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Aviation History
1957
1957 - 1520.PDF
610 A stroboscope being used to study the propagation of fatigue cracks in tail-fin ribs while the structure is subjected to vibration in simulation of the sideways gust loads met with in flight. (Right) For calibrating vibration pick-ups, the Strobotlood on the left is used to "slow down" the motion of the image of the arcuate scale, as reflected into the telescope by a small mirror on the vibrator. distribution can be achieved, so that the jig can readily be adaptedfor tests on different components. The static weight of the linkage is taken up by the bungee cords at the top, so that the fin issubjected only to the lateral alternating stresses. The object of the test in this case is to study the effect of alter-nating stresses on the internal ribs, and in particular to observe the propagation of fatigue cracks at points of stress concentrationround the flanges of the lightening holes stamped in the ribs. During the test the stroboscope is directed at the rib underexamination as in the illustration, and the speed control adjusted until the vibration is "slowed down" so that the rib can be seento move slowly throughout its comparatively large amplitude of movement. Calibrating Pick-ups. Apart from the many detailed investiga-tions of components which must precede the construction of any aircraft, the full-scale vibration testing of complete airframes andmajor units like wings and fuselage, statically and in flight, is nowadays a most important part of any development programme.For this purpose large numbers of vibration pick-ups of various types are attached to parts of the structure to give an indicationof the distribution and magnitude of alternating stresses induced under operating conditions. To ensure that these gauges give anaccurate quantitative indication of vibration amplitude they must be calibrated against a standard. The most convenient way to dothis is to subject the pick-up to vibration from an accurately known source and to calibrate the output of the pick-up against the knownamplitude of its movement. At Chadderton, an ingenious method has been devised in which a strcboscope is used to overcome thedifficulty of measuring the amplitude of the calibrating vibration dynamically to a high degree of accuracy. The stroboscope enablesthis to be done without imposing any additional load on the vibration source, which might affect its behaviour and give riseto misleading results. The calibration set-up is seen in the fourth illustration. Thepick-up itself, which is apparent towards the right of the picture, consists of a rigid U-shaped member embracing a flexible beambetween its arms. In normal use, the entire unit is clamped to a rigid structure placed near the aircraft and the beam is deflectedby the vibration it is desired to measure, the amount of movement being recorded by means of a strain-gauge, whose output is fed toan amplifier and recorded on an oscilloscope. For calibration, the pick-up is mounted on an electromagnetic vibrator, the flexiblebeam being connected by an extension rod to the vibrator spindle. The strain-gauge output is fed to the oscilloscope at the rear,which operates with a zero time-base so that the amplitude of vibration is indicated simply by the height of a vertical trace. A large arcuate scale is mounted at the left-hand end of thebench in such a way that the image of a portion of it is reflected by a small mirror, mounted on a lever attached to the vibratorspindle, into a telescope sighted on the mirror. The armature of the vibrator is first moved manually through increments ofO.OOlin, checked by micrometer. The resulting deflection of the mirror produces a greatly magnified movement of the scale asseen through the telescope. The scale can therefore be calibrated, statically and with great accuracy, in terms of actual amplitude ofmovement at the pick-up. The vibrator is then excited at mains frequency and the input adjusted by means of a Variac control,until the total excursion, as indicated by the movement of the scale image, is 0.020in. It is, of course, impossible to check thiswith the unaided eye, because the magnified reflection of the scale is oscillating at 50 c.p.s. and would appear to the operator as ablur. The difficulty is overcome by illuminating the scale with a stroboscope, and adjusting the flash rate until the image appearsto be moving very slowly, so that the limits of its apparent move- ment can be clearly defined. When the excursion is shown by this means to be 0.020in, the oscilloscope is adjusted to give a verticaldeflection of 10 cm on the cathode-ray tube, which is then in effect calibrated against the actual amplitude at the pick-up.A number of similar pick-ups, which incorporate a means of adjusting their strain-gauge output, can then be calibrated to thesame standard, simply by adjusting their output to give the same C.R.T. deflection for the same vibration amplitude. This pro-cedure has been found to give very accurate results, its particular attraction being that the use of the stroboscope permits a dynamicphenomenon to be compared precisely with a static measurement. It will be noticed from the picture that the Stroboflash itself isnot directed at the arcuate scale, which is instead illuminated by a larger instrument, because the light output of the former,although adequate for most purposes, is not sufficient to show up clearly against the bright background lighting. The actual lightsource is a Type 1201 Stroboflood, incorporating a special three- electrode xenon-filled discharge tube which, operated through acondenser, produces a high-intensity white light the duration of which varies between 25 and 40 microseconds according to theflash rate. This unit has no speed control, and must be triggered by a contactor, or by the speed control of the Stroboflash, asin this case. The two instruments described form only part of a range of stroboscopes with wide application to vibration research. Resonance Search Tests. Perhaps the most interesting instru-ment in the range, from the point of view of vibration investiga- tion, is the Type 1206 Slow Motion Stroboscope, which has beenspecially designed for resonance search testing. It consists of an oscillator which drives, through an amplifier, a vibration generatoras well as a stroboscope. The stroboscope, however, does not flash at exactly the same rate as the vibrator but is triggered througha phase shifter at a constant difference frequency, which can be selected at \, 1, ly or 2 c/s., irrespective of the frequency of thevibrator. In practice, the subject of a resonance search is mounted on the vibrator, while the frequency control is swept through therequired range. Illuminated by the stroboscope, the component under study appears to vibrate at the preselected slow motionthroughout the test, irrespective of the true frequency of excita- tion. Should any resonance occur, the part concerned will beseen to vibrate with greatly increased amplitude but still at the same slow rate. The frequency of the resonance is indicatedaccurately on the oscillator control dial and, if it falls within the range of vibrations to which the component may be subjected inservice, the need for modification is indicated. The wide fre- quency-range of this instrument—from 10 c.p.s. to 500 c.p.s.—provides coverage of virtually all vibration conditions likely to be met with in the aircraft field. ICE AIRFIELDS TPHE United States Air Force has succeeded in landing wheeled-•• aircraft weighing more than 60 tons on Arctic sea ice little more than 4ft thick, according to a recent announcement by the U.S.A.F.Air Research and Development Command. Trials made near Thule A.F.B., in Greenland, showed thatheavy transport aircraft could be put down on sheet ice without melting or breaking it. Neither the weight of the aircraftnor jet heat caused any serious deterioration of the ice. Using pickaxes for tools and inner tubes for runway markers,four men hacked out an 11,000-ft runway for the tests in two days. The purpose of the programme is to compile accurate tables ofthe bearing strengths of both fresh and sea-water ice, and to develop an airborne indicator of ice thickness and strength usingseismic, impact or electro-magnetic principles.
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