THE LEIGH LIGHT FOR NIGHT ANTI-SUBMARINE OPERATIONS
The Leigh Light was a carbon arc searchlight carried on an aircraft and used in conjunction with A.S.V. to illuminate surfaced submarines at night. The searchlight operator was situated in the nose of the aircraft from where he could control the searchlight beam in both elevation and azimuth. Indicators were fitted which showed the direction in which the beam would shine. The operator was thus able to train the searchlight in approximately the right direction and distance before the light was exposed. A lens giving a spread of 10° in either a horizontal or vertical plane was provided which made searching for the target easier, but some expert operators prefer to use the light without the lens. The maximum effective range in ordinary -weather was about two miles.
The arc lamp was fully automatic in operation, the rate of feed of the Carbons being automatically controlled. Power for the arc, which ran at 120 to 150 amperes, was obtained from seven 12 volt 40 ampere hour type D accumulators and a-trickle charge fitment -would maintain them fully charged provided the arc was not run for more than three half minute periods per hour. Fully charged accumulators ?would maintain the arc for about six minutes without recharging.-
There were two types of Leigh Light in service:-
(1) The Turret type, fitted on Wellington aircraft; in this a 24 inchsearchlight was mounted in a retractable under-turret and the controls werehydraulic. The maximum beam intensity was 50 million candles without the
. Spreading lens and about 20 million candles with the lens. Total weight of the installation was 1,100 Ibs.
(2) The Nacelle type, fitted on Catalina’s and Liberators; in this a 20inch searchlight was mounted in a nacelle 32 inches in diameter slung fromthe bomb lugs on the wing. The controls were electric and the maximum beamintensity was 90 million candles, without the spreading lens and about 17million with the lens. Total weight of the installation was 870 Ibs.
The trickle charging current for the accumulators was obtained from an engine-driven generator on the Wellingtons, windmill driven generator on Catalina’s and a motor generator set on Liberators.
The control system on the Wellington was similar to that used for turrets. The turret itself was rotated by a Vane oil motor for movement in azimuth and the projector was moved by a ram inside the turret for movement in elevation.
The maximum speed of rotation when the control column was turned to the limit was 40 degrees per second, the limits of movement were 60 degrees to Port and 180 degrees to Starboard. In the Nacelle type, the control was electric by moans of two small motors built into the nacelle. The controller operated in three steps and gave a maximum speed of about 5 degrees per second. The limits of movement were 50 degrees on either side and 48 degrees downwards from the horizontal. On Liberators the nacelle was attached to the wings by a quick release mechanism so that it could be jettisoned by the pilot in an emergency.
Tactical Instruction for Aircraft fitted with Leigh Lights Operational height
The best height to fly on patrol is between 1,500 and 2,500 feet. Whenthe target is picked up on the A.S.V. the height of the aircraft should be notedand the aircraft brought down to 500 feet at a range slightly exceeding one mileto ensure that the aircraft is level when a range of one mile is reached. Inorder to manoeuvre into this position it is considered that maximum height shouldnot exceed 1,500 feet at three miles. If the height exceeds this figure it laadvisable to complete a 360° turn, losing height, but not closing range, in orderto avoid missing the target.
Direction of approach
If possible approach into wind this type of approach have the following advantages:-
(i) No drift, and it is therefore much easier for the operator to home the aircraft.
(ii) The sound of the 'engines is less likely to be picked up by the enemy. (iii) It gives more time for the final attack.
On moonlight nights approach, if possible, toward the moon beam. It is not considered advisable, however, that aircraft should make a tour in order to approach up wind, as it is essential that the attack should be delivered without delay.
Allowance for wind
In order to carry out a successful attack it is important that both the Captain and A.S.V. operator have a clear idea of the wind direction and speed. As soon as A.S.V. contact has been made, the navigator, taking into consideration the homing course, should announce the drive over the inter-communication. When attacking down wind the Captain must appreciate the high speed of approach and make the necessary allowance.
When attacking cross wind, allowance must be made for drift. When drifting to Starboard, the A.S.V. operator must keep the target to Starboard and vice versa. It should be noted that the searchlight is situated in the Starboard wing of a Catalina and in line with the pilot's eyes. This will tend to dazzle the pilot if the beam is brought across to the Fort side. It will be further advantageous and assist considerably in the success of the attack if the target can be placed dead ahead or very slightly to Port of the aircraft.
Setting of Lens
It has been found that use of the 10 degree vertical diverging lens produces the most successful results on Catalina’s.
Use of A.S.V.
The success of the attack depends mainly on the technique developed by the A.S.V. operator. The original corrections should be bold, bringing the aircraft quickly on. The A.S.V. operator should also state the ranges over the intercommunication every quarter of a mile. At a range of approximately three quarters of a mile the light is switched on. Before this the light operator should have ascertained by means of the indicator that it is aiming straight and is- depressed to the correct angle - usually 6. Only a small degree of light movement • should be necessary to illuminate the target. Immediately the j target is lit up the aircraft should reduce height to 50 or 100 feet and the f normal depth charge attack made, dropping flame floats at the same time.
The successful operation of this form of equipment is entirely dependent on crew drill and it is essential that the A.S.V. operator in particular shall have plenty of practice in homing on to targets, oven when the sea is rough and sea returns consequently high. A skilled light operator should have little difficulty in illuminating the target within two or three seconds of switching on.