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Aviation History
1931
1931 - 1256.PDF
FLIGHT, NOVEMBER 27, 1931 In this way the pilot is immediately warned of his deviation and of the direction in which he should steer for its correction. In effect, he has only to " steer towards the shorter line," until the reeds are again balanced, in order to regain his course. The vibration of the reeds is scarcely observable on the dial of the instrument, the visual effect being that of white lines, the relative lengths of which give the required guidance. The Croydon Beacon.—Visitors to Croydon will shortly see the special aerial system of the Marconi visual-type course indicator in course of erection. It will consist of two Bellini-Tosi triangular " loops " supported by a lattice-steel tower, 100 ft. in height. The base of each loop will be approximately 300 ft. in length, and the outer extremities will be carried on steel jury masts some 20 ft. from the ground, and kept taut by a system of counter- weights. At mid-point in each bottom wire of the loops a further supporting jury mast and insulator will be erected to take part of the weight of the span and to preserve the directional characteristics of the loops under windy conditions, by the prevention of side sway. The transmitter will comprise a drive circuit and two intermediate amplifier stages, each followed by a power amplifier stage. The drive will consist of a high-frequency oscillation generating circuit employing one valve and tuned to 306 kilocycles, the oscillations of which will be applied to the grids of each intermediate amplifier. These will each em- ploy a single valve, the anode being fed by high-tension direct current upon which will be superimposed an alter- nating potential generated by separate alternators for each intermediate amplifier, at frequencies of 65 and 86.7 cycles respectively. The outputs of the amplifiers, modulated to these frequencies, will then be passed to the power amplifiers, each of which will have an input of 1 kilowatt to the anodes and will operate into one of the goniometer stators. Receiving Equipment.—Receiving equipment for the air- craft will employ a vertical rod aerial approximately 10 ft. in length of stream-lined cross section mounted on the fuselage, or in any other convenient situation. The components of the receiver will be mounted on a screened metal chassis contained in a metal case with shock-absorber suspension, the receiver circuits comprising three tuned high-frequency amplifier circuits with ganged controls, and a detector, followed by low-frequency magnification. AIRCRAFT VIBRATION From the paper under this title read by H. Constant, M.A., A.F.R.Ae.S.. before the Royal Aeronautical Society on November 19 we have space to quote only the following passages, the paper being of such a highly technical nature that it cannot, however valuable in itself, be assumed to be of very wide general interest.—ED. The Reduction of Vibration f III "'HE most effective method of reducing vibration is v3 III *ne reduction or complete removal of its source. HI In ungeared, unsupercharged power plants the engine is probably the cause of more serious vibration than is the airscrew. The introduction of gear- ing has had the effect of magnifying the airscrew's contri- bution to vibration, while supercharging of the engine has resulted in the decrease of a considerable part of the engine's vibration, probably due to improved distribution. As a result, the airscrew is now very frequently the chief source of bad vibration. The most effective method of reducing airscrew vibration is the fitting of a three- or four-blade airscrew. The re- sult of this is the elimination of the vibration due to aero- dynamic and gyroscopic couples, the chief source of vibra- tion on the turn and in other manoeuvres. If, in addition, the airscrew be dynamically balanced, the result should be an airscrew with practically no inherent tendency to cause vibration. With regard to the engine, a reduction in vibration will follow on improvements in the balance, more particularly the secondary balance. The addition of a supercharger will improve the distribution and decrease the vibration due to torque reaction. The correct tuning of the car- burettor, the supply of adequate heat to the air intake, carburettor and induction pipe, all serve to reduce the vibration caused by the harmonics of the engine torque reaction. When all possible has been done to reduce the sources of vibration, a further improvement may be obtained by the insulation both of the sources and of individual parts of the structure or fittings of the aircraft by the use of rubber or some other medium for absorbing vibration. It is also probable that the introduction of suitable vibra- tion dampers would have a beneficial effect. A study of the effect of these is, however, beyond the scope of this paper. Summary and Conclusions We will conclude this survey of aircraft vibration with a brief summary of the main points that have been made. Starting with a study of the modes of vibration of a fuse- lage, we have shown how, for flexural vibration, the fuselage may have several modes of vibration in each of its two planes of vibration. When any source of vibra- tion synchronises with the frequency of one of these modes, a resonant vibration is set up. Thus for flexural vibration in both the horizontal and the vertical planes there are several engine speeds at which resonance of some source of vibration occurs. The same state of affairs applies to the fuselage torsional vibration. Vibration of parts of the aircraft other than the fuselage has not been considered. In order to obtain some idea of the magnitude of the forces and couples that might be applied to an aircraft without infringing some arbitrary limit of comfort, some tests were carried out on the physiological effect of vibra- tion of different frequencies. As a result of these tests limiting values for the magnitudes of most sources of vibration were defined. These limits were found to depend on the frequency of the vibration. Their lowest values occurred at a frequency between 1,000 and 1,400 per minute, and limited the forcing impulses to 50 lb. and 1,200 lb. in. for a force and couple respectively, in an aircraft of 4,000- to 5,000-lb. total weight. The actual sources of vibration existing in aircraft were then considered ; the magnitude of these sources were compared with the limits already laid down. As a result of this comparison it became possible to say which sources were the principal cause of unpleasant vibration. It was found that, whereas the engine primary unbalance was insufficient to cause serious vibration, the engine secondary unbalance might do so when the specific unbalance was large. Of the harmonics of the engine torque reaction, those of half-order and first order were the only ones of importance. Normally neither of these were responsible for serious vibration, but either or both might be accen- tuated by a derangement of engine tune or by faulty distribution or ignition. The chief source of airscrew vibration was the effect of cross-wind on the blades of the airscrew. When on the turn, or when climbing with large incidence, this source of vibration might become very serious. Other sources of airscrew vibration were gyroscopic couples on the airscrew blades when turning, and airscrew static and dynamic unbalance under all conditions of flight. During the consideration of a number of specific problems it was pointed out that vibration was greatly reduced as a result of replacing a two-blade by a four- blade airscrew, and the reasons for this were discussed. The increased tendency of a geared engine to vibrate was attributed, not so much to the increase in magnitude of any of the sources of vibration, as to the lowering toward the conditions of resonance of the frequency of the forcing impulses due to the airscrew. The stresses imposed on the structure due to vibration were found to be usually small ; but in cases where the vibration greatly exceeded the limits of unpleasantness, the stresses might assume serious proportions. In conclusion, it is considered that by various means most instances of serious vibration can be cured, bu the cost of this cure, whether in airscrew or engine effi ciency, may, in some instances, be greater than is justifiec. by the increasing comfort of the aircraft. 1178
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