilt! And, this might present some clearance problems for the prop! But, this might be one solution to be able to get good power stability with a LTL.

Here's another way to envision how the HS works to compensate for an offset prop thrustline (HTL or LTL). Picture the prop and HS as one unit or package - thrust and offset deflection of propwash. The HS is simply a "vane" directing some of the prop thrust. Considering this as one unit, the deflection of the "vane" can change the actual thrust vector of the "unit". And, when the HS "vane" deflects some of the prop thrust, its component of the resultant thrust is both re-directed and re-positioned aft of the actual prop! This is a bit like the thrust re-direction vanes of a VTOL Harrier jet - the engine thrust vector in the jet is actually completely re-directed and re-positioned to the aft position of the deflector vanes.

In the gyro version, using the HS as a part of the "prop/HS unit", angling the HS relative to the propwash actually re-directs and slightly re-positions the resultant thrust of the whole "unit". This is much like Bensen did by tilting the whole engine. The HS can be utilized to re-direct the actual thrust of the whole "unit" so that it passes through the CG, or anywhere we might like for it to be directed. This "prop/HS unit" can actually perform nearly the same function as raising or lowering the engine to minimize prop offset or achieve the "CLT" without actual prop CLT or tilting the engine! In this way, the HS "vane" can provide the power stability by re-directing the "unit's" resultant thrust vector through or close to the CG.

This doesn't change the fact that this HS "vane" also needs to do double duty to "balance" the airframe aerodynamic moment - it is not like the Harrier jet vanes which are not actually exposed to the ambient airflow. The HS on the gyro is exposed to the ambient airflow and serves the second purpose of the HS - balance the other moments on the airframe.

If I am going to change my mind that an uploaded HS is as able to do all the HS jobs a down loaded HS can - I will tell you. Please keep trying!

- Greg