Back to the old "I give up"......

[Aussie_Paul]

....this was posted on the Oz forum. It seems that the Oz people don't take me seriously because I am developing a product to sell, and they say I have a barrow to push. Hopefully you guys know me better.

Do we take the attitude that we design for the .000001% !!!!! that may be the exception to the rule!!!!!!!!!!! and forget about the majority? Geez, I hope not. :eek: :eek:

Aussie Paul. :)

I know,everybody is getting sick of the argument but I'v got a genuine reason for my persistance on the subject.
Firstly,I don't argue against CLT,mainly coz it's logical and makes sence.[and dose make machines safer]
Wot concerns me is peoples blind faith in stabelisers.
A stab essentualy takes some of the "flying" out of flying.It is a device that dampens the machines pitch attitude changes caused by inappropriate pilot input and keeps the machine pointing into the virtical airflow.[the same as a vertical fin keeps it pointed into the horisontal airflow.]This affect will be of benifit to most pilots in MOST conditions.But no pilot should belive it can make the machine safe in ALL conditions.
I'll liken it to "anti skid breaking" on a car.In most conditions,ASB helps the driver,by avoiding locking up the wheels in a hard break event.BUT,the computer that controlls the breaks dosn't know what soft sand is,and the quickest way to stop in sand is to lockup the wheels.Another less common, but more dangerous condition is loss of steering.[I'v had it happen twice to me,I turned the steering wheel and nothing happened].Your only defence in this situation is to lock up ALL wheels,ASAP.ASB also takes away the need to train yourself to "DRIVE" the machine,instead of being a passenger behind the wheel.

There's some conditions where a stabeliser on a gyro can be dangerous.
Strong ,abrupt wind shear.[That can occure at any altitude]
In a stable FW like a 172,these conditions are only uncomfortable.
The 172 will only try to drive you through the seat as it lurches up then try to rip your shoulders off as it drops faster than gravity.
[assuming your wearing the shoulder harness]
The gyro,not being FIXED to it's flying surface,but only HANGING from it like a pendulumb,is a little different.
The gyro will try to drive you through the seat too,but when you hit the down draught immediatly after,with the machine piched nose down[wich is wot the stab will make it do] and the throttle still on cruise power and the pilot caught off gaurd,the rotors become unloaded.Need I say any more.??
Wind shear isn't common,but nore is death,it only happens to us once.
Those who know me would understand that I encounter alot of extream flying conditions,mainly coz I fly when I need to,not when I want to.
And out of this experiance I can say that wind shear can occure in any conditions anyware.The most supprising was twice,late in the evening[after sundown]on perfectly calm days over FLAT country.Both times I was cruising streight "n" level,@ 100' and the machine suddenly lurches up then drops like a brick with no warning at all.The transition from up to down was so sudden that a stab would have the machine hit the down with a nose down attitude,commpleatly unloading the blades while on cruise power.


I AM NOT a salesman trying to flog something.
What I am trying to do is make people aware of what can happen.[If you fly long enough you WILL eventually experiance wind shear].
What I do get upset about is people telling inexperianced pilots that a stableised gyro is safer than a nonstabelised one.It's not ,it's only a LITTLE easer to fly in good conditions,and that's all.[remember,I'm not mentioning CLT]
You are flying a gyro,it's different to any other aircraft,so it must be treated as such,and not like a STABLE FW.[for the simple reason,it's not a deadly situation to go negative in a FW]
The gyro is a very simple machine.Baseically it's an autorotating wing,being propelled by the"machine".Neither will fly without the other,but they ONLY need each other to fly.Everything eles that is added to it is going to complicate the machine.

If you dought the word of a SCG then read wot the WORLD LEDGEND has to say about it.[Ken Wallis]He's been there and done everything.He evan got caught in a typhoon in his STABLESS gyro and not only survived but said that a stab would have been DANGEROUS and DESTABELISING.

If you drive a late modle car with all the driver assist crap on it,then you will keep it on sealed roads.
If you fly a "stable" gyro then please,don't take it "off road".

I feel better now.

[Heron]

Whatta . . .???? :eek: :confused:

[chuter]

I guess it takes all kinds to make a world. At least he feels better now. He really should get a spell checker though.

[jucie]

Freedom of speech is good, but I think that's a dangerous text. Some newbie can jump right here trying to learn something and get confused.

Would be good if someone (anyone is a better writer than me) explain why that is flat nonsense.

[Doug Riley]

The poster is confused about the effects of various durations of zero and negative G.

The very short-term neg "G" that can occur when the gyro passes from a column of rising air to a column of descending air is not a problem with an autorotating rotor as long as the frame does not have a residual nose-down pitching moment once the rotor is unloaded. Such a moment can come from thrustline well above CG, draggy landing gear, a pod with a certain shape, or other sources, IF those sources are not neutralized by an appropriate HS. If this moment is properly neutralized, the gyro will continue to track straight and level for the split second it takes to re-orient itself to the new relative wind. If this moment isn't properly neutralized, the gyro will pitch over so violently that the rotor will be unable to follow, will hit the tail and so on.

PPO is not caused by zero G alone; it is the result of nose-down pitching moments that are unopposed during zero G. A half-second or a second of zero G is not enough to cause a catastrophic loss of RRPM (the RRPM decay rate was posted and discussed on the old forum). The HS will have kicked in well within that amount of time.

If we could see updrafts and downdrafts out ahead of us, then perhaps a machine with CLT (and no other net pitching moments) but no HS would be the perfect "expert's" machine. The "expert" could always time his/her stick inputs to anticipate the direction of the next patch of moving air and therefore be "ahead of the curve." With CLT-plus-zero-moment, there would be no sudden pitchover upon momentary zero G.

But we can't see the air -- not even the pros can guess right every time about what kind of air is coming in the next second. The HS doesn't know in advance, either. It reacts after the fact, but with 100% accuracy and at least as fast as you or I can.

Anecdotes don't prove much, but I've been pulled up against my seat belt in both stable and unstable gyros. Again, the rotor will tolerate these short-duration low G events. OTOH, sustained low- or zero-G could slow the rotor enough to result in uncontrolled flapping agaist the teeter stops and the refusal of the rotor to speed up again.

[jucie]

Doug, you are the man. Thanks.

[Hognose]

Doug -- that is an excellent and crystal clear explanation of why both CLT and a stab are safer.

I have heard this "stab is bad for maneuverability" stuff and "only the rotor is flying, what the fuselage does doesn't matter" from RAF adherents for a while. It's rubbish, of course.

When one loses RRPM to the point of hitting teeter stops, the blades are usually flexing enough that they make contact with other stuff -- fin, props, pilot's head -- at that point they have decayed beyond recovery, so it's a bit of a moot point whether the irrecoverable decay happens before the blade strike or consequentially to same -- it all happens in small fractions of a second. There is a famous photo of one of Wing Commander Ken Wallis's stabless wonders doing exactly that.

As far as Wallis flying in a typhoon -- well, there's no fool like an old fool.

cheers

-=K=-

[mrford61]

MMMMM... The Author may not be an expert at explaining what he means on a typewriter but knowing who it is I suspect he, as Wallis, is no fool.

[Hognose]

It's also not "a deadly situation" to "go negative" in a gyro -- it is fatal to go to or beyond zero G for any length of time in any teetering rotor system. Yet many thousands of these machines fly safely, while some types experience anomalous safety (or unsafety) records.

The reason these events are .05% of fatal mishaps in Bell helicopters and something like 90% of the much higher rate of fatal misaps in Mini-500 helicopters (which share a designer with the killer Air Commands) is 2% pilot training and 98% aircraft design.

The inarticulate fellow who Paul quoted makes the argument that a stab is not a panacea, and takes pains to try to separate that from the CLT argument. (Discrete issues? Yes and no. Most of the real problem gyrocraft combine thrustline and stab problems). I follow this and agree to a point. A stab is only one ingredient in rotorcraft stability, and a smaller one than thrustline offset.

I think Greg Gremminger is on to something with the Forrest Gump stability rule: "stable is as stable does." If an a/c is statically and dynamically stable, and not prone to shed this stability in normal anticipated flight regimes, then it is a matter of small import how the designers made it stable.

But I read the initial article as arguing not so much against stabs, but against stability. That suggests a profound misunderstanding of how, exactly, stability and maneuverability are related.

(and... in what circumstances is one not a fool for flying a sport rotorcraft in a cyclone?)

cheers

-=K=-

[MGman]

Hey Paul - I have been reading on this forum for some time and have come to have a lot of respect for what you (and many others) are trying to do to encourage safe Gyro design. Could it be that this fellow was just trying to push your buttons, knowing how you feel about the subject :confused: ?

I'd recommend just ignoring this guy's comments - maybe he just wanted to get a rise out of you - but please keep on getting your message out there. There are many of us wannabe Gyro pilots who are listening and learning, and when we can afford to take the jump, will incorporate the wise comments of this forum into our decision of what Gyro to build or buy.

Thanks for your input and have a good day - Dave :)

[PW_Plack]

This guy seems to be saying what I think I've heard from the RAF guys, that a sudden updraft will cause the nose to drop because of the upward pressure on the horizontal stab. Won't the moment of the stab almost certainly be less than that of the fuselage, even if that fuselage is an open frame with a pilot?

Perhaps if you used a flat plate stab, you'd have to make it so big it might behave this way, but it's a little hard to imagine.

[Aussie_Paul]

You need to have a quick look at the Oz forum to see what the feeling is by some down under.

http://www.asra.org.au/forum/active.asp and go to the Theory of Flight section.

Thanks guys.

Aussie Paul. :D

[mceagle]

Wallis flying in a "typhoon" has little bearing on the stability arguement. Some of the smoothest flying I have enjoyed has been in extremely strong winds. Conversley I have experienced some shockingly rough air on a relatively calm day.

The stabilizer's job is to re-align the airframe with the relative airflow after any vertical air disturbance. This is what it is supposed to do.
Without it, the cabin or frontal area can do the opposite and place the airframe further out of alignment making the joystick pitch control closer to one of its stops.
I believe that those that perceive a stabilizer to be de-stabilizing are unappreciative of this fact.
Either that or the machine is so far out of balance that the stabilizer is fighting against the natural straight and level flight attitude of the airframe and this is incorrectly perceived as instability (and the stabilizer gets the blame on the sound theory that the aircraft is more pitch stable without it).

[birdy]

Hmmmmm........ so Paul posted it over here for me.Gee Paul,if you'd put me name to it most of these people wouldn't have bothered to even read it.They'd have seen the "birdy" and thought,not this dickhead again.

Chuter,I take it that because you could'nt understand the spelling,you missed the point.I appoligise for my spelling mistakes.

Jucie,I posted it for the newbys.The newbys I'v spoken to get the impression you can fly these machines anyware anytime,.........hands off.

Doug,I'm not going to pick a fight with you coz you put up inteligent arguments.
Supposing you do understand wot I mean when I say "severe",you say that a machine,stabbed or not,would not push over in the situation I'v stated.????
To make the situation crystal clear for everyone,I mean the machine has just hit a strong updraught.It's attitude is very nose down,the power is still set at cruise,the rotor disc is at a 45 degree angle[nose down] to the gravitational pull and you are climbing.[remember,you were pinned to the seat,so you are climbing]
Now,you hit the opposing,severe down draught..........with the machine in that attitude.
From wot you posted Doug,you know the machine won't push over ???Even thou you'v got unloaded rotors,wich are not directly opposing gravity[45 degree angle to virtical],The prop is still pushing cruise thrust and you'v got an almost tail wind.[your tail is up ,and you are traveling up,in a strong down draught.]The rotor rpm hasn't decayed because one,it's a very short time frame,two,they'v nealy no load[drag].Rotor rpm dosn't matter here coz they are unloaded and trying to thrust almost forward,not up.The airflow relative to the disc is ,at best,parallel to the disc,at worst above it.
I dought anyone would survive this situation to tell,but I reckon the minimal rotor thrust at this point would'nt be sufficiant to counter the prop thrust.And because the stab is now at a major dissadvantage because of it's relitive airflow,it wouldn't have sufficiant power to stop the machine pushing over.Also,an imersed stab,in the prop blast ,wouldn't have the leaverage either.
I see your only defence in this situation is CLT,not stabs.I'v always said that.
A properly balanced machine,with clt and no stab wouldn't pitch nose down in the first instance.

