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KDTWflyer
Topic Author
Posts: 825
Joined: Tue Jun 29, 2004 12:51 pm

Aircraft Flutter

Fri Jul 23, 2004 2:40 pm

I have no idea what "flutter" is. Can someone shed light on this? I have heard of airliners participating in flutter tests, but have yet to understand what it is.

Ah... the extensive knowledge of these forums


 
Vorticity
Posts: 324
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RE: Aircraft Flutter

Fri Jul 23, 2004 3:00 pm

A detailed little page from the good folks at NASA about flutter...

http://www.dfrc.nasa.gov/Education/OnlineEd/Intro2Flight/nasflut.html

In essence, flutter testing is testing the dynamic properties of the aircraft. A structure can resonate at a certain frequency, where a vibration increase in amplitude instead of damping out. It can inrease out of control and lead to structural flavor.
Thermodynamics and english units don't mix...
 
KDTWflyer
Topic Author
Posts: 825
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RE: Aircraft Flutter

Fri Jul 23, 2004 3:07 pm

Thanks for the information.... are there any recorded incidents of flutter causing severe damage or failure?
 
Boeing4ever
Posts: 4479
Joined: Thu Apr 26, 2001 12:06 pm

RE: Aircraft Flutter

Fri Jul 23, 2004 3:27 pm

I believe the VFW 614 is one incident of flutter. Airframe was lost.

B4e-Forever New Frontiers
 
liamksa
Posts: 301
Joined: Mon Oct 29, 2001 1:13 pm

RE: Aircraft Flutter

Fri Jul 23, 2004 4:47 pm

There's a few interesting videos here ( http://www.airspacemag.com/ASM/Mag/Supp/FM01/Hammer.html ) which demonstrate flutter.

View Large View Medium
Click here for bigger photo!

Photo © Jeremy Irish/Cactus Wings


One of the main reasons engines are mounted so far forward beneath the wing is to act as a mass balance in an attempt to reduce the tendency to flutter.
 
miamiair
Posts: 4249
Joined: Thu Jul 15, 2004 9:42 pm

RE: Aircraft Flutter

Fri Jul 23, 2004 7:27 pm

The Lockheed Electra (L-188) suffered from early flutter problems involving the nacelles.
Molon Labe - Proud member of SMASH
 
411A
Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

RE: Aircraft Flutter

Sat Jul 24, 2004 12:00 am

Technically, the Lockheed Electra did not have 'flutter' problems.
It had whirl-mode vibration incidents with the outboard engine nacelles, which resulted in wing separation.
All nacelles were re-inforced/rebuilt, problem solved.

Flutter can be a very large problem with control surfaces, especially ailerons and elevators, and sometimes with the rudder as well.
Improperly designed or poorly balanced control surfaces can be induced to rapidly vibrate or flutter at higher speeds, which if left unchecked, can result in control surface separation or wing/tail separation.
For example, on the Douglas DC-6, the elevator was noticed to enter a mild flutter at speeds approaching Vne. The fix was actually quite simple. An additional hinge was added at the end of each elevator...problem solved.
 
citationjet
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RE: Aircraft Flutter

Sat Jul 24, 2004 2:34 am

One of the simpliest example of flutter in everyday life is the stop sign twisting in a strong wind. The sign post lacks sufficient torsional stiffness.
Boeing Flown: 701,702,703;717;720;721,722;731,732,733,734,735,73G,738,739;741,742,743,744,747SP;752,753;762,763;772,773,788.
 
phollingsworth
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RE: Aircraft Flutter

Sat Jul 24, 2004 3:14 am

Technically, the Lockheed Electra did not have 'flutter' problems.
It had whirl-mode vibration incidents with the outboard engine nacelles, which resulted in wing separation.
All nacelles were re-inforced/rebuilt, problem solved.


That actually fits into the definition of flutter, which does not have to be aerodynamic in mode. Flutter is basically a dynamic structural instability. The term for the Electra's problem is wirl-flutter, as the propeller shaft surrond structure was not stiff enough. Flutter can also include limit cycle behavior, which may not lead to catastrophic failure but will lead to reduced fatigue life. Associated with flutter is divergence which is a static structural instability. Forward swept wings have a serious divergence problem since the aero-structural interaction is unstable under all conditions.

When dealing with vibrations, divergence, flutter, etc. associated with aero-structural interactions the interdisciplinary field is usually referred to as aeroelasticity.

You start talking about all of this and you get me thinking about Hopf bifurcations, limit-cycle behavior, catastrophes, and chaos.
 
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Starlionblue
Posts: 20650
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RE: Aircraft Flutter

Sat Jul 24, 2004 3:29 am

Citationjet, that's a great example.

But for planes, adding enough stiffness to stop all such vibrations would make the wings and tailplanes too heavy. So they are designed to only vibrate and swing in certain ways and with limitations, if you know what I mean.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Boeing4ever
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Joined: Thu Apr 26, 2001 12:06 pm

RE: Aircraft Flutter

Sat Jul 24, 2004 3:38 am

Okay, to follow up from my first reply, here's the info on that VFW 614. The crash was caused by tab flutter, but apparently the crew managed to parachute out (one still died however).

http://aviation-safety.net/database/1972/720201-1.htm

B4e-Forever New Frontiers
 
SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Aircraft Flutter

Sat Jul 24, 2004 10:28 am

Adding torsional stiffness as a solution to aerodynamic flutter is a little like adding onboard fire supression equipment as a solution to a leaky fuel line. Better to solve the original problem.

IIRC there was a C-119 air tanker lost in the western US a few years back due to aileron flutter failing a wing. Anyone add to that?
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
411A
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RE: Aircraft Flutter

Sun Jul 25, 2004 12:12 pm

Despite the very best efforts of the design engineers, new types always do have problems.
Some of these show up rather rapidly.
Types that come to mind immediately are...

DC-6. Fuel vent postioned in front of the air inlet for the cabin heater, resulting in two accidents...and grounding for a short while.

Lockheed Electra. Nacelle/wing failures.

DeHavilland Comet. Fatigue caused by pressurization cycles.

Some structural problems show up at a later date, sometimes with rather expensive fixes...

Boeing 707. Severe fuselage crown skin cracks, requiring (first) external straps and (later) re-skinning.

Boeing 747. Severe fatigue cracks in the section 41 fuselage area.
Engine fuse pins.

Lockheed TriStar. Wing inboard and outboard aft spar cracks, resulting in a very expensive mod after cycle limits are reached.

A300B4. Section 45 fuselage skin/stringer failures/fatigue cracks.

DC10. Where to begin...many problems.
Cargo doors, lack of hydraulic system fusing, poor design of leading edge slats...etc.

If one looks at the AD list for all jet transports, it can go on for many pages.

And, don't expect this to change, anytime soon.

 
modesto2
Posts: 2731
Joined: Sun Jul 16, 2000 3:44 am

RE: Aircraft Flutter

Tue Jul 27, 2004 4:16 am

One of our aircraft was experiencing vibrations, so we replaced the rudder actuator (allowed too much movement) - seems to have solved the problem.
 
SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Aircraft Flutter

Tue Jul 27, 2004 9:44 am

So if a plane wags its tail that does not mean that it is happy?
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
modesto2
Posts: 2731
Joined: Sun Jul 16, 2000 3:44 am

RE: Aircraft Flutter

Tue Jul 27, 2004 12:01 pm

SlamClick, if it wags the tail on the ground, then it's happy. If it wags the tail while in-flight, then you got problems...

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