Still trying to understand

Sir Real

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In reference to the ever-popular thrustline question, would a gyro with a low thrust line and insufficient stabilizers have a potential to do a Power Push UP? If a PPO can start a PIO cycle, would a PPU have the same tendency? Also, would a PPU dangerously reduce rotor load?

Kurt Franz
 

C. Beaty

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With CG well above the propeller thrust line, the rotor thrust vector trails the CG by a significant amount.

A reduction of rotor thrust causes a nose-up response, tilting the rotor noseup and restoring rotor thrust.

I don’t know the limits to LTL; Glasgow recommends an offset of no more than ±5 cm in either direction but a Dominator violates this rule by a fair amount with no ill effect.
 

Doug Riley

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The situations of HTL and LTL aren't mirror images of each other. The behavior of a gyro on each side of CLT is different.

Low G (= low rotor AOA) conditions are the ones that require close design attention in gyros. The reason? Unlike a FW plane, persistent or worsening low G in an autorotating rotor becomes irreversible after you pass a certain point. You can low-G (or reverse-G) a fixed wing all day and yet the minute you reapply pos-G, the wings flies on as if nothing had happened. That's not true of gyro rotors.

Therefore, we want the machine to resist getting into low G, and to try to get out of it ASAP. Low prop thrustline does precisely these things. As G is reduced, the prop thrust causes the nose to rise, tending to restore positive rotor angle of attack and therefore positive G. HTL perpetuates and worsens any low G situation.

Can the LTL design tactic be overdone? Yes. Mildly over-doing it results in a machine that must be trimmed rather actively as you change throttle settings. The gyro speeds up when you cut power and slows down when you add power. To hold airspeed through your power changes, you'll need to use either in-flight trim or a change in your stick pressure as you work the throttle.

How about severe LTL? It will cause large overshoots in rotor angle of attack with power changes. Therefore a power "chop" will cause the nose to drop radically (the rotor thrustline behind the CG will pull the tail up). This is unlikely to cause the craft to pitch all the way over, because all the time that the nose is dropping, the rotor is losing angle of attack, and with it the ability to pull the tail up any further. This process will at least result in a loss of RRPM, however, which is not helpful.

With radical LTL, something similar (excessive nose drop) is apt to happen in a thermal updraft.

Again with radical LTL, in a downdraft or with application of hard forward stick, the rotor will lose thrust and the low prop thrust will then cause a sharp uncommanded pitch-up. With any amount of airspeed, this will just re-load the rotor. At zero airspeed and wide-open throttle, there's a possibility (on paper anyway) of the machine going over on its back if it has insufficent H-stab power.

The moral is that large, continuous moments about the gyro's CG should be avoided -- including both extreme LTL and HTL and airframe centers of pressure far below CG. An effective H-stab is a buffering or "calming" device in all these cases.
 

gyroplanes

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>>I don’t know the limits to LTL; Glasgow recommends an offset of no more than ±5 cm in either direction but a Dominator violates this rule by a fair amount with no ill effect.<<

I did the math, it's just a few thousandths short of 2 inches. I'd like to thank: http://www.worldwidemetric.com/metcal.htm and my Aurora Big number calculator for their help.
 

karlbamforth

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Which is the reason why the UK CAA now insist on +-2" from CLT thrust on all new built (single seat) gyro's. Alternatively you can demonstrate stability, easier to go CLT +-2"

They get a lot of stick about it but will probably be seen as a good thing in years to come.
 

Doug Riley

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A 2" thrustline offset is well within the ability of a 5-6 square foot H-stab to provide a countering force without exceeding 2-3 degrees of angle of attack. IOW, it's a nice, conservative and realistic number.
 

Gyro_Kai

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As a tid bit of information: The German MT-03 has an HTL of more than 3" but the British test pilot found the dampening effect of the large H-stab sufficient and granted approval for this model in GB.

Kai.
 

Doug Riley

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Damping (remember "dampening" means getting something wet) is a separate issue from positive static stability of the airframe. A very small H-stab will produce noticeable damping (and hence more pleasant handling) without actually curing static instability of the frame.

PPO is a problem of static instability of the airframe, not inadequate damping. There is no reason why a 3" thrustline offset cannot be fully compensated by a practical H-stab. The test for this accomplishment, however, will be different than it would be for damping.

Apples and oranges.
 

bpearson

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Which is the reason why the UK CAA now insist on +-2" from CLT thrust on all new built (single seat) gyro's. Alternatively you can demonstrate stability, easier to go CLT +-2"

They get a lot of stick about it but will probably be seen as a good thing in years to come.
I presume the reason they allow a higher than 2" offset on two seaters is that the pilot will spend the first twenty hours training with an instructor in the back. A 50 hour pilot can fly single seaters with a greater than 2" offset but Chuck has said many times that given the right (wrong) conditions a loss of rotor thrust can cause a bunt in a HTL machine in a split second.

It could be argued that the only differance between single and two place machines is the number of fatalities. However, the current crop of tandem gyros (mt, Magni) have a greater than 2" offset but don't push over ! They may on paper but not in reality.

