Aeronautical Engineering - The Electro-Hydraulic Turret

Reproduced from : THE AEROPLANE

No: 1654
FEBRUARY 5, 1943

Britain can justly be accused of having failed to develop and bring to maturity many of the more important innovations introduced by her aeronautical engineers. The history of stressed skin construction, of flaps, variable-pitch airscrews, and retracting undercarriages, is a sad recital of this country's lost opportunities. But the record is not all black. The power-operated turret was a British invention, and a British development. No other country saw in it a new factor in air tactics, or a new branch of aeronautical engineering, but it has proved to be both.

With the rise in the bomber's operating speed which followed the arrival of higher-powered aero-motors and better aerodynamic forms came fresh problems for the air gunner. Where before the a gale had raged there now raged a tornado, and the gunner needed to supplement a straight eye with the strength of Hercules if he was to hold his guns steady against the slipstream. He needed shelter from the storm, and aid in sighting. The power-operated turret gave him both.

Among the first to give attention to the problems was the firm of Boulton and Paul Ltd., whose aircraft whose aircraft business was later taken over by Boulton Paul Aircraft Ltd. This firm designed a fully-enclosed turret for its Overstrand two-motor bomber, and thereby set a fashion which is becoming standard practice on all large bombers. This early turret was fitted to the nose of the Overstrand, carried one gun and was originally driven by compressed air. Compressed air had its disadvantages. It set short limits to the periods for which the turret could be in continuous use. Every spell of action had to be followed by an interval of rest while the compressed air bottles were re-charged by a motor-driven compressor—a difficulty which proved to be one of the larger boulders in the path of the pioneers. They turned, instead, to a system which combined electricity and hydraulics, and set off afresh.

The article which follows describes the construction and working of the B.P. electro-hydraulic turrets now being built in large numbers for many types of bomber serving in the R.A.F. and Allied Air Forces. It is the first full description of these turrets yet made public.



(Above) THE VENOMOUS CELL - A Boulton Paul electro-hydraulic tail turret carrying four .303 Browning machine guns. Turrets of this type are fitted to the Handley Page Halifax and the Consolidated Liberator.

THE ELECTRO-HYDRAULIC SYSTEM in the Boulton and Paul turrets was selected because it made for compactness and simplicity with sensitivity of control and response, and has a number of advantages both from the operation and installation standpoints.

A most important feature in the design is that the hydraulic system is confined to the rotating portions of the turret. This avoids difficulty of transmitting high-pressure fluid from the power source through continuously rotating joints.

The consequence of using low pressure is the need for large-diameter pipes with the associated troubles of space wastage, because of the large bend radii required.

The Boulton Paul system avoids this trouble along with others such as pipe dilation, differential thermal expansion between pipes and fluid, and leakages and fractures caused by vibration. This latter benefit results from the short pipe lengths and the ability to bend them to conform with neighbouring structure to which they can be conveniently cleated.

The whole of the power units are built into the turret, leads for electric power and other services being taken to a slip ring unit on the axis of rotation, so that installation consists only of dropping the turret into position, lining up. Bolting down and coupling up electrical services with, in some cases, an external oxygen supply. All ground tests and adjustments can be made before installation in the aircraft, and little time is needed in removing and replacing a damaged turret.


Operationally, an outstanding advantage is that power is only used as and when required, and there is no appreciable difference in speeds of operation either up or down wind. A brief description of turret operation will help to make these points clear. The control handle incorporates a grip lever or "dead man's handle," which, when depressed, energies an electric motor and, since this operation is performed only when the gunner desires to put his turret into operation, no power is consumed when the turret is idle. A typical power control system is shown below.



(Above) HOW IT WORKS (Fig. 1) - The principal units of a B.P. 4-gun turret, showing the operation of the gun elevating ram and the method of rotating the turret. The power is supplied by a hydraulic generator driven by an electric motor.

