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PIO ! ( pilot induced oscillation )

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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

Yes how is the SHORT COUPLED actually defined ?

I think tail volume is defined with chord and moment arm and area based calculation.

So if the elevator length is longer than the moment arm...are we then usually close to "short coupled" ?
 

Lendo

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Short Coupled is defined as less than 3 time the MAC Chord length between C of G MAC and 25% of HT MAC Chord. It seems this is where the HT Volume increases exponentially to make up for less Lever Arm length. Probably 25% Wing MAC to 25% HT MAC for preliminary estimates - a little extra never hurts.
George
 

Speedboat100

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Short Coupled is defined as less than 3 time the MAC Chord length between C of G MAC and 25% of HT MAC Chord. It seems this is where the HT Volume increases exponentially to make up for less Lever Arm length. Probably 25% Wing MAC to 25% HT MAC for preliminary estimates - a little extra never hurts.
George
Short Coupled is defined as less than 3 time the MAC Chord length between C of G MAC and 25% of HT MAC Chord. It seems this is where the HT Volume increases exponentially to make up for less Lever Arm length. Probably 25% Wing MAC to 25% HT MAC for preliminary estimates - a little extra never hurts.
George

So all fast jets are automatically short coupled and need a computer to steer it.

How about business jets ?

Does taper make a difference...or the sweep ?

g650xx.jpg
 
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Aesquire

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"So all fast jets are automatically short coupled and need a computer to steer it."

Not so. Short coupled and unstable are different things.

An aircraft unstable in pitch can be uncontrollable without active computer stabilization and/or control. It's a bit of a sliding scale ... At very low levels of stability pilot workload increases, as the aircraft doesn't automatically correct as fast. At neutral pitch stability you need to fly 100% of the time, and at negative stability, if the nose goes up it wants to go up more, etc. This can range from barely able to keep from losing control for a short time to crash on take off.

The F-16 is deliberately unstable and when the computers fail, you eject. But most successful jets from the first Heinkel to 747s are reasonably stable. The "relaxed stability" ( less scary than "unstable monster" ) fighters etc. are relatively new.

Short coupled is a relative term. Not necessarily related to stability at all.
 

Map

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I flew a directionally unstable small (LSA) airplane during flight tests. When I released the rudder pedals, the airplane started to yaw one way or the other and would have continued to who knows how far. The rudder also deflected on its own (negative force gradient)
I did net let it get very far but used the rudder to bring the yaw back to zero. It was flyable but required more attention.
With some modifications it was later made directionally stable. It was not "short-coupled".

I have made some videos on this topic.

https://youtu.be/qK2UxLwFMiE Horizontal tail

Vertical tail
 

wsimpso1

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I think tail volume is defined with chord and moment arm and area based calculation.
You missed a term. It is dimensionless and thus can be used to look at tail volume in airplanes varying in size.

Tail volume coefficient is (Sh*L)/(Sw*MAC) where Sh is tail area, Sw is wing area, L is distance along the FS axis from airplane CG aft to 1/4 of tail, MAC is mean aerodynamic chord of the wing. So it can be looked at as the product of the ratio of tail to wing area times the number of wing chord the tail is aft of CG. It can also be looked at as tail area times arm (tail volume) normalized by wing area and MAC.

So if the elevator length is longer than the moment arm...are we then usually close to "short coupled" ?
Elevator length? I can not even guess where that came from. My apologies if my description got you there. The "arm" we are interested in is how far back the tail is from the wing, because the tail is on the airplane to null out the pitching moment from the wing itself and from weight ahead of the neutral point. To just get enough pitch control to null out pitching moment, the airplane does not care much if the tail are is big but the arm is short, or if the arm is long and the tail smaller. Looking at total induced drag, yeah, a longer arm let's you have a smaller tail and that reduces the tail load needed, which also reduces the load the wing has to carry and so a longer tail arm reduces induced drag - very important in sailplanes, which is why you see longish tail arms and relatively small tailplanes on them.

Going further, pitch damping goes with length of the tail boom squared, and pitch damping helps to settle the airplane if it is oscillating in pitch, like in a PIO.

Billski
 

Hephaestus

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I flew a directionally unstable small (LSA) airplane during flight tests. When I released the rudder pedals, the airplane started to yaw one way or the other and would have continued to who knows how far. The rudder also deflected on its own (negative force gradient)
I did net let it get very far but used the rudder to bring the yaw back to zero. It was flyable but required more attention.
With some modifications it was later made directionally stable. It was not "short-coupled".