Kevin,you'v no need to tell me that the RAF's thrust line is way too high.I have the same attitude towards the standard RAFs as most everybody.As far as your oppinions about Ken,..........mmmmmmmm.[He didn't fly into it,he got caught in it,a big difference]
Stretching the shoulder straps on your seat belt IS -G,floating on the seat is 0G.
[I realy must better my typing if you interpreted me as being against stability,coz it couldn't be farther from the truth.]

With all due respect Tim,I don't reckon that a typhoon would be very smooth.

Mark,I wunder how many of these posters have spent a full day mustering in mountains during a storm??????

[jucie]

I should read the thread at Aussie forum to get the entire picture before emiting any opinion.

Bird, your intent to separate issues (CLT effect analisys apart from HS effect analisys in extreme conditions) is a valid effort to improve the general understanding of the subject.

[Udi]

Birdy - The first Autogiros flew in every weather condition both in the US and in Europe. Those machines had a stab the size of a barn door. If your description of events were true, we would have known about it. Not a single autogiro fell out of the sky (as far as I know) due to wind shear.

Your analysis of a stabbed gyro in extreme weather is not true to the real aerodynamics of a stabbed gyro. When a stabbed gyro hits an up draft, both the stab and rotor (should) react in the same direction. The stab would lower the nose, and the rotor, being AOA stable, would also pitch down. At the same time, the rotor would accelerate due to the higher G. During a transition from am up draft to a downdraft, both the stab and the rotor would pitch up, and the combined reaction would be much quicker than a non-stabbed gyro. A sustained negative G is not likely.

A real strong wind sheer will take down a Boeing 747, not just a stabbed gyro. Large jets have crashed more than once due to winds hear. A strong enough wind sheer would unload the rotors of any gyro, stabbed or not.

I can understand your argument that a non-stabbed gyro just plows through the sky, regardless of vertical currents. But you have to remember that if the gyro does not pitch up into a down-drought, there is no automatic mechanism, other than pilot reaction, for re-loading the blades when they become unloaded. This may be ok for you and any other ace pilots who prefer it this way - but 99% of the pilots are better off with a gyro that is doing the right thing on it's own.

Udi

[Doug Riley]

Birdy, did you mean to suggest that negative G itself causes a pushover? Bensen used to claim that, but, as with his claims about the VCG location on the B-8M gyro, I think he was being inaccurate.

I believe the pushover issue has been tossed about enough that there's general agreement about the force that results in a pushover: it's not a force created by the rotor, it's a force created by the propeller.

Bensen's (discredited) explanation of pushovers was that the rotor, when it assumed a negative AOA, sort of "dug in" like a chisel in soft wood, causing a nose-down pushover moment. This notion sounds plausible until you take a closer look at it. The rotor blades are ordinary airfoils: they're just wings, same as you find on a Cessna. A Cessna wing, and a gyro wing-blade, (and a sailboat sail, for that matter) will generate lift on the side of the airfoil opposite to the side the relative wind hits. If the relative wind hits the bottom of the airfoil, the lift will pull upward on the top surface. If the relative wind hits the top of the airfoil, the "lift" will pull on the bottom of the airfoil. That's how airplanes fly upside down.

If your rotor blades experience a relative wind from above, they will dutifully create "negative lift" -- the same as any other wing would. They even will continue to create the usual autorotative force, and will not abruptly "try to turn the other way," as is sometimes thought.

I'm not suggesting that we ought to fly gyros upside down. With the blades negative, we have reverse coning. We also have the teeter hinge undersling now working as "oversling" -- which ought to create a severe, destructive 2/rev vibe. Worst of all, we have reverse control: during true neg G, forward stick causes a nose-up reaction, and aft stick pushes the nose down.

The point is that the rotor does not simply quit flying the moment you go negative. Neither does it, by itself, precipitate a pushover. If there's nothing ELSE on the airframe that tends to cause a pushover (prop, frame drag), then you won't have one just because you went negative for half a second.

[Randall]

I don't know squat, but wouldn't the same "forces" that worked to pitch the nose down per the original author's comments still apply when the gyro hits the down side of the wind shear? And, thus tend to "swing" the nose back up from it's previous downward direction? I thought that this was explained somewhere earlier on this site as one of the benefits of HS on a properly configured gyro - during upward and downward gusts, the machine will try to "point" itself in the appropriate direction to counter the gust w/o operator interaction. Am I mistaken?

Randall

[pwendell]

I think all of this talk about a few hypothetical -- meaning unverified and based only on anecdotal evidence -- instances where an Horizontal Stabilizer might be detrimental is a bit silly. It reminds me of people who argue against seatbelts or motrocycle helmets by trying to come up with the one-in-a-million (or billion) scenario when they do more harm than good. And I'm not convinced that a properly sized HS is EVER a liability.

I'm not trying to tell anyone else what they should fly or what criteria they should use to judge an aircraft. In the US our laws concerning experimental aircraft are based on the principle that as long as you are only endangering yourself, and, to a lesser extent, informed passengers, you can do whatever damn fool thing you want to. I think that's the way it should be.

What I am saying that a stable gyro is safer than an unstable one. Period.

[birdy]

Udi.
The early autogyros you mention,would they be the tractor type????
You also state,"there's no automatic mechanism to reload the rotors".So what dose the trim spring do???[when you hit a downdraft,there's less weight on the toque tube offset,the trim spring automaticaly,and instantly,pulls the tube down,increasing the disc AOA......no???]This is assuming that the pilot hasn't "choked" the stick and stopped it's intended reaction.[in other words,this can only happen if the pilot is "flying the rotors"]
BTW,as I'v stated on the Oz forum,experiance will have no effect in this situation.
I do thank you thou Udi,you showed me how to spell draft.LOL

Doug,to your first question,no.That's why I kept mentioning the power level.

I love this forum,you said that rotor blades will still fly with "negative" airflow.[Thou with much less efficiancy I gess]See,evan a dumb bastered like me can be enlightened.This explaines why the harness gets so tight in strong downers.Thanxs for that info Doug,it's appreciated.[a simple oversight on my part,I'v often wundered why the gyro falls so much faster than me to jam me harness into me shoulders,It's momentarely "flying down".]
So supposing the rotors did experiance negative airflow in the "in transition"[from up to down draft],Then the thrust vector would now be reversed,[pushing down]and because of the disc angle,it's a reversed thrust vector forward of the COM.That combined with the prop thrust would definatly cause a PPO.No???This reversed rotor thrust would cause the offset to react in a positive way,but would it be enough to beat the prop thrust????

Randall,with respect,if you say you don't know squat,wouldn't it make sence to just read.I'd never say I know all[I'v just learned another thing off Doug]but I do have alot of questions I'd like clearly explained from the likes of Doug.Questions steming from hundreds of hours of flying in less than faverable air.I know I post some idears that most think a little radical,but it's haveing the deasired effect,I think.
Yes,the stab dose keep the machine pointed into the relative vertical airflow,but the fact is,I spend alot of time in some extream vertical airflow changes,and I'm picking the "knowlagable" brains from that perspective.Is that stupid or a crime.If so,I'll stop.

Wendell,In a way,PPO is hypothetical[or theroretical],no-one has ever tested it and survived to tell.
Your not "convinced" that a properly sized HS is ever a liability.Well I'm not convinced that it's NOT.That's why I'm posting to the Knowligable,from my EXPERIANCESE.
Simply,I'd like to know,without actualy testing it.
You say a stable gyro is safer than an unstable one.So have I,and always have.

I'v said a hundred times,but I'll say it again,I'm all for stability.

[Doug Riley]

Birdy, yes, if the rotor is tilted well back, it may be that the rotor thrust line will be ahead of the CM and will try to shove the nose down if you go negative. OTOH, if the rotor is nearly level (stick well forward), the rotor thrust line ought to be behind the CM and cause a nose-UP moment! I think that your stick position will determine which reaction you get. If I'm right about this (haven't tried it lately), then the notion that you can induce an uncontrolled pushover in a CLT gyro simply by throwing the stick hard forward is nonsense. If you do this and manage to attain negative G, the rotor down-thrust with the disk level will jack the nose back up in a hurry. Again, this is speculation... not an idea to be flight-tested this weekend!

All of this talk concerns how the rotor itself behaves... something that's true whether you have a HS or not. Given the rotor's behavior, what differences can you predict with/without a HS (that is, with/without a tendency of the fuselage to weathervane into the relative wind)?

Take the fast transition from updraft to downdraft that started this whole discussion. The HS-equipped craft's fuselage will have reacted to the updraft by dipping its nose. This, in turn, will have lessened the ballooning effect of the updraft by lessening the increase in angle of attack that the updraft creates. That is, the gyro won't be rising in the updraft as fast with a HS as without. Hence, the transition to the downdraft will involve a less drastic change in the vertical component of craft's airspeed. That, in turn, means that the change in angle of attack will not be as great with the HS as without. I think, therefore, that the rotor is LESS likely to "go negative" (with all the weird consequences that THAT brings on) with a HS than without.

I can testify that a craft like a Dominator (which weathervanes into up-and down-drafts pretty aggressively) smooths out the bumps in turbulence as compared to a no-HS craft. By nosing into the relative wind, the HS reduces the G changes far better than a pilot attempting to do the same thing with the stick -- and with the added advantage that, unlike a stick which is hooked to the rotor, the HS's control power doesn't vary with G loading. It take some getting-used-to when you first fly a HS equipped craft and the nose dips sharply into an updraft -- but in fact it takes the "balloon" effect right out of the updraft if you let it.

Naturally, the fact that HS craft's nose dips scares people who've grown up with fear of PPO and/or with the flawed Bensen model of rotor behavior. Think about it a bit, though, and you'll realize that the nose-down position simply gives the rotor the correct angle of attack to stop the ballooning. By stopping the gyro from building up a wild upward velocity, the HS will make the transition to the downdraft that much less dramatic.

The engineers talk about the steepness of velocity gradients and the frequency of shifts in the air's vertical speed when they get into this area. I would guess that, for each combination of rotor and airframe, there would be one or more frequencies and severities of these numbers that, at a given airspeed, would get in phase with the gyro's reaction time and produce a catastrophic result... but that's at least equally true for a no-HS machine (whose feedback mechanisms involve pilot reactions and/or the gimbal head's reactions, PLUS no damping; all feedback loops will resonate at some frequency or other unless suitably damped).

Some controlled experiments (not fly-it-and-see stuff) on this issue would be very useful to put numbers on the ideas.

The one thing that's clear in all this is that a high thrustline/low drag center with inadequate HS and zero-negative G will kill you very quickly. The fine points of reverse flow in rotors won't matter once the rotor flaps into the tail. You won't have a rotor to worry about at that point.

[Heron]

Yes . . .that drough thing was getting me all confused . . .
Lets see . . .up drought means it did not rain up river? And Down . . .forget it!!!
Does draft have anything to do with Bill Clinton?
:)
Heron

[Doug Riley]

Answer to Randall:

Yes, the HS will cause the nose to swing up to meet the downdraft. What people worry about is the time intervals between (1) the very instant that you hit the downdraft (2) the instant when the nose has swung up and (3) the instant when the rotor has followed the airframe to the nose-up position. The frame and the rotor both have mass, so neither of the follow-on events #2 and #3 happens exactly at Time Instant #1. The question is, will something very bad happen during these short, but measurable time intervals?

That's the crux of the argument as I understand it.

[Heron]

Doug:
Doesn't the disc suffers the action of the draft? How so?
Is it possible that the whole gyro reacts in sequence? stab>nose>disc
The machine will ascend nose down and descend nose up?
When the entire cicle happened it should look like PIO with inverted nose movement?
How long can it last? Is it sufficient to download the rotor with significance?
Thanks
Heron

[Doug Riley]

Heron, I'm not sure I followed all of your questions. A rotor disk mounted on an offset gimbal head does try to orient itself with the relative wind. The pilot must use a very light grip ("floating the stick") for the head forces to do their job. A firm grip on the stick will effectively "lock" the head and defeat its function. In my experience, however, the action of the gimbal head alone is not sufficient to lessen the G changes in turbulence as effectively as a HS mounted on the frame. I've found this to be true even in a tandem gyro, which will have a slower airfarme pitching reaction to angle of attack changes than a single-place or side-by-side.

The order of events when you fly from updraft to downdraft will depend on a number of design factors... especially the relative inertial qualities of the rotor and the frame. It's possible (in theory, anyway) to have a rotor that reacts faster than the airframe, or vice versa. Again, though, even the slow-reacting tandem airframe with HS seems to smooth out the G's better than a one-place with no HS and only a "floated" stick.

