Learning PIO

[automan1223]

I know there are unstable machines and there are good pilots. I also know there are stable machines and not so good pilots.

From what I can see, and this bothers me alot.....

It seems that this demon called PIO can sneak up on you at any time and place, at any number of hours and punch your ticket for you. It seems not to matter how many hours of experience you have or which way you wound your watch that day. It is like all of a sudden your dead cause you moved the stick the wrong way for a second......

To me that is not cool. I have been a die hard gyro enthuiast going back to the first mad max movie, started cutting metal back in 1998....


We should have a way as students to learn hands on about PIO. You cannot really do this because if you start it and let it progress say bye bye. We do not have any flight simulators, and even if we did they would be useless because of all the different machines, different response rates, weather conditions etc. Of course you can limmit your risk by flying a stable machine in calm conditions. Now I know I am opening a can of ugly worms here because I will get a bunch of responses like it was the pilots fault, weather, machine etc. But there has to be a way to allow a student to identify a pio situation and stop it. That one second you freeze seems to be too much.
Or am I all wrong ? is it that enough training in a somewhat more stable machine than an RAF will prevent you from entering a pio situation and therefore it should be drilled into you while you are training.


This is just really starting to bother me that these high time pilots, over the years bit the dust. Seriously I would like to hear more about this because I am wondering how we can train for a condition that could end our flying. All of our training comes down to a taught skill but how can we excercise that skill when it could kill us ? They say a high time atp pilot spends a total of 8 mins in the flare over the course of their career. The flair of course the most critical of all manuevers but I am thinking that a highly needed skill, practiced so infrequently can only seem like you are looking for trouble if that one skill you need will kill you if you do it wrong.

Jonathan Weis
Oriental NC

[Doug Riley]

Jonathan: PIO is PILOT induced oscillation. It is caused by ill-timed control inputs by the pilot. I know of no experienced gyro pilot who has ever had a PIO accident (other than one or two in which the pilot was allowing a student to fly and the machine got away from the student).

Once you get the rhythm of a machine, it's almost impossible to PIO it, even if you try deliberately to induce PIO. Your reflexes are in tune with the machine's reaction time.

PPO (power pushover) is a different animal. It's the result of a design flaw. A skilled pilot can compensate for certain design flaws under certain conditions. Some flaws are beyond anyone's ability to compensate. With those flaws, pilot experience can't save you.

Gyros can be made highly PIO-resistant by design. The helpful features are the usual HS, centered thrust, stable blades and gimbal head. Gyros probably can't be made completely immune from a beginner's bad timing, however. (You can PIO just about any device by timing your inputs incorrectly. I went sailing in a fairly heavy cruising sailboat a couple weeks ago. My inexperienced friends who came along were PIO-ing the boat when they took their turns steering -- they got behind the boat's reaction time, overcontrolled and did a few unintentional tacks. It can happen on a bike, in a car with sloppy steering, in the shower trying to adjust the water temp or just about any other gadget having any amount of lag in its reaction time.)

[automan1223]

I think I am beginning to understand what you are saying. I think its kind of like learning to ride a bicycle. At first it seems to be impossible but once you learn its almost impossible to fall off. I would be nice to put a machine in a wind tunnel and simulate all sorts of modes of flight.


I also read part 1 of Greg Gremmer's article in rotorcraft mag and I read it about 3 times and I think I am starting to understand what he is saying about cook book clt issues are not standard, or so straightforward for every gyro. I will prob apologize to him about being secuced by the magni gyro. From what I have read, and I dont think he is blowing smoke, that you can have a totally whacked out looking machine that looks like it would fly like a death trap, but if all the forces are calculated correctly it will fly the way you want it to and be a safe machine.

Doug, when you said stablility was about balancing all the forces. And that started me thinking. There are a heck of a lot of forces acting on a gyro. Throwing a halfed assed clt and a hs and all does not automatically make a safe machine. I would love to be able to figure all of this stuff out for the machine I am flying, and in general I am just starting to understand that there is a LOT more than meets the eye, or has ever been bounced around on the conference over the years.

