Experiments on Transsonic Flutter Using a 2-Dimensional Supercritical Wing.

Kurzfassung

In the recently modified DLR transonic wind tunnel in Goettingen, flutter measurements were performed primarily in the transonic flow regime. The 2-dimensional wing (section: NLR 7301, span: 1m, chord: 0.3m) is suspended at both sides using a plunging and a torsional spring. Thus the rigid carbon fibre model is allowed to oscillate in two degrees of freedom. Between the outer plate springs (plunging) and inner crosssprings (torsion) there is on both sides a piezoelectric balance for measurement of the steady and unsteady forces. The flutter tests were performed in the Mach No. range 0.5<Ma<0.81 and the stagnation pressure was varied for 0.38<p(ind&#610)<1.2 bar. The curve of the transonic dip can be characterised as fol- lows: The minimum of the transonic dip is a very sharp one and is located at Ma&#610.77. The dynamic pressure for flutter at this minimum is roughly half the subsonic value (Ma is approx. equal to 0.5). Three types of flutter were observed: -In and near the minimum there are limit cycle oscillations in plungeand torsion. The limit amplitude in heave is h prime &#61 +- 1.2 mm and in pitch alpha prime &#61 +-0.3. The phase difference between heave andpitch was phi &#61 180 (mean angle of incidence alpha bar &#61 13). -Pure limit cycle pitch oscillations (alpha prime is approx. +-0.3) were measured when the mean angle alpha was alpha bar &#61 0.9. -A little bit away from the minimum, where the dynamic pressure is slightly increased, an alternating oscillation between heave and pitch was observed (peak amplitudes limited). Pure heave oscillations were not found. A Hopf-diagram (i.e. limit cycle amplitude upon the Mach No.) was measured for four Mach numbers.