CHAPTER XXIV. The Wright Biplane (Camber of Planes)
Now that the internal-combustion engine had arrived, the Wrights at once commenced the construction of an aeroplane which could be driven by mechanical power. Hitherto, as we have seen, they had made numerous tests with motorless gliders; but though these tests gave them much valuable information concerning the best methods of keeping their craft on an even keel while in the air, they could never hope to make much progress in practical flight until they adopted motor power which would propel the machine through the air.
We may assume that the two brothers had closely studied the engines patented by Daimler and Levassor, and, being of a mechanical turn of mind themselves, they were able to build their own motor, with which they could make experiments in power-driven flight.
Before we study the gradual progress of these experiments it would be well to describe the Wright biplane. The illustration facing p. 96 shows a typical biplane, and though there are certain modifications in most modern machines, the principles upon which it was built apply to all aeroplanes.
The two main supporting planes, A, B, are made of canvas stretched tightly across a light frame, and are slightly curved, or arched, from front to back. This curve is technically known as the CAMBER, and upon the camber depend the strength and speed of the machine.
If you turn back to Chapter XVII you will see that the plane is modelled after the wing of a bird. It has been found that the lifting power of a plane gradually dwindles from the front edge - or ENTERING EDGE, as it is called - backwards. For this reason it is necessary to equip a machine with a very long, narrow plane, rather than with a comparatively broad but short plane.
Perhaps a little example will make this clear. Suppose we had two machines, one of which was fitted with planes 144 feet long and 1 foot wide, and the other with planes 12 feet square. In the former the entering edge of the plane would be twelve times as great as in the latter, and the lifting power would necessarily be much greater. Thus, though both machines have planes of the same area, each plane having a surface of 144 square feet, yet there is a great difference in the "lift" of the two.
But it is not to be concluded that the back portion of a plane is altogether wasted. Numerous experiments have taught aeroplane constructors that if the plane were slightly curved from front to back the rear portion of the plane also exercised a "lift"; thus, instead of the air being simply cut by the entering edge of the plane, it is driven against the arched back of the plane, and helps to lift the machine into the air, and support it when in flight.
There is also a secondary lifting impulse derived from this simple curve. We have seen that the air which has been cut by the front edge of the plane pushes up from below, and is arrested by the top of the arch, but the downward dip of the rear portion of the plane is of service in actually DRAWING THE AIR FROM ABOVE. The rapid air stream which has been cut by the entering edge passes above the top of the curve, and "sucks up", as it were, so that the whole wing is pulled upwards. Thus there are two lifting impulses: one pushing up from below, the other sucking up from above.
It naturally follows that when the camber is very pronounced the machine will fly much slower, but will bear a greater weight than a machine equipped with planes having little or no camber. On high-speed machines, which are used chiefly for racing purposes, the planes have very little camber. This was particularly noticeable in the monoplane piloted by Mr. Hamel in the Aerial Derby of 1913: the wings of this machine seemed to be quite flat, and it was chiefly because of this that the pilot was able to maintain such marvellous speed.
The scientific study of the wing lift of planes has proceeded so far that the actual "lift" can now be measured, providing the speed of the machine is known, together with the superficial area of the planes. The designer can calculate what weight each square foot of the planes will support in the air. Thus some machines have a "lift" of 9 or 10 pounds to each square foot of wing surface, while others are reduced to 3 or 4 pounds per square foot.