To secure maximum climbing ability, we must determine at what velocity the margin of motor power is the greatest. In the above-mentioned machine we know that the horsepower required for support is least for a speed of near 55 miles per hour, and it is near speed where therefore the excess margin of power is greatest and at which climbing is best done. An airplane designed chiefly for climbing must have low values of motor power necessary for support, namely, must have small resistance, therefore small size, therefore small weight.
Fig. 26.—Performance curves for typical training airplane.
Gliding Angle.—Gliding angle denotes the angle at which the airplane will glide downward with the motor shut off and is spoken of as 1 in 5, 1 in 6, etc., according as it brings the airplane 1 mile down for each 5, 6, etc., miles of travel in the line of flight. The gliding angle of a machine may be found by dividing the total resistance into the weight:
Gliding angle = Weight/Total resistance.
In the above-mentioned airplane it is one in 6.6 when the resistance is 288 lb., that is, when the speed is 57 miles per hour. At any other speed the resistance increases and hence the gliding angle decreases. Hence the importance of putting the airplane into its proper speed in order to secure the best gliding angle.
The Propeller.—The propeller or “screw,” by screwing its way forward through the air, is able to propel the airplane at the desired velocity. Regarding principles of propeller action the matter can be hastily summarized in the following brief lines. The propeller blades may be regarded as little wings moving in a circular path about the shaft; and they have a lift and drift as do the regular wings. The lift is analogous to the thrust; to secure this thrust with least torque (drift) the blades are set at their most efficient angle of incidence, and while the blade appears to have a steep angle near the hub, it actually meets the air in flight at the same angle of incidence from hub to tip.
Propeller Pitch.—Pitch is best defined by analogy to an ordinary wood-screw; if the screw is turned one revolution it advances into the wood by an amount equal to its pitch. If the air were solid, a propeller would do the same, and the distance might be 8 ft., say. Actually the air yields, and slips backward, and the propeller advances only 6 ft. Its “slip” is then 8 minus 6, equals 2 ft., or 25 per cent. Such a propeller has an 8-ft. pitch, and a 25 per cent. slip.
This “slip stream” blows backward in a flight so that the tail of an airplane has air slipping past it faster than do the wings. Hence the air forces at the tail are greater than might be expected. The rudder and elevators therefore give a quicker action when the propeller is rotating than when, as in the case of a glide, it is not.