[Randall]

Uh, did I do something wrong guys?

David, it's no crime, apparently unlike my asking a question around here.

I was just presenting things as I understood them. My comment that I don't know squat was my disclaimer that I didn't intend to provide any hard facts. Just as you, I was trying to understand all this by "picking the "knowlagable" brains" by providing what I understood to be the case from reading this forum and the old defunct forum - again my interpretation not intended to be stated or taken as fact. Hence the question at the end of my post.

[Randall]

Thanks Doug!

[Heron]

My questions are due to the fact that is always mentioned nose and tail movements and nothing much about the disc the major presence in the machine.
So lets do this slow motion: the nose hits first, little up, the frame second and starts going up, the stab comes last and it is the major directional component on that axis (or should be) then goes up forcing nose down stance.
the machine is raising on the UD nose down, the rotor does what?
thanks
Heron

[rehler]

The rotor tilts down in front.

In an updraft the nose tilts down. In a downdraft the nose tilts up.

[Heron]

So . . .the whole machine lines up in seconds or less facing the UD and the rotor speeds or decais?
Now comes the DD . . .in both directions the machine is been forced and can we have a negative G in any way?
Heron

[rehler]

The rotor speed stays the same. I don't see how a negative G can occur (which would decrease the rotor speed over time), except for a brief few seconds caused by a strong downdraft. Any decent gyro will just keep on flying like other aircraft do, just not feel as big a bump as you would feel in a fixed wing.

[birdy]

I applaud Doug for his persistance and understanding of what I'm trying to say :D :D :D .Yes ,I am refering to the TIME intervals.[in Extreme conditions.]

The reason I'v started the subject again is because of the constant "you can't PPO a stable machine." :mad: This is nonsence,any machine CAN be PPOed.[otherwise there'd be manufacturs lineing up to proov there machine can't]
People who are tolled they can't WILL get into serious trouble in EXTREME conditions if they don't have an adequate understanding of the possabilitys.The perception of people who constantly hear the"CAN'T" line could think that these machines are fool proof,and they definatly are not,stable or otherwise.

I also feel I'm obliged to make aware to the general gyro public the POSSABILITYS of what may happen in extreme conditions.[conditions that I'm familiar with and even "weekend warriors" could get caught in.] Posting my thoughts and experiances provokes deeper thought towards the less common[but deadly] scinariouse by the likes of Doug who may not think of it as revelant to us.[no offence meant Doug] :)

One last one Doug,in "the " extreme event,would it be "possible" for the stab to create such an erratic pitch change so as to cause the rotor to bottom on the stops,[like flapping]because the disc dosn't have the time to change attitude???

Maybe there should be a poll to find out how many people want me and all my dribble off the forum :) .Or maybe I should take the hint and get off anyway. :D

[KenSandyEggo]

"Or maybe I should take the hint and get off anyway."

Right. Go out in the shed with a Victoria's Secret catalogue and get off anyway.....that you can. :D Oh! I almost forgot. The preceeding statement may be offensive to some people. If you find it offensive, don't read it.

[chuter]

Hi Birdy,

You said, “This is nonsence,any machine CAN be PPOed.[otherwise there'd be manufacturs lineing up to proov there machine can't]

Maybe you’ve got your terms confused; we went over this on the old conference. PPO, or Power Push Over, is caused by a thrust line that is above the center of gravity. The moment of this relationship causes the craft to tumble forward when other stabilizing forces are removed.

So, a craft with the thrust line below the center of gravity can not PPO. In this case the thrust would tend to rotate the craft nose-up. This doesn’t mean it can’t tumble forward, or BUNT, due to other causes, like a drag-over caused by the center of pressure being below the center of gravity. A proper HS would help prevent this.

Is this correct Doug?

[Heron]

Power push over = the engine forcing the machine over
Tumble forward = can be ppo or pilot induced tumble
In the second case I think all machines can be made to rotate by the pilot.
Right?
Heron

[mceagle]

I agree Heron.
Also "power" can be positive or negative. In a craft with the thrustline below the center of gravity, an application of negative power (engine failure), could cause PPO.

[Dean_Dolph]

Tim, the first P in PPO stands for power. Without Power there can be no Power Push Over. I don't think 'negative power' is a valid term. I think that some of our experts have made reference to some undesirable things that can happen with a below the thrust line machine during sudden loss of power but PPO can't be one of them.

[chuter]

Maybe in that case, thrust line below the cg, power is cut at higher airspeed, it would be another form of "drag-over"? Plus, it seems like the airspeed would have to be relatively high for this to occur, while PPO can happen at any speed.

[Heron]

That hypotesis was presented some time ago when a Dominator crashed and we especulated about that kind of reaction.
I think it will be dragged under, rotational forces under de CG rather than over like the PPO. Power Drag Under maybe? PDU power provided by two sorces drag and rotor inercia.
Heron

[birdy]

Well,no-one has told me to F off so I'm still here.

Michael,your right,I stuffed it up[noth'n unusual for me].
I should have said "can't go over in a ........"
The machines I have in mind in paticular are the likes of RAFs[mainly] wich employ large stabs to counter the high thrust line.
The stabs I realy worry about in the extreme conditions are the realy big ones.Having too much corrective power for the weight of the machine.In the same way as low wing loadings [coupled with low airspeed]in fixed wings makes them "rougher".

My concern is,in a quest to make the ultimate stable machine,are we sacrificing the safty and more importantly ,the controlability in extreme conditions.? I know controlability has been covered before but remember I'm only refering to large stabs in extreme conditions.[the stab is a control surface we can't turn off or adjust when we get caught in the extreme conditions.]
I hope I'm mak'n some sence.

[Aussie_Paul]

With a correctly designed CLT(CoM within 2" of the thrust line) gyro you do not need an enormous stab!!! that is the point!!!!!!!!! :rolleyes:

This is the order in which to design a stable gyroplane

1) get the machine CLT, :eek:

2) use an effective aerodynamic stab, which can be reasonably small with
CLT, to fine tune the variations of weights conditions etc, :eek:

3) have the drag line as close to the thrust line as you can manage, not as
critical. :eek:

It is in this order that a stable gyroplane is designed and produced!!!!!!!!!!!! :eek:

Aussie Paul. :)

[Hognose]

Well,no-one has told me to F off so I'm still here.

Can't have a discussion without an honest exchange of views and ideas, eh?


The machines I have in mind in paticular are the likes of RAFs[mainly] wich employ large stabs to counter the high thrust line.

Right... the stab of course is not a factory-approved gadget on the RAF; they say it flies better without 'em (most people who have flown it both ways disagree. Even most of RAF's loyal instructors say it is easier to train a novice in a stabbed machine).

The stabs I realy worry about in the extreme conditions are the realy big ones.Having too much corrective power for the weight of the machine.

I'm not sure I really follow you here, Bird-man. Let me see if I can rephrase what you're saying. (After all, the US and OZ are two nations of similar heritage, divided by a common language. Heh. Guys who learned English 2nd, like Udi and Jucie, have got to be hanging on by their fingernails sometimes when we get going).

Are you saying that, in extreme wind/thermal/storm conditions, a sudden violent (say) updraft will cause the stab to pitch the aircraft fuse (in this case down), maybe without similarly pitching the rotor disc?

I am not too sure this would happen. It happens in fixed wing; I've been thrown suddenly nose-up or nose-down when, presumably, a gust affects the wings more than the tail or viice versa. When your machine passes through the shearline between up and down. In really bad turbulence in a lightly wing-loaded machine, you slow down and do your best to keep wings level and just hang on. Gyros don't take that kind of beating because their equivalent wing loading is much higher... there isn't so much flat plate area for a draft to beat on. Also... the rotor conventionally extends beyond the stab... if say an updraft starts to raise the tail (but not the forward part of the rotor) it will still affect that part of the rotor disc that is behind and around the stab.

My concern is,in a quest to make the ultimate stable machine,are we sacrificing the safty and more importantly ,the controlability in extreme conditions.?

I suspect, using the example of an RAF, you would find it much better to be in these conditions in an AAI conversion like Ken Janulewicz's than in a factory RAF, which has not-quite-enough vertical (hence the directional control problem flying with the doors on, which is not unique to the RAF2K) and no horizontal stab. On a cabin gyro your cabin is always going to present a certain flat plate area to each aspect and you can't turn that off either.

You raise some good questions. I am going to try to model some of this in X-Plane and see; I also hope to discuss it with Martin Hollman at Oshkosh if we have time. Like I said, Martin is a very smart guy. He has forgotten more about aerodynamics that I have ever been able to learn, and he retains a lot more than he's forgotten.

Questions:
1. Can a stabilizer be "too large?" How large is that, and what would the effect of an oversized horizontal stab be?

2. How do you size a stab for a gyro? References? Formulae?

3. Can a stab's positive effects in smooth air turn against you in turbulence?

Any more?

cheers

-=K=-

[Dean_Dolph]

You raise some good questions. I am going to try to model some of this in X-Plane and see; I also hope to discuss it with Martin Hollman at Oshkosh if we have time. Like I said, Martin is a very smart guy. He has forgotten more about aerodynamics that I have ever been able to learn, and he retains a lot more than he's forgotten.


Kevin, Mr. Hollman seems to be held in high regard by most of the aviation community and I believe just recently (the past year or so) he was honored by the EAA. However there are those in the gyro community that question his knowledge when it comes to gyros. And this is despite his gryo development/design efforts.

Martin had an article published about gyros recently (again, in the past year or so) that a relatively unknowledgeable gyro enthusiast (me!), questioned the validity of, on Norm's conference. I don't recall the publication and this is another case where having access to Norm's archives would be a help.

It didn't take long before the people I accept as experts, C. Beaty, Raghu and others, responded with an in depth analysis of his article and really took him to the woodshed.

I don't know Mr. Hollmann and have never met him but when I see him present 'stuff' that doesn't ring true and then have my observations validated by people like the ones I mentioned then for me it makes anything he presents about gyros suspect. I do know one thing, in his published articles he isn't bashful about patting himself on the back.

I believe (old age keeps me from making flat statements!) that G. Gremminger has also had discussions with him where Greg didn't agree with his conclusions. However, Greg has acknowledged Martin's sons contributions to the gyro world by citing his spreadsheet that I believe dealt with determining gyro stability.

I would say that you have the background to sift the wheat from the chaff so it will be interesting to see what you make of his response to your questions.

[rfi]

Birdy,
I don't think that there are any RAFs with stabs that are "too large". The ones that I have flown seem to have about enough area to counteract the area of the cabin, so that in gusty conditions the airframe does not pitch nose down--it just goes straight up and straight down with no pitch bucking.

The stock RAFs with most of the horizontals that we have over here don't really provide a lot of positive pitch stability but do have enough to make it much easier for a novice to fly without getting into a PIO at the drop of a hat.

There is not a long enough moment arm and an overly large horizontal stab would weigh enough to throw the CG out of whack. My stabilizer when mounted on a keel extension just barely allows the nose wheel to come down on the ground with one person on board but it does provide more positive pitch stability due to the 20 inch longer moment arm. It is hard to get a sufficiently long moment arm on a typical pusher gyro to meet Cierva's formula for adequate pitch stability.

[Hognose]

another case where having access to Norm's archives would be a help.


Amen.

people I accept as experts, C. Beaty, Raghu and others, responded with an in depth analysis of his article and really took him to the woodshed.

Disagreeing with Martin is part of talking with Martin. He is a man of very firm opinions. I don't know enough rotary-wing aerodynamics to take him on. I do have a good (if somewhat dormant) knowledge of metallurgy, from misspent days in machine tools, and I had to differ with him on the subject of the BD-10 inflight failure mishap. Or to put it another way, flutter, my arse.

for me it makes anything he presents about gyros suspect. I do know one thing, in his published articles he isn't bashful about patting himself on the back.

We have to try to look at this the way Chuck or Raghu would -- not let feelings for or against the guy influence what we think about what he's saying. We have to take the matter and analyse the facts in it -- as it sounds like they did.

I don't believe anybody in the gyro world knows everything about stability and control: not Greg, not Jim Mayfield, not Hollman or Bensen (RIP) or Wallis. So if I'm going to take my own advice I'm going to have to knock the rust off my maths and physics and get ready to thimk (old IBM joke) like an engineer again.

