There is as much danger in providing too much lifting-surface in your aëroplane as too little. This fault is well illustrated in an exceedingly clever French model ([Plate I]). Although the model is well constructed, and appears ship-shape at first glance, it nevertheless has far too much surface and will not fly well. If the depth of the wings were reduced fully one half, it would have a much better chance.
The best lifting-planes are those which present a broad front or entering edge, but with comparatively little depth. The successful flying-machines, whether monoplanes or biplanes, use these very wide but shallow planes forward. The theory is of course that the air is caught for an instant beneath the plane and before it has a chance to slip off the sides, the wing has caught its very slight supporting power and moved on to new and undisturbed air.
With this rule in mind examine the model’s front plane once more. It will be seen that, as the air is caught under this broad surface, it will try to escape in all directions and set up currents of air. Instantly the broad plane loses its balance and tilts to one side or the other. No weighting of the plane can overcome this. If the plane were forced through the air at a very high speed a steady flight might be possible, but it is useless to try to overcome this tendency to tip and wabble.
The planes again are badly designed. A perfectly straight front or entering edge gives the best results. A certain stability is gained by curving the front plane slightly, this will be discussed later, but there is no excuse for the semicircle described in this case. Every inch of surface cut away from the front edge of the plane directly reduces its lifting power. The arrow like form of the rear plane does not matter because this is a stability plane, not a lifting plane. In this case the rear plane is twice the size it should be.
PLATE I. A Clever Folding Model. The Wings Are Broader than Need Be.
The propeller of this model is much too small, even if the size of the planes was correct. It is well placed however at the front of the model where it may turn in undisturbed air. The passage of these large planes, or any planes for that matter, is likely to cut up the air just as a ship churns the water into a wake behind it and the propeller does not work effectively in these eddies. The motor seems powerful and well braced, although it might be made even longer by carrying it to the extreme rear.
Several very useful ideas may be borrowed from the construction of the frame of this model. It is made entirely of metal, so jointed that it may be folded up into very compact form like an umbrella. The amateur model builder should not attempt anything so complicated, but an old umbrella frame may be used with good results in building a rigid frame. Use the steel rod of the umbrella as a backbone, and cut away the ribs you do not need. The others may be bent into various shapes to form the front or sides of the planes, the skids or braces. Such a construction is light and perfectly rigid.
A very effective monoplane may be made by curving the front and rear edges of the forward plane, while keeping the rear or stability plane rectangular in shape ([Plate 2]). The curve of this model may be imitated to advantage, as well as the general proportions. Such a plane is less likely to be deflected by air currents than a straight entering-edge and insures longer and steadier flights. Should you be troubled by your model twisting from side to side in flight try curving the front edge of the forward plane.
This model is one of the easiest to make and is an excellent one for beginners. Build the two planes separately making the larger one about thirty inches in width and ten inches in depth, and the second one fifteen inches in width and ten inches in depth. The curved sticks may be worked up by using bamboo or dowel-sticks, soaking them in water and fastening them in a bowed position while damp and leaving them to dry. It may be found a good plan to use a heavier stick for the rear edge of the plane to gain stability.