HTL vs CLT

[JAL]

I have started this thread because I dont want to particpate in a similar thread that has developed that really has nothing to do with original thread posted. I dont want to speculate about something that might or might not have happened and to be frank I dont think anyone should until the all the facts have been published.

Also I want to be provocative and ask a simple question? Would you prefer to fly a CLT machine without a HS (or even a small HS) or a HLT machine with a large effective HS. Now I realise that the a CLT machine with a small or no HS would be a lot more maneuverable and maybe a lot more fun, but is it safer?

Before you answer this. if you were to observe the amount of youtube and other media outlets as a gauge of what are the dominant gyros flown around the world it would seem that HTL machines with large HS would dominate and yet yet seem to have pretty good safety record.

I think it that it would be more important that machine meets a set standard in build quality and standard in flying quality rather than a design criteria or configuration. I say this because there is the possibility sometime in the future that there may be a superior configuration not known to us now. This might sound idiotic but remember maybe 20 years ago a HS or CLT were not thought as important and just how obvious does it seem now!

[Jazzenjohn]

I think it that it would be more important that machine meets a set standard in build quality and standard in flying quality rather than a design criteria or configuration. I say this because there is the possibility sometime in the future that there may be a superior configuration not known to us now. This might sound idiotic but remember maybe 20 years ago a HS or CLT were not thought as important and just how obvious does it seem now!

How about jotting down the set standards for build quality and flying quality you envision.

[dabkb2]

I own and fly both, CLT, KB2 with no HS and HTL, KB3 with an effective HS. The CLT is much more nimble and a lot more fun to fly with no instability in pitch. The HTL with HS is still a blast, and still no instability in pitch it just takes more stick and rudder to overcome the HS.

[RotoPlane]

In my opinion:

A bunt can occur with any thrust line….if there is no HS or an ineffective one.
A PPO can occur under certain conditions with a HTL and no HS or even with an HS, unless there is a long moment arm and a large HS surface area.
The reason a HS restricts some nimble response is because that design loads it up, either by being HTL or LTL, or by weight and balance.

I have absolutely no doubt that a gyro that is close to CLT, has an effective HS (large enough moment arm and a large enough surface area) and have a weight and balance that only lightly loads the HS (taking in to account the RTV)……will be pleasure to fly….and safe.

[WHY]

Hi JAL

Thanks for the question. Here is my point of view along with a related question. If you have a HTL machine, you have a potential longitudinal problem, especially if you are at very low foward speed and very low rotor thrust (lift), your engine thrust offset can produce a PPO very quick, I think this is an established fact.

Now if you add a LARGE horizontal stab with sufficient down load this "COMPENSATES" for that offset thrust, however I do not think it "ELIMINATES" the original design flaw only compensates for it.

Now apply the same situation as first mentioned , low foward airspeed and low rotor thrust, or one might say low rotor load. With low rotor load and low airspeed over the HS do you now have a situation where the HS is ineffective and the HTL is a strong potential for a power push over (PPO).

Now apply the exact same situation to a CLT machine with even a small HS. With CLT it appears to me that the reactive result from the same situation would be a more or less vertical drop until sufficient airspeed is obtained either from the vertical drop or from foward motion which intern loads the rotor with lift, the difference is a vertical drop rather that a potential PPO..

It appear to me that the CLT machine remain under control (even without a HS) while the HTL is set up for a PPO. I see the HS on a CLT machine as just reducing the sensitivity of the control inputs rather than trying to compensate for a design problem, altho I would sure want a HS on the CLT for that very reason.

Am I on base of off base on this analysis ????

Tony

[Lspav8r]

Why hasnt someone ever posted a list of the good bad and ugly gyros. Names of models would help all of us idiots out here. Dont be afraid to call a spade a spade. If its a killer I want to know about it.

All of the brainiacs talk about the problem, but they never mention the machines by name. I say grow a pair and name em.

Thank you vey much.

[RockyMeLad]

Yo JAL,

I avoid birds without an HS, but have flown the DesertBee in a slightly high thrust line and counteracting negative incidence, as well as CLT with "0" incidence using the same HS.
Both configurations were stable as verified by Greg's pitch tests. The CLT config has better efficiency as seen in climb rate and fuel burn. Doug can probably explain it better, but I think it's that the counteracting force of the neg incidence generates "effective weight" working against lift.

[Greg Mitchell]

I think it that it would be more important that machine meets a set standard in build quality and standard in flying quality rather than a design criteria or configuration. I say this because there is the possibility sometime in the future that there may be a superior configuration not known to us now. This might sound idiotic but remember maybe 20 years ago a HS or CLT were not thought as important and just how obvious does it seem now!


Jordan, this lack of importance attributed to the HS 20 years ago was due to some folk designing it out of the equation, just like they did when they strapped on bigger engines and decided to swing bigger props....everything went to hell in a hand basket...

Cierva had it right and so to did Bensen to a lesser degree, we moved away from the safe/good relationship of thrustline to C of G and use of adequate HS's.

What is wrong with going back to the future!

Tony,

Your post is spot on.

RotoPlane,

I have absolutely no doubt that a gyro that is close to CLT, has an effective HS (large enough moment arm and a large enough surface area) and have a weight and balance that only lightly loads the HS (taking in to account the RTV)……will be pleasure to fly….and safe.

Sounds a lot like my Butterfly Monarch Gyroplane


Jordan, your statement below....

However I put this to you it seems to me that a typical gyro accident involves a single seat gyro and the majority have a 2 stroke engine? What does this mean? I am being a bit to simplistic, doesnt anyone see a pattern?


........yes very simplistic and does not include the type or design parameters ( ie relationship of thrustline to C of G and use or non use of HS etc.....but loaded really.

Now for all the anti CLT crowd.

When most (if not all) of the advocates for CLT are stating their cases, they are all including the use of an adequate HS.

A CLT gyroplane to me is a gyroplane with a HS and in the case of the Butterfly we use an inline HS with no angle of incidence and is in the middle of the prop was on a long moment arm. Thrustline is just below the C of G and the gyro is a joy and delight to fly and is extremely pitch stable, very safe and suited to newbies and lowtime piolts because it is very stable and forgiving.

