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In my opinion based on my experience a gyroplane can never have too much tail Jean Michel.

My favorite gyroplane (the American Ranger) has a huge vertical stabilizer and rudder placed well back from the center of gravity. The AR can do things easily that may be a challenge with some other gyroplanes.

I feel placement has much to do with how well the rudder works.

I prefer a vertical stabilizer and rudder to a full flying tail.

I feel weight is the only penalty for building a large empennage.
 

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Make a scale drawing of your gyro’s side profile on cardboard and cut out with scissors or whatever is convenient.

Balance on pencil point and the balance point is approximately the aerodynamic center of your gyro (technically, it’'s the “centroid of area”).

The aerodynamic center should be well behind the CG. If not, you need more vertical tail.

For best propeller torque compensation, the vertical tail should span the propeller disc.

Be careful if using a full flying vertical tail rather than a fin-rudder combination. If a full flying vertical tail is used, it must have a proper anti-servo tab; otherwise, it will try to swap ends if feet are removed from the rudder pedals.
 
jm-urbani;n1125719 said:
thanks Guy's, understood
and what about the Horizontal stab, jean fourcade told me to put it sligthly under the prop center of rotation, I trust jean and will certainly do it but what are the pro's and con of fitting the hs at the same level then the prop center of rotation/vs sligthly bellow ?

I prefer a horizontal stabilizer below the propeller thrust because I have found it is less affected in power off descents.

I feel the idling propeller may block the airflow to the stabilizer reducing its effectiveness.

It may need to be a little larger to work well not being in the propeller blast.

I am not an aeronautical engineer so I am speaking from flight experience rather than education Jean Michel.
 
In the gyro land of make-believe, Jean Fourcade is one of the very few real engineers.

Tail surfaces are roughly 2x as effective when located within the propeller slipstream. The reason for locating the horizontal stabilizer slightly below the propeller center is that the slipstream velocity is higher than when on exact center.

Propeller slipstream velocity is the vector sum of propeller induced velocity and free-stream velocity. If propeller induced velocity is 50 mph and free-stream velocity is also 50 mph but at a yaw angle of 10 degrees, the resultant propeller slipstream* is ~100 mph at a yaw angle of ~5 degrees. Since the lift produced by an airfoil varies as the square of velocity, the effectiveness of tail surfaces located within the propeller slipstream is 2x as great as when located outside of the propeller slipstream. (4x the lift at ½ the angle = 2x as much)


*Simplified trigonometry
 
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I like this one....
 

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Normally, an all flying tail is designed with its hinge line set a bit ahead of its aerodynamic center so that it trails downwind feet off the pedals; in which case, you have something that tries to swap ends “feet off.” It is also a bit wobbly “feet on.”

That’s the reason an anti-servo tab is critically important in such applications. If you don’t understand this, stick with fin-rudder tails.

tail.JPG
 
In my experience, a H-stab in the exact centerline of the prop disk does, in fact. lose its effectiveness when the throttle is closed.

Any H-stab that is energized by the prop blast will lose SOME effectiveness when that prop blast disappears. This problem gets worse, however, if the remaining free-stream flow over the H-stab is turbulent (a.k.a. trash air). The centered H-stab is likely right in the middle of the turbulence cased by the upstream portions of the aircraft, especially the engine. A H-stab set below (or above) center may receive better flow, power off.

I believe that a H-stab mounted down on the tail tube completely misses the opportunity to receive fast airflow from the prop. The slipstream coming off the prop is significantly smaller in diameter than the prop itself any time the aircraft is moving much slower than the slipstream (which is it usually is).
 
My guess is that those “designers” who opt for a keel mounted horizontal stabilizer don’t understand that the propeller slipstream is the vector sum of free-stream and propeller induced velocities, believing it is something like the exhaust from a rocket engine. yaw.JPG
 
C. Beaty;n1125748 said:
My guess is that those “designers” who opt for a keel mounted horizontal stabilizer don’t understand that the propeller slipstream is the vector sum of free-stream and propeller induced velocities, believing it is something like the exhaust from a rocket engine.

My preference for a keel mounted horizontal stabilizer is based on flight experience with various designs and is not influenced by a misunderstanding about what a propeller slipstream is like.

I am not a gyroplane "designer" or a trained gyroplane test pilot.
 
Poor Cierva; without Vance to guide him, centered both vertical and horizontal tail surfaces in the propeller slipstream.

Vertical tail surfaces were equally disposed in the propeller slipstream, eliminating throttle-yaw coupling.

Horizontal tail surfaces were also centered in the propeller slipstream with differential incidence to balance propeller torque.

Cierva C-40.jpg
 
Vance;n1125752 said:
My preference for a keel mounted horizontal stabilizer is based on flight experience with various designs and is not influenced by a misunderstanding about what a propeller slipstream is like.

I am not a gyroplane "designer" or a trained gyroplane test pilot.

Very true ,Vance.

I feel my short tails and keel mounted stab fly's as good if not better than a tall tail with center stab.
True, there is a better possibility for slow speed bunt over, but the difference is minimal and both systems are capable of a bunt over.
I do test fly and design,I've learned that ALL gyros have an envelope of safety that the operator should be aware of that.
Seems as though there are many successful gyro designs that do not have a center stab and tall tail.
Hey, but what do I know, I'm a hit and miss, pot smokin, gyro buildin swamp billy with the I.Q. of a squash !!
 
There usually is more than one way to skin a cat and absolutes are rarely found in aerodynamics. However, there are tried and true solutions which are known to work and deviating from them requires that you either know what you're doing or carry a good life insurance.

