THE LAUNCHING OF THE MODEL.
§ 1. Generally speaking, the model should be launched into the air
_against the wind_.
§ 2. It should (theoretically) be launched into the air with a
velocity equal to that with which it flies. If it launch with a
velocity in excess of that it becomes at once unstable and has to
"settle down" before assuming its normal line of flight. If the
velocity be insufficient, it may be unable to "pick up" its requisite
velocity in time to prevent its falling to the ground. Models with
wooden aerofoils and a high aspect ratio designed for swift flying,
such as the well-known Clarke flyers, require to be practically
"hurled" into the air.
Other fabric-covered models capable of sustentation at a velocity of 8
to 10 miles an hour, may just be "released."
§ 3. Light "featherweight" models designed for long flights when
travelling with the wind should be launched with it. They will not
advance into it--if there be anything of a breeze--but, if well
designed, just "hover," finally sinking to earth on an even keel. Many
ingenious pieces of apparatus have been designed to mechanically
launch the model into the air. Fig. 50 is an illustration of a very
simple but effective one.
§ 4. For large size power-driven models, unless provided with a
chassis and wheels to enable them to run along and rise from the
ground under their own power, the launching is a problem of
considerable difficulty.
§ 5. In the case of rubber-driven models desired to run along and rise
from the ground under their own power, this rising must be
accomplished quickly and in a short space. A model requiring a 50 ft.
run is useless, as the motor would be practically run out by that
time. Ten or twelve feet is the limit; now, in order to rise quickly
the machine must be light and carry considerable surface, or, in other
words, its velocity of sustentation must be a low one.
[Illustration: FIG. 50.--MR. POYNTER'S LAUNCHING APPARATUS.
(_Reproduced by permission from the "Model Engineer."_)]
§ 6. It will not do to tip up the elevator to a large angle to make it
rise quickly, because when once off the ground the angle of the
elevator is wrong for actual flight and the model will probably turn a
somersault and land on its back. I have often seen this happen. If the
elevator be set at an increased angle to get it to rise quickly, then
what is required is a little mechanical device which sets the elevator
at its proper flight angle when it leaves the ground. Such a device
does not present any great mechanical difficulties; and I leave it to
the mechanical ingenuity of my readers to devise a simple little
device which shall maintain the elevator at a comparatively large
angle while the model is on the ground, but allowing of this angle
being reduced when free flight is commenced.
§ 7. The propeller most suitable to "get the machine off the ground"
is one giving considerable statical thrust. A small propeller of fine
pitch quickly starts a machine, but is not, of course, so efficient
when the model is in actual flight. A rubber motor is not at all well
adapted for the purpose just discussed.
§ 8. Professor Kress uses a polished plank (down which the models slip
on cane skids) to launch his models.
§ 9. When launching a twin-screw model the model should be held by
each propeller, or to speak more correctly, the two brackets holding
the bearings in which the propeller shafts run should be held one in
each hand in such a way, of course, as to prevent the propellers from
revolving. Hold the machine vertically downwards, or, if too large for
this, allow the nose to rest slightly on the ground; raise (or swing)
the machine up into the air until a little more than horizontal
position is attained, and boldly push the machine into the air (moving
forward if necessary) and release both brackets and screws
simultaneously.[46]
§ 10. In launching a model some prefer to allow the propellers to
revolve for a few moments (a second, say) _before_ actually launching,
contending that this gives a steadier initial flight. This is
undoubtedly the case, see note on page 111.
§ 11. In any case, unless trying for a height prize, do not point the
nose of the machine right up into the air with the idea that you will
thereby obtain a better flight.
Launch it horizontally, or at a very small angle of inclination. When
requiring a model to run along a field or a lawn and rise therefrom
this is much facilitated by using a little strip of smooth oilcloth on
which it can run. Remember that swift flying wooden and metal models
require a high initial velocity, particularly if of large size and
weight. If thrown steadily and at the proper angle they can scarcely
be overthrown.
FOOTNOTE:
[46] Another and better way--supposing the model constructed with a
central rod, or some suitable holdfast (this should be situated at the
centre of gravity of the machine) by which it can be held in one
hand--is to hold the machine with both hands above the head, the right
hand grasping it ready to launch it, and the left holding the two
propellers. Release the propellers and allow them a brief interval
(about half a second) to start. Then launch boldly into the air. The
writer has easily launched 1½ lb. models by this means, even in a
high wind. Never launch a model by one hand only.