Aeroplane Construction and Operation

by John B. Rathbun

Resistance of Seaplane Floats. The usual type of seaplane with double floats may be considered as having about 12 per cent higher resistance than a similar land machine. Some forms of floats have less resistance than others, owing to their better streamline form, but the above figure will be on the safe side for the average pontoon. Basing our formula on a 12 per cent increase on the total head resistance, the formula for the floats and bracing will become: Rt = 0.00436V² where R1 = resistance of floats and fittings.

Body Resistance. This item is probably the most difficult of any to compute, owing to the great variety of forms, the difference in the engine mounting, and the disposition of the fittings and connections. The resistance of the pilot's and passenger's heads, wind shields, and propeller arrangement all tend to increase the difficulty of obtaining a correct value. Aeroplanes with rotary air-cooled motors, or with large front radiators have a higher resistance than those arranged with other types of motors or radiator arrangements. Probably the item having the greatest influence on the resistance of the fuselage is the ratio of the length to the depth, or the "fineness ratio." In tractor monoplanes and biplanes, of the single propeller type, the body is in the slipstream, and compensation must be made for this factor.

If it were not for the motor and radiator, the tractor fuselage could be made in true dirigible streamline form, and would therefore present less resistance than the present forms of "practical" bodies. The necessity of placing the tail surfaces at a fixed distance from the wings also involves the use of a body that is longer in proportion than a true streamline form, and this factor alone introduces an excessive head resistance. The ideal ratio of depth to length would seem to range from 1 to 5.5 or 1 to 6. The fineness ratio of the average two-seat tractor is considerably greater than this, ranging from 1 to 7.5 or 8.5. A single-seat machine of the speed-scout type can be made much shorter and has more nearly the ideal proportions.

The only possible way of disposing of this problem is to compare the results of wind tunnel tests made on different types of bodies, and even with this data at hand a liberal allowance should be made because of the influence of the connections and other accessories. Eiffel, the N. P. L., and the Massachusetts Institute of Technology have made a number of experiments with scale models of existing aeroplane bodies. It is from these tests that we must estimate our body resistance, hence a table of the results is attached, the approximate outlines being shown by the figures.

As in calculating the resistance of other parts, the resistance of the body can be expressed by R = KxAV², where Kx = coefficient of the body form, A = Cross-sectional area of body in square feet (Area of presentation), and V = velocity in miles per hour. The area A is obtained by multiplying the body depth by the width. The "area of presentation" of a body 2’ 6" wide and 3’ 0" deep will be 2.5 x 3 = 7.5 square feet.

The experimental data does not give a very ready comparison between the different types, as the bodies not only vary in shape and size, but are also shown with different equipment. Some have tail planes and some have not; two are shown with the heads of the pilot and passenger projecting above the fuselage, while the remainder have either a simple cock-pit opening or are entirely closed. The presence of the propeller in two cases may have a great deal to do with raising the value of the experimental results. The propeller was stationary during the tests, but it was noted that the resistance was considerably less when the propeller was allowed to run as a windmill, driving the motor. This latter condition would correspond to the resistance in gliding with the motor cut off. In all cases, except the Deperdussin, the bodies are covered with fabric, and the sagging of the cloth in flight will probably result in higher resistance than would be indicated by the solid wood or metal model used in the tests. The pusher type bodies give less resistance than the tractors, but the additional resistance of the outriggers and tail bracing will probably bring the total far above the tractor body.

In the accompanying body chart are shown 7 representative bodies: (a) Deperdussin Monocoque Monoplane Body, a single-seater; (b) N. P. L.-5 Tractor Biplane Body, single-seater; (c) B. F.-36 Dirigible Form, without propeller or cock-pit openings; (d) B. E.-3. Two-Place Tractor Body, with passenger and pilot; (e) Curtiss JN Type Tractor Body, with passengers, chassis and tail; (f) Farman Pusher type, with motor, propeller and exposed passengers; (g) N. P. L. Pusher Body, bare. Body (a) was tested with a 1/5 scale model at a wind tunnel speed of 28 meters per second, the resistance of the model being 0.377 kilograms (0.83 pounds). Body (d) in model form was 1/16 scale and was tested at 20.5 miles per hour, at which speed the resistance was 0.0165 pounds. Model (e) was 1/12 scale and was tested at 30 miles per hour. These varying test speeds, it will be seen, do not allow of a very accurate means of comparison. The resistance of model (e) was 0.1365 pounds at the specified wind-tunnel air speed.

The speeds given in the above table are simply translational speeds, and are not corrected for slipstream velocity. With a slipstream of 25 per cent, increase the body resistance by 40 per cent. It would be safe to add an additional 10 per cent to make up for projecting fittings, baggy fabric, and scale variations.

Since a body of approximately streamline form has a considerable percentage of skin friction, scale corrections for size and velocity are even of more importance than with wing sections. No wind-tunnel experiments can determine the resistance exactly because of the uncertainty of the scale factor. The resistance as given in the table is also affected by the proximity of the wing and tail surfaces, and by projections emanating from the motor compartment. It will be noted that the dirigible form B.F.-36 is markedly better than any of the others, being almost of perfect streamline form. The nearest approximation to the ideal form is N.P.L.-5, which has easy curves, low resistance, and is fairly symmetrical about the center line. Because of their small size, the pusher bodies or "nacelles" have a small total resistance, but the value of Kx is high.