AERONAUTICS - THE AIRSCREW OR PROPELLER The use of a screw as an instrument of proportion has long been familiar, and various theories have been proposed to account for its action.
The earliest of these, due originally to Froude and Rankine, is known as the momentum theory; the screw is regarded as a disk which in some way imparts momentum to the column of fluid which passes through it.
Imagine the machine to be at rest and the air where uninflu enced by the screw to be approaching it with velocity V.
In reality, of course, the air is at rest, and the machine moves forward with velocity V, but the forces brought into play are identical in the two cases. The velocity of the column of air which passes through the airscrew disk—the area of the circle swept out by its blades—is increased according to Froude by an amount v so that the column leaves the screw with a velocity VI-v. It follows as a further consequence of the theory that half the addi tional velocity is acquired before the screw is reached. So that the velocity with which the air arrives at the screw is V-1---1-v.
Thus we have a column of air of density p and area of cross section A approaching the screw with speed (V-Hv) and the mass which traverses the disk in I sec. will thus be Ap(V-1-1v). This mass gains a velocity v. and the momentum generated in one second is Ap(V-1-1v)v. This must be the force exerted by the screw on the air and therefore the thrust or force driving the screw through the air.
Imagine now that the aerofoil, instead of moving uniformly forward, is spinning about one end round an axis parallel to the direction of lift, a similar result will follow, each section perpen dicular to the length of the aerofoil is moving forwards, though with a velocity which varies as its distance from the axis ; each section therefore experiences a "lift" parallel to the axis and a "drag" acting so as to check the motion of rotation.
The lifts combine to produce a resultant thrust on the aerofoil in the direction of the axis, the drags to form a torque about the same. To maintain the motion a torque must be applied sufficient to overcome the drag and the action of the aerofoil transforms the torque into a thrust on the air. We have an elementary screw.
In order that the thrust may act along the axis and not at a poipt on the aerofoil which changes its position as the blade ro tates, a second aerofoil is added on the other side of the axis and with its front edge pointing in the opposite direction to that of the first.
Again, if the blade were of the same section from root to tip, since the tip is moving very much faster than the root, the thrust on an element at the tip would greatly exceed that near the root. To obtain a more uniform grading of the thrust the shape of the blade is altered as we pass from root to tip in such a way that the angle of attack, the angle be tween the blade and the direction of motion relative to the air, is greater at the root than the tip and thus we arrive at the two-bladed screws in common use.
Reference has been made in the section dealing with aerodynamics to the vortex theory of the aerofoil ; this theory has been applied by Glauert to the action of the airscrew with results which explain most of the outstanding difficulties of th( Airscrew Efficiency.—Theabove account will serve to show in a general way how the energy supplied by the rotation of the engine is transformed into that required to drive the aircraft ; by the efficiency of the screw we mean the ratio of the power de veloped as thrust to the horse power delivered by the engine.
To utilize the power of the engine to the greatest advantage the pitch of the airscrew employed should change with changes in the speed of the machine; for if an airscrew be designed to be most efficient when the engine is running at its normal rate, and the machine flying at full speed, it follows both from theory and ex periment that when stationary on the ground the rate of rotation is very appreciably less than that required for efficiency under these conditions and the airscrew is not giving its full thrust ; it would be an advantage then to vary the pitch of the screw so as to permit the engine to exert full power on the ground as well as when in flight at various altitudes ; efforts are being made to dis cover a satisfactory solution to the engineering difficulties involved.
In general, airscrews are made of hard wood, but in a number of cases metal has been employed and is likely in time to supersede the use of wood. Fig. i i gives a drawing of a typical airscrew.