Harvard Carries on Aeronautical Research . . .

Few people know that Harvard is a leader in many aspects of aeronautical engineering. For though the number of men teaching in the field is small, their research achievements have been great.

In line with all study of engineering in the University, aeronautics is pursued mainly as a pure science, rather than as a practical one. Administered under the Division of Applied Sciences, aeronautical engineering follows the same dictum that guides its parent Division: never duplicate the facilities available at M.I.T.

For this reason, the Harvard group specializes in problems of fluid dynamics and aerodynamics. Such practical subjects as aircraft structures can be learned by cross registration at the school down the river.

Supersonic Wind Tunnel

Though the University does not have the vast Guggenheim Memorial Laboratory, it does have the world's first supersonic speed wind tunnel. From this wind tunnel and other equipment have come experimental data that revolutionized jet engine design.

The tunnel was built with funds provided by the Pratt and Whitney Company, aircraft engine and propeller manufactures, for use in discovering the aerodynamic characteristics of the rotors used in the compressors of jet engines.

The joint Harvard-Pratt and Whitney project was begun almost a decade ago, when design work on jet engines was just getting under way.

The goal of the project was to find out what went on inside the compressor of a jet engine. At the time the project was begun, this seemed like an impossible task. Jet engine compressors may have a dozen or so rotating "fans" and a dozen or so stationery ones. The air flow through the compressor is amazingly complex.

But engineers suspected that the compressor was the most critical part of the engine, and suspected that small changes in compressor performance would mean large improvements in the overall engine performance.

Three-Part Tunnel

Pratt and Whitney provided three pieces of equipment, the tunnel, an axial flow compressor rig, and a centrifugal compressor rig. The tunnel is divided into three parts. The first, or front, is for subsonic speeds and is used to calibrate the instruments used in the testing program. This subsonic section, with a diameter of six inches, narrows down to a throat--shaped section where supersonic speeds are produced. Behind this is a subsonic cascade tunnel, used in the rotor tests.

One of the most significant results pro- duced by the apparatus is that a much clearer understanding of the flow phenomenan called "surge" has been gained. When air is sucked into the compressor of a jet engine, there are certain air speeds at which the whole air mass pulsates. "Surge is a disaster when it occurs," according to Richard Kronanuer, instructor in Applied Science and a researcher on the project. "When the air in the jet engine begins to pulsate, the pilot has only one choice: he must land or have his engine put out of commission."

One result of the project was the elimination of surge in a centrifugal compressor produced a few years ago. The improvement of jet engine compressors is tremendously important because of the vast amount of work the engine does to drive the compressor compared with the work done in driving the airplane forward. Thus, a one per cent increase in rotor efficiency means a five percent increase in total engine efficiency.

New Techniques

Other important results have been the development of new instrumentation techniques. Probably the most unique among these is the hot wire aenomometer, a device which measures the velocity of an air stream by measuring the cooling effect of the stream on a hot wire. With this instrument, the phenomenon called rotating stall, where the air flow across a rotor blade is destroyed, was first discovered here.

Director of the Pratt and Whitney-Harvard research project is Howard W. Emmons, Gordon McKay Professor of Mechanical Engineering, Emmons is presently in England on a Fulbright grant and his place is being taken by Carl J. Pearson, Instructor in Applied Mechanics. Pearson did fundamental work on "surge"

All apparatus used in the project, though originally provided by Pratt and Whitney, is now the property of the University. The supersonic tunnel has been used in a variety of different ways, such as the study of super-sonic shock waves.The axial flow compressor shown here supplies volumes of air up to 30,000 cubic feet per minute to the wind tunneis. Besides the axial flow compressor, there is a centrifugal compressor. Axial flow has largely taken the place of centrifugal flow on the power plants that drive modern jet aircraft. The large size of the above compressor can be observed by comparing it with the boards on the floor of the laboratory.