HELICOPTER RESEARCH by Grover Loening Consultant, N.A.C.A. Editors note: Speech delivered before the Second Annual Forum of the American Helicopter Society-Franklin Institute, Philadelphia, on March, 1946. The opinions herein contained are those of the author. Patience and persistence are still greatly needed in the field of helicopter development. We might as well frankly admit that back in 1938 we expected by 1946 to have the sky pretty well filled with direct-lift hovering aircraft. Actually as of today, our industry has as yet, delivered only one commercial privately owned machine. The war, of course, steered us into other courses and may be blamed for a measure of delay in successful development by requiring the Army and Navy Services to concentrate on production of a model which had not yet grown to maturity in order to meet a crying need for something in numbers at the front. Considering the mandatory nature of such requirements and the usual hesitancy of the military mind in new fields, the Services have been remarkably encouraging to the helicopter. We are now getting into our peace-time stride, however, and can take stock for a moment to see where we are: over 500 helicopters have been built in the United States-not only more than all the world put together, but the only production of any consequence. In our country we have amassed a record of 40,000 hours of flying. Lately the Sikorsky R-5 type garnered the world's records, establishing maximum high speed at 114.6 mph and altitude at 21,000 feet with little payload, but still doing 105 mph at 11,000 feet with 1650 lbs. payload. Another milestone was accomplished by the Bell helicopter recently, when the two-seater Bell design was awarded a C.A.A. airworthiness certificate-the first of its kind to put helicopters into those coveted classes of aircraft that drop the "X" and proudly wear the "NC." In the path of our progress many other milestones mark our American way. Piasecki has put the tandem rotor type successfully into the air. Kellett has continued his progress with daring, originality and competence; Platt-LePage and McDonnell have explored exhaustively the twin-rotor type. Many other experimenters, almost a score of them, have stepped into new helicopter realms-coaxial types, reaction propelled types and various other configurations. But we cannot escape the conclusion that there is much stumbling in the dark. We did not realize until research lighted the way, how important blade surface finish and shape maintenance really was. Nor did we know how delicate the chord wise balance of forces would prove to be. In fact we seem only now to be realizing that structurally the helicopter blade tip is really a 400 mile-an-hour airplane, and its ability to hold its shape, angle, rigidity, and balance is in the same class as a "Mustang" tail surface. We have perhaps failed to date to organize our fundamental concept of the aerodynamic problem to be solved into the right path. The helicopter problem of hovering or lifting loads was so easily solved and enthusiastically publicized that we lost sight of the real line to attack. The real problem of helicopter development-the tough one and the commanding one-is the aerodynamic $64.00 question of how to drive a rotor edgewise through the air at high speeds and do so without excessive instabilities, vibrations or stresses and at a reasonable expenditure of power. Our thoughts, our research, our very language should concentrate on the fast horizontal translation of a turning lifting rotor. The problem is posed thus because of that one overwhelming requirement of speed in travel, without which no aircraft will prove itself of any serious wide utility. The cruising speeds of present-day helicopters are too slow. No aircraft comes into its own that has not at least 150 miles per hour cruising speed. We are far from that as yet. And our limited market will prove that we are. Not only are our present-day helicopters too slow, they are also too hard to fly. And we need much more knowledge of the source and nature of vibrations in order to smooth them out to a usable level. Fortunately, there are as yet so many untrodden paths that need exploring and solution, that even now helicopter development must be considered in a very primitive state and the wise would not appraise it yet-it is too early. There is the development of the tip jet drive, now giving an entirely different and very promising slant to this field. The N.A.C.A. laboratory at Cleveland, is doing much work along this line to investigate the fundamentals, now under the direction of Abe Silverstein, and already many problems have come to light and solutions worked on. The mathematical and theoretical fundamentals are clear and workable - many mechanical, heat, and structural problems remain to face our best scientific brains. Other new configurations, coaxial effects, and rigid blade forces challenge us at every turn. Furthermore, we are, in great need of strain gauge testing and spanwise load distribution determinations. These we could not do until we had metal blades. We now are about ready to test such blades. The N.A.C.A., mindful of all these mysteries to be solved, (Continued on Page 45)