Pitch Control Benefits of Elevators for Autogyros in Low-Speed Forward Flight

kolibri282

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JJ Campbell

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Would the elevator control be linked to the cyclic or a separate control? If linked, would it be live all the time or engaged as needed?
 

Jean Claude

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"Autogyro pitch control is usually exercised by adjusting the main rotor tip-path-plane. However, in the lowspeed flight regime, all available rotor tip-path-plane tilt back is expended to maintain lift, and nose-up pitch control is handicapped."
Is the author is saying that when the steady flight is slow, the position of the control stick is close to the backstop?
Then this is a false starting hypothesis, in the simplest case of Centered Thrust Line.
And it's the opposite when LTL !
With false datas, the correct results are only obtained by error
 
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Doug Riley

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J.C. is correct, of course. A gyro uses only a fraction of its longitudinal cyclic control range in flight. Bensen-style gyros have a very large (30 degrees aft) tilt limit. This limit is seldom, if ever, reached in flight, and not usually even in landing. Bensen provided this large tilt to assist with starting the rotor in the absence of a powerful (or ANY!) prerotator. Air & Space and McCulloch gyros, both with full-RPM prerotators, have less available aft tilt, I believe, without creating any control problems.

Someone should ask Jim Vanek if he pulls the stick all the way back to the stop to execute his loops. I doubt it.

Whether you use elevators, a swashplate or a tilting spindle, in the end you still change the rotor's orbit via cyclic pitch change. The interesting things about elevator pitch control in gyros are that (1) the airframe LEADS the rotor in changing its angle of attack, rather than following it, and (2) elevators work equally well in all G-loading situations, while direct-tilt systems lose their power to control the airframe at low G. The second issue is the root of the problems with low-G control of gyros that have direct cyclic control and centered flapping hinges. One function of the H-stab on a direct-cyclic gyro is to make sure the frame follows a stable flight path during time intervals when there is little or no pitch control, thanks to low/zero G.

PPO is an extreme example of the frame NOT following a stable flight path during low/zero G.

The great weakness of elevator control is that it loses effectiveness at low airspeeds. Immersing the elevator in the prop blast helps, as long as the engine is running. Control will still be poor during engine-out flight, though.
 

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This is an interesting topic.
If I understand the premise, the elevator is being used as the primary pitch control, not the rotor?
I agree with Doug's assessment, but I'd also like to hear some discussion on using elevator position as a TRIM control (rotor used for primary pitch, elevator trimmed for fuselage positioning in cruise).
Brian
 

Vance

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It appears to me that the basic hypothesis is flawed.

In a wind tunnel the air may continue to blow in at the same angle, in the real world if I get slow I sink and the air is still going up through the rotor even if the angle of the disk is nose down relative to the earth.

When I release back pressure flying a gyroplane nothing bad happens and she quickly picks up air speed.

I don't see a trim advantage with an elevator unless the whole control scheme was changed.
 

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Jean Claude

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Brian,
The TRIM of the airframe position vs to airflow would have no interest. It action would be marginal on the drag of the gyrocopter, or on the equilibrium of the stick effort.
 

Doug Riley

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The angle of the airframe to the airflow has some significance, even though it does not directly affect the angle of attack of the rotor disk.

First, the angle of the airframe relative to the airflow (in a gyro with direct-cyclic controls) affects the rotor head's trim-spring tension. The more nose-down the flight stance of the airframe, the less the trim spring tension. An airframe that strongly noses down as airspeed increases can actually overcome the proper functioning of the spring. This, in turn, will tend to affect stick pressures in an unwholesome way.

Second, the primary control stops are attached to the airframe. The stance of the airframe therefore affects the position of the control system within its stop limits (and therefore the position of the stick as experienced by the pilot). Again, if the airframe noses down significantly as airspeed increases, the position of the stick may actually be farther aft at high speeds than at low speeds -- a reversal of the normal stick-position gradient, and quite dangerous. My old, lowrider, no-HS Air Command behaved this way at high speeds. The subjective experience at high airspeed and wide-open throttle was of "holding the nose up" with the stick.

Of course, it's better to change the layout of the airframe (and fixed HS) than to use a flight-adjustable HS as a patch. The flight behavior I described above indicates defective design -- either a substantially high prop thrustline (HTL) or a low fuselage center of pressure (CP), or both. A properly-designed gyro with an offset-gimbal, direct-control rotor head shouldn't need a trimmable HS.
 

Alan_Cheatham

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The late Johnny Miller, who was privileged to have flown both winged and direct control autogiros, stated he preferred the winged versions with their aerodynamic controls. His concern was that a direct control rotor that is unloaded does not allow for attitude control and hinders the pilots ability to recover.

Given the safety record of the aerodynamically controlled autogiros I believe the safest configuration would be a combination of both aerodynamic and direct control, elevator (perhaps taileron) for normal flight and direct control for low speed and ground operation. Mixing to two seamlessly would be the real challenge.

How many lives could have been spared if the pilot had control-ability in a low G event.

