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Aviation History
1947
1947 - 0255.PDF
FEBRUARY 13TH, 1947 FLIGHT 171 Anticipation of ©verspeeding New de Havilland Presetting Airscrew Qovernor AN important design developmentto minimize the risk of momen-• tary overspeeding in engineswhich would be adversely affected by it is announced by de Havilland Pro-pellers, Ltd. Known as the pre- setting airscrew governor" this deviceanticipates the danger of overspeeding and assists the constant-speed unit toprevent it. All airscrew governors operating oncentrifugal - governor - controlled metering valve principle depend upon Fig. 1 *^Ph in r.p.m. This is achieved by theaddition of a mechanism which re- sponds to the lack of oil pressure in theconstant-speed unit following pro- longed underspeeding, but which isnormally held inactive by an auxiliary oil accumulator. This new governor switch (Fig. 4) isnot very different from other de Havilland designs, but has the fol-lowing alterations and additions: A piston against which bears a governorspring is introduced into the constant - the throttle is opened suddenly andthe r.p.m. rise rapidly, the governor assumes the overspeed condition, rnd,therefore, supplies oil to the airscrew at an r.p.m. value below that selected,due to the reduction of load on the governor spring. As the speed con-tinues to rise, the pressure builds up in the airscrew delivery line and istransmitted to the pre-setting piston in the governor rack by way of thebleed and the accumulator. The piston is thus returned to its normal the departure from the governor set-ting in the form of an increase or de- crease of r.p.m. to bring them intoeffect. In operation this often results in a quite appreciable r.p.m. under-speed or overspeed. Under normal conditions this characteristic is not ofgreat importance, but when a sudden large opening of the throttle is re-quired, such as would be used for an emergency overshoot after an ap-proach to land, the momentary ex- cessive r.p.m. may become very im-portant. Use in Overshoot Considering an overshoot from the point of view of the airscrew and constant-speed mechanism, the cir- • cumstances are that on the approach • an engine is throttled back, the air- screw blades are in the full fine-pitch position, and the high-pressure side of the pitch-change mechanism is open to drain (Fig. 1). On fully opening the throttle to give maximum power the ligh-pressure side of the mechanism remains without pressure (Fig. 2) un- til the r.p.m. rise above the maximum figure selected. The governor then, for the first time, delivers oil to oper- ate the mechanism and coarsen the blade pitch (Fig. 3). Since the pitch- increase side of the mechanism can only be primed after the r.p.m. have passed the selected value, over- speeding occurs during the brief time lag between beginning of charging the system and the completion of the pitch-increase movement. The effect of the new pre-setting governor is to prepare the constant- speed unit to react to the sudden rise speed rack, and the cavity in the rackbehind the piston is connected by way of an accumulator and bleed to thehigh-pressure delivery line between the governor metering valve and air-screw. The oil connections are all contained within the governor unit,and the new accumulator is integral with it. The operation of the pre-settinggovernor in the case, for example, of an overshoot, is as follows: (Fig. 4illustrates in diagrammatic form the condition with the pitch lever and air- CHARACTERISTIC OF NORMAL GOVERNOR position in the rack and the originalgovernor-spring load value is restored. The time taken for the piston inthe rack to re-set the governor-spring is determined by the dimensions ofthe bleed and the accumulator stroke. This time lag is sufficient to allow theoil system to become fully primed and to bring the airscrew blades into apitch sufficient to hold the engine at the selected speed by the time ther.p.m. actually reach that value. The pre-setting function is illustrated ingraph form in Fig. 5. For all normal small fluctuations ofr.p.m. the speed control mechanism behaves as if the governor spring wereseated directly on the rack, the accumulator maintaining the pre-set-ting position at the bottom of it's travel in the rack. y Fig. 5. The pre-setting function shown in graph form. screw blades in full-fine (and thethrottle closed for a landing approach). Since there is no pressure in thegovernor delivery line to the airscrew, the pre-setting piston in the rack willbe displaced by the governor spring, thus reducing the load on the springand the governor r.p.m. value repre- sented by the control setting. Thepiston of the accumulator will also be forced forward by its spring. When FLYING SHOWROOM A MINIATURE "Britain Can Make,Jt" exhibition is to leave for an aerial tour of Northern Europe early next month. It will travel in a specially equippedBristol Freighter which will virtually be the world's first flying showroom and willcontain a series of "stands" 2ft 4m square, displaying light engineering andelectrical goods. Top weight allotted to each exhibitorwill be 1 cwt, but accommodation will be provided for a restricted number ofexhibitors' representatives, engineers and —most important of all—an air hostessin charge of a cocktail bar! A '' stand '' in this enterprising showwill cost £300 for the entire trip and this will include the services of a qualifiedengineer to answer technical queries en route.
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