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Aviation History
1932
1932 - 1015.PDF
FLIGHT, OCTOBER 6, 1982 STOCKTAKING /—If]—"HE Annual Report of the Aeronautical Research (^ III Committee for the year 1931-32 was issued on III September 27 (H.M. Stationery Office, price 2s. net), and if it contains little tnat is new, it does at any rate give a reasonably good picture of the present state of the art and science of aeronautics. As in previous years, the publication is divided into two sections, of which the first is a report to the Secretary of State for Air, and is signed by Sir Richard Glazebrook, Chairman of the A.R.C., while the second is a supplement and deals in rather greater detail with the various subjects. Aerodynamics Sir Richard Glazebrook calls attention to the satisfactory progress made with new equipment for aerodynamic research, and refers to the new compressed-air tunnel at the N.P.L. being in working order, while No. 1 tunnel is being replaced by a new tunnel with elliptical cross section and a top speed of 200 ft. per sec. At the R.A.E. the outstanding innovation is the installation of the 12-ft. vertical tunnel for free-flight tests on spinning models. The new 5-ft. open-jet tunnel will have its speed raised to 215 m.p.h. Of the new 24-ft. tunnel being built by Boulton & Paul it is stated that it is hoped to have it installed and in working order before the end of 1933. Spinning Sir Richard considers that satisfactory progress has been made in the investigation of spinning. He refers to the effect which deepening the body towards the rear, and raising the tailplane with respect to fin and rudder, has on the ease of recovery from a spin. On the other hand, in writing on the subject of " buffeting," Sir Richard points out that the tailplane of a monoplane should be placed " in the lowest practicable position in relation to the wings." The unfortunate aircraft designer will be hard put to it to find a working compromise between these two desiderata. The interesting information is given in the supplement to the report that a suggested way of bringing an aircraft out of a spin when the usual control move ments have failed might be to attach a parachute to the outer wing tip, the parachute being released to act as a brake on the spinning aeroplane. The scheme has been tested on a model in the R.A.E. vertical spinning tunnel, where it was found to work satisfactorily. Full-scale tests are reported to be in progress. If these are a suc cess, it is held that the dangers of carrying out spinning tests on new machines of unknown properties would be considerably reduced. Stability and Control at Stall The problem of obtaining adequate lateral control and stability- for stalled aeroplanes is considered to be solved by the use of slots and interceptors near the wing tips, but it cannot be said that the whole question of lateral control is completely understood, and it is interesting to find Sir Richard Glazebrook making the statement that " it seems possible that if the matter were thoroughly understood a sufficient degree of safety for many purposes might be achieved, merely by correctly shaping and arranging the wing and tail organs without using addi tional mechanism on the wings." He thinks, however, that it is probable that the degree of stability and con trol that can thus be secured never approaches that obtained by the use of slots and interceptors. To eluci date the subject a research has been initiated at the N.P.L. on the effect of fitting wing tips of different types on a wing whose general properties are well known. The Effect of Gusts Some time ago we announced that in the near future a vertical gust condition was likely to be imposed soon as a stipulation in strength factors for certificates of air worthiness. Sir Richard Glazebrook, in referring to the subject, states that there are two main types of vertical gusts. Those due to atmospheric conditions, such as thunderstorms, line squalls, etc., and those due to un dulations in the contours of the ground. In the former type vertical gusts with a velocity up to 30 ft. per sec. may be experienced. From the point of view of strength of aircraft, Sir Richard points out, the danger depends not so much on the absolute vertical velocity of the gust as upon the rate at which the aeroplane enters the gust. This, in turn, depends upon the horizontal speed of the aeroplane and upon the sharpness with which the boundary of the gust is defined. A few aeroplanes equipped with continuously recording accelerometers have been allocated at the R.A.E. to fly in conditions in which strong verti cal gusts are likely to occur. In the supplement to the report it is stated that flights made at the R.A.E. re sulted in maximum and minimum values of the normal accelerometer readings of 2.3 g. and 0.3 g. respectively, both occurring when flying in the neighbourhood of cumu lus clouds. On the hynothesis of a simple sharp-edged gust, the corresponding gust velocities were 17 ft./sec. upwards and 9 ft./sec. downwards. It is considered that there is little likelihood of meeting bumps of greater severity over S.E. England. Similar measurements were made in Scotland, and the maximum and minimum read ings obtained were 3.0 g. and 0 g. respectively, corre sponding for the machine in question to an up-current of approximately 30 ft./sec. and a down-current of approxi mately 15 ft. /sec. respectively. None of the flights was made in winds of more than 20 m.p.h., and the work is to be continued in stronger winds. It is also proposed to fit maximum reading or statistical accelerometers to passenger aircraft operating over established air routes, and to obtain records extending over considerable lengths of time. A statistical accelerometer has been devised by Mr. W. S. Farren, of Cambridge, and is an instrument which counts and records the number of times an accelera tion of pre-determined magnitude is exceeded. On the theoretical side the relation between vertical currents and the stresses they induce has been discussed by various authorities, and it has been found that for a moderately stable aeroplane the hypothesis of a sharp- edged gust gives a gust velocity within 10 per cent, of the truth, provided the time taken to reach maximum acceleration is less than a certain amount, depending on the mass and dimensions of the aeroplane. There is a corresponding upper limit to the distance travelled by the aeroplane in this time, which amounts to 44 ft. for the Bristol Fighter and 80 ft. for the Gloster " Gorcock." The accuracy of estimating a given gust on the sharp- edged hypothesis is independent of the velocity of the aeroplane. The acceleration due to a gust which begins by increasing slowly and then rises very rapidly may ex ceed that given by the sharp-edged hypothesis for the same maximum gust velocity. The simplest hypothesis for a linear gust, which neglects the length of the aero plane and any pitching motion, greatly under-estimates the acceleration due to a comparatively slowly-rising gust, owing to the increased incidence resulting from the differ ence in gust strength at wings and tail. Increased wing loading decreases the acceleration due to a sharp gust, but increases it in a slow gust, unless the aeroplane is unstable. " Buffeting " Sir Richard Glazebrook is somewhat reticent on the sub ject of tail " buffeting " (a slightly unfortunate word, selected by those who looked into the causes of the Meopham air accident, now used by British aero- dynamicists to denote an effect, whereas to the ordinary individual it clearly indicates a cause), and confines him self to stating that a lengthy investigation has been carried out by the German D.V.L. In the supplement to the report a rather more detailed reference is made to the final D.V.L. report on the investigation of " buffeting," and it is stated that in the main the wind tunnel results confirm the N.P.L. wind tunnel results. It is, however, admitted that the German report states that although it is conceivable that the aeroplane which crashed at Meopham was a victim of buffeting, there appears to be a possibility that the accident was primarily connected with the wings, or that wing fracture occurred at the same time as the collapse of the tail, since at a speed of about 215 km./h. the wing is stressed up to the limit if it attains the stall ing angle of incidence. The remedy for " buffeting " suggested in the supple ment is to place the tailplane as low as possible, to stiffen the fuselage and tail, and to avoid anything which may cause premature breakdown of the flow over the central part of the wing. It is pointed out that the features which cause premature breakdown are precisely those that promote high interference drag. 943
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