FACTORS THAT MAKE FOR SAFETY
It has been calculated that nearly half the aeroplane disasters of the
early days were due to a structural weakness in machines, or to
mistakes either in their design, or in such details as the position,
shape, and size of their surfaces. To-day, thanks to science, and to
the growing skill and experience of aeroplane designers and
constructors, this risk of the collapse of a machine in the air, or of
its failure to respond to its controls at some critical moment through
an error in design, has been to a large extent eliminated. That such
risks should be eliminated wholly is, as yet, too much to expect.
One of the factors making for safety has been the steady growth in the
general efficiency of aircraft: in the curve of their wings which, as
a result largely of scientific research, has been made to yield a
greater lift for a given surface and to offer a minimum of resistance
to their passage through the air; in the power and reliability of
their engines; in the efficiency of their propellers; and in the
shaping of the fusilage of a machine, and in the placing and
"stream-lining" of such parts as meet the air, so as to reduce the
head resistance which is encountered at high speeds. Such gains in
efficiency, which give constructors more latitude in the placing of
weight and strength where experience show they are needed, have gone
far to produce an airworthy machine. In the old days, when machines
were inefficient, a few revolutions more or less per minute in the
running of an engine meant all the difference between an ascent and
merely passing along the ground. But nowadays, through the all-round
increase in efficiency that has been obtained, a machine will still
fly upon its course without losing altitude, and respond to its
controls, even should the number of revolutions per minute of its
engine be reduced considerably.
When given a greater efficiency in lifting surfaces and
power-plants--and profiting also from the lessons that had been learnt
in the piloting of machines--constructors were able to devote their
attention, and to do so with certainty instead of in a haphazard way,
to the provision of factors of safety when a craft was in flight. With
a machine of any given type, if driven through the air at a certain
speed, it is possible to estimate with accuracy what the normal
strains will be to which it is subjected. But even if such data are
obtained, and the machine given the strength indicated, this factor of
safety is insufficient. It is not so much the normal strains, as those
which are abnormal, that must be guarded against in flight. A
high-speed machine, if piloted on a day when the air is turbulent, may
be subjected to extraordinarily heavy strains; rising many feet in the
air one moment, falling again the next, and being met suddenly by
vicious gusts of wind--in much the same way that a fast-moving ship,
when fighting its way through a rough sea, is beaten and buffeted by
the waves. Air waves have not of course the weight, when they deliver
a blow, that lies behind a mass of water; but that these wind-waves
attain sometimes an abnormal speed, and have a tremendous power of
destruction, is shown in the havoc that is caused by hurricanes.
It seems astonishing to many people that such a frail machine as the
aeroplane, with its outspread wings containing nothing stronger often
than wooden spars and ribs, covered by a cotton fabric, should be
capable of being driven through the air at such a speed, say, as 100
miles an hour, encountering not only the pressure of the air, but
resisting also the fluctuations to which it may be subjected. But,
underlying the lightness and apparent frailty of such a wing, when one
sees it in the workshop in its skeleton form, before it has been
clothed in fabric, there is a skill in construction, and an experience
in the choice, selection, and working of woods, that produces a
structure which, for all its fragile appearance, is amazingly strong.
And the same applies, nowadays, to all the other parts of an
aeroplane. That it should have taken years to gain such strength, and
to reduce so largely the risk of breakage, is not in itself
surprising. Men had to devise new methods in construction--always with
the knowledge that weight must be saved--and to create new factors of
safety, before they could build an airworthy craft.
To-day, when a man flies, he need have no lurking fear, as had the
pioneers, that his craft may break in the air. Even when it is driven
through a gale, plunging in the rushes of the wind, yet held straining
to its task by the power of its motor, the modern aeroplane can be
relied upon; and not in one detail of its construction, but in every
part. Experience, the researches of science, and the growing skill
with which aircraft are built, stand between the airman and many of
his previous dangers. The aeroplane to-day, one of the structural
triumphs of the world in its lightness and its strength, has a factor
of safety which is sufficient to meet, and to withstand, not merely
ordinary strains, but any such abnormal stresses as it may
encounter--and which may be many times greater than the strains of
normal flight.
