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Aviation History
1958
1958 - 0547.PDF
3 563 ANGL E 'ELLE B BLAD E ft2 60°- iff- 40° • Wf. u.O # 110- 100 90- 80 7& 0 -1000 -1000- -3.000 -4.000- -5.000 -6.000 -7,000 -11,000 \ R.PM. V \J 3 10 FUEL GOVERNOR INOPERATIVE /^MECHANICAL PITCH t^ LOCK ENGAGED \/____ MECHANICAL PITCH \^ LOCK SIGNALLED \ THRUST \y BLADE ANGLE Z-0 30 4 0 50 6-0 TIME (SECONDS) 5. Behaviour of a propeller following failures in c.s.u. and drag-limiting system. Most advanced de Havilland propeller is that for the 5,3/5 e.h.p. Rolls-Royce Tynetwo-spool turboprop. This is the Tyne installation in the nose of a Lincoln test-bed. gear of the engine-reduction gearbox by a mechanical linkage to aservo valve in the propeller controller, which operates the coarse- pitching valve. It is to be noted that the independent electricallydriven feathering pump is not brought into action by this auto- matic drag-limiting system, thus rendering the system entirelyhydro-mechanical, and that the coarse-pitching valve is so posi- tioned in the hydraulic circuit that it overrides any contrarysignals that may exist in the constant-speed unit. The response in teims of propeller drag is shown in Fig. 3.With the engine power lever in positions from low cruising power to take-off, the setting of the drag-limiting system is at a slightlypositive value of torque. Thus in the event of engine failure in this range the propeller feathers. With the engine power lever atflight idle, the torque setting of the drag-limiting system is -3,900 lb-ft compared with a normal torque of —1,000 lb-ftdelivered by the engine at flight idle. In the event of engine failure with the power lever in the flight idle position the enginetorque will at first drop below the setting of the drag-limiting system, thus causing the blades to coarsen until a value of— 3,900 lb-ft is restored. The blades will then continue to move to govern the torque to this value under the varying flight con-ditions. The drag is thus automatically limited to the values shown on Fig. 3, which may be compared with the unrestrictedvalues given in Fig. 2 (b). Manual feathering can, of course, be accomplished at any convenient time to reduce the drag to aneven lower value. Hydraulic and Mechanical Pitch Locks. Although the automaticdrag-limiting system provides a valuable first line of defence, it is clearly dependent upon the continued existence of oil supply to thepropeller. It is therefore necessary to arrange for more positive protection against this failure. It is also desirable, in cases inwhich the unrestricted propeller drag or r.p.m. would be dangerously highj to provide protection against the occurrenceof a contributory failure of the automatic drag-limiting system at the same time as a primary failure such as fuel supply failure. One means by which protection is provided against loss of oilpressure is the hydraulic lock, which ensures that when pressure is lost the oil present in the fine-pitch side of the pitch-changingpiston is trapped, thus arresting any further movement in the fine- pitch direction. This lock has been in use on de Havilland pro-pellers exclusively for several years, and has proved its worth on a number of occasions. Due partly to the difficulty of checkingthe functioning of a hydraulic mechanism of this kind, however, the Tyne propeller also incorporates a mechanical pitch lock,which on engagement mech?nically precludes any further move- ment of the blades towards fine pitch. This lock consists of two rings of ratchet teeth mating in stepsof approximately 2i deg of blade angle, one attached to the rotating pitch-change cam and the other to the hub. The rotating ratchetis free to move axially on splines in the cam and is mechanically engaged by multiple springs, being held out of engagement by thenormal operating pressure in the pitch-change mechanism. The lock is brought into engagement either directly by loss of oil pres-sure or through a signal indicating the existence of an overspeed. This signal is derived from an overspeed governor mounted on thepropeller (in order that it may directly sense propeller r.p.m., and not engine r.p.m. in the event of a failure in the transmissionbetween the propeller and the engine). When the propeller r.p.m. exceed the setting of the overspeed governor, the governor valvereleases the oil pressure holding the lock out of engagement, thus allowing it to engage under the action of the peripheral springs.It is clearly essential that the overspeed governor shall transmit its signal and engage the mechanical lock before the blades havemoved to a low pitch, and it is therefore desirable that the operating setting of the governor shall be only slightly above the normalr.p.m. in use at the time. This condition is met on the Tyne propeller by providing two datum settings on the overspeedgovernor, the choice of setting being coupled with the engine power lever to ensure that the setting in use at any given timeis that which is nearest to the normal operating r.p.m. at that time. The change in datum from the low to the high setting of theoverspeed governor is effected by bringing into operation a second governor spring by means of a servo-piston connected to a separateoil feed line in the engine shaft. When the engine throttle lever is advanced above the cruise position high pressure oil is fed tothe servo-piston via this third oil line, thus bringing into operation the second governor spring and raising the governor datum.This feature also provides a method of checking the functioning of the pitch lock, for by providing a manual override it is possibleto select the lower overspeed governor setting at a time when the propeller r.p.m. exceed this setting, thus causing the lock to beengaged. Indication to the pilot of the correct functioning of the lock is then provided by the fall in r.p.m. of the locked-pitchpropeller occurring when the engine power is reduced by the use of the fuel-flow trim lever. The action of all these features issummarized in diagrammatic form in Fig. 4. Fig. 5 shows the behaviour of the propeller in terms of thrust,blade angle, and r.p.m. following the primary failure of the con- trol valve in the constant-speed unit, with a contributory failureof the automatic drag-limiting system. Detailed studies of the effect of various single and double failure cases involving theengine and propeller have shown that a failure of the constant- speed unit control valve in a manner which permits a continuousfine-pitch signal (e.g., seizure of the valve in the maximum fine- pitch delivery position) is potientially the most dangerous type offailure which can occur. The combination of such a failure with an unrelated contributoryfailure of the automatic drag-limiting system may be claimed to be an extremely remote possibility, but is nevertheless consideredhere in order to illustrate that the mechanical pitch lock still pre- vents the occurrence of catastrophic drag or r.p.m., although per-mitting a higher order of drag than the very low values provided by the automatic drag-limiting system. In Fig. 5 the failure isassumed to occur at a true airspeed of 370 knots at sea level with the throttle in the flight-idle position (e.g., at the end of a long,fast descent). After li sec the r.p.m. have risen to the setting of the overspeed governor, which signals the engagement of themechanical pitch lock. The lock is engaged i sec later. With the blades locked at this angle, 18 deg finer than the initial operatingangle, the engine cannot accelerate up to an r.p.m. greater than 10 per cent above normal maximum, with an associated drag of5,900 lb. The design of the safety features described in this article hasbeen founded on a comprehensive series of analyses of the above type, covering various combinations of single and double failuresof the powerplant and its control system. The inclusion of these features in the latest de Havilland propellers assures completeprotection against the particular dangers associated with those failures of high-speed propeller-driving engines affev-ting thepropeller system.
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