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Aviation History
1948
1948 - 0398.PDF
53* FLIGHT MARCH 25TH, 1945 HIGH-SPEED FLIGHT RESEARCH Its Value in Determining Compressibility Effects on Drag, Lift, Stability and Control W IND tunnel research can do muchtowards solving the problemsassociated with flight at high subsonic speeds, as was indicated by the paper read by Mr. W. G. A. Perring, director of the Royal Aircraft Establish- ment at Farnborough, before the Royal Aeronautical Society some weeks ago. In this he dealt with the work of the vari- able-density high-speed tunnel, a descrip- tion of which we published in our issue of March 4 th. There are, however, definite limita- tions to what the wind tunnel can do, and free flight testing, either by pilotless or by piloted aircraft, makes a valuable contribution towards the solution of problems of high-speed flight. In his written paper on the subject of flight re- search at high subsonic speeds, Mr. H. Davies, of the Royal Aircraft Establish- ment, covered in considerable detail the developments which have taken place during the past few years in the methods and technique of flight testing at high Mach numbers. In order to save time and so provide an opportunity for a good discussion, Mr. Davies read an abbrevi- ated version to the Royal Aeronautical Society on March 18th, and it is this summary which we publish below. Mr. Davies began by explaining that the investigation of compressibility effects in flight involves four factors which are not usually present in flight testing at lower speeds. They are: — (1) The variations in aerodynamic characteristics which occur at high Mach numbers are often severe and unpredict- able, and may have catastrophic effects on the behaviour of the aircraft, so that a very high standard of piloting skill and determination is called for. (2) The speeds needed are often higher than can be achieved in level flight, so that the development of a dive-testing teeKnique becomes necessary. O'O4 1- UJO u_ O i OO3 002 001 EXTENSION OF CURVE TO HIGHER MACH N* USING DIVE RESULTS DIVE 1 12,000 R.RM. " 2 14,000 " 3 14 OOO " 4 12,000 " 5 10,000 '• 6 10,000 MAX. MACH NO REACHED IN LEVEL FLIGHT = O-8tl Fig. 2. Meteor IV ££ 454. Comparison of aircraft drag measured in level flight and in dives. The full curve up to 0811 has been obtained from level flight results at 25,000ft without reference to the dive points. All points obtained from dives at vary- ing engine speeds, the peak Mach number being reached at approx. 25,000ft in each case, at a lift coefficient ranging from 0.06 to 009. OCXO-5 i -I- * */* 'a * / a * - O6 MACH O7NUMRFR Fig. I. Meteor I (short nacelles). Typical set of dive results, using the energy method. Each symbol refers to a separate dive. (3) Owing to the high speeds, verysmall changes in the quantities being measured may be of vital importance, sothat an exceptional accuracy has to be achieved. (4) At high Mach numbers the aero-dynamic coefficients and derivatives, being measured are functions of two inde-pendent variables, A.S.I, (or C L) and M,which greatly complicates the tests needed. These difficulties are illustrated in theproblem of measuring compressibility effects on drag. Compressibility effects on drag werefirst measured in this country on a Spit- fire, about 1942. It was done by record-ing speeds and altitudes on an automatic observer at regular intervals during asteady dive from high altitude, and de- riving the drag from the con-servation of energy. This was not a very accurate method,as will be seen from Fig. 1. This is a fairly typical resultand is reasonably representa- tive of the accuracy obtain-able when a steep dive is needed. It wasn't good enough, ofcourse, and a more accurate method was soon found byobtaining a direct measure- ment of longitudinal accelera-tion in the dive. With a little care a very high standard ofaccuracy can be achieved in this way. The full line inFig. 2 is the result of measure- ments of the drag in levelflight. The most important factorsin achieving this good accuracy are probably, firstthe accuracy of piloting, and second the careful adjustmentof the damping of the accelero- meter. The ac'celerometer it-self need not be elaborate. A number of measurements of thiskind have been made and some of them are illustrated in Fig. 3. Notice that theMeteor drag starts rising later than that of the Spitfire, but eventually rises morerapidly. Notice also the large effect of cannons and bulges of the Spitfire. These measurements of overall draghave been very useful, but they do not 300? LATEST FLIGHTRESUUTS OF METEOR IV 0-9 O-8 Fig. 3. Percentage drag rise against Mich number from fight tests at R.A.E. help us to understand the nature of thedrag rise. To do this it is better to measure the local drag coefficient on awing section. Two methods have been used formeasuring local drag coefficients: — (1) The well-known one used exten-sively in low-speed research, of measur- ing the distribution of total head in thewake, and deriving the profile drag of the section. (2) By measuring accurately the pres-sure distribution round the section and deriving the form drag by integration. The first method has two disadvan-
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