The aviator knows also that his engine, as it gives him power to
combat successfully his treacherous enemy, the wind, represents the
fruit of many tests and of many failures, and of the spending of
hundreds of thousands of pounds. Many of its defects have revealed
themselves, and been rectified; it is no longer light where it should
have weight of metal, nor weak where it should be strong. So far as
any piece of mechanism can be made reliable, consisting as it does of
a large number of delicate parts, operating at high speed, the
aeroplane motor has been made reliable. But, so long as one motor is
used, there must always, as we have said, remain a risk of breakdown.
It is for this reason that, thanks largely to the stimulus of the
war--which has created a practical demand for such machines--aeroplanes
are now being built, and flown with success, which are fitted with
duplicate motors. With such machines, which give us a first insight as
to the aircraft of the future, engine failure begins to lose its
perils--particularly in regard to war. More than once during the great
campaign, when flying a single-engine machine, an aviator has found
his motor fail him, and has been obliged to land on hostile soil; with
the result that he has been made prisoner. But with dual-engine
machines it has been found that, when one motor has failed
mechanically, or has been put out of action by shrapnel, the remaining
unit has been sufficient--though the machine has flown naturally at a
reduced rate--to enable the pilot to regain his own lines.
In peace flying, too, as well as in war, the multiple-engined
aeroplane brings a new factor of safety. If one of his motors fails,
and he is over country which offers no suitable landing-place, the
pilot with a duplicate power-plant need not be concerned. His
remaining unit or units will carry him on. There are problems with
duplicate engines which remain to be solved--problems of a technical
nature--which involve general efficiency, transmission gear, and the
number and the placing of propellers; but already, though this new
stride in aviation is in its earliest infancy, results that are most
promising have been obtained.
To those who study aviation, and have done so constantly, say from the
year 1909, one of the most striking signs of progress lies in the fact
that, though unable at first to fly even in the lightest winds, the
aviator of to-day will fight successfully against a 60 miles-an-hour
wind, and will do battle if need be, once he is well aloft, with a
gale which has a velocity of 90 miles an hour. He will ascend indeed,
and fly, in any wind that permits him to take his machine from the
ground into the air, or which the motor of his craft will allow it to
make headway against. And here, though machines are still experimental,
there is removed at one stroke the earliest and the most positive
objection of those who criticised a man's power to fly. When the first
aeroplanes flew the sceptics said: "You have still to conquer the wind,
and that you will never do. Aeroplanes will be built to fly only in
favourable weather, and this will limit their use so greatly that they
will have no significance." But to-day the aviator has ceased, one
might almost say, to be checked or hampered by the wind. If the need
is urgent, as it often is in war, then it will be nothing less than a
gale that will keep a pilot to the ground, provided he has a
sufficiently powerful machine, and a suitable ground from which to
rise--and granted also that he has no long distance to fly.
Wind-flying resolves itself into a question of having ample
engine-power, of being able to launch a machine without accident, and
get it to earth again without mishap; and of being able to make a
reasonable headway against the wind when once aloft; and these
difficulties should solve themselves, as larger and heavier machines
are built.
Apart from the growing skill of the aviator, which has been bought
dearly, science can now give him a machine, when he is in a wind, that
needs no exhausting effort to hold it in flight. Craft are built, as a
matter of certainty and routine, which have an automatic stability.
Science has made it possible indeed, by a mere shaping and placing of
surfaces, and without the aid of mechanical devices, to give an
aeroplane such a natural and inherent stability that, when it is
assailed by wind gusts in flight, it will exercise itself an adequate
correcting influence. To understand what this means it should be
realised that, when such a machine is in flight say in war on a
strategical reconnaissance, and carries pilot and passenger, the
former can take it to a suitable altitude and then set and lock his
controls, and afterwards devote his time, in common with that of his
passenger, to the making of observations or the writing of notes. The
machine meanwhile flies itself, adapting itself automatically to all
the differences of wind pressure which, if it had not this natural
stability, would need a constant action of the pilot to overcome. All
he need do is to maintain it on its course by an occasional movement
of the rudder. With such a machine, even on a day when there is a
rough and gusty wind, it is possible for an airman to fly for hours
without fatigue; whereas with a machine which is not automatically
stable, and needs a ceaseless operation of its controls, the physical
exhaustion of a pilot, after hours of flight, is very severe.
So, already, one sees these factors of safety emerge and take their
place. There is no longer a grave peril of machines breaking in the
air; there need be no longer, with duplicate power-plants, the
constant risk of engine failing; while that implacable and treacherous
foe, the wind, is being robbed daily of its perils.