Re: Flutter & Swept Wing Aircraft

From:         jack@larc.nasa.gov (Jack Dunn)
Organization: AAOB/SySD LaRC NASA
Date:         14 Apr 94 12:33:56 PDT
References:   1 2
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In article <1126@orchard.Chicago.COM>, estock@gandalf.rutgers.edu (Richard G. Estock) writes:
>I would appreciate an e-mail reply to the following query:
>
>I enjoyed reading and recommend to all Clive Irving's "Widebody: The
>Triumph of the 747".  The author mentions the problem of flutter with
>all of the swept wing aircraft that Boeing (and presumably all other
>manufacturers as well) developed.
>
>I am confused about what this problem actually is, and how it
>manifests itself.
The term "flutter problem" covers a large set of dynamic load limits
that elastic aircraft are designed. Classical flutter involves
the exchange of energy between the pitch and plung  motion of the surface
and the air.  The system can be modeled as two uncoupled second
order systems coupled by the aerodynamics. The magitude of the
aerodynamic energy is proportional to the dynamic pressure, qbar,
of the flow, (high speed low level flight == high qbar). There exist
a value of qbar that this system will become unstable, that
is the motion of the wing cause forces to be applied to the
system that cause the motion to increase. The load limit of the wing
is quickly exceeded, (5 cycles). There are other forms of flutter
such as stall flutter which involves the stalling of the aerodynamic 
surface. Transsonic flutter can be influenced by the nonlinear
characters of the aerodynamics at transonic speeds. Panel flutter
can cause problems with skin panels. Each of the different types
of flutter have its own analysis method. Some of the methods are
better than others. Transonic flutter is so bad that dynamicly
scaled models are used in wind tunnels to verify that the analysis
methods will work on the full scale aircraft.
>
>What, then, is flutter?  What is the cause?  What happens to an
>aircraft that is experiencing flutter?  If an observer were in a
>chase plane, what motion would the observer see with an aircraft
>ahead that is undergoing flutter?  
The observer whould only see the wing breaking apart. 
>What corrects flutter?
The basic cure for flutter is to make the wing stiffer. Some
solutions try to seperate the natural frequencies of the two
modes that are involved with limited success. Some designs try
to place the cg of the structure forward by placing the engins out
on pylons.
>
>As I understand it, on a 7X7 wing there is in succession an:
>	inboard flap
>	inboard spoiler
>	tabs (2)
>	aileron
>	outboard flap
>	outboard spoiler
>
>>From one source, flutter was described as an oscillation
>at some critical speed of the aileron.  Why just the aileron?  Or in
>this case are the flaps, spoilers, et al, collectively considered an
>aileron?
All vibrating surfaces will flutter. The question is at what speed and
altitude.
>
>Is the problem of flutter more pronounced with swept wings than
>straight wings?  Or is it more a function of speeds approaching Mach
>1 regardless of wing geometry?
Sweep by itself does affect flutter. However, swept wings are stiffer.
But, aerodynamic effects lead the designer to use more sweep for
higher speed aricraft. There is no simple answer. The major design parameters
that have an effect on flutter, are 1) mass ratio (mass of wing to the mass of the air);
2) sweep angle; 3) taper ratio; 4) aspect ratio; 5) chordwise position of cg;
6) elastic axis position; 7) pitch radius of gyration; and 8) bending
torsion frequency ratio.