Browsing by Author "Alexander, A. J."

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    The aerodynamic characteristics of the jet wing and its application to high speed aircraft
    (College of Aeronautics, 1961) Alexander, A. J.
    The slender wings and bodies suitable for supersonic flight have, in general, relatively poor aerodynamic characteristics especially at lay speeds. In order to improve their performance the use of edge blowing has been explored. In this scheme high velocity air jets in the form of thin sheets are used to fix separation lines on the wing or body and to favourably influence the external stream. Thus the equivalent wing, called here the "Jet Wing” is composed of the wing-body itself plus the extended curved jet sheets which spring from its edges. A limited use of edge jets has been proposed in the 'Jet Flap' concept but the efficiency of this device fails off with decreasing aspect-ratio and the problem of trimming could be severe as most of the increased lift is generated near the trailing edge. At very law aspect-ratios a considerable part of the lift is contributed by the leading-edge vortices which dominate the flow field at moderate incidence. It follows therefore that leading edge blowing is particularly useful for small aspect-ratio wings and the trimming problem can be avoided by a suitable jet arrangement which does not disturb the conical nature of the flow. When sufficiently large auxiliary thrusts are available peripheral jet sheets can be deflected downwards close to the ground, the air craft becoming a Ground-Effect-Machine, with substantial reductions in both take-off and landing speeds and distances.
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    Experimental investigation on a cropped delta wing with edge blowing
    (College of Aeronautics, 1963-06) Alexander, A. J.
    Low speed wind tunnel tests have been made on a 70° cropped delta wing with edge blowing both in the plane of the wing and at a downward deflection angle of 30°. The tests include six-component force and moment measurements, the distribution of static pressure at four chordwise stations, and quantitative measurements of the flow in the leading edge vortex. At a constant incidence, blowing increases the size and strength of the leading edge vortices and moves the vortex cores outwards and upwards. Blowing also tends to suppress the secondary separation due to the entrainment effect of the jet. Blowing from the streamwise tips and trailing edge was relatively ineffective and most of the tests were made with blowing from the swept leading edges only, with tips and trailing edge sealed.
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    Experiments on a delta wing using leading edge blowing to remove the secondary separation
    (College of Aeronautics, 1963-05) Alexander, A. J.
    It is found that the entrainment effect of a thin jet emerging from the leading edge of a delta wing is sufficient to remove the secondary separation. The minimum jet momentum required is small, but increases with incidence. Tests made without blowing, and with minimal blowing, include balance measurements, pressure plotting and tuft studies, over the range of incidence α = 0[degrees] - 20[degrees] . It is concluded that the presence of the secondary separation does not affect the lift or the vortex height appreciably, but is at least partly responsible for the large discrepancy between theory and experiment with regard to the spanwise position of the vortex.
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    An investigation of vortex breakdown at Mach 2
    (College of Aeronautics, 1963-11) Craven, A. H.; Alexander, A. J.
    Wind tunnel tests have been made at M = 2 on a 75° swept delta wing in order to study the progressive breakdown of the leading edge vortices, which occurs at high incidence. The incidence at which vortex breakdown occurred at supersonic speeds was somewhat less than at low speeds, but the pattern of breakdown appeared to be similar, although the spiralling region of flow is much larger at supersonic speeds. Quite slender objects placed downstream of the wing had appreciable upstream effects, causing vortex breakdown to move upstream by as much as 20% of the root chord for the same incidence.
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    A low-constraint, high-pressure air feed system for wind tunnel balances
    (College of Aeronautics, 1963-10) Alexander, A. J.
    A high-pressure air feed system, suitable for use with blowing models mounted on a six-component virtual-centre balance, is described. Air is supplied to the balance by a flexible ring main feed so that no air is lost as with an air-bearing system. Constraints on the balance are very low; at an internal pressure of 30 p.s.i. gauge with a mass flow of 0.5 lbs/sec. all force corrections were less than 1 lb., and the pitching moment correction less than 0.5 lb.ft. Rolling and yawing moment corrections were 2.0 lb.ft. and 2.7 lb.ft. respectively. The constraints were measured using a calibrator Which enabled the desired mass flow to be passed through the system without additional constraints.
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    Preliminary low speed wind tunnel tests on a 70 degrees cropped delta wing with blowing at all edges
    (College of Aeronautics, 1960-09) Alexander, A. J.
    Wind tunnel tests have been made on a 70° Cropped Delta Wing with slot blowing from all edges in the plane of the wing. The tests include six-component force and moment measurements and the distribution of pressure at four chordwise stations on the upper surface. Some results are also given with slot blowing from one wing-tip. At a constant incidence blowing increases the size and strength of the leading edge vortices and moves the vortex cores outboard. It also eliminates the secondary separation. Extra non-linear lift is obtained from the increased vortex strength and the lift coefficient at a = 25 o is increased from 0.98 without blowing to 1,16 and 1.25 for C values of 0.123 and 0.278 respectively with blowing from all edges. The corresponding movement of the centre of pressure is less than one per cent of the root chord. If allowance is made for the wind-off jet drag due to the poor blowing distribution, the values of drag obtained with blowing at all edges are lower than those for the unblown case at a given lift and the resultant lift-to-drag ratios are much improved. Without blowing the maximum ratio is 6.5 at a CT of 0.13 but blowing from all edges with Ca's of 0.123 and 0.278 increases-this value to 13.8 and 18.7 respectively at approximately the same lift coefficient. At incidence, with blowing at all edges, the jet sheets roll up o form stable leading edge vortices but near zero incidence (a < 0.25°) the jet sheets oscillate from one surface to the other, due to a downwash lag effect, producing a sinusoidal lift.