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Aviation History
1963
1963 - 1153.PDF
,»^e-s^esB=a >)) N j^ •^=^='// SECTION A.A CUSHION LEAKAGE ( CROSS FLOW* ' PRESSURE FED SYSTEM THICKER JET. Fig 9 Waves as a factor in the design of the air-lift system SOUTHAMPTON SYMPOSIUM . . . although somewhat worse over flat water when it does not matter, would be better in rough conditions when all drags are at their maximum. This is illus trated in the lower left-hand corner of Fig 9. However, if the overall economics are taken into account, is the compli cation in cost and weight of piping air right round the craft justified for the dif ference in efficiency between a craft part Fig 11 Pitch response excitation by head, angled and following seas NATURAL RESONANT FREQUENCY (AT ZERO FORWARD SPD) JNCREASING HOVER -HT. SRN.2. SRN1. VA3. FREQUENCY OF ENCOUNTER CYCLES/SECOND 0 010 0 20 0 30 0 40 0 50 FREQUENCY OF ENCOUNTER - CYCLES / SECOND 0 0-10 0-20 0 30 0A0 0 50 Air-Cushion Vehicles annular-jet sealed and part cross-flow sealed, as against pressure-fed annular jet sealing all round? As flexible tech niques improve, the balance may well tip towards simplicity at the expense of the ultimate in air curtain efficiency. Now, one of the effects of increasing the body clearance of the craft is to decrease the natural damping and stiff ness, and the variation of stiffness with increasing height-to-length ratios is shown in a general way in the top picture of Fig 10. It will be seen that VA-3 and SR.N2 are operating at height-to-length ratios in the stable region, but future craft with advances in flexible wall technique will be operating at height-to-length ratios more of the order of 6 per cent to 8 per cent, and will have lower and inadequate values of natural stiffness. Since the craft will have length-to-beam ratios of about 2| to 3, the height-to-beam ratio will be in the 15 per cent to 24 per cent region when over the trough of a big wave at speed, and under these conditions the natural roll stability will be very marginal or even negative. As development proceeds, therefore, it seems certain that it will be necessary to augment the natural stability in both roll and pitch by automatic centre-of- pressure shifters, and as a bonus such devices may be made to look after datum trim as well, whether due to wind forces, nose-up attitude due to motion, imperfect loading or the movement of passengers. Since the natural damping of a craft is less than critical (about 0.25), it will exhibit resonance phenomena when excited at its natural frequency, and Fig 10 (lower) shows a typical pitch response envelope of a craft when running over waves, obtained from model tests. If this trouble were only to occur at 10 to 20kt as shown in the diagram, it might be uncomfortable for the pas sengers, but would not be dangerous; but unfortunately this excitation can occur at any speed when travelling over quite ordinary sorts of waves. However, the same c.p. shifters, incorporated to augment the static stability, can be designed for dynamic operation, and, with appropriate black boxes, could be made to iron out the resonance whether in pitch or roll, as shown by the dotted curves. It could also be made, if it is considered worthwhile, to improve the less dangerous heave resonance charac teristics. That the excitation of the fundamental resonances of the craft can come about all up the speed range with quite ordi nary length waves is shown in Fig 11. Here the frequency of encounter is plotted against craft speed, for some examples of head, angled and following FLIGHT International supplement, 23 May 1963 DEVELOPED CRAFT." Fig 10 Top: Curve showing decreasing pitch-stiffness with increasing hoverheight. Lower: Typical pitch resonance of hovercraft (similar resonances occur in roll and heave) < X seas. The natural period of the SR.N2 in pitch (measured at zero forward speed—this is probably a slightly speed dependent term) is about 0.38 cycles per second, and future larger developed craft are likely to have even lower natural stiffnesses and dampings lead ing to pitch natural frequencies of around 0.2c/s. Such a control system will also reduce the overswing if the craft is knocked up out of attitude by the odd extra-high wave top striking the bow, but really good control can only be expected if the body of the craft can be run high enough to keep it sensibly clear of green water. These devices will cost power, but the overall power penalty of about 20 per cent is currently accepted in aircraft for control purposes. Since motor cars are fitted with shock absorbers and ships are being fitted with anti-roll stabilizers, there seems no good reason to shy away from the complication involved in the fitting of c.p. shifters to hovercraft. When, over the coming years, all these and other various improvements have been developed and incorporated, it should be possible to reduce the total power requirement of a given craft having a given performance by at least as much as 50 per cent. In addition there will be improvements in efficiency with increasing size. Some people may feel that this is over-optimistic, but it must be remembered that ship propulsive efficiency, which one would have con sidered to be near the limit, has in fact been improved by some 25 per cent in the last 25 years; and that aircraft 84
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