I also understand that the 2 critical differences between the fixed wing and gyro can kill you in a gyro if you fly them like a fw when pio starts.


I should go back to my old motto,:
"ears open, mouth shut"

Jonathan

[Doug Riley]

I should have added that the one time I tried to fly a sailplane, I overcontrolled it badly. I would have spun in for sure if an instructor hadn't been aboard to bail me out. One minute the nose was pointing straight down, the next it was pointing at the sky and the whole ship was trembling on the verge of a stall. Gyro instincts will get you in trouble in FW plane as well as vice versa.

[Hognose]

We do not have any flight simulators, and even if we did they would be useless because of all the different machines, different response rates, weather conditions etc.



Jonathan and all:

There is a simulator that does gyros, and can simulate both all kinds of weather AND many types of gyros. You can even design your own, which will respond in aerodynamically sound ways. It's called X-Plane and runs on Windows, Linux, and Mac.

It is what was used by the CarterCopter team.

http://www.x-plane.com/

Not as polished as MSFS but for real-world simulation, infinitely more useful.

cheers

-=K=-

[gyrogreg]

Jonathan,

You said you would like to be able to figure out all the stuff about "balancing the forces". It's really not that hard! We are talking about "balancing the static moments". You don't need to figure it all out on paper - especially if you already have a gyro that is built and flying - you just need to perform some simple STATIC flight tests!

(These tests should be performed by a pilot who has developed experience and proficiency in that particular gyro, but the tests not difficult. These tests do not involve introducing dynamic excitations that might result in dynamic PIO divergences. The tests should be conducted in the order presented in the article refered to below. And, under no circumstances, should a non-professional attempt to perform DYNAMIC flight tests that require actually trying to initiate a short-period oscillation. Dynamic flight tests are probably not required if the static flight tests are "passed". And the dynamic flight tests should only be conducted by a professional and experienced test pilot - only after the static criteria are assured!)

The STATIC flight tests may not tell you everything about the PIO issues with your gyro, but it will tell you whether there is potential for a buntover (or a PPO, or "precession stall") - the STATIC fatal event after PIO gets so extreme that a static instability takes over. (PIO does not kill you - the buntover, after the PIO gets to extreme, is what kills you!). If there is little tendency to buntover, there may be little tendency that a PIO will result in a buntover. And, good results on the STATIC flight tests will likely mean there is little tendency for a DYNAMIC PIO event! And, good results in the static flight tests probably prevent any disturbances from being so severe as to excite the pilot into over-reactive control inputs that might initiate a PIO.

Now, this is a bit of a simplified answer - there are a lot more specific tangent issues the technologists can argue about, but the evidence and technology is supporting the fact that if you have a statically well "balanced" gyro, you have much less risk for any bad things to happen - Dynamic or Static, PIO or buntover.

How do you tell if your gyro is statically "well balanced". Please go back to my article in Rotorcraft: THRUSTLINES and HORIZONTAL STABILIZERS. (I'm not sure which issue that was printed in, but you can get it at this link: http://www.magnigyro.com/USA/feature_articles/THRUSTLINES and HS.pdf)

The article tells you how to do the three STATIC stability flight tests.

Now, this might not completely assure the gyro will be immune from a "pitch stability event", but it certainly reduces any potential - probably by at least an order of magnitude. To pass these tests, it will certainly require a very effective, and properly tuned pitch stabilizer (HS!), and a reasonable (and "balanced") prop thrustline offset. There are many of us that suggest that passing these tests will nearly assure there will be no "pitch instability events" - static or dynamic (buntover, PPO, precessions stall or PIO!). If your gyro, with a good, effecive HS and a reasonable prop thrustline offset does not pass each of these three static stability flight evaluations, it should be easy to "tune" the HS configuration to make it better. Even if it does not "pass" all of these static flight tests, those tests will help identify what flight envelope extremes (speed and power) should be avoided!

