A CHAPTER OF LESSONS ABOUT THE AEROPLANE
Early the following morning there was intense activity in the camp, and
Ralph was among the first to learn the cause of the excitement. Several
army divisions had come up in the night, and were bound for the Verdun
sector, and from the information obtained it was the intention of the
commander to make a terrific assault in order to break the German lines
north of the city.
The lieutenant, when approached, admitted that they were now in for
double duty. "Every man in the corps must report and be ready in the
morning. There will be two days of observation, and then look out for
some business."
"When will we go to Verdun?" asked Ralph.
"We are expecting orders at any moment," was the reply.
During the previous afternoon more than fifty machines had assembled,
and some of those were now leaving, a few going directly east, on
observation tours, while others were circling about and testing the
engines.
"I am expecting a big Farman machine," said Lieutenant Guyon, as he
returned from headquarters an hour afterwards, "that is equipped for
carrying bombs, but I am told it is one of the speediest machines in the
service. It will easily carry six hundred kilos in bombs (1,200 pounds),
and we are ordered to wait here until it arrives."
When the boys were alone, Alfred, with a glint of joy in his eyes,
remarked: "I think that will give us an opportunity to do some flying
with the lieutenant."
"Do you think so?" replied Ralph, elated at the thought.
"I am sure of that," said a voice behind them.
They quickly turned and saw the lieutenant, who had entered unobserved.
They saw by the smile on his face that the remark was an agreeable one,
for he continued:
"Yes, and the machine is now here; the men are at work setting it up; so
we might as well go over and help out."
A second invitation was not necessary. Ralph, who was outside in one
bound, rushed across the field, but Alfred accompanied the lieutenant to
the commandant's office, where they were provided with the receipt for
the machine. The lieutenant remained here while Alfred carried the
document to the warehouse. Within two hours the machine was in condition
to receive the fuel and the supplies usually carried in the machines for
emergencies.
When the lieutenant returned he made a careful inspection, and on this
occasion the boys followed every movement of his to learn what were the
essential requirements in inspecting. Naturally, the most important
thing was to know that every part of the frame is not only properly set
up and the wires made taut, but that the fastenings, the turnbuckles,
are in good condition. That necessitated a minute examination of every
one of them.
Then the planes were sighted to ascertain whether they were properly
aligned. Sometimes when the planes are not exactly parallel with each
other, the end of one, for instance, being set a little higher or lower
than the other, it is usually corrected by letting out one or more sets
of brace wires and taking up on others. This observation was followed by
a careful look at the control planes. These are the sensitive parts of
the plane, and may be likened to feelers, for the slightest warping of
the horizontal tail planes will frequently cause the machine to fly with
a skidding motion, not dangerous, but exceedingly uncomfortable when
flying at a high rate of speed.
"The wires leading to the control levers are always a source of worry to
me," remarked the lieutenant, and he drew himself up into the rear part
of the chassis. "You will notice that this one rubs along the side of
that brace. I could not permit that," he said, as he glanced toward one
of the workers.
"Do you think it would weaken the brace?" asked Ralph.
"No, it's not that," replied the lieutenant. "The difficulty is that
all these wires are stranded, and as soon as one of the wires wears out
by the frictional contact, another will give way, and the control wire
is liable to part at the most critical time."
The boys had occasion to remember the lieutenant's warning shortly
thereafter. After an examination of the airplane structure an
investigation was made of the engine. The mechanic turned it over to be
sure that the compression was all right. Self-starters are provided on
all these machines, as the French learned at an early day that it would
be unwise to depend on cranking.
Then a thorough inspection of the pipes and tubing for the fuel and
lubrication was made. The dashboard of an aeroplane is a much more
wonderful contrivance than the dashboard of an automobile. The aviator
must look out for several things not necessary when traveling on land.
The machine which they were examining had on it the most improved
contrivances, which had been found desirable, some of which were
entirely new to the boys, one of them being the inclinometer.
"That must be a dandy thing," said Alfred, pointing to it.
"I think there are only a few of the military machines which are
supplied with them, but the order has been given to put them on all the
new machines," said the lieutenant.
"What are the two rods for?" asked Ralph, as he looked at the
inclinometer.
[Illustration: FIG. 6. INSTRUMENT BOARD OF STANDARD AIRPLANE.
1. Control Lever, with thumb switch. 2. Watch. 3. Altimeter, for
registering height. 4. Compass. 5. Pressure Gages for two gasoline tanks
6. Dial to register engine revolutions. 7. Switches for two magnetos. 8.
Air Speed indicator. 9. Clinometer. 10. Oil Pulsator. 11. Gasoline
supply system. 12. Engine crank and fly-wheel.]
"The two rods which swing past the vertical bar, on which you see the
numbers, indicate the angle at which the machine is going. All that is
necessary is to observe the crossing point of the rod over the marked
bar, and it will instantly tell what the angle of ascent or descent is.
You see an aviator now has so many things to think of that he must have
devices of various kinds which will constantly tell him anything or any
condition without asking for it."
"Why, this also tells how fast the engine turns," remarked Alfred.
"Of what use is that?" asked Ralph.
