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Aviation History
1958
1958 - 0579.PDF
FLIGHT, 2 May 1958 PROPULSIONENGINES LIT Suggested programmes for take-oft and landing for a Griffith VTO airliner; comments on the procedures were published in Part 1 last week. The Griffith Airliner Explained A CRITICAL DIGEST OF DR. C. PART 2 T. HEWSON'S R.Ae.S. LECTURE ' ~ BY JAMES HAY STEVENS, A.F.R.Ae.S. THE speaker next dealt with the mechanics of engines andairframe, elaborating the guiding design principle whichapplies throughout die project—"optimization" as opposed to compromise. It is this which is the really radical aspect of theGriffith proposals; instead of compromising to make the features of the aircraft suitable for a range of conditions, each is designedto approach the ideal for its particular task. The RB.108 lift engines were designed for the lowest possiblespecific weight by making them to fit the lowest point of the square/cube law curve as put into practice by Rolls-Royce experi-ence. This occurs at an engine diameter of about one foot, which achieves the minimum amount of material for the air mass-flowand pressures involved—anything smaller results in parts that are oversize to stress considerations. A specific weight of 0.1 lb/lbthrust appears feasible. Against this has to be set the colossal jet-lift fuel consumption, of approximately two per cent of theaircraft gross weight per minute, or about four per cent of the take-off gross weight for the three minutes of jet-supported flight;considerably less fuel is required for the descent, where the aircraft is lighter and the thrust rather less than the landing weight. Stability and control while hovering—the corollary to the liftengine—was studied in parallel by Rolls-Royce, and was proved with the "Flying Bedstead" during 1953-54. In Dr. Hewson'swords: "The stability of the 'Bedstead' in roll and pitch is main- tained by an automatic control. Gyroscopes detect angular move-ment in terms of attitude and velocity. These signals are fed into an auto-stabilizer, which adjusts the control nozzles to give stableresponse to the pilot's requirements. The required control moment is given by an equation of the form M = as + bO+c$; whereM = control moment, s = stick movement, 6=angular displacement, e= angular velocity, a, b and c are constants. For a given moment THIS is the conclusion of the article—Part 1 of which appeared lastweek—in which our contributor discusses points made by Dr. C. T. Hewson of Rolls-Royce, Ltd., in his Section Lecture to the Royal Aero-nautical Society on April IS. The subject was Dr. A. A. Griffith's "supersonic VTOL dart" scheme. of inertia and maximum control moment, b and c can be chosento give satisfactory stability." Height can be similarly treated, with the thrust/weight relation-ship as the control force and the aircraft mass in place of the moment of inertia. Dr. Hewson said that although widi practicea pilot can keep the height of the "Bedstead" almost constant and can control the rate of descent, it should be realized that the degreeof skill required approximates to driving two cars at a constant distance apart at 60 m.p.h. with only the accelerator pedal control.In the belief that height control must be an automatic function Rolls-Royce is developing a hovering device. Dr. Hewson was unable to discuss transition other than tomention the Short SC-1; but it is evident that the "flat riser" has its own peculiar problems. The writer feels diat the momentumdrag of die inflowing jet-lift air, which as a linear flow must also interfere widi the development of circulation lift in the Griffithconfiguration, is a critical factor of which the solution is as yet by no means certain. In the SC-1 with fuselage engines, one imaginesinterference with the development of circulation would be small. It would be impossible to accelerate from the jet-lift stage on thepropulsion engines alone, hence the tilting lift engines and, one feels, something more in the nature of entertainment, such as adevelopment of die jet flap, may become necessary to achieve circulation. Examining the supersonic VTOL aircraft in comparison witha conventional one of similar performance, Dr. Hewson made the 10 LIFT DRAG S o // CONVENTIONAL // AIRCRAFT / / V NARROW \ DELTA On the left are curves of L/D plotted against angle of attack (de- scribed as incidence) at high Ma ch numbers. INCI DENCC The curves on the right relate LID to fuel requirement and range. These curves are discussed in col. 1 overleaf. w o« O7 O6 O-S O-3 O2 ^^ «oo 0 ^"^^"N^y MILES <^ 3OOO ^^w/ Hues ^^—-^ / MIL ^^ W" »I»C«»FT ALL-UP WEISHT ES . -^ ~* * . _ *-. 1 LIFT / DRAG RATIO
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