Thrust line vs center of mass

giro5

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fara, this is another one I will have to remember " It doesn’t suit your age nor is it civil." Reminds me of one my boss said many years ago - a person can be unaware or unconcerned - translation ignorant and a dumbs__t.
 

jm-urbani

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I've been away from computers for a few days.

Fara, you don't know me. I don't know you. I do, however, know Raghu.

As a newcomer to this activity, you may not be aware of the tests we conducted at Bensen Days a number of years ago on the airspeed actually experienced by gyro horizontal stabilizers. I designed the experiment, and got several of the guys to lend us gyros and help with testing and recording. I contributed an article detailing the setup, and our results, to the PRA magazine (back when there was one).

The highest airspeed over the H-stab is located at about the 2/3 prop radius point. This airspeed, in a full-throttle static test, varies from a low of about 80 mph for a slow-flying Gyrobee, to over 100 mph for a faster gyro such as an RAF or powerful MAC-powered Bensen.

The propwash tapers down from the full prop diameter, right at the prop, to about this 2/3 diameter by the time it reaches the H-stab; the exact amount of taper is a function of the ratio of slipstream to freestream speed (the taper is less at higher gyro airspeeds and greater at low ones). Especially on gyros with bulky components mounted on the engine block near the prop, there is a significant central "dead air" cone inside the propwash.

Certainly not after that test, but even before it, I would not, did not, and do not demand that a H-stab be centered on the prop hub. If it is to counteract HTL, however, it shouldn't be a full prop radius away from that center, either, because of the tapering effect I just mentioned. At a full prop diameter, the slipstream speed over the HS is little more than the freestream speed. Look up my article for numbers on the three gyro models we tested.

Despite the "dead air" zone, there's an advantage to placing the H-stab near the center of the slipstream, however. A more-or-less centered airfoil can be used to counteract the reaction torque created by the prop, which can roll a gyro in a zero G situation. Cierva developed this technique. Watch the (rare, but they exist) films of gyro PPO's, and you'll notice the craft executes a combined pitchover and rollover. Prop torque is over 100 ft.-lb. at full throttle in even in a small gyro with a redrive. Rolling to inverted is no healthier than pitching to inverted.

No, I do not posit that a heavy, high-powered gyro will need a continuous 600 lb. of thrust to stay airborne. PPO's, however, very frequently occur on climbout or coming out of a climbing pattern turn. In those flight regimes, the gyro will indeed be operating at full throttle at modest airspeed and the HS down-load needed to counteract the PPO moment will be as I described it. It's a significant phantom weight to carry unless you have a 15-foot tail boom.

I don't know the thrustline offset of a Magni; there are none in my part of the country. Based on gyros that I have measured, however, and on test figures given me by people I trust, unless the rotor is extraordinarily heavy or there something else heavy at the top of the mast, a gyro's prop thrustline needs to be at or below the crew's navel(s) for the gyro to be non-HTL. 1-2 inches of HTL is fairly easy to compensate for, however, with a properly designed H-stab.

I devised the "double hang test" in the early 90's to check the thustline location of my lowrider 503 DC Air Command. The test is based on a grade-school science-fair experiment. My gyro's prop thrustline came out about 5-6" above the CoM. This explained a lot of things that had mystified (and worried) me about this gyro's behavior. Adding the stock A.C. H-stab (located down at the 2/3 prop radius point where that good fast air is found) went a long way toward taming this gyro. Again, though, it does not neutralize the roll torque problem, and it may not be adequate to compensate fully for the PPO moment of a 532 engine and and added body pod.

A relatively slow, heavy and overbalanced rotor has a higher level of rotor damping than a light, fast one such as a Bensen rotor. This effect helps to provides a margin of safety against PPO in otherwise vulnerable gyros. I would not choose to depend on it, however.

thanks a lot for this long message, do you know if your article on the PRA magasine is available somewhere on the internet please ?
 

fara

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I won't make any Archaeology in the forum, I am waiting for the same kind of paper then Jean fourcade's one, I am sure he can do this in order to reassure every body showing that a gyroplane in Zero G at full power with a magni/mto/ela rudder can't flip up-side down on it's rolling axis
Look at the summary
 

jm-urbani

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the link does not anwser my question :

" I am waiting for the same kind of paper then Jean fourcade's one, I am sure he can do this in order to reassure every body showing that a gyroplane in Zero G at full power with a magni/mto/ela rudder can't flip up-side down on it's rolling axis"

Mr Raghu is here and will anwser this
 

Doug Riley

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This is the most revealing PPO video that I've seen.