Give me a correctly configured CLT type craft any day and as RckymeLad says....I avoid birds without HS's.

Mitch.

[All_In]

Hi JAL ...

Am I on base of off base on this analysis ????

Tony
Right on the money!!!!

[phantom]

I will only fly gyros with a HS because I have no experience without one, I started with a glider in the 70's and when I went to power gyros Jerry Barnett who always answered my radiotelephone calls even when he knew that I had a bensen( bensen aircraft would not talk to me on a radio telephone where you had to say over at the end of each sentense so the operator knew when to move the switch from transmit to recieve) told me that my gyro was not safe to fly without a HS and it had to be large because of the short arm.I may have tried to fly a little without one but mother nature steped in and stopped any chance with a big snow storm so I had to put the gyro on skis and on the first test run without the rotor when I got off the machine rolled over backward as the tail sank in 4 feet of soft snow.I made a nice large HS 2 feet by 6 feet and bolted it to the keel 12 inches under the VS and 12 inches out beyond the keel, it worked well and without it there is a good chance that I would not be around to write this now.

[Friendly]

I fly a drop keel Bensen with Ernies Tall tail, A good friend of mine flys a Bensen with a very large HS under the keel but slightly shorter moment arm than the stock Bensen. He like to fly along side of me but a couple hundred feet higher than me when I take off. I was adjusting my trim not long ago on take off so I left the throttle in and was taking note of my stick and pedals. He told me that I left him at 70 mph and just flew away from him. My gyro seems to hunker down rather than get twitchy when I flew a stock Bensen. My friend has many years of experience flying his gyro but he say's it is a handful after 70 and no fun at all. At 70 my only concern is my face shield coming off the helmet. I would not go back to flying without the HS, it just is not worth it to me to get a few tighter willies .

[Heron]

Apples to apples
First untangle the causes. Most accidents are iniciated by pilot error.
After that, the second cause is lack of stick time to correct from unwanted inputs, be that weather related or design flaw (first pilot error, go up like that)
Smaller machines tend to allow the pilot more aerial shenannigans thus causing envelope departure coupled with low time . . . .you add it!
Get a semi right machine, slap a rocket on it and see what happens if the pilot has the guts to open up . . .
Heron

[C. Beaty]

The other side of the coin for those who care. It’s a bit technical.

http://eprints.gla.ac.uk/4962/1/Advances_in_the..._Flight_Dynamics.pdf

[Aussie_Paul]

The other side of the coin for those who care. It’s a bit technical.

http://eprints.gla.ac.uk/4962/1/Advances_in_the..._Flight_Dynamics.pdf

Thanks Chuck.

Aussie Paul. :)

[JAL]

Well that paper certainly spells out that the CoG location compared with the thrust line is really the most dominant configuration to consider. It also states that the HS and pod really are not that influential and only have some effect at high air speeeds.

So it would seem LTL has a stabilising effect while HTL has destabilising effect (which is what most people are saying here anyway).

What I found interesting was that the magni thrust line was measured to be only 1" above the thrust line (TL) and that it was thought that the thrustline could vary by as much as -3" to +3" depending on the individual machine. The Montgomerie TL was +3" CoG.

But when you look at them you would never pick that the magni has a lower TL. Just goes to show you really cant tell by looking. The article also pointed out that Magnis large tail went a long to mitigate against the destabilising effect of HTL.

Now to my question, considering in this case the Magni was measured to have reasonable 1" HTL, have they managed to move the CoG higher and closer to the thrustline by using a heavy rotor. So the heavy rotor maybe what adds to the stability more than the HS particularly at slower airspeeds?

[Redbaron]

a pusher gyro that is styled after a benson requires careful attention to aerodynamics to make it stable! the only truely stable benson style gyro is slightly high thrust line with a horizontal stab with negative pitch! my gyrobee flies just like a very very heavy cessna! the cierva tractor gyros are truely stable! even when building model gyros a horizontal stab way back didn't take a degree in aeronotical knowledge to understand this! :wacko: the love of the benson style gyros requires more understanding of aerodynamics so you don't buntover or ppo! short coupled gyros are more attrative cause of their compact size, cierva gyros were simply airplanes with a rotor instead of a wing to keep people from dying from stall accidents! :wave:

[JAL]

I was commenting on the UK technical paper, which stated that testing they did in the wind tunnel on the magni model showed that pod or tailplane aerodynamics have a
very limited effect on longitudinal dynamics. They concluded The only significant configurational effect observed was that of vertical location of centre of gravity
with respect to propeller thrust line.

This was testing a model in a wind tunnel, whether the real world is different I dont know? Just that magnis are reported to be pretty stable and dont seem to ppo or pio and wind tunnel testing shows the tail or pod do not have have any signifcant effects, but the authors do say the large tail area may mitigate the low HTL (1" HTL) to some degree.

Also they just didnt state that it was the tail area but the amount of tail area compared to the pod area. Does the pod area detract from the effectiveness of the tail area, do they work against each other? Can you design a pod to be lifting surface to work in tandem with the tail?

Just asking questions out loud. I dont know the answers I am also thinking that the rotor may have a bigger role to play in stability than I previously thought and maybe the HS less.

[Jean Claude]

I think Magny M16 is HTL > 10 " above that thrusline (including heavy rotor) The study by the University of Glasgow seems questionable on several points: the RPM seems to increase when the stick is pushed! The model seems tested without the rotor! (you test a FW without wings?)

[JAL]

Jean that is what I thought but the UK report pretty clearly states that the M16 they used the C0G was 1" below the thrustline. That is what I am trying to work out, have they measured it wrong? They have the Magni being 1" HTL and the Montgomerie as being 3" HTL and looking at them you wouldnt believe that the montgomerie is more HTL. So if they have measured right then the M16 is only just HTL and could near CLT at light weights, if they measured wrong then it does throw a bit of doubt on their conclusions.

