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Aviation History
1964
1964 - 0268.PDF
fllOHT International supplement, 30 January 1964 A RACING ACV Thoughts on Design Problems by David Stevens Air-Cushion Vehicles IN THE SEPTEMBER ISSUE of Air-Cushion Vehicles I discussed the advantages of racing ACVs within a carefully pre- pared formula which would limit the engine capacity, dimensions and weight of the vehicles. Now we can go a stage further and consider some possible layouts. Apart from the entertainment value, there is only one really good reason for producing racing ACVs, and that is to speed up development of the means of controlling machines intended to travel primarily on land. The usual worries about running costs can, thank goodness, be forgotten for the time being at any rate. A racing car constructor is most concerned about suspension, power transmission, weight reduction, streamlining and reliablity. In the same way the racing ACV constructor must think in terms of using the least possible amount of lift power and the most efficient methods of propulsion; he must decide, too, how to cut down excessive drift on corners, how to slow down the vehicle regardless of the type of surface, and how to package the whole machine in the best aerodynamic envelope. Let us look at each of these points in turn to see what a designer might do about them. Minimum Height with Minimum Thrust If you remember, the suggested regu- lations stipulated a minimum "flying" height of 4in, to be attainable with maximum propulsion power in full operation. This would prevent the design of machines that normally cruised only an inch or so from the surface but which could be lifted to 4in when called upon by the scrutineers to do so. And because the regulations would also include a minimum weight limit, the designer would be left with no other choice than to produce efficient fans and ducting. But there seems to be no simple answer to guarantee the highest effic- iency. One fan, as in Project No 3, means a large amount of ducting, with a consequent reduction in efficiency through internal air friction. Twin fans, as in Project 2, could halve the distance the air has to travel inside the vehicle, but create problems in merging the two airflows and double the drag of the transmission gear. Project No 1, with 11 four centrifugal fans, cuts down in- ternal drag even further, and the use of two engines avoids extra transmission drag. The logical conclusion would be that multi fans and multi engines are theoretically more efficient, but in practice this could easily be cancelled out by the extra weight of the multi- plicity of drive shafts, fuel systems, and fan ducting. One would normally expect that the lift fans would be run at a constant speed throughout the vehicle's oper- ating cycle, but if the designer could use ram effect to help lift the vehicle when it was travelling at speed, then the power saved could be transferred to the propulsion system. It might even be possible to stop the fans completely— very intriguing thought! Another difficult decision is whether to have horizontal or vertical fans. Horizontal fans take up a large amount of lateral space, but allow the frontal area to be kept to a minimum. Vertical fans could be fitted facing outwards at the four corners of the vehicle, looking something like the humps over the wheels of a very low sports-racing car. Alternatively they could be forward- facing so as to make the maximum use of ram effect. Power for Propulsion At the moment there seem to be only three basic methods of propelling an ACV—pro- pellers (ducted in the case of a grand prix machine in the interests of safety); Project Two Spaceframe box centre- section incorporating engine, cockpit and fuel tank; l^-litre BMC transverse power unit and transmission; two axial fans; con- trol via steerable variable pitch airscrew and two forward-mounted tail fins; pro- pulsion from single ducted airscrew at rear. Project One Spaceframe backbone "chassis" with sleeved-down 750 c.c. Mini engines slung one on each side; four centrifugal fans with rearward facing take- off ducts for propulsion; twin fuel tanks, one feeding each engine, placed immed- iately below top deck between fans; control via four individual duct nozzles and single rear tail-fin. tilting the machine forward like a glider; or by air ducted away from the lifting fans. Project 2 uses a single propeller mounted at the back, and Project 3 uses two propellers placed well forward, where they help counteract the weight of the engine. In both cases the propellers are driven by an extension from the fan transmission and would therefore need to be constant-speed, fully feathering units. On Project 1 air is ducted directly from the four centrifugal fans. This arrangement is pleasantly simple and makes the trans- fer of power from lift to propulsion relatively easy. The only big reservation about this layout is that ducted drive has never, in the past at any rate, proved as efficient as propeller drive. But might not the considerable reduc- tion in transmission drag and the easier control make up for this? It is amusing to think of the performance of this vehicle if fuel were injected into these ducts and fired: a racing ACV Project Three Monocoque construction; rear-mounted Corsair GT engine and gearbox; single axial fan; fuel tanks mounted just ahead of fan at centre of gravity; control via two mid-mounted steerable ducted airscrews and rear fin; propulsion from same two airscrews.
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