Of interest:

The craft has a H-stab but its volume is insufficient.

The pilot porpoises a little during his fast low pass.

The airspeed at the top of the zoom climb appears not to be great.

At the top of the zoom, the pilot pushes the stick forward, but soon yanks it back. The rotor follows the aft stick command but this has no effect on the developing pitchover.

Raghu's observation that a downdraft should not lead to a PPO in a gyro with an AOA-stable airframe is correct, with certain assumptions: (1) the H-stab is of sufficient volume (area x moment arm) and receives airflow unobstructed by other components (2) the rotor spindle stays at the same angle to the frame throughout the downdraft encounter (i.e the pilot does not "float" the stick, nor does he push it forward because he has gotten behind the aircraft), and (3) RRPM loss during the event is not great.

I like immersion of a downloaded H-stab in a HTL gyro because (if properly designed) it can more completely compensate for the pitching effect of the HTL when forward stick is applied. If the H-stab is not downloaded and/or is outside the propwash and airspeed is modest, then, when the stick is pushed forward, the nose of the craft will drop, amplifying the effect of the control input until the H-stab achieves enough of a negative AOA to arrest further nose-down rotation. Obviously, if the H-stab lacks sufficient potency, it will NEVER reach an AOA at which it arrests the PPO -- instead it will stall and the result may well be as seen on the video. A number of gyros WITH h-stabs have PPOed -- though AFAIK they've had either very small H-stabs (Bensens, etc.) or they have occuured in gyros with large engines, large HTL offsets and modest H-stab volume.

All of this talk concerns pitch stability only. Roll stability is another topic. Fortunately, roll stability can be achieved not only with immersed H-stabs, but just as well with immersed vertical fins, or immersed fins at angles between vertical and horizontal. Immersing fins for this purpose is especially handy, in that, if an anti-torque incidence is built into them, the counter-torque that they continuously apply to the frame will vary directly with throttle setting (or more precisely, with slipstream speed). The counter-torque of a non-immersed fin will vary with aircraft airspeed -- not always a good proxy for throttle setting.
 

WaspAir

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all certification standards even including Part 23 have provisions for exceptions but they do not apply to something as fundamental as longitudinal stability. Sec T longitudinal stability requirements are more stringent than Part 27 or CAR 4. The type certificates Gyroplanes in the US from the 60’s may not pass those requirements. Though that’s an educated guess only on my part.
My guess is they would pass. Calculations I did years ago suggested that the J-2 and 18A are both CLT, plus they have generous horizontal stabilizers well back, plus articulated rotors that are relatively insensitive to low-g compared to the Bensen derivative style teetering systems. I recall no reports of any PIO or PPO in them. In flight, they are as pitch stable as any Cessna.
 

fara

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My guess is they would pass. Calculations I did years ago suggested that the J-2 and 18A are both CLT, plus they have generous horizontal stabilizers well back, plus articulated rotors that are relatively insensitive to low-g compared to the Bensen derivative style teetering systems. I recall no reports of any PIO or PPO in them. In flight, they are as pitch stable as any Cessna.
I was thinking of the long term stability requirement but you are likely right. I didn't think of the articulated rotor system
 

fara

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This is the most revealing PPO video that I've seen.


Of interest:

The craft has a H-stab but its volume is insufficient.

The pilot porpoises a little during his fast low pass.

The airspeed at the top of the zoom climb appears not to be great.

At the top of the zoom, the pilot pushes the stick forward, but soon yanks it back. The rotor follows the aft stick command but this has no effect on the developing pitchover.

...