Or the third option, they measured 10" and missprinted in the report as 1"

I am not trying to suggest anything. I dont know what is right or wrong, I was initially trying to make he point that there seemed to be a lot of HTL machines flying out there like the magni M16 which dont have a history of PPO or PIO and that regulating that a gyro must be CLT for certification might not be necessary and that set of flying standards might be a better way to avoid bad designed gyros. But after Chuck provided the link to the UK report where they clearly state that the most signifcant factor in a gyro configuration is the location of the thrustline in regards to CoG (HTL is destabilising, LTL is stabilising) and not the pod or HS kind suggests maybe that it really should be a consideration (even though it seems to defy the safe history of the Magni types).

But then the report also says that M16 is only 1" HTL so this kind of goes against my point that there are lots of these macines flying around nice and safely as an example of HTL but in fact they are pretty close to CLT if the report is to believed.

My point about the weight of the rotor in the previous thread was that if they have measured right and you compare both gyros in the report it would look like the Magni thrustline is much higher in relation to where where the pilot sits and that Magni have moved the CoG higher (further from the ground) than the Montgomerie by using a heavier rotor. Is it a valid idea if you have a gyro that is HTL that you could improve it by putting on a heavier rotor, and that the difference in weight does not have to be that much greater because of the distance the rotor (leverage) is way from the CoG? and is the heavier rotor that is helping the stability more so then the HS which the report says is ineffective in all but high speed conditions.

[Jean Claude]

JAL, Even in the case of a heavy rotor (90 lbs) and a single passenger (150 lbs), Magny M16 is still HTL 7" (According to my calculations). On the other side, HS is completely surrounded by the deflection of the rotor (induced speed): ignore it in an study of stability is a serious negligence.
So, with HTL suspicions values, and HS A.o.A suspicions values, delay negliged disk angle, this study could begin with "Suppose negligible the main phenomena..."

[JAL]

OK the study seems to be riddled with errors, not much can be taken from it then I guess and the Magni is flying around happily with 10" HTL.

Is that the HS is surrounded by the prop wash or rotor wash?

So what I get from this is that there really is no reason for HTL at all and that the large HS is only a aerodynamic fix for the destabilising effects of HTL. Conversely a CLT machine should also have a HS but it can be smaller therefore reduce weight and drag.

[Jean Claude]

Is that the HS is surrounded by the prop wash or rotor wash?.It is in the deflection caused by the rotor: If it is 0° relatively to the general airflow, it is -5° in the local flow. This increases the stability.

Conversely a CLT machine should also have a HS but it can be smaller therefore reduce weight and drag. this is so my opinion. But I suppose that the stability is also get by the heavy nose and low stiffness of the blade twist.

[karlbamforth]

I think the reason that Greg keeps saying elusive CLT is because any gyro will only be CLT at one moment. IE when everything is in a set condition. As the fuel quantity changes so will the CofG, if the pilot wears shorts, t shirt and sneakers in the summer and cold weather clothing and boots in the winter then the Cof G will vary. More so with 2 seaters, low fuel, solo with a lightweight pilot is a lot different to 2 crew and full fuel.

What we end up with is a range of thrust offset, that most seem to agree +-2 inch is acceptable. This is similar to the CofG range of a fixed wing. The Cof G of the empty aircraft is calculated and then a graph can be produced to ensure the aircraft stays within certain limits when loaded. (+- X number of inches)

Reading the Glasgow report it says that they "calculated the aircraft thrust offset" This can only reasonable be done if you know the expected min and max load and you would end up with a range. IE fully loaded it is 8 inch, min load it is 1 inch. The fact that they state a fixed offset of 1 inch makes me think that it was calculated with the aircraft empty in line with fixed wing convention. This would aslo go a long way to explain the discrepencies we are seeing.

Once you have the basic figures it is possible to calculate for changes in fuel and crew loads but they do not explain how or if they did it.

[C. Beaty]

We have here a bad case of wandering zeros.

The Magni gyro used in the Glasgow research was one from the 1990s with 2-stroke Arrow engine that was in fact near CLT.

Current Magnis are entirely different machines with thrustline offsets of 8.4-11 inches.

From the CAA’s Airworthiness approval note No. 28705:

[Heron]

Manyfacturers should use the same source of data to market their machines.
Heron

[JAL]

Thanks for the answers! There is one thing about all this there is that ther are so many levels of understanding. You think you understand something on one level then you try educate yourself on the next level and then a whole bunch more variables are introduced until you get so confused you realise you never really had a good grasp of it the in first place.

Things like thinking older versions of the Magni would be the same as a recent Magni, since when has any aircraft type ever evolved over time without getting fatter! That should have been my first assumption!

[All_In]

Thanks for the answers! There is one thing about all this there is that ther are so many levels of understanding. You think you understand something on one level then you try educate yourself on the next level and then a whole bunch more variables are introduced until you get so confused you realise you never really had a good grasp of it the in first place.
... :suspicious:
Yaw Mon!!! That's how I feel, just when I think I got a handle on it I discover I still don't have a clue.

[Jean Claude]

The Magni gyro used in the Glasgow research was one from the 1990s with 2-stroke Arrow engine that was in fact near CLT.
Chuk, the report mentions M14. The design has exactly the same height as M16. It would require major changes to climb C of G to 10-inch
Jean Claude
http://img42.xooimage.com/files/4/5/9/m14-m16-1c433ca.jpg (http://autogire.xooit.fr/image/42/4/5/9/m14-m16-1c433ca.jpg.htm)

[Mayfield]

Hi Jean Claude,

I don't think the CG climbed. Since the propeller thrust line is at the same place I suspect the CG was lowered with the additional weight below the thrust line of the M-16.

Jim

[C. Beaty]

I think part of the confusion has to do with model numbers. The older designation when Arrow engines were being used (I think) was VPM-x. The present designation is M-x with Rotax engines.

Here’re side by side photos of an M-16 being measured by a scruffy looking crew of Australians with strings and a crooked stick and the VPM-16 tested by Glasgow. Clearly not the same aircraft. The Australians’ numbers were almost dead-on so it’s inconceivable to me that a team of university engineers couldn’t get it right.

[Greg Mitchell]

Here’re side by side photos of an M-16 being measured by a scruffy looking crew of Australians

Geez Chuck.....be nice! Tim and I 'dressed up' for that pic. LOL!

Mitch.

[C. Beaty]

Sorry, Mitch; you and Tim do look lovely. Perhaps rough and ready?