Test flying a gyro will allow you to discover many design flaws if you know how to test fly and what you're looking for. But some dangerous habits will escape even rigorous flight tests simply because one can't go right to the edge of the envelope in all cases. It's simply too dangerous.

If I were to design a gyro I would definitely use a HS with a differential angle of incidence between the left and right halves. I would also center it on the prop. I would make it wide enough so that it samples enough "clean" ear on the outboard side so that it remains effective when the engine is stopped. With these measures you will compensate prop torque nicely while also having enough downward authority to compensate any pitching moment due to a vertical thrust line offset.

This alleviates the need for a tall tail to compensate prop torque. But I would still use a split (stabilzer-rudder) configuration simply for stability. I am also a fan of having a healthy clearance between rudder and the ground. It allows me to come in and touch down in a very nose high attitude which makes for lower energy landings. It's for this reason that I don't like Magnis. You have to land them almost in a three-point touchdown, like a taildragger.

But that's by no means the only way to go about it. And, beware, I am an experimental physicist and not an engineer.

Greetings, -- Chris.
 
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Short vertical tails, dipping in the lower ½ of the whirlwind coming off the propeller get swatted in whichever way the lower ½ of the prop is moving; requiring the rudder pedal toe dance during the takeoff roll.

Once mastered, nothing to it; most people may not even be consciously aware of performing the toe dance; it having become a reflex. The same may be said for riding a unicycle.

The problem is that throttle-yaw coupling may lead to rollovers for beginners who haven’t yet acquired the skill.

The first versions of the Cierva C-30 didn'’t even have a rudder since the airfields of that era were just that; open fields so that takeoffs and landings could always be directly into the wind.The vertical stabilizer of the C-30 was carefully proportioned between dorsal and belly fins to eliminate throttle-yaw coupling.

Even so, with takeoffs and landing always directly into the wind, most would have ground looped with fixed vertical tails dipping in the lower 1/2 of the propeller slipstream.
 
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jm-urbani;n1125765 said:
what is a slow bunt over please ?

I suspect Jake is referring to a power push over at low speeds.

In my opinion aerodynamic force is related to the square of the velocity of the air passing over the control surface.

I feel a horizontal stabilizer dampens pitch divergence and in the case of a gyroplane with the thrust line offset from the center of gravity it counters the torque created by that offset and thrust.

At low indicated airspeeds to manage this torque a horizontal stabilizer needs to have a larger volume to be effective (often defined as area times the distance from the center of gravity to the 25% chord line) than at high speeds.

Without thrust there is no pitch toque created.

In my experience when a pilot adds power at low speed in a high thrust line gyroplane it pitches nose down and a horizontal stabilizer with reasonable volume helps to counter this pitching moment.

A horizontal stabilizer placed in the propeller slipstream sees an increase in airspeed from the thrust of the propeller and it helps to counter the nose down pitching moment.

If this force is not countered what is commonly called a power push over or a bunt over may occur and it may be a non-recoverable event with the aircraft pitching nose down beyond the rotor control limits and entering a forward tumble.

In this case from a theoretical standpoint a horizontal stabilizer centered in the propeller stream would be more effective for a given size. I have not tested this because I feel going to full power in a nose down attitude at slow speeds is a bad idea.

This is why I teach"recognition and recovery from low airspeed and high rate of descent" (an FAA mandated situation) as; lower the nose to pick up airspeed and raise the nose before adding power. I feel when power is added becomes less important with a low thrust line or near centerline thrust gyroplane. I am teaching people to fly all gyroplanes and I don't want them to develop bad habits.

In my opinion minimizing thrust line offset and having a horizontal stabilizer with enough volume is a reasonable way to manage power/pitch coupling in a gyroplane.

I have a challenge with focusing on one aspect of gyroplane aerodynamics because a gyroplane operates as a system and there are compromises.

The gyroplanes I have flown with a horizontal stabilizer centered in the propeller stream were low thrust line and had a full flying rudder. Pulling the engine to idle the nose pitched down and pitch stability felt vague to me because I am used to a horizontal stabilizer with more volume and a gyroplane closer to centerline thrust. The power/pitch coupling was not dangerous; it is just not what I prefer.
 
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I think Vance is on to something when he says; “In my opinion, water flows downhill.”
 
No CB:
I was taught, Hot on the left, Cold on the right, and $hit runs downhill.
 
Gyro28866;n1125788 said:
No CB:
I was taught, Hot on the left, Cold on the right, and $hit runs downhill.

But that statement is well known scientific fact, not merely opinion.

It is also one of the questions on the test for obtaining a journeyman plumber’s certificate.
 
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C. Beaty;n1125768 said:
I think Vance is on to something when he says; “In my opinion, water flows downhill.”

It is my observation C. Beaty that you sometimes get caught up in the arrogance of "facts" and lose sight of how things actually work as a system or how they interface with people.

I have not personally measured the effectiveness of a particular horizontal stabilizer in a particular location so I don't know how it changes with airspeed or how the rest of the aircraft affects it. I feel that makes my conjecture an opinion.

It is my observation working with people who understand aerodynamics on a much higher level than you or I do; they use wind tunnels to verify their opinion and are open to understanding the weakness of "facts" when it comes to application.

I am not a plumber and I have not articulated an opinion on which way water flows. I have found the flow of water may be affected by much more than "downhill".
 
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You may be right Vance, that the downhill flow of water is only an opinion and not a scientific fact.

Perhaps down under in the Land of Oz where everything is upside down and people drive on the wrong side of the road, gravity is backwards and water runs uphill.
 
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