REFLECTOR: How Fast-flutter

[steve korney]

Hi Al...

I read the same article... Interesting subject that no one knows too much 
about...  I have a friend that worked on the F-18 Hornet...  He said that 
the rudders had flutter problems early on and they just made them stiffer to 
solve it...  They did not want to change the aerodynamic design and loose 
performance...

Best... Steve



----Original Message Follows----
From: "Al Gietzen" <ALVentures at cox.net>
Reply-To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
To: "'Velocity Aircraft Owners and Builders list'" <reflector at tvbf.org>
Subject: Re: REFLECTOR: How Fast-flutter
Date: Thu, 22 Mar 2007 12:07:26 -0800

Since everyone is putting their 2 cents worth; I may as well also.  Most
likely arguing IAS only or TAS only would both be wrong.  The argument for
the IAS only based on the forces on the surfaces is only looking at parts of
the issue; i. e., the static force, and the surface.  The flutter problem
is/becomes a very dynamic problem, and once in play what's happening to the
air and vortices generated by, and generating, the flutter are the other
important aspect of the problem.  I know just enough about Karmen vortex
generation to have an opinion that the temperature, density, and inertial
issues involved could make the TAS a factor.  I'd also have the opinion that
the degree to which TAS is determining, depends on airspeed and altitude,
and that the regime in which we fly - where compressibility is less of a
factor - IAS is closer to the right measure than TAS.



So there ya go - another opinion not backed up by any data or references:-).



If you're going to test the limits; approach very cautiously, and use the
'tapping' (sudden mild input) method, where the incipient flutter can be
felt, but it damps out -  and be ready to pull up and drop speed quickly.
(I've never done it; nor did I stay at a Holiday Inn Express last night)

The aerodynamic forces which can induce flutter are related to the dynamic
pressure, or airspeed, of the airplane. If flutter-inducing forces are
present they will increase as the airspeed is increased. Flutter
characteristics can be explored by "tapping" the surface at progressively
faster airspeeds, then watching how fast the vibrations decay or damp out.
The vibrations will take longer to decay as the airspeed approaches a
possible resonant condition. In this way potential flutter can be approached
safely without actually reaching the resonant condition and experiencing
sustained flutter.

The method for "tapping" the surface varies. On some airplanes a sharp
control pulse is sufficient to excite the natural frequency of the surface.
In most cases a special flutter excitation device is installed. This device
will use either an aerodynamic vane or an unbalanced mass which is driven
back and forth at the known natural frequency of the surface. The device is
abruptly turned off and the natural damping characteristics of the vibrating
surface are revealed. The analysis is similar to the frequency and damping
analysis discussed under the "control pulse" maneuver, except that the
structural (or flutter) frequencies are much higher.

Al





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