Attention Tailless Nerds!

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RPM314

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And what human is going to be able to duplicate a snap roll in a flying wing - with no vertical surface?!
It's not me.:eek:
Yeah, me neither. I don't see how to do that without strong yaw control, or flat spins for that matter either. Knife edge is of course out of the question. But many key staples of an aerobatic routine can be done with just bank, yank, and a lotta power. Loops, immelmans, rifle rolls, etc. Hammerheads are kinda possible if you enter the climb a little off vertical, but are not very reliable in my experience. Or maybe I'm just not good enough for that:rolleyes:.
 

Doggzilla

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When people say "flying wing" I think of the Horten version with sweep. With sweep and washout they can be made nearly stall-proof as long as the CG is far enough forward. For a Fauvel type some type of belly fin makes sense, but that's best done by having the vertical rudder/s extend below the wing a bit.

The problem is that without an elevator if the CG is forward on a flying wing it can cause tumbling after a stall. There is a video of this happening at an airshow where one stalls and as the nose comes down it just starts flipping forward end over end and crashes.

If the CG is forward on a flying wing or trike, when a stall occurs and that weight is up in the air and comes back down too quickly can gain enough inertia to tumble the aircraft end over end. This is caused by a combination of reduced elevator stability and the mass being so tight together. It makes it easier to rotate it quickly.
 

Aerowerx

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This started out as a notice to Tailless Nerds that the Nickles book was available at a reasonable price, but has turned into a treatise on the subject.

I find this interesting. It has been several years since I have actively pursued the topic myself (Real Life again). There must be some interest out there, though, as every so often the topic gets a spurt of enthusiasm.

My intuitive insight tells me that it is possible to have a pure flying wing with acceptable stability/damping/"whatever you call it", by using the proper combination of sweep, taper, twist, and anhederal/dihedral, with no obvious vertical surfaces. To quote a well know TV program from a few years ago, "The Truth is Out There"!
 

Aerowerx

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The problem is that without an elevator if the CG is forward on a flying wing it can cause tumbling after a stall.....
Which is exactly why flying wings need a larger than typical static margin! Maybe as high as 20%.

The Tailless Aircraft book discusses this, the problems with tuck, and how to avoid it. Which is why flying wings sometimes get a bad rap---people make changes because "it doesn't look right---cg too far forward", and then their families have to make funeral arrangements.
 

Heliano

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May I add something to the thread: As Aerowerx mentioned, a flying wing does need a nice static margin. Having a marginal static stability with a flying wing, among several considerations, is different from a conventional airplane because a conventional airplane has far more pitch damping. As far as flying qualities are concerned, given the same static margin, a conventional airplane is more benign. Another aspect: the longitudinal C of G envelope of a flying wing tends to be shorter than that of a conventional aircrafft.
 

henryk

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"The Tailless Aircraft book discusses this, the problems with tuck, and how to avoid it. "

-but Kaspers tailles glider BKB-1 have not this problem

(tuck resist !)
 

Doggzilla

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Not quite. See post #20 and:
This site for some pictures
I think maybe we are misunderstanding one another.

What Im referring to is not the aerodynamic stability, but the inertial stability. When an aircraft is either very short or very short wingspan it can suffer from inertia coupling, which only occurs in supersonic fighters with little wings and flying wings that are very short in length. Thats why its not very well known, because its not an issue with normal subsonic aircraft with proper wings and tails.

https://en.wikipedia.org/wiki/Inertia_coupling

When aircraft have all components close to one another on a single axis that axis can become unstable because its much easier to rotate a mass that is close together. This is almost never a problem with most GA aircraft because they are wide and long, but it becomes and issue if the aircraft is either very short or has a very short wingspan.
 

Hot Wings

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I think maybe we are misunderstanding one another.
No, I don't think you understand inertia coupling. There is no gyroscopic coupling with a tumbling flying wing. It's purely a dynamically divergent phenomena about the lateral axis, due to static stability, and insufficient damping. Moving the CG forward for more static stability also provides a longer arm for the aerodynamic bits giving them more damping power. It's a resonances thing.

A plane that tends to go flat in a spin is responding to inertia coupling. Here is a demo that might help with the visualization?;

Specifically at around 4:25
 

Doggzilla

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Thats not what Im saying.