I have made some videos on this topic.

https://youtu.be/qK2UxLwFMiE Horizontal tail

Vertical tail
Hey Sonya,

Add to your youtube descriptions, topics covered in the video etc, the "small aircraft design series" is too general and not going to get you much traffic, add words like tail sizing, horizontal stab, etc etc etc and you'll end up in more search results.
 

wsimpso1

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How many aircraft has negative directional stability ?




Is directionally unstable plane with "short coupled" looks prone to kill pilots ?
Few do as negative stability in pitch and yaw are difficult to fly and almost impossible to fly for long. They require the efforts to the pilot continuously to keep pointed straight on. Usually, it will only get away from you once, as it tumbles after that. There are some tactical aircraft that are unstable, with a suite of rate and AOA sensors feeding data to an autopilot system. You tell the airplane what you want it to do with pressure on stick, rudder pedals, and throttle, and the airplane takes it from there. The goal was very fast transition for maneuvering, and it does that...

There are stories about that Rutan's early Vari-EZ and/or Long-EZ were pretty much neutral on yaw axis stability until Burt increased tip sail area.

Negative stability is why we pay so much attention to CG before flight - Forward CG may require longer take off roll to lift the nose gear and force the airplane on in landing, but forward CG results in the airplane trying to drive the nose up or down or to one side with no hope of bringing it back...

Instability in pitch and/or yaw axes is sufficient to kill you all by itself. No need to include a short tail arm too...

We have one more axis, and roll stability is not needed and is considered undesirable by many pilots. Roll instability builds up roll slowly enough that most pilots keep up with it naturally. If the bird is stable in roll, it can be both unpleasant to fly and difficult to fly precisely. There are a number of birds with anhedral to make them better airplanes. AV8 Harrier, C5 Galaxy, C17 Globemaster, Vari-EZ, B-52. Steve Wittman's Tailwind and Buttercup both have straight high wings as do a number of Cessna designs as dihedral effect would be too high with any visible dihedral added to their high wings.

Billski
 

Speedboat100

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You missed a term. It is dimensionless and thus can be used to look at tail volume in airplanes varying in size.

Tail volume coefficient is (Sh*L)/(Sw*MAC) where Sh is tail area, Sw is wing area, L is distance along the FS axis from airplane CG aft to 1/4 of tail, MAC is mean aerodynamic chord of the wing. So it can be looked at as the product of the ratio of tail to wing area times the number of wing chord the tail is aft of CG. It can also be looked at as tail area times arm (tail volume) normalized by wing area and MAC.


Elevator length? I can not even guess where that came from. My apologies if my description got you there. The "arm" we are interested in is how far back the tail is from the wing, because the tail is on the airplane to null out the pitching moment from the wing itself and from weight ahead of the neutral point. To just get enough pitch control to null out pitching moment, the airplane does not care much if the tail are is big but the arm is short, or if the arm is long and the tail smaller. Looking at total induced drag, yeah, a longer arm let's you have a smaller tail and that reduces the tail load needed, which also reduces the load the wing has to carry and so a longer tail arm reduces induced drag - very important in sailplanes, which is why you see longish tail arms and relatively small tailplanes on them.

Going further, pitch damping goes with length of the tail boom squared, and pitch damping helps to settle the airplane if it is oscillating in pitch, like in a PIO.

Billski
I checked that Gulfstream G650 has 37% length in the elevator of the main wing...that is huge.

Grumman Bearcat F8F might even have a larger elevator....lookin sorta short coupled as well.

Grumman_F8F-2_Bearcat_BuAer_drawings_1949.png
 
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Speedboat100

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Does "short coupled" term somehow seem to refer to small planes only ?

Like BD-5 and Pou Du Ciel etc ?
 

Aesquire

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Elevator is the movable control surface usually hinged on the back of the Horizontal Stabilizer.

when you are asking technical questions, the terminology has to be consistent.

On a Canard like a Vari-EZ, the Horizontal Stabilizer (HS) is in front, with the Elevator hinged to it. On a "tandem" wing like a Quickie, it's basically a Canard ( French for duck, as the early planes with forward HS like the Wright Flyer looked, if you squint, like a long neck duck or goose ) with more equal sized surfaces.

Some planes, the F-100 & F-16 for example, have an "all flying" Horizontal Stabilizer that pivots on a fitting in the fuselage, so it's Stabilizer and Elevator at the same time. There may or may not be hinged surfaces on that "all flying stabilizer / elevator" but the important part is that the whole thing moved to control pitch. The F-100 is normally pitch stable, the F-16 is not, so the configuration isn't the difference .

