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Aviation History
1955
1955 - 0277.PDF
FLIGHT, 4 March 1955 CORRESPONDENCE..* JET LIFT RAISES A CONTROVERSY 277 IN Flight of February 4th you have produced a very interestingarticle on jet lift, but I cannot leave some of your proposals unchallenged. You write of the weight of lifting motors being less than thesaving in wing and undercarriage weight. This is certainly pos- sible, but with a specific fuel consumption of 1.5 lb/hr/lb theweight of the extra fuel required for take-off and landing could easily be 10 to 15 per cent of the all-up weight.You also write of wing loadings of about 300 lb/sq ft associated with Mach numbers of 1.5 and altitudes of 60,000ft. I suggestthat this speed and altitude would not be achieved with a wing load- ing of much more than 80 lb/sq ft on a straight thin wing or about50 lb/sq ft on a highly swept delta. With these wing loadings demanded for the cruising condition, the prospects of jet lift beginto fade, since the take-off and landing problem is not really severe. Another point with which I cannot agree is the suggestionthat the same engines might be used for take-off and cruise since "some aircraft designed for Mach numbers over two will needthrust exceeding their all-up weight." Unless this sort of aircraft can be designed to achieve a cruising weight/thrust ratio of atleast 4, it can scarcely be considered as a reasonable project and it is becoming increasingly apparent that engines designed forthese high speeds and altitudes cannot at the same time give very high static thrusts; in fact, they must ultimately approach ramjetperformance. I think that Rolls-Royce are absolutely right in their contentionthat the two engines—lifting and thrusting—should be of different design.I do not wish to give the impression that I am at all critical of the great stride forward which Rolls-Royce have made, but Iam not at all sure that the future of jet lift lies in the fields which your article suggests. Belfast. SLIPSTICK. CJETTING aside the more startling of the statements made by^ the author of "Jet Lift"—such as that the pressure distribution over a wing is a Newtonian "reaction" to the deflection of therelative airflow, or that jet lift involves no deflection of the surrounding atmosphere—his analysis of a theoretical aircraftto his own specification is optimistic and disingenuous. For some reason, he stipulates that his aircraft should remainjet-borne until it reached a speed of 150 kt, at which speed the wing would begin to take its share of the burden. But as thewing would not be called upon to support any weight at all at take-off, its loading would not be 300 lb/sq ft, as he says, butnil; it would never be stalled, and would therefore be able to generate lift immediately it started to move forwards. As thebattery of lift-turbines would cost something of the order of £5 per second to run, one hopes that the pilot would start to cutthem out at the earliest possible moment. The author light-heartedly dismisses the problem of the fuelconsumption of his lift-turbines by saying that it would be "of little importance for such short duration." His large aircraft wouldhave fifty lift-turbines of 10,000 lb thrust each, so that on his own figures their gross consumption would amount to 750,000 lb/hr—enough fuel to run a diesel car from London to Edinburgh and back five thousand times, or fifteen times the distance from theearth to the moon. To accelerate the 300,000 lb aircraft from rest to 150 kt would take something more than 20 seconds, so that ifwe allow him a total of 30 seconds' running time for the lift- turbines at take-off we are being generous to him. For landing, of course, other problems arise. He has con-veniently overlooked the fact that when his aircraft's speed has fallen to 150 kt and it has once again become wholly jet-borne,it has still to be brought to rest, and to be manoeuvred directly over the landing-point. If we make him a present of fifty per centreverse thrust from his main engines without weight penalty, it will still, I think, be generous if we allow as little as 60 secondsfor this process. The actual let-down must obviously be per- formed more gingerly than the take-off: let us say 75 secondsfor landing, or a minimum of If minutes' running-time per flight. There is another point to be considered. When the lift-turbinefuel is exhausted, we have an aeroplane whose minimum safe speed is about 200 kt, with almost certainly vicious stall characteristics,an inadequate undercarriage, and, by hypothesis, no runway to land on, so that disaster is comfortably assured. Before allowingthis potential flying lethal-chamber to take the air one hopes that the responsible authorities would insist upon at least a 100 percent reserve of lift-turbine fuel being carried, so that we have to allow for not less than 3i minutes' running-time. This entailstne carrying of 44,000 lb of fuel, without allowing for the weight of tanks, etc.