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gmcjetpilot
Joined: 04 Nov 2006
Posts: 170
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 Posted: Wed Mar 28, 2007 9:52 am Post subject: TAS and VNE? (the expert speaks, ha ha) |
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<?xml:namespace prefix = o /> FLUTTER:
BUT WHY DOES ALTITUDE AFFECT IT? Simply put
there is thinner air and it provides LESS Dampening.
Period, end of story.
There is no debate TAS and VNE are related or more
accurately:
Your Flutter margins go down with altitude.
OR you could say.
Vne goes down with altitude.
But what is it and how do we determine it?
Well it would help to have an engineering degree but
I can explain away some of the mystery. Engineers
use terms like, natural frequency, stiffness, forcing
function and dampening. I'll explain with a example
below. Flutter is a sub part of vibration. There are
100's of flutter MODES or types.
To explain it is easy.
To analyze and determine the dampening, forcing
function and natural frequency of a structural system
(flight controls) can be a challenge for even the best
engineers. Computers help but we are talking about
physical things. The computers answer is only as
good as the data you give it.
How is Flutter margins or speed calculated? Very
carefully. That is why they use wind tunnels and flight
test. I actually know a test pilot that broke a factory
plane in HALF in a Vne Dive Test due to flutter. It was
an existing design that went through some mods.
Technically FLUTTER is special field of vibration
engineering dynamic response or Aeroelasticity. The
math gets pretty involved but with computers they can
solve the theoretical. However a computer MODEL
answer is only as good as the data you give it. When it
comes to flutter flight test still rules.
Now they use FEM and CFD (computers):
FEM= Finite Element modeling
CFD= Computational Fluid Dynamics
In the end Flutter is an illusive thing that must be in the
end tested on the plane in flight. Boeing does this and
adds shakers to the wing. On a long flexible winged jet
the whole wing can flutter. They excite the wing and
dive it to Vne PLUS an additional margin. Some times
extra material and strength (stiffness) is added to the
final design just for flutter margins. Not until the test
pilot does this do they really know. However w/ the new
computer tools predictions and initial designs are much
better.
YOUR RV AND FLUTTER
Before you fly passengers you should do a Vne test as
Van suggest. Wear a Chute and Helmet. Go up dive.
Plan the dive where you are level or just slightly
climbing again when you let go of the stick. THAN
you WRAP the stick! Wrap meaning give it a hit or quick
yank. You repeat the test over and increase the speed
up to 1.10 of your final Vne. Of course Vne is based on
TAS. Repeat with aft CG and different wts.
You may want to omit the WRAP and just have the
death grip on the stick. Holding the stick increases the
flutter speed because you are damping the control.
RV's relatively tight or stiff push pull roller/ball bearing flt
controls is Goodness. A sloppy cable control system is
not as good for control surface flutter dampening.
So what is flutter. Well it is a DYNAMIC instability that is
effected by air density, structural stiffness, control
stiffness and of course the geometry of control
THINGS THAT AFFECT FLUTTER
Ways to increase or change flutter speed:
Stiffer structure
Counter balance
Add flt cont'l mass
Aerodynamic changes</o:
Different hinge or control geometry
There are so many things that can cause flutter. The
late great Steve Whitman and his wife died from flutter.
Going home after Oshkosh, aileron flutter caused by
the fabric coming undone from the frame destroyed
the plane almost instantaneously.
In engineering terms, there are more than 100 different
types of flutter- or "modes"- in which metal can vibrate.
The "mode" that destroyed the Electra's was "whirl
mode. Early Lockheed Electra's crashed for no
seemingly reason at the time. The props gyroscopic
tendency is for the axis to remain spinning in one axis
or plan'(s). So in turbulence the combo of the prop
spinning, gyroscopic rigidity and the engines structural
stiffness (flexibility) all ganged up to create a vibration
that got larger and larger until the structure failed.
THE FLUTTER EVENT (all simular)
The structure deflects and than spring back. If the prop
forces where timed right (frequency) it could add to that
spring back. Eventually the deflections get so large the
structure fails. In this case turbulence could be timed
just at the Natural frequency of the structure so the
frequency of the turbulence (prop forces) and the natural
vibration freq of the structure are in sync. If you don't
have damping the oscillations get larger. As the deflections
get larger the forces get larger until the structure fails from
overload.
In every vibration problem there is a forcing function,
damping, stiffness (springs) and geometry. Add to
that Aerodynamics for aircraft. This is a special field
in aeronautical engineering called Aeroelasticisty. It
combines the analysis of aerodynamics, structural
stiffness and vibration (flutter).
Bottom line is that as much as we know it is still a
mysterious phenomenon but it has disastrous effects
and can distroy an aircraft in a second. Keep in mind
strength and stiffness are different. One is how much
load a part can take, stiffness is how much it moves
for a given load. There is a relation but either can be
manipulated by engineers to TUNE the structure
for both strength and resistance to flutter.
Observer Vne as TAS.
George
Feel free to correct the expert. I have been flying
for the airlines for years and hung my engineer hat
awhile ago.
ago.
[url=http://us.rd.yahoo.com/evt=49678/*http://smallbusiness.yahoo.com/domains/?p=BESTDEAL] Get your own web address.[/url]
Have a HUGE year through
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Vanremog(at)aol.com
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| Posted: Wed Mar 28, 2007 7:41 pm Post subject: TAS and VNE? (the expert speaks, ha ha) |
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In a message dated 3/28/2007 10:53:22 A.M. Pacific Daylight Time, gmcjetpilot(at)yahoo.com writes:
| Quote: | Engineers
use terms like, natural frequency, stiffness, forcing
function and dampening. |
===============================================
Only if he/she intends to bring out a hose to wet something down. Otherwise a real engineer would use the term damping. ;o)
GV (RV-6A N1GV O-360-A1A, C/S, Flying 840hrs, Silicon Valley, CA)
See what's free at AOL.com.
[quote][b]
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