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Aviation History
1928
1928 - 0954.PDF
16 OCTOBER 11, 1928 the pitch of the propeller at normal, suppose the forward speed of the machine to increase. This will involve an increase of R.P.M. of the engine and governor, which then causes oil to flow into pipe 1 and thence to the front cylinder resulting in an increase of pitch accompanied by an increase of propeller torque, and this process will continue until the R.P.M. is brought down to normal. Similarly, a reduction' of forward speed causes a reduction of pitch such that normal R.P.M. is resumed. Thus, under all conditions of flight, there is only one possible R.P.M. for a given setting of the control wheel. The maximum travel of the cylinder in either direction from the normal position is limited by suitable stops, and in the event of either of these positions being reached, very high pressures would occur in the oil system without some provision for releasing the oil. This possibility is avoided by incorporating relief ports Px and P,, in the piston, which communicate with the third pipe. When the cylinder approaches its extreme position, one of these ports is opened, and the oil is permitted to circulate via pipe No. 3 through the non-return valve and back to the pump. The possibility of a failure of the hydraulic system is remote, but should this take place, a powerful spring S, contained in the sleeve G, will return the propeller to normal pitch, irre- spective of the position of the blades at the time of failure. It will be noted that there are no glands or packings any- where in the system, which is kept full of oil by arranging a connection between pipe 3 and an oil tank (usually the engine supply tank), any leakage which may occur being made up by a compensating amount passing one of the two non- return valves, i.e., the valve which at the moment happens to be on the suction side of the pump, and any oil escaping from the system can be led back to the engine crankcase, thus ensuring that there is absolutely no waste or loss of oil due to this hydraulic circuit. The use of oil for the hydraulic medium has one obvious advantage, in that all the working surfaces are perfectly lubricated and wear is practically eliminated. The Flight Tests The Gloster Hele-Shaw Beacham propeller has been tested in actual flight, fitted to a Bristol " Jupiter VI " engine in a Gloster " Grebe " machine. Owing to the fact that, with a variable pitch propeller, there are more variables, it is rot yet possible to publish performance figures, but certain results and observations have already emerged. It had been foreseen by the designers that with a governor control, any sudden change in setting would result in the speed of revolution at first " overshooting the mark," then swing a little to the other side, and so on, until the mechanism finally settled down to the predetermined speed. An esti- mate was made of the amount by which the revolutions would ' overshoot " the desired, for instance, in suddenly opening the throttle. Actual flying tests have demonstrated that this does occur, but that even with the most sudden change in control setting which it is possible to effect, the speed settles down to the desired in something like two seconds. For a reasonably gradual advance of the lever, such as a pilot would normally use, the response of the governor is practically instantaneous. Among the things observed during test flights were the following : With the pitch set to give the maximum per- missible engine r.p.m. (1,850) dimbs were carried out, at round about the 19,000 ft. (5,800 m.) level, followed by a dive to the terminal velocity. In the case of level flight and climbs, the r.p.m. remained absolutely constant, and during the dive they advanced approximately 20 r.p.m. and then returned to within 10 r.p.m. of the previous reading. This dive, it might be mentioned, was continued for some 6,000 ft., and with the throttle in the fully-open position. During a loop with constant throttle, the engine speed did not vary by more than about 20 r.p.m., and at the end of the loop the r.p.m. were the same as at the beginning. An increased rate of climb was observed, but owing to the fact that with a variable pitch propeller, it is possible, within limits, to fly, for a given airspeed and with fixed throttle, at any r.p.m., it is likely that even better results will be obtained once the best combination of r.p.m. and airspeed has been found. At any rate, the Gloster Hele-Shaw Beacham variable pitch propeller has now been proved a most promising mechanism, and doubtless much will be heard of it in the future. HANDLEY PAGE, LTD. THIS pioneer British aircraft constructing firm is not exhibiting a complete machine, but their exhibit, while THE SLOTTED WING AT BERLIN : The action of the Handley-Page slotted wing device is demonstrated veryeffectively by the model shown above. In front of the model, which is fitted with a miniature slotted wing, is an electric fan, producing an air current past the model. The latter can be inclined through varying angles ofincidence until the stalling angle is reached, when the slots automatically open. 884
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