All present-day aeroplanes are a compromise between Climb and horizontal Velocity. We will compare the essentials for two aeroplanes, one designed for maximum climb, and the other for maximum velocity.
ESSENTIALS FOR MAXIMUM CLIMB:
1. Low velocity, in order to secure the best lift-drift ratio.
2. Having a low velocity, a large surface will be necessary in order to engage the necessary mass of air to secure the requisite lift.
3. Since (1) such a climbing machine will move along an upward sloping path, and (2) will climb with its propeller thrust horizontal, then a large angle relative to the direction of the thrust will be necessary in order to secure the requisite angle relative to the direction of motion.
The propeller thrust should be always horizontal, because the most efficient flying-machine (having regard to climb OR velocity) has, so far, been found to be an arrangement of an inclined surface driven by a HORIZONTAL thrust—the surface lifting the weight, and the thrust overcoming the drift. This is, in practice, a far more efficient arrangement than the helicopter, i.e., the air-screw revolving about a vertical axis and producing a thrust opposed to gravity. If, when climbing, the propeller thrust is at such an angle as to tend to haul the aeroplane upwards, then it is, in a measure, acting as a helicopter, and that means inefficiency. The reason of a helicopter being inefficient in practice is due to the fact that, owing to mechanical difficulties, it is impossible to construct within a reasonable weight an air-screw of the requisite dimensions. That being so, it would be necessary, in order to absorb the power of the engine, to revolve the comparatively small-surfaced air screw at an immensely greater velocity than that of the aeroplane's surface. As already explained, the lift-drift ratio falls with velocity on account of the increase in passive drift. This applies to a blade of a propeller or air-screw, which is nothing but a revolving surface set at angle of incidence, and which it is impossible to construct without a good deal of detrimental surface near the central boss.
4. The velocity being low, then it follows that for that reason also the angle of incidence should be comparatively large.
5. Camber.—Since such an aeroplane would be of low velocity, and therefore possess a large angle of incidence, a large camber would be necessary.
Let us now consider the essentials for an aeroplane of maximum velocity for its power, and possessing merely enough lift to get off the ground, but no margin of lift.
1. Comparatively HIGH VELOCITY.