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a.s.elliott(at)cox.net
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 Posted: Sat Jan 27, 2007 8:35 am Post subject: NOT wing flutter! |
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(long post)
Whoa! One needs be careful saying "wing flutter" on a list devoted to aircraft!
In the aerospace world, flutter is a specific phenomenon defined as "dynamic instability of an elastic body in an airstream" (from Aeroelasticity - Bisplinghoff, Ashley and Hoffman - the standard reference text). In a wing-type body, there is a speed *above* which the oscillations of the wing can go unstable, that is, they get bigger and bigger with each cycle.
The only solution for this is to either slow down, change the wing's aerodynamic characteristics, for example by deploying spoilers or other controls, or change the wing's structural characteristics, for example by buckling stuff. If you are below that speed, the wing can not flutter.
It can shake, rattle and roll for sure, but not flutter! 140 indicated is about 150 true, which should give you plenty of margin. (I am assuming mph. If you were going 140 knots indicated at 5000', that's over 170 mph true, and it's a different story.)
Since you sped up and the wing did not come off, what you saw was not flutter, but simply some excitation of the wing bending. A two inch oscillation on a 601-length wing is not even close to being a structural problem. The wing is stressed for +/- 6 G's meaning that it can hold 6 times the gross weight of the aircraft without any failure.
This is tested in production aircraft by loading up the wing with distributed sand bag weights, so in a typical 601, that would be about 5000 lbs of sand on each wing. You can bet that it will deflect a lot more than 2 inches!
Of course, it is possible to get flutter of just a control surface. This is more likely in aircraft like a 601, which does not have mass balanced ailerons or elevators. But again, since you sped up and this did not get worse, it is very unlikely to have happened.
There are a number of scenarios which could lead to an oscillation like you described, excited by the turbulence - including a low frequency control surface vibration linked through the aileron controls to your hand (the soft servo), a vibration caused by a structural change like a loosening cable or control arm, feedback or other failures in the aileron trim system, feedback in the autopilot if you have one (which should be designed to kick off in that situation).
You should certainly check all those things and the wing attach bolts, and whatever else you can! But it could just have been the wing and/or aileron vibrating after being excited by the strong turbulence, like a guitar string being strummed. Note - when you encounter airframe vibrations, a good response is always to make small control movements and *slow down*, instead of speeding up.
FWIW - standard disclaimers, etc.,
Andy Elliott, Mesa, AZ
(MIT '74, Ph.D. '87, all in Aerospace Engineering)
N601GE (reserved)
601XL/TD/QB, Corvair, building...
[quote][b]
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pilot4pay
Joined: 01 Jan 2007
Posts: 171
Location: Louisville, KY
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| Posted: Sat Jan 27, 2007 9:56 am Post subject: Re: NOT wing flutter! |
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Dr. Elliot, could you perhaps comment on the practicality of VGs on a stabillator? You may contact me off-list at pilot4profit(at)nospamsbcglobal.net
of course remove the nospam
do not archive
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raymondj(at)frontiernet.n
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| Posted: Sat Jan 27, 2007 10:50 am Post subject: NOT wing flutter! |
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Please, please! Keep the discussion on the list. This is the stuff I get the
most benefit from since I haven't decided what to build yet.
Raymond Julian
Kettle River, MN
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JAPhillipsGA(at)aol.com
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| Posted: Sun Jan 28, 2007 8:07 am Post subject: NOT wing flutter! |
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Andy, thanks for the input and I don't doubt your a ton smart on the subject. Still I saw what I saw, heard what I heard, and felt what I felt. I have flown enough to have seen a few wings bend in bumps. Even put a wrinkle in a Warrior wing once, but that's another story. If you extend your arm straight out to your side and move the end of your fingers up and down about 2 -3 inches as fast as you can until they blur and hum very loud as you do this you can recreate what I saw and heard. Some scientist I am ? Oh, well. I knew I was in trouble when it did not change sound when I cut power. That's when I rolled over to dive away and it ended somewhere in the roll and dive. I will recheck everything and cable stretch as well. Thanks again, Best regards, Bill [quote][b]
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Tim Juhl
Joined: 21 Mar 2006
Posts: 488
Location: "Thumb" of Michigan
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| Posted: Sun Jan 28, 2007 2:07 pm Post subject: Re: NOT wing flutter! |
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Bill,
You've got us all interested so please don't take our questions as criticism. I've never seen the wings move in the strut-braced airplanes that I've owned but when I've been bounced around violently I'm afraid I never had the presence of mind to look at the wings until after the event had passed.
Would you mind sharing what you experienced in the cockpit while this was happening? Were you being thrown up and down? Was the stick jerking or reacting to forces on the aileron cables?
Tim Juhl
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