FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1952
1952 - 0175.PDF
FLIGHT, 18 January 1952 n FLUTTER . . . before reaching the limit set for that flight. To return to the former example, the limiting speeds for each flight might be successively 300, 400, 430, 460, 480, 500 kt, and the increments might be overlapped to repeat the last two records in the preceding flight. Where continuous excitation was being used, the frequency range should be covered as in ground resonance tests at each increment of air speed. It had been found adequate to record at 1 cyc/sec increments but, in addition, if visual monitoring was possible the frequency should be adjusted so that records could be taken on the resonance peaks. If a rapid cut-off of the excitation was possible, a decay curve could be recorded from each resonance peak, so that the damping obtained from the resonance curve could then be checked by the damping obtained from the decay curve. An alternative procedure was to accelerate slowly through the frequency range and to take a continuous recording. From limited experience it appeared that the slight saving of time in flight was more than offset by the corrections necessary to the results before the amplitude-frequency curve could be drawn, and the damping values obtained. When the control-jerking method was being used, it was suggested that two or three jerks of each control should be made at each increment of air speed if the length of recording film permitted. There was then a better chance of exciting the potential flutter modes and obtaining satisfactory records. It was essential that the friction in the control circuits should be reduced to a minimum to prevent variations in the response of the aircraft from flight to flight, which would give rise to erratic results or even to a dangerous situation. In this connection the effect of temperature on control-circuit friction should be considered when, for example, the flutter clearance tests were combined with a flight to high altitude. The backlash in the controls should be representative of the amount that might be anticipated in subse quent production aircraft. Means of stopping the applied excitation rapidly in the event of trouble in flight were essential. When inertia exciters driven by electric motors were being used, a rapid stop could be achieved by friction brakes or regenerative braking of the motor, or both. Care should be taken to ensure that impulsive loads, which might arise during rapid braking, were not excessive. The exciting equipment should produce forces sufficiently large to overcome any friction in the control-surface hinges, and to produce amplitudes distinguishable from the normal level of vibration in flight, but at the same time the possibility of damage to the aircraft structure due to fatigue or repeated loadings obviously imposed a maximum permissible amplitude. It was considered that amplitudes at resonance should not in general give rise to alternating stresses in the aircraft structure higher than those arising from the application of ± 10 per cent of proof load, and, if particularly high mean structural loads occurred in any of the flight conditions, it would be advisable to reduce this limit. Some approximate estimate of the maximum amplitudes per mitted by this stress limitation could be obtained from theoretical calculations based on the modes obtained in ground resonance tests, but the work involved would generally be prohibitive. The approach used in recent tests was to assume that the funda mental bending mode of the part of the structure of primary concern (e.g. wings, tailplane) approximated to the deflection form adopted during normal static strength tests, and to use the tip deflection at 10 per cent proof loading as a guide to the limiting amplitude in the fundamental bending mode. The excitation applied at higher frequencies was reduced in the degree that experience had shown to be advisable in ground resonance tests. It was fully appreciated that the approach left much to be desired, and such amplitude would not, in fact, be excited unless it were necessary in order to obtain intelligible records. Rigorous inter- flight inspections were made to detect any signs of structural damage. No clear-cut guidance could be given as to what force the pilot should apply when jerking the controls. As with continuous forcing, it should produce the minimum structural amplitude consistent with adequate results, and the same general precautions applied. One or two preliminary trials by the pilot, starting with small forces at the low end of the air-speed range, were usually adequate to establish the order of force required. The problem did not appear to be very real in practice, and the authors knew of no case where structural damage had occurred during such tests. 550 KT 360KT » T70KT Hypothetical damping'air speed curve, showing approach to critical condition. If the damping in a particular mode was seen to be falling seriously during the flight tests, the question arose as to whether it indicated an approach to a critical condition, or whether, in fact, the damping would subsequently rise at higher air speeds. This point was illustrated in Fig. 9. Usually, the slope of the damping/ air speed curve might give a general indication of the probability of a critical condition arising, but too much reliance could not be placed on the slope, since it was known that it might change rapidly. The pre-flight calculations which gave solutions for the critical condition only were of little help in deciding whether to proceed to higher air speeds, because there was no means of checking their validity from the experimental evidence obtained up to this stage in the tests. The particular value of making pre-flight calculations, which were not solved for the critical condition, but for rates of decay in the sub-critical region, became apparent in such a situation. The damping/air speed curves obtained experimentally could be compared with the theoretical curves, and if reasonable agreement was evident, reliance could be placed on the behaviour of the theoretical curves at higher increments of air speed. Although the primary aim of the flutter clearance tests was to establish safely whether the aircraft was free from flutter or not, it was obviously desirable from the long-term point of view that experimental results should provide evidence of the validity of the flutter theory. From the foregoing, it would be evident that calculations solved for critical conditions, although invaluable in the design and development stage of an aircraft, were of little help either in the conduct of the flight tests or in subsequent correlation of theoretical and test results. Unfortunately, the amount of work required to calculate the damping in the appropriate modes over the air-speed range would be prohibitive from the viewpoint of the aircraft designers. However regrettable, such calculations could be justified at present only in relation to long-term research programmes, although the advent of flutter simulators might make damping prediction a reasonable proposition for prototype aircraft in the near future. FORTHCOMING R.Ae.S. LECTURES THE next "main" meeting in the Royal Aeronautical Society's lecture season takes place on Thursday, February 7th, at the Institution of Civil Engineers, when there will be a discussion on The Availability and Use of Aeronautical Information. This exchange of views will be made jointly with the Aeronautical Information Group of ASLIB (Association of Special Libraries and Information Bureaux). Periodically the Society's main lectures are delivered at pro vincial centres, and the succeeding fixture, on February 21st, will be of this kind; at Hull (Electricity Showrooms), Messrs. K. G. Hancock, B.Sc., A.F.R.Ae.S., and P. Person, M.A., A.F.R.Ae.S., will talk on Power Steering for Aircraft. The next R.Ae.S. Graduates and Students Lecture (held in the Society's library at 4 Hamilton Place, London W.i) will be on January 22nd, when Mr. P. H. Southwell, Grad. R.Ae.S., will speak on Aircraft in Agriculture. On January 31st, also in the library, there will be a Section lecture, devoted to The Problem of Short-haul Air Transport; the lecturer will be Mr. P. W. Brooks, B.Sc, A.C.G.I., A.F.R.Ae.S. As a general rule, and unless any special previous announce ment is made, non-members are permitted to attend R.Ae.S. lectures on tickets obtainable through members. MONITORS—NEW STYLE T WO big U.S. Navy cruisers, the 13,600-ton Canberra and Boston, are to be converted into guided-missile ships; they will be the first such vessels to be commissioned. "Mothballed" at Bremerton, Washington, since 1946, they were to be moved last week to Atlantic ports, where their main batteries and fire-control equipment will be replaced by launching devices and radar. The U.S. Navy Department expects this work to take two years and announce that, if it proves successful, other warships may be similarly converted.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
