How the blade-pitch of an aeroplane propeller is automatically varied to give maximum engine efficiency
A WIDE RANGE OF SIZES of controllable-pitch airscrews is made by the De Havilland Aircraft Company, Limited, under Hamilton-Standard licence. The large three-bladed propeller shown in this photograph has a radius of over seven feet; the smaller propeller is one suitable for engines up to about two hundred horse-power such as the Gipsy Major or Gipsy Six.
THE introduction of the controllable-pitch airscrew is one of the greatest advances in aeronautical engineering of recent years. A close parallel to the controllable-pitch airscrew is the gearbox of the motor car. Both gearbox and controllable-pitch airscrew permit the engine concerned to do its work in its most efficient manner.
An internal-combustion engine works most efficiently at a certain number of revolutions a minute. As the revolutions fall below this number the power output of the engine rapidly diminishes; as the revolutions increase above the most efficient number the power increase is small in relation to the increase in fuel consumption, and the number of revolutions rapidly approaches the point at which damage to the engine may occur.
If a motor car were provided with only one gear — a gear equal to the middle ratio of a three-speed gearbox — the motor car would not perform well. It would be slow in picking up speed from a standstill and would have difficulty in climbing steep hills. On the level, once speed had been picked up, the engine would tend to revolve too fast and the car would not have a fast top speed.
To a lesser extent the same effects apply to an aeroplane with a fixed-pitch airscrew. There are two ways of dealing with the difficulty when a fixed-pitch airscrew is used. The pitch of the airscrew may be made fine and a good take-off obtained. To prevent the engine revolutions from increasing too much during cruising, the throttle will have to be considerably closed; the full power of the engine will not, therefore, be available for cruising. Alternatively, the airscrew pitch may be chosen so that full power can be transmitted to the airscrew at cruising speed. During the take-off the engine will be unable in this instance to develop its full power and must, therefore, be of sufficiently high power to provide a large margin of output. In practice, the usual procedure with a fixed-pitch design is to compromise. The engine is made somewhat larger than would be necessary for a satisfactory take-off if full revolutions were developed and the engine is throttled back a certain amount for cruising.
The first controllable-pitch airscrews to be introduced in large numbers were of a two-position variety in which a choice of a fine pitch and a coarse pitch was provided. (The pitch of an airscrew indicates the rate at which it would screw itself through the air if the air were a solid and the airscrew were forced into it like a screw into a piece of wood.)
The two-position controllable-pitch airscrew enables the engine to develop full power whether cruising or taking off. Fine pitch is used for the take-off and coarse pitch for cruising.
Hamilton two-position controllable-pitch airscrews were the first to be made in any numbers. They are built in Great Britain by the De Havilland Company. In these airscrews, instead of the blades being attached solidly to the airscrew boss, provision is made for them to turn and thus alter the pitch of the airscrew.
Attached to levers which turn the propeller blades are weights which by centrifugal force turn the blades to coarse pitch. Working in opposition to these counterweights is a piston actuated by oil pressure applied from the engine through the hollow airscrew shaft. By opening a valve the pilot permits oil pressure to move the piston and bring the blades of the airscrew into fine pitch. When the oil pressure is cut off, the blades automatically return to coarse pitch. This is the position they would take up if failure in the engine lubricating system cut off the oil pressure which controls the airscrew mechanism.
The angles of the airscrew blades, when in their fine or coarse settings, may be altered independently by preliminary adjustment. The two-position controllable-pitch airscrew is a compromise like the fixed-pitch airscrew, although a less undesirable one. Better than an airscrew that can be made to change pitch in one step is an airscrew that continuously varies its pitch as the load on the engine varies and as the engine tends to speed up when taking off. This effect is obtained in the constant-speed controllable-pitch airscrew.
By means of a simple governor mechanism the flow of oil from the engine to the pitch-changing mechanism of the two-position controllable-pitch propeller may be regulated. The pilot may set the governor mechanism for any particular number of engine revolutions a minute, after which the pitch of the propeller will automatically vary so that the load on the engine keeps the revolutions constant.