[karlbamforth]

Thanks Chuck,

That certainly resolves some of the questions.
I was begining to wonder how they could have got the thrust offset so wrong but the results (CofG/thrustline) so right.

I think everyone is arguing pretty much the same point here too.

We all agree a HS of sufficient size is required.
That CLT fixes a problem.
If not CLT then you need other measures to stabalize, IE. overbalanced rotor, HS etc.
CofP is important and should be CL or balanced with adequate tail feathers.

Most of the dissagreements here are simply different ppl falling different sides of the fence. We are all standing close to the fence, just that some prefer one side to the other. Strange analogy sorry its early here. :suspicious:

[JAL]

I wouldnt mind asking a few more questions just about general gyro aerodynamics if people dont mind answering them.

First is the centre of pressure (CoP), in a fixed wing normally applies at the point 30% of the wing chord which is easy to visualise in regards of CoG. Not so easy if the wing is rotating 350 times a minute. Is the teeter bolt the location at which this force is applied and does it act perpendicular to the plane of the rotor disk.

Can the CoP be offset from CoG. Is the vertical line which the CoG is located is measured from the top of the mast or from the teerbolt. (Side question: what is the offset between the top of the mast and the teeterbolt referred to?). IOW Is it possible to leverage CoP or is always acting through the CoG.

In typical case of 2 seat gyro with a pilot in the front seat and full fuel where the fuel is located foward and below the CoG what happens to the CoG as the fuel is used. Does the CoG move upwards along the vertical line. Also should the nose of this type of gyro start lower and move upwards as the fuel is used (if the fuel was located behind the CoG then the nose would start higher and move lower as it is used).

Where does the weight of the rotor act? Through the top of the mast or the teeterbolt? IOW is possible to leverage the rotor weight.

I think I know the answers and why but I am just want to make sure that I do understands the basics.

[Jean Claude]

Here’re side by side photos of an M-16 and the VPM-16 tested by Glasgow. Clearly not the same aircraft.
You are right, Chuck, it's not exactly the same aircraft, but the frame is very near, and it is not crédible that HTL is 11" for one and 1" for the other (with Arrow 100 lbs or with Rotax 170 lbs)

it’s inconceivable to me that a team of university engineers couldn’t get it right.
However, it is certain that this team of university engineers armed with a powerful hardware measurement have obtained only futile results.

[C. Beaty]

I agree, JC, judging from the photographs, there can not be so great a difference in CG location between the Arrow powered 2-stroke models and the Rotax powered 4-stroke models.

I speculate the rotors are identical so that differences would have to come from weight of fuselage and undercarriage. But everything is speculation; there is almost no factual information available. Apparently, no one knew (or cared) about the CG location until some Australians with sticks and strings measured it.

For research, Glasgow would certainly have measured the CG as it was during flight testing. To a large extent, this would have been affected by disposition of test instruments, pilot weight and location.

[bpearson]

I'm not saying it's incorrect but....looking where the cg is on that photo, there seems to be an awful lot of weight above it...half the pilot, the whole engine and a very heavy rotor nine feet up.

The airframe, pod etc on Magni's are very light. Don't attack me, just my thoughts.

[Vance]

It looks to me like most of the pilot and a lot of the passenger is below the thrust line.

The middle of people is usually around their belly depending on how they are sitting.

When I see a Magni fly with a heavy pilot, particularly with a high performance takeoff it is not hard to see the nose drop with a power application.

In my limited experience flying a Magni; I found a definite tendency to pitch nose down with the application of power at the lower speeds. As the speeds rise in my opinion they pitch less with throttle.

Thank you, Vance

[bpearson]

I was relating the pilot to where they had found the vcg to be Vance. Just seems that there is a great deal of weight above this point. Granted, the whole of the pilot is below the TL.

In my 'old style Magni' one up it climbs on opening the throttle. I havent flown in a later model.

[Jonathan Mylrea]

I fly a Magni M16 which is an HTL machine with a very good horizontal stabiliser. I personally feel much more comfortable in this aircraft than one with CLT and no stabiliser. I used to design missile systems and, believe me, a rocket has CTL, but without tail fins it can go very unstable and will need a far more responsive autopilot system to maintain it's flight stability. An airframe needs damping to make it controllable and that is what a horizontal stabiliser provides in the pitch axis. CLT designs out any pitch eccentricity due to motor thrust, but does not provide much assistance if the pitch disturbance is caused by other inputs such as pilot commands to the flight control system. A large horizontal stabiliser will quickly damp out pitch instability. CLT hopes to avoid its occurrence, which in my opinion is hoping for the best if not backed up by some effective damping. I guess that CLT and a good horizontal stabiliser is the best solution, but I am happy that an HLT gyro with a good horizontal stabiliser is very stable in pitch, whilst any gyro without a horizontal stabiliser will require far more pilot input and, given my reactions, I would not attempt to fly one. This is not to say that a proficient pilot cannot fly one quite safely, just as unicycles are perfectly stable with the right cyclist.

[C. Beaty]

If one has 400 ft-lb of nose down torque from offset propeller thrust trying to make a gyro behave like a rotary lawn sprinkler, then the horizontal tail must make 400 ft-lb of nose up torque.

A needless burden.

If the horizontal tail can’t make 400 ft-lb of nose up torque, then the rotor thrust vector must pass aft of the CG to supply the balance.

[Jazzenjohn]

I fly a Magni M16 which is an HTL machine with a very good horizontal stabiliser. I personally feel much more comfortable in this aircraft than one with CLT and no stabiliser. I used to design missile systems and, believe me, a rocket has CTL, but without tail fins it can go very unstable and will need a far more responsive autopilot system to maintain it's flight stability. An airframe needs damping to make it controllable and that is what a horizontal stabiliser provides in the pitch axis. CLT designs out any pitch eccentricity due to motor thrust, but does not provide much assistance if the pitch disturbance is caused by other inputs such as pilot commands to the flight control system. A large horizontal stabiliser will quickly damp out pitch instability. CLT hopes to avoid its occurrence, which in my opinion is hoping for the best if not backed up by some effective damping. I guess that CLT and a good horizontal stabiliser is the best solution, but I am happy that an HLT gyro with a good horizontal stabiliser is very stable in pitch, whilst any gyro without a horizontal stabiliser will require far more pilot input and, given my reactions, I would not attempt to fly one. This is not to say that a proficient pilot cannot fly one quite safely, just as unicycles are perfectly stable with the right cyclist.

