Learning to fly in the U.S. Army

by E. N. Fales

PRINCIPLES OF AIRPLANE EQUILIBRIUM

Fig. 28.—Balances of forces in an airplane.

Weight forward of lift, thrust below resistance. Thrust equals resistance, weight equals lift.

Introductory.—Under this head will be discussed: (a) features of airplane design which tend to maintain equilibrium irrespective of the pilot; (b) matters of voluntary controlling operations by the pilot. As regards (a) the tendency of the airplane toward inherent stability acts to oppose any deviation from its course whether the pilot so desires or not. The more stable is a machine, the less delicately is it controlled, and the present consensus of opinion among pilots is that a 50-50 compromise between stability and controllability is the best thing.

In questions of airplane equilibrium the starting point is the center of gravity; obviously, if the center of gravity were back at the tail or up at the nose there would be no balance; the proper place for it is the same spot where all the other forces such as thrust, lift and resistance act; there it is easy to balance them all up. But it is not always easy to bring the line of thrust and the line of total resistance into coincidence, because the line of thrust is the line of the propeller shaft and when this is high up as in the case of some pushers it may be several inches above the line of resistance. And as the thrust is above the resistance there is a tendency to nose the machine down; to balance which the designer deliberately locates the center of gravity sufficiently far behind the center of lift so that there is an equal tendency to tip the nose upward; and all four forces mentioned completely balance each other. But things may happen to change the amount or position of these forces during flight, and if this does happen the first thing to do is to restore the balance by bringing in a small new force somewhere. In an actual airplane this small restoring force is supplied at each critical moment first, by the tail, etc., of the airplane and second, by voluntary actions of the pilot. The center of gravity of any airplane may be determined easily by putting a roller under it and seeing where it will balance, or by getting the amount of weight supported at the wheels and tail, according to the method of moments.

Longitudinal Stability.—Longitudinal stability has to do with the tendency of an airplane to maintain its proper pitching angle. It was said above that the four forces of lift, resistance, thrust and weight always exactly balanced due to their size and their position. Now the first consideration about longitudinal stability is that while the centers of gravity and other forces remain in a fixed position, the center of lift changes its position whenever the angle of incidence (that is the speed) is changed. The phenomenon of shift of center of pressure applies only to the wings and to the lift (the position of center of resistance remains practically fixed at all angles).

Note the effect on center of pressure position of a change of wing angle (see Fig. [20]). The wing used on the U. S. training machine has a center of lift which is about in the middle of the wing when flying at a small angle of maximum speed; but if the angle is increased to the stalling angle of 15°, the center of pressure moves from midway of the wing to a point which is about one-third the chord distance of the wing from the front edge. The lift may travel about ½ foot, and it is equal in amount to the weight of the machine (that is, nearly a ton), and the mere effect of changing the angle from its minimum to its maximum value therefore tends to disturb the longitudinal equilibrium with a force which may be represented as 1 ton acting on a lever arm of ½ ft. Suppose that the airplane is balancing at an angle of 2° so that the center of gravity coincides with the center of lift for this angle; now if a gust of wind causes the angle to increase for an instant to 2¼°, the center of lift will move forward and tend to push the front edge of the wing up, thus increasing the angle further to 2½°. Then the center of lift, of course, moves further forward to accommodate the increase of angle, and in a fraction of a second the wing would rear up unless it were firmly attached to the airplane body and held in its proper position by the tail. Similarly if for any reason the proper angle of 2° were decreased, the same upset would follow, only this time tending to dive the wing violently to earth. This tendency is neutralized in an airplane by the “Penaud Tail Principle.”

There are certain shapes of wings in which the center of pressure travels in the reverse direction; a flat plate, for example; or a wing having its rear edge turned up so that the general wing shape is like a thin letter “S.” Such wings as these would not tend to lose their proper angle, because when the angle is changed for any reason the center or pressure in these wings moves in just the manner necessary to restore them to their proper position; but these wings are inefficient and are not in present use on airplanes.

Fig. 29.—Diagrams illustrating theory and application of longitudinal dihedral angle.

