Quantcast

PIO ! ( pilot induced oscillation )

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

flitzerpilot

Well-Known Member
Joined
Apr 19, 2017
Messages
115
Location
Hirwaun, Aberdare, S.Wales, UK.
Defining 'short-coupled' as a 3:1 ratio, 1/4 MAC to 1/4 HT with the implication that PIO may result from breaching this figure is not consistent with practice and experience, assuming adequate tail volume. Without checking my own figures relating to the prototype Flitzer Z-1, I can say that the tail arm is a lot less than this ratio and yet the a/c is fully stable in pitch hands-free at Vc and will remains so indefinitely and without an active trimming system (all unnecessary systems eliminated in the quest for utter simplicity) only small adjustments to RPM are needed as fuel is burned off, the tank being slightly ahead of the average CG.

The prototype tail area is smaller than on the plans-built Z-21 type, which was only increased in an attempt to achieve a full g-break with a light weight pilot. The foil used is the USA 35B which has a CP travel, migrating forwards from 45% to 30% approx at Vs and CG does not breach the 30% MAC position. Full (control deflection) pitch-excursions, positive or negative from level flight at cruise result in the a/c returning hands-free to a level attitude with zero pitching oscillation around its original attitude: ie dead-beat damped at Vc.

The natural in-flight stability which provides for relaxed X-country flying transitions to extreme agility with positive pitch input with the mobile CP approaching the CG, creating a highly manoeuvrable machine for a little recreational aerial combat, turn rate being 360 in 7 secs.

So such a hard and fast rule on tail arm is not necessarily correct.
 

Speedboat100

Well-Known Member
Joined
Nov 8, 2018
Messages
1,891
Location
Europe
Defining 'short-coupled' as a 3:1 ratio, 1/4 MAC to 1/4 HT with the implication that PIO may result from breaching this figure is not consistent with practice and experience, assuming adequate tail volume. Without checking my own figures relating to the prototype Flitzer Z-1, I can say that the tail arm is a lot less than this ratio and yet the a/c is fully stable in pitch hands-free at Vc and will remains so indefinitely and without an active trimming system (all unnecessary systems eliminated in the quest for utter simplicity) only small adjustments to RPM are needed as fuel is burned off, the tank being slightly ahead of the average CG.

The prototype tail area is smaller than on the plans-built Z-21 type, which was only increased in an attempt to achieve a full g-break with a light weight pilot. The foil used is the USA 35B which has a CP travel, migrating forwards from 45% to 30% approx at Vs and CG does not breach the 30% MAC position. Full (control deflection) pitch-excursions, positive or negative from level flight at cruise result in the a/c returning hands-free to a level attitude with zero pitching oscillation around its original attitude: ie dead-beat damped at Vc.

The natural in-flight stability which provides for relaxed X-country flying transitions to extreme agility with positive pitch input with the mobile CP approaching the CG, creating a highly manoeuvrable machine for a little recreational aerial combat, turn rate being 360 in 7 secs.

So such a hard and fast rule on tail arm is not necessarily correct.
I wholeheartedly agree with you.

Many things affect to short coupledness.

I bet the stiffness of the main wing can be one aspect.
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
7,641
Location
Saline Michigan
I wholeheartedly agree with you.

Many things affect to short coupledness.

I bet the stiffness of the main wing can be one aspect.
Let's see if anyone else knows of evidence in the literature. You could do your own search too. The place I would start would be in texts on the topic of stability and control, and look for effects of whatever stiffness you have in mind.

So, what do you mean by wing stiffness? Are you speaking of load versus deflection in bending, load vs deflection in torsion, skin panel q vs deflection, aeroelastic stiffness in pitch? What is the basis for proposing that this matters? If you think that it does and no one else is all excited about it, you might just have to go get data on the topic and see if it really does contribute to PIO.

Airplane structures tend to be defined by minimum weight to get to strength for the bird's mission, and then whatever stiffness is needed to keep flutter at bay for the resulting airpseeds. This is because Weight is The Enemy in airplanes. Most of what I have seen on the topic of wing deflection and stiffness is aeroelastic effects on flutter. Beyond that I have seen concern for panel deflection in order to preserve laminar flow on wings and tailplanes.

