Aeroplane Construction and Operation

by John B. Rathbun

Foot Rudder Bar Used in the Standard H-3. Courtesy "Aerial Age."

Rudder Control. Foot bar control for the rudder is standard with both the stick and Dep controls. The foot bar is connected with the rudder in such a way that the aeroplane turns opposite to the movement of the foot bar in the manner of a boat. That is, pushing the right end of the bar forward causes the machine to turn toward the right.

Automatic Control System (Sperry) Installed in Fuselage of Curtiss Tractor Biplane.

CHAPTER XVI. HEAD RESISTANCE CALCULATIONS.

Effect of Resistance. Resistance to the forward motion of an aeroplane can be divided into two classes, (1) The resistance or drag due to the lift of the wings, and (2) The useless or "Parasitic" resistance due to the body, chassis and other structural parts of the machine. The total resistance is the sum of the wing drag and the parasitic resistance. Since every pound of resistance calls for a definite amount of power, it is of the greatest importance to reduce this loss to the lowest possible amount. The adoption of an efficient wing section means little if there is a high resistance body and a tangle of useless struts and wires exposed to the air stream. The resistance has a much greater effect on the power than the weight.

Weight and Resistance. We have seen that the average modern wing section will lift about 16 times the value of the horizontal drag, that is, an addition of 16 pounds will be equal to 1 pound of head resistance. If, by unnecessary resistance, we should increase the drag by 10 pounds, we might as well gain the benefit of 10 x 16 = 160 pounds of useful load. The higher the lift-drag efficiency of the wing, the greater will be the proportional loss by parasitic resistance.

Gliding Angle. The gliding angle, or the inclination of the path of descent when the machine is operating without power, is determined by the weight and the total head resistance. With a constant weight the angle is greatest when the resistance is highest. Aside from considerations of power, the gliding angle is of the greatest importance from the standpoint of safety. The less the resistance, and the flatter the angle of descent, the greater the landing radius.

Numerically this angle can be expressed by: Glide = W/R, where W = the weight of the aeroplane, and R = total resistance. Thus if the weight is 2500 pounds and the head resistance is 500 pounds, the rate of glide will be: 2500/500 = 5. This means that the machine will travel forward 5 feet for every foot that it falls vertically. If the resistance could be decreased to 100 pounds, the rate of glide would be extended to 2500/100 = 25, or the aeroplane would travel 25 feet horizontally for every foot of descent. This will give an idea as to the value of low resistance.

Resistance and Speed. The parasitic resistance of a body in uniform air varies as the square of the velocity at ordinary flight speeds. Comparing speeds of 40 and 100 miles per hour, the ratio will be as 40° is to 100° = 1600: 10,000 = 6.25, that is, the resistance at 100 miles per hour will be 6.25 times as great as at 40 miles per hour.