SPEED AS A MEANS OF STABILITY.
It is recognized on all hands that speed is a great factor in the
problem of stability. To begin with, a machine going at high speed
would be practically untouched by gusts of wind, different densities
of air strata, holes in the air, etc. Also its greater momentum would
tend to keep it in a straight line, not only relative to its course
but also relative to itself. That is to say, its wings being started
in a horizontal plane, would tend to keep in the same plane and would
not easily tilt or sway out of it. Both these effects of natural law
show that a high speed machine must be more stable than a low speed
machine. How then are we to design a high speed machine?
Leaving aside the question of higher power, the first point that
suggests itself is to lessen the head resistance. All fast things,
boats, birds, arrows, even motor-cars, are made long and narrow. It
will be objected that a bird with its wings outspread is not long and
narrow, but in the sense in which this illustration is meant, the
bird’s wings, being merely its propelling apparatus, do not count, and
when the bird is at its fastest, as in the swoop of a hawk or an
eagle, the wings are shut tightly to the body so as to offer no
resistance to its lightning passage through the air. If we are to
follow previous experience in Nature’s laws, our aeroplanes must be
considerably reduced in span. To drive through the air at a high speed
with a machine of 40 foot span is a practical impossibility, both
because of the tremendous power it would require and also by reason of
the great strength the plane must have to withstand the resistance of
the air.
In reducing the span, however, we reduce the lifting surface of the
machine. But on the other hand it must be remembered that the lifting
efficiency is increased by increasing the speed. Lift is the product
of supporting surface and speed. A small plane driven at a high speed
will give as great a lift as a large plane driven at a low speed.
Speed, again, is the difference between the propelling power and the
head resistance, and we can increase the speed by decreasing the
resistance. It follows, then, that we need not necessarily give up
lifting power by reducing the span of the wings, since the shorter
span gives greater speed, and the increase of efficiency by reason of
the greater speed would go to make up for the loss of span.
It is, then, quite possible to design a short span machine which shall
be as efficient for lift as a long span machine, and which will have
the advantage of possessing, by reason of its speed, much greater
stability.
But the span is not the only factor in the speed problem. In the low
speed machines at present in use we have found it necessary to curve
the planes to get greater efficiency. This efficiency is also gained
at the expense of head resistance, and it is already recognized that
the higher the speed the less is the need of camber. This is the same
problem over again. A high speed flat plane will give as much lift as
a low speed cambered plane, and we gain in stability with every
additional mile per hour.
The third point to be considered in the problem of speed is the
resistance caused by the multitude of struts and wires, the body of
the pilot, the tanks, engine, and all the other impedimenta projecting
in all directions from the body of the aeroplane. It has occurred to
our builders that if the whole of these things could be collected
together and enclosed in a light covered-in car of a proper shape, the
skin friction of such a car would be much less than the total head
resistance offered by the different obstructions so covered. And there
is another advantage to be gained here, for if, at 40 miles per hour,
the force of the wind is very seriously uncomfortable for the pilot,
the position at such speeds as 70 or 100 miles per hour would be quite
impossible.