The Penaud Tail Principle.—Rule.—The horizontal tail must have a smaller angle of incidence than the wings. The upsetting force above mentioned must be met by a strong opposite righting force, and this latter is furnished by the horizontal tail surface. In the angle of equilibrium of 2° above mentioned, the flat horizontal stabilizer will perhaps have no force acting on it at all because it is edgewise to the air and its angle of incidence is zero. When the angle of the wing increases to 2¼° and the lift moves forward tending to rear it up, the wing being rigidly fastened to the body pushes the tail downward so that the tail now begins to have a small lift force upon it due to its angle of ¼°; and this newly created force, though small, acts at such a long lever arm that it exceeds the rearing force of the wing and will quickly restore the airplane to 2°. This action depends upon the principle of the Penaud Tail or longitudinal “Dihedral” which requires that the front wings of an airplane make a larger angle with the wind than the rear surface. This principle holds good even when we have rear surfaces which actually are lifting surfaces in normal flight, the requisite being that the wings themselves shall in such cases be at an even greater angle than the tail. No mention has been made of the elevator control, because its action is additional to the above-mentioned stability. The elevator is able to alter the lift on the tail; such alteration requires, of course, immediate change of angle of the wings so that equilibrium shall again follow; and this equilibrium will be maintained until the lift at the tail is again altered by some movement of the elevator control. Thus the elevator may be considered as a device for adjusting the angle of incidence of the wings.

The air through which the wings have passed receives downward motion, and therefore a tail which is poised at zero angle with the line of flight may actually receive air at an angle of -2° or -3°. In the above case we would expect an actual downward force on the tail, unless this tail is given a slight arch on its top surface (for it is known that arched surfaces have an angle of zero lift which is negative angle).

Longitudinal Control.—Steering up or down is done by the elevator, which as explained above is merely a device for adjusting the angle of incidence of the wings. The elevator controls like all the other controls of an airplane depend for their quick efficient action upon generous speed; they can not be expected to give good response when the machine is near its stalling speed. The elevators like the rudder are located directly in the blast of the propeller and in case the speed of motion should become very slow, the elevators may be made to exert considerable controlling force if the motor is opened up to blow a strong blast against them. This is good to bear in mind when taxying on the ground because if the motor is shut off at the slow speed of motion the elevator and rudder will lose their efficacy. The propeller blast, due to a 25 per cent. slip, adds 25 per cent. of apparent speed to those parts which are in its way, and therefore the tail forces are affected as the square of this increase, that is, the forces may be 50 per cent. greater with the propeller on than off.

Lateral Stability.—This depends upon the keel surface or total side area of an airplane. The keel surface includes all the struts, wires, wheels, wings, as well as body, against which a side wind can blow. Skidding and side-slipping have the same effect as a side wind, and the resulting forces acting against the side of the machine should be made useful instead of harmful. This is done by properly proportioning the keel or side surface. If keel surface is low, the side force will rotate the airplane about its axis so that the windward wing sinks; if high, so that it rises. But if the keel surface is at just the right height (i.e., level with the center of gravity) the side forces will not rotate the machine at all and will simply oppose the skidding without upsetting equilibrium.

Fig. 30.—Diagram showing effect on lateral stability of dihedral angle and non-skid fins.

(a) Machine flying level. (b) Machine tips and side-slips: excess pressure is created on windward wing and fins, (c) Machine has side-slipped and rotated back to level.

Lateral Dihedral.—Now when an airplane appears to have its keel-surface center too low, the easiest way to raise it level with the center of gravity is to give the wings a dihedral angle, that is make them point upward and outward from the body. Thus their projection, as seen in a side view, is increased, and the effect is to add some keel surface above the center of gravity, thus raising the center of total keel surface.

A further advantage of the lateral dihedral is that any list of the airplane sideways is automatically corrected (see Fig. [30]). The low wing supports better than the high wing, because a side slip sets in, hence will restore the airplane to level position.

Non-Skid-Fins.—Where for the above-mentioned purposes an excessive dihedral would be needed, resort may be had to non-skid-fins erected vertically edgewise to the line of flight above or beneath the topwing. These are used in marine machines to balance the abnormally large keel surface of the boat or pontoon below.

