The body resistance is by far the greatest item. A great part of the body resistance can be attributed to the motor cooling system, since in either case it is diverted from the true streamline form in order to accommodate the radiator, or the rotary motor cowl. The body resistance is also influenced by the necessity of accommodating a given cargo or passenger-carrying capacity, and by the distance of the tail surfaces from the wings. A body is not a streamline form when its length greatly exceeds 6 diameters.
Calculation of Total Resistance. The nearest approach that we can make to the actual head resistance by means of a formula is to adopt an expression in the form of R = KV² where K is a factor depending upon the size and type of machine. The true method would be to go over the planes and sum up the individual resistance of all the exposed parts. The parts lying in the propeller slip stream should be increased by the increased velocity of the slip stream. The parasitic resistance of biplanes weighing about 1800 pounds will average about, R = 0.036V² where V = velocity in miles per hour. Biplanes averaging 2500 pounds give R = 0.048V². Machines of the training or 2-seater type weigh from 1800 to 2500 pounds, and have an average head resistance distribution as follows:
| Body, radiators, shields | 35.5 percent. |
| Tail surface and bracing | 14.9 " |
| Landing gear | 17.2 " |
| Interplane struts, wires and fittings | 23.6 " |
| Ailerons, aileron bracing, etc. | 8.8 " |
The averages in the above table differ greatly from the values given for the high speed fighting machine, principally because of the large control surfaces used in training machines, and the difference in the size of the motors.
With the wing drag being equal to D = Kx AV², and the total parasitic resistance equal to R = KV², the total resistance can be expressed by Rt = KxAV² + KV², where K = coefficient of parasitic resistance for different types and sizes of machines. The value of K for training machines will average 0.036, for machines weighing about 2500 pounds K = 0.048. Scouts and small machines will be safe at K = 0.028. The wing drag coefficient Kx varies with the angle of incidence and hence with the speed. For example, we will assume that the wing drag (Kx) of a scout biplane at 100 miles per hour is 0.00015, that the area is 200 square feet, and that the parasitic resistance coefficient is K = 0.028. The total resistance becomes: R = (0.00015 x 200 x 100 x 100) + 0.028 x 100 x 100 = 300 + 280 = 580 pounds. The formula in this case would be R = KxAV² + 0.028V².
Strut Resistance. The struts are of as nearly streamline form as possible. In practice the resistance must be compromised with strength, and for this reason the struts having the least resistance are not always applicable to the practical aeroplane. From the best results published by the N. P. L. the resistance was about 12.8 pounds per 100 feet strut at 60 miles per hour. The width of the strut is 1 inch. A rectangular strut under the same conditions gave a resistance of 104.4 pounds per 100 feet. A safe value would be 25 pounds per 100 feet at 60 miles per hour. If a wider strut is used, the resistance must be increased in proportion. With a greater speed, the resistance must be increased in proportion to the squares of the velocity. When the struts are inclined with the wind, the resistance is much decreased, and this is one advantage of a heavy stagger in a biplane.
The "Fineness ratio" or the ratio of the width to the depth of the section has a great effect on the resistance. With the depth equal to twice the width measured across the stream, a certain strut section gave a resistance of 24.8 pounds per 100 feet, while with a ratio of 3.5 the resistance was reduced to 11.4 pounds per hundred feet. Beyond this ratio the change is not as great, for with a ratio of 4.6 the resistance only dropped to 11.2 pounds.
Radiator Resistance. For the exact calculation of the radiator resistance it is first necessary to know the motor power and the fuel consumption since the radiator area, and hence the resistance, depends upon the size of the motor and the amount of heat transmitted to the jacket water. An aeronautic motor may be considered to lose as much through the water jackets as is developed in useful power, so that on this basis we should allow about 1.6 square feet of radiation surface per horsepower. This figure is arrived at by J. C. Hunsaker and assumes that the wind speed is 50 miles per hour (73 feet per second). The most severe cooling condition is met with in climbing at low speed, and it is here assumed that 50 miles per hour will represent the lowest speed that would be maintained for any length of time with the motor full out. For a racing aeroplane that will not climb for any length of time, one-half of the surface given above will be sufficient, and if the radiator is placed in the propeller slip stream it can be made relatively still smaller as the increased propeller slip at rapid rates of climb partially offsets the additional heating.
In the above calculations, Hunsaker does not take any particular type of radiator into consideration, merely assuming a smooth cooling surface. The Rome–Turney Company states that they allow 1.08 square feet of cooling surface per horsepower for honeycomb radiators, and 0.85 square feet for the helical tube type. The surface referred to means the actual surface measured all over the tubes and cells, and does not refer to the front area nor the exterior dimensions of the radiator. While a radiator may be made 25 percent smaller when placed in the slipstream, the resistance is increased by about 25 per cent, with a very small saving in weight, hence the total saving is small, if any. Side mounted radiators have a lower cooling effect per square foot than those placed in any other position, owing to the fact that the air must pass through a greater length of tube than where the broad side faces the wind.
In the radiator section tested by Hunsaker, there were about 64 square feet of cooling surface per square foot of front face area, but for absolute assurance on this point one should determine the ratio for the particular type of radiator that is to be used. The Auto Radiator Manufacturing Corporation, makers of the "Flexo" copper core radiators, have published some field tests made under practical conditions and for different types and methods of mounting. The four classes of radiators described are: (1) Front Type, in which the radiator is mounted in the end of the fuselage; (2) Side Type, mounted on the sides of the body; (3) Overhead Type, mounted above the fuselage and near the top plane; (4) Over-Engine Type, placed above and connected directly to the motor, as in the Standard H-3.