Thrust line vs center of mass

in addition in zero G situations the HS which are not placed in the prop wash can't produce the necessary force... coz in Zero G conditions there is no more airspeed ... no air speed no lift .. ( as far as I know)
I feel you may be conflating zero Gs with zero airspeed.

In my opinion it is impossible to sustain zero Gs in a gyroplane and low Gs may be unrelated to airspeed.

NASA says: “The lift equation states that lift L is equal to the lift coefficient Cl times the density r times half of the velocity V squared times the wing area A.”

As can be seen from this simple equation as airspeed is reduced an aerodynamic control surface becomes dramatically less effective.
 
in addition in zero G situations the HS which are not placed in the prop wash can't produce the necessary force... coz in Zero G conditions there is no more airspeed ... no air speed no lift .. ( as far as I know)
There is no way you will have a zero g situation unless you encounter a very very strong downdraft. The funny thing is, a very strong downdraft will keep the rotor spinning, except all the sudden, the lift might by point down. Now you are in real trouble regardless if you have CTL or HTL.
 
There is no way you will have a zero g situation unless you encounter a very very strong downdraft. The funny thing is, a very strong downdraft will keep the rotor spinning, except all the sudden, the lift might by point down. Now you are in real trouble regardless if you have CTL or HTL.
Please explain how a downdraft strong enough to produce even momentary zero Gs would keep the rotor spinning.
Thank you.
 
a certain number of low profile European gyros were found crashed with their rudder sliced by their rotor blades , and we are talking about gyros with huge pitched down HS

fortunately it does not happened often , but the guys who died and their family don't mind statistics @ all

btw what you say and what is said here my choice is done :


sorry but btw Jean Fourcade and you my choice is clearly done

this paper says :

"It is possible to compute the time for such gyro to do a 180-degree roll over. Letus take for example a single place gyro which weights about 330 pounds. Let us suppose that the CG is I0 inches below the propeller thrust line and that thepropeller thrust is about 200 pounds. To totally unload the rotor blade you need to hit a downward gust of about 40 feet per second. This is a strong gust but itcan exist.If the pilot does not react immediately by reducing throttle, it takes less thanone second to do a 180-degree bunt over. One can understand the danger ofthis situation. "

Safety is not a matter of design compromise,

so again you did not convince me that low profiles gyros are safe even with huge HS

Jean cites a UK study without mentioning that the same UK study determined Magni M16 is close to CLT when everyone else believes that that gyro is 10 inch high thrust line. Enough said That study from the UK got answered by Finnish university researchers and also by its own CAA who certified almost all high thrust line gyroplanes under BCAR Sec T. The record of accidents there shows hardly any PPO in those machines in the UK.
Chopping off tail does not equate automatically to PPO. It means you went zero or negative G. Give me your gyro and I can write you a script to execute that will definitely take you close to zero G and chop your tail. Doesn’t matter if your engine is below the wheels.
 
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Please explain how a downdraft strong enough to produce even momentary zero Gs would keep the rotor spinning.
Thank you.
Say you are doing a vertical descent and all the sudden, and you driffed into a downdraft column of air, and the speed of downdraft air is faster than your descent rate. You might even see negative G at this point.
 
The trim drag created by fully compensating a very HTL gyro is significant in a pusher.

By "fully compensating," I mean down-loading the H-stab enough to prevent any nose drop when the pilot commands full power. Another way of saying the same thing is that the H-stab must generate enough of a nose-up moment at any throttle setting so that the rotor thrustline intersects, or lies behind, the CoM.

Think about what this means in a heavy, Subaru-powered pusher gyro with, say a 12" HTL and a HS whose aerodynamic center is 6 ft. aft of the COM. Pick a thrust number for WOT -- likely 600 lb. or more. With the 12" HTL, at WOT the power unit imposes a nose-down moment on the frame of 600 foot-pounds.

The H-stab has a 6-foot lever arm. To counteract the 600 ft.-lb. nose-down moment, the H-stab must generate 100 lb. of down-load (600/6). This down-load adds to the total load that the rotor must support, just as if you brought along a 100 lb. bag of cement.

But it gets worse. If the H-stab operates at a L/D of 10, then its 100 of down-lift also adds 10 lb. of drag, on top of the extra rotor drag created by the need to lift the 100 lb. of phantom weight.

In a tractor gyro with a longer tail cone, the H-stab has a larger lever arm, and the down-loaded need to make 600 foot-pounds is much less. But, in designing a tractor gyro, it's not as tempting to build a low-rider in the first place. You want the pilot centered right behind the prop for streamlining purposes, not below it.

Are you telling me that you think the thrust of this gyro at say a steady state S&L non accelerating cruise of 70 mph is going to be 600 lbs? Are you sure about that?
 
As an aside, if one unloads the rotor in flight how long is it before the rotor speed would decay to an unrecoverable speed ?
 
As an aside, if one unloads the rotor in flight how long is it before the rotor speed would decay to an unrecoverable speed ?
You don't need the rotor to slow down to get yourself in trouble. On a HTL gyro, simply unload the rotor, (unless you are at idle) will flip the you over. If you know the thrust generated by the prop, high school physic can tell you how fast you can flip 180 degrees.
 
