The airman's emergency exit
Although the parachute is one of the earliest of aerial inventions it is only in comparatively recent years that it has been considered seriously and used as a necessary adjunct to aviation.
Before the war it was commonly regarded in the same light as the trapeze and lion taming—a means whereby the sensation-seeking spectator could be thrilled but which was otherwise useless.
This attitude changed, of course, when the war came and parachutes proved their worth as life-savers in the kite-balloon section of the R.F.C. But while this was so, their use in connection with aeroplanes at that time was only on an experimental basis and was finally discarded after tests had shown them to be none too certain in action or easy in manipulation. It was one thing to make a jump from a captive and stationary balloon, but quite another thing to get clear from a spinning, aeroplane falling at 130 m.p.h. or so. The difficulty was that the time of opening of the parachute was not controlled by the wearer with the result that his ‘chute was liable to become entangled in the empennage of the machine from which he might be attempting to escape. And in addition to this, the apparatus was somewhat bulky and encumbered the airman to a considerable degree. The doubtful practicability of the parachute then for use in aeroplanes coupled with other more pressing needs of the war, influenced the authorities in ceasing their efforts in this direction.
Private individuals and concerns continued the work of developing the parachute, however, and eventually a form was produced which overcame the original difficulties. This type, which was the forerunner of the present standard equipment parachute of the R.A.F. (the Irvin), was known as the “free-type, manually-operated.” It was termed the ” free-type ” because it was carried complete in one unit, strapped to the body of the airman. In this way it differed from the earlier apparatus where the container of the chute was fitted to the machine and the ‘chute itself operated by being pulled out as the airman fell from the plane. Thus, at once the danger of the parachute opening too early and fouling the aircraft was done away with as the ” free ” type operated independently of the plane and could be timed to open by the wearer by a ” pull-ring ” located on the harness.
Superficially considered, the production of such a contrivance might appear a moderately simple matter, but in point of fact there were many problems and many factors to be taken into consideration before the Irvin type of ‘chute was brought to the high standard of efficiency which one finds to-day. In the first place it had to be of such size and so disposed as to give a reasonable degree of comfort to the wearer and allow him to leave his machine with the least difficulty or delay. It was essential, too, that it should open promptly and be able to withstand the sudden shock incurred by the falling of the airman at a high velocity. The harness had to be designed so that the shock of opening was transferred in such a manner as to obviate physical injury, and to prevent the wearer falling out, regardless of his position in the air when the opening occurred.
The possibility of landing in a high wind or in the water had to be considered also, so that it was necessary that the harness be made quickly detachable. Finally the axiom “a chain is as strong as its weakest link” had to be borne in mind, so that the overall strength had to be uniform from the lowest coupling of the harness to the top of the parachute itself.
In the Irvin service type, which, as previously mentioned, is standardised in the R.A.F., the parachute is 24 feet in diameter, the fabric used being the finest Japanese silk. This is attached to the harness by “shroud lines” of silk cord which has the extraordinarily high tensile strength of 400 lbs. These lines are continuous from the point of fixing on one side of the harness to the other and they pass through and over the top of the parachute, without any knots or splicing. The harness is made of specially woven linen webbing and is attached round the shoulder, body and legs of the wearer in such a manner that no bodily injury results when the ‘chute opens at a high speed. The harness webbing has a tensile strength of 3,000 lbs., and is reinforced at all metal parts.
Attached to the apex of the main ‘chute is a pilot ‘chute of 30 inches diameter, which, actuated by a springloaded frame, leads the main ‘chute out into the line of flight as soon as it is released from the pack. While not essential, this pilot ‘chute has been proved to be valuable in reducing the time required for the complete opening of the main ‘chute. The average time required for opening and to assume normal descent, by the way, is but one and three-fifths seconds after the pull-ring has been pulled.
Probably the most ingenious part of the Irvin is the pack or container, wherein is housed the whole of the body of the ‘chute and its shroud lines. This is made of special fabric and so constructed that the instant the pull-ring is actuated the pack flies open and allows the ‘chute to free itself immediately, led by the pilot chute. One may best describe the pack, perhaps, as a parcel, with four flaps meeting centrally on one face and held together by locking cones passing through grommets. Located through these cones is the rip-cord locking-pin, which is attached to the pull-ring by a cable.
Fixed externally to each flap are elastic strips which fling the flaps open the instant the rip-cord pin is pulled from the cones.
Needless to say the packing of the parachute in the container is of the utmost importance and the interior of the latter is so arranged that fouling or entangling is impossible. The shroud lines are led through pockets in zig-zag formation so that on the ‘chute being released they flow out easily and evenly and the silk body of the main ‘chute is folded and so positioned as to be clear of the pilot ‘chute and other parts. In manufacture, every detail and every piece of material is subjected to the strictest scrutiny and constant test both by the makers and the Aeronautical Inspection Department of the Air Ministry and every parachute issued to the R.A.F. is subjected to two gruelling” dummy” tests, before final acceptance.
The usual type of ‘chute in use at the present time is the ” seat-pack “, which as its name implies is arranged so that the wearer sits on it when in the cockpit of a plane. There is also the “lap-pack,” which has been developed for the use of aerial photographers and observers and the ” back-pack,” for use on lighter-than-air-craft which permits complete freedom of movement for walking about.
The standard Irvin ‘chute weighs 18 lbs. and its rate of descent is 16 feet per second. In all something over 15,000 ” live ” drops have been made without failure, the heights from which they were attempted varying from 20,000 feet to a mere 150 feet.
G. G. O. M.