aero engines

powerplant accessories
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power plant accessories

The development of power plant accessory equipment goes back to a period immediately following the first World War, when flying was a haphazard and dangerous occupation. The only flight instruments were a compass, bubble, altimeter and tachometer. With these instruments, flights were occasionally made through clouds but not for protracted periods. Very few pilots were capable of flying blind even for the shortest period. After the World War and at the beginning of commercial aviation and the inauguration of airmail service in this country most of the flying was of the contact type. The development of the turn and bank indicator gave the first real help in blind flying. Then came the artificial horizon and gyro compass. The development of the radio range beam and directional radio made it possible and practical to fly without contact with the ground. With these instruments available, the art of blind flying developed rapidly and increased to the point where experimental flights entirely with instruments from "take-off" to landing have been successfully conducted.

Similarly, the development of the single engine airplane of the World War period, with its engine of 30 horsepower, to the huge multi-motored transport of today, wherein the output of a single cylinder is often equal to or more than the total output of the early engine, has brought about the development of power plant accessory equipment to its present stage. The days of engine starting, wherein the propeller was pulled through by hand, have long since disappeared to be replaced by electrically operated Inertia or Direct Cranking Electric Starters capable of transmitting 1500 pound feet of torque for the cranking of engines rated up to 2000 horsepower.

In a like manner, the development of generating equipment from the early wind driven types having an output of approximately 50 watts, to the engine driven types of 3000 watts capacity, has also taken place. In addition, the Venturi tube, which was subject to clogging with dirt and ice, has long since been replaced with engine driven vacuum pumps with capacities sufficient to provide proper suction for the operation of navigating instruments and pressure for the operation of wing and tail surface de-icers. Whether the need has been for aircraft engine starting or for the generation of electric power, accessory units have proved vital necessities to the operation of the present day air transport and military airplane. Electric motors for the operation of retractable landing gear, wing flaps, tail wheels, etc.; hydraulic pumps, valves and control units for actuating hydraulically, full feathering propellers, retracting mechanisms, etc.; radio dynamotors for the operation of radio receivers and transmitters; fuel flow meters for determining fuel consumption; these are only a few of the many accessory units which have played an important part in the development and dependability of the present-day airplane. To the old time pilot the airplane may have consisted primarily of the airplane and its engine, but to the experienced air transport pilot of today aircraft accessory equipment and flight instruments play a major part in establishing flight security.

The fundamental requirements of any form of aircraft power plant accessory are dependability, minimum weight, simplicity of installation, ease of operation, effectiveness, serviceability, and low initial cost. As the number and type of power plant accessories are, necessarily, limited to the number of drives available on the rear or accessory drive section of the power plant, careful consideration must be given to the number of operations to be performed, and the type of

service to which the airplane is to be subjected, prior to the selection of accessory equipment. In addition, the capacity and type of accessory units to be installed on a given power plant are definitely limited to the physical characteristics, operating speed and torque limitations of the accessory drives available.

Swinging the Prop

Although swinging the prop was the earliest common form of starting for aircraft engines and the method still in limited use for present-day light engines, there has always been considerable danger to the operator when effecting engine starting by this method. Swinging the prop, in most cases, has been replaced by the use of either a hand turning gear, air-injection starter, or direct cranking electric starter, the latter two methods providing quick and convenient starting by the pilot without need for external assistance. The need for mechanical aid in starting, evolved many methods which were short lived. Several, however, were successful to a certain degree.

The Hucks Starter

An early advance in the development of aircraft engine starting was the invention of the Hucks starter, which was named after its inventor. This unit was an external mechanical starter, employing a Ford motorcar chassis. A special chain from the gearbox was made to drive a layshaft mounted high up and adjustable. At the forward end of this shaft was a coupling which could be made to engage with a special "dogged" fitting attached to the propeller hub in the same manner as a motor-car crank handle was used to engage the engine crank shaft. Starting was effected by engaging the coupling and rotating the layshaft. Disengagement of the driving dog from the engine was automatic upon starting of the engine. The Hucks starter, although an improvement over the method of hand swinging was in turn, very impractical. Delays in starting occurred when there were a large number of airplanes to be started. Also, it was sometimes out of action or did not exist at places where a start was required. At the same time, it must be remembered that engines were increasing in size and compression ratio to the point where swinging of the propeller by hand as well as turning the propeller mechanically by means of the Hucks starter were impracticable. This brought about the development of the hand turning gear, which is used in a modified form at the present time for certain aircraft installations.

Hand Turning Gears

The development of the band turning gear type of aircraft engine starter was primarily due to the need for an aircraft engine starter capable of cranking engines rated up to 600 horsepower not equipped with batteries or generators. The hand turning gear consists, in general, of a gear reduction unit, which operates an automatic engaging and disengaging mechanism through an adjustable torque overload release. Mechanical features are incorporated in the unit to safeguard both the operator and starter from injury in case of engine backfire. In case of the engine backfiring, the torque overload release automatically disconnects the starter drive, thus preventing damage to the starter mechanism. As a further protection to the operator, a ratchet is provided in the hand crankshaft to preclude the possible transmission of any reverse motion to the crank-handle. Unusual care is exercised in the design and manufacture of a hand turning gear to eliminate friction, thereby assuring maximum even cranking. In addition, the ratios of the gear reductions used have been carefully determined in order to permit the highest engine cranking speed consistent with the average manual effort which can be expended in cranking. Thus, the operator is assured of a prompt start with minimum effort, provided the other factors which influence engine starting are normal.