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Aviation History
1954
1954 - 0035.PDF
1 January 1954 21 it allowed higher wing loadings for a given take-off or landing performance. There were many ways of comparing aircraft in the theoretical design stage, but, Mr. Johnson said, two factors would always have to be taken into account. These were "real estate" or runway length available, and the "mission"; two aircraft designed to one specification would have to be capable of carrying the same equipment or payload. It was in relation to runway lengths that the low maximum lift of the delta became a serious disadvantage. Drag Factors in Configurations.—Mr. Johnson now dealt with the drag factors involved in various configurations. He said that doubling the Mach number increased the drag of the aircraft to 520 per cent of the value at 0.8 Mach. Wave drag became extremely important; and trim drag, due to the pitching moments of the wing and tail, became noticeable. Furthermore, extremely thin wings assumed major importance. This could be shown by a diagram [Fig. 2] and by the fact that at Mach 1.5 a wing 3 per cent thick had only 63 per cent of the drag of a 5 per cent wing of the same planform. For such thin wings there was the problem of leading-edge flow separation (when no leading-edge droop was employed); it was at first thought, during wind tunnel tests, that this separation would disappear with increased Reynolds number, but it had not done so. The low-aspect-ratio wing caused high drag over an important range of speeds, particularly on approach and take-off; there was nothing good, aerodynamically, about low aspect-ratio, but it was required in high-speed aircraft in order to obtain the low thickness ratios which would be otherwise impossible. The delta suffered considerably in maximum L/D ratio because of the upturned trailing-edge flap required for longitudinal control. A delta wing thus trimmed had a ratio only 63 per cent of its ratio without trim, comparing unfavourably with a straight wing of similar aspect and T/C ratios. Lift Considerations.—The delta achieved its maximum lift at an angle of attack which was unusable with practical ground angles and landing-gear lengths. It was possible to improve this situa tion, however, by fitting a conventional horizontal tail surface and employing normal high-lift flaps. It was far easier, however, to obtain maximum lift from a straight wing, which was far more amenable to the use of high-lift devices and the shorter aero dynamic chord of which reduced the tail load required for trim ming. If the maximum usable lift of the straight wing was taken as 100 per cent, the tailed delta developed 70 per cent, and the pure delta only 48 per cent. When these values were considered in the design of several aircraft for comparable take-off and landing performances, it was obvious which type had the advantage. Even on longer runways the straight wing proved superior to the delta. COMPARISON OF SURFACE AREAS Aircraft F-80A F-86 XF-92 F-94C F7U-3 Avro 698 Avro707A Gloster Javelin Boulton Paul PS-120 ,Boulton Paul P-111 Over all length (ft) 34.5 37.54 42.5 40.08 43.17 109 34.17 57.0 29.63 26.1 Wing sq ft 237.6 287.9 425 232.8 535.3 4230 426.5 905 291.0 area °/ of wing ar^a 100 100 100 100 100 100 100 100 100 Horizontal tail area sq ft 43.5 33.9 — 60.3 — 79 — % of wing area 18.3 11.8 25.9 8.7 — Vertical tail area sq ft 22.5 33.4 76.1 28.0 147.4 294 29.9 61 33.5 % of wing area 9.5 11.6 17.9 12.02 27.5 6.9 7.0 6.7 11.5 Total area of wing and empennage sq ft 303.6 355.3 501.1 321.1 682.7 4524.0 456.4 1045.0 324.5 %of wing area 127.8 123.4 117.9 138.0 127.5 107.0 107.0 115.5 111.5 Stability and Control.—On the subject of stability and control, Mr. Johnson said that stability problems would always arise on an aircraft with low aspect-ratio, sharply swept wings, and that yaw and roll dampers would probably become standard equip ment. Some aircraft would suffer from rudder reversal or Dutch roll at high angles of attack and low speeds. Trim changes at transonic speeds would, however, automatically disappear with improved aerodynamic forms and thinner wings. Any aircraft, he said, which could exceed Mach 1 in level flight would also have greatly improved flight characteristics. With thin wings, however, some degree of transonic buffeting might have to be accepted. Increasing size of fuselage, both in length and cross-sectional area, would, despite better fineness ratio, have an increasing effect on airflow over the wing and tail at high angles of attack. Claims had been made that centre-of-pressure travel was less on a delta than on a straight wing: this was true in percentage of chord travel, but not .in actual linear distance which was the same for both wings. The low