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Subject: Flutter
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Dan and Don are both right -- flutter may be a mild buzz (which will incre=
ase 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 flut=
ter 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 str=
ucture *and* the aerodynamic characteristics.  The model is "flown" at var=
ious virtual airspeeds to determine what the aeroelastic modes are.  "Aero=
elastic" is a big word that we use to make people think we are smarter tha=
n we really are.  It includes "aero" -- the aerodynamic effects which gene=
rate the forces on the airframe, and "elastic" -- the fact that the struct=
ure 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 deflecti=
on 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 test=
s" 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 whe=
re Hollmann and Lancair disagree.  Both have done these tests and they got=
 different results.  This testing is beyond the capability of virtually al=
l 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 test=
s were done on different airplanes so maybe that's a factor.  Hollmann als=
o 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 reinf=
orcement 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)=20

Then you go fly.  Those modes are always there, but they are heavily dampe=
d (and therefore not a flight safety issue) until you go fast enough.  (Th=
e 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 sti=
ck 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 positi=
ve 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 inno=
cuous, like hinge wear.  It could also be something more.  Find out and fi=
x 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 (T=
his is not a snide comment -- I applaud him for that).

Hope this helps, or is at least educational.=20

- Rob Wolf

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<font color=3D'black' size=3D'2' face=3D'arial'>
<div>Dan and Don are both right -- flutter may be a mild buzz (which will=
 increase with airspeed) or it may be an explosive event.&nbsp; The big bo=
ys (by this I mean the aircraft companies with the resoures to do this) ap=
proach flutter clearance in a build-up fashion as follows.&nbsp; Sometimes=
 they do part of this and somtimes they do all of this.</div>


<div>&nbsp;</div>


<div>An analytical model is generated which includes the flexibility of th=
e structure *and* the aerodynamic characteristics.&nbsp; The model is "flo=
wn" at various virtual airspeeds to determine what the aeroelastic modes=
 are.&nbsp; "Aeroelastic" is a big word that we use to make people think=
 we are smarter than we really are.&nbsp; It includes "aero" -- the aerody=
namic effects which generate the forces on the airframe, and "elastic" --=
 the fact that the structure deforms under load, and therefore the aerodyn=
amic forces change.</div>


<div>&nbsp;</div>


<div>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 def=
lection and the changing aerodynamic loads that results in it shaking back=
 and forth (oscillating), well, that's an "aeroelastic mode".&nbsp; The ob=
vious one is the wing flexing up and down.&nbsp; But there are other modes=
, too, such as the vertical fin flexing back and forth, or the tailcone tw=
isting and the horizontal stabilizer driving that oscillation.&nbsp; The=
 latter is the mode that Martin Hollmann claims for the large-tail Lancair=
 320/360, and which Lancair claims is not present.&nbsp; There is a writeu=
p of this somewhere on Marv's web site.</div>


<div>&nbsp;</div>


<div>Okay, once you analytically calculate modes, you do "ground vibration=
 tests" to explore the modes that are significant.&nbsp; You shake the air=
plane with accelerometers mounted in strategic places.&nbsp; (This is done=
 on the ground)&nbsp; This confirms the modes and their frequencies.&nbsp;=
 As I recall, here is where Hollmann and Lancair disagree.&nbsp; Both have=
 done these tests and they got different results.&nbsp; This testing is be=
yond the capability of virtually all homebuilders, and costs something lik=
e $25,000 to contract out. Probably more.</div>


<div>&nbsp;</div>


<div>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.&nbsp; Ho=
llmann also claims that the critical speed for that mode is around 150 kno=
ts.&nbsp; If it were really a problem, the ground would be littered with=
 Lancair parts.&nbsp; Hollmann suggests a reinforcement.&nbsp;Most people=
 have not done that, but I wonder if that obviously highly damped mode (if=
 it exists at all)&nbsp;might be related to the fatigue cracks at the vent=
ral fin that some high time 320/360's have experienced.&nbsp; (Myself, I'm=
 not losing any sleep over this but it is curious. I have also done a diff=
erent 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".&nbsp; Personall=
y, I have no concerns about flying in a 320/360 which does not include Mar=
tin's reinforcement) </div>


<div>&nbsp;</div>


<div>Then you go fly.&nbsp; Those modes are always there, but they are hea=
vily damped (and therefore not a flight safety issue) until you go fast en=
ough.&nbsp; (The venetian blind is stable until the wind gets too high, th=
en it flutters.)&nbsp; But you measure those frequencies and, using electr=
onic instrumentation and computers, you calculate the damping.&nbsp; 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.&nbs=
p; Then, only when you have the damping number and it's greater than 2%,&n=
bsp;you increase the speed 5 knots and do it again.&nbsp; When the damping=
 drops to 2%, you stop.&nbsp; That testing is aso beyond the capability of=
 virtually all homebuilders.</div>


<div>&nbsp;</div>


<div>Free play can cause the flutter speed to drop dramatically.&nbsp; On=
 the positive side, this type of flutter is normally the non-explosive kin=
d.&nbsp; If your airplane is buzzing, slow down.&nbsp; If it diminishes as=
 you slow down, see what's looser than it used to be.&nbsp; It could be so=
methiing apparently innocuous, like hinge wear.&nbsp; It could also be som=
ething more.&nbsp; Find out and fix it before you fly again.&nbsp; Don't=
 assume that it won't suddenly get worse -- airlanes don't usually fix the=
mselves.&nbsp; 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).</div>


<div>&nbsp;</div>


<div>Hope this helps, or is at least educational.&nbsp;</div>


<div>&nbsp;</div>


<div>- Rob Wolf</div>
</font>

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