Rumour is that flight testing on long established Benson clones (small prop) has shown acceptable stability, but it could be argued that weather conditions were not severe on a particular day. History and tens of thousands of hours of trouble free operation have proven a safe a machine in many peoples eyes.

So we have a very confused situation here in the UK. If machines are deemed acceptable through flight testing then what was the point of the Glasgow report. Why not just use flight testing ? But, the more I read the less faith I have in the American ATSM tests.

Of course the obvious solution would be to let the individual decide on what he/she chooses to fly. Seems to work in the rest of the world OK. But of course theres no jobs in that route.
 

Doug Riley

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Determining the size and incidence of a horizontal stabilizer (or "stabiliser," if you prefer your beer warm) sufficient to eliminate PPO in a HTL machine is matter of grade-school arithmetic.

The reasons for the continuing, unnecessary complication of this extremely basic issue escape me.

One is left to conclude that it's some combination of politics, bravado and plain ignorance.
 

C. Beaty

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But Doug, don’t you know that jacking up a gyro completely spoils its appearance?

“Don’t you just love that low-slung Ferrari look? Ought to hold the road like it’s on rails.”
 

karlbamforth

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It could be argued that the only differance between single and two place machines is the number of fatalities. However, the current crop of tandem gyros (mt, Magni) have a greater than 2" offset but don't push over ! They may on paper but not in reality.
What is the thrust offset of an MT, Magni Brian ?

The VPM16 was shown to be close to the 2" offset for most of the flight envelope.

I believe that the Benson was designed to be very close to CLT. Its the so called clones that are not.
 

Doug Riley

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Sure, I like the low-slung look as much as the next guy. I have legs like a giraffe and have trouble getting out of extremely low-slung vehicles, though. I can get onto the seat of my Dominator with a bottom-first hop of a couple inches; my lady friend's low Subaru wagon is more trouble for me.

Enough chatter. The point I was fuming about was that, if you must have HTL because you want an airplane that looks like a go-kart, fine. Compensate for the maximum nose-down moment that your HTL is going to cause with a H-stab of approriate size, shape, position and incidence. Just don't tolerate a potential PPO, doing nothing about it, for the sake of looks or supposed entry-exit ease.

If you can't do the arithmentic involved in this relatively simple design task, you are LETHALLY under-qualified to be designing anything that flies. Either bone up or get someone who knows (a technical person, not some supposedly "crackerjack" pilot) to assist you.
 

bpearson

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Enough chatter. The point I was fuming about was that, if you must have HTL because you want an airplane that looks like a go-kart, fine. Compensate for the maximum nose-down moment that your HTL is going to cause with a H-stab of approriate size, shape, position and incidence. Just don't tolerate a potential PPO, doing nothing about it, for the sake of looks or supposed entry-exit ease.

If you can't do the arithmentic involved in this relatively simple design task, you are LETHALLY under-qualified to be designing anything that flies. Either bone up or get someone who knows (a technical person, not some supposedly "crackerjack" pilot) to assist you.
To loosely quote the Glasgow report Doug.

"The size and location of a Horizontal stabilizer has little affect on longitudinal stability". Maybe they should go back to school ?

My guess is that we would probably all go the CLT route given the freedom to choose but when I approached our authorities with the Dominator it was pointed out that the nose wheel would fail the sec T tests for starters !

Until someone "double hangs" a Magni Karl I remain a sceptic about the v c of G location. I think the MT people have and it came out at 3" worst case scenario.

Until we started putting 5ft props on Bensons without any other modifications and importing Air Commands the record of gyroplanes in the UK was good. The one machine that sticks with the Benson small prop has an impeccable safety record.
 
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Harry_S.

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But Doug, don?t you know that jacking up a gyro completely spoils its appearance?

SPOT ON...I AGREE.


?Don?t you just love that low-slung Ferrari look? Ought to hold the road like it?s on rails.?


Chuck, I do believe you and I are getting closer in agreement. My machine performs just about the way you described. ;)

Why not ride with me in my machine for a nonsensical S/L fly?! I promise...no shenanigans.

COME FLY WITH ME................


Cheers :)
 

karlbamforth

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Brian,

Sect T is readily available, work out what your gyro of choice needs. Speak to the manufacturers, they may have info or modifications to help.
Poll ppl here for further info then approach the CAA and either meet the sect T, demonstrate why your gyro of choice doesn't need to meet sect T or get a dispensation.

The CAA will not guess what you want, its up to you to go do it Brian.

If you approach the CAA with a sensible proposal and the evidence to match then there is a good chance you will get what you want.

I work with the CAA 7 days a week, if you approach them with a sensible idea, the right information and have scientifically proven facts, they will help you. Its time to stop the "them and us" culture, and work together for the benefit of all aviators.
 

C. Beaty

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Thanks, Harry, but I already know how a HTL gyro flies; statically unstable vs. angle of attack.

Unless the stick is floated, it will pitch nose up in an updraft and nose down in a downdraft.

The Bensen rotorhead provides an illusion of stability if allowed to float.
 
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