Billski
 

rv7charlie

Well-Known Member
Joined
Nov 17, 2014
Messages
1,251
Location
Jackson
It has the thickest point at the wing neighbourhood.
As in 'area rule'? Only became a thing with transonic/supersonic flight. Biggest potential fuselage 'gotcha' is reducing width too soon, in front of the wing's trailing edge. (Ex: Globe Swift)
 

Speedboat100

Well-Known Member
Joined
Nov 8, 2018
Messages
1,891
Location
Europe
As in 'area rule'? Only became a thing with transonic/supersonic flight.Biggest potential fuselage 'gotcha' is reducing width too soon, in front of the wing's trailing edge. (Ex: Globe Swift)
Will this cause a separation at the low speed too soon ?
 

rv7charlie

Well-Known Member
Joined
Nov 17, 2014
Messages
1,251
Location
Jackson
Warning: non-engineer words about engineering stuff. (My version of what an actual aero engineer explained to me.)

It causes drag; the aft section of the wing upper surface is pulling air down; if the fuselage pinches inward in the same area, it is trying to pull the air 'in'. Result is a local low pressure area near the intersection, flow separation, and turbulence. Bigger issue the faster you go. The 'fix' (using the term loosely) on some a/c (like the Swift) is a large and increasing fillet at the wing aft end/fuselage intersection.
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
7,641
Location
Saline Michigan
Warning: non-engineer words about engineering stuff. (My version of what an actual aero engineer explained to me.)

It causes drag; the aft section of the wing upper surface is pulling air down; if the fuselage pinches inward in the same area, it is trying to pull the air 'in'. Result is a local low pressure area near the intersection, flow separation, and turbulence. Bigger issue the faster you go. The 'fix' (using the term loosely) on some a/c (like the Swift) is a large and increasing fillet at the wing aft end/fuselage intersection.
I use that description too. This issue is prevented way better by simply carrying vertical fuselage walls parallel to the long axis of the airplane through the whole root chord of the wing. Look at any modern jetliner or business jet. It may not look the best, but the air thinks so...
 

rv7charlie

Well-Known Member
Joined
Nov 17, 2014
Messages
1,251
Location
Jackson
Yeah, that's what I meant by 'fix' (in quotes). Swift (and others) owners get what the designer gave them, so the compensation is the big fillet as the intersection approaches the trailing edge.
 

Speedboat100

Well-Known Member
Joined
Nov 8, 2018
Messages
1,891
Location
Europe
Back to PIO: Pilot-induced oscillation - Wikipedia

Over correcting seems to be main issue.

Pilot-induced oscillations, as defined by MIL-HDBK-1797A,[1] are sustained or uncontrollable oscillations resulting from efforts of the pilot to control the aircraft and occurs when the pilot of an aircraft inadvertently commands an often increasing series of corrections in opposite directions, each an attempt to cover the aircraft's reaction to the previous input with an over correction in the opposite direction. An aircraft in such a condition can appear to be "porpoising" switching between upward and downward directions. As such it is a coupling of the frequency of the pilot's inputs and the aircraft's own frequency. During flight test, pilot-induced oscillation is one of the handling qualities factors that is analyzed, with the aircraft being graded by an established scale (chart at right). In order to avoid any assumption that oscillation is necessarily the fault of the pilot, new terms have been suggested to replace pilot-induced oscillation. These include aircraft-pilot coupling, pilot–in-the-loop oscillations and pilot-assisted (or augmented) oscillations.[2]



How to recover:

PIO tendency jet Folland Gnat :

Folland_Gnat_Mk_I.svg.png
 
Last edited:

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
14,176
Location
Memphis, TN
Correct. The pilots job is to recognize this and stop correcting. Stop chasing the problem. Then the pilots brain can reset on the sensitivity of the aircraft. The pilot is stuck dealing with it until he lands. The designers are stuck with it after he lands.
 

Aesquire

Well-Known Member
Joined
Jul 28, 2014
Messages
2,540
Location
Rochester, NY, USA
It's not necessarily a problem with the airplane. At all. PIOs, are Pilot Induced.

it's not flutter. It's not close coupled. It's not a stability thing.

it's the pilot expectations out of sync with reality.

It isn't even required that the plane responds faster than the pilot expects, although that is probably the most common "cause". That Vickers Vimy replica had some PIO issues with some pilots, because it responded so Slowly to input.

