FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1963
1963 - 2339.PDF
Air-Cushion Vehicles FLIGHT International supplement, 26 December 1963 APPROXIMATE MEAN AIR CLEARANCE REQUIRED (ft) FOR ASSUMPTIONS SEE TEXT. FIG 10 RESTORING RESPONSE OF FLEXIBLE SEAL 40 60 SPEED - KNOTS Fig 10 Approximate mean air clearance required over probable maximum waves worst conditions') COCKERELL ON HOVERCRAFT Fig 11 Air-seal power required over probable maximum waves AIR SEAL POWER [EXCLUDING MOMENTUM DRAG POWER) FIG 11 POWER LEVELS FOR MINIMUM SAFE AIR SEAL AT 100 kt 40 60 100 200 400 600 1,000 WAVE LENGTH (ft) POWER TO OVERCOME DRAG OF FLEXIBLE SEALS 2C FIG 12 40 60 100 200 400 600 1000 WAVE LENGTH (ft) Fig 12 Power to overcome drag of flexible seals over probable maximum waves Fig 13 Influence of flexible-seal material weight and ift/drag ratio on power POWER TO OVERCOME DRAG OF FLEXIBLE SEALS FIG 13 23456789 10 DRAG FLEXIBLE SEAL iifl RAT 10 of the flexible sealing. It should be noted, however, that the actual shape of the gap will vary with the type and design of the sealing, and the way in which the restoring forces are brought about. However, this shows that there is a minimum mass-flow requirement for the cushion air supply at a given speed, and assuming that the mean air clear ance required at lOOkt is equal to half the gap (further and more detailed work is needed here), it is possible from Fig 9 to construct Fig 10. It is thought that this shows at least the order of mean air-clearances required to be safe in these very severe conditions. The clear- FLEXIBLE SEAL DRAG LIFT RATIO ance at 50kt would appear to lie between a half and a quarter of that required at 100, and again the law will depend somewhat on the design of the flexibility. It would appear that for a lOOkt craft with a mean air-clearance of 1ft, a restoring response of 64g would be required from the seals. In fact, it should be possible to design a graduated system with 1ft mean air-seal, 2ft of flexibility with a 64g response, 4ft of 16g, 8ft of 4g, and 16ft of lg, which would do more than would be required at lOOkt over the worst conditions. The air-seal power-requirement curves must be of much the same form as those of Fig 14 Effect incidence of water-contacting surfaces of flexible seal FIG u 0 INCIDENCE OF WATER CONTACTING SURFACES OF FLEXIBLE SEALS the peak air-gap curves of Fig 9, and are shown in Fig 11. For the practical values of restoring responses (for the bottom 2ft or 3ft of the flexible system), the maxima occur for wave lengths of less than 100ft. Curves of power to overcome the drag of the flexible seals (Fig 12) increase with increasing wave lengths, until the waves are so long that the craft itself begins to heave. There is a very wide hatched area, and it is suggested that very considerable and worthwhile im provements are possible on present methods of engineering the flexible suspension system of Hovercraft. The simplest systems are knocked up out of the way by waves. The surface of the water or land is used to actuate the system, and the cushion pressure is used as the restoring force. Halving the specific weight of the material either halves the work done for a given restor ing acceleration, or doubles the restoring acceleration for a given restoring force. The drag, and therefore the power to overcome the drag, is a function of both the work done and of the efficiency with which it is done, and this brings in the lift : drag ratio of the lifting system. Lift : drag ratio of a well-designed planing surface is about 8 (Fig 13), but that of a piece of canvas trailed flat behind a boat is less than 1, due to viscous effects. The design of simple 88
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