There is much more to this than this simple answer - but these flight tests should pretty well eliminate any severe risks. But, even then, you should not push the flight envelope of your machine until you have gradually developed good proficiency outside the moderate (speed and wind turbulence) envelope. If your gyro does not pass all of these flight tests, even a proficient pilot should not venture into the flight envelope corners where those STATIC stabilities are not verified by the flight tests.

For the record, these static flight tests will likely be refined somewhat as more testing data is accumulated. For instance, the speed tolerances of test #1 may be able to be relaxed after we have more data - these criteria may be conservative at this time.

One more thing: I do not believe you can practice PIO!!! If you are flying a machine that has a potential to PIO, if you excite that PIO, I believe it can diverge so quickly that proficincy does not matter anyway. I believe that when people say they have observed PIO or "saved it from PIO", they are not talking about the true PIO that kills you. They are probably talking about "long-period" oscillations that do not rapidly diverge into PIO (rapid, short-period) oscillations. PIO can not be practiced, because that would mean you would need to practice it in a machine that is capable of PIO - that is VERY dangerous! The best way to avoid PIO is to fly a machine that has a low potential to PIO in the first place. As said above, a truly statically stable gyro probably has very little or no tendency to PIO. And as suggested on previous posts, development of pilot proficiency would probably avoid any residual tendencies for PIO in that pilot/gyro combination.

Thanks, and safe flying - Greg

[automan1223]

Greg,

First of all I want to thank you for responding to my posting.

Second I want to publicly apologize for any rude or unbecomming remarks I made about you having been corrupted just to sell a machine. I also am sorry I got you mixed up with Gary, Gary and a few other people.....

I have been around, and studying gyros since 1998 and your article In the Feb 04 Rotorcraft issue that goes off the farm so to speak. But I have read it many many times and I have begun to visualize and understand what you are trying to explain.

The link to above is not working and the usa section on the magni website has no places where I can find the article.

You said
One more thing: I do not believe you can practice PIO!!! If you are flying a machine that has a potential to PIO, if you excite that PIO, I believe it can diverge so quickly that proficincy does not matter anyway. I believe that when people say they have observed PIO or "saved it from PIO", they are not talking about the true PIO that kills you. They are probably talking about "long-period" oscillations that do not rapidly diverge into PIO (rapid, short-period) oscillations"

TRUE PIE'O whole new ball of wax ?

When I was training with maxie in my own machine, on 2 different occassions I started doing what I would consider to be long period oscillations. I started chasing the machine with the stick and with the long lag with long rotor blades turning slowly this student was way behind the control curve. Maxie on the second up yelled at me and he stopped it, however I have a vivid memory of my manuever, it was a long time up and down kind of thing, kind of like what you might have seen on a dominator demo video. I cant say that I would not have gotten into serious poop if Maxie had not been there to save my ass.

Short period oscilliations: Or what I will call PIO/Buntover. Occurs so quickly that no pilot has the reflexs to catch it. Classic examples are the often referenced air show disaster over in england, a nellie gyro during a highspeed pass, and our RAF machines here...... among other designs..... I think the england air show is a classic pio that we are referencing.

So how do we figure out if a machine has a spot for a short period oscillation ?

(answer watch them drop from the sky)

ok I know that was a bad joke but seriously Greg, is there any way to fully understand a machines tendency to short pio ? Is it just a linear equation, if it longs, then it will short, and the more=more and you are in trouble under some circumstances.. or can a machine just up and short you out of the sky ?

Jonathan

[PW_Plack]

Pilot-Induced Oscillation is not unique to gyros, especially for student pilots. I figured out how to stop over-controlling on landings pretty early on in my tailwheel training, but the problem resurfaced when I started flying in turbulence. A very nice older lady CFI saw me fighting the controls, took over the plane, trimmed it, and let go of the stick. It was still bumpy, but the plane pretty-much stayed on course. "See," she said, "you were creating your own turbulence!"

I will say, though, that the longer control lag time on gyros was a whole new experience the first time I flew in one.

[automan1223]

Paul,

How long have you been flying gyros and what types ?

