A Survey of Unsteady Hypersonic Flow Problems

46 -
APPENDIX IV Review of Flutter of Kivpersonio Vehicles
Differences between the flutter behaviour of vehicles flying at
hypersonic speeds and the behaviour at lower speeds will be due to changes in
the nature of the air flow at hypersonic speeds and the changes in the
stiffnesses Of the vehicle structure which result from heating of the structure.
This report is mainly concerned with the effects of changes in the nature of
the flow but the importance of the second factor should be borne in mind and
is illustrated in a general way by Fig. 35(a), (b) and (c) from Ref. 55.
In Fig. %(o) the lower line represents the value of the speed parameter U/&o,
along the flight profile, and the upper line represents the values of U/bw,
at which flutter could occur. Its importance is emphasised by the fact that
for sMne flight profiles the maximum heating rate can be combined with the
maXimum ValUe of dynamic pressure on the trajectory - as in the case of the
SUperCirOdar lifting re-entry trajectory shown in Fig. 4.
4.1 Types of Flutter and Values of Flutter Frequency Parameter
The types of flutter than can tse expected on hypersonic vehicles are
discussed in Refs. 5 and 6. These discussions and the conclusions presented
here can, of course, only be deductions from the likely structures of the
vehicles based on general technical considerations. On this basis, flutter
involving vibration modes of the main structure IS likely to be met only on
slender vehicles for hypersonic cruise, and lifting re-entry vehicles having
high L/D and, if it occurs,
it seems likely to be of a form involving the
longitudinal bending modes of the body, even for the winged vehicles. The
slenderness of the wing, and the degree of integration of wing and body, make
it unlikely that vibration of the ting can be considered in isolation from the
response of the body. Simple bending-torsion flutter could occur for certain
types of control surface. Panel flutter could occur on lifting surfaces, heat
shields, and in inlet and propulsion ducts for power units, where panels are
heavily loaded, aerodynamically and thermally. Membrane flutter and large
amplitude distortions could be met if lightweight structures are used to give
lift or drag in re-entry. It is suggested in Ref. 12 that, for slender bodies,
using representative values of the overall structural frequencies, an upper
limit of 3000 rad.ft/seo can be set on the product wb where w = %f, b is
a representative length and f is a frequency, so that at M = 5,
UC 5000 ft/sec, the maximum value of frequency parameter will be about 0.6,
but values are likely to be much lower than this in most practical cases.
For bending-torsion flutter of control surfaces it seems likely,
again using representative frequencies, that the maximum value of ob will be
around 600 rad.ft/sec, giving maximum frequency parameters around O-1. And, in
a similar way, it can be deduced for panel flutter that frequency parameters
based on a representative length of the order of the wavelength of the panel
mode, will be in the range of values up to 0.5.
4.2 Flutter Investigations
Most of the reports of analytical and experimental work on flutter
at hypersonic speeds that have been published have been concerned with the
standard basic ease of flutter of a two-dimensional section with pitching and
plunging flexibilities, or the closely comparable case of flutter of a rigid
half sing with root flexibility in pitching and plunging or flapping, both of
which are related to the bending/torsion flutter Of wings. There has been a
much smaller amount of work on flutter of low aspect ratio wings with chordwisebending
as well as torsional and spanwise-bending modes of vibration, and with
flutter/