The hydraulic generator incorporates relief valves set to a blow-off pressure, which, at full output from the generator, limit its power to that obtainable from the electric motor. There is, therefore, no danger of either the electric motor or the hydraulic generator being overloaded. Movement of the control handle imparts stroke to the generator pistons, the power output being proportional to

a) the stroke imparted, which governs the speed of the turret movement, and

b) the pressure built up in the hydraulic system (up to maximum blow-off pressure) determined by the external resistance to turret movement.

At any rate of the turret rotation the power output from the hydraulic generator, and consequently power input into the electric motor, is a function of turret speed and of the pressure required to overcome external resistance to turret movement at any instant—limited to the maximum determined by the setting of the blow-off valves. The speed of the turret is virtually unaffected, whether the aeroplane is flying upward or downward.



(Above) THE PRESSURE BUILDER (Fig. 2) - A sectional drawing of the hydraulic generator, and diagrams explaining the control responses. The mechanism of the turret is highly sensitive to the controls and allows the gunner to bring his guns to bear on a target quickly and accurately.

The Hydraulic Generator

The hydraulic pumping unit (known as the generator) consists of two separate pumps---one connected in a closed circuit to a hydraulic motor for turret rotation, the other, in a similar closed circuit, to a hydraulic ram for elevation and depression of the guns. A pressure regulator is interposed in the pipe lines to and from the gun-elevating ram, to provide a hydraulic lock on the guns in elevation and depression and thus prevent gun creep.

Two banks of five cylinders of 9/16 in. bore formed radially about the central vertical bore make up the cylinder block of the hydraulic generator. (See Fig. 2)

The block is of phosphor-bronze and the pistons are of case-hardened steel, their outer ends being domed and held centrifugally in engagement with a shallow groove formed in the inner race of a ballbearing slide ring surrounding the pistons, movement of which gives stroke to the pump in either direction. The generator mechanism is contained in a magnesium alloy casting, which also incorporates built-in make-up or suction valves and adjustable blow-off valves.

The cylinder block rotates on a fixed steel spindle having two opposed ports for each cylinder bank. Each of the control rings slides in guides in the generator casing and has a range of movement of .13 in. to either side of the central position concentric with the cylinder block. In operation the pistons, in addition to oscillating in their respective cylinders, have also a rotary motion arising from a very slight offset of the point of contact between crown and grooved track in the ball race. This combination of sliding and rotary motion has the effect of equalising wear and the prevention of line scoring of pistons or cylinders.

The Operation of the Controls

The control slides for the two pumps are connected to a control handle, so that movement of the handle to left or right turns the turret in the same direction, and movement of the handle backwards or forwards elevates or depresses the guns.

With the control in the central or neutral position, the control slide ring is concentric with the cylinder block and no delivery takes place, because the pistons have no stroke. When the column is displaced from the neutral, the slide is displaced from the concentric position in relation to the cylinder block, imparting stroke to the pistons, which move inwards during one halt of the cylinder block revolution and outwards during the other half of the revolution, producing suction while they are in communication with one port in the distributor and delivery when they communicate with the other port.

A supply of oil is carried in a sump in the generator for making up internal leakages as needed through suction valves in each circuit. Overload protection is given by blow-off valves in each line, these valves being set at pressures of approximately 1,000 1b. per square inch.

The hydraulic generator, which runs at 1,230 r.p.m. is driven by a constant speed 24-volt electric motor running at 4,000 r.p.m., coupled through epicyclic gearing to the generator to form a single unit. Both pumps are controlled independently and both azimuth and vertical traverse can take place individually or simultaneously.

Small electric resistance heaters of approximately 50 watts loading are incorporated in the hydraulic system, and are automatically cut out when the turret is in operation. Others are fitted to the hydraulic generator and motor. These heaters help to guard against faulty operation at low temperatures.