Lateral Control.—By means of ailerons, lateral control is maintained voluntarily by the pilot; the aileron on the low tip is given a greater angle of incidence while on the high tip a less angle of incidence thus restoring the proper level of the machine. Notice that the efficacy of the ailerons depends upon speed of motion of the airplane, irrespective of propeller slip because the propeller slip does not reach the ailerons. Therefore, at stalling speeds the ailerons may not be expected to work at their best, and when lateral balance is upset at slow speeds it is necessary to dive the machine before enough lateral control can be secured to restore the balance.

Fig. 31.—Deperdussin control.

System used in U. S. training airplanes.

Directional Stability.—Directional stability has to do with the tendency of an airplane to swerve to the right or left of its proper course. To maintain directional stability the “vertical stabilizer” is used, which acts in a manner analogous to the feather on an arrow. Thus in case of a side slip the tail will swing and force the airplane nose around into the direction of the side slip so that the airplane tends to meet the relative side wind “nose-on” as it should. The vertical stabilizer should not be too large, however, as then any side pressure due to deviation from a rectilinear course will cause the machine to swerve violently; the wing which is outermost in the turn will have preponderance of lift due to its higher speed; that is, the airplane will get into a turn where there is too much bank and a spiral dive may result.

Directional Control.—The rudder gives directional control in exactly the same way that it does on a boat; it should be said, however, that the rudder is sometimes used without any intention of changing the direction, that is, it is used simultaneously with the ailerons as a means of neutralizing their swerving tendency. The ailerons, of course, at the same time that they restore lateral balance create a disadvantageous tendency to swerve the machine away from its directional course; that is what the rudder must neutralize. Moreover, the rudder is frequently used against side winds to maintain rectilinear motion.

Banking.—Banking combines the lateral and directional control, which should be operated simultaneously so as to tilt the machine and at the same time maintain the radius of turn. The wings are tilted in a bank because in going around a curve of a certain radius the weight of the machine creates a centrifugal force in a horizontal direction and if the curved path is to be maintained this centrifugal force must be neutralized; and this is done by inclining the force of lift inward until it has a horizontal component equal to the centrifugal force. That is why the angle of bank must be rigidly observed, or else the inward component of the lift will change. Now as soon as the wings bank up, the lift force is no longer all vertical and therefore may not be enough to support the weight of the machine. To offset this have plenty of motor power for speed in a bank; and do not try to climb while banking.

It is better to bank too little than too much; too little results in skidding which may be easily cured; too much results in side slipping inward and if the tail surface is too great in this latter case, a spiral dive may result—so look out for over banking.

It is better for the beginner in banking to move his ailerons first and then move the rudder; for if he moves the rudder first there will be skidding outward, forward speed will drop and a stall may result. On high angles of banking, over 45°, it should be noted that the elevators are now more nearly vertical than horizontal and operate as a rudder; similarly the rudder’s function is reversed, and to turn down the rudder will be used.

Damping in an Airplane.—Above have been mentioned the restoring forces which tend toward airplane equilibrium. Now these restoring forces tend to push the machine back to equilibrium and even beyond in exactly the same way that gravity causes a pendulum to swing about its point of equilibrium. This can sometimes be noticed in the case of an automobile when travelling at high speed along country roads where a sort of slow oscillation from side to side may be noticed due to the forceful maintenance of equilibrium of the body in its forward motion. This oscillation in an airplane would be serious unless there were means of damping it out and these means are: first, the wings; second, the tail surfaces; third, the weight and inertia of the machine itself. Regarding inertia it should be said that a machine with weight distributed far from the center of gravity, such as the double-motor airplane has a large tendency to resist the rolling motions associated with lateral stability. But from the same sign airplanes with large moment of inertia are difficult to deviate from any given attitude, and therefore have the name of being “logy.” The proper proportioning of an airplane’s parts to secure first, the restoring forces; second, the proper damping force; third, the proper amount of moment of inertia, is a very delicate matter and beyond the scope of the present chapter.

