ically more complex than the single rorored craft and has the same inflexibility in center of gravity location. 3. LATERAL AND TANDEM CONFIGURATIONS - A comparison of these two configurations shows them about even in hovering flight, but as they start forward, the lateral configuration, having the larger span in the direction of motion, will climb faster, while at top speed the structure of the tandem configuration will produce less drag and go faster. Both have more flexibility in center of gravity location than any of the other types, but the tandem configuration has the greater usable space for disposable load for, it is conceivable that the entire disposable load could be moved any place in the fuselage between the two rotors and the ship could still be flown. 4. INTERMESHING ROTORS-This configuration allows the same torque balance for stability, and increase in thrust for the amount of torque that the coaxial has. It gains over the coaxial, however, in having the rotor centers covered by the intermeshing of the blades. In spite of these many approaches, we have today a machine that will carry a load, that will do a job of work, that is controllable and maneuverable far beyond the possibilities of either the airplane or the autogiro. This in itself is a major accomplishment, after being so long concerned with merely making the machine fly, and it is understandable that the advocates of the helicopter should see in this accomplishment fantastic and sensational possibilities for its immediate use. It is equally obvious, however, why the skeptics view its present limitations with distrust, for as compared with the finished refinement of the present-day airplane, the helicopter has some problems that have not been adequately perfected as yet. Chiefly those concerned with, fall into the following aspects: A. VIBRATION-It is indisputable that all rotary wing aviation has long been plagued by vibration. Yet when one realizes the number of things that can cause this vibration, it is quite conceivable why the problem has not yet been completely solved. For instance, in order to avoid the building up of oscillations in the plane of rotation on a hinged rotor, it is necessary to include a mechanical damping device. Vibrations can be caused if these dampers result in too much restriction, too little restriction, or uneven restriction of the blades. Vibration can also be set up if the blades are out of track (where they are not following in the same flight path), out of pattern (when not evenly spaced), or out of balance (where the blades are not of equal weight). There is, moreover, the possibility of aerodynamic unbalance when any change takes place in the individual aerodynamic characteristics of the blades under load, or where there is deflection of the blades under load, or where there is deflection of the blades caused by dissimilar structural characteristics. Coupled with all this, the entire machine enters the problem since some pylon structures tend to absorb rotor vibrations while others accentuate them. B. STABILITY-Stability, for all intents and purposes can be defined as an inherent tendency for an aircraft to stay in a given flight path with little or no effort on the part of the pilot. The helicopter, as we know it today, does not have that tendency. However, the instability is not so bad that it cannot be corrected by control, and there is at least one helicopter flying today (namely the Bell machine) that has a mechanical stabilizer to take some of the work of correction away from the pilot. C. CONTROL-lf we think of control as we know it in the airplane, we must realize how much greater is the range of flight that is possible for the helicopter. Actually control in the majority of helicopters that are flying today is adequate to overcome all tendencies toward instability and to maneuver the aircraft satisfactorily throughout the flight range. The methods of obtaining that control, however, are numerous and varied, and the problems of each are multiple. For when you realize that control is accomplished by a relationship between the lift vector of the rotor and the center of gravity location of the machine, you can appreciate the many methods that can be used to accomplish this. Basically, control of the pitch angles of the blades is used for both rising and descending as well as for directing the flight of the machine. In other words, to rise or descend, the pitch angles of the blades are changed collectively, i.e., increased or decreased in all blades simultaneously; for translational flight, the rotor as a whole is tilted by means of changing the pitch angles cyclically. Thus to tilt the rotor forward, the pitch angle of the blades is increased in the aft position of the rotor and decreased in the forward position. These two types of change are superimposed so that one or the other or both can be used at any given time. Although these problems still exist, nevertheless, we have at least attained a measure of success in circumventing them. Helicopters today are actually being used-in many fields and in many places. There will be failures, of course, and probably a good deal of back-tracking, too, before final perfection is attained. But the important thing is that perfection is definitely assured and we are passing into the final stage of development. What that final development portends, I cannot even guess. It is possible that the helicopter will someday completely replace the airplane, although I do not expect to live long enough either to have to eat my words or be acclaimed as a prophet. It is quite likely that the ultimate machine will little resemble what we know today. It is entirely certain that, as an aircraft, it will open up fields of flight little dreamed of by those who have so far envisioned flight forever limited by speed and distance. But this is the future. For the present, the helicopter is a workable, usable and potentially very valuable aircraft, enthusiasts and skeptics notwithstanding. It rests entirely with us how soon we realize and make use of those potentialities.