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.
AERIAL NAVIGATION
=Effect of Wind.=—Navigating in an airplane is complicated only on
account of the fact that there is a wind blowing which may not be in
the desired direction. While on the sea navigation is simple through
the assistance of the magnetic compass (because side winds can not
materially drift the ship sideways), in the air this is not the case;
for if the pilot using the compass points the nose of the airplane
directly north while a west wind is blowing, this wind will cause the
machine to drift in an easterly direction so that in an hour of flight
the airplane will be off its course by an amount equal to distance
which the wind travels in 1 hr.; and the joint result of the motion of
the airplane forward and the motion of the wind sideways will cause
the machine to drift in a northeasterly direction at a speed quite
different from its rated velocity, and in this case somewhat larger.
Victor Carlstrom in his Chicago-New York flight found while he was over
Cleveland that a side wind was deviating his course 17° away from what
it should be, and if he had not had such landmarks as the shore of Lake
Erie for guidance he might easily have lost considerable time.
The question of making allowance for this wind drift is very important
where there are no landmarks, as in the case of night flying, flying
over the sea, or flying over the clouds; and the only way the pilot can
make allowances for these conditions is to figure them out before he
starts from the airdrome, and plan to circumvent them. That is to say,
the pilot in flight has no means, aside from visual observation of the
ground, to determine whether or not the wind is blowing him off his
course. He must determine the whole situation before he starts, and the
process of doing so is as follows.
=Graphical Method for Determining Direction to Steer.=—The pilot will
ascertain from the weather vane and anemometer of the airdrome (1)
the velocity and (2) the direction of the wind, (3) the speed of the
airplane he is to fly, (4) the compass bearing of the actual course
which he desires to follow. With this data it is possible to construct
a simple diagram and to determine the direction to be steered and
the actual velocity which will result in the proposed journey. A
draftsman’s scale, protractor and dividers, a pencil and a piece of
paper are the necessary equipment.
When the wind blows at an angle with the desired course it is necessary
to steer the airplane in such a direction that its own forward motion
will neutralizer the side effect of the drift of the wind from moment
to moment. The problem is to determine this direction for steering, as
it is not known. We are not concerned with distances in this problem,
for the direction is going to be the same whether our flight is of 100
or 200 miles. We are, however, vitally concerned with velocities; and
we will assume that the velocity of the airplane is known to be 75
miles per hour, and from observation on a local anemometer the velocity
of the wind is known to be 20 miles an hour. We also know, of course,
the direction of the wind, which should be given in terms of an angle
whose other leg points directly north. Now if the flight is to be made
at a height of 2000 ft., as is usual in cross-country flight over
average country, we will find that the speed of wind will increase as
we rise up; moreover, that its direction will change. In the present
case the wind will be 88 per cent. higher in 2000 ft. than it is on
the ground; that is to say, the velocity at the altitude we are going
to use is twenty times 1.88, or about 38 miles per hour. Moreover,
as the height increases the direction of the wind changes, shifting
around always in a clockwise direction as the height increases, in the
present case shifting around 16° from its ground direction. (The change
of velocity and direction for various heights is indicated on the
subjoined table.) Thus a west wind becomes at a height of 2000 ft. a
slightly northwest wind, or, to be exact, blows from a direction which
is 74° west of north.
Our treatment of the problem then has for starting points: velocity
of wind, 38 miles per hour; direction of the wind, 74° west of north;
velocity of airplane 75 miles per hour; desired direction of flight
(which has been determined by laying out on the map and reading the
compass bearing with the protractor), say 60° east of north. In 1 hr.
of flight the machine would travel in this unknown direction a distance
of 75 miles were it not for the wind, but for every hour of such flying
the wind is blowing it 38 miles sideways; and the desired direction
must be such that its joint effect, together with the 38 mile sideways
wind, will leave the machine exactly on its proper course at the end of
the hour.
On the map or piece of paper denote the starting point by _A_ (see Fig.
37). From _A_ draw a line parallel to the wind (that is to say, 74°
west of north), and let this line represent, to any convenient scale,
the speed of the wind, 38 miles per hour. The far end of the line may
be called _B_, and may be given an arrow to represent the direction of
wind. Now draw on the map a line from _A_ to the desired destination
(_C_), giving it, of course the proper compass bearing. Take the
dividers, and with _B_ as a center, describe an arc at such distance
as to represent 75 miles per hour, the speed of the machine; this arc
will intercept the line _AC_ at _D_, and _BD_ then gives the direction
to steer, for it is that direction which will permit the airplane in
1 hour exactly to neutralize the sidewise drift of the wind. The
distance _AD_ on this diagram can be measured off and will give the
actual velocity of movement along the line of flight in miles per hour.
Notice that it is 97 miles per hour, quite different from the speed of
the airplane.
[Illustration: FIG. 37.—Graphical method for determining direction to
steer to counteract wind-drift.]
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 3.29_x_, a total time of _x_ + 3.29_x_ which we
know equals 180 min. before the gas runs out. Solve the equation _x_ +
3.29_x_ = 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.
Such a drift indicator is, of course, useful only when the ground is
visible. The pilot knowing the angle between the airplane axis and the
line of motion and therefore knowing the deviation between the supposed
course and the actual course is able to make corrections and steer
the machine in its proper direction. This may be done by altering the
“lubber-line” or his compass just enough to offset the side drift of
the machine; after which the desired course may be followed by simply
keeping to the proper compass bearing. An instrument has been devised
wherein the rotation of the drift-indicator telescope simultaneously
alters the lubber-line zero. The operator then has merely to take
an occasional observation of the apparent drift line of the ground,
which observation automatically shifts the lubber-line and navigation
proceeds as if there were no side wind blowing whatever. Knowing the
angle between the direction of movement and the airplane axis, the
pilot may then compute the speed of motion in a manner analogous to the
graphical method previously mentioned; or he can make use of a chart
for the determination of this speed.
=Navigation over Water.=—In flying over water the presence of waves is
a valuable guide to the aviator, for he knows that these waves extend
in a direction normal to the wind. Moreover, he knows that the velocity
of the waves bears some relation to the velocity of the wind. In order
to estimate the velocity of the waves it is only necessary to know
their wave length, that is, the distance between two consecutive wave
crests. The rule is that for a wave length of 10 ft. the velocity is 10
miles per hour, and will vary as the square root of this wave length;
that is, if the wave length is half, the velocity will be 10 divided by
the square root of 2, or 7.1 miles per hour.