CHAPTER IV
FLYING THE AIRPLANE

Starting Off.—The first thing to do before starting off in an airplane is to inspect carefully everything about the machine and assure yourself that it is in perfect condition.

When all is ready to start turn the machine directly against the wind; this is done in order that the rise from the ground may be more quickly made with the assistance of the wind under the wings, and it has a more important advantage in the fact that if you try to get off the ground across the wind the machine will be very hard to balance. Birds also take the air directly against the wind even though for the moment this carries them in a direction toward some supposed enemy, and it is a fundamental principle in airdromes. Keep the machine pointed into the wind for the first 200 ft. of altitude (and similarly in landing face the wind when within 200 feet of the ground). In case the engine should fail before a height of 200 ft. is reached, never turn down wind as this is extremely dangerous.

Assistance will be had for the start from the mechanics, or if away from the airdrome from bystanders. Have each assistant in his proper place before starting the engine; one is to start the propeller and the rest to hold back the machine until ready to let go.

(From “How to Instruct in Flying.”)

Fig. 32.—Airplane in flying position just after starting.

This cut also illustrates proper landing attitude, since airplane is just skimming the ground.

In order to get off the ground you will want good engine power; it takes considerable thrust to accelerate an airplane on the ground to its flying speed; in fact the first flying machine of the Wrights had to use an auxiliary catapult to furnish the thrust necessary to get them into the air. Making sure that the motor is giving full power raise the hand as a signal to the attendants to remove the chocks and let go. As you start rolling forward push the control lever forward which will raise the tail off the ground and place the wings edgewise to the wind while they will not offer resistance to the acquiring of good rolling speed. Within a few seconds the machine will have attained on the ground a velocity not less than the low flying speed; it will not rise, however, until the tail is lowered by pulling the lever back. When the necessary rolling speed is attained pull the lever softly backward; the tail at once drops, the wings increase their angle and lift and the machine will rise, the lever being held in a fixed position (see Fig. [32]). The distance between the point of starting and rising will be 100 yd. or more and will occupy from 5 to 10 sec. depending on the wind.

The change from flying position to climbing position is only a slight modification involving only a slight pulling back of the control lever and holding it in fixed position; the motor may in some machines simply be opened out when its increased power will make the machine rise; however, there is only one speed at which the climb will be fastest and therefore it is well to know what is the proper speed for climbing; the motor is then opened out full and the airplane operated to give the proper speed corresponding.

The pupil should rise to the height of at least 100 ft., as any less is useless and nothing will be learned from landing. In the case of cross-country flying the pilot will rise to the height of 2000 ft., circling over the field rather than flying off in a straight line so that preparatory to his start he always has the flying field in reach.

(From “How to Instruct in Flying.”)

Fig. 33.—Airplane in gliding position, approaching a landing.

Note that its attitude relative to line of flight is similar to “flying position,” line of flight however being inclined.

Landing.—Proper landing is the most important thing in airplane flying. The pilot in turning his machine downward toward a landing spot from flight will choose a distance from the field equivalent to the proper gliding angle of his machine. If the gliding angle is 1 in 7 he must not turn downward any further from the field than a distance greater than seven times his altitude or he will fall short. It is safer to come closer to the field before turning downward for two reasons: first, because you may not be gliding at the best gliding angle; second, because you can always kill extra height by a spiral or two better than you can regain it. Have height to spare when landing.

To come down throttle down the engine and push the lever softly forward until the proper gliding angle is obtained (Fig. [33]). The reason for throttling down the engine is: first, that you do not need its thrust when you are coasting down because gravity furnishes all the necessary velocity; second, if you glide or dive with the motor wide open high speed will result, resulting in strains on the machine especially on the moment of leveling out again; third, at this high speed the controls become stiff to operate.

Fig. 34.—Attitudes of an airplane in flight.

