08-16-2017, 09:10 PM
HOVERCRAFT
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INTRODUCTION
A hovercraft is a vehicle capable of traveling over most surfaces on a cushion of air trapped under the body for lift. Air propellers, water propellers, or water jets usually provide forward propulsion. Air-cushion vehicles can attain higher speeds than can either ships or most land vehicles due to lower frictional forces and use much less power than helicopters of the same weight. Figure 1 below illustrates the operational principles and basic components of a typical hovercraft.
Thrust System :
The air not directed to the cushion and skirt is propelled backwards, providing forward thrust to the craft. The size of the propeller, rpm output of the engine, and height of the lift/thrust divider are the determining parameters for the thrust force. A thrust duct channeling the air into the propeller can provide up to a 15% increase in efficiency [Universal Hovercraft]. The limiting factor for the thrust is the air flow available to direct backwards since our primary concern is providing pressurized air for air cushion and lift. As a result, our forward speed is limited but maintainable.
Steering System:
Since a hovercraft lacks the same frictional and drag effects as boats or cars, steering must be approached without precise control in mind. This is especially true in our case as the power supply is limited. Rudders are a main source of steering and are attached to the rear of the duct to direct the flow of air and the direction of the subsequent momentum transfer from the air to the craft. The driver controls the movement of the rudders through a joystick located in the front of the craft. A throttle on the engine situated next to the driver allows him to vary the speed of the craft, allowing for a smaller radius of turning once proper driving techniques are mastered.
Post-war progress:
When the war ended a number of groups took up development of air-cushion vehicles again. The Soviets, specifically Rostislav Alexeyev and his Central Hydrofoil Design Bureau, returned to the ground effect design pioneered by Levkov, and produced a wide variety of such craft over the next 30 years. However, Alexeyev's systems were always experimental, and never entered production. Most famous among these is the Caspian Sea Monster, as it was known in the west, a massive missile-firing boat powered by eight jet engines. Unlike Levkov's design, however, these boats generally lacked a lift engine, used wings exclusively, and were less similar to western hovercraft than Levkov's design.
Skirts and other improvements:
Testing quickly demonstrated that the idea of using a single engine to provide air for both the lift curtain and forward flight required too many trade-offs. A Blackburn Marbor for forward thrust and two large vertical rudders for directional control were added, producing the SR.N1 Mk II. A further upgrade with the Armstrong Siddeley Viper produced the Mk II. Further modifications, especially the addition of pointed nose and stern areas, produced the Mk IV.