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I hate to think how many Paras broke their jaw dropping through it, something known as ringing the bell.
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Why the RAF installed belly hatches in Bristol Beverleys is a complete mystery to every other army??????
That whole "ringing the bell" tradition started during early World War 2, with hastily converted Whitley Bombers. When Churchill ordered the rapid establishment of a parachute force, the British aircraft industry could not produce enough transports, so they converted obsolete bombers.
First they tried gutting the tail turret on a Whitley and had paratroopers do pull-offs with slightly modified pilot emergency parachutes. After too many refusals, George Quilter develop the X type static-line parachute and they removed the belly turret from Whitleys. Dropping out the belly proved far easier, but the turret ring was too small for heavily-laden paratroopers, so any less-than-perfect exit saw them whacking their heads on the turret ring! Ouch!
aka. ringing-the-bell.
This led to the Sorbothane helmet which was little more than a ring of sponge rubber wrapped in cloth. That padding soon developed into the liner for the Para-Helmet that remained in service until 1980.
The other problem was capacity, limiting Whitleys to 6 or 8 paratroopers.
Britain never developed enough transport airplanes to carry enough paratroopers for D-Day, so most Brit and Canadian paratroopers jumped out of C-47 Dakotas on D-Day and Arnhem. They were glad to receive Curtiss C-46 Commandos before jumping across the Rhine River because C-46 could carry more jumpers and with two streams exiting both side doors, could empty the plane in fewer seconds.
Early SAS had it easier with the side doors on Bristol Bombays. In the long run, side doors proved easier because you can stand on your hind legs - like a gentleman - and simply walk out the door. Standing vertically is doubly important when jumping with a rucksack, rifle and snowshoes that weigh almost as much as the paratrooper.
Rear cargo ramps are the easiest to jump, but you can still unload double side doors quicker. Amazingly, Russian paratroopers jump from all four doors on IL-76 jet transports, even the personnel doors ahead of the engines! Fortunately they drop away so steeply that there is little risk of fowling a jet engine..