Aeroplane Construction and Operation

by John B. Rathbun

Burgess Seaplane Scout.

Calculation of Area: Let us assume that we are confined to the use of an aspect ratio of 6, a speed of 50 miles per hour, weight = 2500 pounds, and wish to obtain the area that will give the most efficient surface (Least lift-drag ratio.) The equation can be now transposed so that the area = A = KyV². On examination of the table it will be seen that the greatest lift-drag ratio is 6.4 at 5 degrees, and that the Ky at this angle is 0.00103. Substituting these values in the equation for area, we have A = L/KyV² = 2500/000103 x (50 x 50) = 971 square feet.

Wind Tunnel at Washington Navy Yard in Which the Air Circulates Continuously Through a Closed Circuit

CHAPTER IV. EXPERIMENTAL LABORATORIES.

Test Methods in General. As already explained, the behavior of a body in an air stream cannot be predicted with any certainty by direct mathematical calculation, and for this reason, each and every aerodynamic body must be tested under conditions that are as nearly similar to the actual working conditions as possible. Prior to Professor Langley's first experiments in 1887, mechanical flight with a heavier than air machine was derided as an impossibility, even by such scientists as Navier, Von Helmotz, Gay-Lussac, and others, who proved by the most intricate calculations that a body larger than a bird could not be supported by its own energy. Such calculations were, of course, based on a wrong understanding of air flow, and as no experimental work had been done up to that time, the flow was assumed according to the individual taste and belief of the demonstrator. The presence of a vacuum on the back of a plate was not understood, and as this contributes full two-thirds of the lift, it is an easy matter to see why all of the early predictions fell short of the actual lifting forces. To quote one classic absurdity, the scientist Navier proved mathematically that if mechanical flight were possible, then 17 swallows would be capable of developing one horsepower.

In spite of these discouraging computations, Langley proceeded with a very carefully conducted series of experiments, first investigating the laws of surface sustenation on various forms of plates, and when the data collected was sufficient for his needs, he started to construct a number of model flyers with various wing arrangements and aerofoil forms. It was Langley's experiments upon aerofoils that cleared the way for the Wright Brothers, who started a further and more complete investigation in 1896. Experiments were made on the effect of curvature, aspect ratio and angle of incidence, and the results obtained in their "wind tunnel" were afterwards applied to their successful full size machine. During 1901 to 1902 the Wrights investigated the properties of at least 100 different aerofoil forms. Both Langley's and Wrights’ experiments were with models, although they were made in a different manner. It was in this way that experimental evidence gained precedence over theory.

Langley's specimens were mounted at the end of a revolving arm, so that with the arm revolving, a relative air stream of known velocity could be had. The aerofoil was mounted in such a way that the lift and drag could be measured. In the early experiments of the Wrights, the models were placed in an enclosed channel through which a stream of air was maintained by a fan. The model was attached to a balance system so that the lift and resistance could be measured. This is what is now known as a "wind tunnel," and at present is almost exclusively used in model tests. Several investigators immersed their model aerofoils in running water so that the direction of flow could be visibly observed. While this latter method is of great service in determining disturbances, stream line flow, and general characteristics, it is qualitative rather than quantitative, and cannot be used in obtaining accurate numerical results. A more accurate method of mapping out the direction of flow, eddies, etc., is to introduce smoke into the air stream.

Full Size Experiments. The old "rule of the thumb" method of building a full size machine without model test data or other experimental evidence to begin with has seen its day. It is not only exceedingly expensive, but is highly dangerous, and many a flyer has met his death in the endeavor to work out untried principles on a full size machine. The first cost of the machine, the continual breakage and operating expense, to say nothing of the damage suits and loss of time, make a preliminary full size tryout an absurdity at the present time. Again, the results of full size experiments are not always reliable, as so much depends upon the pilot and weather conditions. The instruments used on a large machine are far from being as accurate as those used in model tests. These are also likely to be thrown out of adjustment unknowingly by falls or collisions. The great number of variables that enter into such a test make it almost an impossibility to obtain accurate data on the result of minor alterations, and, in fact, it is almost impossible to get the same results twice without further alterations than changing the pilot. Full scale tests are necessary after sufficient data has been obtained and applied in a scientific manner to the design of the machine, but successful performance cannot be expected from a powered machine built by guess work.