I agree stability in Pitch & Yaw are the important axes. When the tail passes you it's probably a bad day, unless you're doing aerobatics and so intended.

Most of my flight time is in swept wing hang gliders, which often have anhedral to produce neutral or even slightly negative roll stability. That's a pilot preference, and usually can be tuned to suit.

You turn a Lot in gliders, and with weight shift control, roll on a 10+ Meter wing takes muscle. And wing warping.
 

wsimpso1

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I checked that Gulfstream G650 has 37% length in the elevator of the main wing...that is huge.

Grumman Bearcat F8F might even have a larger elevator....lookin sorta short coupled as well.

View attachment 107635
You asked us questions, we presented the standard theory on the topic, showed you the math used, defined terms.

"37% length in the elevator of the main wing" sorry, not following you. What is this "length in the elevator of the main wing"? To our usual definition, the elevator is on a tail surface and is not part of the main wing. Where is this elevator of the main wing? Yes, both the G650 and the F8F appear to have the tail a little close to the CG in terms of how many MAC fit in the interval, but they also have big tail plane area compared to wing area, and so get sufficient tail volume that way.

Are you proposing a new term or coefficient? Or are you perhaps misunderstanding the ones we have been trying to help you understand?

Bill
 

rv7charlie

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SB100, are you confusing tail *distance* (from the wing) with tail *length* (span)?

BTW, Why do you think that the Venture fuselage (egg) 'isn't the most aerodynamic' (for subsonic flight)?
 

Speedboat100

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SB100, are you confusing tail *distance* (from the wing) with tail *length* (span)?

BTW, Why do you think that the Venture fuselage (egg) 'isn't the most aerodynamic' (for subsonic flight)?
It has the thickest point at the wing neighbourhood.
 

Speedboat100

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You asked us questions, we presented the standard theory on the topic, showed you the math used, defined terms.

"37% length in the elevator of the main wing" sorry, not following you. What is this "length in the elevator of the main wing"? To our usual definition, the elevator is on a tail surface and is not part of the main wing. Where is this elevator of the main wing? Yes, both the G650 and the F8F appear to have the tail a little close to the CG in terms of how many MAC fit in the interval, but they also have big tail plane area compared to wing area, and so get sufficient tail volume that way.

Are you proposing a new term or coefficient? Or are you perhaps misunderstanding the ones we have been trying to help you understand?

Bill

Yes I undestand the formula quite well, obviously in need to save weight etc you have to make the elevator longish, which is a bit challenge....and sweeping it is a must..like did already Fokker in his triplane.
 

TFF

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I think the real point is one spec does not make a design. A flying wing is as short coupled as it gets. It also has to have compromises to make it work like impeccable CG. A flying wing has the least amount of leverage per elevator defection, the more you stretch the tail out back the more efficient it all works.

Reynolds Numbers, size, length, weight, speed, airfoils, sweep, incidence and more answer the question. It’s easier to stay away from short coupling because you don’t have to be spot on with the details.

One of the worst flying RC planes I ever built was short coupled with a small tail. It was a scale model. It was just about un flyable. I doubled the stabilizer area and it was more flyable, but it was a huge disappointment. Unusable for competition. If I had been a real airplane and it didn’t kill me, it would have broken my heart. If you looked at it, you would think it was some sort of pre Cub Cub. It practically wanted to tumble out of the air in pitch.
 

Speedboat100

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I think the real point is one spec does not make a design. A flying wing is as short coupled as it gets. It also has to have compromises to make it work like impeccable CG. A flying wing has the least amount of leverage per elevator defection, the more you stretch the tail out back the more efficient it all works.

Reynolds Numbers, size, length, weight, speed, airfoils, sweep, incidence and more answer the question. It’s easier to stay away from short coupling because you don’t have to be spot on with the details.

One of the worst flying RC planes I ever built was short coupled with a small tail. It was a scale model. It was just about un flyable. I doubled the stabilizer area and it was more flyable, but it was a huge disappointment. Unusable for competition. If I had been a real airplane and it didn’t kill me, it would have broken my heart. If you looked at it, you would think it was some sort of pre Cub Cub. It practically wanted to tumble out of the air in pitch.

Yes PIO is a dark ghost...I assume long movement in the stick while only moving the elevator little is a must ?

I recall FiAF had Folland Gnats that were notorious for the PIO.
 

Erik Snyman

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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
...........beauty, is in the eye of the beholder.....
 

Erik Snyman

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...........beauty, is in the eye of the beholder..... I believe you have that wrong, Charlie. It should read "......beauty is in the eye of the BEER-holder......."

Regards,
Erik in Oz.
 
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