What kind of fw ?

Jonathan

[gyrogreg]

Hi Jonathan,

First, the above link to the article is a bit tricky because it has some spaces in the URL. http://www.magnigyro.com/USA/feature_articles/THRUSTLINES%20and%20HS%20.pdf If this link does not work do this: on the USA section of the Magni website, click on "FEATURES". Then, look for the THRUSTLINES and HS article. I'm sorry, I do not know how to place a link in the text of this forum.

You asked: "So how do we figure out if a machine has a spot for a short period oscillation ?" First, as I've said, if you meet the static flight test criteria, there is probably very little risk of PIO - short period natural oscillation tendencies. The reason is fairly complex, but the way I describe it is that you cannot achieve the static criteria without a significantly effective HS. And a good HS is the way to minimize the potential of PIO (poorly damped short period oscillations).

The ASTM Gyroplane Standard development process is suggesting some specific DYNAMIC stability criteria that would identify if the gyroplane has any residual tendency to oscillate at rates that are difficult for a human pilot to actively try to stop - the best thing would be to stop moving the stick if true PIO is a concern! But, if it is true PIO, once started, would be very quick and can probably not be stopped! This is a bit involved - technically, so I suggest you review a forum thread in which this is discussed a bit more technically - and in mathematical terminology I am not sure I can understand either. Please review this thread for more background:
Gyro stability Test Criteria and Methods:
http://www.rotaryforum.com/forum/showthread.php?t=1038

I had always thought that a gyro (or any aircraft), would be most susceptible to PIO if the natural pitch oscillation tendency was in the very short-period frequency range - less than 5 second period. The reasoning is that any attempt by a pilot to actively stop such fast oscillations would probably be "behind the curve", or in the wrong phase and would actually amplify the oscillations. This would be true PIO. So, the ASTM standard currently requires that there be no natural oscillations with periods less than 5 seconds long. Professional test pilots would have flight test methods to determine this - but such tests would be very dangerous if there were a true propensity for short-period natural oscillations. (That is the reason we implore people to not try dynamic tests and that the static criteria must be met before any attempt to test the gyro dynamically!)

What we are learning on the "Gyro stability Test Criteria and Methods" thread above is that this may not be exactly the case. In that thread and in a couple of other threads, the suggestion is made by some very mathematically competent people that "HIGHLY DAMPED" natural oscillation frequencies in the range of periods under 4 seconds (very rapid), are actually a desirable attribute in a gyro - makes it quick responding! The suggestion is also that the long-period oscillations (such as you describe may have happened to you) are not truly dangerous because normal piloting skills can readily "damp" such slow oscillations. It is doubtful that such long period oscillations, even if not properly damped by the pilot, would diverge into a "pitch instability event" as long as the three static stability flight test criteria are met - long-period dynamic oscillations will not diverge to an extreme where the gyro becomes statically unstable (buntover) because the static testing has already verified that it won't do that! The comment by professional test piulots is that such long-period oscillations are not a major factor for pilots, but it can drive autopilot designers nuts!

The " Gyro stability Test Criteria and Methods" thread does suggest that the desired short period performance (highly damped short period oscillations) are actually assured by the G-Load static stability criteria (don't ask me how that is - it's highly mathematically complex!). But, this also suggests that even such a gyro, apparently depending on how far back the HS is positioned, can have an intermediate short-period oscillation (combination of the long and short period natural oscillations) that is not well damped - in the range of 7-10 seconds period - that could be the true root of PIO. Apparently, there is a theoretically remote potential that this can occur even in a gyro with a HS, even in a gyro that meets the three static flight test criteria! This needs to be explored more - but this might be the natural oscillation tendency that is the root of PIO and should be investigated by some very special flight testing. But, let me stress, this is suggested to be a very remote possibility, and that a gyro that meets the three static stability tests will be very resistant to PIO.

