Airscrew

AIRSCREW, the designation of all those devices which, when rotated, tend to screw their way forward, sucking the air from in front and throwing it away behind, or which, when placed in a, moving stream of air, are themselves rotated. The term covers (I) fans of a certain type, (2) screw propellers working in air, (3) helicopters, (4) windmills, (5) gyroplanes.

(I) If the airscrew is rotated but restrained from forward movement, air is drawn in from the front and thrown back behind, and the airscrew acts purely as a fan.

(2) If the airscrew is not so restrained, the thrust caused by moving the air backwards drives forward the airscrew and tha aircraft, or hody, to which it is attached. The airscrew is then acting as a propeller. The thrust exerted, and the volume of air moved, have their maximum value when the airscrew is acting purely as a fan. As the airscrew moves forward, the volume of air delivered and the thrust exerted decrease, and at a certain speed become nothing. At this forward speed the distance moved in one complete revolution is known as the "experimental pitch" of the airscrew. The thrust acts approximately in a horizontal di rection.

(3) If, however, the airscrew is placed so that it exerts its thrust upwards in an approximately vertical direction, it is then acting as a helicopter.

(4) In the above conditions the airscrew is driven by some engine or motor, but if it is placed freely on a shaft in a wind, or if it is moved forward through the air, it will rotate and some of the energy in the moving air will be transformed into useful work, and transmitted to whatever mechanism is attached to the airscrew. The airscrew is then acting as a windmill. The wind blows approximately in the direction of the airscrew shaft; i.e., at right angles to the plane of rotation.

(5) If the wind blows at a small angle to the plane of rotation, the airscrew will not only be caused to rotate, but in doing so will exert a thrust. It is then acting as a gyroplane. The best known example of this use of the airscrew is the autogiro, invented in Spain by Senor Don Juan de la Cierva. In this case the plane of rotation is approximately horizontal and the thrust approxi mately vertical.

General Description.

Anairscrew consists of a boss or hub with spokes radiating therefrom—like a wheel without a rim. The number of spokes or blades, as they are technically termed, is usually two or three, though four are sometimes used, particu larly on the larger British transport craft. They have seen little use in the United States, however.

In fig. i is shown the plan and elevation of a two-bladed air screw intended for use as an aircraft propeller. Below them are shown the sections which would appear if the blade were cut through at points A, B, C, etc. The terms used in connection with airscrews are also shown. Note the term "diameter," the distance from the tip of one to the tip of the opposite blade. For aircraft propellers the diameter varies from as little as four to as much as 16 feet. Note also the characteristic shape of the cross sections of the blade. Sections of this shape are called aerofoil sections; their properties will be found described in any standard work on aerodynamics. The shape of these cross sections has an impor tant influence on the efficiency of the airscrew. It will be noticed that the blades are twisted ; the angle is greater at the root than at the tip. This twist is necessitated by the fact that in order to work most efficiently, an aerofoil section must move at a small angle to the relative air flow.

Now any point in the blade has two simple motions : (I) In the direction of rotation, (2) The forward motion at right angles to (i). In fig. 2 these two simple motions are illustrated to scale as AB and AC for a point near the tip of the blade, and in fig. 3 for a point near the root of the blade ; AD is the resultant direction and speed of movement. The speed AC of the forward movement is the same for both points, but the speed AB in the direction of rotation is proportional to the distance of the point from the centre. Its value is zero at the centre and a maximum at the tip.

Thus as will be seen from figs. 2 and 3, the resultant direction of movement of the point near the tip is at a smaller angle to the plane of rotation than that of the point near the centre. Since the aerofoil section must be placed at a small angle to its direction of movement, the blade has a smaller angle at the tip than near the centre. The distance travelled in one complete revolution, when the direction of the resultant motion is along the line joining the leading and trailing edge of the section (see fig. r, section DD), is the geometrical pitch. It has approximately the same value for all parts of the blade, and is usually referred to by the word "pitch." Problems of Design.—Themain difference between various airscrews is in the diameter and pitch, and it is the designer's object to choose these for the attainment of certain specified re sults. He has three aims : (I) To ensure that the airscrew runs at a certain rate of rotation when driven by a certain power, when the aircraft to which it is attached is travelling at a certain speed at a certain height above sea level. (2) To ensure that under these conditions the airscrew will give its maximum efficiency. The efficiency is the ratio of the useful work done by the airscrew to the work done by the engine in turning it. Efficiencies of from 75 to 87% are commonly attained. (3) To ensure that the airscrew is reliable and safe. Airscrews rotate at a high speed. Their tips travel at speeds varying from 70o to as much as r,iooft. per second—a velocity exceeding that of sound. This sets up a centrifugal pull in the blade of as much as from ro to 25 tons, in addition to which the blade must withstand the bending caused by its own thrust.

To satisfy these various requirements, the designer will specify an airscrew of a certain diameter and pitch, with blades of a cer tain width and thickness. If any one of these conditions alters— the rate of rotation, the power or the aircraft speed—a different airscrew will be required. Hence the infinite number of designs involved.

Controllable Pitch Alrscrews.

