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The Autogiro As A Military Craft

By Lieutenant Commander William Nelson (C.C.), U. S. Navy
August 1931
Proceedings
Vol. 57/8/342
Article
View Issue
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Body

The Army and Navy have always in­quired into the possibilities of any new machine and have endeavored to adopt it to military use. The history of aviation is replete with the part taken by the services. Each new phase of aircraft development has found the government vitally interested in the prospects presented by that development. It would appear in many cases that the inter­est extended could not have been founded upon the grounds of premeditated adapta­tion to service use. However, experience has indicated that the military organization has used its imagination to good advantage in furthering those ideas necessary for mod­ernized strategy and tactics.

The successful conduct of warfare is de­pendent on machines to a much greater ex­tent than ever heretofore. The military forces must extend themselves to determine the military value that lies in each new in­vention in order that they may not be sur­prised on the field of battle by the enemy’s advantage in this respect. The work done with the early aeronautical experimenters has been the foundation for the present-day air fleet. This fleet grants an advantage of position and speed not attainable in any other manner. It is not readily conceivable how land or sea forces could succeed in any offensive or defensive action without the as­sistance of a full quota of aircraft.

Without analyzing the detailed duties of military airplanes and dividing them into their classes it can be stated generally that they have two outstanding functions—one, to place men and materials in a position of advantage; two, to transport men and ma­terials at high speed. The first of these is usually a matter of altitude. The second is of particular importance on account of the great speed practicable compared to other modes of transportation. The combination permits forms of attack and defense against other things than aircraft which are peculiar to aircraft. Proper analysis of modern war­fare introduces considerations regarding ac­tions against other aircraft; and in such cases maneuverability, armament, and armor have a bearing.

Military airplanes heretofore have been of the conventional type. Helicopters, orthopters, autogiros, and even those introducing lesser digressions from the ordinary airplane have been viewed with much scientific inter­est but without practical application. Lack of development or particular disadvantages in comparison with the airplane have pre­vented their adaptation. Out of this group, however, there has emerged the autogiro. This type of craft has certain characteristics which seem to give it justification for con­sideration with other types of military air­planes.

The autogiro is essentially in a class by itself. It approaches the airplane in what it can do, and yet it differs materially there­from. It is not a helicopter. In the ordinary airplane, motion forward in the air is ob­tained by the thrust of the propeller and lift is produced by the movement of the wing through the air. In a helicopter motion and lift originate in the horizontally placed pro­peller. In the autogiro motion forward in the air is obtained by the thrust of the pro­peller as in an airplane; the lift comes pri­marily from the rotor and to a small degree from the fixed wing. In vertical descent all the lift is given by the rotor and in horizon­tal flight about 80 per cent of the lift comes from the rotor. The rotor is free to assume its own speed of rotation in flight there be­ing no connection to the engine. The rotor might be considered as-at freely rotating wing.

Excepting for the rotor and the adapta­tion of other parts of the craft to suit this rotor and its functioning, an autogiro is very much like the common type of heavier-than- air machine in construction. The fuselage, landing gear, tail surfaces, power plant in­stallation, and other details having to do therewith are quite similar to any airplane using the same kind of construction. Even as to the location of the useful load there is little that can be regarded as different. Ex­tending from the lower part of the fuselage is an ordinary airfoil-section fixed wing of about three-quarters the area one would ex­pect to find in the lower wing of a biplane. This wing has the tip turned up at a large angle for reasons of stability. A conventional unbalanced aileron is fitted to this fixed wing.

Extending up from the fuselage where the upper wing of a biplane would ordinarily be, is a pylon with its pivot at a point above the fuselage at a height equal approximately to the depth of the fuselage. The apex of the pylon acts as the axle and bearing of the rotor. The lift of the rotor blades is trans­mitted via the hub and this axle through the pylon to the fuselage. The rotor consists of a hub rotating on ball thrust bearings at the apex of the pylon and four hinged blades mounted on the hub. The rotor as a whole revolves about an axis approximately per­pendicular to the longitudinal axis of the fuselage and rotates freely under the aero­dynamical pressure of the relative wind in­dependent of the power plant. The speed of rotation of the rotor is governed by the de­sign, being practically constant for all flying conditions. Experiments have been made varying the rotor speed from 110 r.p.m to 170 r.p.m. The lesser speeds of rotation are now used.

The rotor blades approach the streamline in section, are constant in section, and have a relatively high aspect ratio. The circle scribed by the blade tip is somewhat greater in diameter than the span of a biplane of like size.

