From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of rwolf99@aol.com
Sent: Saturday, December 05, 2009 2:02 AM
To: lml@lancaironline.net
Subject: [LML] Flutter

Dan and Don are both right -- flutter may be a mild buzz (which will increase with airspeed) or it may be an explosive event.  The big boys (by this I mean the aircraft companies with the resoures to do this) approach flutter clearance in a build-up fashion as follows.  Sometimes they do part of this and somtimes they do all of this.

An analytical model is generated which includes the flexibility of the structure *and* the aerodynamic characteristics.  The model is "flown" at various virtual airspeeds to determine what the aeroelastic modes are.  "Aeroelastic" is a big word that we use to make people think we are smarter than we really are.  It includes "aero" -- the aerodynamic effects which generate the forces on the airframe, and "elastic" -- the fact that the structure deforms under load, and therefore the aerodynamic forces change.

By "modes" we mean that there are several things on the airplane that will deform under load, and when there is an interaction between that deflection and the changing aerodynamic loads that results in it shaking back and forth (oscillating), well, that's an "aeroelastic mode".  The obvious one is the wing flexing up and down.  But there are other modes, too, such as the vertical fin flexing back and forth, or the tailcone twisting and the horizontal stabilizer driving that oscillation.  The latter is the mode that Martin Hollmann claims for the large-tail Lancair 320/360, and which Lancair claims is not present.  There is a writeup of this somewhere on Marv's web site.

Okay, once you analytically calculate modes, you do "ground vibration tests" to explore the modes that are significant.  You shake the airplane with accelerometers mounted in strategic places.  (This is done on the ground)  This confirms the modes and their frequencies.  As I recall, here is where Hollmann and Lancair disagree.  Both have done these tests and they got different results.  This testing is beyond the capability of virtually all homebuilders, and costs something like $25,000 to contract out. Probably more.

As an aside, I have no explanation for the different results, but the tests were done on different airplanes so maybe that's a factor.  Hollmann also claims that the critical speed for that mode is around 150 knots.  If it were really a problem, the ground would be littered with Lancair parts.  Hollmann suggests a reinforcement. Most people have not done that, but I wonder if that obviously highly damped mode (if it exists at all) might be related to the fatigue cracks at the ventral fin that some high time 320/360's have experienced.  (Myself, I'm not losing any sleep over this but it is curious. I have also done a different version of Martin's reinforcement which he siad "will probably work but I can't say for sure until you pay me to analyze it".  Personally, I have no concerns about flying in a 320/360 which does not include Martin's reinforcement)

Then you go fly.  Those modes are always there, but they are heavily damped (and therefore not a flight safety issue) until you go fast enough.  (The venetian blind is stable until the wind gets too high, then it flutters.)  But you measure those frequencies and, using electronic instrumentation and computers, you calculate the damping.  You fly at one speed, excite the structure with either an electric shaker or else just rapping the stick or rudder, and you calculate the damping.  Then, only when you have the damping number and it's greater than 2%, you increase the speed 5 knots and do it again.  When the damping drops to 2%, you stop.  That testing is aso beyond the capability of virtually all homebuilders.

Free play can cause the flutter speed to drop dramatically.  On the positive side, this type of flutter is normally the non-explosive kind.  If your airplane is buzzing, slow down.  If it diminishes as you slow down, see what's looser than it used to be.  It could be somethiing apparently innocuous, like hinge wear.  It could also be something more.  Find out and fix it before you fly again.  Don't assume that it won't suddenly get worse -- airlanes don't usually fix themselves.  You might also consider Randy Stuart's cautious approach of flying solo if you have to ferry it home (This is not a snide comment -- I applaud him for that).

Hope this helps, or is at least educational. 

- Rob Wolf