Power Push Over (PPO)

Thank you Vance and Leigh. Good posts.


Can you not when flying S/L at cruise power, pull full aft stick and hold it till the nose reaches for the sky...then give it full fwd. stick and hold it till the nose dives toward the ground...then full aft stick again? Would this not simulate PIO?

But Harry what would be learned. If I did that in my aircraft all I would need to correct, is to stop doing it. In addition it is my understanding that a PIO starts as a much subtler action than what you describe.
 
But Harry what would be learned. If I did that in my aircraft all I would need to correct, is to stop doing it. In addition it is my understanding that a PIO starts as a much subtler action than what you describe.



Joe, what would be learned, IMO, the visual realization of what is PIO...the up and down, or, the "pendulum" or "rocker" feeling of the action...and the learning experience, if I may.

That's exactly right Joe. You would stop doing "it." How you would stop doing "it" is from info gained from this forum? Have you actually experienced PIO thru a demonstration? Believe me, it's much more "scarier" than you can imagine...especially when you're lookin' at nothin' straight down but the ground. Think about that!!

No...PIO is not subtle in that regard. Your reaction would be firm and instantaneous, You're goin' up, you push the stick forward...you're goin' down, you pull the stick back. That's normal reactions/reflexes. To stop it..."reduce power to idle and center the stick." Really simple!!

May I say that I'm refering to the low time gyro pilot, not necessarily the experienced pilot. There is a difference, IMO.


Cheers :)
 
Ok Harry. I have given my opinion and not persuaded you, I have read yours and it has not persuaded me, I guess we just disagree on this point.
 
Interesting conversation concerning training. I only have about 10 hours in a mac powered Parsons 2 place and another hour in a honda powered SnoBird 2 place (several years ago). I never had a problem with PIO-ing the parsons (even though it had no HS and probably had a high thrustline). But the one thing I remember startling me and striking fear in my heart more than anything else was flying through turbulence. IMO I think it would be much more meaningful for a student pilot to practice dealing with unexpected gusts and turbulence with the safety of an instructor behind them, than trying to force an inherently stable machine into PIO (unless the student pilot displayed PIO prone behavior).

I also thought power off landings were pretty scary with that steep descent angle and learning when and how to flare just right without ballooning or running it into the tarmac (especially with a crosswind present). Takeoffs weren't so scary as they were difficult, what with throttle management, rotor spinup, and then balancing on the mains while multi-tasking yaw and roll simultaneously (without the front wheel making contact and skittering around) :twitch:.
 
Takeoffs weren't so scary as they were difficult, what with throttle management, rotor spinup, and then balancing on the mains while multi-tasking yaw and roll simultaneously (without the front wheel making contact and skittering around) :twitch:.
Tall tails and steering with differential brakes remove those difficulties, Sno.

But definitely not cricket; gyros are supposed to require exceptional skills.
 
Thank you Vance and Leigh. Good posts.


Doug,

The machine don't PIO...the PILOT does. :cool:
Harry, a person could be trained for PIO in an automobile.

All that would be necessary would be to replace the steering shaft with a rubber rod and a high inertia flywheel on the far end that required a steering wheel windup of 2-3 turns before anything happened followed with a large amount of overshoot.

You might call the resulting oscillation, using the literal meaning of Pilot Induced Oscillation, PIO. I’d have to call it dumb for trying to drive such a contraption.

And why do you suppose you never hear of lateral PIO? The control mechanism is the same; the pilot displaces the rotor thrust vector laterally about the CG which begins a roll acceleration. Care to guess?
 
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Chuck, I had 2 students out of 33 that induced lateral PIO. Funnily, those students never had trouble with pitch PIO. I never really pondered why but I would like to know.
 
I'll venture a guess. I don't know how to put this correctly, but there's not as much mass far from the cg in the roll/lateral direction, so it responds quicker. Not as much lag for the pilot to deal with that gets him into PIO.....?
 
The reason lateral PIO is so rare, Tim, is that roll axis moment of inertia is significantly lower than pitch axis MOI. Lag and overshoot are reduced. Your exceptional students evidently got the rhythm of pitch axis response but were out of tune with roll axis timing.

Of course, there’s a whole lot more to it than that. A major factor in pitch axis PIO is angle of attack instability with HTL gyros which causes no adverse effects about the roll axis.

The way to avoid PIO is to have a gyro that is stable vs. angle of attack and one that also has high rate damping.

A gyro is controlled in pitch and roll by applying torque about the CG that begins an acceleration about the relevant axis. Humans are more accustomed to controlling rate than to controlling acceleration.

Damping converts acceleration to rate; as the machine accelerates about a particular axis, damping torque will eventually equal accelerating torque and thereafter, the machine rolls or pitches at a uniform rate dependant upon stick input. Of course it might be upside down before that happens.

The rotor itself is the only source of damping about the roll axis; as the machine begins to rotate about the roll axis, the rotor lags behind by an amount determined by roll rate. High inertia blades lag farther behind and provide a higher level of rate damping.

A correctly designed horizontal tail is perfect for pitch axis damping if the machine is otherwise stable.

Chuter, you nailed it.
 
I guess my sense of rhythm is out of whack (my musical bandmates would agree and offer tapes to prove it). The first time I had my Bensen gyroglider off the ground, I got it into a lateral PIO that resembled a drunk driving a dump truck with worn-out steering. I eventually encountered "external damping" in the form of a collision with a snowbank on the side of the runway.

