The angles at which the planes are set may vary from 1 in 6 to 1 in 20. One in ten might be called the "happy medium." If the planes are given too great an angle, the drift becomes so great that the propeller thrust is severely taxed. The smaller the angle, the less will be the drift and consequently the greater the speed. However, if the surface is curved the angle must not be made too small or not much lift will result.
FIG. 19. The various ways two planes may be combined to secure stability or form a biplane.
The angle of the tail planes should be adjustable. If too great, the machine will slow down and the tail will drop, destroying the equilibrium of the machine and consequently the flight. If the lift of the tail is too great, however, it will cause that part to rise and the machine will dive downwards.
Elevators and Tails are usually made of thin wood or fabric stretched over a rattan or wire framework. They are usually rectangular or elliptical in shape.
In case they are made of wood one of the best methods of attachment is to fasten the plane to a small stick by means of two or three small rivets. The stick is secured to the framework of the machine by two small rubber bands. Then in case the machine strikes head on in alighting, the band will absorb the shock and permit the elevator to move so that it is not damaged by the fall.
Vertical Fins. It is a much mooted question whether or not a vertical fin is of any value on a model aeroplane since a good model should be so designed that it will fly in a straight line without the use of a rudder. It has been the author's experience that it is often of decided advantage in correcting the flight of an "erratic machine" or in compensating any little difference that there may result in the drift of the two halves of the planes.
FIG. 20. Fins.
The fin should be placed well toward the rear of the machine and, whenever possible, stretched both above and below the centre line of the machine, so that the pressure due to cross winds will be equal both above and below and there will be no tendency for the machine to twist about its longitudinal axis.