CHAPTER IX
A CHAPTER OF LESSONS ABOUT THE AEROPLANE
Early the following morning there was intense activity in the camp, andRalph was among the first to learn the cause of the excitement. Severalarmy divisions had come up in the night, and were bound for the Verdunsector, and from the information obtained it was the intention of thecommander to make a terrific assault in order to break the German linesnorth of the city.
The lieutenant, when approached, admitted that they were now in fordouble duty. "Every man in the corps must report and be ready in themorning. There will be two days of observation, and then look out forsome 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, onobservation tours, while others were circling about and testing theengines.
"I am expecting a big Farman machine," said Lieutenant Guyon, as hereturned from headquarters an hour afterwards, "that is equipped forcarrying bombs, but I am told it is one of the speediest machines in theservice. 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 flyingwith 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; sowe might as well go over and help out."
A second invitation was not necessary. Ralph, who was outside in onebound, rushed across the field, but Alfred accompanied the lieutenant tothe commandant's office, where they were provided with the receipt forthe machine. The lieutenant remained here while Alfred carried thedocument to the warehouse. Within two hours the machine was in conditionto receive the fuel and the supplies usually carried in the machines foremergencies.
When the lieutenant returned he made a careful inspection, and on thisoccasion the boys followed every movement of his to learn what were theessential requirements in inspecting. Naturally, the most importantthing was to know that every part of the frame is not only properly setup and the wires made taut, but that the fastenings, the turnbuckles,are in good condition. That necessitated a minute examination of everyone of them.
Then the planes were sighted to ascertain whether they were properlyaligned. Sometimes when the planes are not exactly parallel with eachother, the end of one, for instance, being set a little higher or lowerthan the other, it is usually corrected by letting out one or more setsof brace wires and taking up on others. This observation was followed bya careful look at the control planes. These are the sensitive parts ofthe plane, and may be likened to feelers, for the slightest warping ofthe horizontal tail planes will frequently cause the machine to fly witha skidding motion, not dangerous, but exceedingly uncomfortable whenflying at a high rate of speed.
"The wires leading to the control levers are always a source of worry tome," remarked the lieutenant, and he drew himself up into the rear partof the chassis. "You will notice that this one rubs along the side ofthat brace. I could not permit that," he said, as he glanced toward oneof the workers.
"Do you think it would weaken the brace?" asked Ralph.
"No, it's not that," replied the lieutenant. "The difficulty is thatall these wires are stranded, and as soon as one of the wires wears outby the frictional contact, another will give way, and the control wireis liable to part at the most critical time."
The boys had occasion to remember the lieutenant's warning shortlythereafter. After an examination of the airplane structure aninvestigation was made of the engine. The mechanic turned it over to besure that the compression was all right. Self-starters are provided onall these machines, as the French learned at an early day that it wouldbe unwise to depend on cranking.
Then a thorough inspection of the pipes and tubing for the fuel andlubrication was made. The dashboard of an aeroplane is a much morewonderful contrivance than the dashboard of an automobile. The aviatormust look out for several things not necessary when traveling on land.
The machine which they were examining had on it the most improvedcontrivances, which had been found desirable, some of which wereentirely 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 aresupplied with them, but the order has been given to put them on all thenew machines," said the lieutenant.
"What are the two rods for?" asked Ralph, as he looked at theinclinometer.
FIG. 6. INSTRUMENT BOARD OF STANDARD AIRPLANE.
1. Control Lever, with thumb switch. 2. Watch. 3. Altimeter, forregistering height. 4. Compass. 5. Pressure Gages for two gasoline tanks6. Dial to register engine revolutions. 7. Switches for two magnetos. 8.Air Speed indicator. 9. Clinometer. 10. Oil Pulsator. 11. Gasolinesupply system. 12. Engine crank and fly-wheel.]
"The two rods which swing past the vertical bar, on which you see thenumbers, indicate the angle at which the machine is going. All that isnecessary is to observe the crossing point of the rod over the markedbar, 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 havedevices of various kinds which will constantly tell him anything or anycondition 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, tomake eighteen hundred revolutions a minute, and the dial showed that theshaft was turning only one thousand, you would quickly get the idea thatsomething was wrong," answered the lieutenant.
"Oh, yes; and then you would start to make an investigation at oncewithout waiting for the engine to stop," said Alfred.
"Exactly; it is there as a warning, just as a headache is a warning thatsomething is wrong, or a pain, which is nature's way to indicate that aninvestigation should be made without delay," answered the lieutenantwith a nod.
Besides the foregoing, the dash had on it a watch. Now, a watch may be aconvenient thing to have on an automobile, but it is not a necessity. Ona 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 alongwithout 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 inthe distance, and shortly thereafter heard a boom, the distance could bedetermined almost instantaneously by roughly calculating 1,200 feet forevery second. You see, the clock there has a second hand, which is veryplain, for that very purpose."
"But suppose there is a regular battle on, it would be hard to tellabout 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 toflash the distance by means of second intervals."
"How is that done?" asked Alfred, now growing intensely interested inthe details of the dashboard mechanism.
"It is an easy matter to signal numbers," answered the lieutenant and afavorite plan is to fly ov
er the area where the enemy's battery islocated, then flash at that altitude. The gunner will take the angle,set his piece and fire, the aviator, meanwhile, noting the course andeffect 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 elapsedtime between the flash of the gun and the landing of the shot.
The altimeter is another very important part of the equipment. It is adevice which tells at a glance how high the machine is flying, and Ralphlooked at it with a somewhat doubtful expression. The lieutenant saw thequestioning 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 upand took the readings, we found we were actually hundreds of feet lessthan the altimeter actually showed, so I am wondering if it is possiblereally to tell the exact height by that apparatus," remarked Alfred.
"Yes, I understand what you mean," replied the lieutenant. "The otherday, when we made the long flight from Bar-le-Duc, the device we hadshowed 900 meters, for it was an automatically registering barometer, asall of them practically are. When we returned I noted on the report book872 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, asindicated by the topographical charts, was 28 meters above sea level soI merely subtracted 28 from 900, and that showed how far up above theland we really were," said the lieutenant.
"Of course, it's a satisfaction to know how high one is flying," saidRalph.
"It is not only a satisfaction, but it is an absolute necessity for anaviator to know how high he is flying, if his mission is to convey anyinformation 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 havean opportunity to use the system, and I will explain it then so you canquickly grasp the subject."
The air speed indicator was another interesting device, but at that thelieutenant 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 severalnew things enter into the problem, which makes it difficult for theinstrument to be truthful."
"What is the main trouble?" asked Alfred.
"Well, suppose we are starting a flight against a wind, which is movingat the rate of, say, ten miles an hour," answered the lieutenant."While in flight we note that, going against the wind, the air speed issixty miles. Now, if we happened to stop suddenly so we were poised inthe air, the indicator would show the air speed to be ten miles, so thatwe were really going only fifty miles. If, now, the machine is turned sothat we are going at right angles to the movement of the wind, althoughwe may be going actually through the air at a speed of seventy miles anhour, the indicator does not register the air speed at all, because itis in such a position that the air does not affect it and the onlyindication we have is the speed of the air relative to the movement ofthe machine,--or, to put it in a better way, I might say that theinstrument only registers, in reality, the speed at which the aeroplanegoes 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 machineafter it has been properly tested in a calm, and also in wind currentsof different velocities, will show a record of speed with the enginerunning at certain revolutions. An experienced aviator can judge prettywell, by experience, what the speed of the wind is, and, knowing thepropeller speed, and the direction he is going relative to the wind, hecan make a fair estimate of the speed of the machine."