Learning to fly in the U.S. Army

by E. N. Fales

Assuming that the pilot has determined the proper angle toward which the airplane nose must be pointed, has maintained this angle throughout his flight by means of the compass and has safely reached his objective; for the return trip this diagram must be completely reconstructed (unless the wind is exactly parallel to his course). The pilot should not make the mistake in returning to the starting point of steering the airplane nose in a direction exactly opposite to the outward trip; the reader may make this clear to himself by drawing the return diagram and comparing it with the outward-bound diagram.

To summarize flying when a cross wind is blowing, it will be said that the direction of actual travel will not be the direction indicated by the axis of the airplane; and that therefore while in a picture of the situation the airplane appears to skid sideways along the whole course it must be borne in mind that actually there is no skidding whatever but the air is meeting the airplane in normal manner. The situation is analogous to that of a fly going from one side to the other of the cabin of a moving ship, where the actual course through space of the fly is an apparent skid, due to the resultant of its own and the ship’s movement.

Variation of Velocity and Direction With Height
(25 miles per hour wind)

Height in feet	At surface	500	1000	2000	3000	4000	5000
Velocity change in per cent	100	135	172	188	196	200	200
Clockwise deviation in degrees	0	5	10	16	19	20	21

Effect of Wind on Radius of Action.—Not only is the direction of flight altered by the wind but also the radius of action from a standpoint of gasoline capacity is altered. In the above machine the gasoline capacity is sufficient for 3½ hr. of flight. How far can it go across country and return before the gasoline is used up? Always allow ½ hr. gasoline for climbing and for margin; this leaves 3 hr., which at 75 miles an hour is 225 miles, or 112 miles out and 112 miles back. Now suppose that a flight is to be made across country directly in the teeth of a 40-mile wind; the radius of flight will be altered as indicated by the following calculation: Speed outward is obviously 75 minus 40 or 35 miles per hour. Speed on the return trip is obviously 75 plus 40 or 115 miles per hour—3.29 times as fast—and occupying a time which may be designated by the letter x. The time on the outward trip may be designated by 3.29x, a total time of x + 3.29x which we know equals 180 min. before the gas runs out. Solve the equation x + 3.29x = 180 and we find that x is equal to 42 min., that is, the return trip requires 42 min., and the outward trip requires 138 min. The distance covered on the outward trip is then ¹³⁸/₆₀ of 35, which equals 80.5 miles. The radius is then reduced from 112 miles to 80.5 miles.

In cases where the wind is not parallel to the line of flight the actual velocity of course can not be obtained by adding up the airplane and wind velocities, but must be obtained by the graphical method mentioned above; thenceforward the calculation is the same.

Effect of Height.—Of course if one has to fly in the teeth of a wind and can choose one’s own altitude, it is desirable to fly low where the head wind has its smaller velocity, and when flying with the following wind to rise to considerable altitudes. The proper height at which to fly will be about 1500 to 3000 ft., for cross-country trips over ordinary country; but may be increased when the wind is unsteady or decreased when there are low-lying clouds. The steadiness as well as the speed of the wind increases with the height. The character of the country should be carefully investigated from the profile maps before starting; all hilly parts should be marked on the map as a warning against landing. Contour is not readily distinguished from a height of 2000 ft. and for this reason points may be indicated on the map where poor landing places make it desirable to fly high. The character of the country or the scarcity of landing places may make it advisable to fly at high altitudes for the following reasons: (1) in case of engine failure a good margin of height is necessary to provide length of glide to reach distant landing places; (2) there is then plenty of space for righting the airplane in case of bumps, side slips, etc.; (3) eddies or local currents due to inequalities of the ground do not exist to great heights; (4) landmarks can be better distinguished from high altitudes because the vision is better (however, one must never trust to landmarks only in navigating but should constantly use a compass if only as a check, and especially in passing through clouds). Having selected in advance the proper height to use during the trip climb to this height in circles; note the direction of wind drift meanwhile to check up your estimate. Pass directly over the point of departure and when over it point the nose of the airplane for a moment directly toward the desired objective (which can be done with the aid of the magnetic compass); select some distant object which is dead ahead, and therefore directly in the course; then head the nose of the machine up into the wind just enough so that the direction of movement will be straight toward this distant object. The direction of the nose of the machine thus set by a method distinct from the graphical method above mentioned should exactly correspond, however, with the calculated direction; and thus a means of checking is obtained.

Effect of Fog.—The effect of fog upon navigating an airplane is that it prevents the use of landmarks in aiding the pilot; also that it upsets the pilot’s sense of level. These two effects are, of course, independent of the fact that proper landing places are obscured, with resultant peril in case of engine failure. Therefore, a fog should be avoided whenever possible; when one comes up, the airplane should descend, and should never attempt to get above it, as in certain localities it may turn out to be a ground fog. If the fog is very bad, land at the earliest opportunity. It is on account of fog that the pilot avoids river valleys where frequently there is a haze from the ground up to a height of 700 ft., preventing the view of proper landing places in case of necessity.

Effect of Clouds on Navigation.—Flying in or above the clouds is a similar case, inasmuch as landmarks can not be seen. It is not wise to go above the clouds when on the sea coast, as offshore winds may, unknown to the pilot, carry him out to sea; and any flight over the sea which is to a distance greater than the safe return gliding distance is, of course, perilous.

Navigation by Means of the Drift Indicator.—The drift indicator is an instrument for determining directly the side drift of an airplane. It enables the pilot by looking through a telescope at the ground to determine exactly what his direction of motion is with relation to the ground. The telescope is mounted vertically and is rotatable about its own axis; it has a cross-hair which appears in the field of view during the pilot’s observation of the ground. As the airplane speeds overhead objects on the ground will appear through the telescope to slip backward in the given direction; and when accustomed to the use of this instrument the pilot can rotate the telescope until the cross-hair is exactly parallel to the apparent line of motion of objects on the ground. The telescope cross-hair is parallel to the axis of the airplane normally and the scale attached to the telescope will in this case read zero. When the pilot rotates the telescope so that the cross-hair becomes parallel to the relative backward motion of the ground the scale will read something different from zero and will give the angle between the actual line of motion and the axis of the airplane.