SHORT SPAN AND AREA.
Both on account of speed, and also on account of stability, with a low
centre of gravity, we are forced in the direction of the short span
machine. How are we to construct a machine with a span short enough to
damp out swaying and yet with sufficient lifting surface to raise the
machine and its load?
The position is somewhat simplified, as already pointed out, by the
fact that though the lift is decreased by the decrease in span, it is
to a great extent compensated by the increase in speed. Also another
compensation is offered by the fact that fore and aft stability
requires a lifting tail.
Lift is largely in proportion to the length of the entering edge of
the plane, but it does not always follow that this entering edge must
be at right angles to the direction of flight. The Dunne machine
obtains its lift with an entering edge that is entirely at an angle of
some 45 degrees, and its shape is an exact replica of the arrow head
of prehistoric man and the paper darts of our schooldays, a design, by
the way, that was patented in 1860.
At first sight it would seem that the lift on a plane shaped thus
(Fig. 18), would only be equal to the lift given by a plane with an
edge as long as the distance between A and C, thus (Fig. 19), but this
is not so. Although the lift is not so great as it would be if the
edge was straight in one line (Fig. 20), it is very much greater than
it would be on Fig. 19. The probability is that it is about half-way
between (19) and (20), but probably nearer to (20) than (19). There
are no exact data to go on, but the efficiency of the Dunne machine
would seem to show this.
[Illustration: FIG. 18.]
[Illustration: FIG. 19. AND FIG. 20.]
Again, in seeking for planes that offer the least resistance to the
air, one of the best that suggests itself is the T-shape (Fig. 21),
and this may be improved by cutting off useless corners (Fig. 22). A
plane of this shape lends itself to great strength of construction
owing to its small extending parts. It is compact, it gives an
entering edge half as long again as its span, and gives a lift in
proportion to that edge, and it is in itself stable. Having thus
evolved a suitable plane for the front of the machine, the best thing
to do is to base the back plane on the same design, and join the two
planes together to form the supporting surface of the machine,
allowing sufficient space between them to avoid any interference or
overlapping. The design then stands thus (Fig. 23), when the back
plane is a slightly smaller copy of the front one. The position of the
centre of gravity in this design would be coincident with the centre
of pressure longitudinally and laterally, and would be situated about
at A. A paper model on these lines with a low centre of gravity may be
easily constructed and will prove useful in illustrating the different
points here stated. The paper should be cut out sufficiently wide to
allow of a central longitudinal fold (Figs. 24 and 25), and a roll of
paper should be made for ballast and pushed through the fold as shown
in Fig. 26 at the point marked A.
[Illustration: FIG. 21. AND FIG 22.]
[Illustration: FIG. 23.]
[Illustration: FIG. 24. AND FIG. 25.]
[Illustration: FIG. 26.]
The writer, when exhibiting at Olympia this year, distributed 500 of
these paper models, and the almost uncanny way in which they righted
themselves when started from all sorts of impossible positions
greatly interested the visitors. In fact, numbers of persons spent
considerable time and ingenuity in trying to force the little glider
to turn over or dive, but quite without success.
In order to test the turning capacity of this design, a rudder should
be fixed to the tail, and the model launched at a moderate speed, when
it will be found that it turns quickly and without any pendulum
motion, and without any perceptible tilt. And although the writer’s
experiments with the paper model and with many larger ones on the same
plan have run into thousands, none of the models have ever been
induced to come down in any other position but on their feet. The
largest model, which measured 6 ft. 6 in. in length, was launched both
upside down and with its head pointing vertically to earth from a
height of 30 ft., and in each case righted before it reached the
ground and landed on its skids.
As a further lifting surface, a very simple expedient offers itself in
the shape of a duct built on the box-kite principle. The
diamond-shaped box has been proved over and over again to be a very
efficient lifting device, but it has not yet been tried on an
aeroplane (Fig. 27). It is also a great stabilizer, since the air
entering into the diamond-shaped opening is collected and compressed
into the top angle there, and the whole box is thus practically
suspended from its apex line in absolute stability. The lifting
efficiency of such a box--or rather the top portion of the box, for
the bottom part is not needed on our machine--is considerably
greater than the value of the entering edge, and if run the whole
length of the machine it forms a triangular girder of great strength,
giving rigidity to the whole structure. The lifting efficiency is
doubled by allowing the centre third of the girder to be open, as the
dead air from the front part escapes, and the back part forms a new
entering edge.
[Illustration: FIG. 27. AND FIG. 28.]
There is also another lifting factor to be considered, and this is the
car. If the car is formed with a flat bottom, this at once becomes an
efficient lifting plane, and if the car is suspended with an open
space between it and the under surface of the plane, the loss caused
by the negative angle of the upper portion of the car front is
compensated for by the lift given by the deflected air to the under
surface of the main plane (see Fig. 28).
It will be recognized that in the design here being gradually evolved,
the great lifting surfaces of the ordinary machine have not been
largely reduced, they have simply been broken up into several smaller
surfaces, each of which retains its efficiency. Something of the same
nature happened in prehistoric days, when our first navigators at last
made up their minds to abandon the flat-bottomed raft with its huge
supporting surface, for the new-fangled and dangerously narrow boat.
When all the different surfaces here mentioned are taken into
consideration, it will be found that the lifting surface in a
monoplane machine of this design, with a span of 20 feet, is equal to
the lifting surface of an ordinary bi-plane with a span of 40 feet.
And as the head resistance is less than half that of the bi-plane the
speed should be very much greater. At the same time the increased
speed renders the planes more efficient, area for area, than the
planes of the slower machine.