Gyroplane CLT

GrantR

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I hope the following makes sense and is correct. This is the way I see how CLT is important and how some people may misinterpret its important in gyroplane design and how some may falsely think a HTL machine is stable due so some of the flying characteristic it may have.

When a Gyroplane is in normal steady flight the rotor is loaded and it is holding up its own weight and the weight of the airframe. The weigh of the rotor however is separate for the weight of the airframe. The weight of the airframe hangs below the rotor by the control hinge bolts therefore the airframe is like a pendulum or a swing hanging from a limb unless the pilot has a death grip on the stick and has it locked in place.

So with that being said, any additional thrust added from the engine in this steady state of flight would cause the airframe to swing forward regardless of the engine thrust line since the airframe is rotating around the pivot point in the control block. This would appear to be stable since this is a nose up tendency with power increase and a nose down with power reduction.

This can be though of like a pusher vs tractor airplane. Pushers have the thrust about the lifting force which causes a nose down pitch with power increase and tractors have the thrust line below the lifting force which causes a nose up pitch with thrust increase.

Of course a gyroplane will never have the engine thrust above the rotor so no downward pitching in stable flight.

Since the rotor supports its own weight when it is producing lift the rotor weight is not relevant and neither is the vertical cg during steady flight.

Now all this changes when the rotor loses lift and gets unloaded. If the gyroplane experiences 0 or negative lift the rotor quits pulling on the airframe at the pivot point(no lift no pull) so the engine thrust is not acting around this pivot anymore. The vertical cg of the airframe changes or moves up the airframe as the weight of the rotor is added since it is not being supported by lift. In this state the gyroplane airframe and rotor become one as they are essentially floating along. Now this is the critical point where CLT is important. If the thrust line is high, engine is producing a lot of thrust and there is little to no rotor lift the gyroplane will rotate around its vertical cg and bunt over.
 

gyromike

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Not even close Grant.

Gyroplanes are not pendulums hanging from the rotors. They don't swing from the gimbal.
They rotate about the CG of the aircraft.

Thrust above the CG causes nosedown pitching (High Thrustline).
Thrust below the CG causes noseup pitching (Low Thrustline).
Thrust through the CG causes no pitching (Centerline Thrust).

And rotor weight is always relevant whether it is producing thrust or not.
If an object is attached to the airframe though a rigid mount, gimbal mount, duct tape, or bubble gum it is part of the machine and affects the CG.
The only time the CG would change is if something fell off.
 

GrantR

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No not from a CFI. Yes I have read the CLT threads.

Just looked at your stability video Chuter.

The reason I think this is because the rotor is lifting the airframe from the top of the mast where it’s attached.

If you hold a pencil from the end and push on it anywhere it tilts the same direction. Even if the rotor was fixed it would do the same

A powered parachute swings forward when power is added. A tractor configured airplane noses up with power a HTL pusher airplane noses down with power.

It seems to me like the same should occur with a gyroplane.

Maybe I am just looking at it all wrong. Ok what would happened if you hoisted up the gyroplane like doing a hang test but with the rotor installed so the vertical cg would be in place and fired up the engine and ran it up to full power? Assuming you could keep it going in a straight line wouldn’t the gyroplane swing forward and around the hoist pivot point? Or would it rotate around the gyroplanes VCG?
 

gyromike

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The difference is that if you hoisted it up and attached the gimbal to the rafters it would be "massively constrained" (connected to the rafters, the walls, and the Earth).

In real life you have a 500 lb. gyro hanging from a 50 lb. set of rotors.

Here's an experiment for you. Go find an overhead rail with a lightweight trolley, preferably one about 10% your bodyweight. Attach a rope to it and try to swing. If you swing across the rail's direction of travel, it's constrained in that direction. You have a pendulum because the trolley can't move.

Now try to swing in the same direction that the trolley travels. The trolley will immediately center itself over your body as you travel along the rail. That's how the rotors react, but in both directions.
 

RotoPlane

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What Mike said….plus, you left out two other important variables; airframe drag and the rotor thrust vector.

The RTV balances the averaged weight/drag/thrust vectors around the CG, in level flight.
 

Udi

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In addition to what Mike said...

