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A unified approach to flutter, dynamic stability and response analysis of high aspect ratio aircraft

Cal, A. A. (1992). A unified approach to flutter, dynamic stability and response analysis of high aspect ratio aircraft. (Unpublished Doctoral thesis, City, University of London)

Abstract

A unified method of flutter, dynamic stability and response analysis of deformable aircraft is presented. Normal modes and the generalised coordinate approach are used to develop the equations of motion. Strip theory and Theodorsen's expressions for unsteady lift and moment are utilised to generate the generalised aerodynamic forces. Theodorsen's function C(k) for harmonic motion is employed for the flutter analysis.

A comparison is made between generalised unsteady aerodynamic forces and flutter quantities obtained from lifting line theory and those obtained from conventional strip theory, for both a rigid and elastic wing undergoing binary flutter. This investigation shows good agreement between the two theories particularly at high aspect ratios and suggests that the lower limit of aspect ratio for good flutter prediction, using strip theory, is about 6.

The unified analysis is carried out on three particular aircraft of which two are high aspect ratio sailplanes, the Kestrel 22m, the tailless Ricochet and the Cranfield A1 a moderate aspect ratio aerobatic aircraft. A symmetric flutter analysis, including the effects of rigid body modes is carried out on all three aircraft. The Kestrel is found to suffer from classical wing bending/torsion flutter. The tail plane aerodynamics is seen to have a marginal if not stabilising influence on its flutter behaviour. The Ricochet in the absence of a tail plane is found to suffer from body freedom flutter involving coupling of the short period mode with the first wing bending mode. As the A1 is very stiff and of comparatively low aspect ratio it is found to be virtually flutter free.

Making use of the generalised Theodorsen's function for convergent motion and the current body fixed axis system, the analysis is extended to evaluate the short period mode of an aircraft. Stability derivatives using the normal classical rigid body approach are also used for comparison. The introduction of flexibility and unsteady aerodynamics is seen to have a destabilising influence on the short period mode. Close to the flutter speed, classical rigid body assumptions are found to be inadequate in predicting the short period characteristics of the Kestrel and the Ricochet, but not the relatively stiff Al. An unsteady wing wake is then introduced at the tail plane where this is seen to have a negligible effect on flutter of the Kestrel.

An analysis of the aircraft response to continuous atmospheric turbulence and discrete gusts is carried out using the Power Spectral Density method (PSD) and Statistical Discrete Gust method (SDG) respectively. The introduction of flexibility is seen to substantially increase the overall aircraft response, especially at subcritical speeds. From the analysis carried out on all the three aircraft, an SDG-PSD overlap does appear to be characterised in this investigation not by a 10.4 factor, but rather by a 10.4 plus or minus approximately 17% factor when rigid body modes are considered. For the flexible case this range is found to be plus or minus 31% factor.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TJ Mechanical engineering and machinery
T Technology > TL Motor vehicles. Aeronautics. Astronautics
Departments: School of Science & Technology > Engineering > Mechanical Engineering & Aeronautics
School of Science & Technology > School of Science & Technology Doctoral Theses
Doctoral Theses
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