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Old May 12, 2013, 09:28 PM
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New Zealand, Waikato, Hamilton
Joined Jul 2011
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Question
Tail Wag

I've noticed a lot recently that when I'm in cruise mode searching around - that the tail quite often "wags" left/right by quite a lot - its not drawing one way or the other - just wags.

there is no indication of lift - anyone know what it could be? just turbulance?
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Old May 12, 2013, 09:34 PM
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United States, AZ, Arizona City
Joined Sep 2001
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Could you be seeing a wing lift that causes the tail wag?

Gary
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Old May 12, 2013, 09:41 PM
Duane
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No. VA
Joined Nov 2004
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That can be one of the indications of lift. Passing through a gust blowing towards a thermal can cause that tail wag. Look for the direction of initial excursion, and turn that direction.
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Old May 12, 2013, 09:50 PM
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gtrbaby:

I've seen the same thing, under either of two conditions.

First, at high speed between thermals, almost every one of my DLGs through the past several years has exhibited some directional instability. At high speed, I've attributed the behavior to some sort of flow separation around the vertical. I don't have any proof - I'm just supposing. My Taboos did it at high speed, so did my Blasters, and now so does my new Neos. I was scooting along at high speed just yesterday, and sure enough.

Second, at cruise speeds I have also seen some "tail wag", and finally realized I was flying too slow. The airplane was actually on the verge of a stall, and the directional behavior came about as one wing or the other momentarily stalled. Solution: a click or two of down-trim. My airplanes then exhibited less tail wag, greater legs and longer times. It's a lesson I've had to learn several times - it's actually difficult to fly too fast.

I am sure that others with greater technical insight may have better answers than mine.

Yours, Greg
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Old May 13, 2013, 10:24 AM
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Las Cruces, New Mexico, United States
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Maybe your issue is just a case of having a happy plane? I know I wag my tail when I get to go flying

OTOH - I second Greg's suggestion of flying faster. I find that I have a bad habbit of flying too slowly when trying to max my times. Once I relax and let the glider run a bit faster, flight times extend as if by magic. The magic of better L/D.
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Old May 13, 2013, 10:48 AM
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Denver CO
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All my planes do this too! I just figure it's either small shear layers I'm flying through or the type of tail since I use foiled tails like swindells.

Brandon
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Old May 13, 2013, 11:02 AM
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Just an added thought.

My airplanes (when properly set up) indicate lift in the usual manner: surging upward, lifting a wing tip. The tail wag is different: it's cyclic, and more what I would think of as Dutch Roll (which we dealt with a lot in jet fighters, when I was a flight test engineer at Edwards). (Yes, that was a long time ago, and most flying things flapped their wings.) (And also yes, my test pilots were Icarus and Deadalus.)

I still see the tail wag (Dutch roll?) in my remaining Blaster (which has Swindell tails), at high speed. Also the Neos, at high speed. I have not noticed that behavior in my Concept. When I see directional issues at cruise, it is nearly always an indicator (to me) of flying too slowly.

Yours, Greg
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Old May 13, 2013, 11:15 AM
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United States, CA, Tehachapi
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Just to add my 2c here, dutch roll is caused by high dihedral and small vertical stabilizers. Since DLGs have such huge verticals for launch, I doubt that dutch roll is what's being experienced.

I've noticed the same behavior on my DLGs before and just attributed it to turbulence since it doesn't appear to be periodic or constant in any way.
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Old May 13, 2013, 11:25 AM
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Yeah I just figure our planes are so light there just isn't much mass to keep the tail stable at higher speeds. I have never noticed this on launch however. It might have something to do with using directional verticals?
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Old May 13, 2013, 05:04 PM
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Quote:
Originally Posted by 1900 Driver View Post
Yeah I just figure our planes are so light there just isn't much mass to keep the tail stable at higher speeds. I have never noticed this on launch however. It might have something to do with using directional verticals?
We see a lot of tail wagging when we are DSing. The drag on the wing tips is increasing at an exponential rate with increasing speed and if any turbulence hits the ship a yaw is introduced and the wag is generated. For DS you just try to ignore it in an attempt to prevent a crash (a little DS humor) and some ships the wag much more (Destiny's) and they all have small tails. Even the Blade with its big V-tail wags as it goes through the boundary layer.
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Old May 13, 2013, 05:09 PM
G_T
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Tip airfoils with significant change in absolute drag when operating near the desired operating point can cause wag in turbulence; easily so actually. I would also expect that planes with greater than average dihedral will also have greater than average succeptibility to this as there is greater coupling between yaw and alpha at the tips. Smaller vertical tails or could contribute as well, since they allow greater yaw excursions, all else being equal.

Vertical tails with higher aspect ratio have reduced damping compared to lower aspect ratio. This can also encourage the wag behavior.

Really the only planes I've seen where this is a real problem, when flown correctly, are ones with combinations of all these issues conspiring to make a sensitive plane.

Gerald
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