The Romance of War Inventions Part 22

At intervals round the edge are fastened twenty-four V-shaped tapes.

These are only a few feet long and the lower end of each V-shaped pair is attached to a long main tape. There are twelve of these main tapes, and their lower ends unite in a metal disc from which is suspended the sling and harness by which the man is supported.

[Ill.u.s.tration: THE "GUARDIAN ANGEL" PARACHUTE.

(1) Shows the airman in the harness by which he is attached to the parachute. By means of the star-shaped buckle he can instantly release himself. (2) Shows the parachute two seconds after the airman has jumped from the aeroplane. In (3) he is seen nearing the ground.

(_By permission of E. R. Calthrop, Esq._)]

So the twenty-four short tapes form twelve V's to the points of which are attached the twelve long tapes which support the man. The reason why tapes are used in this particular parachute and not cords will be referred to later.

In the crown of the silk body there is the usual hole for the purpose of forming the air-rudder to steady the parachute in its descent.

And now we can consider the first great feature of this wonderful invention and ask ourselves these questions: "By what means is it made to open?" "What makes it more reliable than others?"

To answer that we must first see why the others sometimes refused to open. In whatever way an ordinary parachute may be packed it must, when coming into use, a.s.sume the state of a shut umbrella with a hole in the top.

In this condition it is a.s.sumed that as it falls the air will find a way in through the lower end and will blow the parachute open in precisely the same way that a strong wind will sometimes blow out the folds of an umbrella.

But, as a matter of fact, the loose folds of a parachute, when the edge of the gussets is gathered in, are sure to overlap and enfold each other more or less. Thus, when in the shut-umbrella state, it sometimes happens that air which is inside can escape upwards through the hole more easily than fresh air can get in from below. The parachute, in such a state, is, let us imagine, falling rapidly through the air. The result is just the same as if it were still and the air were rus.h.i.+ng upwards past it. And the upward rush past the top hole tends to _suck air out_ through the hole faster than fresh air can find a way in at the bottom.

This is the principle of the ejector, which engineers have put to many uses. For example, the vacuum brakes employed on many large railways owe all their power to stop a train to a vacuum caused by an ejector. There is a short tube or nozzle, placed in the centre of another tube through which steam blows. The action of the steam in the outer tube as it rushes past the end of the inner tube drags after it the air which is in the inner tube so effectively as to produce quite a good vacuum. And in precisely the same way, the upward rush of air past the parachute, or what is just the same, the falling of the parachute through stationary air, can suck the air from inside the latter and create a vacuum in it if the gussets gathered together at the mouth unfortunately overlap one another and are thus locked together by the pressure of the air striving to get in. Thus, instead of the downward fall causing the ordinary parachute to open, as in most cases it will do quite well, the fall under these particular conditions actually binds its folds together and prevents it from opening. It is true this does not often happen, but the risk is _always_ present at every drop, and this unreliability has cost the lives of brave men and women, and the knowledge of this constant risk has led others to write down the parachute a failure, by reason of its known unreliability to open instantly. Even when it does open the depth it falls before it opens is so variable, by reason of the fight between vacuum and pressure, that it may be one hundred feet one time and one thousand feet next time with the same parachute.

Now the "Guardian Angel" is designed so that those conditions cannot occur. Its silken covering is first laid out on the ground and into the centre is introduced a beautifully-designed disc of aluminium, somewhat like a large inverted saucer, of exceeding lightness but of ample strength for what it has to do. Then the silk body is pleated and folded back over the upper part of this launching-disc and gradually packed so that it occupies but a very small s.p.a.ce upon the upper surface of the disc. It is so folded that its edge comes in the topmost layer and also in such a manner that on the tapes being pulled the silk unfolds easily and regularly, flowing down as it were over the edge of the disc almost as water flows if allowed to fall from a tap upon the centre of an inverted saucer. After the folding is complete another aluminium disc is placed above the packed silk body which s.h.i.+elds it from the enormous air pressure when it is being released from an aeroplane flying at top speed. The upper and lower fabric covers are then superimposed and sealed and the "Guardian Angel" parachute is ready for use.

The tapes, likewise, are folded up, in a special way upon the bottom cover, which is sprung over the bottom of the disc. The bottom cover with the tapes upon it, is pulled away by the weight of the airman as he makes his jump to safety, and the tapes are so arranged that a pull upon them causes them to draw out steadily and smoothly, almost like water falling from a height.

