Aeroplane Construction and Operation

by John B. Rathbun

When performed in connection with a wind tunnel, or based on dependable data from other sources, full size wing tests are very instructive and useful if care is taken to have the tests conducted under uniform and known conditions. Many full size experiments of this nature have been carried out by Saint-Cyr University in France, and by the Royal Aircraft Factory in Great Britain. Both of these institutions have a wind tunnel and an almost unlimited fund of performance data, and last but not least, have the services of skilled observers.

At Saint-Cyr, the full size wings, or the entire machine, are carried on an electric car or "chariot." The speed of the car, the lift and drag, can be determined at any moment during the run through suitable recording devices. Actual flying tests have also been made, the measurement of the propeller thrust giving the drag, while the lift is known as being equal to the weight of the machine. The R.A.F. have carried out a very extensive series of flight tests, the experiments on the old "B. E.-2" probably being the best known.

The greater part of the experiments performed with the car at Saint-Cyr differed considerably from the results obtained by model tests, and apparently these differences followed no specific law. According to theory, and the results obtained by different laboratories, the performance of a full size wing should be better than with a model, but the Saint-Cyr tests showed that such was not always the case. The center of pressure movement differed in almost every case, and as a direct result, the pressure distribution of the large wings was materially different than with the model. The lift-drag ratio results varied, sometimes being better for the model than for the large wing. These differences can probably be explained as being due to variation in air currents, side winds, etc.

Model Tests. Since lift and resistance are due to relative motion between a body and the air stream, a model can either be towed through the air, or it can be held stationary while the air is forced past it. There has been some controversy on the relation between the results obtained by the two methods, but for the present we will accept the common belief that the results obtained by either method are the same. In testing ship models, they are always towed through the tank, but in the case of aero-dynamic bodies this is complicated and not desirable. In towing models through the air a very high velocity is needed and this necessitates either a very long track or a short time length for making the observations. Again, it is almost impossible to avoid errors because of vibration, inequality of movement due to uneven track, or air eddies caused by differences in temperature and by the movement of the towing device. In fact, the same difficulties apply to towed model tests as to the full size "electric chariot."

The whirling arm method of testing as used by Langley, Maxim-Vickers, and others, is a form of "towed testing," but is also open to serious objections. Unless the arm is very long, every part of the model surface will not move at the same velocity, the outer portions moving the faster. As the forces produced by an air stream vary as the square of the velocity, this may introduce a serious error. The fact that the body passes repeatedly over the same path introduces error, as the body after the first revolution is always working in disturbed air. The centrifugal force, and the currents set up by the arm itself all reduce the accuracy of the method.

When a model is placed in a uniform current of air in a properly designed channel or tunnel, the greater part of the errors due to towed tests are eliminated. The measuring instruments can be placed on a firm foundation, the air stream can be maintained at a nearly uniform speed and with little error due to eddies, and the test may be continued under uniform conditions for an indefinite period. While there are minor errors due to wall friction and slight variations in the velocity at different points in the cross section of the tunnel, they are very small when compared to the errors of towing. For this reason the wind tunnel is the accepted means of testing.

Eiffel's Wind Tunnel. The Eiffel Laboratory at Auteuil, France, is probably one of the best known. The results in Chapters III and V were obtained in this laboratory and thousands of similar experiments have been carried out at this place. Two tunnels, a large and small, are placed side by side in the main laboratory room, the tunnels being supported midway between the floor and ceiling. The air is drawn from this room into an airtight experimental chamber through a bell-mouthed circular opening. A grill or honeycomb baffle is placed in the opening to straighten out the flow, and from this point the air passes across the chamber and exits through a circular duct to the suction side of a large fan. From the fan the air is discharged into the room. The same air thus circulates through the tunnel continuously. The test chamber is considerably wider than the openings so that the walls do not influence the flow around the model. A cylinder of air passes through the chamber at a remarkably uniform velocity, and without any appreciable eddies. Diameter of the stream approximates 6.6 feet in the large tunnel and 3 feet in the smaller. In the large tunnel the maximum velocity is 105 feet per second, and 131 feet per second is attained in the smaller. A 50-horsepower electric motor is used with a multiblade fan of the "Sirocco" type.

The observer and weighing mechanism are supported above the air stream on a sliding floor, and a standard extends from the model in the wind stream to the balances on the weighing floor. These balances determine the lift and drag of the models, the center of pressure, etc.

The N. P. L. Tunnel. The National Physical Laboratory at Teddington, England, has a remarkably complete and accurate aerodynamic equipment. This consists of a large tunnel of 7 square feet area, a small tunnel of 4 square feet, and a whirling table house. The large tunnel is 80 feet in length with an air flow of 60 feet per second, the air being circulated by a four-bladed propeller driven by an electric motor of 30 horsepower. The velocity is uniform within one-half per cent, and the most accurate of results have been obtained. The smaller tunnel is about 56 feet long and the wind velocity is about 40 miles per hour maximum. The propeller revolves at 600 revolutions and is driven by a 10-horsepower electric motor. There is no chamber and the models are suspended in the passage half way between the "Diffuser" in the entering end, and the baffles in the exit. The Massachusetts Institute of Technology, and the Curtis Aeroplane Company both have similar tunnels.

United States Navy Tunnel. In this tunnel the air is confined in a closed circuit, the return tunnel being much larger than the section in which the tests are performed. The cross-sectional area is 8 square feet at the point of test, and the stream is uniform within 2 per cent. The balance and controls are mounted on the roof of the tunnel, with an arm extending down through the air stream to the model, as in the Eiffel tunnel. The balance is similar to Eiffel's and is sensitive to less than 2/1000 pound. A velocity of 75 miles per hour may be attained by the 500-horsepower motor, but on account of the heating of the air stream through skin friction, the tests are generally made at 40 miles per hour. Models up to 36-inch span can be tested, while the majority of models tested at M. I. T. are about 18 inches.