Analysis of dynamic response of a very flexible Delta wing model in a wind tunnel

Limit cycle oscillations involving Delta wings are an important area of
research in modern aeroelasticity. Such phenomena can be the result of geometric nonlin-
earity, aerodynamic nonlinearity or under-wing store nonlinearity. In this paper, a flexible
half-Delta wing without stores is tested in a low speed wind tunnel in order to investigate
its dynamic response. It is found that, at several combinations of airspeed and angle of
attack, the wing undergoes high amplitude limit cycle oscillations. Three types of such
oscillations are observed. Type 1 oscillations occur only at low angles of attack and are
the result of a Hopf-type bifurcation. Type 2 limit cycle oscillations occur at intermedi-
ate angles of attack and are the result of an atypical bifurcation. In other words, these
oscillations appear as the airspeed is increased but disappear at even higher airspeeds.
Type 3 oscillations occur at even higher angles of attack. A bispectrum analysis shows
that type 3 limit cycle oscillations feature quadratic phase coupling. No such coupling
was measured for type 2 oscillations, leading to the conclusion that the nonlinearity must
be of higher order.