There is a little plate for HS. Yeah that won't work.
 

jm-urbani

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This is the most revealing PPO video that I've seen.

Raghu's observation that a downdraft should not lead to a PPO in a gyro with an AOA-stable airframe is correct, with certain assumptions: (1) the H-stab is of sufficient volume (area x moment arm) and receives airflow unobstructed by other components (2) the rotor spindle stays at the same angle to the frame throughout the downdraft encounter (i.e the pilot does not "float" the stick, nor does he push it forward because he has gotten behind the aircraft), and (3) RRPM loss during the event is not great.
Doug, the very problem is the word should (not lead to a PPO),

In my opinion pushing under the center of gravity in zero g situation eliminates the pitch down moment the engine thrust is responsible for when it pushes above the center of gravity

this is a certainty (unless one contests the newton gravity laws) ... objects rotate around their centrer of gravity hence on LTL or CTL gyros the engine generate no nose down moment

it is also a certainty that on HTL gyro the engine produces a nose down moment

it is not enough to say that as there are very few accidents on htl gyros the ppo2 hazard is nill, I would be reassured on this point if there was a stability study provided for each HTL gyro showing that the ppo hazard does not exists, and as far as I know makers do not issue the studies that they must have done for their HTL gyros

I personally prefer to stay on the safe side, no pitch moment no ppo, and generally speaking a prefer to get rid of a problem then to adress a problem

as for the rolling moment generated by the engine , it is easy to place the gyro on scales and to put cruise engine rpm and to adapt the tail design so that the rolling moment is reduced

I ll do it promised and I will issue a video comparing the results of a tall tail and of a classic tail and we will see .
 
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Jean Claude

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There is a little plate for HS. Yeah that won't work.
When the forward speed is almost zero at the top of the trajectory, how large a not too little plate?
 
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Vance

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I feel that safety is an important measure of a gyroplane and no doubt power push overs are dangerous.

Theory is great and people much more informed than I may be able to accurately predict what is going to happen under specific conditions with a gyroplane.

Reality is much better because there is no guess work.

The reality is that a line of gyroplanes made by Magni with the one of the highest thrust line in relation to the center of gravity has one of the best safety records in this country.

I have flown an M24 in winds that should have kept us on the ground and found it surprisingly docile.

Most of my clients passed their proficiency check ride in a Magni M16 after having been trained in a gyroplane with near centerline thrust (The Predator) and so far I have a hundred percent pass rate.

The Flight instructor I use most often for proficiency check rides flew The Predator and felt it handled very similar to the Magni. He flew to commercial standards with no demonstration and very little instruction from me.

Another gyroplane with the thrust line well above the center of gravity (RAF2000) has one of the worst safety records.

This is the problem with imagining that a single feature governs the flight characteristics of a gyroplane.

In my opinion flight characteristics of a gyroplane are the result of a complex system.Predator and Paul' Magni.jpg
 
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jm-urbani

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Dear Vance,
when a gyroplane enters in a ppo situation , it can't recover this means : death of the pilot
when a gyro get's out of it's flying envelope it can't go back in the envelope like it can be the case for a fixed wing aircraft ( for example : stall limits)
a test pilot can't go to the limit without risking it's life because there are no parachutes on the gyros
hence it is impossible to define the flying envelope of a gyro otherwise then by making a stability study and taking big margins
the fact that most of stock htl gyros have prooved to be stable because there have been zero or very few people that suffered this kind of accident simply means that if the maker does not make a stability study the clients have been the long terms test pilots ... i accept to beta test a soft but not a gyro
but I am sure that the serious makers have conducted serious stability studies they should issue for each of their machines to put an end to the discussion
 
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Vance

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Dear Vance,
when a gyroplane enters in a ppo situation , it can't recover this means : death of the pilot
when a gyro get's out of it's flying envelope it can't go back in the envelope like it can be the case for a fixed wing aircraft ( for example : stall limits)
a test pilot can't go to the limit without risking it's life because there are no parachutes on the gyros
hence it is impossible to define the flying envelope of a gyro otherwise then by making a stability study and taking big margins
the fact that most of stock htl gyros have prooved to be stable because there have been zero or very few people that suffered this kind of accident simply means that if the maker does not make a stability study the clients have been the long terms test pilots ... i accept to beta test a soft but not a gyro
but I am sure that the serious makers have conducted serious stability studies they should issue for each of their machines to put an end to the discussion
You are going to be the long term test pilot for the aircraft you designed jm-urbani and the maker did not do a stability study.

