SOME OF THE PROBLEMS THE INVENTORS HAD TO SOLVE
Every American must feel a glow of pride when he stops to think that it
was two of his fellow-countrymen, Wilbur and Orville Wright, who
invented the airplane. But it is largely to France, our great ally and
friend, that the credit must go for improving upon the invention of the
Wrights, and making possible the wonderful aerial feats, the marvelous
flights and accomplishments of the airplane of to-day. From the first
day they saw an airplane flown, the French were wildly enthusiastic.
They gave freely of their money and their encouragement to help the good
cause along. French inventors attacked the problems of the
heavier-than-air machine with a will, and their unfailing determination
and refusal to accept defeat or failure made final victory inevitable.
But before we could have the powerful fighting machines, the big cross
country fliers and the seaplanes of to-day, there were many difficulties
of construction which had to be met and solved.
First of all the pioneer designer had to choose between the monoplane,
the biplane and the triplane. The monoplane was light in weight and
could fly faster with the same powered engine than the biplane. But it
was difficult to know just how to brace and strengthen the single pair
of wings. In the biplane the struts between the wings gave strength and
firmness. The wings of the monoplane were braced by wires to the body,
but often they did not prove strong enough and the airplane collapsed in
mid-air. In spite of this danger the monoplane was much in favor because
of its speed.
Slower in speed, but stronger and a better weight lifter was the
biplane. And in addition to strength it possessed more natural
stability, a much sought after quality in the pioneer days.
Even more stable and with greater lifting powers than the biplane was
the triplane, but the difficulty here was the lack of an airplane motor
of sufficient strength to drive it. Until clever engineers came to the
rescue with an improved aircraft motor, the triplane was very much in
disfavor.
The monoplane, indeed, captured most of the early records for speed and
it was this type of machine that was generally built by the sportsman
type of airman, while men like the Wright brothers and others whose aim
was to develop an airplane of unusual reliability and suited to many
purposes, turned to the biplane and gave many hours and months and years
of their time to its improvement.
Once the choice of a _type_ had been made, there were countless other
problems. _Stability_ was of prime importance and the airmen of a few
years ago labored desperately to attain it. They knew all too little
about the airplane from a scientific angle. We have seen in our brief
study that the method of obtaining balance in a glider or an airplane is
to see that its _center of weight_ coincides with the center of the
_upward pressure_ of air. How to bring this happy state of things about
was a source of much debate. Some suggested that instead of a tail at
the stern a tail in front of the main planes of the machine would help
to balance it in flight. Some placed the pilot's seat above both planes
of the biplane, while others thought he should sit below. Many of these
queer ideas were tried out and by dint of hard practise and many
failures certain simple elementary facts were finally weeded out and set
down.
Probably the addition of a "fuselage" or body to the modern airplane has
had something to do with helping in the proper distribution of its
weight and increasing its stability. Larger at the bow and tapering
toward the stern where a fixed tail piece or horizontal stabilizing
plane is attached, it resembled more or less closely the general
outlines of a fish or bird. And this "streamline form" greatly reduces
the _head resistance_, another important subject on which there was very
little known when the first of the airplanes was built. In addition to
having only a very slow and inefficient engine the early machine
suffered from the head resistance it created as it pushed forward
through the air, and this check to its progress ate up the little speed
its motor could develop. For if the airman of 1908 or 1909 was made
miserable by his fear of winds, gusts and aerial whirlpools which might
upset him in mid-air, his fears in this direction were completely
overshadowed by his worries about a suitable motor. If the design of his
craft was faulty and it proved "balky" when he attempted flight, he had
only himself to blame. But for an engine he had to rely entirely upon
some one else. The airplane could be a "home-made" article, but the
engine had to be chosen from such as were on the market.
The Wright brothers in their first flying machine used a made-over
automobile engine of 12 horsepower. It was not long before this was
improved upon, and later Wright machines had a four-cylinder,
water-cooled engine developing 35 horsepower. Its weight had been
reduced as far as possible and its simplicity of design was its greatest
recommendation.
Undoubtedly the engine problem has been the big one in the history of
aviation. The coming of the internal combustion engine might be said to
have placed practical aviation within the range of possibility, but at
that it took a long time to evolve a motor especially suited to the
needs of aircraft. There were three things needed in an airplane motor:
_Light weight_, _high power_, and _absolute reliability_. How important
the third factor is we can imagine if we stop to think that nothing
keeps the heavier-than-air machine afloat but its own speed, creating an
air pressure beneath its wings. Like the boy who runs with his kite in
order to make it go up, the airplane must "go" if it would rise, and the
moment its engine fails there is nothing to prevent it from falling to
the earth. The driver of a motor car, can, if his engine goes wrong, get
out and go over it carefully until he finds what the difficulty is. The
pilot of an airplane, soaring thousands of feet above the earth, is at
the mercy of his motor's reliability or lack of it. Engine failure was,
and still is, one of the greatest dangers the airman has to fear.
