Improvement in the wing characteristics is principally a subject for the wind tunnel experimentalist, since with our present knowledge, it is impossible to compute the performance of a wing by direct mathematical methods. Having obtained the characteristics of a number of wing sections from the aerodynamic laboratory, the designer is in a position to proceed with the calculation of the areas, power, etc. At present this is rather a matter of elimination, or "survival of the fittest," as each wing is taken separately and computed through a certain range of performance.
Wing Loading. The basic unit for wing lift is the load carried per unit of area. In English units this is expressed as being the weight in pounds carried by a square foot of the lifting surface. Practically, this value is obtained by dividing the total loaded weight of the machine by the wing area. Thus, if the weight of a machine is 2,500 pounds (loaded), and the area is 500 square feet, the "unit loading" will be: w = 2,500/500 = 5 pounds per square foot. In the metric system the unit loading is given in terms of kilogrammes per square meter. Conversely, with the total weight and loading known, the area can be computed by dividing the weight by the unit loading. The unit loading adopted for a given machine depends upon the type of machine, its speed, and the wing section adopted, this quantity varying from 3.5 to 10 pounds per square foot in usual practice. As will be seen, the loading is higher for small fast machines than for the slower and larger types.
A very good series of wings has been developed, ranging from the low resistance type carrying 5 pounds per square foot at 45 miles per hour, to the high lift wing, which gives a lift of 7.5 pounds per square foot at the same speed. The medium lift wing will be assumed to carry 6 pounds per square foot at 45 miles per hour. The wing carrying 7.5 pounds per square foot gives a great saving in area over the low lift type at 5 pounds per square foot, and therefore a great saving in weight. The weight saved is not due to the saving in area alone, but is also due to the reduction in stress and the corresponding reduction in the size and weight of the structural members. Further, the smaller area requires a smaller tail surface and a shorter body. A rough approximation gives a saving of 1.5 pounds per square foot in favor of the 7.5 pound wing loading. This materially increases the horsepower weight ratio in favor of the high lift wing, and with the reduction in area and weight comes an improvement in the vision range of the pilot and an increased ease in handling (except in dives). The high lift types in a dive have a low limiting speed.
As an offset to these advantages, the drag of the high lift type of wing is so great at small angles that as soon as the weight per horsepower is increased beyond 18 pounds we find that the speed range of the low resistance type increases far beyond that of the high lift wing. According to Wing Commander Seddon, of the English Navy, a scout plane of the future equipped with low resistance wings will have a speed range of from 50 to 150 miles per hour. The same machine equipped with high lift wings would have a range of only 50 to 100 miles per hour. An excess of power is of value with low resistance wings, but is increasingly wasteful as the lift co-efficient is increased. Landing speeds have a great influence on the type of wing and the area, since the low speeds necessary for the average machines require a high lift wing, great area, or both. With the present wing sections, low flight speeds are obtained with a sacrifice in the high speed values. In the same way, high speed machines must land at dangerously high speeds. At present, the best range that we can hope for with fixed areas is about two to one; that is, the high speed is not much more than twice the lowest speed. A machine with a low speed of 45 miles per hour cannot be depended upon to safely develop a maximum speed of much over 90 miles per hour, for at higher speeds the angle of incidence will be so diminished as to come dangerously near to the position of no lift. In any case, the travel of the center of pressure will be so great at extreme wing angles as to cause considerable manipulation of the elevator surface, resulting in a further increase in the resistance.
Resistance and Power. The horizontal drag (resistance) of a wing, determines the power required for its support since this is the force that must be overcome by the thrust of the propeller. The drag is a component of the weight supported and therefore depends upon the loading and upon the efficiency of the wing. The drag of the average modern wing, structural resistance neglected, is about 1/16 of the weight supported, although there are several sections that give a drag as low as 1/23 of the weight. The denominators of these fractions, such as "16" and "23," are the lift-drag ratios of the wing sections.
Drag in any wing section is a variable quantity, the drag varying with the angle of incidence. In general, the drag is at a minimum at an angle of about 4 degrees, the value increasing rapidly on a further increase or decrease in the angle. Usually a high lift section has a greater drag than the low lift type at small angles, and a smaller drag at large angles, although this latter is not invariably the case.
Power Requirements. Power is the rate of doing work, or the rate at which resistance is overcome. With a constant resistance the power will be increased by an increase in the speed. With a constant speed, the power will be increased by an increase in the resistance. Numerically, the power is the product of the force and the velocity in feet per second, feet per minute, miles per hour, or meters per second. The most common English power unit is the "horsepower," which is obtained by multiplying the resisting force in pounds by the velocity in feet per minute, this product being divided by 33,000. If D is the horizontal drag in pounds, and v = velocity of the wing in feet per minute, the horsepower H will be expressed by:
H = Dv / 33,000
Since the speed of an aeroplane is seldom given in feet per minute, the formula for horsepower can be given in terms of miles per hour by:
H = DV / 375