The hydraulic motor (see Fig.3) is of the seven-cylinder elliptical-cam type 5/8in. bore and .45-in. stroke, with the cylinder block as the driven member rotating on a stationary distributor spindle. Power strokes occur in opposite 90-deg. Sectors, and the motor is reversed by reversing the supply of hydraulic fluid from the generator by the movement of the control handle. The materials of construction are similar to those used for the hydraulic generator, with delivery and exhaust ports formed in the distributor spindle, provision being made for adjustment of port timing by partial rotation of this spindle. The approximate maximum speed of the hydraulic motor is 200 r.p.m. and connection is made to the internally cut main driving gear through epicyclic gearing.

Gun and seat rams, where the latter are fitted, follow normal practice, the only point for comment being the construction of the piston in which all screw joints for retaining the synthetic rubber cap washers are eliminated. The washers are retained by a "dead" spring ring and "D" washer, avoiding any tendency to squeezing out or damage to the cup washers.

The pressure regulator or metering valve interposed in the pipe run to gun rams serves to maintain a small back pressure on the return line and this unit also provides a means for short-circuiting the gun ram to move the guns without the use of power.

The hydraulic system is virtually a closed circuit. i.e., return fluid from the hydraulic motor and rams is fed back to the inlet side of the hydraulic generator and not into the sump. The make-up valves function only under normal operation to permit replacement of fluid lost through interport and piston leakage, and to feed the intake side of the hydraulic generator when the blow-off valves are operating.



(Above) THE TURRET TWISTER (Fig. 3) - The hydraulic motor, and the manner in which it rotates the turret. The motor incorporates a reduction gear.

Gun Fire Interrupter

Each gun is electrically fired by a separate solenoid, a single firing button to control all guns being mounted in the top of the control column. A safety isolating switch is included in the circuit.

One of the first problems which had to be solved was the automatic protection for surrounding aircraft structures, such as tail and airscrew, from rounds fired from the turret. To supply this, an electrical fire interrupter device is fitted in the firing circuit. As the guns are some distance apart independent interruption was arranged for left-hand and right-hand guns so that there should be the maximum field of fire.

For this purpose, Boulton Paul turrets use a brass drum rotated at the same speed as the turret. Two spring-loaded sliding brushes are located one above the other on a frame and are held in contact with the drum and made to slide up and down the drum surface with gun elevation and depression. Firing circuits for the left-hand and right-hand pairs of guns are fed by the upper and lower sliding brushes respectively, a fixed brush in permanent contact with the brass drum forming the other connection for the circuit.

On the upper half of the drum shadow patterns corresponding with the obstructed areas of the left-hand pair of guns are cut out and filled in flush with moulded insulating material so that in passing over these areas the upper brush is isolated and firing of the left-hand pair of guns is interrupted. In the same way the lower half of the drum surface carries similar insulated areas corresponding to the obstructed areas of the field of fire of the right-hand guns, and when the lower brush makes contact with these areas the firing circuit is broken.

These shadow patterns are usually designed from a turret mounted in an aeroplane, allowances being made for aircraft deflections and gun characteristics. The advantage of this method of protection is that the aircraft shape, no matter how complicated, can be adequately protected with a minimum loss of field of fire.



(Above) BULLET HOISTER (Fig. 5) - The ammunition feed assister which lifts the belted ammunition to the tail guns. Its position can be seen in Fig. 4; its weight is more than balanced by the advantages its brings.

Ammunition System

Ammunition systems generally follow normal practice, but that for tail turrets deserves special mention. In order to minimise weight in the tail the ammunition is carried in the fuselage and led to the turret in steel runways or tracks, through the turret base centre and up to the guns (see Fig.4). This arrangement makes possible a greatly increased ammunition capacity, but has the disadvantage that the guns cannot be fed under their own power. To overcome this power-driven feed assister (see Fig.5), in which the ammunition belt runs over toothed sprockets carried on a hinged arm, is direct-driven through suitable gearing from the turret electric motor. Normally the belt sprockets are free, but when the tension in the ammunition belt between its feed sprocket and the gun exceeds a predetermined figure the hinged arm is actuated by the belt pull, applies a band brake to an epicyclic gear unit in the drive between the main motor and the belt sprocket and provides a power drive to the sprocket which tends to overfeed the guns.