I think most everyone would agree with you, and would agree that the Magni is a safe, good flying gyro, however, I have to ask, How many missiles are designed with off center thrust compensated by extra large fins with a built in incidence?

I understand from a styling, ease of entry, and possibly a more stable ground handling perspective why they might design the gyro that way. It is an understandable set of tradeoffs that require some expertise to keep safe, Magni so far has demonstrated that expertise, some other manufacturers have not.

[Doug Riley]

Chuck, in Post 41, did you mean "aft" of the CG or "forward"? A typo, I imagine.

The rotor thrustline must be forward of the CG to hold the nose up if the tail can't do it alone.

[C. Beaty]

Thanks, Doug. A case of my fingers outpacing my not so speedy brain. I've made the correction.

[Jonathan Mylrea]

Hi Guys, actually the first successful guided missile, the V1 Flying Bomb, had a fairly significant off centre thrust line and was quite stable and controllable with a very simple bang bang type autopilot. As long as the flight control surfaces are correctly designed there is no problem with stability.

In fact a gyro is a far more complex situation as the rotor thrustline is the stabilising factor during normal flight. I think that the Magni horizontal stabiliser is doing very little when the gyro is trimmed for level flight and is thus no big drag burden at all. If you tip the nose 30 degrees down the tailplane will suddenly provide a very significant correcting moment.

I agree that in a situation where the gyro has very little forward speed, an unloaded rotor and full thrust applied even a Magni would bunt. But that is a bit like saying that an automobile doing 100 Mph down a highway is unstable if you suddenly turn the steering wheel full lock to the left. What I would be much more concerned about is an automobile that takes constant careful control of the steeing wheel when doing 60 Mph down the highway to prevent it from swaying from lane to lane.

So the question was, would I prefer to fly a CLT without an HS or an HTL with a correctly designed HS?

The second every single time.

[Jean Claude]

If one has 400 ft-lb of nose down torque from offset propeller thrust trying to make a gyro behave like a rotary lawn sprinkler, then the horizontal tail must make 400 ft-lb of nose up torque.

A needless burden.

If the horizontal tail can’t make 400 ft-lb of nose up torque, then the rotor thrust vector must pass forward of the CG to supply the balance.
However, despite RTV pass forward the CG, it is stable if dF pass aft the CG. I agree with you: A needless burden (and trajectory disturbed with variation propeller thrust)
dF is the lift added by the variation of the general flow angle
dF rotor is the lift added no change r.p.m (time short)
dF tail is the lift added (same with Cz<0, =0, or >0)
Jean Claude
http://img41.xooimage.com/files/7/2/f/gyro-is-stable-1c61976.jpg (http://autogire.xooit.fr/image/41/7/2/f/gyro-is-stable-1c61976.jpg.htm)

[Heron]

The question explains what we are discussing: what to fly? gyro A or gyro B (both with deficient configuration)
Then comes the sequel: what to build? what to regulate?
Simple to be or not to be situation.
thanks
Heron

[brett s]

It's the wrong question to begin with though, near CLT + effective horizontal stab would be the obvious choice.

There's no good reason to have large thrustline offsets or for not using a horizontal stab, so why do either?

[Jazzenjohn]

Hi Guys, actually the first successful guided missile, the V1 Flying Bomb, had a fairly significant off centre thrust line and was quite stable and controllable with a very simple bang bang type autopilot. As long as the flight control surfaces are correctly designed there is no problem with stability.

The V-1 is definitely off center thrust, not all that many examples of off center thrust missiles though, and the V-1 was a bit more like a plane than a missile. If it were a superior configuration, I would expect the majority of missiles to be off center thrust, wouldn't you?

In fact a gyro is a far more complex situation as the rotor thrustline is the stabilising factor during normal flight. I think that the Magni horizontal stabiliser is doing very little when the gyro is trimmed for level flight and is thus no big drag burden at all. If you tip the nose 30 degrees down the tailplane will suddenly provide a very significant correcting moment.

When you trim your Magni for level flight, what exactly are you trimming and how is it removing the drag burden from the Horizontal stab?

I agree that in a situation where the gyro has very little forward speed, an unloaded rotor and full thrust applied even a Magni would bunt. But that is a bit like saying that an automobile doing 100 Mph down a highway is unstable if you suddenly turn the steering wheel full lock to the left. What I would be much more concerned about is an automobile that takes constant careful control of the steeing wheel when doing 60 Mph down the highway to prevent it from swaying from lane to lane.

So the question was, would I prefer to fly a CLT without an HS or an HLT with a correctly designed HS?

The second every single time.

I would take the HTL with the good stab over a CLT without one, but that is a False Choice. What bizarre situation would ever actually force that decision on someone?

I agree that you Need a correctly designed horizontal stab to have a stable gyro, you don't Need HTL to have a stable gyro.

[gyromike]

It's the wrong question to begin with though, near CLT + effective horizontal stab would be the obvious choice.

There's no good reason to have large thrustline offsets or for not using a horizontal stab, so why do either?

Exactly.

It seems that "CLT and no stabilizer" is just a strawman. I don't recall anyone saying that if the gyro is set up properly with CLT that you can just throw the stab away.

My Bensen is within 1/2" of being CLT as measured at Bensen Days a few years ago. I have the butterfly rock guard slid all the way back to the tail wheel as a stabilizer. I have flown it back to back against an identical machine without a stabilizer and the stabless gyro had a noticable pitch instability.

[Doug Riley]

The situation of low airspeed, full throttle and zero G (that is, zero rotor-disk AOA) isn't as unlikely as it may first seem.

A full-throttle takoff in turbulence, where the pilot allows the airspeed to drop, can produce these conditions.

However, the same downdraft that produces the zero G also slams into the H-stab, producing a nose-up torque. Therefore, it's not necessarily true that a Magni would PPO in such a situation. The nose might try to dip a bit during the brief time interval between the moment the downdraft reaches the rotor and the moment the downdraft reaches the H-stab. A fraction of a second.

