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Aviation History
1963
1963 - 1170.PDF
FLIGHT International supplement, 27 June 1963 Air-Cushion Vehicles discusses his findings, reference being made to Figs 10, 11 and 12, all repro duced here:— Jx was first calibrated, and the cali bration is shown in Fig 10, curve A. J2 was then calibrated, and this is shown in Fig 10, curve B. Jx was then supplied with air at Win water gauge, and this resulted in a cushion pressure as shown in Fig 11 A, with the usual stagnation peak pressure at PS^ The supply to the inner jet J2 was then gradually increased. When the pressure ofJ2 was only slightly above the cushion pressure (less than 0.5in water above) the following effects were obtained:— (1) The cushion pressure throughout had increased by some 20 per cent. (2) The wool tufts, which we use to give a visual picture of the airstreams, showed that the primary vortex had been suppressed by the exhaust air from J2, resulting in (3). (3) The stagnation-pressure peak PSt of Fig 11, curve A, had practically disappeared. (4) The maximum value of the velocity of the exhaust air had decreased. All these points are sheer gain. The pressure supplied to the inner jet J2 was then further increased in steps. Under these conditions the outer cushion hardly altered, while the inner-cushion pressure increased exactly according to the calibration of the inner jet shown in Fig 10, curve B. The fact that the outer- cushion pressure did not increase shows that it ought to be possible to bleed off from the outer cushion all the air which is flowing through J2, without materially affecting the outer or inner cushion pressure, and rather rough tests showed that this is so. A hole was then cut in the back of the test gear, so that the outer cushion was connected to a third jet Jz, outside Jly as shown in Fig 12. With this arrangement, the exhaust of J2 fed into the outer cushion, and this air then flowed out ofJs almost at outer-cushion pressure. The effect ofJ3 was to increase both the inner and the outer cushion pressure. The gain is not, however, only this increased cushion pressure, but is the reduction in the quantity of air flowing through the jets due to the higher pressure into which they have to work. Fig 9 from Hovercraft Report No 7/58, the reference to which appears near the foot of column three opposite Figs 10 (top) and 11 from Hovercraft Report No 7/58, in which are reproduced actual results obtained with the rig sketched in Fig 9 In Fig 12 the flow system is operating at its design condition. Increases or decreases in height would produce changes in the amount of air being recirculated, and would result in a non linear height-to-lift characteristic. Fur ther work was done on this rig in June 1958. A simple two-dimensional single- stage recirculation system was set up, in which some flow was recovered within the cushion and directed to an outer nozzle. The rig was modified as shown in the sketch. A substantial increase in cushion pressure was ob tained as shown by the results for a height/jet thickness value of 2. Cockerell then draws attention to a further method of power-saving, which he had commented on earlier in his Ripplecraft Report No 12/57: that the most economical jet is a multi-partitioned let which is supplied from stepped pressure sources, so that each bit of the jet is supplied with the minimum pressure appropriate for its position from the inside of the jet. For example; suppose for a certain height a pressure of Win water gauge supplied to a lin jet will produce a 6in cushion pressure. Half-way through the jet (it will not be exactly half-way unless PjjPc is large) the pressure on the air will be not 6in but 3in, and so the outer half inch of the jet could be replaced by a lin jet supplied with air not at Win water but at 5in pressure. Such an arrange ment yields a saving of 20 per cent in power at the cost of the complication involved. To the limit, this method halves the power—for an infinitely wide jet!— and, in practice, savings of about 30 per cent could be achieved. Cockerell goes on to consider multi stage systems. Two four-stage systems are discussed, one in which air is "motored uphill" in stages, and one in which air is "expanded downhill" from the top pressure. The latter approach would appear to be the more promising, especially from the point of view of mechanical practicability. Fig 14 shows this system, and Fig 15 the same system with each jet having a high- and low- pressure part as discussed previously— air cannot escape from the inner cushion \ y^ • i 1 HOLE CONNECTED WITH r f f J3 "£35 mu\unu\\n\n \-T-TT VAUH Fig 12 from Hovercraft Report No 7/58, which is commented upon at the top of column two of this page of such a system without being put to work on the way out. From simple power considerations such systems look good; but it is noted that, for a given diameter of craft, and given internal cushion pressure, the step system gives an inte grated lift less than that of the basic system. Therefore, depending on the geometry, the final cushion pressure for the step arrangement will have to be more than the cushion pressure of the basic arrangement. Figs 14 and 15 from Hovercraft Report No 7/58, the text reference to which will be found at the extreme top of the column above W/.O v \ \ \ \X \ \ v v^T v *, \ u \ \ \ \ v \ \ •;\\\\\\\ v \\ \U\ \\ \ v \\ v i \ \ \ ' s \( 6 STAGE "DOWNHILL* DIAGRAMMATIC RE PRESENTATION. 2 PRESSURE JETS-ABOUT 20V. BETTER THAN FIG".. 99
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