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PIO ! ( pilot induced oscillation )

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Speedboat100

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Reno 2002 acccident with a Questair Venture M-20.




Here is Bonney Gull.




Can this happen to any plane or just short coupled ones ?

How can it be avoided ?
 

jedi

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"Can this happen to any plane or just short coupled ones ?" Yes/no.

"How can it be avoided ? " Fly within the design and flight test limits. Do the flight tests with appropriate test planning and and safety precautions.

Do not fly on a "wing and a prayer" unless you are wanting to go to heaven.
 

rv7charlie

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Your question poses a false condition; at least with the Tommy Rose accident. The Venture is not 'short coupled'. My understanding is that the ratio of wing chord to tailplane distance is well within the range of traditional designs. It only *looks* short coupled, because of the relatively high aspect ratio (skinny) wing.

Charlie
 

Aesquire

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PIOs happen when the pilot responds to the disturbance of his intended attitude with corrections that are out of sync with the frequency of the plane's response to his inputs.

a fancy way of saying you get all tense and jerky. It can happen on any airplane, but is more common in ones that respond quickly to input. You can PIO a 747.

my limited experience shows it happens to me when flying a new craft that is faster ( or slower ) to respond to inputs than I'm used to. Usually just a momentary bobble, if I'm paying attention and can relax into the motion. Unclench the death grip on the controls and it should smooth out. Two fingers instead of full fist, often helps.

PIOs are not flutter. But can cause destructive oscillations in extreme cases.
 

wsimpso1

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PIO is normally suppressed at a design level by having enough tail volume coefficient and enough pitch damping. Tail volume coefficient is ratio of tail area to wing area times ratio of tail arm to chord, while damping goes with the second term squared. Yes, more tail arm is the better way to get both...

Then there is pitch sensitivity, stick force per g and stick force per knot speed change.if these are too low, PIO can be entered pretty easily. Elevator area and chord have big influence on control forces as do aerobalance of the surfaces. Once surface sizes are fixed, you can go for servo tabs to decrease feel, antiservo tabs to increase feel, bob weight, centering springs, downspriings and opposing tabs, and so on to tune control feel, etc
 

bmcj

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The Questair may have been PIO, flutter, a single sudden control movement, or simply a control or structural failure. Any or all are possible since the tail failed, the plane was flying well past its allowable design speed, and the FAA found issues with the construction and the choice to reduce the strength of the control stick force spring. Probably nothing to learn here for someone who flies well within their plane’s allowable flight envelope.

Personally, I think it might have been due to flying an ugly plane design.
 

Speedboat100

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The Questair may have been PIO, flutter, a single sudden control movement, or simply a control or structural failure. Any or all are possible since the tail failed, the plane was flying well past its allowable design speed, and the FAA found issues with the construction and the choice to reduce the strength of the control stick force spring. Probably nothing to learn here for someone who flies well within their plane’s allowable flight envelope.

Personally, I think it might have been due to flying an ugly plane design.
I think it kinda looks personal, but not super fast.

How about propwash induced need to correct fast that caused stresses beyond structural limits.


Wings and tail feathers are really long and thin on Venture M-20.....prone for flutter ?

 
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Speedboat100

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Your question poses a false condition; at least with the Tommy Rose accident. The Venture is not 'short coupled'. My understanding is that the ratio of wing chord to tailplane distance is well within the range of traditional designs. It only *looks* short coupled, because of the relatively high aspect ratio (skinny) wing.

Charlie

It has 310 hp engine ..side by side seating and under 5 m long.



Questair_Venture.jpg
 

wsimpso1

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Your question poses a false condition; at least with the Tommy Rose accident. The Venture is not 'short coupled'. My understanding is that the ratio of wing chord to tailplane distance is well within the range of traditional designs. It only *looks* short coupled, because of the relatively high aspect ratio (skinny) wing.

Charlie
Agreed. Look at the plan view. The proportions are virtually the same as the Piper Malibu line, as are the tail volume coefficients and damping coefficients, allowable CG positions, etc.

What is different is with small chord surfaces, control moments were low, leading to really light input forces for what you get in responses. The design team corrected for that with centering spring system, and that was changed to a much lower gradient system in the accident airplane. Then it was flown above known Vne. After that we do not really know if it was PIO or flutter, but either could have been present and damaged the tail as it did.

Billski
 

wsimpso1

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It has 310 hp engine ..side by side seating and under 5 m long.
So what? Tail volume coefficient and pitch stability correlate almost perfectly. Pitch stability is a measure of how stiff an airplane is - that is how much elevator movement is required to get how much deck angle change.