Rotation of the rotor is essential for lift. In flight this rotation is derived from move­ment through the air. In taking off, the auto­giro can be taxied until the rotor gains speed; or, as has been done in the later ma­chines, the rotor is geared to the engine crank shaft through a manually operated clutch. A brake can be used to stop the rotor after landing.

The rotor blades are hinged at the hub so that they can move up and down within limits to suit the forces imposed. If the blades were fixed considerably more lift would be received from the advancing blade than from the one receding. This inequality would cause rolling and the results would likely lead to disaster. Being free the blades move up and down so that the lifts on dia­metrically opposite blades balance each other and stability results. The advancing blade automatically rises, decreasing its effective angle of incidence, and thereby adjusts its lift to suit that of the opposite blade moving at a lower relative speed. In vertical descent all blades act approximately the same on the air so the hinging action is not essential in that maneuver. The blades are prevented by cables from going below a plane at right angles to the plane of rotation. In flight they cone up slightly, the tendency to rise due to air pressure being opposed by the centrifugal force of the blades. Equilibrium is produced automatically.

Each rotor blade is hinged also in the plane of rotation. This hinging is not entirely free. It is introduced to permit the varying loads imposed to be absorbed gradually and smoothly. Centrifugal force tends to hold the blades in the proper position when in flight, cables between the blades holding them in position when the rotor is at rest. The rotor blades are quite flexible and, were it not for the hinging actions the movements produced would require exceedingly heavy construc­tion. The articulation of the blades also re­moves the gyroscopic action of the rotor, which force would otherwise be violent in results. In general, it can be stated that many tests and experiments and much study has given data for the development of an ex­ceedingly efficient rotor.

Autogiros have been built weighing 1,000 to 3,000 pounds gross. Apparently there is no limit to the size that can be built insofar as the autogiro principles are concerned. Small high-powered autogiros as well as large patrol types appear practicable.

The outstanding feature of the autogiro is its very low landing speed. The speed ra­tio is given as 6 to 1. It can be readily seen then that with a high speed of 150 m.p.h. the landing speed is about 25 miles per hour. The speed ratio of the ordinary airplane is about 3 to 1. The low landing speed of an autogiro permits operation from areas much smaller than those which an airplane re­quires. The advantage of this is obvious. Very short runs to take off—about 100 feet for present-day autogiros—and vertical landings give a great freedom of action to simple cross-country operation. The value of this feature to a seaplane is questionable for usually landing areas in seaplane operation are expansive.

Maneuvers of aircraft can be segregated into those necessary for safe navigation and those utilized for other purposes. In the first class are the banked turns, climbs, glides, dives, etc. In the second classification are the special turns, rolls, loops, and other acro­batic maneuvers. The acrobatic maneuvers are a necessary part of certain combat air­planes and some commercial craft. To others these operations in the air are incidental. The autogiro responds readily to the naviga­tion maneuvers much in the same manner as an airplane, other than in its ability to hover and land practically vertically. The autogiro does not stall and acrobatic maneuvers are impracticable in this craft. Insofar as mili­tary use is then considered it becomes suit­able primarily in those functions where navi­gational maneuvers control.

The ceiling of an autogiro is about the same as that of an analogous airplane.

The autogiro in its present stage of de­velopment has more parasite resistance than its compatriot, the airplane. Consequently its top speed is less than that of an airplane similarly loaded and powered. Theory seems to indicate that the autogiro can be brought up to the same efficiency as the airplane in this respect. At intermediate speeds the ad­vantage of efficiency rests with the airplane.

In comparing the weight of the autogiro and that of an airplane it is only necessary to consider the difference between the weight of the rotor and its mounting and the upper wing of a biplane. It is believed that they will be practically the same when the autogiro reaches a higher degree of development. For the present then it is an opinion that the airplane has the advantage in use­ful load and accordingly in range.

The autogiro as a military machine logi­cally falls into the group where navigational maneuvers are paramount. In this group it seems to have all of the features of the air­plane with the added characteristics (of par­ticular benefit to a landplane) that it can land and take off in severely restricted areas. Also its ability to hover might be utilized to advantage. One then comes to the con­clusion that excluding the classes involving fighters, pursuits, scouts, and certain obser­vation machines the autogiro can compete favorably with land-type military airplanes.

 

Digital Proceedings content made possible by a gift from CAPT Roger Ekman, USN (Ret.)

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