Greg has acknowledged Martin's sons contributions to the gyro world by citing

Here is that paper by Eric Hollman for anyone interested: http://aircraftdesigns.com/gyro3.html

What is interesting is that the paper seems to answer Birdy's question (from another thread? I have lost track of all the threads where we are talking stability) of what is the effect of a large tail on oscillations caused by a sudden gust. As Dave Bird seemed to be concerned about, when a gust causes an oscillation or "porpoising," the movement of the fuselage and the rotor are not exactly the same and they may diverge enough for a rotor strike on the prop, tail, cabin or just the teeter stops which can be bad enough. But Eric's computer studies show that an "oversize" tail produces the best (safest) damping... which, if this model is an accurate model (big IF there, eh?) means that Birdy's concern about stabs in storms is misplaced. Hmmm. There are penalties for a stab that's way too big (way more drag, as if a gyro wasn't draggy enough already) but they're not safety problems. Hmmm... maybe worth reproducing his data and then extending it, working it out for ridiculously large stabs to see if you reach a point where the safety benefit reverses.

The paper's a bit dense and written in the normal scientific pyramidal format (dull implementation stuff first, conclusions at the bottom). Maybe I ought to work it through, translate it into plain English and put it in journalistic inverted-pyramid format (conclusions first, information in most-to-least important order).

cheers

-=K=-

[Vance]

Hi Kevin, I think your translation would be very interesting to many of us on this forum. Please do it. Thank You, Vance

[birdy]

Yes kevin,that last paragragh sounds a bit contradicting.
An oversized stab could................................but his computer sim says...... .

Thankyou for seeing my points.
BTW,you asked for anymore questions.
Q,should weight[of the machine] be taken into account when desigening a stab for a paticular machine.?

A stab of ,say 3 square feet, would have more affect on a light machine than a heavy one.[less weight to "move"means it will move it quicker.]The same as the length of the moment arm.

The pitching of the rotor disc is in reaction to the movement of the frame,this means the frame moves before the rotors.Add to that ,the rotors need time to "fly" into their new plain of flight.
So not only do the rotors react slightly after the machine starts to pitch,they also can't change pitch as fast.A light machine with a large stab could pitch too fast for the rotors to keep up,no?????

[mcbirdman]

NO - (worry) The stab isn't on a hinge that allows changing angle of attack / pitch that can suddenly divert the airframe from its original direction the way a rudder can. Even if you are sinking, the airframe is still moving forward and the stab is a follower..... jtm

[Hognose]

Birdy --

You're right that, other things being equal, a larger area stab has more effect than a smaller area one,. Now, you can change the arm the stab is on (i.e. move it further back) to give a smaller stab the effect of a large one without making it too draggy. You'll see that the tail on an AAI (both horizontal and vertical) is quite large, but it is also close in.

Well, why didn't they put it ten feet back and then it can be a fraction of the size? Well, one of the big things you run into is structure. You'll pay a weight penalty to put a smaller fin further out. I am not aware of the design thought process that went into the AAI tail, mind you, but the design they chose is very light. Weight is always your bitterest and most insidious enemy in anything that goes into the sky. But by hanging the tail fins out on the end of a fuselage structure of some kind, you can make them smaller (ergo, lower induced and parasitic drag)

So how can weight (mass, really) of the aircraft can work at cross purposes to the stabilizer? Basically by the simple inertia of the mass itself. Remember that the stab acts by applying a force, which is a moment at a certain arm or distance from the CG (resisting motion == applying a counter force). Mass doesn't want to move and once moving it doesn't want to stop. But all forces inflight act about the centre of mass of the aircraft.

It appears from looking at the limited data set in Eric's paper that a light machine with an oversize stab, the pitch of the fuselage would diverge lessfrom the rotor disc and would damp out the oscillations more quickly.

Eric has identified a long period or phugoid in gyros that is considerably shorter than in fixed wing aircraft. Indeed, in most common fixed-wing planes you don't even notice it (Mooneys are a well-known exception). IIRC the phugoid on a Mooney is on a period of about 18 or 20 seconds, whereas Eric has the gyro at six point something. That may be why we hear about gyros PIOing to their doom, and Mooneys not.

I probably won't be able to do much on the paper till after Oshkosh. Busy time.

cheers

-=K=-

[Doug Riley]

I think Birdy's scenario is theoretically possible, though unlikely in the real world of gyro design. You have to rig the assumptions in a ridiculous way to get the "fuselage outruns the rotor" scenario in conditions OTHER than PPO.

OK, let's rig the assumptions anyway. Assume a very slow-turning, truly massive rotor (say, a couple hundred pounds). Assume further that the blades have a really crude airfoil whose lift curve is nearly flat (that is, the airfoil's lift doesn't increase very much as its angle of attack increases). In short, assume a massive, sluggish rotor. Further assume a huge HS that has a really steep lift curve. Put the HS on a short lever arm to reduce the damping effect. Further assume that the airframe has a very low moment of inertia -- that is, that its mass is all concentrated right at its CG, not spread out in various directions around the CG. A tandem won't cut it -- instead picture a tiny single-place pusher in which the pilot sits right against the mast and the engine is light and also close to the mast. The mast itself must be very short.

Finally, to wind up the trap even more, assume the craft has a spindle head or a gimbal head with very little offset and hence no trim spring.

If you exaggerate these qualities enough, you can, I think, get a condition in which the frame reacts too quickly relative to the rotor in aligning itself with the wind. In a severe wind shear, the frame could move so fast (compared to the rotor) that either the teeter stops would slam or the rotor might experience a precession stall, or both.

Some combination of those things apparently happens in a PPO. In that case, however, the events are being powered by engine thrust along a mismatched thrustline, often amounting to hundreds of pounds of nose-over torque. What's more, the PPO process is classically unstable: as the nose starts to go over, the rotor loses lift, which allows for more net nose-over torque, and so on. The magnitude of the forces, and the self-energizing instability, aren't present in the sluggish-rotor model. The aggressive pilots will tell you that they sometimes move the stick fast and far enough that they get teeter stop contact, and live to tell the tale... so we know that that itself is not automatically fatal.

Real gyro rotors aren't that heavy, have reasonably decent airfoils and respond to imbalances of lift across the disk within a rev' or two. The quick response is true even at partial RPM: sit in your gyro with the blades turning at prerotator speed and jab the stick around, while watching the tips. The reaction is a fraction of a second. Try it in flight (carefully, please) and see how fast the reaction is.

Can a real-world airframe and HS beat the rotor to a new orientation by enough to be catastrophic when the relative wind shifts? I think it's possible on paper, but not a practical concern.

[raghu]

Just to add to Doug R.'s comprehensive description, the "big fact rotor" would also add a damping force to the fuselage as soon as it overshoots the rotor. This force would slow down the fuselage pitching motion considerably, allowing the rotor to catch up.

[birdy]

Doug,
Instead of exagerating the gyro configuration,exagerate the conditions.It'll have the same result and is more likely to be real.[remember I'm still talk'n about extreme conditions]
You also said,
The aggressive pilots will tell you that they sometimes move the stick fast and far enough that they get teeter stop contact, and live to tell the tale... so we know that that itself is not automatically fatal.
Hmmmm..........and I thought I was smooth on the stick.[I felt the stops again today]
Mind you,the "pressure" I put on the stops through the stick is nothing compared to what the stab could,with the extra leaverage.And I'v never felt the stops in pitch,only in roll,and I'v never heared of anyone PPOing or PIOing sideways.

Kevin,
It appears from looking at the limited data set in Eric's paper that a light machine with an oversize stab, the pitch of the fuselage would diverge lessfrom the rotor disc and would damp out the oscillations more quickly.
:confused: :confused:
I'm not sure if you agree with that,but I don't.I can't see how a smaller one would make the airframe diverge more from the rotors than a large one,wich is how I read it.
I'm not talking about oscillations,I'm refering only to virtical wind shear.

rfi says,

I don't think that there are any RAFs with stabs that are "too large". The ones that I have flown seem to have about enough area to counteract the area of the cabin, so that in gusty conditions the airframe does not pitch nose down--it just goes straight up and straight down with no pitch bucking.

I assume[hope] your refering to horisontal gusts only.
I'v always been refering to virtical ones on this thread.


raghu Just to add to Doug R.'s comprehensive description, the "big fact rotor" would also add a damping force to the fuselage as soon as it overshoots the rotor. This force would slow down the fuselage pitching motion considerably, allowing the rotor to catch up.

:confused: :confused: :confused:

[Doug Riley]

Birdy: Nope, the severity of the conditions doesn't affect the analysis. Either the fuselage reacts so fast that the rotor can't follow or it doesn't. Both of them (fuselage and rotor) experience a proportionally stronger weathervaning moment if (1) the wind shear is more abrupt (by virtue of a thin, sharp division between up and down air and/or high gyro airspeed) or (2) the difference in velocities of the two columns of air is greater. Their reactions will both be faster in an abrupt, severe shear.

If memory serves (ha!), the tech lingo sometimes used for these reaction times is "compliance rate." What I was calling a "sluggish" rotor is, in the jargon, one with a slow compliance rate. A slow-complying rotor and a fast-complying fuselage are not an ideal combination in turbulence.

The only kind of PIO-ing I've ever done is in the roll axis. There are no tail surfaces to damp out the oscillations, so you have to depend on the (rather weak) damping effect of the rotor and/or you own wits to stop it.

Raghu, I believe that rotor damping depends on the G-load on the rotor. In a momentary zero G, with no rotor thrust, I would think that the rotor's ability to influence (that is to slow) the pitching of the frame would be gone. The damping effect of the HS (which is a function of its size and especially its lever arm) would still slow down the pitching, however... that's why my "rigged" example specifies a short tail arm. I doubt a Little Wing or similar craft would pitch too fast, no matter how big the HS was. Perhaps that's the effect that Eric's paper captures and that seems wrong to Birdy.

In the real world, the fact that the damping effect of the HS isn't G-dependent seems to me one of the best arguments for the use of the HS.

[Heron]

How does all of this relates to the Rudder and the rudder pedals?
The rudder is there for the same purpose of correction (yaw) and has amplifying devices (pedals) to enforce its action.
The HS is fixed and the relation between that movement and the proximity of the rotor seems to me the greater danger.
If the engine is not centered in any of the axis the machine will turn around the CG.
The limit of turning forward is the disc and its angle of performance (point of reversing rotation) wich causes catastrofic failure of the machine as a whole.
A little lost but trying to track it down . . .
Heron

[Doug Riley]

Heron, the rudder pedals should not be used as "amplifying devices." A proper FIXED vertical fin performs its main function just fine without any pilot input: it keeps the aircraft pointed into the relative wind. In this function, the vertical fin works the same way as a horizontal one. It does not need "amplification" nor any input at all from the pilot -- if pilot input is neceessary just to keep the craft pointed into the wind, the vertical surfaces are too small or are otherwise improperly designed.

On a gyro, we have a vertical fin that can be moved (via the pedals) for three reasons:

(1) to steer the craft at higher speeds on the ground

(2) to compensate for the changing yaw effects produced by the engine as the throttle is adjusted (unless you have a tall tail), and

(3) to turn the craft AWAY from the relative wind when we wish (as in a crosswind landing or "trick" flying, such as sideways flight, vertical spins and so on).

I see no analogy between any of these other functions of the rudder and the function of a HS. The HS is flight adjustable on elevator-equipped craft in order to vary the angle of attack of the lifting surface. With direct cyclic pitch on a gyro, we have another way to do this and don't need a adjustable HS/elevator. The HS on direct-cyclic gyros provides the pilot with a platform whose angle to the airflow tends to stay constant.

[raghu]

Raghu, I believe that rotor damping depends on the G-load on the rotor. In a momentary zero G, with no rotor thrust, I would think that the rotor's ability to influence (that is to slow) the pitching of the frame would be gone. The damping effect of the HS (which is a function of its size and especially its lever arm) would still slow down the pitching, however... that's why my "rigged" example specifies a short tail arm. I doubt a Little Wing or similar craft would pitch too fast, no matter how big the HS was. Perhaps that's the effect that Eric's paper captures and that seems wrong to Birdy.

In the real world, the fact that the damping effect of the HS isn't G-dependent seems to me one of the best arguments for the use of the HS.

Well Doug, like you say, the damping does rely on the fact that the rotor is producing thrust, but then again if the rotor is not producing thrust the rotor will not follow the motion of the fuselage either. I recall in an earlier post you said the rotor will align with the fuselage with in a couple of revolutions; true, but obviously if there is no thrust you are not going to have that motion either. If you have one you have the other.