Ctv = Sh/Sw x arm/MAC

Two airplanes with same Ctv have the same pitch stability basics. After that, you can still mess up pitch stability by moving the CG aft towards the NP, getting rid of your static margin.

Griswold and his team copied proportions from the very well mannered Piper Malibu. Now if the 1/4c point on the tail was only 2 MAC aft of the 1/4c point of the wing, then that might qualify as close coupled, but the tail really is pretty far back from the wing counting MAC's, which is what matters.

Billski
 

rv7charlie

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IIRC, Griswold actually participated in the design of the Malibu, prior to designing the Venture. Proportions are not so much of a coincidence.

For a little local lore... Tommy Rose lived about 65 NM from me. While he wasn't a close friend, I did know him, and attended some of the same local events including his own flyin at his private strip. Local stories after the crash was that that particular Venture had been 'rode hard & put up wet' (in air racing) before he bought it, and he was advised *not* to purchase it. Note that the Venture is a cross-country a/c; never intended to be flown in that environment.

Having said all that, ugly, like beauty, is in the eye of the beholder. I'm a firm believer in 'form follows function', and as a cross country screamer, the Venture pushes all the right buttons. To me, it looks exquisite. Everything needed is there, and nothing else. It has an incredibly comfortable cockpit, with something like 46-48 inches of width. It has the widest flight envelope of any homebuilt I'm aware of (stall to max speed). If I could justify one, I'd own one. I've lusted after one since a friend started building one while I was still a student, in the early 1990s, and I regret that in the one or two chances I had to purchase in-progress kits, I was unable to pull the trigger.

For one more small world/local lore detail, Questair Aviation (and all the tooling, etc for the Venture) now resides at John Bell Williams Airport (JVW) near Raymond MS, only about 15 miles from my home strip. And no, I have no financial relationship with the company or its owners. They do, however, attend our EAA chapter meetings from time to time,

Charlie
 

Aesquire

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"Pitch stability is a measure of how stiff an airplane is - that is how much elevator movement is required to get how much deck angle change."

Not arguing, but I don't understand. That's not how I learned it. ( doesn't mean I'm right, just that I have a different view )

Elevator movement is related to chord percentage and lever arm lengths, not inherent stability, I thought? I'm sure airfoils and thus stagnation layers figure in there too. More for dead band than what we're discussing, I suppose.

Can you explain it for the old kite flyer?

as to the Venture & looks, I Like the Eggs. Once you fix the wimbly landing gear I don't know of any really bad habits. It is basically a Malibu front & back end with the middle seats removed...

But with any plane operating well over Va speed, you can break stuff with a sudden yank.
 

wsimpso1

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Stability and control IS complex. Sorry.

Sum of forces = zero. It will fly in 1 g;
Sum of moments = zero. You can null out attitude, so it will not be changing pitch attitude;
First derivative of pitching moments is negative. The nose wants to stay pointed forward - when disturbed it wants to return to where it was;

These are usually achieved by having:
Wing making a little more lift than the airplane weighs;
Tail makes some down force to stabilize the airplane, and is equal to the excess lift;
The CG is forward of the Neutral Point - which is where the first derivative of pitching moments is zero.

Now let's go further. Just barely making it stable does not make a good flying bird.

The tail area times its distance aft of the CG must be big enough that the airplane is easily controlled, can set attitude anywhere between negative alpha stall and positive alpha stall, and can do the positive alpha stall while CG is at our forward limit, while flaps are in worst case position, while in ground effect. This is making sure you can do a full stall landing... we usually do this by using tail volume coefficient that is big enough depending upon things like flaps and how big and how maneuverable you want the airplane. See Pazmany's book and Thurston's book for help on this. The coefficient is Sh/Sw*arm/MAC, where Sh is horizontal tail area, Sw is wing area, arm is distance from CG to 1/4c of the tail, MAC is mean aerodynamic chord. Paz and Thurston both have mIn reccomended Ctv info. This is all based upon Ctv of other well behaved airplanes.

The CG is far enough forward of the Neutral Point that the airplane has some stiffness in pitch. If the CG is barely ahead of the NP, it can be pitched up a lot with very little force from the tail. It is stable, but not stiff. If the tail is really well balanced, you can get a lot of elevator with little force on the stick. And if the CG is only barely ahead of the NP, little force on the stick can drive the nose way up. Do yo you want that? Most of us do not... so we want about the right amount of resistance to move the elevator and lift the nose. Well, we do not fully balance the elevator and we set the aftmost CG forward of the NP. Now it takes some elevator force to get travel on it, and the plane has a moment that wants to return the nose to where it was when the stick force is eased off.

