Pilot induced rotor oscillation

SPOON

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Could someone give me the facts on this situation and if improvments have been made from the early days of gyros to present days. :argue:
 

MichaelBurton

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Search for PIO it is full discuses on the forum.
 

willisbr

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I don't think it's rotor oscillation in PIO. Is this correct? I think it is the aircraft that oscilates. Can someone confirm or correct me?
 

gyromike

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Correct Brian.
It's the airframe that oscillates.

More specifically, an undamped airframe that oscillates.
 

Doug Riley

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Autorotating rotors are very stable if left to themselves, with a bit of weight concentrated just beneath the center of rotation. Maple seeds don't have problems with oscillations (as I was reminded when cleaning out my gutter recently).

If the direction of the rotor's thrust can be pointed this way and that by a pilot with a control stick, though, things get more complicated. The rotor's thrust then can pull up first one one end of the craft, then the other. If the natural frequency of the craft's rotations matches the pilot's reaction time, the aircraft can end up porpoising nose-up, then nose-down. It can also do the same thing in the roll axis, tipping from a left bank to right and back.

Aerodynamic damping works like a shock absorber to counteract these oscillations. A properly-designed gyro has adequate built-in damping and is not prone to pilot-induced oscillations.

Some old designs, based on the Bensen gyrocopter,* were prone to oscillations that could result in a crash if not stopped. Experienced gyro pilots generally don't get into pilot-induced oscillations, even in machines that are prone to it. There's no reason (aside from ignorance) to design an oscillation-prone gyro, however.
_____________________

*Igor Bensen tolerated a tendency to oscillate in his gyros. He was a skilled engineering test pilot who was well able to handle and stop oscillations. He apparently felt that it would be no big deal to pass along these skills to his customers. He ended up with a lot of dead customers, though. His self-training method required more patience than many people could muster.
 

birdy

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I don't think it's rotor oscillation in PIO. Is this correct?
No.
It IS the rotor that oscillates.
As Doug said, unless the pilot induces the oscillations, the rotor will happily stay steady.
The oscillating rotor makes the airframe buck, not the other way round.
But, an unstable machine with an unstable pilot will make the rotor oscillate, maken the frame buck, which feeds greater oscillations to the rotor and so on till it all goes to sh1t if not stoped by the pilot..
 

Heron

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Bauru - Sao Paulo - Brasil
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Ther airframe moves the rotor, using the mast as leverage, tilts the disc.
The forces that cause these movemenst are: pilot input and thrust placement.
The pilot can input in two ways: stick and weight shift (slow response)
The thrust line can move the disc up or down, according to its placement.
An unexperienced pilot, will try to counteract any non intended input by using the stick, thus causing a movement in the other direction.
As a result, the nose bobs up and down, this can increase untill the disc can no longer follow and the divergent travel of nose and disc front arc causes its rear arc to make contact with the airframe.
If you eliminate pilot´s interaction by locking the stick, the disc will turned forward and the machine will follow increasing the speed of the movement, a classic tumble foward.
Dampening devices will slow the movement, giving time for pilot input and correction, as trained.
Eliminating thrust displacement will end airframe/disc unintended interaction.
Heron
(just practicing english, please feel free to correct me)
 

birdy

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open frame single seat & a 'wasa' RAF, among other types.
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Ther airframe moves the rotor, using the mast as leverage, tilts the disc.
Also incorrect.
You could never make a mast strong enuf to 'leaver' a spin'n disc.
The only part the frame/mast plays in rotor control [ besides applyn load] is to give the cyclic systm sumthn to leaver against.

If you eliminate pilot´s interaction by locking the stick, the disc will turned forward and the machine will follow increasing the speed of the movement, a classic tumble foward.
Compleatly ass about Heron.
The frame [ though the cyclic control] makes the disc change AOA, and the frame follows.
As Doug said, the disc will not diverge [ within reson] without cyclic input.

The real cause of 99% of Pilot Induced Oscillation is simply the pilot not be'n aware of the slight lag time between cyclic input and rotor/frame reaction.
 

Doug Riley

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The lagged reaction that Birdy mentions can be larger or smaller, and can take more or less time, depending on the design of the airframe.

The frame will be slower to react to control inputs if it lacks an adequate horizontal stabilizer (HS). All gyros that I know of designed in the last 15 years or so have HS's -- most of them adequate. The old Bensen had almost none, and some gyros patterned after the Bensen had absolutely none.

The HS speeds up the frame's reaction to control inputs by forcing the frame to line up quickly with the gyro's direction of travel. It also reduces the magnitude of the frame's reaction by preventing overshoot. Overshoot and lag are frame "behaviors" that make porpoising (PIO) much more likely.

A second design feature that affects the likelihood of PIO is the amount of damping supplied by the rotor itself. To over-simplify a bit: heavier, slower-turning rotors supply more damping. Bensen's rotors were very light, and turned relatively fast (over 400 RPM). They had very little damping, making PIO more likely. More modern gyros have generally trended toward heavier, slower rotors with a consequent increase in rotor damping.

Too much rotor damping can make a gyro handle like a dump truck. There's a happy compromise in this area. A very large H-stab, OTOH, does not compromise the gyro's handling -- except in the case of a few "trick" maneuvers that rely on lag or on the frame NOT pointing in the direction it's going.
 
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