Using a loaded H-stab and/or CLT would eliminate even the small "nose initially goes the wrong way" reaction. In the case of the Magni, these temporary "backwards" reactions appear to be harmless -- but some people might find the idea a little ... untidy.

It's not much of a safety problem, though, given the record.

[Jean Claude]

The situation of low airspeed, full throttle and zero G (that is, zero rotor-disk AOA) isn't as unlikely as it may first seem.
Doug, not exactly. If AoA rotor is 0 °, then g = .5, not 0 (because positive blades pitch, gyro becomes temporarily helico). 0g, requires approximately AoA rotor = -10°.
Jean Claude

[Doug Riley]

Yes, Jean, you are correct. I was simplifying to make the post shorter!

A rotor doesn't really generate exactly zero lift until the disk angle is a few degrees negative; the exact number of degrees depends on the blade incidence.

The point I wanted to make is that "zero G" is not some strange place where gravity mysteriously goes away -- it is simply a reduction of rotor angle of attack to the point where lift is zero.

For me, low angle of attack is easier to picture than gravity disappearing. I could not have dreamed up the floating mountains in "Avatar."

[Jean Claude]

I agree Doug. My point was just to give an order of magnitude. With the usual pitch and 50 mph, it takes about -10 ° to 0g. It is a difference of 20 ° for SH. If the profile is thick, it can be very effective in preventing this, but a thin sheet of plywood stalls: ineffective.

[Heron]

Is it possible to quantify (put numbers to it) the situation where a machine pitching will get in to the danger zone and what is the timing involved?
Lets say x wind and y angle will produce z effect.
thanks
Heron

[Murray Barker]

So, basically it would take an extrordinary amount of foward stick movement to unload a rotor and produce a buntover WITHOUT the engine running?

[C. Beaty]

One can always attempt an outside loop, Murray, but the sort of discontinuity, springing the mouse trap, that tumbles HTL machines doesn’t exist without propeller thrust.

[bpearson]

I remember asking my instructor how hard you can push the stick forward when you lose the engine on climbout...it was fairly hard...

[Jonathan Mylrea]

Hi I think that we are all agreeing that CLT, or as close as possible, as a design goal is a good thing. As Jazzenjohn mentioned, sometimes there are tradeoffs that have to be made with any design and a slightly high thrustline can easily be compensated for by a correctly designed HS.

The Magni definitely pitches slightly nose down if you suddenly apply power, which is why I believe it to be HTL.

[Jean Claude]

The Magni definitely pitches slightly nose down if you suddenly apply power, which is why I believe it to be HTL.
There is no doubt: HTL 11". The HS and inertia in pitch (about 10,000 lbs. square foot or 500 kg.m2) slow motion and the pilot can react before being in a critical situation.
Jean Claude

[Jonathan Mylrea]

Wow! 11 inches is a lot more than I would have guessed. I assume the inertia in pitch is due to a heavy rotor system?

[C. Beaty]

According to the UK CAA:

[Murray Barker]

So us dumb-arsed Aussies got it right!!

I have now flown the new Magni M24 and although pleasant to fly does nose over at low speed when power applied.

[Doug W.]

In regards to the nose dropping on a Magni when adding power from a slow speed. What speed are you trimmed for, slow or fast? In mine, if I am trimmed for 40 and add power, the nose rises and it climbs. If its trimmed for 100 the nose will drop because it wants to find 100 mph. It will drop the nose without adding power in the same situation if you let the stick go and will find 100 mph and stay at 100 mph.

Doug

[Jean Claude]

I assume the inertia in pitch is due to a heavy rotor system?
Jonathan, Inertia: sum of mass x square radius. If Driver far ahead of C o G, engine behind CoG, Rotor far above CoG, then pitch inertia is large.
Gyroscopic delay of the rotor gives a favorable damping. But damping is not inertia
http://img49.xooimage.com/files/c/e/b/analogy-of-pitch-1c902d3.jpg (http://autogire.xooit.fr/image/49/c/e/b/analogy-of-pitch-1c902d3.jpg.htm)

[Doug Riley]

Doug W., the fact that you can get the nose to rise by adding rotor pitch trim does not address the basic issue of balancing airframe moments. Trim that adds or subtracts spring tension to the rotor head is simply a device that pulls the stick back for you.

IOW, you're using rotor thrust to compensate for an otherwise imbalanced airframe. Using the rotor as a trim device is an inadequate solution, because the rotor's thrust can disappear a strong downdraft. We would like to see the nose-up moment provided entirely by the HS, not the rotor, because the HS does not lose power in a downdraft, as the rotor does.

(The HS also gains power as the craft noses over. Apprently it does so suffciently in a Magni -- the nose "dip" does not proceed to a full-blown PPO. Still, it's better not to have any initial "dip" of the nose. If the HS were mounted higher, partially in the prop sliptream, the "dip" could be eliminated. Aligning the CG and prop thrustline would accomplish the same thing.)

[Doug W.]

Doug Riley, I follow what you are saying. One thing I have noticed on the magni when adding power, that may be the same in others, is that the nose seems to stay in the same plane and it is a tail up feeling not a nose down feeling. Granted it is a slight nose down attitude but not a nose dip it's a tail rise. Is this typical for other HTL's?

I'm always trying to figure out what Magni's done to make them so stable. I still think the forward angle of the mast adds leverage in some odd way like the forward angle of that a claw hammer has when pulling a nail straight up.

Doug

[Doug Riley]

The gyro will pitch around its CG. If you're seated well below the CG, the sensation from the pilot's viewpont may be more aft-swinging than nose-down.

The angle (or construction method) of the mast won't make any difference as long as the rotor head ends up in the same location. You could use a truss, a single tube, aft rake, forward rake, no rake or a mast with half dozen zigzags in it like a "crazy straw" -- if the head ends up in location X, it'll fly the same because the rotor's forces and mass will land in the same place.

Structurally, a truss has the most strength for its weight, but it makes the mast rigid, when it really needs to flex.

[C. Beaty]

So us dumb-arsed Aussies got it right!!Mitch, you dumb arsed Aussies are a pack of spoil sports with your cold, hard facts.