I think all these arguments boil down to esentially: " is is it possible for a severe gust/wind shear to unload the rotor sufficiently." The answer: no one knows for certain! Unfortunately there are no hard numbers on this. Nothing like getting a rotor instrumented and tested on the back of a pickup. Perhaps the likes of Ernie can help?

As for Eric's paper, I must say, I agree with the quote on the PIO/PPO safety website by an helicopter aerodynamic specialist; the paper is, in my humble opinion, A LOAD OF RUBBISH. It starts with a false premise and tries to find a stability mode that does not exist (only exists if a gyro is a pendulum). Of course all this is cloaked in essentially flawed differential equations, resulting in many people not verifying and accepting the model as gospel. I am afraid to say, at least as far as the theoretical analysis goes, it is as invalid as 2=3. Does anyone know where eric got his PhD from? and in what? I would not be so critical if this was just a lay opinion, but when some one tries to pass of something on the pretext of expert knowledge, then it I feel it is important to be critical.


ps. As an aside, I am also having diffculty conceptualizing a zero G situation where there is any appreciable aerodynamic force. By definition, in a zero G situation, the body must be freely falling under the influence of gravity. Negative Gs are another matter, the downward motion should help load the rotor and bring it back to positive Gs, unless offcourse you have a high prop thrustline. The high thrusline in combination with the negative rotor G will push the gyro into a PIO, in a flash of an eye.

[raghu]

Well Doug, like you say, the damping does rely on the fact that the rotor is producing thrust, but then again if the rotor is not producing thrust the rotor will not follow the motion of the fuselage either. I recall in an earlier post you said the rotor will align with the fuselage with in a couple of revolutions; true, but obviously if there is no thrust you are not going to have that motion either. If you have one you have the other.

I think all these arguments boil down to esentially: " is is it possible for a severe gust/wind shear to unload the rotor sufficiently." The answer: no one knows for certain! Unfortunately there are no hard numbers on this. Nothing like getting a rotor instrumented and tested on the back of a pickup. Perhaps the likes of Ernie can help?

As for Eric's paper, I must say, I agree with the quote on the PIO/PPO safety website by an helicopter aerodynamic specialist; the paper is, in my humble opinion, A LOAD OF RUBBISH. It starts with a false premise and tries to find a stability mode that does not exist (only exists if a gyro is a pendulum). Of course all this is cloaked in essentially flawed differential equations, resulting in many people not verifying and accepting the model as gospel. I am afraid to say, at least as far as the theoretical analysis goes, it is as invalid as many of those "2=3" trick math proofs. Does anyone know where eric got his PhD from? and in what? I would not be so critical if this was just a lay opinion, but when some one tries to pass of something on the pretext of expert knowledge, then it I feel it is important to be critical.


ps. As an aside, I am also having diffculty conceptualizing a zero G situation where there is any appreciable aerodynamic force. By definition, in a zero G situation, the body must be freely falling under the influence of gravity. Negative Gs are another matter, the downward motion should help load the rotor and bring it back to positive Gs, unless offcourse you have a high prop thrustline. The high thrusline in combination with the negative rotor G will push the gyro into a PIO, in a flash of an eye.

[Vance]

It looks like double talk to me!!!

[Doug Riley]

Raghu, I believe cyclic pitch still works in zero G (with the rotor producing no NET lift), in the sense that movements of the spindle still cause the rotor to follow the spindle. Consequently, the rotor also follows the frame -- because of cyclic pitch changes caused by the rotation of the frame, which then carries the spindle with it (assuming the rotor head is more or less fixed relative to the frame, by friction, trim spring, control stops or the pilot holding the stick still relative to the frame). Even with no NET rotor thrust, a cyclic change will increase the AOA of one blade and decrease the AOA of the other, causing the rotor to tilt. The tilt won't cause the frame to follow, however, if there's no rotor thrust. It's a one-way street, if you will.

I'm sorry to have to drop out of this most interesting discussion. I'm going to the big ultralight camp/flyin in Greenland, NH for the next week. No Internet in my tent by the pond!

[raghu]

In case anyone is interested with my take on the Hollman model of gyro stability, here is brief post. Without going into the math, here is a physical description. I have made some simplification by only considering the pitching motion of the rotor and fuselage and ignored other factors, to provide insight of the flaw.

The fuselage and rotor, because of the existence of the universal joint, have separate degrees of motion that interact with one another- move the rotor and the fuselage will follow suite after a slight delay, and conversely move the fuselage and the rotor will follow suite and align with the fuselage after a short delay. This much is true and well understood.

Now, Martin and Eric believe that these two pitching motions (namely the rotor and the fuselage) can potentially (in bad gyro designs) get in-phase with each other, and result in each of the motions feeding on to the other, causing a growing oscillation that results in a rotor strike with the fuselage and a tumble. Can this really happen? Is this physically possible?

For both these motions to feed onto each other they must be capable of oscillating on their own accord. Can the fuselage oscillate: yes! If a gust disturbs the fuselage it will try to return to equilibrium ( if it has a HS) but in doing so unless the HS has sufficient damping it will overshoot and then return back.

What about the rotor? Here is the catch! The rotor can only have a pitching motion if it there is a force acting on it. Take out the force and the motion stops instantly, no overshoot. Thus the rotor motion is non oscillatory and only adds a damping term to the fuselage motion.

In control system theory all this can be summed up by the principle, you need to have at least a second order system for an oscillatory response. The gyro can off course be unstable but the Hollman model is not how- the rotor pitching motion is a first order one. Steward Houston’s model is the correct one, and it turns out the instabilities are very similar to other aircraft and have nothing to do with the pendulum motion conceptualized by the Hollmans.

[raghu]

Even with no NET rotor thrust, a cyclic change will increase the AOA of one blade and decrease the AOA of the other, causing the rotor to tilt. The tilt won't cause the frame to follow, however, if there's no rotor thrust. It's a one-way street, if you will.

pond!


Not sure how Doug! The way I see it, if there is no rotor thrust then it means the blades are producing no lift forces, so the change in relative pitch of the blades will not cause any force. If the cyclic change cause a force then we are not in zero G anymore, ofcourse this caannot happen.

Have a good one at the fly in.

[Udi]

Good to have you back, Raghu.

I agree with Doug that cyclic pitch (and roll) still works in zero G. As long as the rotor is rotating, it will respond to aerodynamic inputs. Changing the airfoil AOA immediately changes the lift. The cyclic is not floating in space; it is bolted to the airframe.

As for the question whether an updraft may drive the tail into the rotor, I think the answer to this question must have something to do with the individual response times of the airframe and the rotor.

Any given updraft results with angular acceleration of the airframe (tail-up, nose-down). The rate of acceleration is related to the stab lift coefficient, stab damping, aerodynamic forces (drag) about the airframe, airframe MOI, and rotor feedback.

The rotor has it's own response time. As Doug said above, the rotor response is related to the rotor weight (momentum) and aerodynamic response (lift coefficient).

If the rotor response is significantly slower than the airframe response than, in theory, the tail may fly into the rotor. Is this a practical concern? I don't know. It sure is a spicy argument for those who don't want the stab.

Udi

[Heron]

How long zero G can last? Is it significant?
How strong has the up gust be to have that kind of impact in a moving machine?
The examples kinda remove the relative wind for a moment . . .
Oh brother I need to fly so bad . . .:(
Heron

[raghu]

Good to have you back, Raghu.

I agree with Doug that cyclic pitch (and roll) still works in zero G. As long as the rotor is rotating, it will respond to aerodynamic inputs. Changing the airfoil AOA immediately changes the lift.

Udi

Udi,
If the lift immediately changes, then vola we have some G forces so we are not in zero G. Thats the point I am trying to make. If we are not in zero G then rotor damping comes into the equation. What say you?

[C. Beaty]

To meet your definition of zero "G" Raghu, the rotor would have to be operating in a vacuum. A rotor immersed in any fluid having mass; whether air, water or hydrogen, will respond to cyclic input even though the net thrust and therefore damping applied to the airframe is zero.

Imagine a helicopter mounted on gimbals with rotor thrust set at zero. The rotor will follow airframe motion with the usual lag but will offer no resistance to such motion without offset flap hinges. Therefore; no airframe damping. Rotor damping in this case doesn't matter.

I've put a series of articles by Prof. J.A.G. Bennett (former chief engineer of Cierva Autogiros) on CD. Send your mailing address to: cabeaty@att.net and I'll mail you a copy. Loads of sheet music (differential equations) to entertain you but nothing about zero G flight.

[raghu]

Chuck,
You are right! How could I be so dense? You, udi and Doug are absolutely right. I have been 'not thinking' gyros for a bit ( they work me too hard at CMU) ....got to blame it on something!

Anyway glad that I messed up, at least I got you posting.

I would love to get hold of the CD. I appreciate it! I have emailed you my snail mail address. Thanks!

[Hognose]

OK, let's see if I have it straight here --

1) Basic premise of the Eric Hollmann paper is that, stimulated in pitch by an external force, the rotor and the fuselage will essentially oscillate, and that a stabilizer has a profound influence on whether those oscillations are divergent or damped out.

2) Raghu calls el toro poo poo. Says sure, fuselage will oscillate in pitch but rotor will not. When force leaves rotor rotor no longer moves. Ergo whole paper is drivel, because it describes phasing between a pair of oscillations where in physical world only one oscillation exists.

Raghu, Chuck, and Doug (when you get back -- I will not make Greenland NH, duty, but you may see my brother and his bro-in law there): Have I got this understood?

Raghu: if gyro instability is more similar to other classes/cats of aircraft than we have assumed, then solutions similar to those machines would be effective, would they not? Therefore we have a bit of a paradox where, if your criticisms are correct, Hollman père et fils are arguing a correct conclusion (stab) from a faulty premise.

No wonder Dave Bird (and many others who lack his assertiveness but are lurking here scratching their heads) is confused. If the PhDs are at each other's throats what hope for us dumbass B. Sc.s?

cheers

-=K=-

[birdy]

Doug,
Assuming there was such a machine that you described[the exagerated one],and it would be a handful to fly in anything but the calmest conditions,I'm wundering how close the follwing machine is to the same configuration.??

A light weight[<300lb] ,2 seat machine with a high power [to weight] engine.

MTOW=880lb.[machine 300lb,2x 250lb people and 80lb of fuel]

Now, the same machine with only a [120lb] pilot and 10 lb of fuel.

A lot lower loading on the same rotors,same amount of avalable power and much less weight[mass] for the stab to move[pitch] in a sudden virtical wind direction change.

BTW,I undrestand the -G no thrust thing.
So long as the rotors are spinning[flying],you do have pitch control over them,regardless of the amount of thrust [or lack of ]you have.

[Chopper Reid]

Glad you got on here before I did Birdy as I might have defended your integrity a little more vigorously!!
Just because someone isnt articulate and doesnt use the spell check, it doesnt make him an idiot. Read what he says and in the context its written. I don't know how many hours Birdy has in gyros but I do know he uses his like most of us use our everday vehicle and spends long ours in the air mustering and "mustering" means flying everyday, no matter what the weather/wind conditions so a variety of conditions are experienced. I know from personal experience the feeling of my gyro going from straight and level on a reasonably calm day to suddenly being thrust straight up like a missile for some 800 feet and then being dropped off just as quickly back to the original altitude. The feeling of negative g's and the seat belt cutting into my shoulders scared me, as all through training we were taught negative g's weren't desirable and the loss of rotor revs was foremost in my mind !!
My machine is not one of Paul's "safe" CLT etc but it behaved beautifully with no great attitude change except a small nose down as we were violently lifted up then adopted a slight nose up on descent.
The scenario Birdy puts is very possible.
Aussie Paul has been what could be described as being very vigrously pushing this CLT/safe gyro he is going to put into production plus slightly intimidating some owners of "unsafe" gyros on the Australian forum.
Theories and elaborate diagrams are all very well but many hours of practical experience sometimes prove otherewise. Gyros here in outback Australia are being used every day in a working environment, being flown in demanding conditions accumulating thousands of hours each year, so we have a "solid" base to make comments from.

[birdy]

Brian[chopper] mate,are you the same one I woz talk'n to on the Oz forum??

I appreciate your support,I'v been told that self praise is no garantee of truth.

I think these blokes reckon I'm just a "knowall" from Oz try'n to stir ****.Nothing could be further from the truth,my aim is to open some eyes to the potential of these magnificant machines.
And if "DESIGEN,ENFORCED BY LAW" wasn't posted on the Oz forum[by Paul B. I think] I wouldn't even be on this stupid !@*^%@$(%@ computer.
When I saw that I felt someone was trying to step on me toes,and I got tender toes.

