Canadian Navy Timeline
Originally published in Crowsnest Magazine - Vol 17, Nos 3 and 4 March-April 1965
THE PILOT moves the helicopter slowly ahead, keeping pace with the ship. He is about 50 feet above a heaving, rolling deck. He releases a thin wire messenger. It brings back a heavier wire from the flight deck. The slack is taken upit tightens. Slowly the helicopter descends on its umbilical cord. The descent quickens. As the helicopter touches down, steel jaws grip it.
What is this?
Beartrap they call it the new haul-down system for landing helicopters on destroyer escorts.
Why is it?
Basically, to make possible the landing and securing of heavy helicopters on destroyer-size ships in rough weather.
The project had its beginnings nearly 10 years ago, when the helicopter-destroyer combination was selected by the Royal Canadian Navy as a promising antidote to the high-performance nuclear submarine.
To start with, the Navy fitted a small, experimental flight deck to a frigate, HMCS Buckingham. Trials were successfully carried out, using a Sikorsky HO4S-3 helicopter. The next move was to put a platform on the destroyer escort HMCS Ottawa. Further trials were conducted, using an RCAF Sikorsky S-58. On the basis of the trials, the concept of operating helicopters from destroyers was recommended and received approval in principle.
Two things were needed. One was a helicopter capable of all-weather day and night operation (the HO4S-3 was not). The other was a system for handling and securing a helicopter on a small flight deck in rough seas.
The former was found, in the 9.5 ton Sikorsky CHSS-2 Sea King. The landing-handling problem was solved by the beartrap.
During the trials, it was found that landing was not so much a problem as was the handling of the helicopter after it had landed. Manhandling was neither quick enough nor certain enough to establish the measure of control necessary to ensure that, in certain circumstances, the helicopter will not take charge, and go over the side.
The Navy went to the drawing boards and came up with a scheme that promised to make the concept practicable. Conceived by the RCN, the haul-down and beartrap system was engineered by Fairey Aviation, Dartmouth, N.S. A prototype was designed and built by Fairey, under RCN supervision, and was installed in HMCS Assiniboine during her 1962-63 conversion.
Trials with a newly-acquired Sea King began late in 1963. By mid-1964 the daytime trials were completed and pronounced successful. Using the new system, no manhandling was needed to get the helicopter on the deck and in or out of the hangar. The helicopter was solidly secured on landing and remained so until the next take-off.
In conjunction with the helicopter carrying features and hangar facilities, roll-damping fins were added to the destroyers being so built or converted. These fins reduce the roll of the ship and aid landing and take-off operations during rough weather.
An average landing doesnt take any more than five minutes from approach to the snapping shut of the beartrap. The approach is made from the stern of the ship. When in position, an operator in the helicopter lowers a wire rope messenger. To this messenger a man on the flight deck attaches a heavier hauldown cable. (A pair of grounded tongs discharges any static electricity in the messenger so that the man wont get a rude jolt.) The messenger and hauldown cable then are drawn into the helicopter through a probe in the helicopters belly. After the haul-down cable has been locked in position inside the probe, the slack in the cable is taken up. A landing control officer (LCO) on the flight deck controls the haul-down and landing from this point onward.
The pilot keeps his helicopter hovering in the correct position over the trap. Like an angler reeling in a jumping trout, the LCO slowly begins to reel in the helicopter. The LCO regulates the rate of descent of the helicopter on the control console. When it is in a position just off the deck, he can then increase the rate during a lull in the ships motion. He plays the helicopter quickly into the beartrap where steel jaws snap around the probe and hold the chopper securely against any motion the ship might offer.
This operation can be performed with the ship rolling as much as 310 and with pitching motion as much as 8°.
Breaking the system into its component parts, the largest and most complex arc in the destroyer escorts, with the lightest and smallest in the helicopter.
The helicopter contains the main probe, a tube-like structure protruding from the underside of the fuselage, through which the messenger cable is paid in and out. The winch operating the messenger sits on top of the probe, and is controlled by an operatur in the helicopter. The probe incorporates pins to engage the haul-down cable and lock it in position. A series of micro-switches then actuate the locks, disengage the messenger from the haul-down cable, and stop the winch
equipment is divided into three sectionswinch unit, power
unit and beartrap rapid securing device.
Motive power for the system comes from a 60 hp electric motor. This operates a hydraulic pump and motor which in turn actuate a double drum winch through reduction gears. Each drum is operated independently and has its own clutch and braking system. The entire hydraulic system operates at 3,000 psi and is rated at 4,000 psi.
The system maintains constant tension in the haul-down cable. This is of great importance for, without it, the helicopter would be dragged down and jerked drastically whenever the ship pitched to any appreciable degree.
Constant tension in the cable is ma intamed by an intricate system of black boxes, or modules. Basically, they compare selected tension on the control console with actual cable tension. The difference is measured and fed to a valve which controls the paying in or out of cable. The sensing devices, in company with the five control modules which make up the constant tension equipment, are so sensitive to change that narrow limits are achieved even in the roughest of weather conditions.
absorber is built into the system as well, to absorb snatch loads
in cable tension. These loads occur particularly when the slack
in the cable is being taken in before haul-down. The shock absorber
is a piston-cylinder arrangement with double sheaves on either end
around which the haul-down cable passes. The cylinder is charged
with air under pressure.
The beartrap rapid securing device sits in a slot in the flight deck and travels fore and aft in response to a command signal from the LCOs control console. It secures the helicopter immediately upon landing by engaging the main helicopter probe.
The six-foot square beartrap secures the helicopter when the LCO pneumatically fires two parallel beams equipped with steel, spring-loaded teeth. The beams prevent the probe from moving port or starboard and the teeth prevent probe movement fore and aft. The end of the probe is swaged so that it cant jump out of the beartrap.
The beartrap has a centring device. Centring is accomplished by traversing the beartrap unit aft. The beams are equipped with a fail-safe device which keeps them together in case of system failure.
The entire beartrap mechanism travels in a slot along the centre-line of the flight deck. It can be traversed with its captive helicopter the full length of the flight deck, in or out of the hangar. This eliminates the dangerous manhandling problems which could exist with a 9.5-ton aircraft, particularly in rough seas.
While the haul-down cable is operated from one of the twin drums on the winch unit, the traversing system is controlled by the other.
With the landing complete the LCO centres the helicopter, the rotor blades and tail pylon are folded by the pilot and the helicopter is stowed in the hangar.
Safety and ease of handling are the keynotes in this system. Day landing trials on the Assiniboine were completed last summer and Experimental Squadron 10 (VX 10) pilots have begun a series of night landing trials.