Maintain the proper gliding speed to within 5 miles an hour of what it ought to be as it is the speed which determines the proper gliding angle. The revolution counter will indicate what the speed is or the air-speed meter may be used. Arrange to come on to the field facing directly into the wind, which may be observed by watching smoke or flags below. In landing against the wind you are again copying the practice of the birds. When you come to within 15 ft. of the ground pull the lever softly back until the machine is in its slow-flying position, which should be attained 5 ft. above the ground (Fig. [34]). Hold the stick at this position of horizontal flying; no further movement of the lever is necessary except to correct bumps, for which purpose it would be held lightly for instant action. The aileron control must be used here to keep the machine level and it may be necessary to operate the rudder after touching the ground in order to avoid swerving; in fact some machines are provided with a rear skid which steers for this purpose.

In rolling just after landing keep the tail as close to the ground as possible without causing undue bumping, so that the maximum resistance of the wings may be presented to the air and the machine be slowed up rapidly. Some machines are fitted with brakes on the wheels to assist in the quick retardation of the roll. Landing is one of the biggest problems in aviation and is a hard thing to learn because it is done at a high speed especially in the fast military machines such as the Fokker, Nieuport, etc. Landing is more of a problem than it used to be in the early days when, for instance, the Wrights were able to land without any wheels at all on mere skids because their machines were not fast.

The following are examples of bad landings:

1. The pancake results from allowing the machine to get into its rising position when it is landing (Fig. [35]). There will be a perpendicular bounce and on the second bounce the running gear will break. In order to get out of an imminent pancake open up the engine to keep machine flying, put the machine into a flying position, then throttle down again and land.

2. Another type of pancake results from bringing the machine out of its gliding position at a point too far above the ground when the machine will drop due to lack of speed and break the running gear. To avoid this open motor full, thus regaining speed and flying position; afterward throttle down and reland.

(From “How to Instruct in Flying.”)

Fig. 35.—Bad landing, Type 1—the “pancake” landing.

Line of flight is downward; angle of incidence large, hence speed is slow; but there is too much downward momentum and landing gear will break. Should line of flight arrow point upward, airplane as shown would then be in climbing position.

3. A third type of bad landing results from failure to turn the machine out of its glide at all, so that it glides straight downward until it touches the ground. This is the most dangerous case of all the bad landings. To cure it open up the engine after the first bounce, regaining flying speed before the second bounce; then reland.

(From “How to Instruct in Flying.”)

Fig. 36.—Bad landing Type 4—machine not level.

Wheels do not touch ground at same time, and one may smash.

4. If at the moment of landing the rudder is turned causing machine to swerve, or if the machine is not level, a side strain will be placed upon the landing gear and the wheels will buckle (Fig. [36]).

CHAPTER V
CROSS-COUNTRY FLYING

Cross-country flying differs from ordinary airdrome flying in that it takes you a long way off from your landing field. On the airdrome your chief anxiety is to learn how to fly, how to work the controls, how to bank; but in cross-country work, you are supposed to have all the technique of airplane operation well in hand, so that you do not have to think much about it. In cross-country flying, then, your chief anxiety will be to arrive at your destination and to be constantly searching out available landing fields in case of engine failure. The first cross-country flight you make may be a short, easy one, in which there are plenty of available landing places, and on which you will be able to make a regular reconnaissance report. Further experience in cross-country work will involve more and more difficult trips, until you will think nothing of flying, for example, on long raiding tours over unfamiliar enemy country.

Equipment.—Knowing that you may have to land far away from any headquarters, you must take a complete set of tools and covers for the airplane. Your clothing need not be different from usual, and will comprise helmet, goggles, leather suit, and gloves. Do not forget your handkerchief, which you frequently need to clean off your goggles.

The instruments needed on a cross-country trip are: a compass, which should be properly adjusted before starting and the variation angle noted. A wrist watch is necessary; ordinary dashboard clocks go wrong on account of the vibration. Take an aneroid barometer with adjustable height reading. Of course you will depend upon a revolution indicator, for no matter how experienced a pilot may be in “listening out” faulty engine operation, after a long flight his ear loses its acuteness, and he will fall back on the revolution indicator for assistance. The air-speed meter, whether of the Pitot type or pressure-plate type, will prove invaluable in flying through clouds or mist when the ground is obscured. Also the inclinometer is able to give the angle of flight when the earth is not visible, although the speed indicator usually is sufficient to give the angle of flight, for an increase of speed means downward motion and decrease of speed means upward motion. Additional instruments may be used.

