AERONAUTIC RESEARCH.1

By JOSEPH S. AMES, Ph. D., LL. D.

Professor of Physics, The Johns Hopkins University, Baltimore, Maryland; Chairman, Executive Committee, National Advisory Committee for Aeronautics.

[With 5 plates.]

Progress in the navigation of the air is being made constantly along two quite distinct and independent lines; one is the art of flying and the other is the science of flight. To this last is given the name aeronautics; and by an aeronautical investigation is meant a research which has a direct bearing upon our knowledge of the properties of solid bodies immersed in a stream of air or moving through the air. We wish to know the forces and moments acting upon such solid bodies; how these vary with the shape and characteristics of the bodies, and how they are affected if the velocity of the air is changed. Another important inquiry refers to the stability of the solid body when in flight; if the attitude of the body is changed by some gust or otherwise, does it tend to return to its previous attitude, or on the contrary does it continue to depart more and more from its original attitude? The question is like that referring to a body balanced on a table. If it is pushed slightly, will it simply oscillate to and fro, or will it turn over? These matters and similar ones make up the subject of aeronautics; and in order to investigate them the same methods must be applied as in any department of physics.

Experiments must be performed; a theory is evolved; deductions. are made from the theory and tested by experiment; the theory is modified and improved, etc. During all the process knowledge is being gained, and the facts being made known help the designer of aircraft to make improvements in speed, in carrying power, in safety, in stability.

One most important fact should be emphasized, and this is that without the series of scientific studies just outlined not only would flight itself have been impossible, but also all progress in the art would

Presented at the meeting of the Section of Physics and Chemistry of the Franklin Institute held Thursday, Oct. 6, 1921. Reprinted by permission from the Journal of the Franklin Institute, January, 1922.

cease.

Scientific investigation forms the most important feature of aviation, and it can be conducted only by trained students. The best pilot in the world may know very little about the scientific principles underlying flight, and he would therefore be unable to make any marked improvements in his machine. Aeronautics is in no sense a function of an engineer or constructor or aviator, it is a branch of pure science. Those countries have developed the best airships and airplanes which have devoted the most thought, time and money to the underlying scientific studies. When the physical facts are known, the engineer can design his aircraft, the constructor can make it, and the trained man can fly it; but the foundation stone is the store of knowledge obtained by the scientist.

Before describing the types of investigations in progress in aeronautics and the methods pursued, it may be interesting to see some illustrations of the two types of aircraft now in use-the airship and the airplane. If I had time, I would like very much to say something about the helicopter, a type of aircraft to which we have given a name before constructing one. Up to the present no such machine, worthy of the name, has been made; but beyond a doubt one will be constructed, and in the near future.

An airship owes its flying power to the fact that it is made "lighter than air" by being filled with a gas lighter than air. Hydrogen is the gas always used, although helium may be. The lifting power of the latter gas is about four-fifths of that of hydrogen. As the airship moves through the air, it meets with

opposition as the air flows along its surface. Forces are required to move the control surfaces, i. e., the rudders and elevators. We must determine these forces, and must investigate the changes in them as we change the shape of the airship, e. g., its length or its cross-section. A great deal may be learned by studying theoretically the way in which air flows around a solid body shaped more or less like an airship. A most interesting mode of attack on this problem was devised by Admiral Taylor of our Navy, and was applied by him to the design of ocean vessels. The drawings illustrate how a uniform flow superposed upon a source and sink produces a condition like the flow around an airship. This type

FIG. 2.-Air flow due to superposing a uniform rectilinear flow upon a combination of source and sink.

of flow may then be studied mathematically; the pressures may be deduced, etc.

Similarly, in the case of airplanes, we must know the character of airflow past the struts, the fuselage, and the wings. It is the difference in the pressure on the two sides of a wing that produces the upward force required to support the machine. The figures on plate 3 illustrate the type of flow around the aerofoil.

Great progress has been made in recent years by Prandtl and other German physicists by showing how a flow of air around an aerofoil could be produced in an ideal frictionless gas similar to that observed in air by imagining vortices or whirls in the gas. The method is not unlike that mentioned above as useful in the case of airships, only vortices are used in place of sources and sinks.

Following this process, Prandtl and his associates have shown how one could calculate the influence of one wing of a biplane upon the other, so that if the behavior of one wing is known that of two may be deduced; and they have proceeded much further and made aeronautics into a beautiful theoretical science.

But in the end the function of aeronautic research is obviously to learn all there is to be known about the forces acting on an aerofoil or wing. How can this be done? It is not possible to make actual airplanes of various types and test them, nor would this help us much if it were practicable. We must actually measure the forces involved in any one case, and must vary our conditions in every conceivable way, but in a systematic manner. There are several methods open to us. One is to make a model of a wing, say, as nearly full size as possible, suspend it by wires 30 feet or

FIG. 3.-Diagram of flow past an aerofoil showing an increased pressure below and a decreased one above.

more below an airplane which can carry it in flight, and then, by inserting measuring instruments in the wires, study the forces under varying conditions. This method is being used with marked success at Langley Field, near Old Point Comfort, Va., by the staff of the National Advisory Committee for Aeronautics. It is called the "free-flight method."

A second method is to make a small model of a wing or a fuselage or an entire machine, say, one twenty-fifth the size of the actual part, and then to investigate the forces acting on it when it is placed in a rapidly moving stream of air. This is known as the wind-tunnel method, and is now in general use in all countries. England has 10 or more such tunnels, France has several, Germany has a large number, etc. In this country there are 12, and more are being made.

Other methods have been used in the past but are no longer. Langley attached his models to one end of a long arm which could

be made to revolve rapidly in a horizontal plane. Others have studied models of different shapes by dropping them from heights inside buildings, so as to avoid wind.

Air Stream

LA

D

Chord

Angle of Attack

FIG. 4.-Diagram illustrating forces acting on an aerofoil.

Owing to the importance of wind tunnels, I have introduced plates 4 and 5, which show some photographs of the latest one made in this country, that of the National Advisory Committee for Aeronautics at the Langley Memorial Laboratory, Langley Field, Va.

FIG. 5.-Diagram of wind-tunnel in its house.

Investigators in these different laboratories all over the world have studied the greatest variety of problems-forces on models of wings of different shapes at different air velocities, forces on models of airships, the effect of disturbing elements on these forces, etc. A long series of investigations has been made in particular upon air propellers. The blade of a propeller may be considered as made