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Aviation History
1944
1944 - 0337.PDF
FEBRUARY 17TH, 1944 FLIGHT *75 Flight Testing Difficulties Introduced by Modern High-performance Aircraft Flying at Qreat Altitudes WHAT will she do? Has she any vices? " These are the two questions which the young pilot is apt to ask when a new aircraft type has been produced. Just how vast is the labour needed in order to answer those two apparently simple questions was brought out very fully in the admirable paper read by E. T. Jones, M.Eng., A.F.R.Ae.S., before the Royal Aero nautical Society last Thursday. The paper deserves to be read in full by everyone seriously interested in his subject, which was: " Flight Testing Methods." Unfortunately the complete paper would occupy some 30 pages of Flight, so we hope our readers will forgive us if we confine ourselves to a mere outline of the numerous subjects with which the lecturer dealt. Later, when the paper is published in the Society's Journal, many will be able to study it there. Mr. Jones began by pointing out the difficulties introduced by the fact that in moving "yhrough the au" an aircraft sets u§ local pressure and velocity fields around it. The pressure contours within this field become steeper in gradient as they ap proach a part of the aircraft, and they change with wing incidence, with the shape of external com ponents, and with the Mach Number, which is the ratio of aircraft speed to the speed of sound at the particular height at which the aircraft is flying. We reproduce herewith a graph which shows how the Mach Num ber has jumped between 1939 and 1944. The speed of sound, it might be pointed out, de creases with height, and the graph shows that in 1944 we have reached nearly 75 per cent. of that speed at about 35,000ft. Locally, speeds well above the speed of the complete aircraft may be reached, and some recent fighter aircraft have shown symp toms of this "compressibility Effect," and in some cases re- ifcjvery from dives has been diffi cult. The scope of flight tests is in general limited to determining the characteristics and "safeness" of the aircraft, and the data required by the operator to enable him to determine fuel and pay load for a given range, and journey time at selected speed. Flight Tests Required Flight tests required to establish performance and behaviour were classified by the lecturer under four groups: com plementary tests, performance and allied quantitative tests, tests for flight characteristics, and miscellaneous tests. In the paper each of these four groups was given a separate part. The wide scope of the paper will be realised from the subjects in each group. They are as follows: In group I, measurement of air temperature; difference between atmospheric pressure and pressure at the static head. In group II: take-off; measurement of speed; max. speed-height curves; cruising speed-height curves; minimum speed for continuous cruising; partial (or saw-tooth) climbs; ceiling climbs; range; cooling tests of engine installations; landing and approach. In group III: take-off; longitudinal, lateral and directional trim; longitudinal, lateral and directional stability; longitudinal, &teral and directional control; stability and control at the stall; spinning; steadiness in controlled flight; behaviour with one or more engines cut; manoeuvrability ; dives ; the baulked landing; determination of acceptable e.g. limits. In group IV: c a bin and gun heating; cabin contamination; noise; structure. Take-off distance and distance to clear a 50ft. screen are measured by means of the F.47 camera, which is virtually a phototheodolite. Speed, excluding the stall region and gliding flight, is measured by registering the differential pressure between the pitot and static pressure heads. The lecturer explained in detail the troubles introduced by position errors and the means adopted for assessing or overcoming them. He made the inter esting disclosure that some improvement is likely to follow the decision to replace the norrnal underwin'g pitot static heads by a leading-edge pole projecting approximately half a chord length ahead of the wing leading edge. In America, Wright Field had made a succesj of the use of calibrated aircraft formating the machine on test, but this method was un economical in aircraft, in this country. Radio ground-speed equipment might be used after the war, but at present there is too much interference. Stalling speed is determined by flying level at best climbing speed, closing the throttle slowly and at the same time gradually moving the control column back wards. A speed is reached at which the nose or one wing can no longer be kept up, and this speed is taken as the stalling speed. For great accuracy the static pressure head is suspended below the aircraft, clear of the pressure field, and a swivelling pilot head is placed away from local interference. Gliding Flight Speed in gliding flight is detei - mined, for multi-seaters, by the use of a trailing static head or trailing air log. In single-seaters the easiest method is flight in formation with a calibrated air craft. Full-speed tests at altitude present the difficulty that the full-throttle period for the engine is limited to five minutes, and it takes longer than that to reach top speed from cruising speed. The solution is for the pilot to obtain first at each height the approximate A.S.I, before the proper test is made. He will then make his approach from cruising speed to top speed by increasing speed and losing height as he gradually opens the throttle. As he approaches the pre-determined A.S.I, he will have regained level flight with the throttle fully open. The leadings of the instruments are then taken during the last two minutes of the five-minute, full-throttle period Speed-height tests for maximum cruising power are more easily made because each run can be maintained to ensure stabilised speed. Near the ceiling patience is required, par ticularly as the ratio cruising power to weight decreases. Engine powers and airframe characteristics are not entirely consistent, so that what is needed is an easily fitted toique- meter. Information is also needed concerning the effect on speed of external surface finishes, and a method of categorising the finishes. fn long-range operations a knowledge of the minimum speed for comfortable continuous cruising is of great importance. It has been found that this speed is most reliably determined during cruising speed and fuel consumption measurements. A good approximation to minimum speed for cruising is 1.35 Vj„,p, where V, mp is the speed for minimum drag power in level flight. Cruising ceiling may be assumed as the greatest height at which the speed 1.35 Vj mp can be maintained at maximum cruising power. Before measuring climb to ceiling it is necessary first to determine the A.S.I, for the maximum rate of climb by making 50 4.0 so H61CHT- -THOUSANDS OF F6ET 2P IO CORRESPONDS WITH TOP SPEED (1939) > 1 CORRESPONDS WITH TOP soeecj (1944) CORRESPONDS VvtTM • CONSTANT Ot VIN1G A S.I. OF 550MPH/ A •A. -3 -6 -7 -S -J MACH NUMBER(M) This graph, height plotted against Mady Number, indicates the jump in performance and height between 1939 and 1944.
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