It’s a whole lot more fun to speculate and invent new terminology; effective offset, perceived offset, dynamic offset, static offset, imaginary offset, non-linear offset, intangible offset and so on.

[Jonathan Mylrea]

Hi Doug, if I am trimmed for say 70 mph and add quite a bit of power, the nose dips for maybe 3 seconds before the airframe returns to its original attitude. It is not very marked, I doubt that a passenger would even notice and although I may instinctivly compensate, I am sure that the aircraft stabilises quite quickly without any pilot input. So it is not hazardous as far as I am concerned

I have not played around with it, but at a lower speed it may be that the nose rises more quickly, or even immediately. This may be due to having the disk tilted more back, which would result in the rotor drag being higher and additional lift being obtained more quickly, both of which tend to raise the nose.

Jean Claude, yes I meant that a heavy rotor system situated at the end of a long lever arm from the COG would result in a significantly higher moment of inertia around the pitch axis which may add some stability to the Magni design. The angular acceleration would be reduced giving the HS and other factors time to provide a correcting moment.

[Heron]

Unphatomable offset . . .

[Doug Riley]

Jonathan, actually "more lift" alone should, in an aircraft that is G-stable (or AOA stable, if you prefer; they are the same unless you have collective pitch) cause the nose to drop. The aircraft should trend back to one G. A nose lift that occurs when rotor lift increases exaggerates the G increase. The offset gimbal rotorhead is designed to flatten the rotor disk angle when lift increases.

An increase in airspeed, OTOH, should cause the nose to rise. The aircraft should trend back to its trimmed airspeed.

The complication with rotors is that rotor blowback increases with airspeed, which cause an increase in rotor disk AOA and a nose rise, as you say.
what you should be seeing is the rotor is reacting to the increased airspeed, though, not to increased lift.

As always, it's important not to rely on the rotor as a trimmimg device to counteract pitching effects arising from an offset prop thrustline (HTL). Rotor thrust varies too much for rotor-countering-HTL to be a reliable strategy.

[Greg Mitchell]

Mitch, you dumb arsed Aussies are a pack of spoil sports with your cold, hard facts.

It’s a whole lot more fun to speculate and invent new terminology; effective offset, perceived offset, dynamic offset, static offset, imaginary offset, non-linear offset, intangible offset and so on.

G'Day Chuck,

I get it . You crack me up!;)

However, I think you intended to direct your comments to our ASRA President Muzza.:boink:

From one of the local spoil sports.

Mitch

[C. Beaty]

Don’t feel too bad Mitch. At certain passionate moments, I used to get the name of my girlfriend confused with that of other girlfriends or of ex-wives.

It never went over too well.

[Friendly]

Don’t feel too bad Mitch. At certain passionate moments, I used to get the name of my girlfriend confused with that of other girlfriends or of ex-wives.

It never went over too well.

Now I can say that I have made some of the same mistakes Chuck B. has made and I am wiser for it.:humble:

[Jonathan Mylrea]

Hi Doug. That is very interesting, I am still getting my head around all of the forces and reaction forces involved with a gyro, much more complex than they first appear. Are you saying that with constant airspeed and increased lift the nose of the gyro would be left behind due to the inertia of the airframe with the COG ahead of the pivot point? In a pure CLT (no HS) this would equalise due to gravitational forces once the gyro stopped accelerating vertically. In the case of an HS stabilised set-up the HS would react far sooner, I suspect, and lift the nose.

I agree with you in terms of relying on rotor thrust to provide stability. I am a little concerned about a no HS design even with pure CLT, but I may be completely wrong on that score.

[Doug Riley]

Jonathan, a given gyro may react to a pure increase in lift by nosing up, down or not all. A stable reaction (the kind we want) would be to nose down, since that would tend to decrease lift and return us to one G's worth of lift.

How do we achieve that result?

A solid body in space, if pushed or pulled by a force acting along any line that doesn't pass through the body's CG, will tend to rotate around the CG. The CG is the pivot point -- there is no other.

Therefore, the usual answer to G-stability is to arrange the forces on the airframe so that the rotor thrustline is aft of the CG. That way, when rotor thrust (and hence lift) increases, it pulls the tail up, and the nose goes down.

LTL forces the rotor thrustline aft as long as there's prop thrust. A downloaded HS forces it aft as long as there is airflow over the stab.

A theoretical, perfect CLT (and centerline drag) gyro with a neutral (or no) HS will have its rotor thrustline right through the CG. A pure increase in lift will not cause the nose either to rise or to fall.

All this is a bit abstract if you don't have collective pitch on your gyro. You can't HAVE pure changes in lift alone. Lift will increase if either the disk AOA increases or airspeed increases. Therefore, you'll end up worrying more about airspeed stability or AOA stability than pure G-stability.

Rotor blowback gives us some airspeed stability automatically, if we don't ruin it with bad design.

An autorotating rotor on an airframe lacks AOA stability with respect to deliberate control inputs, without the use of some auxiliary gadget. When the stick is pulled back, the rotor thustline moves forward relative to the CG, pulls up the nose and causes a further back-tip unless the stick is "floated." The Bensen-style gimbal head, with excess offset and a trim spring, at least provides control pressures that tend to limit this overreaction. A H-stab does the rest.

You're right; a CLT gyro with no H-stab is not an optimum design.

[hillberg]

Question,Is it the configuration of the rotor system a bigger factor in Low G push over? A simi ridgid type free to flap in low G loading will move to pilot command with no reaction to the body hanging below,the fuslage rolls uncommanded and "balisticly" goes its own way, mast bumping or blade strike on the way down? Gyros in WWII were doing loops with articulated systems. any thoughts?

[brett s]

Sure, that would help - an articulated system still has control power at zero g loading, unlike a teetering system.

But it's also a lot more complex & you also lose the stick-free stability added with an offset gimbal rotor, it'll handle like a helicopter

[Jonathan Mylrea]

Hi Doug, thanks for the concise description, the balance of forces is more or less as I had imagined it.

[Vance]

Also I want to be provocative and ask a simple question? Would you prefer to fly a CLT machine without a HS (or even a small HS) or a HLT machine with a large effective HS. Now I realise that the a CLT machine with a small or no HS would be a lot more maneuverable and maybe a lot more fun, but is it safer?

