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Aviation History
1961
1961 - 0421.PDF
FLIGHT, 6 4pn7 1961 429 THE SUPERSONIC TRANSPORT Dr A. E. Russell's Lecture to the Guild of Air Pilots and Air Navigators ONE of the most important papers of recent years was thatdelivered on March 28, before the Guild of Air Pilots andAir Navigators, by Dr A. E. Russell, CBE, DSC, FRAes,FUS, technical director of Bristol Aircraft Ltd. Special importance attaches to the lecture by reason of the fact that Dr Russell ispopularly regarded as the master mind behind the British supersonic airliner project, for which British Aircraft Corporation have aMinistry of Aviation study contract. The prospect of a supersonic transport—said Dr Russell—hashad to await an advance in the field of aerodynamics and the awaken- ing of interest by an authority able to find large sums of money insupport. Arguments in favour of such a project vary from an emotional complex of national prestige to claims of economic meritand the consequent inevitability of such developments emerging around the end of this decade. In some unexpected places, judg-ment seems to be dominated by over-emphasis on the merits of the highest conceivable speed. Starting with a wide open field, it has become apparent that twosharply diverging tracks are being set, the followers of which tend to be recognized by their nationality. Many on the one side arepuzzled by a strong indication that the other advocates a cruising speed around M3. Nobody denies that M3 poses formidabletechnical problems; but these need not be regarded as uncrossable barriers. The present point at issue is whether such effort is worthundertaking, for a less ambitious speed seems to offer a consider- ably better chance of approaching typical costs in airhne operation.Alternative subsonic transport provides the standard of reference. It is well known that there is an inevitable sharp drop in lift/dragratio in the neighbourhood of Ml. Aerodynamic solutions now available limit this drop to approximately half the value achievedby the best subsonic aircraft. Thereafter the continuing fall with increasing Mach number is very gradual. Fortunately, withincreasing speed the propulsive efficiency rises (Fig 1), so that—in spite of the higher drag—the overall efficiency first falls sharply atMl and then rises again with increasing Mach number. With rising speed a complication to be faced is the rapidly increas-ing generation of heat, for which there are two fundamental causes. In the first place, the forward motion of the aircraft compresses theair by impact, the areas most affected being the leading edges and engine intakes. The second effect is the exchange of heat from the t Of BEST OVERALL THERHAl EFflCttHCY 2 3 4 CRUISE MACH Fig /. A plot of overall powerplant thermal efficiency over a wide range of cruising Mach numbers. The im- provement with in- creasing Mach num- 5 her counteracts the reduction in lift/drag ratio layer of air to which energy has been transferred by friction, bycontact with the external surfaces. Choice of Engines Typical results show that, whereas up to about M2 the turbofanshould have a higher efficiency than the comparable turbojet, at somewhat higher speeds this advantage is reversed. As speedscontinue to increase, the higher temperature in the intake reduces the allowable pressure rise in the compressor, and after passingthrough a hybrid form the most efficient engine eventually becomes a ramjet. While it is essential that the engine performs efficientlyin cruise, it must also be as economical as possible at lower speeds and at the lower altitudes through which it must pass and, onoccasions, have to loiter. Generalities may therefore be unreliable, so comparison should be made on the basis of specific enginedesigns which have passed through successive adjustment to achieve the best overall compromise. An example can be quoted where a turbofan and a turbojet,each employing reheat only for transonic acceleration, have been carefully matched to suit an aircraft having a cruise speed of M2+.An optimum overall cost analysis, taking into account the effect of variations of the diverse factors affecting aircraft operating econo-mics, led to the choice of a turbofan having slightly less cruise thrust than the turbojet. Consequently the cruise altitude wassomewhat lower and the climb fractionally slower. With a large turbofan and accompanying higher installed weight and drag thetrends showed increased operating costs. For a sector distance of 3,000 n.m. (zero wind), the best results showed that an equalquantity of fuel—37 per cent of take-off weight—was required by both the turbofan and turbojet. In establishing the total quantity of fuel that must be carried it isnecessary to decide what reserves are required. At present, this approaches an arbitrary choice based on the experience of indivi-dual airlines. Ideally, a rational basis would be used, making due provision for the probability of occurrence of all possible adverseevents. Generally, an attempt is made to take account of engine and airframe deterioration, engine failure en route, navigational errorsand air traffic control requirements. As regards engine failure, an analysis has been made, for six-engined supersonic aircraft, of the available range remaining after single or double engine failure at any point on the North Atlanticroutes. Comparison with a similar study for a four-engined sub- sonic aircraft shows that the same airfields are as readily withinreach in both cases. The same proportions of fuel reserves to cover engine failure should, therefore, be adequate. Winds, either heador cross, cause progressively less deviation from the desired track as aircraft speed increases, and improved navigational systems arecontinuously being developed. Current methods of valuation of the allowances in the foregoing respects should be more than adequate.Should circumstances impose a temporary reduction in the flight- plan altitude, then, for a given height loss, reserves are consumed atrather more than twice the rate of a subsonic aircraft. Many contingencies are sometimes lumped together in theterminal holding reserves. It may be assumed either that the aircraft is held for 80min at 30,000ft or, alternatively, 45min at 20,000ft and15min at 5,000ft, with a 200 n.m. diversion. The equivalent reserves would then be 20 per cent (turbofan) and 21 per cent (turbojet)of the fuel consumed on a normal 3,000 n.m. flight. With an ad- ditional allowance for de-icing, this leads to the total fuel to becarried being equal to 44.7 and 45 per cent of the take-off weight, the small advantage resting with the turbofan. Overall powerplant efficiency in a supersonic aircraft is greatlydependent on the design of the variable intake and exit nozzle. As the range of operating conditions is very wide, this leads to com-plex arrangements. The intake must control shocks at the lips and throat, and pass the appropriate mass flows through a long diffuser.Subsidiary vents are necessary to augment or spill, according to the main-intake capacity and the needs of the engine. The condition
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