Myself,Brian and countless others in Oz use these machines for practical aplication,and I don't reckon too many would take too kindly to being told what to do by someone who "hasn't done it".

And if others on this forum don't care what we do ,know or have to offer,then I'll gladly turn off this machine and keep it all to meself.

I feel better now.

[StanFoster]

Birdy: Keep posting guy....I love reading the stuff you do with your gyro. :)

Now...get out there and muster them cows....oh..and send some pictures. :D

Stan

[Heron]

Camomila Tea and Passion fruit juice is good for calming :D
Heron (the closest one to the Amazon jungle)

[Harry_S.]

Birdy and Chopper:

I'm with you guys. Experience speaks volumes. I suspicion you're going to get ripped, because the inexperienced and/or newby will be guided by what you say.

Maybe you need a disclaimer on your posts. :rolleyes:

[Heron]

Newbies that do their home work are less prone to get their arces in trouble!
Post away Birdy, we have fun and keep the keetle steaming . . .
How many times did you cheat death?
Heron

[birdy]

Harry said,
I suspicion you're going to get ripped, because the inexperienced and/or newby will be guided by what you say.

With respect to everyone Harry,anyone who takes anything posted on this thread as fact is a fool.The subject is hypothetical,for the simple reason ,no-one has ever survived a PPO or bunt over to tell anybody wot exzactly happened.[even if someone had a miraculous escape,I dout he'd remember anything coz it'll happen too fast.]

The aim of this thread is to probe the excelant minds out there ,and to try to get them to think out side the normal.
There have been some accidents about,in stabed machines, that resemble PPO or bunt overs.[I suspect speed has a big part to play sometimes]Now,if the machine is clasified as "stable" ,the pilot will almost always be blamed to be at fault.He could be too,but I'd like to know if there's any other possabilitys.

"Witnesses" to these accidents are almost all we have to go by to determan wot happened,and the witness isn't in the best place to tell an accurate story of wot happened.[bent, physical evidence only tells wot happened,not always wot caused it]
I'll make a hypothetical ecsample,
The machine is flying S&L at 300',on the downwind leg of an airfield.The witness in a hanger,on the other side of the pattern,says the machine just appered to start oscilating,tumble forward and disintergrate.
Now,lets assume the machine DID hit a clear air windshear,and go through the motions of the situation I origionaly posted.The witness,from his position,[angle to flight path and distance from the machine]wouldn't notice the slight ELEVATION changes,only the pitch changes.Consiquently,the pilot would be at fault for PIO and not arresting it.If he did hit a clear air windshear,he COULD have been trying to fight the over powered stab.[and shouldn't be blamed wholy for his own demise.]

Remember,this is all hypothetical,and should be treated as such.

I have trouble understanding how a machine can go over in calm conditions, even with an inexperianced pilot,without some other "input" from outside.
And having flowen in some very crappy air,for a full day many times, and had some narsty supprises on calm days,I'm concerned that the stability thing may be blinding[blind faith] us to some other less common, but more dangerous possabilitys.
I hope I'm wrong,but my experiance makes me dout,that's why I'm here asking questions.

Heron,
How many times did you cheat death?
Mate,I'm no different to anybody,we all cheat death when we git out of bed.[smily]

[Chopper Reid]

Hi Birdy and all similiar minded pilots,
Yep, chopper Reid and Brian are the same fellow, we actually breed a few cows here too , nowhere near as many as you do Birdy.
I feel very strongly about whats going on in regard to this CLT/cog/ safe machine stuff and see you do also and I know you have a lot of experience plus a "GrandChampion " tittle at the Nationals up your sleeve so that tells me you know how to fly extremely well. I had the privalige to do some judging at the Nationals some time ago and you have to do extremely well to take the big tittle. Congratulations !.........besides, any rum drinker has to be a top guy !! :)
In defence of Aussie Paul, there is no doubt he has a great design/idea and a machine that looks as if it could be a real winner, however, you have to be careful that you dont get carried away with the moment. If his gyro is really good, it will sell itself and his safety ideas will filter down through the ranks and I reckon thats the best way to go. Its a bit like respect, you cant buy it, you have to earn it !!
You only have to look at GA here in Ozz to see what overegulation can do to a thriving industry so you have to be careful to keep any industry/sport free from overregulation or you are going to strangle it.
Great forum guys.
Brian.

[Aussie_Paul]

Birdy said,"I have trouble understanding how a machine can go over in calm conditions, even with an inexperianced pilot,without some other "input" from outside."

The "input" Birdy, is the out of balance forces of the thrust line to the VCoG.



Brian,

In defence of Aussie Paul, there is no doubt he has a great design/idea and a machine that looks as if it could be a real winner, however, you have to be careful that you dont get carried away with the moment. If his gyro is really good, it will sell itself and his safety ideas will filter down through the ranks and I reckon thats the best way to go. Its a bit like respect, you cant buy it, you have to earn it !!

You are so right Brian. I was for stability long before I started down the road to selling a product. What I am saying is that in my product I have incorporated all the "experts", and most of these experts have flown their gyros, and tested, various ideas.


Someone mentioned experience. Experience can work against coming to the right conclusions because of the "automatic skill" that comes with experience.

Experience in "controlled testing" though, is worthy stuff.

Aussie Paul. :)

[birdy]

Paul,
Doug said this back on page 4,

"Can a real-world airframe and HS beat the rotor to a new orientation by enough to be catastrophic when the relative wind shifts? I think it's possible on paper, but not a practical concern.

He hit the nail on the head.He's refering to the situation as a hypothetical one.
"Theroeticaly it's possible".
In case you havn't been paying attention again Paul,we can only go on therory,coz of the PPO fatality rate.[100%]
"But not a practical concern"
Practicaly no-one will have to concern themselves of it,because of the unlikelyhood of ever getting in to the situation..........but it's still possible.

My argument is,"IF" it's possible,how many times has it happened and been misinterprited as a simple PIO.

You can bet that if it is possible,you'll only do it once,but you won't know about it.

You said,
The "input" Birdy, is the out of balance forces of the thrust line to the VCoG.

The machine I'm refering to was a highly regarded clt desigen with a stab.And out of respect to the people involved,I won't say anymore about it.

If it wasn't a clt [or close to it] machine Paul,I wouldn't have started this thread.[I thought that would have been obviose]

You also say,
Experience can work against coming to the right conclusions because of the "automatic skill" that comes with experience.

Paul,that's correct,but as I'v said on this thread ,and the same Oz one,experiance has nothing to do with this situation.
"Automatic skill" may well blur the pilots perception of the stability of his machine,but experiancing the conditions,and reading the extremes on the flight gauges isn't perception,it's reality.My concern is born from my experiance in these extreme conditions,and the lack of understanding of these conditions by the general recreational pilot.Conditions that none of us are immune to ,solong as we fly.

BTW Paul, I thought you started this thread by stating,"I GIVE UP".
I quietly wish you would,because your mind is closed and not helping the disscussion.
I'm getting alot more constructive conversaion from your mates.

[Harry_S.]

Birdy, I hope everyone reads your #78 post. It speaks volumes.

[mcbirdman]

I thought realative wind is - Realative to flow of air across the fuselage. Since the aircraft is being pulled along at say 50-80 mph what kind of downdraft are we talking? Would it be So Focused that it would only hit the tail? I would assume that if the tail with 0 degree angle of attack was flying balanced with the rotors and you put more or less air into both - the forces should still remain balanced. Also, I suppose if you put a big tail on the back it will try to weathervane into the appaerent wind which is propably coming past the blades same as the fuselage. What are we trying to prove? Better not to have a tail? Whether you can get too little or too much of something? I suppose the too little or too much means just that - otherwise it wouldn't be too this or too that.... It just seems that you are working really hard to set up something that just isn't a likely or practical or demonstrated problem. Thousands and thousands of hours logged on tailed machines and then years later the new designs were rolling and rolling. I think you are right it is tail and I believe (arm length) related but Not in a bad way. It is far easier to make a tail or find a tail on an aircraft TOO small or ineffective than to make or find a tail TOO big.

I feel that a longer arm on a fuselage with a stab will be less likely to pitch the aircraft up or down to far not only because of the moment but just how far a tail would have to diverge from the vertical center of gravity compared to a short arm. The long arm and big tail just follow the long fuselage as it moves through the direction of air flow.

Also, if someone feels a little inadequate because they realize their machine is not configured properly and knows it - what do you expect? Should someone just not say anything about the problem and hope for the best? I guess if you have the desire to learn and figure these things out - you may not like what you learn. Wishing someone away does not make a person right or any smarter.

[C. Beaty]

The Sun rises in the East, moves across the sky and sets in the West, leading one, inexorably, to the conclusion that the Sun revolves around the Earth.

As most any crackerjack mechanic can tell you, removing the thermostat from an engine is likely to cause overheating at high speed, leading to the conclusion that the water is circulating too fast to cool down.

Observation and experience without an underlying knowledge of the laws of nature leads, more often than not, to incorrect interpretation of observed phenomena.

Now Birdy, I know Alice Springs is located on the Todd River where local "sailors" hold boat races by by running along the river bottom carrying their boats but you've probably seen pictures of real rivers in back issues of National Geographic while waiting in a doctor's office, so use your imagination in the following.

Imagine you're floating down a river on an innertube and can't see the river banks. Since your innertube is moving with the flow and water speed is zero, is there any way to tell whether the river is flowing or not?

Suppose the river narrows, causing an increase of water speed. Your innertube increases its speed, keeping your water speed at zero (without inertia). Not being able to see the river banks, you wouldn't know your innertube had increased its speed except by sensing the acceleration. (By the way, inertial navigation systems work by sensing acceleration and converting acceleration to velocity with a mathematical operation called integration.)

Both you and your innertube possess something called inertia; the two of you resist having your velocity changed. As you enter the narrowed section of the river, the flow overtakes you and applies a force that accelerates you to the speed of flow. During that time you would sense some water speed. The amount would depend your inertia, submerged surface area and rate of change of river velocity.

You've suggested that a gyro with a large horizontal stab might tilt 45º nose down upon entering a fast rising air column.

Let's explore the ways in which that would be possible.

Imagine your gyro, moving at 60 mph, is gimbaled to a level skyrail that has supports attached to the Earth's surface. Upon encountering an air column raising at 60 mph (5,280 fpm), your relative wind shifts from head-on to one that is rising at an angle of 45º with a velocity of 85 mph. Of course the gyro tilts 45º nosedown if it has enough horizontal stab to be stable.

Whether or not the nosedown tilt rate is great enough to stall the rotor and cause a collision depends upon the torque applied by the stab, the gyro's moment of inertia, the maximum precession rate of the rotor and most important of all, how narrow the boundary between still air and the rising air column. Still air and rising are air aren't often separated by an invisible wall; the transition generally looks something like Mr. Gauss' famous "bell curve."

But real gyros aren't gimbaled to skyrails. They move through an ocean of air and what that air mass is doing with respect to the Earth's surface is irrelevant. They behave like your innertube floating down a river.

An inertialess gyro, upon entering a rising air column instantly accelerates to whatever upward velocity the air column happens to have. There would be no change of relative wind direction or speed and no pitch response.

A real gyro, of course, possesses inertia and a finite time is required for it to accelerate to the velocity of the air column. During the initial penetration (always the most exciting part), there is a shift of the relative wind and if the gyro has sufficient horizontal stab to be stable, it will tilt nosedown, always heading into the relative wind. Unstable gyros will tilt noseup, increasing the rotor angle of attack, magnifying the disturbance.

After the air column has been penetrated and the gyro has acquired its upward velocity, everything returns to trim; airspeed, airframe attitude and whatever.

A gyro doesn't know it's in a rising air column; for all the gyro knows, the air could be standing still with the Earth falling away.

As earthbound critters, our instincts tell us that everything is somehow related to what's going on at the Earth's surface but it's not.

[Udi]

As most any crackerjack mechanic can tell you, removing the thermostat from an engine is likely to cause overheating at high speed, leading to the conclusion that the water is circulating too fast to cool down.

So what is the true explanation for that, Chuck?

Udi

[gyromike]

Quick question(s) to Chuck B.:

When our gyro (with stab) encounters this updraft and the nose begins dropping to point into the relative wind, the rotor tends to lag behind shifting the Rotor Thrust Vector ahead of the CG, correct? (Assuming that our pilot is not gorilla-gripping the stick). It takes a moment for the cyclic pitch input (reduced trim spring tension) to affect the rotor's orientation.

