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Aviation History
1962
1962 - 0478.PDF
476 FLIGHT International, 29 March 196: SR.N2 . . . lOO LB. OF FUEL PAYLOAD TON MILE IO Ol ipoo M. P H. 00.000 3 5 3 O p 2 5 20 <P2 CONSTANT OVERALL- PLAN AREA CONSTANT CUSHION AREAl" EFFECT OF JET ANGLE 3 20 40 60 SO JET ANGLE (9) DEGREES 3 O 2 5 EFFECT OF JET THICKNESS ri 02 0-25 0-30 035 04 JET THICKNESS / t) HOVER HEIGHT K'K, 3 O LV P 2 5 JO EFFECT OF JET SWEEPBACK FOR PROPULSION ID 20 30 40 SO JET SWEEPBACK (tf) DEGREES Fig 4 basic curves for a typical air-cushion vehicle showing the effect on its efficiency of varying jet area, angle, sweepback and thickness. The parameter for efficiency is LV/P, the product of lift (weight) and forward speed divided by engine power 6 O 4 O 2 O LB OF FUEL PAYLOAD TON MILE i .o OB O 6 O 4 .WIDGEON WHIRLWIND .WESTMINSTER 10 • BEAVER • CARIBOU • BRISTOL FREIGHTER 40 oO BO IOO SPEED- M.PH. 200 20 IO 8 6 PRICE £PER4 LB EMPTY WT (B.OW.) 2 i o 8 6 0 1 "~ 1 HOVERCRAFT IN THIS STUDY \ (©HELICOPTER „,. ~A ^££ 7 *f "2_SH* £5r / «.•-•»»• •• > YACHT/ . g^CAP JpBUS .. TURBO-JET 1 Nt -— 1 hoc?s^DDnD r 1 1 EXECUTIVE TWIN | BJr~PREVIOUS~ HOVEf ESTIM CRAFT ATES 1 IOO 200 300 400 CRUISE SPEED-KNOTS 500 Figs 1-3 In the curves above, the engineers at East Cowes illustrate the manner in which Hovercraft fit in to the spectrum of all transport vehicles. Fig 2 is the centre of Fig I on a larger scale. It should be emphasized that these illustrations are intended to give only a broad picture, and there are some excep tional vehicle designs which fall outside their designated areas approximation suggests that the jet should be directed backwards at about 60°, but the optimum normally lies between 20° and 30\ If this range is exceeded, the jet airflow required to maintain lift leads to unacceptable duct losses, and the fact that the lifting system always generates substantial forwards thrust makes it diffi cult for the machine to hover without excessive expenditure of power on some form of reverse thrust. On the other hand, backwards deflection of the jet through about 20°, which suffices to provide about 20 per cent of the total required thrust, imposes little penalty in lift for a given jet flow. Propulsion provided by the angled jets is applied well below the centre of drag, and, even if the aerodynamic lift normally experienced (Fig 5) were to be discounted, a strong nose-up pitching moment results. This would give the lift force a substantial rearwards component, greatly increasing overall drag. In practice, the Hovercraft is designed to run in an essentially level attitude at all speeds, so that angle a in Fig 5 remains 0°. At the optimum cruising speed the drag is of the order of 10 to 20 per cent of the weight, so that the same relationship applies between the thrust and the lift. A typical percentage breakdown of the principal aerodynamic forces on a Hovercraft having a value of LV/P (lift x forward speed : power) of 5.0 may be tabulated as follows:— Lift Cushion + jet, 90 Aerodynamic, 10 Thrust Propellers, 80 Angled jet, 20 Drag Momentum, 70 Profile, 30 L AERODYNAMIC T ANGLED JET ^*^"=I D INCIDENCE .^./.JtJD MOMENTUM L CUSHION + JET Fig S Basic forces acting on the SR.N2 during forwards running. The angle of incidence a is greatly exaggerated; as noted in the text above it is usually 0°
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