View Full Version : Porpoising
06-05-2005, 09:04 AM
I have absolutely no idea what the heck this guy is saying... but oooooooo, pretty little pictures.... pictures good.... shiny, colored, pictures :D
06-05-2005, 11:16 AM
If you phugoid too much the roundy thing hits the prop thingie... :confused:
(or you cough , or go blind or somethin...)
06-06-2005, 07:23 AM
Actually, the long-period (so-called "phugoid") oscillation in even an unstable gyro is so slow that it's not likely the problem in porpoising accidents. I recall flying my old, no-HS Air Command for long sessions and still feeling that slow oscillation after going to bed for the night! It feels like a ride in a small boat with a following sea (waves overtaking you from behind). Twenty seconds is pretty long for a small gyro, it might have been more like ten -- but that's still slow. Count it out and see.
Gyro porpoising (PIO) is a much faster roller coaster ride, and it's pumped up by pilot inputs (phugoid isn't). I'd guesstimate that a half-cycle of PIO takes only a couple seconds. It can end in a PPO in a machine with high thrustline and inadequate HS. Gyros with severely high thrustline will sometime PPO without PIO-ing first.
06-06-2005, 08:15 AM
Can we assume that porpoising is something the machine does on its own and PIO is what the pilots create (induced porpoising) ?
06-06-2005, 08:23 AM
No. Porpoising and PIO are two different names for exactly the same thing.
"Pilot induced" is a little unfair, I think. It lets the designer off the hook when, in fact, poor design is a huge part of the problem.
Some gyros simply are very user-unfriendly. They practically beg you to get into oscillation. Such machines must be flown using test-pilot techniques (jabs, reverse jabs, stick floating, intentional under-control). Well-designed aircraft do not require these special techniques in order to avoid porpoising.
06-06-2005, 10:14 AM
If you have ever read Dr. Bensons' Gyro Glider construction and pilots manual, ( copyright 1969 ) on pages 44 and 45, he tells you what causes, and how to prevent PIO. He also says short jabs and back to neutral on the stick, which is what I believe Doug is saying.
06-06-2005, 01:23 PM
We should change it from PIO to DIO. Thus the name would be Designer Induced Oscilation. When someone is so arrogant that they refuse to change the design of their aircraft to be pitch stable or, design it to be pitch stable from the start, then the credit should be given to the proper person. The Designer. New term to the Gyro dictionary DIO. Just a thought. :)
06-06-2005, 01:51 PM
Well, neither of those "traditional" terms really captures the flavor of the thing.
The term "PIO" gives the designer a free pass when he should get blamed in many cases. OTOH, "porpoising" scares people into thinking the craft will just randomly stop responding to the stick and go thrashing around all by itself -- and that isn't true either, at least in calm air, even with the bad machines. If a gyro could porpoise all by itself, the standard advice to cure it (hold the stick still) certainly wouldn't be right.
Any piece of machinery that gives you a tangible result with any time lag at all can get into operator-induced oscillations. This is true even of very tame things like walking with a full cup of coffee or adjusting the water temp control in your shower. With devices where the designer has taken the human factor into account, PIO is possible but suitably unlikely. So there's room for the idea of PIO even when we're talking about good designs.
PIO should be a minor, obscure little botheration, though. It shouldn't be the murderous monster ready to jump you any minute, as it has been historically in no-HS, HTL machines. If PIO is THAT huge an issue in a machine, then it really is the designer's fault. Even if it is technically the operator who gets thing swinging with an ill-timed control jab.
06-06-2005, 05:49 PM
You hit the nail on the head.
06-07-2005, 01:20 AM
06-07-2005, 02:56 AM
I have to agree with Doug - "Any piece of machinery that gives you a tangible result with any time lag at all can get into operator-induced oscillations" - and it does not necessarily matter whether its a Dominator or a RAF. Most Instructors will appreciate that many students can and will porpoise to some extent, no matter how stable a machine is. Perhaps it should be called "Pilot Perpetuated Oscillation".
06-07-2005, 06:12 AM
Tim, sure, any device with lag can oscillate. However, the device's relative PRONENESS to oscillation can be controlled through competent design. That makes all the difference.
In the case of gyro pitch stability, the gyro's static angle-of-attack stability, control lag and dynamic stability (damping) all play into this proneness (or lack of it). If the machine reacts to up- and down-drafts by exaggerating them (negative static AOA stability) then the pilot will need to correct all the time. This sets him up to jab at the wrong time. If the machine has significant control lag, timing and sizing your inputs become more difficult. If the machine overshoots back and forth after a control input is applied (weak dynamic stability) then addition reverse "jabs" are needed, adding another complexity.
My old Air Command and, to a lesser extent, my old Bensen, had all of the qualities that make a gyro easy to PIO when they were flown at higher speeds and power settings. In contrast, I have never had a student PIO the Dominator at cruise or higher power-- not the seasoned FW pilots and not the first-ride-in-a-small-aircraft people, either.
Most students do "stairstep" the tandem Dominator for awhile when doing idle-power approaches. I think this is a form of mild PIO, aided by the reduced flow over the HS and the lazy pitch response of a long machine. So, yes, PIO is always possible. PIO at idle is pretty benign, though, and can't honestly be equated with the blown-out-of-the-sky porpoising fatalities of the bad old days of no-HS machines.
06-07-2005, 06:26 AM
IN this initial phase of my training I did some PIO (very large indeed) from the back seat and looking at the instruments it takes a while to notice the nose moving and you try to get it back a little too much.
Gyros fly real good (the stable ones) if you let them! :D
Now from the front seat it is a very different view and I think I can better "see" the moves and counteract just a little more on the "enough" side . . .
So porpoising is something the machine does for bad design and PIO is something the pilot does intentionally of for bad piloting technics, both look alike but have different origins.
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