Map.—The map is essential for cross-country work. It should be tacked on to the map board if the flight is short, but made to run on rollers if the flight is long. In the latter case the map is in the form of a single long strip, while your flight may be full of angles; therefore you will have to practice using this sort of map, in which the corners of your flight are all drawn as straight lines. The scale of maps may be 2 or 4 miles to the inch for long flights. This scale is sometimes spoken of as a fractional figure; that is, 2 miles to the inch is the same as ¹/_127,000 scale. The map should be studied most carefully before the start of the trip. The course which you propose to fly should be marked out on it; all available landmarks which could be of service as guides should be distinctly noticed and marked on the map where necessary. These landmarks will in case there is no wind enable you to make your trip without using the compass at all, and in case of wind, are essential as a check on the compass. Mark off the distance in miles between consecutive points of your course. Mark the compass bearing of each leg of this course.

As landmarks towns are the best guides, and they should be underscored on the map, or enclosed in circles. It is customary not to fly actually over towns. Railways are very good assistance to finding your way, and these should be marked on the map in black wherever they approach within 10 miles of the course. Mark water courses with blue color, and roads with red.

Landmarks.—Only practice can make a pilot good at observing the various features of the ground beneath him. The various features which can be used as guides are those which are most visible. After towns, railways come next in importance. Their bridges, tunnels, etc., make good landmarks. On windy days when relying on the compass, it will be well to keep in sight of a railway even if this be the longer way around, because the railway gives a constant check upon the compass bearing. In this case you will have noted on your map a general magnetic bearing of the railway, which bearing you can readily compare with your compass reading. Moreover, the railway is good in case you become involved in a fog or mist for a time. It should be remembered, however, that on most of the maps no distinction is made between one and two-track roads; also that it is easy to make mistakes where branch lines are not shown on the map because they are dead ends leading to private quarries, etc., and may be taken for junctions. Railways sometimes seem to end abruptly, which means that you are looking at a tunnel.

Water is visible from a great distance. Cautions to be observed are that after a heavy rain small flooded streams may take on the appearance of larger bodies of water or lakes, which you will have difficulty in reconciling with the map. Small rivers are often overhung with foliage, and to follow them in all their curves will waste a lot of time.

The use of roads as guides may be governed by the fact that paved roads are usually main roads, and telegraph wires may be expected along them. In the newer parts of the United States the system of laying out roads provides a very useful means of gaging distances; I refer to the section system which is in use, for instance, in Illinois, where there is a road every mile running north and south, so that the entire country is cut up into squares 1 mile on each side, with occasional roads of course at ½-mile and ¼-mile points.

Navigation by Landmarks.—In all cases of cross-country flying the pilot will have two independent systems of maintaining his proper directions: first, the computed compass bearing; second, the use of landmarks whose position is known. In comparing his computed course with the course actually indicated by passing over these landmarks the rule should be made that, in case of doubt when a landmark is not distinctly recognized, take the compass course; there are many chances that a landmark may be altered or even removed without being so recorded on the pilot’s map, whereas the errors of the compass of course are presumably understood by the pilot who has secured every opportunity to check it when passing previous landmarks.

It is important to note the time of completing successive stages of the flight, that is when passing over predetermined landmarks. Time is a very uncertain condition to ascertain in airplane flying for it seems to pass quickly on calm days but slowly when the journey is rough. If the pilot does not check the time interval between successive objects he is quite likely to expect the next before it is really due.

Landing Fields.—Next to the ever-present worry which the pilot has regarding the perfect operation of his engine, the most important thing about cross-country flying is that wherever he may be he must have available a landing field within gliding distance in case his engine defaults. The question is of course immediately raised, “What if there is no landing field within gliding range?” The answer to this is that the pilot will instinctively learn to keep his eyes open for landing possibilities every minute of his progress whether he expects to use them or not; in cross-country flying the lookout for fields is first and foremost in his mind; if there are no fields, it is up to him to pick out a spot of ground which is the least objectionable for a landing. In the State of Illinois the question of landing fields is almost non-existent, because there are large, flat fields and pastures in almost every square mile of the farming district, and a cross-country flight from Rantoul to Chicago could have no terrors for the beginner as regards the choice of a landing ground.

