General Aircraft Hamilcar - Design

Design

The Hamilcar was constructed primarily from wood, mainly birch and spruce, with fabric-covered plywood forming the skin, and high grade steel reinforcement beams in critical areas. It had a wingspan of 110 feet (34 m), a length of 68 feet (21 m) and a height of 20 feet (6.1 m) to the top of the fin. It weighed 18,400 pounds (8,300 kg) when empty, and could transport a military load of 17,600 pounds (8,000 kg) to give a total weight of 36,000 pounds (16,000 kg). It was so large and heavy that it required the largest and most powerful aircraft to pull it off the airfield and subsequently tow it; four-engined bombers were used, most frequently the Handley Page Halifax. Both wing and cockpit were set above the fuselage to provide the greatest amount of room for the cargo compartment, and to ensure that they did not interfere with the loading of vehicles; the compartment measured approximately 32 feet 31.5 inches (10.554 m), 7 feet 10.5 inches (2.400 m) inches wide and between 6 feet (1.8 m) and 7 feet 7 inches (2.31 m) inches in height. The nose of the glider was hinged and opened to the side for ease of loading vehicles and cargo, and the crew of two pilots were seated in tandem in a cockpit on the top of the fuselage, which was accessed via an internal ladder and was fifteen feet above ground; they were eventually protected by a bullet-proof windscreen and a plate of armour behind the second pilot. An intercom was also added to provide communication between the pilots and the personnel below them. An initial design feature, which was eventually removed prior to full-scale production, was the installation of an under-fuselage hatch which would allow the prone firing of a Bren light machine gun as the glider approached the landing zone.

The ratio between length and wingspan was practically the same as that of an Avro Lancaster bomber, which had a wingspan of 102 feet (31 m) and a length of 69 feet 6 inches (21.18 m), in contrast to modern sport gliders which possess a particularly large wingspan to enhance gliding performance. This was the result of a decision taken by the War Office in early 1940 on how military gliders would be used; the idea was for the glider to be released at a low altitude close to the landing zone and conduct a steep descent to reduce time in the air and exposure to enemy fire. The glider also possessed large flaps which assisted in a steep and rapid descent, and through adjustments of their angle during landing a precise control over descent rate and point of landing could be achieved; they also allowed a slower touchdown speed to be attained. They were operated through a small bottle of compressed air large enough only for a single landing; a small bottle not only saved weight, but gave a smaller chance of it being hit by enemy fire, thereby exploding and damaging the glider. Standard approach speed for the Hamilcar was 100 miles per hour (160 km/h), although for shorter landings this could be slowed to 80 miles per hour (130 km/h), and stalling speeds were 64 miles per hour (103 km/h) with flaps up or 52 miles per hour (84 km/h) with flaps deployed. The Hamilcar was fitted with tailwheel landing gear, with oleo-pneumatic shock absorbers that could be deflated to bring the fuselage nose down for loading or unloading purposes. A jettisonable undercarriage was initially designed for the glider, as it was discovered that it travelled for a shorter distance when it landed only on its skids. However, this was eventually replaced with a fixed undercarriage, as pilots found that they preferred to land on wheels because of the extra control it gave them and the ability to avoid other gliders and potential collisions in the landing area.

When the glider was carrying tanks or other vehicles, common practice was that their engines would be started in the air, usually just prior to the glider casting off from the tug; special exhaust ducts were fitted to the glider to expel exhaust fumes. The Tetrarch and M22 Locust light tanks were so large that they barely fit inside the glider, and as such their crews stayed inside the tank for the duration of the flight. Once the glider landed, the anchorages keeping the vehicle stationary would be released by the driver pulling a lanyard within his reach, and the driver would then drive the tank forward, which automatically pulled a line that operated the swing door release. However, universal carriers and other vehicles relied on one of the pilots operating the door line manually. This was achieved by the pilot sliding down the fuselage and then dropping to the ground. They would then go to each undercarriage leg and release the valves there, which would expel hydraulic fluid and allow the shock absorbers to deflate, and then enter the glider and operate the door release line. If the swing door was jammed after the glider had landed, it was possible for tanks to break through the unopened forward fuselage and drive straight out of the glider, which occurred in both airborne operations where Hamilcars transported tanks.