Another chief cause of trouble in early airplane motors was overheating.
Before actual airplane engines had been designed there was nothing to do
but to use the type of engine which had been designed for the
automobile, with as much reduction in weight as could be secured. But
the automobile engine was never intended to run at top speed
continuously and for long periods, as the airplane engine necessarily
must do. In a car the motor has little stops and rests, as it is
throttled down for a moment or changes in speed are made, and these
breathing spells help it very much indeed in the "cooling off" process.
The airplane engine does not have these little between-time naps. The
result was that the automobile engine installed in the early airplane
invariably overheated and caused serious trouble. Under these
conditions no flights of any distance could possibly be attempted.
Yet at the Rheims Meeting of 1909 Henry Farman surprised the world by
remaining in the air two hours in a continuous flight. Up to that time
the feat had never been equaled or approached. Aviators were amazed and
sought an explanation. The answer was: the Gnome motor.
Anxious to help the airplane in its forward march, French engineers had
good naturedly set to work and the Gnome motor was their first answer to
the anxious question of "What engine?" It involved a new and ingenious
system of cooling which made it possible for Farman to drive his big
machine round and round the Rheims course until stopped by darkness, but
without ever experiencing the slightest difficulty with his motor.
Before attempting to understand the secret of superiority of this first
real airplane motor over others of its day, we must know a little more
about the elementary principles of any internal combustion engine. The
diagram on page 156 shows _one cylinder_ of such an engine in action.
A mixture of gasoline and air--called "carbureted air"--is introduced
through a valve opening into a chamber or cylinder, as shown in figure A
of the diagram. The valve opening then closes, and the piston moves
forward compressing the gases enclosed in the cylinder, as shown in
figure B. An electric spark suddenly explodes these compressed gases,
causing them to expand with the greatest violence and drive the piston
back. This action, which is shown in figure C, is called the "power
stroke," for, transmitted by the piston rod to the crankshaft it
furnishes the power which turns the propeller and sends the airplane
forward through the air. Just before the piston reaches the end of the
power stroke the exhaust valve opens, and the exploded gases are forced
out of the chamber, partly by the force of their own tension and partly
by the upward stroke of the piston, as shown in figure D.
The carbureted air is supplied to the cylinder from a chamber called the
"carbureter." Here it is produced by the mixture of a gasoline
spray--similar to the fine spray of an atomizer--with the air.
[Illustration: DIAGRAM OF AN INTERNAL COMBUSTION ENGINE CYLINDER, SHOWING
PRINCIPLE ON WHICH IT WORKS]
A spark plug is fitted to the cylinder, and a break current from an
electric magneto causes the spark which at the proper instant explodes
the compressed gases.
Since by means of the explosion of the gases the force is produced which
drives the airplane propeller, the violence and frequency of these
explosions determine the power of the engine. Greater power can be
obtained either by increasing the size of the cylinder so that it can
hold more of the carbureted air, making a greater explosion possible; or
else by causing more frequent explosions. The latter is the better
method in an airplane engine, as larger cylinders mean more weight to be
carried. In the average airplane engine from 1500 to 2000 explosions or
revolutions occur per minute.
The combustion cylinder of an aircraft engine is usually built of steel,
and the piston of cast iron or aluminum, which furnishes a very smooth
gliding surface. The piston rod transmits the power to the crankshaft, a
long rotating piece of steel. Every time the piston rod is thrust down
by the explosion in the cylinder, its motion serves to turn the
crankshaft and thus the vertical motion of the piston is transformed
into the rotary motion which sends the propeller whirling through the
air.
Wherever two surfaces of metal must rub against each other, as in the
case of the piston and the cylinder, there is bound to be a great amount
of friction. This friction causes the parts to heat and in time it wears
away the surfaces and destroys the efficiency of the engine. In order to
avoid this, the surfaces must be kept constantly well oiled or
"lubricated." In some engines all the parts are enclosed in one large
box or "crank case" which is filled with oil. Small holes are bored
through to the surfaces to be lubricated, and the oil is splashed upon
them by the motions of the piston rod, the crankshaft, etc., as they
plunge through the oil bath.