If we regard the silk as forming a shallow bag inverted, we may say that it is folded upon the disc inside out and the function of the disc is to cause it to spread and enclose a wide column of air as it is pulled from its folds. To commence with it is nothing more than so much folded-up silk, but from the first moment of action it becomes a bag with a wide-open mouth, for its open mouth cannot be smaller than the disc.

Therefore, from the first instant it begins to grip the air and the ejector action never gets a chance to commence. The pressure of air inside is from the very commencement of the fall greater than that of the surrounding air. Moreover, the disc covers the hole until the parachute is actually open, thereby making ejector action doubly impossible.

The widely-opened mouth of the air-bag (I cannot help repeating that term for it is so expressive) swallows up more and more air as the thing falls rapidly, with the result that the air inside is instantly compressed and the increasing pressure as the silk is more and more fully drawn out causes it to expand until the whole is fully extended like a huge umbrella. The instant compression of the enclosed column of air is what causes it _always_ to open automatically.

When once it is pointed out it is easy to see what a difference the presence of this disc makes. It is so simple that it cannot fail to act and having once produced that open mouth all the rest is due to the action of natural forces which can be absolutely relied upon. The ordinary parachute with its hopeless irregularities has, in fact, been converted into a machine whose action can _never_ fail.

The disc is fastened to the balloon or aeroplane and is left behind when the parachute falls, having done its work.

And now let us consider the tapes. As has already been remarked, a series of coiled cords cannot be relied upon to pull out straight without possibility of entanglement, but a tape, if folded to and fro like a Chinese cracker, will invariably do so. So packed tapes have been subst.i.tuted for coiled corded rigging, with the certainty that they cannot be entangled in the fiercest air current.

And now we come to another interesting feature. The man is not suspended directly from the small disc to which the tapes are attached but by a non-spinning sling which contains a shock absorber. This latter consists of a number of strands of rubber and it is owing to its action that the aviator who trusts his life to the parachute suffers little or no shock; even when the instant opening of the parachute begins to arrest his fall. And not only does it save him from shock, but it also avoids the possibility of too great a stress coming suddenly upon the parachute or its rigging of tapes.

The aviator himself is attached to the parachute through the shock-absorber sling, by means of a harness which he wears constantly throughout his flight, so that in the event of trouble he only has to jump overboard and the parachute automatically does the rest. This harness consists of two light but strong aluminium tubular rings through which he places his arms, combined with a series of straps which can be so adjusted that the stress of carrying him comes upon those parts of his body best adapted to bear it.

This improved parachute is the only one which is capable of being used instantly and without preparation for descent from an aeroplane flying at top speed. It is easy to see that it is one thing to drop from a stationary or nearly stationary balloon and quite another to dive from an aeroplane at one hundred miles per hour. The latter is equivalent to suddenly trusting oneself to a parachute _during the strongest gale_. It has been found, by experiment, however, that high speed is no bar to the use of this parachute since it only causes the parachute to open a little more quickly than usual, which means that it can be used with safety from an even lower height.

Under the worst conditions this wonderful parachute can be relied upon always to open and carry its load at a height of only one hundred feet, and its use is safe in all circ.u.mstances when dropped from two hundred feet above the ground. After it has once got into operation and taken charge of affairs, so to speak, the man descends at the rate of only fifteen feet per second, which is just about the same as dropping from a height of a little over three feet. In other words, he will arrive on the ground with no worse b.u.mp than you would get by jumping off the dining-room table.

But suppose that there were a wind blowing: would not the parachute come down in a slanting direction and then drag the man along? Or may he not alight upon a tree or the roof of a house, only to be pulled off again and flung headlong? Quite true he might, were not proper provision made for such occurrences. Embodied in the harness is a lock which can be instantly undone, by a simple movement of a lever in the hand, and by its aid the man on touching earth or on alighting upon anything solid can release himself instantly, after which the parachute can sail away whither it will, but he will be safe and sound.

What Mr. Calthrop has accomplished by the invention of his "Guardian Angel" parachute may be summarised briefly by saying that he has reduced the minimum height from which a parachute could be dropped from two thousand to two hundred feet, and that he has made it possible to launch a parachute, with the certainty of safety, from any kind of aircraft flying at the slowest or highest speed of which they are capable.

You are only a boy now, but when in years to come you are quite old and have grey hair you may become a Member of the Air Board and--who knows--it may become your duty to decide that this great invention shall be always used on the training grounds to save the lives of the young men, not yet born, who are then learning to fly. During the War, one was killed every day, 365 in a year, many of whom might have been saved had more "Guardian Angels" been in use.