e.g. travel limits of the delta made c.p. travel far more critical for this plan form. Altogether, claimed Mr. Johnson, the Lockheed Corporation had found that the straight wing, in conjunction with a properly designed tail, had stability and control characteristics superior to those of any other form of wing. Whereas most present-day high subsonic bombers and transports used swept, relatively thick wings—for wing fuel carriage and optimum performance up to Mach 1—supersonic aircraft of this size would need a different wing design. For a given mission and with a given power plant, the subsonic delta wing with all its problems appeared inferior to the swept wing in that speed range. The lecturer concluded by saying that with the solution of the aero-elastic problems associated with the unswept, very thin wing, this planform appeared to have substantial advantages when evaluated on a strictly comparative basis and with a view to high supersonic speeds. SAAB 91 C SAFIR (continued from page 3) noticeable trim-change. The flaps came up in like manner. One then immediately had to coarsen pitch to prevent overspeedmg. Since I could see the ground only directly below, I was com mitted to a timed circuit and therefore started a Rate 1 turn to port and levelled off at a few hundred feet. Once settled as nearly as possible on a reciprocal heading according to the com pass, I had my first and only opportunity of sampling the feel of the aircraft straight and level. This was pleasantly light and positive and suggested immediately the direct, sensitive feel of a thoroughbred. The speed built up quickly and smoothly, still without any apparent trim change. It was rather noisy in the cabin, but this Saab are to rectify with soundproofing when the new furnishings are installed. Out of sight of the airfield, and indeed of any recognizable ground feature, I began another turn to port after a due interval. The magnetic compass no longer indicated to within less than 30 or 40 deg and we called the tower to say we hoped we were approaching land. I spotted first the "pundit" and aimed slightly to starboard of it. Then, at something less than 900 yd, the runway lights came into view to starboard and, after some very fast work with power and flaps by Bertil Andersson, we turned sharply on to the runway. Once lined up, the landing was simple, positive and quite uncomplicated. After touch-down the nosewheel came onto the runway of its own accord, and after a short distance we turned off at the first intersection—not without a considerable sensation of relief. It was out of the question to tempt the circuit further in such visibility and in the now-increasing gloom. In such circumstances it was clearly impossible to carry out a full air test, but even in so short and restricted an experience the Safir proved itself most pleasant to handle and inspired con siderable confidence. I should like very much to have tried some aerobatics, since the aircraft is at present cleared for 4g in England and was built for 6g. It is rarely that one finds a four-seater offering such equipment and comfort, with 190 h.p. and a tricycle undercarriage, and yet so compact, so tough and fully aerobatic. The 91C will cruise comfortably at about 150 m.p.h. and, if fitted with full blind-flying panel and A.D.F., should prove a first-rate executive aircraft. Almost every feature of the design shows the care and ingenuity which have been exercised in meeting every likely demand of a private owner. Further refinements which Saab intend to incorporate include manual control for engine cooling-air outlets (these are at present pre-set on the ground) and a number of optional instruments. Except for the instrument layout—which, it was pointed out, is subject to customer's requirements—I had difficulty in finding anything which appeared to require correction or alteration. The 91C seems admirably suited for any task which it is capable of performing—for instance, club, executive, ambulance and light freight flying. I was, of course, unable to try the aircraft at full load with four occupants, but the 190 h.p. engine now fitted should overcome the slight sluggishness that was apparent in the Gipsy-engined 91A under these conditions. Luggage space would, however, prove rather limited with four up; the one small compartment behind the rear passenger seats would, we fancy, hold little more than overnight effects for four people. But the 91C Safir is a most pleasant aeroplane. With its good performance and equipment it must equal the American standards for such aircraft and even surpass them on many counts. As a personal machine for combined business and pleasure it can surely have no equal in its class in Europe. C.M.L.
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