City Research Online

Abstract

A unique characteristic of helicopter stall is the occurrence of stall on the retreating side of the rotor disk. Operating in an unsteady environment, the most severe type of stall encountered by the retreating rotor blade is dynamic stall, which limits the helicopter maximum speed, adversely affect handling qualities and causes excessive cabin vibration. Dynamic stall is characterised by the formation, migration and shedding of a leading-edge vortex. The leading-edge vortex produces a large pressure wave moving aft on the aerofoil upper surface and creating abrupt changes in the flowfield. The pressure wave also contributes to large lift and moment overshoots in excess of static values and significant nonlinear hysteresis in the aerofoil behaviour.

The proposal of the experimental research programme is to study the capability of employing an active flowfield altering device utilising low energy systems known as Air-Jet Vortex Generators (AJVGs) to suppress helicopter dynamic stall as a means to expand the helicopter flight envelope, thereby enhancing the utility of these aircraft. AJVGs consist of small jets of air emerging from an aerodynamic surface via slots/holes that are pitched and skewed relative to the oncoming freestream. The interaction between the air-jets and the freestream flow forms longitudinal stream wise vortices that re-energise the “tired” boundary layer enabling it to negotiate severe adverse pressure gradients as these vortices penetrate downstream.

The aims of the research programme are to experimentally establish:

(a), the potential of operating a spanwise array of AJVGs continuously on an oscillating aerofoil to suppress the formation of the leading-edge vortex. Wind tunnel tests will be conducted on an unswept oscillating RAE 9645 aerofoil section of chord length 500mm in the University of Glasgow Handley Page low-speed wind tunnel (Rec = 1.5 xlO6). The sinusoidal-pitching motion tests will be conducted at a = (15 + 10 sin<ut)deg, for the reduced oscillation frequency range of 0.01 < k < 0.181. The aerofoil section is equipped with an array of 28 equi-spaced, co-rotating AJVGs across the span located at 12% and 62% chord with the AJVGs operating at jet momentum blowing coefficients between 0.0 < < 0.01;

(b) , the effect of aerodynamic sweep on the geometry requirements of the AJVGs, keeping in mind that the retreating rotor blade can experience ±30° variation in sweep angle. Wind tunnel tests will be conducted on a swept wing (A = 35°) half model section with parallel leading and trailing edges, constant chord (c = 232mm), semi-span (b = 958mm) and aspect ratio of 4.5, in an incidence range 0° < a < 20° in the City University T2 low speed wind tunnel (Rec = 0.5 xlO6). Installed in the top surface of the swept wing is a spanwise array of AJVGs located at 10% chord with jet momentum values in the range of 0.0 < < 0.01; and

(c) , the capability of unsteady AJVGs to reduce the blowing mass flux of steady AJVGs required to effectively delay boundary-layer separation and hence improve lift/drag performance. Wind tunnel tests will be conducted on an unswept NACA 23012C aerofoil section of chord length 482.6mm at angles of attack 6° < a < 21° in the City University T2 low-speed wind tunnel (Rec = l.lxl 06). The blowing momentum coefficients and non-dimensional pulsing frequencies employed are in the range of 0.0 < C^ < 0.01 and 0.3 < F+ < 2.0 respectively. The aerofoil section is equipped with an array of 15 equi-spaced, co-rotating AJVGs across the span located at 12% and 62% chord; but only the forward array of AJVGs will be utilised in the experimental study.

The results of the tests demonstrate that it is possible to:

i) Utilise a spanwise array of AJVGs located at 12% chord at relatively low-blowing momentum coefficients (C^ < 0.01) to delay the formation of, and suppress the effects of, a leading-edge vortex on a dynamically stalling aerofoil.

ii) Reattach the stalled flow on a swept wing by directing the jet efflux towards the wing root.

iii) Reduce the steady-state blowing mass flow required for effective boundary-layer separation control by up to 25% by operating the AJVGs intermittently.

[Please look inside the thesis for correct equation format]

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

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