4. A very tapering form of stern, the actual stern only being of just sufficient size to allow of the propeller shaft passing through. In the case of twin propellers some slight modification of the stern would be necessary.
5. Every portion of the body in contact with the fluid to be made as smooth as possible.
6. A body of such shape gives at most only one-twentieth the resistance offered by a flat disk of similar maximum sectional area.
Results since fully confirmed.
Fig. 1.—Shape of Least Resistance.
The design in Fig. 2 is interesting, not only because of its probable origin, but because of the shape of the body and arrangement of the propellers; no rudder is shown, and the long steel vertical mast extending both upwards and downwards through the centre would render it suitable only for landing on water.
§ 5. In the case of a rubber-driven model, there is no containing body part, so to speak, a long thin stick, or tubular construction if preferred, being all that is necessary.
The long skein of elastic, vibrating as well as untwisting as it travels with the machine through the air, offers some appreciable resistance, and several experimenters have enclosed it in a light tube made of very thin veneer wood rolled and glued, or paper even may be used; such tubes can be made very light, and possess considerable rigidity, especially longitudinally. If the model be a biplane, then all the upright struts between the two aerofoils should be given a shape, a vertical section of which is shown in Fig. 3.
§ 6. In considering this question of resistance, the substance of which the aerofoil surface is made plays a very important part, as well as whether that surface be plane or curved. For some reason not altogether easy to determine, fabric-covered planes offer considerably more resistance than wooden or metal ones. That they should offer more resistance is what common sense would lead one to expect, but hardly to the extent met with in actual practice.