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Aviation History
1962
1962 - 0353.PDF
355 f International, 8 March 1962 Problems ofreingestion are examined by injecting oil from each side of the plate beneath the R&.I08 at Hucknall Jet-lift Erosion RESULTS OF RESEARCH BY ROLLS-ROYCE ADEQUATE experience has now been gained with the light weight and compact separate jet-lift engine to provide an acceptable level of confidence in its abilities. Nevertheless, there have been misgivings over certain operational problems— in particular those of ground erosion and hot-gas reingestion. What follows is a precis of a Rolls-Royce paper prepared for the RAeS by Messrs J. R. C. Fearon and D. H. Norman, both of the company's establishment at Hucknall. Once the lift engines are lit and idling, the trained jet-lift pilot must make the deliberate decision to open the lift throttles to the maximum and go. In the absence of ground suction, the aircraft will lift-off at approximately 80 per cent full thrust. The time taken to accelerate from this condition to max lift r.p.m. is around 0.5sec. Thus the full jet efflux will impinge on the surface for only a very short period, and its destructive power becomes rapidly attenuated as the aircraft rises. A lOsec cycle was evolved to simulate a lift-off, and this was used throughout the tests. The cycle included a slam acceleration, lift-off rating for two seconds, followed by a return to idling. The rig (heading picture) comprised an RB.108 engine suspended at any required height in a gantry which could be positioned over various surfaces. Also a Gloster Meteor was used with an RB.108 mounted with its nozzle 30in above the ground. Concrete surfaces were the first to be tested, and a typical sample was selected at random. With the lift-engine nozzle 3ft above the surface, erosion was experienced after one take-off cycle (below). It took the form of flaking to a depth of approximately -k in over an area of 2 sq ft. Water contained in small cavities in the concrete boils under the influence of the exhaust gases and the steam pressure breaks-off the top flakes. After 50 cycles the eroded area had in creased by only some 25 per cent. A lift-off and landing was then carried out by the Short SC.l on a similar area of hardstanding. Only five flakes, each about 1.5in across, were blown off, and no damage was sustained by the aircraft. Concrete is less porous and much stronger if it is cured at a markedly slower rate than when exposed to the weather. If wet sand is placed over the concrete surface for 15 days after laying the moisture is prevented from evaporating, and the concrete is pro tected from tempsrature fluctuations that might be caused by radiant heat from the sun. Water-cured concrete of this type was tested with most encouraging results. To date it has withstood the equivalent of over 200 take-offs with no deterioration whatsoever (but for some oil stains it would be difficult to pick out the area over which the engine had been run). The benefits offered by this type of surface could be further enhanced by the adoption of a more speedy method of curing. Tests were performed on three additional types of hardstanding: firebricks, which withstood the blast satisfactorily; tarmacadam, with which an unacceptable degree of erosion was experienced; and unprotected grass. With the engine nozzle 3ft above the ground, the grass did not withstand even one take-off cycle; during start-up sods were dislodged and, as engine speed was increased, undesirable quantities of subsoil were eroded away. Thus, with the exception of concrete and firebrick, it appeared that protection of the ground would be necessary. As a first step, measurements were taken of the temperature and dynamic pressure on the surface. It was found that the total depth of flow was about 3in, and that this remained fairly constant with distance from the jet axis. As had been suspected, this indicated that rapid and extensive mixing occurred as the flow spread out radially. Peak "scrubbing" velocity occurred over an annulus between 1.67 and 2.0 nozzle diameters around the axis. This agreed well with the erosion pattern observed during the preceding tests, and suggested that purely local protection might be sufficient. Measure ment of the surface temperature showed that the maximum reached was 360°C, on the nozzle axis. This fell to 100°C at 3ft, and to below 50 °C at 6ft. As these surveys indicated that the destructive power of the jet was considerably dissipated only some three or four nozzle dia meters from the axis, it was thought that small metal plates pegged into the ground might provide adequate protection. Initial tests were carried out with a 3ft disc pegged to a grass surface. During a single cycle the plate protected the ground from all but slight scorching of the grass roots around the edge. A 2ft plate was sub- Lift-off conditions for the RB.108 (80 per cent thrust) correspond to a jet temperature of 620°C and velocity of I .SSOftfsec. The effect of a single take-off cycle on a random sample of concrete is seen at left; tarmac (right) shows maximum erosion at 1.67 nozzle diameters
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