It is not generally realised that the ground effectprinciple on which the air cushion vehicle is basedwas first expounded by a Scandinavian scientist somehundred years before Sir George Cayley grasped thebasic principles of flight and two hundred and fiftyyears before the hovercraft went into commercialservice. The basic principle of the air cushionvehicle is extremely simple, but, as in manybranches of engineering, simplicity of principle andease of achievement do not go hand in hand!

How does a hovercraft hover? Let us take a vehicle witha weight of 1000 lb (454 kg) and a flat undersurfaceof 100 sq ft (9m2) in area. If weincrease the air pressure under the machine by 10lb/sq ft (478 N/m2), the total reactionpushing upward from the undersurface of the machinewill be 10 x 100 = 1 000 lb (454 kg). This, beingequal to the weight of the machine, will cause it toleave the ground.