Aeroplanes and Dirigibles of War | Page 9

the

frequent allusion which will be made to the terms hereinafter. Lift is
the word employed to indicate the amount which a plane surface will
support while in flight. Drift is the term used to indicate the resistance
which is offered to a plane moving forwardly against the atmosphere.
Fig. 3. Lift and Drift
In Fig. 3 the plane A is assumed to be moving forwardly in the
direction of the arrow B. This indicates the resistance. The vertical
arrow C shows the direction of lift, which is the weight held up by the
plane.
NORMAL PRESSURE.--Now there is another term much used which
needs explanation, and that is normal pressure. A pressure of this kind
against a plane is where the wind strikes it at right angles. This is
illustrated in Fig. 4, in which the plane is shown with the wind striking
it squarely.
It is obvious that the wind will exert a greater force against a plane
when at its normal. On the other hand, the least pressure against a plane
is when it is in a horizontal position, because then the wind has no
force against the surfaces, and the only effect on the drift is that which
takes place when the wind strikes its forward edge.
Fig. 4. Normal Air Pressure
Fig. 5. Edge Resistance
HEAD RESISTANCE.--Fig. 5 shows such a plane, the only resistance
being the thickness of the plane as at A. This is called head resistance,
and on this subject there has been much controversy, and many theories,
which will be considered under the proper headings.
If a plane is placed at an angle of 45 degrees the lift and the drift are the
same, assumedly, because, if we were to measure the power required to
drive it forwardly, it would be found to equal the weight necessary to
lift it. That is, suppose we should hold a plane at that angle with a
heavy wind blowing against it, and attach two pairs of scales to the

plane, both would show the same pull.
Fig. 6. Measuring Lift and Drift
MEASURING LIFT AND DRIFT.--In Fig. 6, A is the plane, B the
horizontal line which attaches the plane to a scale C, and D the line
attaching it to the scale E. When the wind is of sufficient force to hold
up the plane, the scales will show the same pull, neglecting, of course,
the weight of the plane itself.
PRESSURE AT DIFFERENT ANGLES.--What every one wants to
know, and a subject on which a great deal of experiment and time have
been expended, is to determine what the pressures are at the different
angles between the horizontal, and laws have been formulated which
enable the pressures to be calculated.
DIFFERENCE BETWEEN LIFT AND DRIFT IN MOTION.--The
first observation is directed to the differences that exist between the lift
and drift, when the plane is placed at an angle of less than 45 degrees.
A machine weighing 1000 pounds has always the same lift. Its mass
does not change. Remember, now, we allude to its mass, or density.
We are not now referring to weight, because that must be taken into
consideration, in the problem. As heretofore stated, when an object
moves horizontally, it has less weight than when at rest. If it had the
same weight it would not move forwardly, but come to rest.
When in motion, therefore, while the lift, so far as its mass is concerned,
does not change, the drift does decrease, or the forward pull is less than
when at 45 degrees, and the decrease is less and less until the plane
assumes a horizontal position, where it is absolutely nil, if we do not
consider head resistance.
TABLES OF LIFT AND DRIFT.--All tables of Lift and Drift consider
only the air pressures. They do not take into account the fact that
momentum takes an important part in the translation of an object, like a
flying machine.

A mass of material, weighing 1000 pounds while at rest, sets up an
enormous energy when moving through the air at fifty, seventy-five, or
one hundred miles an hour. At the latter speed the movement is about
160 feet per second, a motion which is nearly sufficient to maintain it
in horizontal flight, independently of any plane surface.
Such being the case, why take into account only the angle of the plane?
It is no wonder that aviators have not been able to make the theoretical
considerations and the practical demonstrations agree.
WHY TABLES OF LIFT AND DRIFT ARE WRONG.-- A little
reflection will show why such tables are wrong. They were prepared by
using a plane surface at rest, and forcing a blast of air against the plane
placed at different angles; and for determining air pressures, this is, no
doubt,