Now, and this is a bit of speculation by me, if the gyro does not meet the static stability flight test criteria, then it is more likely to have some very short-period natural oscillations that are not "highly damped". If a pilot was "excited" into trying to stop such rapid un-damped pitch oscillations once initiated, the pilot would likely respond out of phase with the result being PIO. In other words, if the gyro is not adequatelly statically stabilized, there is a much higher potential for PIO. I suspect that this is the case for probably every PIO occurrance. But, in addition, such inadequately statically stable gyro would also have much more propensity for buntover, PPO or precession stalls ("pitch instability incidents"). So, in such a gyro, if PIO occurs, it can likely quickly diverge into such extreme oscillations that very quickly a static instability would end the PIO with a buntover, PPO or precession stall!

The bottom line is - make sure the gyro meets the static flight test criteria, and you should not have much concern for PIO. But, in all cases, do not explore the extremes of the flight envelope (high power, high speed and turbulence) until you have gradually built your experience and proficiency in these areas. And, if the gyro does not meet all three of the static stability flight criteria, understand what area of the flight envelope this indicates should be avoided - usually fast, high power and turbulence.

Thanks, Greg

[PW_Plack]

Jonathan,

So far, I'm mainly a keyboard flyer when it comes to gyros. Over the past three years, I've been up a total of six times with Jim Vanek in the Sport Copter Vortex II tandem, none of it loggable because Jim's not a CFI. I hope to be training and building this fall and flying next year, once my divorce is final, and I figure out what's left.

I first learned to fly in an Aeronca Champ, and have since flown rented Cessnas, 150, 152, 172. I have about 160 hours total, 85 in tailwheel.

[ventana7]

Greg,

As always thanks for taking the time to help educate us.

Can you give me a good explanation of precession stall, what causes it, what are the aerodynamic factors and what is the result?
Thanks.
Rob

[gyrogreg]

Rob,

The Glossary of Gyro Terms (http://www.magnigyro.com/USA/feature_articles/GyroTerms.pdf) hs the following definition:

Precession Stall:
Refers to the stalling of one or more individual rotor blade(s) due to too rapid and too large of cyclic input to the rotor system. Due to the precession tendency of the rotor disk to maintain its initial disk attitude, a rapid and large cyclic input will create an immediate large angle of attack of at least
one rotor blade – upon cyclic input, one blade will see more positive AOA, while the other blade(s) may see nearly as large negative instantaneous AOA. It is this cyclic unbalance between the rotor blades that initiates the rotor disk attitude to follow the cyclic input. However, if the cyclic input is
too large or too quick before the rotor disk can adjust, it is possible that the large cyclic input will result in individual blade stall AOA. If significant high blade AOA or stall continues too long, the increased drag on the rotor blade can slow the rotor dramatically. The physical limits of 10 degrees on the rotor teeter stops and on the range of cyclic control input is intended to limit or prevent cyclic inputs over about 10 degrees where significant rotor drag would be occurring. Precession stall could be initiated by too rapid and large of cyclic input from the pilot or by a similar uncommanded cyclic input from airframe motions. Precession stall that slows the rotor too much will be unrecoverable as the centripetal forces on the slowed rotor may no longer sustain the weight of the gyro.

In shorter terms, this is when the cyclic input from pilot control or from airframe (spindle) rapid rotation (as in a bunt) are so great that one or more blades stall significantly so that the teeter limits are not adequate. Sever precession stall could be a violent and immediate blade flap as a result of too much individual blade AOA - lots of stalled area! I have suggested that precession stall may be the more than common mechanism for in-flight destruction of the gyro during a buntover - the airframe and spindle rotate so far and so fast as to cause a precessions stall - probably long before the rotor really slows down or the gyro pitches over to inverted. Precession stall may be more common than buntovers - but who can tell the difference from the results?!

Thanks, Greg Gremminger

[Doug Riley]

Hey, Greg, I think there's a typo in there. The second sentence probably should refer to the "inertial" tendency of the rotor, rather a "precession" tendency (which is just the opposite of an inertial tendency). IOW, the rotor disk tries to stay put when the spindle tips. This results in a cyclic pitch change.