When it comes to a cross-country flight like Ruth Law’s, from Chicago to New York, these favorable conditions begin to disappear after the middle of the journey, that is, east of Buffalo. The most ideal condition for cross-country flying would be one like that on the London-Edinburgh route, where landing grounds are so frequent that by flying at a height of a couple of miles the pilot can free his mind completely of the worry of suitable landing places; but in the United States we have very few established airdromes, and the only approach to the London-Edinburgh route is the St. Louis-New York route, where the jumps are approximately 150 miles; namely, St. Louis, Champaign, Indianapolis, Dayton, Sandusky, Erie, Hammondsport, Philadelphia, and New York. That is why long cross-country trips are such an adventure in this country and such an ordinary affair in England.

The beginner will have special difficulty in training his mind to pick out available landing places; first of all because the earth looks so different from the sky that it is only with practice a beginner learns the shades and hues of color which mean certain kinds of ground, or learns to spot the different features of flat and hilly country. Even for an accomplished pilot it is hard to tell whether a field is good or bad from a height of over 1000 ft.; and as it is dangerous to fly this low over unknown territory, you can at once see what is meant by the worry of scanning the countryside for available fields.

Choose the best field that you can get, having a smooth surface and being easy to get out of in all directions. The following considerations are intended as a guide to what constitute the best field, in case you have a choice between several possibilities.

1. Choose a field near a town if possible, or failing that, near a main road or at least a good road. Remember that a field which appears to be near a town from the air may actually turn out to be a long walk after you have landed there and find that there are various trips to be made to and fro between your chosen landing spot and the town for the purpose of securing ropes, gasoline, supplies, etc. If you land near a main road there will probably be telegraph wires along it, which are undesirable in the case of a small field and wind direction such that you have to rise off the field over the telegraph wires. It is often hard to distinguish between main roads and minor roads, and it will be wise to look for the number of vehicles on any road in determining whether or not it is the main road.

2. The best field is a stubble field, and is most numerous of course in the fall when the crops are in. It will have a lightish brown color when seen from a height, and is pretty sure to be smooth, without ditches or mounds. Grass land is next best, but is often full of mounds. Plowed, furrow fields are to be avoided. It might be said that stubble fields will be hard to get out of after a wet night. Vegetable and corn fields have a dark green appearance which the pilot must learn to distinguish from grass pastures, etc. If you choose pasture land, remember that in summer evenings the farm animals will generally be lying down near the hedges.

3. Avoid river valleys for landing over night, as there is liable to be a fog in the morning.

4. Any field which has been previously used for landing with success by an army officer can be wisely chosen.

The final determination of landing field characteristics can be made when your airplane has descended to a height of 1000 ft. off the ground, and in case you are not making a forced landing and your engine is still going, you can check up your estimate by descending to this level.

Proper Dimensions of Fields and Airdromes.—There are three kinds of flying fields. One is the airdrome which is used exclusively for flying, and may be as large as a mile square; very few of these will be found in cross-country flights in the United States. Second, there is what is called the “one-way” field, a long, narrow, open space which is usable when the wind blows parallel to its length. Third, there is the “two-way” field, which has two sufficiently long runways at right angles to each other. A two-way field is very much better than a one-way field, inasmuch as you can always head within 45° of the wind, whereas in a one-way field an extreme case would be 90°. Moreover, two-way fields, such as the crescent-shaped field at Dayton, Ohio, sometimes permit of almost universal direction of flight. The two-way field may be crescent-shaped, T-shaped, or L-shaped. An L-shaped field should have each arm 200 by 300 yd. Under certain conditions there may be buildings located inside or outside the angle which do no harm aside from creating eddies in case of strong wind. A T-shaped field should also have its arms 300 by 200 yd. in size.

