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Aviation History
1948
1948 - 0241.PDF
FEBRUARY IQTH, 1948 FLIGHT 209 AIRSCREW VIBRATION TESTING D.H. Vibration Department Tackles Problems Arising from the Airscrews of the Bristol New Type 170 22.OOO 20,000 1&000 16,000 f KOOO 12.OOO 10,000 aooo 6,000 4.000. 2POO Engine I airscrew vibration diagram. THE preceding pages contain a description of theBristol New Type 170, an._important feature ofwhich is the fitting of the new de Havilland 14ft airscrews with blades of N.A.C.A. Series 16 section. In view of the considerable technical changes involved, re-investigation of the stressing and vibration characteris- tics of the installation was necessary the work was undertaken by the rrbration Department of de Havil- land Propellers, Ltd. It is pointed out by the Vibration Department that the principal stresses in an airscrew blade are due to centri- fugal force plus a small bending due to thrust and vibratory stress. Though the first two are easily calculable, the last can be more easily and accurately measured by means of strain gauges; for this reason, the strain-gauging technique of the airscrew manufactur- ing companies is generally well in ad- vance of those of other organizations. Many years ago they developed in- struments for measuring vibratory strains in their blades with perfect re- liability and high accuracy. An airscrew blade is very stiff trans- versely but relatively flexible in a fore- and-aft direction; thus it has well- defined resonances, the frequencies of which can easily be found in the laboratory with a violin bow or an electro-dynamic exciter. These fre- quencies are somewhat modified by the centrifugal force produced by the rotation of the airscrew and a correction for this effect can be calculated. Usually, a chart Us plotted. The accompanying chart represents a hypothetical case of an engine which produces strong vibration forces of 1st engine, z\ engine, 3^ engine and 7th engine order. These are represented by the sloping lines drawn between enginef eed and vibration frequency. The resonant frequencies i represented by horizontal lines at 3,700 c.p.m. and e harmonics thereof. The points of interference are shown circled. At these engine speeds, there will be an excitation at a particular resonance by an engine vibration order, and, though the approximate effect can be predicted, the exact magnitude can be obtained only by actual running tests. In these tests, strain gauges and con- necting wires are cemented down the length of the blade and brought out into a harness. Couplings to the input circuits and amplifiers are made via rotating slip-rings. As the strain under the gauge varies in sympathy with the strain on the blade surface, weak elec- trical signals are generated, which are amplified until they are strong enough to deflect the very small mirrors cemented to the coils of the recording galvanometer. From these, a point source of light is reflected on to a steadily moving strip of photographic film and a wave-trace is formed and re- jne application corded proportionate in amplitude to of dampers to the stress under measurement. airscrew blades. The de Havilland equipment records from eleven to fifteen gauges simultaneously on a film, 6in wide, also carrying marks indicating separately each revolution of the airscrew and of the engine, and each elapsed time of i/5oth of a second. This enables the relation- ship between the stresses in the differ- ent blades, and the distribution of the stresses along each blade, to be re- corded. In the 14ft airscrew used in the first tests on the Bristol New Type 170 it was found that high vibration stresses were being induced at certain engine speeds, and the present airscrew was designed to overcome this. Dampers were introduced into the blade roots, with tracks designed to suppress the particular troublesome frequency. Each damper consists of a metal ball which is free to roll to and fro on a curved track, as shown in the accom- panying sketch. The radius of curva- ture of the track is made to give a natural period of oscillation equal to that to be suppressed. At this fre- quency, the ball will roll on its track and produce a force opposing the vibration of the blade. By this means, the high stresses at a particular fre- quency can be considerably reduced. Tests now in hand are designed to demonstrate the efficacy of this vibra- tion-stress suppression. The designation of the new 14-ft 5A 4A 3A 2A 1A j 7 -—- 7 / //• •. y y A 1 A i .— 7 f *-- 1.000 2POO ENGINE RRM. 30OO interference laminar-flow, four-blade de Havilland airscrews as fitted on the Bristol New Type 170 has just been announced as 4/4000/6. Their efficiency, compared with the earlier 13ft 3m models used on the "170" exemplifies what can be achieved by objective co-operation between the design technicians and the staff of the Vibration Department. FILMS WITH A PURPOSE FROM last Monday, Mr. Stewart Macpherson, the B.B.C.commentator, is being heard each week for six months by R.A.F. personnel throughout the country and abroad. He isaddressing them—via the sound track of a series of films—on accident prevention. A series of "flashes," to be screenedin R.A.F. cinemas during the normal programme, is intended to drive hoftie all the causes of ground and flying accidents.Bach ft]#f shows a different aspect of accident prevention, froS^a pilot's error to the incorrect way to refuel an aircraft. Thes^raining films branch of the R.A.F. produces more thana hiindrld. reels of training films a year, covers subjects ranging isQrn technical instruction on engine maintenance to films ofceurtft-martial and courts of inquiry procedure. A film now in production deals with the different trades in the R.A.F. andwili lie shown to new recruits as a method of assisting them to choose the branch of the Service most suited to theirexperience and temperament. The duties of officers towards the aflpnen under their command are taught by means of filmson welfare, and on the flying side aircrew are shown films on the principles of flight, map reading and the correct use of air-borne rescue equipment. When a branch of the R.A.F. needs a film to assist it in atraining programme, a special Air Ministry section obtains the necessary details for the outline of the film and then arrangeswith an outside film-producing organization for its production. Professional actors, producers and script writers are employed,but an Air Force officer is attached to act as technical adviser. The script writer and producer visit Air Force units to obtainlocal colour.
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