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.
2. A comparatively _small surface_, because, being of greater velocity than the maximum climber, a greater mass of air will be engaged for a given surface and time, and therefore a smaller surface will be sufficient to secure the requisite lift.
3. _A small angle relative to the propeller thrust_, since the latter coincides with the direction of motion.
4. A comparatively _small angle of incidence_ by reason of the high velocity.
5. A comparatively _small camber_ follows as a result of the small angle of incidence.
[Illustration: ANGLES OF INCIDENCE (INDICATED APPROXIMATELY) OF AN AEROPLANE DESIGNED AS A COMPROMISE BETWEEN VELOCITY AND CLIMB, AND POSSESSING A SLIGHT MARGIN OF LIFT AT A LOW ALTITUDE AND WHEN THE THRUST IS HORIZONTAL.]
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.
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.
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.