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Aviation History
1932
1932 - 0196.PDF
FLIGHT, FEBRUARY 26, 19S2 German and British Experts Disagree The Deutsche Versuchsansta.lt fur Luftfahrtlhas now published its report on experiments and researches carried out to check the findings of the British inquiry into the accident to the Jankers F.13 G-AAZK at Meopham, Kent, in 1930. The D.V.L. finds itself unable to agree with the British view that the accident was due to tail" buffeting." PKOBABLY most of our readers will recollect that, as a result of the inquiry into the air crash at Meopham, Kent, on July 21, 1930, in which several people were killed, the British experts arrived at the conclusion that " buffeting " caused the tail to break, the breakage of the wings and the tearing away of the engine from the machine being subsequent events. German representatives came to England at the time of the crash and examined the wreckage. On their return to Germany they drew up a report in which the main conclusion was that the machine accidentally got into a steep dive in the clouds known to have prevailed over the district on that day, and that in pulling out too suddenly and at high speed, stresses were set up in the wings which proved too great, and the wings collapsed. In the German view, the tail breakage followed the wing breakage and did not precede it. What lent strength to the German view was that a very similar accident had previously taken place, in which the observer was saved by parachute and was able to corroborate the evidence of eye witnesses on the ground. The present German report—or such portions of it as have become known in this country through publication in the " Z.F.M."—is concerned not with assigning the primary cause of the accident, but with testing the proba bility of the British theory being correct. This work has been undertaken in an extremely thorough fashion by static tests on tails in the laboratory, by flying tests on actual aircraft of the same type, by vibration tests of tails, and by tests of models in the wind tunnel. Space does not, unfortunately, permit of referring in detail to the German researches and experiments upon the results of which the German view is based. The work contains much that is of interest to aircraft designers in all countries, even if its main result is of a somewhat negative character in that it expresses what is not likely to have happened. We must confine ourselves to giving the merest outline of the experiments made, and do so in the hope that this brief summary will at any rate serve to show the thoroughness with which the work was under taken and carried out, and that the practical ruling out of the British theory was only arrived at after the most painstaking efforts to get at the truth. The D.V.L. Experiments Static and Dynamic Tailplane Tests.—Static tests of tailplanes were carried out in the laboratory, and were made with the machines so arranged that the undercarriage wheels were rigidly held and the tail skid supported on a universal ball bearing, so that free movement in all direc tions was afforded. The tests included symmetrical and unsymmetrical loading and gave failures at bending moments of 310 mkg. (2,233 ft. lb.) and 388 mkg. (2,800 ft. lb.). The difference in the results was due to the spars of one tailplane being of slightly heavier gauge than the other. In both cases the breaking stress was 35 kg. per sq. mm. (51,333 lb. /sq. in.). Dynamic tests were also carried out in the laboratory in order to determine how the tail would behave when vibrations corresponding to the natural frequency of the tail were set up and permitted to build up. The ampli tude corresponding to breakage was reached in about 300 vibrations, or about 25 seconds. Breakage occurred when the double amplitude measured at the tip of the tailplane reached 12.5 cm. (about 4.9 in.). The bending moment causing breakage was calculated to be 255 mkg. (1,840 ft. lb.), and the maximum stress developed was about 85 per cent, of that of the static tests. The tail which had been broken in the dynamic loading tests was subjected to static tests. Although the spar was broken, and the tail only held together by the metal skin covering, the tail was so placed that the undamaged spar boom was in tension, and in this way the tail supported a load of 125 kg. (275 lb.) at each end of the tailplane. The conclusions from these tests were that if breakage is brought about by high dynamic loads (resonance), the natural period of the tail is at once lowered to the point where resonance no longer occurs. As the strength of the tail under static load is still 60 per cent, of the original, it is concluded that complete static breakage can only take place undei specially unfavourable circumstances. Fatigue tests were arranged to correspond to a bend ing moment of + 74 mkg. (534 ft. lb.), or 24 per cent, of the static bending moment leading to breakage. After approximately 700,000 vibrations the metal covering began to crack, indicating that a breakage was imminent. Actual Flying Tests.—To determine the air flow between wing and tail, the vibrations of the tailplane, and the forces in the tailplane struts, a machine was prepared specially for a series of flight tests. It carried a slow- motion cinematograph camera and an optograph, as well as woollen threads, smoke-producing apparatus, etc., to give visual indication of the air flow. We have not the space to describe in detail this interesting equipment, but must content ourselves with giving the bare results obtained. The flight tests indicated clearly that the slip stream had a great effect on the airflow, and that, with engine throttled right back, the air disturbances and the tailplane vibrations were much more pronounced than with engine only slightly throttled. In one of the flight tests pronounced " buffeting " set in during a side slip to the right. This was not expected from previous experience, and but a small portion of the latter part of the " buffet ing " was filmed. The pilot (not unnaturally) righted the machine in a hurry, and subsequent attempts to reproduce the same conditions failed. From this fact it is concluded that a very special set of conditions of speed, slipstream, gustiness, acceleration and yaw is necessary before " buffeting " can be produced. The flying experiments indicated that vortices are shed by the wing roots on to the tailplane in an unsymmetrical manner, and that the vortices increase with angle of incidence and angle of yaw. The tailplane " buffeting " was experienced in all flying conditions in which vortices from the trailing edge of the main wing were determined, and more particularly during stalled flight or flight at large angles of yaw. The tailplane vibrations were irregular both in amplitude and frequency, and a gradual building up of large amplitudes during a series of vibrations was rarely found. On the other hand, large amplitudes in the form of single vibra tions, or a short series of vibrations, were found more frequently. Asymmetry was found at the frequency of torsional vibrations and symmetry at the frequency of bending vibrations. The amplitude increased slowly with speed, but did not reach its maximum at any given speed. The worst double amplitude reached was of approximately 9 cm. (3.5 in.), corresponding to about 30 per cent, of the breaking strength determined in the static tests. Wind Tunnel Experiments.—As a result of wind tunnel tests on models, it was concluded that resonance between the frequency of the vortices shed by the wings and the natural period of the tail increased tailplane buffeting considerablv. The results of the German model tests do not agree with the English tests, and the view is expressed that this may be due to the fact that in the British tests the wings of the model had to be '* clipped " to enable the tests to be made on a large model in a small tunnel. Also in the British tests the airscrew was absent, while the German tests were made with airscrew running. It is thought that this fact throws considerable doubt on> the probability of the British theory of tail ' buffeting as the primary cause of the accident. The report concludes with the statement that while tailplane breakage due to "buffeting" is not entirely ruled out as an explanation, its occurrence is so unlikely as to be practically ruled out, and in the present state of the technique it cannot, and need not, be designed for. The theory advanced by Hermann Blenk and Heinrich Hertel shortly after the accident, that the wing broke first as a result of too rapid pull out of a dive, is regarded as more probable, and calculations have shown that at a diving speed of 215 km./h. (134 m.p.h.) the pilot can pull out rapidly enough to break the wings. 180
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