aeroplanes-第25节
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way between the tip and the hub。
In such a propeller； if the blade is the same
width and pitch at the two points named； the pull
at the tips will be four times greater than at the
intermediate point。
CHAPTER XIV
EXPERIMENTAL GLIDERS AND MODEL AEROPLANES
AN amusing and very instructive pastime is
afforded by constructing and flying gliding machines；
and operating model aeroplanes； the latter
being equipped with their own power。
Abroad this work has been very successful as
a means of interesting boys； and； indeed； men
who have taken up the science of aviation are
giving this sport serious thought and study。
When a machine of small dimensions is made
the boy wonders why a large machine does not
bear the same relation in weight as a small machine。
This is one of the first lessons to learn。
THE RELATION OF MODELS TO FLYING MACHINES。
A model aeroplane； say two feet in length； which
has； we will assume； 50 square inches of supporting
surface； seems to be a very rigid structure；
in proportion to its weight。 It may be dropped
from a considerable height without injuring it；
since the weight is only between two and three
ounces。
An aeroplane twenty times the length of this
model； however strongly it may be made； if
dropped the same distance； would be crushed； and
probably broken into fragments。
If the large machine is twenty times the dimensions
of the small one； it would be forty feet in
length； and； proportionally； would have only
seven square feet of sustaining surface。 But an
operative machine of that size； to be at all rigid；
would require more than twenty times the material
in weight to be equal in strength。
It would weigh about 800 pounds； that is； 4800
times the weight of the model； and instead of
having twenty times the plane surface would require
one thousand times the spread。
It is this peculiarity between models and the
actual flyers that for years made the question of
flying a problem which； on the basis of pure calculation
alone； seemed to offer a negative； and
many scientific men declared that practical flying
was an impossibility。
LESSONS FROM MODELS。Men； and boys； too；
can learn a useful lesson from the model aeroplanes
in other directions； however； and the principal
thing is the one of stability。
When everything is considered the form or
shape of a flying model will serve to make a large
flyer。 The manner of balancing one will be a
good criterion for the other in practice； and
experimenting with these small devices is； therefore；
most instructive。
The difference between gliders and model aeroplanes
is； that gliders must be made much lighter
because they are designed to be projected through
the air by a kick of some kind。
FLYING MODEL AEROPLANES。Model aeroplanes
contain their own power and propellers which；
while they may run for a few seconds only； serve
the purpose of indicating how the propeller will
act； and in what respect the sustaining surfaces
are efficient and properly arranged。
It is not our purpose to give a treatise on this
subject but to confine this chapter to an exposition
of a few of the gliders and model forms which
are found to be most efficient for experimental
work。
AN EFFICIENT GLIDER。Probably the simplest
and most efficient glider； and one which can be
made in a few moments； is to make a copy of the
deltoid kite； previously referred to。
This is merely a triangularly…shaped piece of
paper； or stiff cardboard A； Fig。 84； creased in
the middle； along the dotted line B； the side wings
C； C； being bent up so as to form； what are called
diedral angles。 This may be shot through the
air by a flick of the finger； with the pointed end
foremost； when used as a glider。
_Fig。 85。 Deltoid Glider。_
THE DELTOID FORMATION。This same form may
be advantageously used as a model aeroplane； but
in that case the broad end should be foremost。
_Fig。 86。 The Deltoid Racer。_
Fig。 86 shows the deltoid glider； or aeroplane；
with three cross braces； A； B； C； in the two forward
braces of which are journaled the propeller
shaft D； so that the propeller E is at the broad
end of the glider。
A short stem F through the rear brace C； provided
with a crank； has its inner end connected
with the rear end of the shaft D by a rubber band
G； by which the propeller is driven。
A tail may be attached to the rear end； or at
the apex of the planes； so it can be set for the
purpose of directing the angle of flight； but it will
be found that this form has remarkable stability
in flight； and will move forwardly in a straight
line； always making a graceful downward movement
when the power is exhausted。
It seems to be a form which has equal stabilizing
powers whether at slow or at high speeds；
thus differing essentially from many forms which
require a certain speed in order to get the best
results。
RACING MODELS。Here and in England many
racing models have been made； generally of the
A…shaped type； which will be explained hereinafter。
Such models are also strong； and able to
withstand the torsional strain required by the
rubber which is used for exerting the power。
It is unfortunate that there is not some type of
cheap motor which is light； and adapted to run
for several minutes； which would be of great value
in work of this kind； but in the absence of such
mechanism rubber bands are found to be most
serviceable； giving better results than springs or
bows； since the latter are both too heavy to be
available； in proportion to the amount of power
developed。
Unlike the large aeroplanes； the supporting
surfaces； in the models； are at the rear end of
the frames； the pointed ends being in front。
_Fig。 87。 A…Shaped Racing Glider。_
Fig。 87 shows the general design of the A…
shaped gliding plane or aeroplane。 This is composed
of main frame pieces A； A； running fore
and aft； joined at their rear ends by a cross bar
B； the ends of which project out slightly beyond
their juncture with the side bars A； A。 These
projecting ends have holes drilled therein to receive
the shafts a； a； of the propeller D； D。
A main plane E is mounted transversely across
this frame at its rear end； while at its forward
end is a small plane； called the elevator。 The
pointed end of the frame has on each side a turnbuckle
G； for the purpose of winding up the shaft；
and thus twisting the propeller； although this is
usually dispensed with； and the propeller itself
is turned to give sufficient twist to the rubber for
this purpose。
THE POWER FOR MODEL AEROPLANES。One end
of the rubber is attached to the hook of the shaft
C； and the other end to the hook or to the turnbuckle
G； if it should be so equipped。
The rubbers are twisted in opposite directions；
to correspond with the twist of the propeller
blades； and when the propellers are permitted to
turn； their grip on the air will cause the model to
shoot forwardly； until the rubbers are untwisted；
when the machine will gradually glide to the
ground。
MAKING THE PROPELLER。These should have
the pitch uniform on both ends； and a simple
little device can be made to hold the twisted blade
after it has been steamed and bent。 Birch and
holly are good woods for the blades。 The strips
should be made thin and then boiled； or； what is
better still； should be placed in a deep pan； and
held on a grid above the water； so they will be
thoroughly steamed。
They are then taken out and bent by hand； or
secured between a form specially prepared for
the purpose。 The device shown in Fig。 88 shows
a base board which has in the center a pair of
parallel pins A； A； slightly separated from each
other。
_Fig。 88。 Making the Propeller。_
At each end of the base board is a pair of holes
C； D； drilled in at an angle； the angles being the
pitch desired for the ends of the propeller。 In
one of these holes a pin E is placed； so the pins
at the opposite ends project in different directions；
and the tips of the propeller are held
against the ends of these pins； while the middle
of the propeller is held between the parallel pins
A； A。
The two holes； at the two angles at the ends of
the board； are for the purpose of making right
and left hand propellers； as it is desirable to use
two propellers with the A…shaped model。 Two
propellers with the deltoid model are not so necessary。
After the twist is made and the blade properly
secured in position it should be allowed to thoroughly
dry； and afterwards； if it is coated with
shellac； will not untwist； as it is the changing
character of the atmosphere which usually causes
the twisted strips to change their positions。
Shellac prevents the moist atmosphere from affecting
them。
MATERIAL FOR PROPELLERS。Very light pr