The constant-pitch governor mechanism weighs only 4 lb. The way in which it works is simple. A fly-weight governor, geared to the engine, operates a valve which controls the admission or release of oil from the cylinder containing the pitch-controlling piston. As the aeroplane gathers speed during a take-off the load on its engine is decreased. The engine therefore tends to revolve faster; this causes the flyweights to swing farther out and operate the oil-control valve. Oil is released from the cylinder, the counterbalance weights have more effect and the pitch of the airscrew becomes coarser. With the coarser pitch the engine tries to pull the aeroplane along faster and this increases the load on the engine, tending to slow down its speed of revolution. A balance is reached which retains the number of revolutions at the desired figure. When the engine revolutions drop an opposite effect occurs.
A controllable constant-speed airscrew enables the pilot of an aeroplane with a supercharged engine to get the best from it at any height.
Racing aircraft require such coarse-pitch airscrews for their fast top speeds that a fixed-pitch airscrew makes the take-off hazardous unless a particularly large aerodrome is available. This is another instance where the constant-speed airscrew proves valuable.
THE AUTOMATIC HUB of a “Hydromatic” constant-speed quick-feathering airscrew. A piston operating in the dome at the top is controlled by oil pressure and moves the mechanism that twists the propeller blades. When the blades are feathered, their flat part is approximately in line with the direction of flight of the aeroplane. In the event of engine failure the blades thus offer negligible wind resistance and there is no turning movement applied to the engine.
In multi-engined aircraft constant-speed airscrews increase the effectiveness of active engines if one or more engines should fail. The load on the active engines would be extremely great, but by the use of a fine pitch for the airscrews the engines are enabled to produce their maximum power. De Havilland controllable-pitch airscrews are available for engines from 130 to 1,000 horsepower, and are being produced for engines of 1,750 horse-power and more.
The controllable-pitch airscrew suitable for engines up to 200 horse-power is known as the 1,000 size. The weight of the airscrew with spinner is about 77½ lb. and the diameter may be between 6 feet and 7 ft. 6 in. The limit of some two-pitch installations is 9 degrees, but ranges up to 11 degrees can be supplied for constant-speed operation. This size of airscrew is suitable for engines such as the Gipsy Major and Gipsy Six.
The most, recent addition to constant-speed airscrews is that known as the “Hydromatic” quick-feathering type. The design was evolved by Hamilton-Standard Propellers, an American company, and will be produced in Great Britain by the De Havilland Company.
The pitch range of the normal large-size De Havilland constant-speed airscrew is 20 degrees. The new propeller gives the range of 35 degrees, a range which will probably be required in aircraft of the future. Beyond this range for constant-speed operation, a further range, of 45 degrees, is available to feather the propeller. An airscrew is fully feathered when the broadest part of the blades is turned to be approximately in line with the direction of movement of the aircraft.
The advantages of feathering are in increased safety in the event of engine failure on a multi-engined aircraft. When an engine develops a fault it is desirable to stop it immediately. Otherwise there might occur damage to the engine or damage to the airframe due to vibration. Friction brakes oil the propeller shaft take a relatively long time to stop the engine revolutions and the drag of a propeller with a coarse pitch is considerable. With the propeller fully feathered the engine will stop almost immediately and the drag is reduced to a minimum.
The reduction in drag produced by feathering the airscrew materially improves both the stability and the performance of the aircraft. This applies especially to twin-engined aeroplanes. The rate of climb, the speed and the ceiling obtainable on one engine are greatly increased. The difference between the ceiling obtainable on a twin-engined aircraft with a feathered airscrew and that obtainable on a normal braked airscrew may be as great as 2,000 feet.
In the “Hydromatic” airscrew mechanism, the use of counterbalance weights to turn the blades into coarse pitch is superseded by oil fed at two pressures and by the tendency which exists for centrifugal force to twist the blades into fine pitch. Provision has to be made to prevent the blades from feathering when the engine is running.
The automatic tendency for the blades to turn into fine pitch is counter-acted by oil boosted to a high pressure by a special governor pump. This oil acts on a piston in a similar way to the piston of the ordinary controllable-pitch airscrew. The oil pressure can counteract sufficiently the tendency of the blades to turn to fine pitch for full coarse pitch to be taken up.
To prevent the blades from turning beyond the full coarse-pitch position, the motion of the piston is converted to a turning movement through a cam arrangement. This cam arrangement is so designed that throughout the normal pitch change of the airscrew the oil-operated piston has a large mechanical advantage over the effects of centrifugal force, but beyond the fully coarse position the cam arrangement gives the centrifugal force the mechanical advantage.