The nose being heavy and swinging downward can induce enough inertia during a stall that the stalled control surfaces cannot counter it, and this flips it end over end. Its happened repeatedly before.

Here is a video of a trike having the same issue. When the nose comes down quickly the inertia throws it into a forward flip.


Almost every single case of flying wings tumbling end over end have been because CG shifted forward and caused the inertia of the nose mass to overwhelm the elevator's ability to counter it.

During a stall the elevator on flying wings has reduced effectiveness, aerodynamic and static stability of the elevator are near non existent during a stall. It becomes purely a pitch inertia issue when the control surfaces are stalled.

The Kasperwing has a very centered CG and is actually far more stable and does not tumble or nose over violently during stalls.
 

Aerowerx

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Thats not what Im saying.

The nose being heavy and swinging downward can induce enough inertia during a stall that the stalled control surfaces cannot counter it,
Then design it so that the control surfaces don't stall!

A properly designed flying wing will not enter a hard stall. Instead it will just sort of mush. A stall occurs when the AoA causes the airfoil to reach it's maximum CL. Ideally a flying wing will reach this first at about 30% of the wing half-span. This is achieved by the proper combination of twist, sweep, and airfoil choice. This can easily be seen with XFLR5 virtual experiments. The outer half of each wing-half is still generating lift, so full control is maintained.
 
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Aesquire

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"Almost every single case of flying wings tumbling end over end have been because CG shifted forward and caused the inertia of the nose mass to overwhelm the elevator's ability to counter it."

I disagree, strongly.

First, the video shows a blown aerobatic maneuver. He went inverted ( probably attempting a loop ) and ran out of energy. The wing failed under negative G. The resulting tumble is not a CG issue, since the wing has radically changed shape.

If you had a Kitfox and pulled the pins/bolts that prevent the wing from folding, and they swept back, In flight, it's not that the CG is wrong, it's that the new position of the wing changed where the CG should be on the different design now flopping about to it's doom. And if flapping, the moving CG. Assuming there is a CG that would stabilize the new lift positions. Probably into a smoother lawn dart path.
 

Heliano

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Perhaps we need to organize this subject a little: when one talks about static margins, inertia is NOT a factor. Inertia is a factor when the subject is dynamic stability. When it comes to dynamic, pitch only stability, inertia is a factor but not inertia coupling. There are two possible dynamic pitch oscillations: short period and phugoid. The video showing the out-of-control trike seems to me more of a control problem (where in fact inertia can play a role) rather than a stability problem.
 

Aesquire

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Nearly every tumble of a flying wing I know of is because of CG too far back.

I don't count flex wing hang glider "over the falls" tumbles out of thermals. There, a horizontal vortex ( the mushroom cloud shown in drawings of ideal thermals ) rotates the glider, by brute force. CG position be damned. Sailplanes experience the same phenomenon, but generally are flying faster and leave the vortex before full inversion. Which they are better at withstanding, having 3axis control, and a pilot that is fixed in position, and can't fall into the wing.

Modern flex wing gliders, post 1980's, have dive recovery/inverted recovery, systems that reshape the wing to a highly reflexed airfoil, then back to the balanced lifting shape upon recovery, automatically. (No pilot input needed, or available ) They work fine. Essentially, automatic full up elevator, on a craft without them, until needed.

These are 2 axis, weight shift craft and while the shape, the swept wing with washout, is a good example of a stable flying wing, the structure is very different from a 3 axis design requires, and the washout is considered excessive for maximum efficiency.

The history of the various Northrop flying wings, shows 2 different accident modes.

1. Tumbles from stall while testing rear CG position.
( Better range with less elevon deflection was the goal on some tests )

2. Inadequate or even negative, yaw stability, sideslip, crash.
In the case of the XP-56, the air stream boosted drag rudders had slow response and induced PIO issues. In the XP-79 the crash was from "a slow roll" which became an irrevocable rolling dive. I bet it was a side slip issue. Even a Kitfox in a 90 degree slip is having a bad day.
 
