Engineering Wonders of the World
Volume III

42 ENGINEERING WONDERS OF THE WORLD. body to the fixed surface without appreciable loss. But in water and air, which can be dis- placed easily, the problem of getting, so to speak, a good push-off is one that has de- manded close investigation and a huge amount of experiment. For moving a ship or a flying machine the angle may counterbalance the decrease in rotary speed, and enable all parts of the blade’s surface to push back the air with an equal velocity. Otherwise, there would be a great waste of power, some portions of tho blade acting as a drag on the others. A propeller blade would, if flattened and set square to the CONSTRUCTING A FOUR-BLADED PROPELLER OUT OF SUPERIMPOSED LAMINÆ OF WOOD. (Photo, London Electrotype Agency.) axis of the propeller shaft, offer a mini- mum turning resist- ance ; if set with its surfaces' in line with the shaft, a maxi- mum resistance. In neither case would it have any lift or thrust. The de- signer has to con- sider how to curve the blades so as to give a maximum thrust for a mini- mum windage, which is the counter- part of drift, and at the same time he must be careful to make the sur- faces as smooth as screw propeller has no rival. The marine propeller has been brought to great perfec- tion ; air propellers are being improved rapidly, but are still, as a class, wasteful of power. The air propeller is in principle closely allied to the curved deck of the aeroplane. As it revolves it strikes the air at an angle, and produces thrust, which is th© counterpart of the lift of a deck. Owing to the fact that the speed of the parts of a propeller blade vary with, their distance from the centre of rotation, it is necessary to increase the steepness of the angle of the blade gradually from the tip to the base in such a way that the increase of possible in order to keep air-friction very low. The efficiency of a screw is gauged by the amount of thrust which it gives in proportion to the force exerted to turn it. The thrust itself is arrived at by multiply- . , Thrust. ing the weight of the mass of air acted on in a second by the velocity in feet per second at which that mass of air is moved. The amount of air engaged varies —tho pitch being constant—as the square of the diameter of the propeller. The velocity in feet per second at which it is moved is the pitch multiplied by the number of revolutions per second. Assuming that the screw is perfectly effi-