Regarding the size of fields it can be said that, while the JN-4 machine will rise off the ground after a run of 100 yd. or so, a field of this length is of course not big enough for frequent use, especially if bordered by trees, telegraph lines, fences, and so forth. A field for temporary use should be at least 200 by 200 yd., about 9 acres. If obstructions at the edges are more than 5 ft. high add to this 200 yd. a distance equal to twelve times the height of the obstruction. For a permanent field 300 yd. is the minimum dimension necessary for clearing obstacles and must be increased if the trees exceed 50 ft. in height. This minimum dimension assumes hard ground and the possibility of starting in any direction. Training fields are ½ mile square or more.

Whatever field is used either temporarily or permanently by the pilot should be absolutely familiar to him over every inch of its surface. The adjacent country should also be absolutely familiar to him from the standpoint of possible forced landings which he may have to make during his flight; he should make a habit of informing himself as to all the woods and hills, etc., which can affect air currents in the neighborhood of the field from which he is going to start.

Guide Posts on Airdromes.—Some fields have pot holes in them, and these holes should be marked in each case with a large high red or yellow flag. Do not use short, small flags, as they will frequently be invisible to pilots taxying on the ground. All telephone wires, etc., should have large blankets or other suitable signals hung over them to warn the pilot away.

Commonly accepted marks for designating a landing spot on airdromes are as follows:

For day use a large letter “T” lying on the ground, made out of white cloth strips 15 by 3 ft. This letter T is shifted with the wind so that its long leg always points in the direction of the wind and the pilot will therefore have nothing to do in landing but approach the letter “T” from the bottom, so to speak.

For night flying a system of four flares is used, so arranged that the pilot in making a proper landing will pass flare A on his left; within 50 yd. further on, flare B; then 100 yd. further on, flare C, also on his left. In passing flare C he will have a fourth flare, D, 50 yd. to his right. That is to say, the four flares make the outline of a letter “L” and the pilot approaches the letter having the long leg on his left. The flares may be made by putting half a gallon of gasoline into a pail. This will burn for 30 min. and will be visible 8 miles away. Sometimes at night instead of flares white sheets can be spread on the ground and a shaded lamp used to illuminate the sheets.

All searchlights on the landing field should point in the direction of landing. All other lights within a distance of a mile should be extinguished, and red lamps should be used at danger points.

On moonlight nights the same signals and guides may be used as in the daytime.

Pegging Down an Airplane.—In landing for the night do not stay up until it gets dark but choose a landing place which will allow you to come down 1 hr. before dark; this amount of time will be needed for laying up the machine over night. As you come to the landing ground note the time so that you can compute the actual duration of your flight in your report, then make a good landing. Taxy the machine to the spot where you intend to leave it over night, such as the lee of a hedge, etc.; or if there is no choice of position taxy the machine to the approximate location from which you will make your start next morning; this will save trouble when you get ready to start.

Dismount from your machine, lift up the tail enough to leave the wings edgewise to the wind, the machine, of course, facing the wind, and jack up the tail in this position by the use of any convenient prop. Lash the control wheel or joy stick fast in a fixed position so that the wind can not flap the control surfaces around and damage them.

Choose a sunken trench if possible in which the wheels may be sunk; if the wind is going to blow and there is no sunken trench it will be wise to dig one so that the effect of the wind on the airplane will be lessened. If the trench is not necessary, at least put chocks under the wheels. Peg down the wings and the tail to stakes driven into the ground using rope if you can get some or lacking this in an emergency fence wires which you can secure by means of your wire cutters. Do not lash tightly enough to induce strains in the framework of the machine.

Next, fill up the tanks if a supply of gasoline or oil is available. Put the covers on the propellers, engine, cowls, etc., in order that rain and dew shall do no damage to these parts. The wings and body are varnished waterproof and will not be seriously damaged by a little moisture; to avoid the collection of moisture in the wings small eyelet holes are sometimes set in the wings at the trailing edge to let out the water.

Of course, you will engage a guard to watch the machine all night; see that a rope is strung around the airplane to keep off the crowd which may collect.