Minimum Angle.
This gives the greatest velocity during horizontal flight at a low altitude. Greater velocity would be secured if the surface, angle, and camber were smaller and designed to just maintain horizontal flight with a horizontal thrust. Also, in such case, the propeller would not be thrusting downwards, but along a horizontal line which is obviously a more efficient arrangement if we regard the aeroplane merely from one point of view, i.e., either with reference to velocity or climb.
Optimum Angle. (Thrust horizontal).
The velocity is less than at the smaller minimum angle, and, as aeroplanes are designed to-day, the area and angle of incidence of the surface is such as to secure a slight ascent at a low altitude. The camber of the surface is designed for this angle of incidence and velocity. The lift-drift ratio is best at this angle.
Best Climbing Angle.
The velocity is now still less by reason of the increased angle producing increase of drift. Less velocity at a given angle produces less lift, but the increased angle more or less offsets the loss of lift due to the decreased velocity; and, in addition, the thrust is now hauling the aeroplane upwards.
Maximum Angle.