—a nasty slice out of the useful load of a 300,000 lbaircraft, even if we have saved the weight of the turbines them- selves by constructional simplification. Of course, with a cruising speed of Mach 1.5 the fuel require-ments for the main engines will be prodigious, but we cannot depend upon their reserve supply to feed the lift-turbines, on thegrounds that by the time the latter are brought into use the possibility of diversion no longer exists. The aircraft may alreadyhave been diverted and the reserve propulsive fuel been expended. Let us examine this remarkable aircraft a little more closely. Itwill hardly be suitable for short stages; let us fly it from London to New York non-stop. The main engines have a total thrustof 120,000 lb and give it a cruising speed of Mach 1.5 at 60,000ft —say, 1,000 kt. For the New York trip it will have to carry fuelfor not less than 3j hours' running, and if we are very generous we may allow the engines a specific fuel consumption of 0.5lb/hr/lb. This amounts to 210,000 lb of fuel. The installed weight of the main engines can hardly be less than 20,000 lb; weknow that the weight of the lift-turbines is 50,000 lb, and of their fuel, 44,000 1b. Thus the weight of engines and fuel aloneamounts to 324,000 lb, which is a high figure for an aircraft of 300,000 lb all-up weight! It is true that I have assumed that themain engines are run at full power continuously, but as the aircraft is to cruise at Mach 1.5 with less than the equivalent thrust of aHunter or Sabre I do not think this is unreasonable. I am not trying to belittle the possibilities of jet lift. No doubtresearch into this question is well worth while; but one may be forgiven for hoping that your contributor will not be in charge of it. Horning, Norfolk. G. YOUNG. Comment on the two foregoing letters, by the author of "JetLift," is as follows:— Jet lift is going to require a considerable re-orientation ofthought; aircraft of this type cannot be considered as conventional fixed-wing machines with an auxiliary device added. "Slipstick" is naturally loth to envisage large aircraft withmissile-like wing loadings; but the values he quotes are not demanded for the cruising condition. Incidentally, I did notadvocate the use of one set of engines for both lift and forward flight; my comment was that ". . . the disadvantages almost cer-tainly outweigh the advantages." W/C. Young's letter does him less than justice; his first twoparagraphs are pure quibbles, which may be left to readers to assess. He actually misquotes the article; after "jet lift involvesno deflection of the surrounding atmosphere," the words "per se" appeared. Of course there is deflection of the whole neighbour-hood; but it has no effect on the thrust, and, if compressed air were fed to the engine from a bottle, the jet would work equallywell (better, in fact) in vacuo. It is also generally understood that wing loadings are not calculated for aircraft at rest; as for the"wing . . . would never be stalled," W/C. Young can try some sums assuming an angle of attack of minus 90 deg. It seems that his own calculations are at fault. Had heseen the "Bedstead" film, he would not suppose that lift-jets are kept at full throttle. Actually, to support 300,000 lb, enginesworking at an s.f.c. of 1.5 lb/hr/lb demand an hourly consump- tion of 450,000 lb, not 750,000. Allowing for gradual throttlingback, the mean lift-fuel consumption amounts to 100 lb/sec, or 3,000 lb for W/C. Young's time-value. It seems to have escapedhis notice that, at landing, the aircraft would be much lighter; again taking his time estimate, the fuel required for landing, with100 per cent allowance, would be 10,000 lb. The total lift-fuel thus comes to 13,000 lb. One of the obvious advantages of the jet-lift aircraft is that itdoes not need a fuel allowance for stacking and diversion. In a conventional aircraft of the size considered, such an allowancemight well exceed 30,000 lb. The penultimate paragraph of the letter suggests thatW/C. Young is unused to calculations of this type. It is also strange that he should take the sea-level thrust rating (30,000 lb)as the high-altitude cruising thrust. His mention of the Hunter and Sabre, both conventional "winged" aircraft, implies that hehas missed the point of the new development. CORRESPONDENCE IN BRIEF ON the "Here and There" page of Flight for February 18th wetold the story of a reader who, on a journey through Norway in a R.N.A.F. Dakota, occupied a seat in the fuselage of a KlemmKl 35, which was being carried as cargo. The correspondent wondered if he could claim to be "the first civilian to land in twoaircraft at the same time." Mr. J. Rasmussen, of Vickers- Armstrongs, Ltd. (Supermarine Works), now writes recalling asimilar occurrence in 1945, when he also was travelling in a Dakota, which on this occasion was transporting a Spitfire fromBurma to India. The incident was the subject of a photograph published in Flight for December 4th, 1947.
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