VARIABLE SPEED AND THE PARACHUTE PRINCIPLE.
Hitherto, on the score of efficiency and also of stability, our
investigations have led us to seek for speed as the grand panacea. But
there are usually two sides to a question, and though, while in the
air, speed may be most desirable, it becomes a source of considerable
difficulty at both starting and landing. A machine built to fly at 80
miles per hour would have to get up something like 60 miles per hour
before it could rise. And this difficulty is nothing like the problem
that presents itself when we consider how it is to land in safety from
a flight at such a speed. It becomes evident that some provision must
be made for starting and landing at some more practicable rates; we
must have a variable speed machine.
To convert a high speed machine into a low speed machine means either
variable surface area, variable camber, or variable angle of
incidence. Any of these is possible, but the choice must be decided by
simplicity of action. To spread extra wings when rising or landing is
a cumbersome suggestion full of pitfalls and liable to accidents
through the failure of mechanical devices, which, experience shows,
always have a way of failing at inopportune moments. To vary the
camber of the planes is easier, but having decided on using flat
planes it would be loss of strength to make these flexible, and an
increase of mechanical complications to have to flex them. It would be
easy to alter the angle of incidence by having the leading edge
capable of a rotary movement, and machines have been constructed
employing this principle. But the easiest plan of all, since it does
away with all moving parts whatever, would be to alter not the planes
themselves, but the whole machine. Thus suppose the angle of
incidence, in order to get an efficient lift, to be 1 in 6, the
lifting plane, all in the same line, would be set on its chassis so
that it presented an angle of 1 in 5. The machine would then lift at a
much slower speed. Naturally, the tail being the furthest from the
centre of gravity would lift first, and as soon as the speed was
sufficient the pilot would alter the elevator, send down the tail on
to the ground, thereby raising the leading edge of the front plane,
and the machine would rise. As the speed increased the tail would
continue to rise, till, at the maximum speed, the plane would be at
the minimum angle with the horizontal, i.e. at its lowest angle of
incidence.
This solves the problem of starting and to some extent of landing, but
we have not yet come to the end of our resources. Most landings are
effected by shutting off the engine and planing down. All flying
machines will glide if put at the proper angle, and it is the
business of the pilot to attend to this when he stops the engine. But
to glide with the same wing area as is used in flying, means to glide
at the same rate. In order to descend slowly it is necessary to have
more area. Is it possible to increase the area used for descent
without interfering with the area used for flight? In the design we
are engaged in considering, it is possible, and without any mechanical
devices. There is a large space between the front plane and the back
plane which is at present unused. It is of very little value in
flight, being in apteroid aspect and having practically no entering
edge. But if this space is covered in it gives no resistance in
flight, and in descent it becomes a very efficient parachute. Further
than this, if openings be cut in this plane immediately under the
centre of the two box-kite ducts, the air under the longitudinal
plane, having offered its resistance to the vertical passage of that
plane, will escape into the duct and again offer considerable
resistance to the descent of this closed-in surface before it escapes
finally out of the end of the duct.
A model made on these lines will not need putting at any angle. It
will assume its proper angle when left to itself by reason of its
design and the way the weight is balanced between the supporting
planes, and it will descend by partly gliding and partly parachuting
at a steep angle but quite slowly. While, if the pilot so choose, he
can, by raising the tail, increase the speed to a glide, which he can
turn into a parachute action at any moment.