When it comes to Magni there are other test pilots I can confer with.

In my opinion there are enough Magni flying that if there was a design defect we would have seen more crashes.

I prefer a near centerline thrust gyroplane because I like how it flies, not because I think it is safer than a Magni with its high thrust line.

So far I have been able to recover from piloting errors in a gyroplane.
 

jm-urbani

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Vance, this is the reason why I have chosen a LTL configuration, unable I am to make the stability study that would be necessary in the case of a HTL configuration to make sure that the HS and all the rest is enough to protect me from a ppo

and as you say I will be the long term test pilot, nobody else will be , I am not going to sell plans or gyros, I am a pure home builder and if I was to become a stock gyro maker ( which is more then unlikely a stability study would be the minimum )

no doubt that the magni's are ppo resistant and that unless the pilot places himself in a zero G situation and pushes the stick ahead keeping full throlle at the top of the trajectory he won't get into ppo

the tests pilots and the thousands of clients have proven this in the long term : none of them have gone out of the flying envelope

nevertheless having been trained on a M16, I can tell you that a great part of the end of my training has been focused on top of ascending trajectory management multiplying the simulated engine failures in initial ascent where I was trained not to push the stick until the gyro returns on a flat attitude,

I don't think that those exercices are done for no reason and that a magni can't technically enter in a zero G situation in case of a big piloting fault, ok this is the same for every aircraft and one have to learn to fly .

Ok the trainee is not qualified on only one gyro and is suppose to fly less ppo resistant gyros ( for example poorly designed home built gyros) but trainees are trained to avoid ppo, and I don't think that instructors are doing it for no reasons.

when you said that you recovered from piloting errors, you certainly don't mean you recovered from a ppo , so thanks of god for you and us you did not get out of the flying envelope of the gyros you have flown

but without stability studies issued , nobody knows the flying envelope of the gyros we are flying

that is not to say that autogyros, elas, fara gyros etcetc are widow makers, I have never said this, no doubt that all those makers have done all that they can to get their gyro ppo resistant ..

that is only to say that getting rid of the engine generated pitch moment permit to get rid of a problem rather then addressing it ... nobody has ever heard about a ppo accident on a CTL or LTL gyro ..
 
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fara

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When the forward speed is almost zero at the top of the trajectory, how large a not too little plate?
jean if you mean what will prevent low to zero G if someone flies a parabola to generate low G. Nothing. But that maneuver is a known maneuver to create low G. It should never be done in a Gyroplane of any thrust line. Gyroplanes are not aerobatic Aircraft just as trikes are not. In both if you do a deliberate parabola, and a tumble or torque roll or more likely a combo of both starts it’s not going to be good thing. That case will likely also be fatal for a LTL Gyroplane. In fact in Florida a few years ago a Dominator had a fatal because the pilot was new and got into PIO on takeoff and flew up, down, up, and then down and done. Do I blame Dominator? No. The pilot was not ready. He unloaded the rotor. This idea that somehow the tail configuration and thrust line will save you when your rotor is gone is a figment of imagination that has never been proven. I believe the effort is to make it easier to avoid getting to that situation.

The volume of tail needed is dependent on the high thrust line offset. For instance we are 4 inch high thrust line and I can dump the stick forward somewhat quickly at idle in testing without any problem with the HS volume we have. Where adequate is enough more is going to be a bit better. So it’s a compromise in what you are willing to accept. The way rigging is setup putting abrupt power input tends to raise the nose slightly and slow you down slightly in an AR-1. However there is no guarantee of any kind when the pilot flies a parabola deliberately unloading the rotor. Without an aerodynamic surface like an elevator, good luck. It’s essentially the same question as asking an airplane designer what will happen if the airplane elevator came off at the top of the parabola. Most likely you are dead.
 