But overheating of the cylinder may cause this oil to decompose and in
order to prevent this a "cooling system" is necessary. For only when the
engine is kept cool and properly oiled can it be expected to run
smoothly or give satisfactory service.
So now we come back to the problem of cooling, which caused so much
anxiety and trouble to the early aviators. With their engines running at
the great speed which was necessary to keep the airplane in the air,
overheating and engine difficulties were sure to arise. Cooling of the
cylinder is accomplished in one of two ways: either by water or by air.
If water is used, a "jacket" in which the water circulates is placed
around the cylinder,--the water as it becomes heated passing out of the
jacket to the radiator, where it is cooled before it returns. The
radiator, at the very front of the airplane body, is exposed to the
swift current of the air as the machine drives forward, and this air
current serves to reduce the temperature of the water.
This method was the one originally employed with the automobile engine,
but in the early models the cooling system, though adequate for the
motor car, was hopelessly insufficient when the same engine was
installed in an airplane.
It was the Gnome manufacturers who first thought of a most ingenious
scheme for cooling the cylinders of the internal combustion engine.
Instead of having the piston and the crankshaft move, it was the
cylinder itself which moved in the Gnome motor, while the crankshaft and
piston were stationary. Thus cooling was very easily accomplished, for
the cylinders, flying through the air, making as many as 1500
revolutions per minute, cooled themselves.
The crankshaft in the Gnome motor had been hollowed out to form a tube
or pipe, through which the fuel or carbureted air passed to the cylinder
by means of a valve in the head of the piston which worked
automatically. The Gnome could be built up of any number of cylinders,
according to the power required. Its cylinders were set in a circle
about the crankshaft, so that the entire engine occupied a minimum of
space in the airplane body. Scouted at first as a freak engine, it soon
proved its superiority over all those in use and was rapidly adopted by
builders of all types of airplanes.
To-day the stationary engine has been greatly improved, its provisions
for cooling have been increased and it is once more looked on with favor
by many manufacturers of aircraft.
The cylinders of an internal combustion engine can be grouped in one of
three ways, and thus there are three main types of airplane engines we
should be able to recognize. They are the _straight-line_ engine, the
_V-type_, and the _radial_. In the straight-line model four, six, or
even a larger number of cylinders are placed in a row in one crank case.
In the V-type of motor they are set instead in two lines, like a letter
V; while in the radial type the cylinders form a circle around the
central crankshaft. The radial motor may be stationary or its cylinders
may revolve, in which case it becomes a rotary engine, as for instance,
the Gnome.
Each of these types of motors has its peculiar advantages. The least
"head resistance" is caused by a straight line engine, and this type
also uses less fuel and oil. But it is usually heavier in weight, owing
to the larger cooling system necessary and the longer crankshaft, and it
takes up more room in the airplane fuselage than a motor of the compact
radial type. The radial engine is very light in weight,--a big item in
the airplane--but it consumes a large quantity of fuel and oil and
besides produces a maximum "head resistance." The V-type motor is a
compromise between the two,--lighter in weight than the straight-line,
less wasteful of fuel and causing less "head resistance" than the
radial.
The rotary engine, because of its appetite for fuel and oil is no longer
used in airplanes which are intended for long distance flights, because
here the weight of the extra fuel carried has to be considered. In short
distance, high-speed machines it works well, but in the larger planes
the vertical or V-type motor has been found to give greater
satisfaction.
When we read of the enormous trouble the pioneers of aviation went to,
in order to find an engine suitable to drive the propeller of the
airplane, we cannot help wondering just how the revolving of the
propeller sends the whole machine flying forward through the air. The
matter is very simply explained. The propeller of a ship is often
referred to as the ship's "screw," and though few people have ever
compared it with the small screws they use about the house, or with the
screw and screw driver in the tool chest, there is in fact very little
difference in principle.
Take a screw and place it against a block of wood, and then commence to
turn it with a screw driver. Straight into the wood its curved edges
will cut their way, dragging the round steel rod of the screw behind
them. With every turn they will cut in deeper and carry the screw
forward through the wood. That is what the propeller of a ship or an
airplane does: it screws its way through the water or the air. But of
course there is this difference, that the wood offers great resistance
to the forward motion of the screw, while the water offers much less
resistance to the ship's propeller, and the air less still to the
propeller of the airplane. If, as in the case of the screw-driver, the
airplane propeller is in front of the airplane and drags its load along
behind it, it is called a "tractor" propeller; but if instead it is
placed at the stern of the airplane, and as it screws through the air it
pushes the airplane along ahead of it, then it is known as a "pusher"
propeller.
