Rohrbach Rotating wing
THE ROHRBACH ROTATING WING AEROPLANE
By W. S. SHACKLETON
Mr. W. S. Shackleton, after designing the A.N.E.C. Monoplanes for the Lympne Light Plane trials some years ago, became chief aircraft designer to the Beardmore Company. Illhealth compelled him to resign that post and go to Australia for some years. Upon his return to England, Mr. Shackleton designed the little S.M.i pusher monoplane, which incorporates many unusual features. While with the Beardmore Company, Mr. Shackleton supervised the building of the " Inflexible,'" and in that way came into close touch with Dr. Rohrbach. It is therefore natural that, when Dr. Rohrbach designed his rotating wing aeroplane, Mr. Shackleton should be interested, and the result of his interest has been that he has secured the sole rights in the machine for the British Empire. Mr. Shackleton wishes us to point out that in the following article, written exclusively for FLIGHT, questions connected with international patent law have made it inadvisable to reproduce general arrangement drawings, stability and performance calculations, and details of the drive which has been designed for rotating the wings. Such details will, of course, be disclosed to anyone seriously contemplating the building of a machine in this country under licence.-ED.
THIS machine, the latest project of Dr.-Ing. Adolph K. Rohrbach, the famous German constructor of large all-metal aeroplanes and flying-boats, has attracted considerable attention in scientific and aeronautical circles throughout the world. It is felt, therefore, that a semi-technical description of the machine, together with an account of the progress already made with this and similar types, will probably be of more than ordinary interest. The fact that Dr. Rohrbach's name is associated with the development will go a long way towards removing that intolerant prejudice with which most novel schemes are received by practical engineers. In this connection, attention is drawn to the 1,000-h.p. Zeppelin " Staaken " four-engined monoplane developed under Dr. Rohrbach's direction in 1919. This aeroplane, constructed entirely of metal and fitted with wheel brakes and front wheel to prevent nosing over, although completed 14 years ago, would be accepted at the present time, if fitted with the engines now available, as a thoroughly modern design. With a wing span of 102 ft., it had a top speed of 131 m.p.h., and with 20 passengers a range of 760 miles. After making some 17 highly successful test nights, it was broken up by order of the Inter-Allied Aeronautical Commission, to their discredit it may be added.
General Principles of the Rohrbach Rotating-Wing Aeroplane
A perspective general view is shown on the next page. It will be seen that the fuselage, undercarriage and tail unit are of approximately conventional type. The usual hxed-wing system is, however, replaced by a rotating system consisting of three or more narrow-chord wings ol normal thick aerofoil section, braced by means of struts and tie rods to a revolving cantilever shaft. This shaft ls continuous across the span and rotates in fully-floating bearings carried in a fixed central casing built rigidly into the fuselage structure. The whole wing system is rotated through worm or spiral-bevel gears from an engine or engines carried in or at the nose of the fuselage, or in separate nacelles over the fuselage. In a transport or similar design, the engines could be accessible during flight I this feature was considered desirable. A free-wheel device, or one-way clutch, allows free auto-rotation of the wings in the event of engine failure and when the engines are being throttled below a predetermined speed. Lift is generated by suitably feathering or oscillating the wings throughout the circle of revolution. This is accomplished in the Rohrbach machine by a simple but extremely ingenious control gear operating the wings through pushand- pull rods, which positively and progressively changes the angles of the wings (measured against the respective tangents of the circle of revolution) to suit the particular operating conditions. The wings rotate in a forward direction at the top of the circle and at a sufficiently high speed to develop useful lift round the lower part of the circle, even with a high translational velocity of the aircraft.
These wings develop a useful lift or propulsive force at practically every position round the circle of revolution. Round the upper portion of the circle the effective lift force naturally reaches its greatest value, as the velocity relative to the attacking air-stream is greatest in this zone. Around the forward portion of the circle, high lift and propulsive forces are produced, whilst around the lower sector of the circle positive lift forces are again produced, as the wings have in this section been oscillated past the " no-lift " angle to a negative angle of attack. The action is illustrated in a diagram. When hovering, ascending or descending vertically with no horizontal air stream directed against the aircraft, the resultant lift from the wings acts in a vertical direction. This resultant force can be changed in direction by a simple fore-and-aft lever movement so as to have a forward component of the amount required to produce the desired forward speed. This force can also be inclined backward of the vertical to produce either a decelerating component on the aircraft or backward flight at any speed which might be considered useful.