Video of the Kay autogiro which uses rotor control for roll and elevator for pitch.

 

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My Bell 47 has a horizontal tail surface that is linked to the fore-aft motion of the cyclic and a has a trim effect; it allows the fuselage to hold a more level attitude through the speed range, so you don't wind up standing on the pedals at high speed or reclining when slow.
 

kolibri282

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The very same system that Wasp describes for the Bell 47 was adopted for the UH-1. The H-stab has a positive incidence for full forward and aft stick while the angle is negative between roughly -5 to +5 inch of longitudinal cyclic. The values are from nasa-tm-73254.
 

Jean Claude

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For the helicopter, the manufacturer want a position of the stick more forward as the speed increases. It's the exact opposite of Traum and Carter, who want a stick position less backwards at a slower speed, i.e more forward.
But it's just not necessary .
 
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I believe the Little Wing had elevator pitch control on the first model, but switched to rotor tilt for pitch later with trim tabs on the horizontal surface set to counteract torque roll. With our modern machines’ ability to land at zero or near zero airspeed, I would think a tilting rotor would give more control.
 

Doug Riley

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A tilting rotor provides better control at low speeds, but (at least with centered flapping hinges), it provides ZERO control at zero G. IOW, the aircraft becomes a ballistic object at zero G. If the airframe is unstable, this can go badly.

It's no use counseling pilots to "avoid zero G," (though that's what the Wise Men told us back in the Bensen era). Aircraft encounter zero G all the time on bouncy summer days. Zero G does not only occur because some foolish pilot throws the stick forward (though that also was part of the tale told by the Wise Men back in the day).

Elevators are one way to maintain control at zero G; the elevator doesn't care about G (or, more precisely, about disk angle of attack).

Another way is to use NON-centered flap hinges. Place them some distance outboard of the rotor's center. Such hinges allow us to aim the centrifugal reaction of the rotor, as well as aiming the blades' lift, to achieve control. Unlike blade lift, the centrifugal effect isn't lost during zero G.

A stable airframe, in any event, will coast through a brief zero-G ballistic event without a sudden diversion from a straight flight path. Control will come back, and all will be well. An unstable airframe will pitch or roll (or both) during these events, which can lengthen the duration of the zero G and result in loss of control.
 

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Doug, would you diagram "NON-centered flap hinges", please?
I'm getting a mental image of a plate/bar rigidly attached to the rotorhead with hinged blades outboard of the bar.
Not sure how 'teetering' or 'underslung' would work thusly, so I'm missing something.
Pictures?
Brian
 

kolibri282

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I think Doug is refering to the type of blade attachment used mostly in three or four bladed helicopters (although you could also build a two bladed rotor that way), which are also called offset hinges. The rotating blades will act as a large gyroscope and the offset hinges provide a lever arm by which the centrifugal blade force will maintain the rotor shaft in more or less the same spacial position. Teetering and offset hinges are mutually exclusive.
 
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Arco

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One remark, the offset hinges helps in the cero g events, because you keep some control, but not full authority. You can applied to the center of the head a moment ( not a force) that comes from the centrifugal force of the blades.

But this is right ONLY with a fix shaft, ( helicopter type head) where the rotor plane tilt by aerodynamic forces (changing the blade pitch with the cyclic), but not for controlling the rotor plane by tilting the shaft.

In our autogiros, we tilt the rotor plane, tilting physically the shaft. In this case, is totally the opposite. The centrifugal force keeps the rotor as it is. The pilot has to fight against this centrifugal force to tilt the shaft always, no matter is a cero event or not..

If this distance is cero ( and no centrifugal moment to fight against), as we have in our teetering rotors, the control forces are low.

De la Cierva do the best to minimize this distance from the flapping hinge to the rotation axis, but always was a positive distance. One the complaints of the Cierva autogiro was the HIGH force done by the pilot to tilt the rotor head.
 

kolibri282

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Quote: .....helicopter type head, where the rotor plane (is) tilt(ed) by aerodynamic forces /Quote
The rotor plane is tilted by aerodynamic forces in a gyro just the same way as with a helicopter. If you assume the controls fixed the gyroscopic moment of the rotor blades would still keep the shaft (and everything attached to it) in the same spacial position.
 

Arco

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Please note the English is not my mother language so sometime is difficult to write something with no room for the misinterpretation, so let me clarify.

The rotor plane tilting is done by aerodynamic forces, period.

But how you change the cyclic blade pitch depends of the mechanic implementation. They are different mechanical solutions to change the blade pitch.

In some solutions the shaft is fix (most of the helicopters), but in others mechanical solutions the shaft tilt, is not fix (most of the autogiros).

The flap axis blade offset is good for fix shaft mechanical solutions. The pilot has a way to control the ship even in cero g events.

The flap axis blade offset is bad for non-fix shaft. The pilot force is high due this offset.

This was my point, the offset distance goodness depends of the mechanical implementation of the rotor head cyclic control, sometimes is good, sometimes is bad.
 
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