On the opposite side of the piston from that to which the boosted oil pressure is applied, oil is fed at normal engine pressure. This oil assists the airscrew to turn to fine pitch when engine revolutions drop and the governor valve automatically relieves the oil pressure on the high-pressure side of the piston.
PERFECT BALANCE IS OBTAINED in constant-speed propellers when they are manufactured. This propeller is mounted on knife-edges and will react to a cigarette-paper placed on one of the blades. In this Hamilton-Standard type of constant-speed propeller the mechanism is simple, compact and operated without any source of power — such as an electric motor — external to the aircraft engine.
If a considerably higher oil pressure than that applied by the boosted oil is available to operate the cam gearing, it will be possible to continue to coarsen the pitch beyond the setting normally used. This extra high pressure is required to feather the propeller, and is supplied by an auxiliary pump mounted between the engine-oil tank and the constant-speed control mechanism. The pump is driven by an electric motor.
The oil pressure fed from the auxiliary pump does a number of things. First it cuts off the oil supply from the governor pump ; then, when sufficient pressure has been built up, it opens another valve which permits the high-pressure oil to pass into the pitch-controlling mechanism where it is at a sufficiently high pressure to produce full feathering. A mechanical stop prevents the blades from passing the fully feathered position. The engine stops revolving with the blades fully feathered, but the oil pressure continues to build up until it reaches 400 lb. per square inch. At this pressure it operates a switch which cuts off the power supply to the electric motor driving the auxiliary pump. Because the blades are fully feathered with the engine stopped, they remain in this position.
Restarting Engine in Flight
If it is desired to restart the engine while the aircraft is in flight, the auxiliary pump is brought into use again. The switch which controls the power to the electric motor is held closed to prevent the oil pressure at 400 lb. from opening it. The oil pressure is thus able to increase to about 550 lb., at which pressure it operates a valve that transfers the oil pressure to the opposite. side of the pitch-controlling piston. The blades are forced into fine pitch and the force of the wind passing them causes the airscrew to rotate.
When a sufficient number of revolutions a minute have been attained by the engine, the switch for the auxiliary pump is released. The propeller continues to turn and the engine may be switched on. The cutting off of the auxiliary pump releases oil from the governor pump and the controllable-pitch mechanism once again functions. The time taken to turn the blades of the airscrew from the normal coarse pitch to the fully feathered position is only nine seconds.
The manufacture of constant-speed airscrews calls for precision work of the highest standard. Every part of the airscrew must be carefully balanced. So carefully is the balancing carried out that a three-bladed airscrew of 12 ft. 9 in. diameter, when mounted on knife-edges, will respond to the weight of a cigarette-paper placed on one of the blades.
All stressed forgings used in the propellers have test pieces integrally forged with them and, until these test pieces have passed a series of tests, the forgings may not be used.
The constant-speed airscrew has proved itself so valuable that manufacturers are busy devising ways of producing it at a cost which makes possible its application to light aircraft. One ingenious control introduced in America and fitted to some of the Stinson Reliant aircraft is electrically operated.
A feature of the control is that it is combined with the throttle lever. The throttle lever is of the push-pull type mounted on the instrument board and the head of the throttle lever is made to turn to the left or to the right. This turning of the head alters the pitch of the airscrew.
In aircraft fitted with constant-speed airscrews, it is not possible for the pilot to check the performance of the engines by looking at the revolution counters. Suppose the control of the airscrew were set for a constant speed of 2,000 revolutions a minute and the power of one of the engines began to drop. The engine would begin to slow down; this would cause the propeller airscrew mechanism to turn the blades to a finer pitch and so remove some of the load from the engine. The engine would, therefore, still show the desired revolutions, although its power output was less. There are several instruments which can be used to show the power output of the engine. Most of them work from the exhaust manifold.
A CONTROLLABLE-PITCH AIRSCREW on a Percival Mew Gull. Racing aircraft generally require a propeller with a steep pitch because of their high top speeds. Without an airscrew on which the pitch could be varied, an exceptionally long run would be required to give the engine time to speed up sufficiently to provide enough power for the take-off.