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jm-urbani

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someone who takes off on a steep ascent and suffer an engine failure and make the error to push the stick too early to regain airspeed as the ground is not far below, will not aim at getting in the danger zone , destiny will have put him in this situation

someone who suddenly panic during ascent and tries to secure his seat belt will unwillingly be in a zero G situation

someone seeing at the last moment a power line and who will pull the stick hard to pass above will unwillingly in a zero G situation

in many small airfield I had to accelerate a lot and then pull hard the stick at the last moment to pass above a row off trees , of curse I was trained to zero G situations and I did reduce engine thrust and carefully pushed the stick ahead, but with no doubt my ass was not pressing the seat anymore

in those situations we are in danger, ok training must focus on taking off with reasonable ascent rates, avoiding flying low so that there is no reason to jump over any obstable and on avoiding taking off from inadequate airfields but sheet happens and we have to be trained to pull back the throttle first and then push the stick gently even if the ground is not far below

this was the heart of my training coz in France in the 90's when gyros were not what they are now we suffered a lot of casualties , and it is not because not htl have now huge HS ( and certain like yours small high thrust offsets) that this hazard must not be adressed
 

Doug Riley

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Based on my own observation, or on detailed reports from experienced pilots whose word I trust, it is possible to throw the stick forward fairly aggressively in either a Dominator or an M-16 without causing a PPO. I did the same (not on purpose!) in my Gyrobee with downloaded, immersed stab at full throttle at 70 mph. I floated up against my seatbelt but nothing else scary happened.

So we have some data points indicating (at the very least) more PPO resistance in these models than in the Bensen in the old Japanese video. That pilot did the same thing and died. But...

Do we know where the edge is? My throttle was WOT on the 'Bee; how about those other two? Would a parabolic flight path unload the rotor longer and precipitate a PPO? Or a loss of RRPM sufficient to cause retreating-blade stall (=catastrophic flapping)? As JM says, no one wants to try this just to see what happens.

As a first step to a rigorous answer, it would be nice if a mathematically-minded person (hey, don't look at me) worked out equations to "model" these types of event. The equations would not be short. To be useful, they'd have to account for such factors as the lift curve of the H-stab, its immersion or not, its lever arm, the mass moment of inertia of the airframe (besides its plain old mass), aircraft airspeed, the thrust and slipstream speed of the prop in flight at the given airspeed, the lift curve of the rotor and the rotor's rate of compliance with cyclic inputs. In the downdraft scenario, the speed and direction (maybe even the velocity gradient) of the downdraft would also be relevant.

Obtaining the raw data to feed into such a model would be a project in itself. Rotor makers, quick now, what's your rotor's compliance rate? [sound of crickets]. Airframe makers, what's the thrust that your powerplant applies to your airframe at airspeed X at WOT (you know of course that it it can be either higher or lower than static thrust)? ...Didn't think so.

My point? Without data and analysis, we don't know what we don't know. We don't know how close to the edge Aircraft X is. Given this level of knowledge (or rather un-knowledge), keeping the thrustline near, at or slightly below the CoM represents a commonsense, abundance-of-caution approach.
 

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A few months back, someone suggested flying some gyroplanes using remote control.
That would allow the finding of 'the edge' w/o undue risk.
Accuracy would seem to be high as it was all empirical data, not 'calculated'.
But...…. you'd have to crash at least one of each type of aircraft unless you were confident in applying the results to others.
Great discussion.
Brian
 

Brian Jackson

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A few months back, someone suggested flying some gyroplanes using remote control.
That would allow the finding of 'the edge' w/o undue risk.
Accuracy would seem to be high as it was all empirical data, not 'calculated'.
But...…. you'd have to crash at least one of each type of aircraft unless you were confident in applying the results to others.
Great discussion.
Brian
I was remembering the exact same thing... the RC tests. I was also wondering if this could somehow be a cooperative effort with the University Student projects the PRA has organized. I wonder if GyroPedia / Phil Harwood would be interested in being involved in such a study.
 
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