 

Speedboat100

Well-Known Member
Joined
Nov 8, 2018
Messages
1,891
Location
Europe
It's not necessarily a problem with the airplane. At all. PIOs, are Pilot Induced.

it's not flutter. It's not close coupled. It's not a stability thing.

it's the pilot expectations out of sync with reality.

It isn't even required that the plane responds faster than the pilot expects, although that is probably the most common "cause". That Vickers Vimy replica had some PIO issues with some pilots, because it responded so Slowly to input.


Right...I think this is why often the plane is blamed....usually small planes that the pilots are not used to fly.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
14,176
Location
Memphis, TN
Some pilots can’t adapt well. Some have no issue. The question comes, what are you? If you design something or just build something out there your ability can’t catch up with, the choice was not good. Low time pilots, especially older, might have issues. High time pilots have been there before. Experience or better design, pick where you want to start the design. Lots build cool planes and find out they can’t fly them. It’s a big reason many homebuilts end up low time airframes, the owners are too scared to fly them.
 

Speedboat100

Well-Known Member
Joined
Nov 8, 2018
Messages
1,891
Location
Europe
Yes and like Folland Gnat becomes a legend as several pilots are able to get the ride of their lifetime and live to tell about it. Our neighbour once had cut a hay stack as he went so low in the bronco wild mode with a Gnat.
 

Doran Jaffas

Well-Known Member
Joined
Jun 25, 2019
Messages
316
PIO...nuts... another opinion?? can be induced in any aircraft. Obviously speaking the smaller coupled aircraft with larger power to weight ratios are more likely to have this happen but again it can happen to any aircraft of any horsepower or size. The pilot, whether older or younger experienced or less experienced should always get a check out and at least a similar aircraft before attempting to fly anything such as the above mentioned especially in the smaller categories such as the Wittman Tailwind, note.. I'm partial to that.. the Pits series of aircraft, the KR series of aircraft, even the extremely popular RV series of aircraft, the older designs such as the Baby Lakes, the Sonerai series etc etc... You get the idea. Most pilots can adapt to these with the proper checkout but some will not be able to and they need to be realistic about their own personal capabilities as well as the person checking them out needs to be able to be upfront with them in a respectful manner. I have flown some what I would call are sensitive aircraft that some would call twitchy but with a little finessing they become fun to fly and pretty soon they're sensitivity becomes a moot point. Anyone out there contemplating any type of experimental except the low and slow types that we all know about should definitely get a check out even if they have flown the type before. it had been 30 years since I had flown anything that I would call sensitive when I purchased my Tailwind W8. The checkout was the best money spent on that airplane. Only took a few hours to get reacquainted with the type of aircraft but had I not had that check out at the least I would have scared tthe living daylights out of myself and at worst I don't want to think about... Now after over 100 hours in the airplane I am very comfortable in it but should I ever decide to sell it and I doubt that I will at this point, I would definitely want to take the perspective purchaser up and give them an idea of what it is like and I would not sell it to him and they agree to get a check out my qualified instructor in the airplane or something similar.
 

Aesquire

Well-Known Member
Joined
Jul 28, 2014
Messages
2,540
Location
Rochester, NY, USA
I've had PIOs in air-craft with no power, and no control surfaces. But different response TIMES to one I was used to flying. Usually it shows as a momentary "bobble" before I've reprogrammed my reactions. Occasionally it took several seconds, quite worrisome!

I'm not sure the Gnat is really the problem. I've watched new Semi-truck drivers out on the old airfield ( closed, I'm probably the last to fly there before they made it a mall ) over correcting as the reality didn't match expectations.

There were trainers, the AT-6, for example, that had a reputation for being more difficult to fly than the full on fighters the students were being trained for. A lesson in one is on my bucket list for after the pandemic eases and I can leave the Empire & return without restraint. Assume rant on politics, religion, idiots, redacted here. I'll let you know if I PIO. I expect I'd bobble anything! ( he said with misplaced ego)
 

Doran Jaffas

Well-Known Member
Joined
Jun 25, 2019
Messages
316
New aviation term.. bobbleheads. I would venture to say we've all done it or will.
I did it my first flight in the Tailwind. Really bothered me but it didn't take long to get the feel of the airplane. I tell people it took me about 3 hours to realize it wasn't going to crash it and another 30 to feel like I was making friends with it.
 
Top