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Aviation History
1963
1963 - 2338.PDF
lushion Vehicles fJT International lement, tcember 1963 CUSHION^.POMjgL- "A PROPULSION POWER a 20 WAVE HEIGHT 10 (ft) *#- >>RS >> ?2! c£ ^ S^ W* W^- FIG A -SPEED — TOO kt FIG 6 10 20 40 60 100 200 400 600 1D00 WAVE LENGTH (ft) Fig 4 Rough water drag-powerjspeed characteristic for material seals [constant wave conditions) Fig 5 Rough water drag-power/'seal-height character istic for material seals (constant speed) PROPULSION POWER Fig 6 Probable maximum wave heights versus wave lengths v50kt 100 kt WAVE LENGTH* 275 ft FIG 7 Fig 7 Response of flexible-material seals IBLE EXTENSIONS FIG 5 -MATERIAL SEAL HEIGHT — 64 32 SEAL 16 ACCELERATION 9 4 ? 1 \* V h V Nfov /Q3 \n. &J Vv?^ *v fifth and seventh components, that is to say, with cushion air supply and momentum drag, and with surface- contact drag. The form of the calm-water lift power against speed, and lift-power against air-clearance characteristics are set out in Figs 2 and 3. Practical values of cushion pressure lie in the range 20 lb/sq ft for small and relatively slow craft (20 to 50kt) to perhaps 100 Ib/sq ft for large multi-decked craft in the region of lOOkt. In any given case, optimization is mainly a function of speed, size, hump drag, shallow-water wave drag, structural weight and structural cost. Rough-water drag power of flexible seals (Figs 4 and 5) increases as the third power of the speed, but linearly with seal height (to a first order). The cushion-air power characteristics of Fig 2 have been superimposed on Fig 4, showing that an air-seal is an expensive way of containing the cushion, except at high speed. Whereas slow-speed craft might have simple plenum air supply, and simple fixed sidewalls with displacement stability, high-speed craft are likely to have annular jets and advanced flexible systems. But all up the speed range it would appear desirable to cut down the cushion air supply to the minimum. Is there a minimum? We must first consider the character istics of a rough sea. The lower curve in Fig 6 represents a pretty severe sea, while the upper curve is believed to be the worst ocean conditions. Ocean waves with lengths of 2,000ft, and of greater height, do occur in the Pacific; but the really long waves are the easier part of the spectrum. WAVE LENGTH (ft) WAVE HEIGHT (ft) 10 15 20 3 40 60 5-5 7-5 100 10 200 15 400 600 1,000 21 25 FIG 8 Fig 8 Seal accelerations to conform with probable maximum idealized waves Consider now the craft depicted on the left side of Fig 7. It is fitted with wave-actuated flexible walls, with a maximum response of lg. These are capable of following the idealized wave at 50kt, but "skip" at lOOkt, leaving large gaps through which the cushion air can escape. If the response of the flexible seal were increased to 4g it would be just capable of following the idealized wave, and the gaps would disappear. More over, if, due to the sea or to the speed, or both, a condition is reached where the area of the gaps becomes larger than the maximum for which the installed air supply is capable of sustaining the cushion pressure, the body of the craft will lose height. If now the response law of the flexibility with deflection is constant, the area of the gaps will not decrease as the craft loses height, and the craft will ditch in. This is not unlike the stall condition of an aircraft, and it would appear to represent a safety boundary for Hovercraft. The response required from the flex ible system to enable it to follow the "probable maximum (idealized) waves" is shown in Fig 8. It is not the high and long waves which are the problem, but the short waves; for practical maximum restoring g's, it is impossible to effect a seal over them by flexible walls alone. As would be expected, a reduction of speed in bad conditions quickly eases the problem. The information of Fig 8 may be presented as shown in Fig 9. Here g-contours are drawn showing the order of gap between the probable maximum wave trough and the bottom 15 10 Fig 9 Peak air gap be neath flexible seal over probable maximum waves (worst conditions) PEAK AIR GAP BETWEEN SURFACI AND SEAL (ft) FIG 9 1C 20 40 60 100 200 WAVE LENGTH (ft) 400 600 1.000
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