The little cutting edge that winds round and round an ordinary screw is
referred to as its _thread_, and the distance between two of these edges
or threads is known as the _pitch_. In some screws the threads are very
close or, to put it another way, the pitch is small, while in others it
is much greater. Each blade of a propeller is really a portion of a
screw. To go back to the example of the screw-driver and the block of
wood, every time the screw is turned once around it will advance into
the wood a distance equal to its pitch. The same thing is theoretically
true of the propeller of an airplane; at each revolution it might be
expected to advance through the air a distance equal to the pitch that
has been given to its blades.
But the air may allow the propeller to slip back and so lose some of its
speed, a thing which was not possible with the screw-driver. This
tendency to slip varies with the pitch of the propeller and the speed of
its revolutions. A propeller which works splendidly when turning at a
given rate, may prove worse than useless when the engine is slowed down
and it is only making half the number of revolutions per minute. And so
we begin to see another of the big problems of the pioneer airmen: to
determine the right pitch for the propeller in relation to the speed
which had been determined upon for the airplane. It is a problem that
has not been wholly solved to-day, because of the fact that an airplane
cannot always be driven at "top speed." If the maximum speed of the
machine is 150 miles per hour, and the propeller has been designed to
deal with the air efficiently at this speed, it is apt to slip and slide
and waste away the power of the engine when for any reason it is
necessary to slow down to 100 miles per hour. The only answer to the
difficulty is a "variable pitch propeller" which may be altered to
conform with alterations in speed, but up to the present time nothing
really satisfactory along this line has been devised.
Another question in connection with the propeller has been of what
material to make it. Wood is most generally used to-day, for although
steel and aluminum have been tried, they have not been found to stand
the strain so well. Imagine for one moment the stress upon an airplane
propeller beating through the air at the rate of 1500 revolutions per
minute. The greatest strength has been secured by building it up of
several pieces of wood which are fastened strongly together and
varnished.
_Materials_ have always presented a source of endless experiment and
differences of opinion in the construction of the airplane. The problem
has come up in connection with the fuselage, the wings and wing
coverings, the landing chassis--in fact, each and every part of the
heavier-than-air machine has raised the old query: "What shall we make
it of?"
In the earlier machines wood was almost entirely used in airplane
construction. For one thing it was cheaper, and for another it was
easier to get wood working machinery, than the complicated and expensive
machinery necessary to construct airplanes out of metal. Metals are
stronger but they cost more and they make the problem of repairs more
difficult.
The wings of the airplane are usually built up on a wooden framework
which gives them their shape and curve. Many have been the disputes over
the matter of wing coverings. In the pioneer machines they were covered
with cotton material which had not been treated to make it water-proof
or air-proof. It gave the poorest kind of service, and an effort was
made to improve it by rubberizing it, but this process did not produce a
wing of lasting durability. Many other treatments were experimented
with, but with little success until the substance known as "dope" made
its appearance.
"Dope" is largely composed of acetyl cellulose. It makes the wing
covering proof against rain, wind, and the oil thrown off from the
airplane engine, and gives it a fine, smooth finish and excellent
durability. Two or three coats of it are usually applied, with a final
coat of varnish on top, to produce a wing that is sure to prove strong
and trustworthy.
The problems of starting and landing the airplane have been many. The
early Wright machine had to run on a little trolley down a track in
order to gain sufficient momentum to take to the air. Later machines
showed an improvement on this. Henry Farman attached wooden skids to the
bottom of his airplane and fastened wheels to them by means of heavy
rubber bands. Thus he could start his motor and run over the ground
until his speed permitted him to rise, while in making a descent the
wheels flew back on their flexible bands and the stout skids absorbed
the shock of the fall. Most of the modern machines have a wheeled
framework below the fuselage, which permits them to run over the ground
in starting and also in making a descent. The danger of engine failure
becomes very important when near to the ground, as the pilot has no time
to get his machine into a gradual glide and avoid a bad accident. This
danger is sometimes averted by installing two engines, so that if one
stops the other will carry the airplane on up into the air and prevent a
smash-up. But the thing which has greatest effect on the ability of the
airplane to land easily is its own design and speed. The wings of the
airplane, its propeller and its whole construction have been planned so
that it can support itself best in the air when flying at a certain
fixed speed. Suppose this speed for a certain type of airplane to be 150
miles per hour. The airplane cannot land while traveling at that rate,
yet its speed while still in the air can only be diminished to a certain
point with safety, and below that point it may not be able to sustain
itself in flight. The pilot must be able to land his machine without
accident and without throttling his engine below this danger line; while
the designer of airplanes must struggle to produce a machine which,
while flying best at its maximum speed, will _fly_ at a much lower rate
of motion, when necessary to effect a landing.