That is the short course. Doing the calcs takes some work. I am not going to teach that now, but I believe I have written it up around here someplace.

Billski
 
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pwood66889

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I had PIO in my Cessna 150. Just out of sync with the bounces, resulting in higher and higher excursions.
Am most grateful I had read a flying magazine that described it. Solutions are: Full power and go around -or- no power and land. The trick is stop the bouncing by purposeful action - select a nose setting and keep it there! Don't try to "catch it." Your inputs just add to the excursions because they will be out of phase from where you want them. Your nerves take too long to respond.
Have not had in preferred ride.
 

Speedboat100

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So what? Tail volume coefficient and pitch stability correlate almost perfectly. Pitch stability is a measure of how stiff an airplane is - that is how much elevator movement is required to get how much deck angle change.

Ctv = Sh/Sw x arm/MAC

Two airplanes with same Ctv have the same pitch stability basics. After that, you can still mess up pitch stability by moving the CG aft towards the NP, getting rid of your static margin.

Griswold and his team copied proportions from the very well mannered Piper Malibu. Now if the 1/4c point on the tail was only 2 MAC aft of the 1/4c point of the wing, then that might qualify as close coupled, but the tail really is pretty far back from the wing counting MAC's, which is what matters.

Billski

Okay...I understand...just wanted to be sure.
 

Speedboat100

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PIOs happen when the pilot responds to the disturbance of his intended attitude with corrections that are out of sync with the frequency of the plane's response to his inputs.

a fancy way of saying you get all tense and jerky. It can happen on any airplane, but is more common in ones that respond quickly to input. You can PIO a 747.

my limited experience shows it happens to me when flying a new craft that is faster ( or slower ) to respond to inputs than I'm used to. Usually just a momentary bobble, if I'm paying attention and can relax into the motion. Unclench the death grip on the controls and it should smooth out. Two fingers instead of full fist, often helps.

PIOs are not flutter. But can cause destructive oscillations in extreme cases.

Especially if the controls are light. Tommy Roses Venture had less artificial elevator resistance than was requested in the handbook.

Cri Cri has very light flying tail and needs also artificial reistance to be able to fly PIO free.
 

Speedboat100

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The Questair may have been PIO, flutter, a single sudden control movement, or simply a control or structural failure. Any or all are possible since the tail failed, the plane was flying well past its allowable design speed, and the FAA found issues with the construction and the choice to reduce the strength of the control stick force spring. Probably nothing to learn here for someone who flies well within their plane’s allowable flight envelope.

Personally, I think it might have been due to flying an ugly plane design.
Yes I referred to this.
 

Speedboat100

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IIRC, Griswold actually participated in the design of the Malibu, prior to designing the Venture. Proportions are not so much of a coincidence.

For a little local lore... Tommy Rose lived about 65 NM from me. While he wasn't a close friend, I did know him, and attended some of the same local events including his own flyin at his private strip. Local stories after the crash was that that particular Venture had been 'rode hard & put up wet' (in air racing) before he bought it, and he was advised *not* to purchase it. Note that the Venture is a cross-country a/c; never intended to be flown in that environment.

Having said all that, ugly, like beauty, is in the eye of the beholder. I'm a firm believer in 'form follows function', and as a cross country screamer, the Venture pushes all the right buttons. To me, it looks exquisite. Everything needed is there, and nothing else. It has an incredibly comfortable cockpit, with something like 46-48 inches of width. It has the widest flight envelope of any homebuilt I'm aware of (stall to max speed). If I could justify one, I'd own one. I've lusted after one since a friend started building one while I was still a student, in the early 1990s, and I regret that in the one or two chances I had to purchase in-progress kits, I was unable to pull the trigger.

For one more small world/local lore detail, Questair Aviation (and all the tooling, etc for the Venture) now resides at John Bell Williams Airport (JVW) near Raymond MS, only about 15 miles from my home strip. And no, I have no financial relationship with the company or its owners. They do, however, attend our EAA chapter meetings from time to time,

Charlie
I agree 100% here. Having said that...the egg shape is not the most aerodynamic form....the water drop then again is closer to such.

Also is worth to mention...that Tommy Rose was very quickly closing on the leader as the accident/breakage happened...so the egg form may have potential.
 
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