Engineering Wonders of the World
Volume III

8 ENGINEERING WONDERS OF THE WORLD. travel, which motive force. The Design of a Flying- Machine. takes to earth in turn is dependent on the When travelling horizontally the machine is practically con- stantly climbing a slope equal to that of the natural gliding angle of descent which it when the engines are stopped. So that in effect the power required to sustain it must be equivalent to the extra power (above that developed on the level) needed to drive a motor car of equal weight at an Fig. 4. An aeroplane travelling horizontally has, weight for weight, to exert as much force to support itself as is required to propel a motor car up an incline having a gradient equal to the gliding angle of the aeroplane. equal speed up an incline equal to the gliding angle of the aeroplane, and, in addition, to overcome the air resistance and skin friction of all parts of the machine. The first factor, the aerodynamic resistance, is decreased rela- tively to the lift by higher speed, since, as we have seen already, the lift increases with the speed ; the second factor, head resistance, increases in the same ratio, as the square of the velocity. Hence one factor tends to counter- balance the other. It follows from this that for any one machine there is a certain speed at which, it will support itself and travel from one point to another most economically—that is, with the least expenditure of force. To improve the speed without increasing the power or altering the weight, the head resist- ance must be diminished, or the design of the decks improved and the inclination reduced. Should the designer elect rather to decrease the supporting area without increasing engine power, he would be compelled to increase also the inclination of the decks—and with it the “ drift ”—which would tend to diminish speed —a very undesirable alternative. An aeroplane must travel at a certain speed to support itself at all. To enable it to rise, the power must be increased. Merely to point an elevating rudder upwards will not suffice, as the increase of inclination will increase the “ drift ” of the supporting surfaces and slow the machine. At the great meeting at Rheims the struggles of competitors to reach the highest altitude—the winner rose but slightly more than 500 feet—proved the difficulty of increasing the steepness of ascent over and above the angle at which the machine must take to maintain a horizontal path. An efficient machine has a gliding angle of about 1 in 8; that is, when influenced by gravity alone, it will descend one foot for every 8 feet it progresses. The power needed to propel the machine on a horizontal course is that required to, say, roll a ball of equal weight up a frictionless incline of 1 in 8, and also to overcome fric- tional air resistance. To maintain stability a speed of from 35 to 40 miles an hour is required. Let us assume that the machine weighs 500 lbs. with pilot, and that it has to travel at 40 miles per hour to sustain itself. Every second 500 lbs. will be lifted (in effect) |th of 60 feet = 7| P°wer needed feet. To effect this will re- A ^°F ,an Aeroplane, quire about 7| horse-power. In order to rise, at least one-fifth more power must be added, making 9 horse-power in all. Owing to loss of power in transmission and to screw inefficiency, a further 50 per cent, more power is required, and to overcome air fric- tion and resistance we must allow a further 30 per cent. The engine for a 500 lb. load should therefore develop some 16 horse-power, or about 1 horse-power for every 31 lbs. of weight.