At the same time, the rotor is also experiencing a momentary load increase due to the change in relative wind, yes?

If these two suppositions are correct, wouldn't this shifting of the RTV ahead of the CG, along with the increase in the load factor, serve to slow down the rate at which the frame pitches forward? Wouldn't it dampen the rate of rotation of the frame?

I'm sure the type of rotors used and their respective weights come into play, but it's just something that I was curious about.

[C. Beaty]

Open impeller pumps cavitate if the suction head is too high. A vapor bubble forms in the eye of the impeller and the pump quits pumping.

An outlet restriction reduces the flow rate and lowers the suction head.

I'm disappointed in you, Udi. Surely you didn't believe that water could move too fast to cool.

Maximum flow of heat to the outside world requires keeping the temperature gradient between radiator inlet and outlet as small as possible, i.e., the fastest possible flow rate.

[C. Beaty]

Sure, Mike. If the airframe begins outrunning the rotor, the shift of thrustline forward reduces the rate of airframe tilt and tends to damp the motion.

[Udi]

Of course I didn't believe that Chuck. I didn't think cavitation though. Damn. This is not the first time cavitation eludes me! Pump cavitation, by the way, is the main reason radiators are pressurized. Most automobile radiators nowadays operate at 13-16 psi.


An outlet restriction reduces the flow rate and lowers the suction head.
It's a little more complicated than that. The reason for pump cavitation is more than just low head pressure. A high flow rate is causing cavitation due to the Bernoulli effect. As the liquid accelerates inside a centrifugal pump the liquid pressure is reduced according to the Bernoulli principle. More velocity --> lower pressure. As the pressure goes below the liquid vapor pressure you get boiling - cavitation. Using a larger - and slower - pump, under the same conditions may also prevent cavitation.

Thanks.

Udi

[Dean_Dolph]

Open impeller pumps cavitate if the suction head is too high. A vapor bubble forms in the eye of the impeller and the pump quits pumping.

An outlet restriction reduces the flow rate and lowers the suction head.

Maximum flow of heat to the outside world requires keeping the temperature gradient between radiator inlet and outlet as small as possible, i.e., the fastest possible flow rate.

Chuck, unless I've been wrong all these years then centrifugal pumps cavitate because the suction head is too low and the liquid vaporizes when it gets below the vapor pressure of the liquid. And reducing the flow rate will raise the suction pressure not lower it. Once a pump gets into serious cavitation the flow rate pretty much drops to zilch and the pump can get extremely hot.

'Ole' petro/chem. plant operators will squeeze back on a centrifugal pump discharge valve to reestablish suction pressure on pumps that start cavitating. Cavitation in centrifugal pumps is easy to spot since it sounds like the pump is trying to pump rocks. It is sorta self sustaining since there is heat build up from the pump drive motor energy that isn't being removed by liquid flow. Consequently the liquid vapor pressure drops even further and perpetuates the problem. The noise comes from the collapsing of the vapor bubbles which can do some serious erosion of the impeller if given enough time.

Heat exchange between a liquid and the 'outside world' is dependent on a buncha factors! Flow rate being one of them. Exchange area is another. But the driving force is temperature differential between the circulating coolent and the ourside world. A small temperature gradient is the end result of high enough flow rate and a large enough exchange area that the temperature differential isn't noticed.

In a closed system, such as our gyro cooling systems, if the flow rate is high enough and the radiator large enough then you wouldn't see any difference, on our relatively crude temperature measuring instruments, between the temp going into the radiator or the temp out.



It's a little more complicated than that. The reason for pump cavitation is more than just low head pressure. A high flow rate is causing cavitation due to the Bernoulli effect. As the liquid accelerates inside a centrifugal pump the liquid pressure is reduced according to the Bernoulli principle. More velocity --> lower pressure. As the pressure goes below the liquid vapor pressure you get boiling - cavitation. Using a larger - and slower - pump, under the same conditions may also prevent cavitation.

Udi

Good point, and one that most people won't know or think about. I was one that didn't think about this since the simple explanation of, and solving, low suction pressure usually works for me!

[C. Beaty]

Suction head, Dean, refers to the height a column of fluid is lifted; pressure head refers to the height to which it is pushed. Total head, naturally, is the sum of the two.

[gyromike]

Sure, Mike. If the airframe begins outrunning the rotor, the shift of thrustline forward reduces the rate of airframe tilt and tends to damp the motion.

Thanks Chuck,

It seemed to be a point not yet mentioned in the discussion.
(Or maybe I just missed it).

[Udi]

Since suction, and discharge, head is expressed in absolute units, Dean is correct. A too-low suction head would result in cavitation.

Udi

[birdy]

Chuck B.,mate, you'v raised one of my favourite subjects. :)


"The Sun rises in the East, moves across the sky and sets in the West, leading one, inexorably, to the conclusion that the Sun revolves around the Earth."

If this is the case,then tell me why I can see the same stars tonight as I can in 6 months. :confused:
In 6 months we're supposed to be on the other side of the sun[half way round our orbit].If we're on the other side of the sun in 6 months, why can I still see the stars out this side??

Highjacked me own thread. :mad:


Back to the origional discussion.

Thank you Chuck B. for your well explained and easy to understand post.[and the odd bit of humour.] :)
I take it that your saying the machine CAN"T POSSIBLY outrun the rotors.???
Irrispective of the power[size or leaverage] of the stab or conditions.

A simple yes or no will do,and would end the thread. :D

You also said,
"After the air column has been penetrated and the gyro has acquired its upward velocity, everything returns to trim; airspeed, airframe attitude and whatever."
Supposing the machine now hits the opposing down draft[of equal power ,ie;5280fpm,].The machine is riseing at roughly 2600fpm,with it's nose pointed to earth at 45*,the downer is sinking at 5280fpm,so the effective down draft power is close to 7880fpm.No??
Will,or should I say COULD anything bad happen.???[I know it all depends on the time frame between up and down,but I'm thinking of severe,so imagine the worst.]
I gess what I'm asking is ,how severe is too severe.??Or dose the compressability of air mean there's no such thing as too severe.[I heard autorotating blades can't stall from over loading coz of the air's compressability]
Thanx. :)

[GyroRon]

Don't forget the Earth is flat as well. :)

[C. Beaty]

If this is the case,then tell me why I can see the same stars tonight as I can in 6 months.
In 6 months we're supposed to be on the other side of the sun[half way round our orbit].If we're on the other side of the sun in 6 months, why can I still see the stars out this side??

That's because, Birdy, Australia doesn't actually exist. It's an imaginary place where strange beings spend their days hanging upside-down from their feet.

It was created by Lewis Carroll for his children's book; "Alice goes Down Under." Alice Springs, by the way, has nothing to do with water; that's where Alice rode a mythical beast called a kangaroo.

You also said,
"After the air column has been penetrated and the gyro has acquired its upward velocity, everything returns to trim; airspeed, airframe attitude and whatever."
Supposing the machine now hits the opposing down draft[of equal power ,ie;5280fpm,].The machine is riseing at roughly 2600fpm,with it's nose pointed to earth at 45*,the downer is sinking at 5280fpm,so the effective down draft power is close to 7880fpm.No??

First of all, I said that a gyro could only tilt 45º nose down if attached to a rail or if the transition from still air to 5280 fpm rising air occurred in zero time. Entering a rising air column with normal gradient, the gyro has enough time to acquire the upward velocity of the air column without being exposed to a 45º shift of relative wind. If the gradient was shallow enough, the gyro wouldn't noticeably pitch nose down at all.

I expect even a Boeing exposed to a wind shear 7880 fpm that occurred in zero time would suffer catastrophic failure. Never mind whether a gyro has a stab or not; something bad will happen.

I gess what I'm asking is ,how severe is too severe.??Or dose the compressability of air mean there's no such thing as too severe.[I heard autorotating blades can't stall from over loading coz of the air's compressability]

Gyro rotor blades can and do stall. In forward flight, a portion of the retreating blade is always stalled; the stalled area spreading outboard as the forward speed increases. The NACA measured the stalled area of the retreating blade on a Kellet KD-1 and found the stalled area extended out to around 60% of radius near the upper speed limit. http://naca.larc.nasa.gov/reports/1939/naca-tn-741/

Blade stalling serves as a speed limiter for gyros. As the upper speed limit is reached, blade flapping angle becomes so great (the rotor disc blows back) that the stick is against the forward stop and additional power causes the machine to climb without farther speed increase.

I don't have the foggiest notion of how severe is too severe but nominate you to find out. Go to Australia's northern coast, Darwin or some such place and penetrate some of those massive black, tropical thunderheads. Let us know what you learn.

[Heron]

Lets exercise some . . .
I think the machine will react as a whole when pushed in any direction, first I believe the rotor will do something, because it is the major component and the "draggy" thing.
After that, two ways: some machines will point the nose to the prevailing air flow or away from it (stable/nonstable?)
By pointing the nose in any direction, the rotors will have to realign with the new attitude and the machine will fly in a correct/incorrect path to position itself according with its design.
How am I doing? (Chuck have mercy) :D
Heron

[birdy]

"If this is the case,then tell me why I can see the same stars tonight as I can in 6 months.
In 6 months we're supposed to be on the other side of the sun[half way round our orbit].If we're on the other side of the sun in 6 months, why can I still see the stars out this side??"
Sorry to dissapoint everybody but that was a serious question.

Chuck B.,
You said,
"Entering a rising air column with normal gradient, the gyro has enough time to acquire the upward velocity of the air column without being exposed to a 45º shift of relative wind. If the gradient was shallow enough, the gyro wouldn't noticeably pitch nose down at all."
[For the record,I have an uncle in town who's been flying sailplanes here for 40 years,and 7880fpm virtical windshifts are not uncommon,even at lower alts.]
I know that is what happens in NORMAL conditions.
But I'v never said anything about normal conditions.
I'm trying to establish what the limits are[if there are any] in extreme conditions.Unfortunatly,conditions arn't always normal,windshear,while not normal,can occure in some unexpected conditions.Some of us spend alot of time in very rough conditions[maby we are idiots] and don't have the luxury of landing when it gets rough.
ie;encountering a strong pressure gradient with no were to put down for miles,
and to a lesser extent,mustering in a dust storm.

While I'm continualy impressed with the gyros controlability in increasingly rougher air[as my confidence grows],I'm trying to establish when I should opt out.
How dose a SCG know when it's becoming dangerous.???

"I take it that your saying the machine CAN"T POSSIBLY outrun the rotors.???

A simple yes or no will do,and would end the thread. "

I think this was your answer.
"I expect even a Boeing exposed to a wind shear 7880 fpm that occurred in zero time would suffer catastrophic failure. Never mind whether a gyro has a stab or not; something bad will happen."

I take it that is a NO answer.So that means a stabbed machine COULD over run it's rotors, in extreme conditions.
Or dose it realy mean know one realy knows.If this is the case I'll shutup.

I'll state my position ................again.
People are touting the STABLE gyro to be a safe machine,without explaining the limits to which it must be flowen within.If there are limits,knowen or unknowen,any "stable machine" claims should be followed by the operating limits of this safty.

Without the "op limits" stressed to a student,a simple one like me could interpret that as,idiot proof."It is safe,even in rough air."So I'll fly it in rough air,any air.

[C. Beaty]

Our Sun, Birdy, is a tiny pebble and its planets are grains of sand in an infinite universe. Some of the stars we see today burned out millions of years ago but the light we see began its journey even earlier.

The Sun and its planets spin about an axis that is fixed in celestial space; the whole affair being a giant gyroscope. The spin axis of our solar system could only be changed by some cataclysmic event.

As long as the spin axis of our solar system remains fixed, the stars are so far away that your view remains unchanged no matter where the Earth is on its annual trip around the Sun.

[birdy]

Your good at explain'n gyro things Chuck , but astronamy is'n your strong point mate. :D
That makes absalutly no sence to me. :confused:
How can it not chang if in 6 months I'm supposed to be look'n in the opposite direction ??[at night in 6 months when we're supposed to be on the other side of the sun,we would be look'n 180 degrees from where we do tonight.]

[Heron]

It is all in the angles Birdy . . .
Heron

[chuter]

Birdy, that's a really good question; I never thought of that. I think maybe it's the secret world government that is projecting a hologram of a universe up into the night sky? :D

We all know the moon missions were faked. ;)

No really..........what is the answer? I figure it's probably real simple and I'm going to feel really dumb when I see it.

[C. Beaty]

One more time, slowly.