I'd write that experience off to the de-stabilizing effects of the towline if I hadn't done almost the same thing the first time I got the Gyrobee off the ground. That time, I think the extra control lag occasioned by a very long overhead stick (it's so insensitive that it takes TIME to deflect far enough for anything to happen) was to blame.

In both cases, the problem appeared only on an initial flight and went away thereafter.

Rotor damping makes a pretty weak stabilizer, especially with light, high-RPM Bensen blades. Slow, heavy blades like McCutchens are very noticeably different.
 
I expereinced some roll PIO in some earlier days on my Air Command - with heavier Skywheels blades. I discovered that the stick would shake a lot less if I tightened the roll and pitch pivot bolts for more and more friction. At some higher friction point, the "sticktion" must have added some delay in my roll response and I unexpectedly PIO'd in roll. Stopped it by not moving the stick. Landed immediately and loosened those bolts a bit!

Another way to experience (or think about) PIO (and buntover) in a car is to drive in reverse fast - faster than you are proficient at. In reverse, the steering is statically unstable which means it tends to stray further from straight quickly - "buntover"! (try not to roll over!) And, if the "pilot" is able to stop the "buntover" with quick reversal steering commands, it is likely that he/she will start to PIO back and forth. Things get worse the faster you go - which may also analageous to flying a statically unstable gyro at higher airspeeds. In this auto case, there is little dynamic damping, but it would be instructive to discover if it is easier to PIO in a heavier car or a lighter one - inertia to overcome is higher or lower, but dynamic damping is nil. (Inertia and damping are components of the inherent dynamic response. But, without inherent damping, the "pilot" is the dynamic damper - just as in a statically unstable gyro without little dynamic damping.) The quickness of steering is also a factor - as it is in gyro cyclic quickness.

Thanks, Greg Gremminger
 
Neat analogy, Greg. Surface vehicles with rear-wheel steering (tail wheel airplanes, Buckminster Fuller's 3-wheeled car) are unstable and try to ground-loop on you unless the weight is right over the front wheels. In a backing-up car, the front wheels become the back wheels.

When I was in college, I worked in the campus cafeteria washing dishes. We loaded stacks of clean dishes into huge, deep 4-wheeled carts that probably weighed 800 lb. full. There was no way you could have steered them if the front wheels were pivoted -- so the back wheels were pivoted. It was amusing to give one of these suckers a good push, let go and watch it ground-loop.

Sometimes they'd even tip over, but that broke an awful lot of dishes and was discouraged.
 
The car that broke the sound barrier at Black Rock Desert was rear steer.

In my opinion cars don’t go backwards well because of the steering geometry.

I agree with Greg that a car with typical steering geometry is dynamically unstable as you go faster in reverse.

It is unusual for a four wheel car to go backwards fast enough for the center of gravity to have much influence on stability.

In my experience, static friction in the control system is often destabilizing and it also removes the natural feedback loop from the pilot.

Thank you, Vance
 
Greg,

I agree with Doug...a really good analogy.

I wonder why it didn't come from CB, the "Analogous One", my mentor also.


Cheers :)
 
The only time that I’ve experienced PIO in a normal gyro was years ago when I was asked to test fly a Bensen that no one could fly without PIOing. That’s the sort of thing you don’t fly higher than prepared to fall.

Sure enough, I too PIOed but recognized the problem; pitch axis friction. The builder had wedged oversized Teflon washers between torque bar and “U” block that produced friction which was difficult to recognize. Teflon has static friction not much greater than dynamic friction so that “sticksion” isn’t obvious.

With Teflon removed and replaced by ordinary washers, the machine flew about like any other Bensen.
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Man and machine form a closed loop not much different than a speed governor on a lawnmower or the cruise control of an automobile. Any such loop will oscillate if phase shift exceeds 90º while the loop gain is greater than unity.

Squeal (howl around) in a PA system is a good example. The microphone picks up the sound from the speaker, the system finds a frequency it likes and oscillates.

Lawnmower governors will surge if the inertia of rotating parts is increased beyond a critical value.

Automobile cruise controls will hunt if vehicle gross weight is increased beyond a critical value.

I once installed an after market cruise control on an automobile that had a 5-speed transmission. It worked fine in 4th gear but surged in 5th gear; too much lag. I had to open it up and increase the time constant of the lead compensation network to make it operate properly in 5th gear.
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It is unfortunate that the particular name, PIO is the standard term for this phenomenon. It is the result of a man-machine combination.
 
Good analogy Greg. Anyone who has had the opportunity to reverse a "T" model (even at slow speed) will understand what you mean. You had to keep a firm grip on that steering wheel for fear of loosing some fingers.
 
The difficulty from backing an automobile comes from having reversed caster. Going forward, the steering axis is inclined so that the tire contact patch trails the projected line of the steering axis. Going backwards, the wheel tries to flip.

Rear end steering works fine on forklifts and boats. That’s what Vance was hinting at but he doesn’t want to offend anyone.
 

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Jim Mayfield feels I have a propensity to sound pedantic.

I am trying to speak to his concern so I didn’t get into the specifics of the destabilizing effects of steering geometry when a typical automobile is driven at high speed in reverse.

I am sorry if I was not clear, I don’t mind offending people Chuck.

It was the weight location being the source of the problem that I pushed back on.

Thank you for clearing that up, Chuck,

Vance
 
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