A Survey of Unsteady Hypersonic Flow Problems

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Most of the informatIon relates to the pitching and plunging flutter
of a twc-dimensional section, or the similar problem of the bending-torsion
flutter of a cantilever wing. There is still a need, in this field, to
investigate the use of theories applicable to Mach numbers higher than the
piston theory range, and to find an adequate method for estimating the
aerodynamic forces on a sectlon with a blunt leading edge; experimentally,
there is a need for studies that explicitly take account of possible non-linear
behaviour, and for further studies on the effects of incidence. But this kind
of flutter is likely to be of comparatively minor importance for hypersonic
vehicles, and there is a great need for more analytical and experimental work
on the flutter of low aspect ratio wings and slender bodies, on panel flutter,
and on membrane behaviour.
Work on slender bodies and low aspect ratio wings is likely to be
analytically complex. For pointed slender bodies and wings with supersonic
leading edges, shock expansion theory should give suitable estimates of the
aerodynamic forces but Its use in flutter analyses may be ocmplicated. For
blunted nose bodies an adequate aerodynamic analysis does not exist (Appendix I
Experimental work on these bodies and wings could include tests on rigd bodies
flexibly mounted to give a simple check on theories, but would need to be
extended to the use of flexible mcdels.
In the case of panel flutter, a theoretical investigation of the use
of piston theory in a steady flow field with large entropy gradients would be
useful since these are the conditions which usually apply downstream of the
strong nose shock on a hypersonic vehicle, and experiments would need to be
carefully planned to show what fluid dynamic effects, if any, require special
investigation.
All the experimental results which have been reviewed show clearly
the need in fiture experimental flutter studies for very careful control of the
experimental conditions if reliable and precise information is to be obtained
on the merits of aerodynamic theories used in flutter analyses.
Finally, the point should be made that the values of aerodynamic
damping coefflclents at hypersonic speeds tend to be low, flutter frequency
parameters tend to be small, and the density ratios at which flight takes place
are high. In these conditions the importance of aerodynamic damping in flutter
analyses may become negligible (Ref. 62, Section 6-6) and it would then be
possible to use quasi-static air forces and the calculation of these forces
would be correspondingly slmpllfled. Clearly, this is a matter which should be
investigated.
Nomenclature/
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