...This can be though of like a pusher vs tractor airplane. Pushers have the thrust about the lifting force which causes a nose down pitch with power increase and tractors have the thrust line below the lifting force which causes a nose up pitch with thrust increase.
You must be comparing standard airplanes with HTL amphibs. The difference is not between tractors and pushers -- the difference is between LTL planes and HTL planes, like amphibs. Some amphibs are tractors, and they still have the nose-down reaction. It makes no difference whether the plane is a pusher or a tractor, what matters is prop thrust LINE offset vs CG.

A powered parachute swings forward when power is added. A tractor configured airplane noses up with power a HTL pusher airplane noses down with power.
The reason a powered parachute swings forward with power is because it is highly airspeed stable. The wing resists accelerating - in effect acting like a large mass - so the airframe swings below. Gyro rotors are not nearly as airspeed stable (which is why gyros are less sensitive than parachutes to wind gusts). The first pitch reaction of a gyro to power change depends on the thrust line vs. CG offset, and only then to other effects, like airspeed stability. That is why, in a properly configured gyro, you use power to control altitude and stick to control airspeed.

Udi
 

RotoPlane

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Perhaps I should add, that the RTV changes with aircraft speed. Say a gyro is configured to fly level at 40 mph and the RTV passes somewhere forward of the CG. If you increase the speed to say 65 mph, keeping the same altitude, the rotor disk angle will decrease, causing the RTV to move aft of where it was, and the nose will now angle down to less than level.

This nose-down angle will increase the faster you go, due to the new RTV location and the increased below CG drag. That is why it is important to keep drag as low as possible below the CG AND to have a horizontal stab that becomes more effective (balancing this drag and RTV movement) the faster you go.

Note that this is my opinion......
 

Udi

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Perhaps I should add, that the RTV changes with aircraft speed. Say a gyro is configured to fly level at 40 mph and the RTV passes somewhere forward of the CG. If you increase the speed to say 65 mph, keeping the same altitude, the rotor disk angle will decrease, causing the RTV to move aft of where it was, and the nose will now angle down to less than level.

This nose-down angle will increase the faster you go, due to the new RTV location and the increased below CG drag. That is why it is important to keep drag as low as possible below the CG AND to have a horizontal stab that becomes more effective (balancing this drag and RTV movement) the faster you go.

Note that this is my opinion......
Sorry, Ed, this is incorrect. With a locked stick, increasing airspeed increases the rotor AOA due to blade flapping (blow back) - and the RTV moving fwd as result. In order to maintain altitude with a higher airspeed, THE PILOT has to move the stick fwd, reducing rotor AOA. Rotor AOA does not become shallower all by itself.

Udi
 

RotoPlane

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Sorry, Ed, this is incorrect. With a locked stick, increasing airspeed increases the rotor AOA due to blade flapping (blow back) - and the RTV moving fwd as result. In order to maintain altitude with a higher airspeed, THE PILOT has to move the stick fwd, reducing rotor AOA. Rotor AOA does not become shallower all by itself. Udi

I assumed he knew the stick would need to be pushed forward as airspeed increased to maintain altitude, ....which would move the RTV further aft.

I know....don't assume....;).
 
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GrantR

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Thanks guys I got it now. Mike your trolley example makes perfect sense.

DUH Uh:humble:!
 

Heron

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Throtle for climb, stick for speed . . . who hammered that in to my brain?
Doug?
Heron
On a pusher, the power plant is ALWAYS trying to get ahead of the frame it is attached to. Minor misalignment will cause twitching.
Tractors are more forgiving as the mass tends to align with the main force.
Heron
 

Passin' Thru

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On a pusher, the power plant is ALWAYS trying to get ahead of the frame it is attached to. Minor misalignment will cause twitching.
Tractors are more forgiving as the mass tends to align with the main force.
Wrong again, Heron!:rolleyes:
 

Joe Pires

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Care to comment?
Thanks
Heron
Mike, did you take over Bensendays.com yet? :)

No but I did. And it will be there when and if Sunstate decides it wants it.
 

Heron

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Thanks Joe!
It was Sunstate´s all along, I meant it as a present, paid for 2 years.
Heron
 
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