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After an incident which, it was suspected, was due to symmetric elevator
flutter, had occurred on the Morane-Saulnier 760 operated by the College of
Aeronautics, Cranfield, a theoretical and experimental investigation of the
aircraft's flutter characteristics was undertaken.
The theoretical investigation consisted of binary and ternary symmetric
flutter calculations with and without the control circuit included. These
showed the aircraft to be liable to flutter for mass distributions similar to
that which existed at the time of the incident.
The experimental work consisted of flight flutter tests using control jerk
excitation with both film and magnetic tape recording. These showed that
the aircraft as supplied by the makers had a critical speed for symmetric
elevator flutter of 380 knots, but that this could be lowered to 240 knots by
the installation of a stick force indicator combined with unfavourable distributions of fuel load and fuselage mass distribution. The tests also showed
the tail unit mode excited by rudder kicks to be safe, but as doubt exists
as to whether this mode is the most critical antisymmetric one, further work
is needed on this aspect. A 75 c. p. s. rudder buzz was encountered that
was not caused by compressibility effects.
As a result of this investigation the elevator mass balance was increased
and the aircraft proved to be free from elevator flutter up to at least 400
knots.
A general conclusion reached in this investigation was that static balancing
of control surfaces should include the effect of components of the control circuit
attached to them if those components contribute to the inertia couplings induced
by vibration in other elastic modes. |
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