FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1958
1958 - 0546.PDF
562 FLIGHT, 25 April 1958 PROPELLER SAFETY 3 2.000- J 1,000- 25,000- LIMIT 100 (a) In the event of failure of the fuel supplyalone, the propeller control system would normally cause the blades to take up a pitchsufficiently low to maintain the pre-selected governed r.p.m. which may be that appro-priate to take-off, cruise or flight idle depending on the part of the flight at whichthe failure occurred. In the event of an additional or related failure of the control system, and in theabsence of any special safety features, it may be possible for the blades to reduce pitch down to the setting of a mechanical stop inthe pitch-change mechanism. Fig. 2 (b) shows that the drag in this case may be very large indeed at high aircraft speeds. Safeguards against Excessive Drag and r.p.m. The values ofdrag of the order shown on Fig. 2 (b) would, at best, severely penalize the performance of the aircraft, and may in some casesbe sufficiently high to cause complete loss of control or structural failure of the aircraft, as shown by the typical limiting lines includedon the diagram. Where a failure can give rise to conditions of this type, it is clearly essential to incorporate safeguards to preventthe assumption of an abnormally low pitch in any circumstances, whilst yet retaining the ability for the propeller to perform itsnormal function of automatic engine r.p.m. control over the whole speed range of the aircraft. In the case of the piston-engine installation, protection is pro-vided by the presence of the fine-pitch stop in the pitch-change mechanism, which is set at a blade angle just below the minimumvalue used in normal constant-speed operation of the propeller. This angle is normally determined by the take-off conditions of theengine/propeller combination, and is in the region of 15 deg to 25 deg. The stop also serves as a positive barrier to inadvertent entryby the blades into reverse pitch, as the stop cannot be withdrawn except by deliberate action of the pilot after touch-down. With increase of aircraft speed and engine motoring powerresulting from the use of turbine engines, however, the flight fine- pitch stop no longer gives adequate protection against excessivedrag as evidenced in Fig. 2 (b), nor is it possible to increase the blade angle setting of this fine pitch stop, or to introduce one ormore additional stops set at a higher angle, without placing upon the pilot the additional burden of manual withdrawal of thesestops at the correct time, simultaneously on all engines, to allow the propellers to continue their governing function in all flightconditions. If stops set higher in the blade-angle range were used as a pro-tective feature, failure to withdraw these stops at the appropriate flight condition would result in the propeller underspeeding,whilst inability to withdraw the stops, caused by a failure of the withdrawal system, would seriously affect the performance of thepowerplant during final approach and landing, particularly in the event of a baulk necessitating rapid acceleration of the engines. The system adopted by de Havilland Propellers in the designof the Tyne propeller to ensure the limitation of drag and r.p.m. following engine or propeller malfunction is, therefore, based 3 a TAKE-OFF R.PM .CRUISE R.PM, IDLING R.RM. 200 100 FLIGHT SPEED (KNOTS) 2.OOO B.H.P PISTON ENGINE 400 Fig. 2. Variation of windmilling drag with aircraft speed for (a) medium-size piston en- gine and (b) large two- spool turboprop. TAKE -Of [ R.PM (b) 400 FLIGHT SPEED (KNOTS) 5.0O0 E.H.P TWO-SPOOL PROPELLER TURBINE the entire range of blade angle used to maintain governed r.p.m.during flight is left unobstructed. In order to give protection against excessive drag or r.p.m. thesafety system must first sense the occurrence of a failure, and then prevent the assumption of an excessively low pitch by, ifnecessary, overriding the normal functions of the constant-speed unit. The dangers to be protected against are essentially dragand r.p.m.; it would therefore be logical to use signals based on these quantities for the failure-sensing system. In the case ofoverspeed detection this is in fact done, but in the case of the drag-limiting system it is inconvenient to derive a mechanicalsignal which is directly sensitive to drag. However, it has been noted that the amount of drag developed after a failure is dependentupon the negative or windmilling torque present in the system. An effective control of windmilling drag is therefore obtained if alimit is applied to the negative torque which can be developed in the propeller shaft, which thus provides the basis for a failure-detection mechanism for the prevention of excessive drag. Automatic Drag-limiting System. Propeller drag-limiting sys-tems based upon an engine torquemeter signal have been in service for some years. Up to the present time the main purpose of the feature hasbeen to provide automatic reduction of propeller drag after an engine failure at the critical point of the take-off, for which creditmay usually be taken in the certification of the aircraft. In such installations, the system is set to operate at a low positive valueof torque; should the torque in the shaft fall to this value, a valve is operated in the propeller controller which transfers all theincoming oil supply to the coarse-pitch line, thus feathering the propeller. Such a system must be cancelled at low throttle settings,since the normal torque transmitted by the engine shaft would then be similar to, or lower than, the torque setting of the safetysystem. To provide complete protection over the whole flight upon the provision of means for immediate automatic detection range for Tyne installations, this system has been extended by of any failure, followed by quick remedial action to prevent varying the torque setting with throttle position to maintain the dangerous conditions being attained. In this way the flight fine- setting at a value somewhat lower than that appropriate to normal pitch stop can be retained at a conventional setting to provide a engine operation, positive barrier to inadvertent selection of braking pitch, while The torque signal is transmitted from a movement of the annulus 4.000-1 5000- 2P00 1,000 AUTOMATIC DRAG-LIMITING SYSTEM SET AT -3.9OO LB-FT SHAFT TORQUE PROPELLER BLADES JSM FINE PITCH STOP Fig. 4 (right). Schematic diagram illustrating operation of mechanical pitch lock. (Below) Part of the mechanical pitch lock assembly for the Tyne propeller, showing ratchet teeth which are forced into engage- ment by multiple springs following loss of oil pressure or overspeeding. FEATHERED DRAG AT SEA LEVEL 200 300 FLIGHT SPEED (KNOTS) Fig. 3. Response values for an auto- matic drag-limiting system. HYDROMECHANICAL SIGNAL FROM ENGINE TORQUEMETER COARSENS PITCH TO LIMIT DRAG AFTER ENGINE FAILURE MECHANICAL PITCH LOCK ENGAGES IF :- - (•) OIL PRESSURE IS LOST, OR (bj OVERSPEED IS REGISTERED FINE PITCH STOP POSITIVELY LIMITS MINIMUM PITCH OBTAINABLE IN FLIGHT
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
