Where V = velocity in miles per hour, D and H remaining as before. The total power for the entire machine would involve the sum of the wing and structural drags, with D equal to the total resistance of the machine.

Example. The total weight of an aeroplane is found to be 3,000 pounds. The lift-drag ratio of the wings is 15.00, and the speed is 90 miles per hour. Find the power required for the wings alone.

Solution. The total drag of the wings will be: D = 3,000/15 = 200 pounds. The horsepower required: H = DV/375 = 200 × 90/375=48 horsepower. It should be remembered that this is the power absorbed by the wings, the actual motor power being considerably greater owing to losses in the propeller. With a propeller efficiency of 70 per cent, the actual motor power will become: Hm = 48/0.70=68.57 for the wings alone. To include the efficiency into our formula, we have:

H = DV/375E

where E = propeller efficiency expressed as a decimal. The greater the propeller efficiency, the less will be the actual motor power, hence the great necessity for an efficient propeller, especially in the case of pusher type aeroplanes where the wings do not gain by the increased slip stream.

The propeller thrust must be equal and opposite to the drag at the various speeds, and hence the thrust varies with the plane loading, wing section, and angle of incidence. Portions of the wing surfaces that lie in the propeller slip stream have a greater lift than those lying outside of this zone because of the greater velocity of the slip stream. For accurate results, the area in the slip stream should be determined and calculated for the increased velocity.

Oftentimes it is desirable to obtain the "Unit drag"; that is, the drag per square foot of lifting surface. This can be obtained by dividing the lift per square foot by the lift-drag ratio, care being taken to note the angle at which the unit drag is required.

Advantages of Cambered Sections Summarized. Modern wing sections are always of the cambered, double-surface type for the following reasons:

  1. They give a better lift-drag ratio than the flat surface, and therefore are more economical in the use of power.
  2. In the majority of cases they give a better lift per square foot of surface than the flat plate and require less area.
  3. The cambered wings can be made thicker and will accommodate heavier spars and structural members without excessive head resistance.

Properties of Modern Wings. The curvature of a wing surface can best be seen by cutting out a section along a line perpendicular to the length of the wing, and then viewing the cut portion from the end. It is from this method of illustration that the different wing curves, or types of wings, are known as "wing sections." In all modern wings the top surface is well curved, and in the majority of cases the bottom surface is also given a curvature, although this is very small in many instances.