Page 559 - Aircraft Stuctures for Engineering Student

Elementary aeroelasticity

Aircraft structures, being extremely flexible, are prone to distortion under load. When
these loads are caused by aerodynamic forces, which themselves depend on the geo-
metry of the structure and the orientation of the various structural components to the
surrounding airflow, then structural distortion results in changes in aerodynamic
load, leading to further distortion and so on. The interaction of aerodynamic and
elastic forces is known as aeroelasticity.
Two distinct types of aeroelastic problem occur. One involves the interaction
of aerodynamic and elastic forces of the type described above. Such interactions
may exhibit divergent tendencies in a too flexible structure, leading to failure, or,
in an adequately stiff structure, converge until a condition of stable equilibrium is
reached. In this type of problem static or steady state systems of aerodynamic
and elastic forces produce such aeroelastic phenomena as divergence and control
reversal. The second class of problem involves the inertia of the structure as
well as aerodynamic and elastic forces. Dynamic loading systems, of which
gusts are of primary importance, induce oscillations of structural components.
If the natural or resonant frequency of the component is in the region of the
frequency of the applied loads then the amplitude of the oscillations may
diverge, causing failure. Also, as we observed in Chapter 8, the presence of
fluctuating loads is a fatigue hazard. For obvious reasons we refer to these prob-
lems as dynamic. Included in this group are flutter, buffeting and dynamic
response.
The various aeroelastic problems may be conveniently summarized in the form of a
‘tree’ as follows

a
Aeroelasticity

Static stability.. . Static Dynamic . . . Dynamic
4-7 I
stability

Load Divergence Control Flutter Buffeting Dynamic
distribution reversal response