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Aviation History
1944
1944 - 2319.PDF
NOVEMBER 9TH, 1944 FLIGHT 505 This sectionalised illustration of the airscrew shows the very neat manner in which the various elements are incorporated. See aiso exploded drawing below. Modern Airscrews—II " Design and Construction Features of the Rotol Electric By C. B. BAILEY-WATSOX T HIS, the second article in the series on Modern Airscrews deals with the Rotol electric airscrew, which is a modified version of the American Curtiss Elec tric, but is made in this country. The British product is a very neat job, Rotols having succeeded in making their tricyclic reduction gearing somewhat smaller than the American original, without ^uiy sacrifice of robustness. They have, in addition, de cided to employ a higher power although smaller size electric motor, its power output being £ h.p. as against the i/ioth h.p. of the Curtiss. In point of fact, pretty well every component of the Rotol airscrew is of different detail design than the corre sponding item in the Curtiss, but the general layout and principle of the two airscrews is the same. The blades fit into the short barrels of the hub, which are internally threaded to accept the bearing housings. Four ball races comprise each blade-bearing stack and are located between the bearing housing and the blade-root adaptor, in the base of which is screwed the bevel seg ment through which pitch change motion is transmitted. The hub shell embodies a central buttressed and inter nally spined sleeve, which protrudes, about half-way into the hub interior to take the airscrew shaft. As is the usual practice, the static and thrust loads of the airscrew are taken on two annular cone seatings, one at the rear and one at the front of the airscrew shaft, t*he splining between shaft and hub merely transmitting torque. Pitch-change Mechanism On the front of the hub is bolted the pitch-change mechanism, consisting of a reversible electric motor and a compound epicyclic reduction gear giving a ratio of 8,880: 1. The output side of this reduction gear terminates in a helical-toothed bevel wheel—called the power bevel— "which is in mesh with the blade bevels. As the power bevel has 39 teeth and the blade bevels have 58-tooth pitch, a gear ratio of 1.49 :1 is produced, making the total reduc tion between motor and blades 13,230: 1, this reduction giving a pitch change rate of T.36 deg./sec. for a motor Here are shown the main features of the airscrew projected to make their unit shape clear. Reference between this drawing and the heading illustration will help in orienting the location of the various parts. speed of 3,000 r.p.m. The series wound 24-volt motor is driven direct from the aircraft batteries, consuming about 10 amps, at 3,000 r.p.m., and has two sets of field wind ings to allow reversible rotation. A drum-type armature is employed, each of the four radial brushes having a spring pre-loading of 27 oz. in order to compensate centri fugal force generated by airscrew rotation. Carried on front of the motor is a solenoid-operated brake consisting of a fixed and a movable plate each faced with a Ferodo lining. When the pitch angle of the blades is changed to effect the maintenance of constant engine speed, the sole noid is energised and attracts the movable brake plate and the motor is then free to turn. Immediately the requi site pitch angle is obtained, the brake circuit and the motor circuit are broken and the brake is automatically and instantaneously applied. It is impossible for the centri fugal twisting moment of the airscrew blades to overcome the brake owing to the mechanical advantage the latter possesses by virtue of the extreme reduction ratio between motor and blades. Current is supplied to the airscrew from the standard aircraft batteries via the constant-speed governor unit. On the rear of the airscrew hub are mounted four slip rings of hardened bronze, separated and insulated by micarta rings. Each slip ring serves a separate function, the cir cuits being (i) Fine pitch ; (ii) Coarse pitch ; (iii) Feather ing; and (iv) Negative return. From the governor unit the leads are taken to a brush gear mounted on the slip LEADS FROM C S.U. .._. BRAKE MOTOR RETAINING NUT / ELECTRIC 7 MOTOR REDUCTION GEARS
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