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Doggzilla

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Perhaps we need to organize this subject a little: when one talks about static margins, inertia is NOT a factor. Inertia is a factor when the subject is dynamic stability. When it comes to dynamic, pitch only stability, inertia is a factor but not inertia coupling. There are two possible dynamic pitch oscillations: short period and phugoid. The video showing the out-of-control trike seems to me more of a control problem (where in fact inertia can play a role) rather than a stability problem.
Its caused by the CG sticking out forward and the inertia gained as the mass swings downwards. This inertia from the sudden downswing is vastly higher than the static stability at low airspeeds, since as the airspeed drops the aerodynamic static stability drops as well, and so the inertia gained by the dropping nose can overpower it.
 

Doggzilla

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"Almost every single case of flying wings tumbling end over end have been because CG shifted forward and caused the inertia of the nose mass to overwhelm the elevator's ability to counter it."

I disagree, strongly.

First, the video shows a blown aerobatic maneuver. He went inverted ( probably attempting a loop ) and ran out of energy. The wing failed under negative G. The resulting tumble is not a CG issue, since the wing has radically changed shape.

If you had a Kitfox and pulled the pins/bolts that prevent the wing from folding, and they swept back, In flight, it's not that the CG is wrong, it's that the new position of the wing changed where the CG should be on the different design now flopping about to it's doom. And if flapping, the moving CG. Assuming there is a CG that would stabilize the new lift positions. Probably into a smoother lawn dart path.
The wing folds after the tumbling occurs, and a wing is useless if its stalled and being spun.

I thought maybe you really did not understand but from your blatantly false statement above its clear that you are intentionally being combative. If you are going to be arrogant for the sake of being arrogant I have no interest in talk to you.
 

Aesquire

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The wing folds after the tumbling occurs, and a wing is useless if its stalled and being spun.

I thought maybe you really did not understand but from your blatantly false statement above its clear that you are intentionally being combative. If you are going to be arrogant for the sake of being arrogant I have no interest in talk to you
Upon review of the video, you are correct, the trike tumbled at least three time before structural failure. Because of the blurring, I'm not clear on what the pilot was trying to do before inverting. Without a frame by frame look, I can't tell the position of the trike chassis in relation to the wing, aka the CG position. Obviously the anti-tumble features I would expect did not work in this case. Therefore my initial analysis was incorrect. There's no elevator involved, no stalling of the control surfaces, ( since there are none ) but I can't tell what CG positions are from moment to moment. Horrible crash.

There is certainly something to the notion that low damping and stalled elevons are a problem/feature of wings without tails. IMHO there is some confusion by some on this thread as to the nature of damping, and would welcome a discussion & better explanation since I fear I lack the ability to make the math & physics concepts clear. ( and could be wrong in my current understanding )

My Strong Disagreement is with the idea that forward CG positions are the cause of the majority of flying wing crashes. I could be wrong, and invite a real world example, to show me the truth of the matter.

Your statement "The nose being heavy and swinging downward can induce enough inertia during a stall that the stalled control surfaces cannot counter it, and this flips it end over end. Its happened repeatedly before." Is essentially correct, but I question the "being heavy" part, as the CG should be nose heavy at stall speed with the elevons not balancing the weight. The nose Should drop. That dynamic balance & imbalance is the same on a Cub as a B-2.

The inertial part of your argument, however, is sound. I question if a too far forward CG is destabilizing on it's own, But, Again, I may be under educated.

Strong disagreement, however was, and is, not meant to be insulting. I cast no mean words upon thy character, truthfulness, or intent. I think no ill thoughts towards you. ( although upon being told I am arrogant, my ill tempered reaction is to think "Kiss my shiny metal...", which is a bit arrogant. I admit. ) Combative? Yes, a bit. should I apologize for disagreeing?
 

Aesquire

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I thought maybe you really did not understand but from your blatantly false statement above its clear that you are intentionally being combative. If you are going to be arrogant for the sake of being arrogant I have no interest in talk to you.
I was wrong and did not intentionally issue a blatantly false statement for the purposes of deception or insult. I invite education. I respect you. This isn't politics, I respect your right to arm bears and want to know how you make the sword hilts. A kind of mitt arrangement for battle axes?

As a current pundit says, I stand by the truth. If I find the truth is different, I go stand by the truth over there. I expect any civil discussion we have to make me smarter, and maybe inform you as well. I deeply apologize for any incivility.
 
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