You don't need the rotor to slow down to get yourself in trouble. On a HTL gyro, simply unload the rotor, (unless you are at idle) will flip the you over. If you know the thrust generated by the prop, high school physic can tell you how fast you can flip 180 degrees.

Not quite that simple.

When in 2014 I came from the airplane and trike world and was trying to learn about gyroplane and their behavior and make sense of them, this same stuff was being discussed. Raghu was one of the members who discussed it. He is a research scientist at a US university (I think in Pennsylvania).

People should read his posts


and this one where Raghu presents a document with analysis if you are the type to go through and understand them and to do that you have to read all of Raghu's posts (download his derivation of tail volume document) and read his answers to JC and Doug R. to correct their misconceptions that lead them to wrong conclusions. Like Doug R contention that immersing the tail in prop wash will do some significant magic and immersion to him seems to mean that HS has to be right at the hub. First vertical placement of HS is not so important. Presence of it is 90% of the solution. Second every engineer working with prop based aircraft knows that flow of the air in a prop disc has a void close to the center. The best flow is usually 60 to 70% of the blade span point at the bottom not at the center. Yet these myths continue to perpetuate with oversimplification for the uninitiated because they will be easier to make sone tech sense. That sounds good but it does not actually work


Just because of persistence and presence of people married to this analysis, this idea that HTL is definitely unstable about to fall out of the sky like flies on any transient gust sustains in this forum even though their horizontal stab gives them effective AOA stability unlike HTL gyros like RAF without a HS. If that was the case Magnis which by far are the highest thrust line gyro models should be falling out of the sky. How long do they have to go without that to prove what Raghu already calculated? Its been 25 years with thousands of units flying. Theory if not done by high school Physics but with deeper understanding predicts this same result. Theory and math agree with real life data. Its only some on this forum who do not and they are the champions here for amateur designs. Pretty amazing and somewhat comical.
 
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As an aside, if one unloads the rotor in flight how long is it before the rotor speed would decay to an unrecoverable speed ?
You don't need the rotor to slow down to get yourself in trouble. On a HTL gyro, simply unload the rotor, (unless you are at idle) will flip the you over. If you know the thrust generated by the prop, high school physic can tell you how fast you can flip 180 degrees.

I want to add that the rate of PPO also depends on mass distribution. The further the mass from the center of mass, the slower it happens.
angular acceleration = touque / sum of all moment of initia

in case of hard turbulences flying level all you can do is to leave the stick stil and to reduce engine thrust when you feel the pressure on your but getting light.
Yeah. The slower you fly, the less blade flapping; and the blades do flap in a vertical descent.
 
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I want to add that the rate of PPO also depends on mass distribution. The further the mass from the center of mass, the slower it happens.
angular acceleration = touque / sum of all moment of initia


Yeah. The slower you fly, the less blade flapping; and the blades do flap in a vertical descent.

It's probably a typo: the blades don't flap in a vertical descent...
 
Aviator168 said:
There is no way you will have a zero g situation unless you encounter a very very strong downdraft. The funny thing is, a very strong downdraft will keep the rotor spinning, except all the sudden, the lift might by point down. Now you are in real trouble regardless if you have CTL or HTL.

Please explain how a downdraft strong enough to produce even momentary zero Gs would keep the rotor spinning.
Thank you.

Say you are doing a vertical descent and all the sudden, and you driffed into a downdraft column of air, and the speed of downdraft air is faster than your descent rate. You might even see negative G at this point.

A gyroplane in un-accelerated flight is at one g. Remover the lift and it falls at 32feet per second per second and would be at zero gs.

How would this downward acceleration keep the rotor going in this extreme downdraft you describe?

How would the down draft push down on the rotor to reach negative Gs?

I feel it is important to understand how a gyroplane rotor works and the limitations of a gyroplane rotor.

It is my observation that a gyroplane rotor will begin to slow down at anything less than one G.
 
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At one G without lift an object will accelerate downward at thirty two feet per second per second.

How would this downward acceleration keep the rotor going in this extreme downdraft you describe?

How would the down draft push down on the rotor to reach negative Gs?

I feel it is important to understand how a gyroplane rotor works and the limitations of a gyroplane rotor.

It is my observation that a gyroplane rotor will begin to slow down at anything less than one G.

An object in free fall is not 1 G, it is 0 G. A gyro in vertical descent is 1 G as in most gyroplanes, at a constant rate of about 1500 to 2000 ft/min. Now if you have a downdraft faster than that speed, the blades will be in negative AoA and you will in less than 1 G and if the downdraft is strong enough, you might even be in negative G. I will draw a picture later when I have time.
 
I agree that was poorly worded.

I should have written a gyroplane in un-accelerated flight is at one g. Remover the lift and it accelerates downward at 32feet per second per second and would be at zero gs.

The point I was trying to make is if a gyroplane accelerating downward at less than 32 feet per second per second is seeing more than zero gs.