Rolling moments are generated by means of a differential oscillation of the two wing sets, this giving increased lift on one side and reduced lift on the other. This differential lift is produced by the pilot in the usual manner by a lateral movement of the joystick, which motion changes the respective oscillation fulcrums. Yawing moments are similarly produced by varying the forward inclination of lift on the two wing sets, the yawing control being actuated by foot pedals in the usual manner. This is designed to give full control when hovering, since with no airflow over the tail surfaces the usual rudder control would be inoperative. In an actual machine the rudder would probably be interconnected so as to damp out any oscillations in normal flight. In the event of engine failure, a free-wheel device installed in the transmission allows the wings to revolve with the engine or engines stopped. When free-wheeling, the wing oscillation controls are automatically set to give the most favourable attitudes for slow descent. Tests on full-scale and model wings have proved that the wings will continue to auto-rotate with the engine power cut off, at a sufficient speed and generating sufficient lift to ensure a controlled descent at a sufficiently low vertical velocity. The flight path under these conditions can be varied at will from the flattest glide (gradient about 1 in 9) to a vertical descent. When hovering or flying at all speeds the torque reaction of the engine drive is compensated by the pendulum effect of the fuselage around the wing shaft. In vertical ascent under full power the stern of the fuselage would be inclined downward by about 6 deg. against the horizon.
With the motor power cut out, however, the bow would be inclined downward at about 6 deg. With other operating conditions, the equilibrium positions of the fuselage are between the two above-mentioned extremes, The possibility of the fuselage as a whole developing a pendulum oscillation about the rotor shaft when hovering has been mentioned by a prominent aircraft designer in England. I t can be asserted with confidence that this could not occur, owing to the damping produced by the great inertia of the rotating masses in the engine and rotor. In order to oscillate, the rotational speeds would have to be accelerated and decelerated at the same period as the oscillation.
At reasonable forward Speeds the damping action of the horizontal tail surfaces is Sufficient to maintain the longitudinal attitude of the fuselage in a substantially horizontal plane. The tail is operated by a hand wheel through the usual irreversible mechanism, no elevator control of the usual kind being fitted. The term " ground angle " has no significance on a machine of this type, as a change in the pitching attitude is not accompanied by such a corresponding change in the angle of incidence of the wing as in an aeroplane of conventional design.
Owing to the relatively high lift obtainable from a given area of wing surface, it is possible to keep the effective angles of attack low, with a corresponding increase in efficiency. Tests have proved that the rotating-wing system is non-stalling.
The wings give little or no lift when the machine is standing at rest on the ground with the rotor stationary. Consequently aircraft of this type could be safely left out on the aerodrome even in high winds. An adjustable brake is provided to prevent the wings revolving under such conditions. The weight of individual components would be carefully checked and adjusted to secure uniformity and correct balance. The question of vibration in the structure due to fluctuating aerodynamic loads has been investigated, also certain out-of-balance forces have been allowed for in stressing the design. The elastic resonance period of the whole structure is quite different from the period induced by air forces on the wings, _ All these points are already confirmed by running tests with rotating wings of full size. The effect of gyroscopic forces and couples has also been investigated mathematically.
FORERUNNERS OF THE ROHRBACH ROTATINGWING AEROPLANE
As far as can be ascertained, airwheels were used for the first time in 1786 with the intention of driving the Bnssy balloon. Since then many inventors have endeavoured, on paper and by actual experiment, to develop improvements in airwheels for flying machines. The following is a short description of the line of development during the las^ 50 years leading up to what is claimed to be the first physically correct solution-the Rohrbach revolving wing-
Revolving surfaces may be broadly divided into two groups: -
(1) Pusher surfaces intended to push the air along as the surfaces move. The absolute inefficiency of this system « recognised.
(2) Surfaces of aerofoil shape which cut through the an with a certain angle of attack. This system, with adeqaadimensions of the suifaces and with appropriate control the angle of attack, gives a high degree of efficiency.
ONE WAY OF DOING IT : An artist's impression of how a machine using the Rohrbach system of rotating wings might appear. This machine was first described in FLIGHT of February 2, 1933.
THE ROHRBACH WAY : Diagram showing the action of air forces on the wing