!

This question diverges from the title of this thread.

I would not want to fly a gyroplane with a large thrust line offset and I would not want to fly a gyroplane without a horizontal stabilizer. I feel I am not alone in this.

I don’t agree that a horizontal stabilizer makes the aircraft less maneuverable. In my opinion a more stable platform to command the rotor from makes the aircraft more maneuverable and have less control lag.

I don’t know why someone would imagine that choosing CLT would change your decision about a large horizontal stabilizer placed well back from the center of gravity.

I would be grateful for someone with this perspective to explain it because I am confused by the suggestion that CLT means deleting the horizontal stabilizer.

Thank you, Vance

[birdy]

I would be grateful for someone with this perspective to explain it because I am confused by the suggestion that CLT means deleting the horizontal stabilizer.
I will. :)

I don’t agree that a horizontal stabilizer makes the aircraft less maneuverable.
I do, coz it dose.
But not to the point where itd be noticable to the average gyro nut.
For starters, a CLT machine physicaly cant PPO, so theres no need to loose sleep over that danger.
A HS ona clt machine dose make the airframe follow the airflow.
This makes it a very stable and power efficiant machine. [ as far as gyros go]
But if you need greater manouverability [higher disc pitch rates], at any ASs, a HS will not allow the frame to over run the airflow. IOW, without a stableising/ damping horisontal tail plane, a gyro is free to swing at wotever rate the pilots inputs command.
Haven no HS means pitch change rates can be equal to roll change rates, coz theres nuthn to stop the frame from 'getn in frunt of the airflow'.
The only way you can get 90* AOA into a disc ona machine with a HS is in a Virtical decent, and even then, its still not 90*, coz a virtical decent ina HSed machine requires sum amount of backstick to hold the nose up.
Without a HS, you can apply a 90* AOA to the disc at wot ever flight attitude/angle you want. Horisontal or virtical, nose up or nose down, level or 90* bank.
And a 90* AOA is handy wen im apparently "flyn to hell ina hand basket", as Chuck Beaty says. ;)

[Vance]

Thank you David,

Vance

[Heron]

"Nothing to get hung about" . . .
Heron

[Doug Riley]

Birdy describes maneuvers in which he uses the engine as a kind of retro-rocket -- pointing the frame away from the direction of travel and hammering throttle to reverse direction faster that the rotor alone could manage. After all, a rotor can only produce so much control power (and that amount is less than an aerobatic airplane).

You can imagine that a H-stab would interfere with such shenanigans.

That type of maneuver is really "out there," though. It's nothing that even an edgy hobby pilot need ever do. People have died doing fast reversals in gyros improperly.

[MichaelBurton]

The Wright Brothers Explained the relationship between stability and maneuverability. They chose to build an aircraft that was more maneuverable than the stable gliders of the day. According to the brothers the more stable the aircraft the less maneuverable. The balance of maneuverability and stability produces an aircraft that is controllable.

For most of us a low stability aircraft would be a death trap. For others like fighter pilots and Birdy this lack of stability is a necessity.

Airplane Stability and Control ([URL="http://www.google.com/url?sa=t&source=web&ct=res&cd=2&ved=0CBsQFjAB&url=http%3A%2F%2Fassets.cambridge.org%2F97805218%2 F09924%2Fsample%2F9780521809924ws.pdf&ei=YVsFTPSsNYLGlQe895DYBg&usg=AFQjCNEruQ4dvun2Osx2w2RrViMerHUMHA&sig2=h4bBLhl85bxPFo21lhAp2A)... " this is a pdf file with historical data on the subject.

There are edges to the flight envelope. Without any warning a highly maneuverable aircraft can cross that edge and produce catastrophic results. Some pilots take these risks with every flight. The fact that they survive is not proof of a design or a maneuver. It simply shows that they have successfully flown within the envelope. With experimental aircraft the envelope is not well defined. This fact combined with environmental influences should lead most of us to choose a more forgiving/stable aircraft and to fly well within the flight envelope.

[birdy]

Birdy describes maneuvers in which he uses the engine as a kind of retro-rocket -- pointing the frame away for the direction of travel and hammering throttle to reverse direction faster that the rotor alone could manage.
Thats sum of it Doug.
Haven a CLT machine with no stab gives it higher rates at idle as well.
Use of throttle and rudder dose assist the rotor in quicker directional change, but the unstabed [ well, the HS is only effective in one direction] clt machine is also much slipperier with power off, usen just hight and inertia, while still be'n ppo proof and stable.
IE;, im sootn along at ground level to get to a position quick, but i dont want to waste valuable time on approch by powering off in advance to allow time for the AS to bleed off without exchangen AS for hight, so i just chop power, put the disc into a 90* AOA and stop within 20' from top speed, without changen hight.
This obviously excites the disc so soons the AS has washed off i just level the disc, sit in an inertia hover for a bit, the power out just before it starts to sink.
[ kinda leaves the heli blokes scratchn their heads. ;)]
A well setup gyro is faster off minimum speed to 80mph, can stop much quicker and do a peddle 180, blast off, stop, natha 180 blast off .............. for ever. An R22 tends to run into over speed and power problems tryn to do it at the same rate.

For most of us a low stability aircraft would be a death trap.
There is a difference between low stability and high rate manouverability Michael.
One is potentialy deadly, the other is efficiant high rate controlability.

There are edges to the flight envelope.
This is true, but my 'near' outside backward loop woulda happened ina Magni if i was flyn one, coz i could just as easy get it into the same situation.
If i did go over, it woulda been pure pilot error.
You cant engineer 'anxiuos' out of any machine.

[Redbaron]

hey bird, ANY reason you don't wanna make her stable? I bet the moos wouldn't mind!

[Friendly]

hey bird, ANY reason you don't wanna make her stable? I bet the moos wouldn't mind!

Birdy,
Save the ink. :der:

[GrantR]

"Ignorance is curable, Arrogance is Terminal"

[birdy]

It is stable.

[MichaelBurton]

It is stable.

By the word of the gods it is so.

[birdy]

Stic it in ya ear dichead.