"Well; if you knew that the engine was capable, under full throttle, to
make eighteen hundred revolutions a minute, and the dial showed that the
shaft was turning only one thousand, you would quickly get the idea that
something was wrong," answered the lieutenant.
"Oh, yes; and then you would start to make an investigation at once
without waiting for the engine to stop," said Alfred.
"Exactly; it is there as a warning, just as a headache is a warning that
something is wrong, or a pain, which is nature's way to indicate that an
investigation should be made without delay," answered the lieutenant
with a nod.
Besides the foregoing, the dash had on it a watch. Now, a watch may be a
convenient thing to have on an automobile, but it is not a necessity. On
a scouting and observation plane it is one of the necessary implements.
Alfred laughed, as he looked at the neat little clock face.
"I suppose," he said, "we could get along without that."
"Of course we could," answered the lieutenant. "But why get along
without it when we now find it such a necessary element?"
"Necessary how?" asked Ralph, in surprise.
"In determining distances, for one thing," answered the lieutenant.
"A watch to tell the distance?" asked Alfred incredulously.
"Yes, indeed," responded the lieutenant. "If you saw a puff of smoke in
the distance, and shortly thereafter heard a boom, the distance could be
determined almost instantaneously by roughly calculating 1,200 feet for
every second. You see, the clock there has a second hand, which is very
plain, for that very purpose."
"But suppose there is a regular battle on, it would be hard to tell
about the boom from any particular gun, wouldn't it?" asked Ralph.
"Quite true; in such a case it would be useless for that purpose.
Another use is in signaling," continued the lieutenant. "For instance,
in sending information to a battery, arrangement is sometimes made to
flash the distance by means of second intervals."
"How is that done?" asked Alfred, now growing intensely interested in
the details of the dashboard mechanism.
"It is an easy matter to signal numbers," answered the lieutenant and a
favorite plan is to fly over the area where the enemy's battery is
located, then flash at that altitude. The gunner will take the angle,
set his piece and fire, the aviator, meanwhile, noting the course and
effect of the shot. By means of the watch he can determine the distance,
counting either by the time of flight of the missile, or by the elapsed
time between the flash of the gun and the landing of the shot.
The altimeter is another very important part of the equipment. It is a
device which tells at a glance how high the machine is flying, and Ralph
looked at it with a somewhat doubtful expression. The lieutenant saw the
questioning lines on his face, and instantly divined the reason.
"You are in doubt whether the altimeter can be relied on? Am I correct?"
he asked.
"Well, I suppose it is all right, but almost every time we have been up
and took the readings, we found we were actually hundreds of feet less
than the altimeter actually showed, so I am wondering if it is possible
really to tell the exact height by that apparatus," remarked Alfred.
"Yes, I understand what you mean," replied the lieutenant. "The other
day, when we made the long flight from Bar-le-Duc, the device we had
showed 900 meters, for it was an automatically registering barometer, as
all of them practically are. When we returned I noted on the report book
872 meters. Did you notice that?"
"Yes, and I wondered at it at the time," said Alfred.
"I knew that the general altitude of the country at that place, as
indicated by the topographical charts, was 28 meters above sea level so
I merely subtracted 28 from 900, and that showed how far up above the
land we really were," said the lieutenant.
"Of course, it's a satisfaction to know how high one is flying," said
Ralph.
"It is not only a satisfaction, but it is an absolute necessity for an
aviator to know how high he is flying, if his mission is to convey any
information to the batteries in the rear," said the lieutenant.
"And how is that done?" asked Alfred.
"By a system of triangulation," he replied. "In a few days we may have
an opportunity to use the system, and I will explain it then so you can
quickly grasp the subject."
The air speed indicator was another interesting device, but at that the
lieutenant shook his head as though doubting its general value.
"Don't you think it's accurate?" inquired Alfred of the officer.
"Oh, yes; it is as accurate as it can be made. In fact all devices,
which are used to indicate the speed of the wind, are nearly perfect.
That is not the difficulty. When it is used on a flying machine several
new things enter into the problem, which makes it difficult for the
instrument to be truthful."
"What is the main trouble?" asked Alfred.
"Well, suppose we are starting a flight against a wind, which is moving
at the rate of, say, ten miles an hour," answered the lieutenant.
"While in flight we note that, going against the wind, the air speed is
sixty miles. Now, if we happened to stop suddenly so we were poised in
the air, the indicator would show the air speed to be ten miles, so that
we were really going only fifty miles. If, now, the machine is turned so
that we are going at right angles to the movement of the wind, although
we may be going actually through the air at a speed of seventy miles an
hour, the indicator does not register the air speed at all, because it
is in such a position that the air does not affect it and the only
indication we have is the speed of the air relative to the movement of
the machine,--or, to put it in a better way, I might say that the
instrument only registers, in reality, the speed at which the aeroplane
goes through the air, and no more."
"But is that the only way to tell how fast we are going?" asked Ralph.
"The best test is to take the engine revolutions. A certain machine
after it has been properly tested in a calm, and also in wind currents
of different velocities, will show a record of speed with the engine
running at certain revolutions. An experienced aviator can judge pretty
well, by experience, what the speed of the wind is, and, knowing the
propeller speed, and the direction he is going relative to the wind, he
can make a fair estimate of the speed of the machine."