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Aviation History
1952
1952 - 0036.PDF
8 FLIGHT INSPIRATION Dr. SeddorCs R.Ae.S. Lecture on Air Intakes for Gas Turbines IN presenting his lecture, Air Intakes for Aircraft Gas Turbines, before the Royal Aeronautical Society on December 6th, Dr. J. Seddon, Ph.D., A.F.R.Ae.S.,* said that the intake aerodynamicist might be said to exist on the eccentricities of side intakes. It was important that the necessity for compromise should be realized and borne in mind. There were not the resources in Great Britain for thorough investi gation to be made of every flow problem that arose, and only by holding firmly to the target of a practical design with good all-round efficiency could British designers keep in the fore front of progress. The lecturer said that his paper was concerned with the air intake as a field of aerodynamic problems and not with overall per formance comparisons. Physically, the air intake.was taken to mean the complete engine air ducting system upstream of the compressor inlet, and was thus normally that part of the installa tion for which the aircraft designer was primarily responsible. On a typical single-engined turbojet aircraft, the total entry area of the intakes amounted to a little under i per cent of the wing area, or about 15 per cent of the maximum frontal area of the body. The total intake volume had in the past usually been between 10 and 30 cu ft, this representing on the average about 5 per cent of the total cubic capacity of the fuselage. The importance of maintain ing high intake efficiency was illustrated by the fact that, at a flight Mach number of 0.9, a 10 per cent loss of intake adiabatic efficiency resulted in from 7 to 10 per cent loss of sea level static thrust, and from 2 to 4 per cent increase in specific fuel con sumption. At a Mach number of 1.2, these penalties were increased by more than 50 per cent. Most of the problems of intake design were connected with the flow processes occurring in the immediate neighbourhood of the entry, where the separation between internal and external flow took place. The distinguishing parameter was the entry velocity ratio (the ratio of flight speed V0 to mean entry velocity V ) and the values typifying the four regimes were: TT=O, I, 2, and 3. For V\ convenience and to indicate where the problems lay, the four regimes would be labelled the static, climb, level flight and dive conditions respectively. In the static case, the air accelerated into the entry from all directions. The velocity over any wetted surface external to the duct was low, and so the effect of external boundary layer was generally negligible. On the other hand, pressure gradients round the lips were severe, and this led to rapid boundary-layer develop ment just inside the entry or, in some cases, to separation. It was clear that lip radius would play a major part in determining the extent of losses arising in this way. As the aircraft speed was increased, with Fj more or less constant, the relative acceleration into the intake decreased rapidly until, in the climb condition, the air flowed into the entry with practically no change in speed or direction. As might be expected, most designs showed a mini mum loss near this condition. The third regime was that of level flight at or near top speed. In this case, the air decelerated to about half speed between the free stream and the entry. Losses in the boundary layer on wetted sur faces ahead of the entry now played an important part, firstly because they accumulated at much higher velocity than that inside the duct, and secondly because the adverse pressure gradient accompanying the pre-entry retardation might cause the boundary layer to separate. This flow-pattern also indicated the high velocity region on the outside of the lip, which determined the critical Mach number of the intake. The excess velocity was accounted for partly by the thickness and shape of the lip, and partly by the pre- entry retardation which effectively set the lip at a positive incidence. In the dive case, with a non-pitot-type intake, there was an increased tendency for the boundary layer to separate in the region of adverse pressure gradient. It was convenient to regard this case as a separate condition, because the practical implications were different from the previous one. The concern was not so much for high intake efficiency as for the possibility of aircraft vibration and other effects of separated flow. Dealing with pressure recovery and the significance of position ratio, Dr. Seddon went on to say that the pressure recovery of a subsonic air-intake was equal to the available isentropic pressure rise corresponding to the forward speed of the aircraft, less the total effect of all losses occurring between free stream conditions *Dr. Seddon is Principal Aerodynamics Officer, Royal Aircraft Establishment. and those at the compressor inlet. A new intake design of doubtful efficiency would usually be given a low-speed wind-tunnel test. This provided a check on the suitability of the chosen entry area, an opportunity to modify the detailed shape of the duct, and the necessary data to decide whether special devices, such as a boundary layer by-pass, would be required. Excluding plenum-chamber installations and turboprops, these measures could normally be expected to take the development up to a stage where all flow-separations were suppressed, and the losses were reduced to those arising from skin friction in a field of varying velocities and pressure gradients. Analysis of past results showed that, in these circumstances, the most important factor which discriminated between one design and another was the quantity and state of external boundary layer taken into the duct from the approach surfaces. Most developed intakes showed efficiencies of more than 90 per cent. To achieve this result often required the use of a boundary layer by-pass, as in the Attacker. A by-pass removed a proportion of the approach boundary layer and so reduced the effective position ratio of the intake. With a full set of measurements of intake loss, both with and without by-pass, the effective position ratio could be deduced for the case with by-pass present. A proportionate reduc tion in SjA\ might be taken as defining the by-pass efficiency. This efficiency for the Attacker by-pass, for example, was about 70 per cent. The use of a by-pass improved the ram efficiency of the intake by about 11 per cent. A 100 per cent efficient by-pass would effectively convert the intake to pitot type. Blade Root Loss on Turboprops Another important form of intake loss which came outside the category of frictional losses was that caused by the blade roots on a turboprop installation having the conventional arrangement of an annular intake close behind the airscrew(s). Wind-tunnel model tests had shown that, in a typical case, the total intake loss might be about 25 per cent ram, of which 15 per cent was attributable to the blade roots. The flow over the roots was complicated in character because of the large thickness/chord ratio of the root sections, and the existence of centrifugal forces acting on the boundary layer. As a result, there was little advantage to be gained by having flared roots with large root chords, and if these involved an increase of actual section thickness, the effect might be detrimental. Modern blade-development was more in the direction of pro ducing thinner roots, and this was a surer way of reducing the intake loss. A good, modern, four-blade airscrew on an engine like the Mamba might be expected to have a blockage ratio of the order of 16 per cent, and would therefore be responsible for the loss of about 8 per cent ram in the intake. An alternative way of obtaining an increase in intake efficiency with a turboprop installation was by means of a ducted spinner. This gave a large reduction in velocity over the blade roots, and effectively converted the intake itself to pitot type. The advantage of the (Napier Naiad) ducted spinner amounted to 19 per cent ram, and resulted in giving 90 per cent efficiency. The main disadvantages of the ducted spinner were its weight, and the difficulty of providing protection against ice. It had been shown that the pressure recovery of a subsonic intake in high-speed flight depended primarily on three factors: (i) the intake position ratio, (ii) whether the engine was of the direct inlet or plenum chamber type, and (iii) whether the method of pro pulsion was by turbojet or by turboprop. If either plenum-cham ber loss or blade-root loss was present, this tended to dominate the picture. But the influence of position ration was just as great as for direct inlet turbojet engines. Dr. Seddon said that to put a few well-known examples into Fig. 1. Diagram of intake flow and velocity variation. *-x
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