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s_thatcher(at)bellsouth.n
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 Posted: Sun Dec 23, 2007 5:34 am Post subject: Wing or Flap or Aileron Flutter |
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In response to both Jay and Larry who are both excellent contributors to the
list, I think they are right in that it is important to provide the correct
terminology when referring to flutter.
Flutter is not necessarily a result of high speed however and does not
necessarily limit itself to control surfaces. Normally, a designer will fly
the aircraft (usually in a dive) to Vne and if there is no flutter, assume
that it is safe. However, if the aircraft encounters a gust while flying at
even cruise speed, and the center of pressure and elasticity of the wing is
such that the lift on the wing changes (causing an increse in lift), the
wing may experience oscillations that can become sufficiently divergent
resulting in failure.
Normally, flutter is seen in control surfaces that are not tight (such as
ailerons, flaps, elevators) and that can be induced to flutter when used
without a counterbalance. As to whether a divergent vibration on a wing is
called flutter, I will leave that to the experts in that field. I do
remember that the Saturn V experienced such severe vibrations during
lift-off that the name used to describe the condition was not flutter (a
seemingly benign condition compared to what was happending) but rather, "the
Pogo effect."
Regardless of what it is called, we have at least two examples of divergent
vibrations in 601XL wings in which the participants lived to tell about it.
These vibrations were not small, rattle your pencil type vibrations. They
were full blown amplitudes of several feet and gaining in severity.
In one case, the pilot tried to slow the aircraft with no change in
amplitude and elected to dive in order to solve the problem, which it did.
In the second case, during a decending right turn, the wings started to
change amplitude so quickly, the passenger (with hundreds of hours of his
own) said he would never get in that plane again. They landed safely and
examined the plane to discover that the flap nylon bearings had extreme
wear. At this point I do not know if that wear was present during pre-flight
but it was definitely present post flight. I am not making any statement or
judgment here, only indicating what has been reported by the participants.
In both of these cases, the pilots stated they were no where near Vne but
rather in cruise only. Although neither has stated this, it is safe to
assume that both did experience a gust or turbulence of some sort, but it
would be helpful to have this down as empirical rather than speculative.
Anyway, getting back to my original questions, I still want to hear from
those persons who have experienced flutter or high amplitude excitations of
their wings. Here again is the list of questions I am concerned with. I
have added a 15th question.
The following are questions I would like those who have experienced flutter
to provide.
1. What was the speed of the aircraft at the time of the flutter?
2. Do you have wing lockers?
3. Were you using flaps?
4. Were you making left or right turns?
5. Were you decending, ascending or in level flight?
6. Had you attempted any aerobatics prior to this flight?
7. Did you notice any smoking rivets along spar?
8. How did you recover from the flutter?
9. Was there any advanced notice of the pending flutter?
10. What make of engine were you using?
11. Did you notice any unusual vibrations prior to the flutter (prop or
engine)?
12. Did you fly through unusual weather conditions?
13. Did you make any structural changes to the wing itself during or after
construction?
14. Any other information you feel is important.
15. Is your aircraft a Plans (scratch) built, Kit built, Quickbuild or
Manufactured?
Please send your answers to me and I will compile the data and report to the
list, or , if you prefer, please answer the questions directly on this
forum. Please indicate if you want your name withheld.
Scott Thatcher, Palm Beach Gardens, FL
601XL with Corvair, Still 99% complete but Registered as E-LSA
N601EL
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robert.eli(at)comcast.net
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| Posted: Sun Dec 23, 2007 6:47 am Post subject: Wing or Flap or Aileron Flutter |
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Scott,
When this flutter issue discussion was in vogue about a year or so ago, I
also recall a 601XL pilot reporting severe flutter of the wing (not the
control surfaces) while overflying a cooling tower at normal crusing speed.
So there may be at least three cases rather than two. It should be in the
archives.
Bob Eli
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amyvega2005(at)earthlink.
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| Posted: Sun Dec 23, 2007 10:11 am Post subject: Wing or Flap or Aileron Flutter |
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All
I wonder how many cases of "Flutter" are just the wings Oil canning. Flutter is a pretty difficulty thing to see, its more feel.. Oil canning is pretty typical on the 601 and is not a problem.
Juan
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tonyplane(at)bellsouth.ne
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| Posted: Sun Dec 23, 2007 11:20 am Post subject: Wing or Flap or Aileron Flutter |
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| Quote: | | Quote: | | The one case "matronics list" report of "wing flutter?" was in Jan this yea in Georgia, and is as copied below. Was it flutter or just a really bad ride in extremely man made turbulent conditions? If flutter, the diving and picking up speed should have resulted in a really bad outcome. In the other case, do not know the circumstances, but I always pull up and down onmy flaps to make sure excessive play is not present during pre-flight. I wonder if the "second" case the descending right turn was made with flaps down - as copied below. |
012345678901234567 |
89 | Quote: | | Was it flutter or just a really bad ride in extremely man made |
0 | Quote: | | Was it flutter or just a really bad ride in extremely man made |
1 | Quote: | | Was it flutter or just a really bad ride in extremely man made |
2[/quote][/quote]
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[quote][b]
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dredmoody(at)cox.net
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| Posted: Sun Dec 23, 2007 11:58 am Post subject: Wing or Flap or Aileron Flutter |
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That was Bill Phillips and is the case in which the pilot chose to dive to escape the turbulence. Not a new case as far as I can tell.
Dred
[quote] ---
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DRAGONFUEL(at)aol.com
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| Posted: Sun Dec 23, 2007 5:49 pm Post subject: Wing or Flap or Aileron Flutter |
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Whenever an airplane goes out of control with flutter, oscillations, porpoise, or whatever you call it, the rapid shift in forces puts great stress on the airframe. The up cycle puts excess positive G forces on the frame and the down cycle puts excess negative G forces on the frame, control surface, flap, elevator, wing or whatever. These excess forces quickly exceed the design limits of the airframe, resulting in structural failure -- it happens pretty fast. The immediate actions needed are to reduce airspeed with power and attitude change. This has traditionally meant to pull up to slow down, along with reduction of power. This is why some folks shook their heads when the 601 pilot in the power plant incident went into a dive rather than pull up. But he got away with it. He probably stumbled into what military fighter pilots have been doing for several generations.
When an airplane gets into an out of control maneuver: a PIO (pilot induced oscillation), a JC maneuver (named for the person whose birth we are celebrating), an unwinnable dogfight in combat, the correct maneuver is often to unload the G forces on the plane and fly out of the maneuver at zero G's. That is to push forward slightly until the weight of your body on the seat is zero, not any positive force and no negative force, equilibrium. This will eliminate overstress on the airframe. It may not break the oscillations immediately, so it is vital not the chase the oscillations (you can never catch up with them and will almost certainly exert excessive forces in the wrong direction), but to continue to hold slightly forward elevator forces to keep your butt light on the seat. Postural sense they call it. Once the plane is back under full control, you can apply power to accelerate out of any stall, or reduce power to avoid an overspeed dive, just don't re-enter another out of control situation.
At zero G's an airplane can accelerate rapidly -- but you are exchanging altitude for airspeed. Since there is no G force on the plane, it has little drag, and engine power seems to be multiplied. Don't overspeed engine, prop, or aircraft limits.
Get the plane on the ground ASAP and check it out for damage. Land at the nearest field, you may have another failure you are not aware of.
Cheers,
Bob Archibald
Dragonfly Aviation
CH601XL/Lyc/Dynons
See AOL's top rated recipes and easy ways to stay in shape for winter.
[quote][b]
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