In my opinion it is unreasonable to imagine a down draft fast enough to reach sustained negative gs outside of a microburst.

In my experience a rotor slows down quickly if not supporting the weight of the gyroplane.

Based on information from flying with a g meter in The Predator a sustained .7 gs will slow my thirty foot eight and a half inch chord Sport Rotor blades below what the manufacturer feels is the minimum safe rotor rpm (275 rotor rpm) quickly.

I could not find a maneuver that would get me below .6 gs even momentarily.

If I encountered a strong down draft the aircraft would soon stop accelerating as the aircraft caught up with the down draft up and my G meter would again be showing one g and I would again be showing in the neighborhood of 315 rotor rpm.

In my opinion the idea of experiencing zero gs or negative gs in a recoverable attitude in a gyroplane is highly unlikely.

In my opinion the rotor slows down as soon as the rotor sees less than one g.
 

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From a practical standpoint, watching your rotor rpm after landing should give a reasonable approximation of how fast your rotor rpm decays.
 
on a LTL gyro the engine does not generate any nose down pitch moment, it creates a nose up pitch moment,

in order to fly level the pilot has to place it's stick more ahead to compensate for the nose up pitch moment, hence the Rotor Thrust vector passes behind the gyroplane centre of gravity

and as any object turns around it's centre of gravity the gyro will raise it's nose in a downdraft because the rotor lift will decrease ,

in big down drafts I have been taught to reduce throttle (my training was done on HTL gyros) in order to avoid the engine thrust to flip over the giro, this is what I am doing on my ctl monoseater

in summer in montelimar we encounter really strong down drafts when we are heading east after leaving the pattern ( result of the strong mistral north wind passing over 2 mountains ), it is really impressive ( I hate this )

I am now wondering what will I have to do in a downdraft when I will be flying my future LTL gyro ? I wonder if it is not better to leave the throttle where it is unlike what I have been taught to do on HTL gyros ?

I have flown a lot of different gyroplanes in windy conditions.

It is my observation that how the gyroplane manages turbulence is an entire system rather than a single design feature like high or low thrust line.

I have flown gyroplanes of the same model that did not handle the same.

In a down draft I may not want to descend so in The Predator (a slightly low thrust line gyroplane) I may add power.

In moderate to extreme turbulence I slow down to around fifty knots and reduce power to maintain altitude.

In most of the gyroplanes I have flown I slowdown in moderate to extreme turbulence.

In an RAF that happens to have a high thrust line in relation to the center of gravity I reduce power when I encounter a wind shear from a down draft.

In your new gyroplane you will need to find what works. It may be of benefit to employ a test pilot to get a second opinion on what may be best practice.
 
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.
 
Eurotubs are the brainchild of Vittorio Magni. Mr. Magni was a helicopter mechanic who learned how to design gyroplanes by having built a Bensen from plans.

Bill Parsons was the first Magni dealer and believed that horizontal tails were dangerous as well as causing the stiff controls. Bill believed that a vertical gust could blow the tail up into the rotor.

Magni published a service bulletin with the title in English; “Frictioining the Controls” that blamed control pivot friction for the stiff controls. I never paid much attention to that until David Bird (Birdy) in Australia flew one and said; “Where does the friction go when the gyro is sitting on the ground?”

I stuck my nose where it didn’t belong and suggested the heavy controls were most likely caused by nose heavy rotor blades. Greg Grimminger, by then the Magni agent, strongly objected; stating that he had a Magni rotor sample section and it was precisely balanced about the ¼ chord point. As it turned out, Greg had a blade sample from about mid span of the blade.

Averso, the French rotor blade builder, published photos of Magni rotorblade sections from both root and tip ends, showing a highly tapered spar and very overbalanced at the tip end.

It seems to me that most Eurotub designers are not engineers and mostly copy one another.

It is possible that the only qualified gyroplane designer on the other side of the pond is Nicolas Karaolides, designer of the Aviomania line of gyros?
 
What is this with you and Eurotubs (definitely not used as a term of endearment by you as far as I can gather) Chuck? What do you have against them. They have proven themselves by the 1000's to be stable in flight against scripted stability tests that came out of BCAR Sec T criteria. Raghu gave you the concepts and the math years ago why they are stable. You simply ignore that knowledge like it does not exist. As far as judging Magni or Nicolas. Its just your opinion and I am well aware of your biases which seem to IMHO not add up when placed against the reality of stats of PPOs in Magni or others with HS nor do they align and make a compelling case against Raghu's math or ideas. If you have the data to counter that, I would be happy to hear or see it.

Doug: Yes I know and I think I stated that prop blast is most between 60 to 70% (or 2/3 radius) as you found out. It depends a little bit on the blade design but generally all will fall within this range. The reason I wrote that is your insistence on placing tails immersed in prop stream. Well the tails like MTO, AR-1 and others like it are exactly around 70% of blade radius. But if your preference is for cruciform tail close to the prop and that to you is a superior tail overall, well so be it. Nothing wrong in that.
 
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