The supporting power of the wings depends partly on their size and
partly on their rate of motion. Small wings moving at high speed produce
the same supporting pressure of air beneath them as large wings flying
at slow speed. The problem of a safe landing could best be solved by
building wings whose area could be altered in mid-air. When traveling
under full power the pilot would reduce the wing spread, as the smaller
wings would then be sufficient to support the weight of the machine and
would create less air resistance. When about to land, he would increase
the spread of the wings, so that at the slower rate of motion through
the air he might take advantage of a larger supporting surface. Nothing
of this sort has yet been worked out on a practical scale, but many have
been the suggestions for "telescoping wings."
The reduction of "head resistance" and the "streamlining" of the
airplane have received their goodly share of attention and experiment.
To-day the airplane fuselage is carefully streamlined, but the landing
chassis beneath it creates a good deal of resistance to motion. Probably
this problem will be solved by devising a landing chassis which, after
the machine has arisen from the ground, can be drawn up inside the body,
and let down again to make a landing, but this is another important
question which is not yet worked out in the airplanes of the present
time.
The coming of the War caused all nations to stop and take strict account
of what had been accomplished in solving the many problems of aviation,
for the war machine had to be as nearly as possible the sum total of all
the best that had been worked out up to that time in the difficult
matter. In aircraft design and in types of engines France undoubtedly
stood foremost, although the knowledge she possessed had not been
sorted, pigeonholed and accurately standardized as was the case in
Germany.
Germany had some excellent aircraft motors of the water-cooled type,
which were light in weight, very reliable and high-powered. The German
government had spent large sums of money for the purpose of encouraging
airplane construction and the improvement of designs and engines.
Yet no country at war found her military airplanes all she had expected
them to be. It was not until actual war service brought definite demands
from the pilots and definite criticisms of the bad features of the
airplanes in use, that the designers were able to turn out machines of
the highest efficiency.
There were many things which the pilots asked for. Speed and climbing
power were among them, greater ease of operation, more protection in the
way of guns and armament, the pilot's seat so located that his vision
was not obstructed above or below, and a uniform system of controls.
Gradually all these requirements have been met by the airplane makers.
By 1917 they had turned out machines which could fly as fast as 150
miles per hour and climb to 22,000 feet, while since then even this
record has been greatly improved upon.
In the field of aviation America can claim one big accomplishment since
her entrance into the World War. That is the Liberty motor, probably the
most successful motor that has ever yet been devised for an airplane.
When it was decided that we should begin work building American
airplanes, there was one important problem: the engine. Foreign types of
engines could not very well be built in this country, as they required
workmen of many years' training in a highly specialized field. It was
agreed that we must have a motor of our own, which could be manufactured
rapidly under the conditions of our present industrial system.
Two of the most capable engineers in the country were summoned to
Washington, and in order to assist them in their work motor
manufacturers from all over the United States sent draftsmen and
consulting engineers. For five days these two men did not leave the
rooms that had been engaged for them at the capital.
Sacrifice was necessary if victory was to be won. Engineering companies
and companies making motors for automobiles, etc., gave up their most
carefully-guarded secrets in order to make the Liberty motor a success.
The result was that an engine was produced so much better than anything
on the market that our allies ordered it in large quantities for their
own airplanes. Twenty-eight days after the drawings were started, the
first motor was set up. It was ready on Independence Day, and was
demonstrated in Washington. The parts had been manufactured in many
factories, yet they were assembled without the slightest difficulty. The
completed engine was sent to Washington by special train from the West.
Thirty days later it had passed all tests and was officially the Liberty
motor.
One of the most remarkable things about the Liberty motor is the way in
which all of its parts have been carefully standardized so that they can
be manufactured according to instructions by factories in all parts of
the United States. The parts can then be rapidly assembled at a central
point. The cylinders are exactly the same in every case, although the
Liberty motor is made in four models, ranging from 4 to 12 cylinders.
They can be interchanged and the parts of a wrecked engine can be used
to repair another engine.
Thus American wit, patriotism and energy were able at a most critical
time to answer the threat of German supremacy in the air. Our aircraft
production has gone forward with speed which almost baffles
understanding, while the airplane motors we shipped abroad in such
overwhelming numbers to be installed in foreign machines gave good
service to the cause for which the Liberty motor was named.