When sufficient overfeeding has taken place to reduce the tension in the ammunition belt the hinged arm returns to its normal position and disengages the power drive. Although the description indicates a definite sequence of operations, in practice the feed mechanism is always working, the belt tension and band brake application adjusting themselves to gun requirements. Safety devices in the form of quickly replaceable shear couplings are fitted to prevent damage to ammunition belt or feed assister mechanism.



(ABove) FOOD FROM AFAR (Fig. 4) - The ingenious manner in which ammunition is fed from the bomber's fuselage to the tail turret to avoid concentrating a great weight in the tail. By this means, also, the gunner can draw upon a larger number of rounds.

Mounting Ring

The unorthodox design of the turret mounting ring is also worth mentioning. This consists of two fully machined magnesium alloy castings assembled to form a double-row cageless ball bearing. The lower row of balls takes care of down loads caused by turret weight and the upper row deals with radial loads, the main driving gear being split, piston ring fashion, for assembly purposes. The bottom race conforms to normal ball-bearing practice, the balls running in a radiused groove in the fixed ring. The track for the upper race is formed in a different manner. There is a small interference fit between the inner and outer races and the balls. The inner race is turned with a smooth finish, the outer race has fine feed tool marks left, and when assembled and run-in the balls roll a track into the ridges formed by the finishing tool.

This produces an excellent assembly free from objectionable diametrical clearance. Some surprise may be expressed at the running of steel balls on a magnesium ally track, but tests have shown that, provided a suitable value for unit ball loading is selected, excellent results are obtained, there being no pick-up of the ball track, which actually improves with use because of the effect of work hardening the material. Similarly, gun trunnion bearings through which the ammunition passes to the guns are of the full ring needle roller type running direct onto the magnesium alloy.



(Above) ELECTRICAL PLUMBING (Fig.6) - A sectional drawing of the electrical distributor unit, which also shows how the gunner receives his oxygen. The electric current passes into the turret through brushes and slip rings; the oxygen through a tube down the centre.

General

The representative distributor illustrated in Fig.6 indicates general constructional features, and the manner by which electrical supply and oxygen are taken to the turret.

In the matter of variety, the Company has produced a wide range for nose, tail, mid-upper and under positions, two- and four-gun models. The cut-away drawing, Fig. 7, shows a 2 by .303-in turret as installed in Lockheed Hudson aircraft. A photograph of the four-gun (.303 in) turret as fitted in the tail of the Handley Page Halifax appears at this beginning of this article.

The first electro-hydraulic turret to go into quantity production was a four-gun .303-in calibre model designed expressly for and in conjunction with the "Defiant" fighter produced by the Company's aircraft division. All Boulton Paul turrets are fully equipped with auxiliary services, e.g., oxygen, inter-communication, point for connecting gunner's clothing heater plug, etc., and reference to the illustration Fig.7 will indicate the layout of items of equipment.



(Above) DOMED DEFENCE (Fig. 7) - A general view of the B.P. turret fitted to the Lockheed Hudson reconnaissance bomber. Some of the features already illustrated, and some of those described in the article, can be seen. Although no space is wasted, this turret is surprisingly roomy. It also commands an excellent view from its position well down the fuselage.

Turret Habitats

Among those aircraft - British and American - fitted with Boulton Paul turrets may be mentioned the following:-
Defiant 4 x .303-in. Mid-upper
Halifax 2 x .303-in. Nose
2 x .303-in. Mid-upper
4 x .303-in. Tail
Lockheed Hudson 2 x .303-in. Mid-upper
Ventura 4 x .303-in. Mid-upper
Liberator 4 x .303-in. Mid-upper
4 x .303-in. Tail