Set a cantaloupe atop a fence post. Imagine that is the Sun.

Take a golf ball in hand and walk around the fence post. Imagine the golf ball to be the Earth.

The real Sun can be imagined to be a distant star.

Does the angle to the real Sun change, depending on which side of the fence post you're on?

At high noon, you can imagine the Sun to be a pole star. In the northern hemisphere, we have the North Star (Polaris), a fairly bright one. There is no pole star in the southern hemisphere of similar brightness.

The pole stars rotate in a small circle, requiring something like 26,000 years to complete because our solar system is wobbling (nutating: precession of the equinoxes) on its axis.

The stars that are not pole stars move across the sky just like the Sun as the Earth rotates about its own axis.

There is, of course, a 12 hour shift in the time a given star appears overhead between summer and winter. A star that is directly overhead at midnight in the winter is directly overhead at high noon in the summer.

[david holmes]

and we're over 550,000 miles from we were just an hour ago --- over 13,000,000 miles per day.
Is the creator big or what?

[chuter]

There it is, I knew I'd feel it...........dumb. I knew that. :cool:

[Aussie_Paul]

..at least I only made you talk slowly for me in aerodynamics!!!!!!!!!!!!!! :D

Joke Birdy, it's a joke!!! :eek:

Aussie Paul. :)

[birdy]

You reckon you'd feel dumb Chuter,I'v been pondering it since I was first told about the setup out there[space] :rolleyes: .When I was a kid,I'd lay in me swag look'n up and reckoned "they'v got it wrong."[I still do] ;)

C. Beaty
You reckon,


Set a cantaloupe atop a fence post. Imagine that is the Sun.

Take a golf ball in hand and walk around the fence post. Imagine the golf ball to be the Earth.

The real Sun can be imagined to be a distant star.

Does the angle to the real Sun change, depending on which side of the fence post you're on?

No,the angle don't change,but when the post is between you and the sun,you wont see the sun[star] coz it's daytime.[the sun{post} is between you and the star{sun}]
Tell me that ain't confus'n. :D

I'll try again. :rolleyes:
Now;
When the sun is between us and the star,we can't see it[coz it's day time]
When we are between the sun and the star,we can see it[coz it's night time]
6 months time;
We can still see the same star,but at night we would be look'n 180 degrees from where we was 6 months back.[coz in 6 months,we're supposed to be on the other side of the sun.]

To look the same direction [relitave to space]as streight up tonight,in 6 months,we'll be look'n past our feet.
How cum we can still see the same star. :confused: [we're look'n the opposite way] :D

[chuter]

I don't know squat about astronomy; are there some stars that are only visible certain times of the year?

[chuter]

It seems like there would have to be some stars that are only visible part of the year.

See the attachments.

[pwendell]

Birdy,

The Earth rotates around it's axis as it revolves around the sun. Imagine, for a moment, that we turn off the sun, but change nothing else. If you were to lie on your back and stare up at the sky you would see exactly the same stars pass overhead day after day. The position of the earth relative to the sun in irrelevant. The same stars are in the sky all year round.

Now, if we turn the sun back on, the only thing that has changed is that, when your particular part of the earth is facing the sun, the sun is so bright that you can't see any other stars, but they are still there. Because the earth's axis is tilted relative to the sun, we have seasons and the length of night and day vary throughout the year. This affects what stars are in our sky at night at different times of the year, but that's all.

Some stars are positioned close to the north pole. Those stars are visible every night, all year round, from the northern hemisphere, but they are never visible from the southern hemispere because the earth is always in the way. There are also, of course, stars that are only visible from the southern hemispere and are never visible from the north.

[Heron]

You can't see the forest from behind a tree . . .get out of there!
Don't think like a sniper = 1 point> 2nd point> target
think like a gunner = range> elevation> curve> target
Are we giving up on the I give up . . .?

[birdy]

Gees,I'll have to paint it. :mad:

[birdy]

In the first setup I can see the suothern cross[duh] coz it's night.

[under the black line]It's the same time of night,6 months later,BUT,in real life I can still see the southern cross???????

[birdy]

Heron,I give up?.Wot's that? :D This is much more fun. :p

[gyroman]

Birdy,

Too quote a famous movie, "your thinking two dimensionally".

[Heron]

Right you are Alvin!
Its not two dimensional and its not lined up with our planet axis.
The stars do not "circle around" like the sun, they stay in a smaller range of sight sorta!
Now do some mental exercise an leave the earth and look from outer space to get a better view.
Now if Chuck B. could be kind to us one more time, the conclusions of this thread are . . .?

[chuter]

I did a quick search and found plenty of info that says some stars are only visible part of the year, which supports the drawings that Birdy and I posted.

So, the answer to Birdy's original question: "why do I see the same stars tonight as I did 6 months ago?" is that you don't see the same stars, just some of them are the same.

Pretty soon they'll prove what I've been saying all along, that the universe is really flat. :eek:

[gyroman]

Hey Heron who is Alvin? I live in Alvin but I don't know anyone named Alvin....except for the chipmunk.. :D :D :D

[C. Beaty]

You would be correct, Master Bird, if the Southern Cross was to your north as shown in your painting or even directly overhead.

But it’s not. It’s at a low elevation to your south, almost a galaxy of pole stars, making it visible to you year round.

Since there is no bright Southern Hemisphere pole star, early explorers used the Southern Cross to indicate south.

[birdy]

Chuck,I used the southern cross coz it's the only one I know.[bad move]

The stars that are directly overhead now are still visable in 6 months.

Still no one has explained to me why,if we're on the other side of the sun in six months,why can I still see these stars here,on this side.??[The stars I can see in my northern hemisphere,would be some of the ones you would see from the equater,and I can see them all year.]

Chuter,The only stars that dissapear here are the ones clost to the horison,and they come and go with the seasons.[earths tilted axis.]
Pretty soon they'll prove what I've been saying all along, that the universe is really flat. Or we're the center of it.!!!

[C. Beaty]

Birdy, the stars that were nearly overhead in the winter may be visible in the summer but they’ll lie near your southern horizon. Likewise, the stars that were nearly overhead in the summer will be on your southern horizon in the winter.

The stars to your north, summer or winter, won’t be visible 6 months later.

The Earth spins about an axis that is tilted 23.5º relative to the axis of its travel around the Sun.

Furthermore, if the Southern Cross is the only one you know, what makes you think you’re seeing the same stars if you don’t know one from the next (I don’t either; all stars look about the same to me.)?

[birdy]

I gotta hand it to you chuck,thanx for your persistance.
I don't know the names of the stars ether,but I'v been watching them all me life and reckognise them.
All you said above is true.Your diagrame can explain the southern cross always beeing visable,but now imagine your head as a star[while look'n at your pic].When the earth is between you and the sun,you would be able to see it,but when it goes around behind the sun ,you can't.but in real life I can.??

[Heron]

Toby . . .like Ron use to say . . .brain fart . . .thats what it is!
I like the chipmunks too!
Birdy: where do you think the stars are located in a 3-D rendition of our solar sistem, and why do you think they are lined up with the Earth-Sun's axis?
Try to see our position on the Via Lactea and imagine that we are rotating around its center . . .
For those stars to move away from our vision they have to travel imense distance.
Heron

[C. Beaty]

Your field of view from the Earth’s surface, Birdy, is a hemisphere that over the course of a 12-hour night, swings through 180º.

This produces considerable overlap between summer and winter views of the sky. But the stars are not in the same position, -azimuth, elevation-, summer to winter at any given time. Many disappear entirely summer to winter.

What became of wind shear?

[Aussie_Paul]

My thread has well and truly been hijacked!!!!!!!!!!!!!! Lucky I am soooo tolerant!!! LOL :D

Aussie Paul. :)

[birdy]

Heron,I never said the stars moved,or are lined up with anything.

Paul,who's thread???
If I remember correctly,you pinched me post from the Oz one.
So I reckon it's my thread,and I'll f..k it up as I please.lol.

[John_Read]

Birdy, you stick to your guns mate!

Remember that in the English language "draft" refers to the depth of water required by a vessel to avoid grounding; a gust of wind is a "draught" - whatever these Yanks say! ;-)

[C. Beaty]

And here I thought draft or draught refered to beer drawn from a keg.

draught (draft, dräft), n.
1. draughts, (used with a sing. v.) Brit. the game of checkers.
2. Chiefly Brit. draft (defs. 1, 3–10, 18–25, 28).
–v.t.
3. Chiefly Brit. draft (defs. 29–33).
–v.i.
4. Chiefly Brit. draft (def. 34).
–adj.
5. Chiefly Brit. draft (defs. 36–38).
[1150–1200; ME draht (c. D dracht, G Tracht, ON dr!ttr); akin to OE dragan to DRAW, dr$ht a pull (at the oars)]
—draughtÆer, n.
—Pronunciation. DRAUGHT is a variant spelling of draft and is normally pronounced the same way, as (draft) or (dräft) or with a vowel somewhere between (a) and (ä). A pronunciation (drôt) is sometimes heard for DRAUGHT, perhaps because -aught is frequently pronounced (-ôt) elsewhere, as in caught and taught.

[Doug Riley]

I laft when I read this. Hope I'm not going daught.

[Aussie_Paul]

I give up on the lot of you!!!!!!!!! LOL

Aussie Paul.:)

[quadrirotor]

don't run away!...

[ymmv]

I laft when I read this. Hope I'm not going daught.

lol

You've seen the old "ghoti" thing, right?

For those who haven't seen it, "GHOTI" spells "FISH". GH as in "enough", O as in "women", TI as in "ambition".

What a dumb language. Then again, there's Hebrew, where they don't bother with vowels, or Japanese where there's no such thing as a plural....

[KenSandyEggo]

I have an interesting book of word origins of the English language somewhere. It seems that most spelling of words came about merely because most of the populace in England was illiterate. Only the muckety-mucks and religious leaders were not. The populace started using words, both English and other languages that they thought they heard or spelled them like they thought they sounded and common usage took over. Most of our words are misspellings. I tried to find the book to show an example, but couldn't find it.

[quadrirotor]

The new ESPERANTO is not english but GLOBISH (global english) that will kill english (in a while...) ;) http://victorian.fortunecity.com/vangogh/555/Spell/globish.html

[KenSandyEggo]

New!!?? It's been around since before me....or did you mean a new version?

[C. Beaty]

Ken J.:

I'm beginning to understand the lack of understanding between André and the rest of us is more of a communication problem than anything else.

I don't think he meant Globish was a version of Esperanto. Globish, as I read the link, is a phonetic spelling of English pronounceable by anyone.

Esperanto is an artificial Latin based based language comprehensible to anyone speaking Spanish, Portuguese, Romanian or Italian.

Many years ago, my #1 wife and I spent Christmas in Italy with her aunt. Her cousin was a doctor; a heart specialist. He subscribed to whatever US medical journal it was that covered that field.

He didn't speak a word of English but at the beginning of each article was an Esparanto summary which was comprehensible to him but also puzzling. He thought it was ungrammatical Italian and couldn't understand why a prestigeous medical journal couldn't hire a better translator.

[Heron]

????
I've got to research about this . . .
Many words in english came from French, lots or radicals are latin,
If you remember Blade Runner the language was a mix of english, japanese, arabic and portuguese, I never understood whay the later, but now I know, it is the fifth language in the world.
Arabic is already part of our vocabulary but we don't realise, all the musical instruments came from arabic words.
Interesting subject . . .
Heron

[John_Read]

Well, English has been around for a while, and seems to have conquered the world in much the same way as we English did a few centuries back! :D

(Those were the days, eh?)

Of course the spelling is irrational, but isn't that part of the charm? You can have plough (which is how "plow" is really spelt) cough, tough, enough; draught and draft; tyre to differentiate tire as in getting weary from the thing you put around a wheel; one could go on ...

However, as the nation whose native tongue this wonderful, powerful, all-conquering language is, we reserve the right to pontificate on orthography until the coughs come hoame! :D

[C. Beaty]

Yes, we do have an interesting mixture.

When in the fields; sheep from the Angles and Saxons; on the table, mutton from the Norman invaders.

The Normans were Danes and Norwegians who settled in Normandy and adopted French language and customs.

Our Latin based words came not from the Romans who left little except the ruins of Hadrian's Wall and a few buried coins but from the Angles and Saxons.

[birdy]

Dunno wot you blokes are onabout,carnt workout wot your say'n.
If we all spoke proper inglish there wood be no argu'n.

[John_Read]

Too right mate!