Fromthe very start of air craft development it has been plain to engineers and inventors that it would be very desirable to alter the pitch angles of the propeller blades during flight. If the blades are set at the proper angle for level flight at relatively high forward speed, the angles are too high at the time of take-off and climb when the forward speed of the aeroplane is low, so that the propeller is operating inefficiently under these conditions. When the internal com bustion engine was adopted universally as an aircraft power plant, there was an additional need for a variable or con trollable pitch airscrew, since a propeller which was set for the proper engine rotation at level flight would not allow the engine to turn at its full rated speed at the time of takeoff, so that there was a material loss in engine power under these conditions.

The effect of the controllable pitch propeller on the engine is analogous to that of the gear shift for the automobile, and the controllable pitch propeller has sometimes been referred to as the "gear shift of the air." A large part of the early pioneering development on the con trollable pitch propeller was carried out at the aeronautical laboratory of the U.S. Army at McCook Field, Dayton, Ohio. These experiments led up to the adoption of the detachable alumin ium alloy blades for the propellers. While the pitch of these early propellers could not be changed in flight, the development of the aluminium alloy blade paved the way to a large degree for the later development of the controllable pitch propeller.

A large number of types of controllable pitch propellers were built experimentally. The Hamilton Standard is one of the leading successful controllable pitch propellers, and may be taken to indi cate the development of this adjunct to flying. The original of this type was built in late 1929, and improvement was rapid after that time. During 1933, the Hamilton Standard came into uni versal use on the air transport lines of the United States, and the propeller was awarded the Collier trophy for the greatest ad vancement in aviation during that year.

This propeller is made with semi-hollow aluminium alloy blades journaled at the end on plain bearings. Centrifugal load on the blades is taken up by means of roller bearings encircling the inner ends of the blades. The actual operation of changing the pitch is carried out by means of an oil cylinder which forces the blades into the low pitch position when oil pressure is applied. When the oil pressure is released, the propeller is forced into high pitch by means of counterweights which are acted on by centrifugal force.

The device is very simple in design and in operation. In the initial form, it utilized two pitch settings, one being suitable for take-off and climb, and the other for level flight and cruising. The latest development of the device incorporates an automatic governor which serves to change the pitch automatically so as to maintain the engine operation at any constant r.p.m. which may be selected by the pilot in flight. This device is known as the "constant speed" propeller.

Materials of Construction.

Atfirst airscrews were invari ably constructed of hardwoods, such as walnut or mahogany. The airscrew is built up of a number of superimposed planks or laminae about one inch thick, suitably cut to shape. In Fig. 1 the cut planks for a two-bladed airscrew are illustrated. The planks are glued together to form the rough block from which the airscrew is shaped. This and the finished airscrew are also shown.

Wooden airscrews are affected by the humidity and temperature of the atmosphere, and have been found unsuitable for tropical climates. Hence there has arisen a need for metal airscrews. Of such there are two distinct types : airscrews with hollow blades, and those with solid blades. The former are constructed of steel and the latter of a light metal, usually an aluminium or magnesium alloy. Both types have met with a considerable degree of success, but both are heavier and more costly than the wooden airscrew. Despite the added expense, however, metal propellers have come into very common use, particularly on engines of higher horse power. In large sizes the weight of the solid type appears likely to be excessive, while that of the hollow steel type approaches very closely to that of a wooden airscrew.

BIBLIOGRAPHY.--W. J. M. Rankine in the Transactions of the Bibliography.--W. J. M. Rankine in the Transactions of the Institute of Naval Architects (1865) ; S. Drzewiecki, Les Comptes Rendus de l'Academie des Sciences (1892 and 1909) ; Le Bulletin de l'Association Technique maritime (1892, 1900, 1901, 191o) ; Riabou schinsky, Bulletin du Laboratoire Aerodynamique de Koutchino (19o9 and 1912) ; L. Prandtl, Handworterbuch der Naturwissenschaften ; A. Betz, Zeitschrift fur Flugtechnik and Motorluftschif fahrt (1915, 1920, 1924) ; L. Prandtl, Nachrichten der Koniglichen Gesellschaft der Naturwissenschaften zu Gottingen (1918-19) ; A. Betz, ibid. (1919) ; M. A. S. Riach, "Helicopters," Aircraft Engineer ing (192o) ; Th. v. Karmcn, "Helicopters," Zeitschrift fur Flag technik and Motorluf tschi ff ahrt (1921) ; Th. Bienen and Th. v. Karman, Zeitschrift des Vereines deutscher Ingenieure (Dec. 20, 1924) ; G. Eiffel, Nouvelles Recherches sur la resistance de fair et l'aviation (1914).

For the more important English work see Reports of the Aero nautical Research Committee. These official reports include the works referred to H. Glauert and A. Fage. For the more important American work see Reports of the Advisory Committee for Aero nautics (U.S.A.) , which include G. de Bothezat's work in Report No. 97 (1921).

See also the following text-books: R. Soreau, l'Helice Propulsive (191 I) ; C. Dornier, Berechnung der Luftschrauben (1912) • P. Bejeuhr, Luftschrauben (1912) ; S. Drzewiecki, Theorie Generale de l'Helice (1920) ; L. Bairstow, Applied Aerodynamics (1920) ; A. Fage, Airscrews in Theory and Experiment (1920, bibl,.) ; H. C. Watts, Screw Propellers for Aircraft (1920) ; H. Glauert, The Elements of Aerofoil and Airscrew Theory (1926). (H. C. W.; X.)