The mirror. (Stillwater, Minn.) 1894-1925, 1918-06-13
Air Boat Design a Success
Continued from page one.)

turns q corner with the aid of friction be-
tween the wheels and road, to which he adds
with chains in slippery going. The motor
cyclist has banked track on which to make
speedy turns. ALI have something solid or
at least liquid heavy with inertia to work
against. The air boat has only thin air.
Consequently, the air boat must be built
so that it won't change its course with every
gust that blows. To do this it must have
more keel surface behind than in front of its
vertical turning axis, like a weathercock,
that the gusts may tend to blow the ma
chimes nose into the wind, rather than away.
This is highly important with this idea in
mind, which was one of several reasons why
they didn't dare fly in a windy. The i'm
portance is found in momentum. Two tons
of flying machine at hundred miles an
hour have enormous momentum-just as
much, as huge touring cars of two tons
weight making that speed on a road. I'm
agine turning suddenly to one side or an
other in such car. The wheels would turn
but the car would 60 straight on-the wheels
would break and bad accident would re-
| suit If the air boat is suddenly turned
as it might be by gust were it not for suf-
fcient keel surface behind the vertical turn
ing axis-it would be flying crabwise through
the air, momentum keeping it on its old
course, propeller and rudder trying to get it
on a new course. This "side slip" is very
dangerous. 1t is bad enough if it is caused
by trying too sudden turn, but if it could
be caused by every gust of wind that blew,
life aloft in breeze would be life constant
ly in danger.
The designer must provide that the plane
have as much tendency as possible to stay
on an even keel. The pilot has control of
his balance, but he cannot be forever balanc-
ing-he has many other things to do. So
the designer helps him by giving the wings
what he calls a dihedral angle-that is,
front view of the air boat shows the wings
as very fiat v. The result, that is when
the whole structure tips in the air, the hori-
zontal equivalent of the lower wing is great
er and that of the elevated wing less than
before. AS the horizontal position provides
the greatest lift, tipping the dihedral angled
wings produce the greatest lift on the down
side, which tends to right the tipping ma
chine. But too much dihedral angle de-
creases the total horizontal equivalent of the
surface, increasing the proportion of weight
to the surface, and also tends to throw the
center Of gravity too low. which results in
a pendulum like motion to the plane.
One other thing the designer can do to
help lateral stability, and about which few
besides pilots and designers know even the
name. The designer and the rigger wash
out one side of the aeroplane and wash in
the other side. To wash out plane is to
decrease its angle of incidence towards the
end of the plane. 1t is done on one side,
and the angle increased or washed in on the
other side, to counterfeit the desire of the
plane to revolve over and over in direction
contrary to that of its propeller. In other
words, the engine tries to turn the propelles
in one direction and incidentally, itself in
the other direction. If it were not for the
wash out and the wash in the pilot would be
continually combating this desire of the
| plane to roll in the air.
Longitudinal stability. too, is problem
of the designers. A properly built plane is
nose heavy, that is, if the power fails aloft,
the plane automatically drops nose first, just
enough to reach the best gliding angle, by
which gravity takes the place of the propel-
Ter thrust and maintains that speed by which
and by which alone, control of the whole is
maintained in the air. But it is not only q
proper placing of the center of gravity
which the designer must consider-not too
low, that there be no pendulum effect and
not too high, that the whole thing not turn
turtle at the first opportunity, he must re-
member that gusts of wind do not blow only
from side to side, but from below up and
from above down.
| The tail surface is relied on here, and it is
a beautifully simple contrivance. 1t is set
at a less angle of incidence than the main
planes. II a gust of wind Tits the plane
nose down from the horizontal, the craft
does not Immediately 60 in the direction its
nose is pointing. but somewhere between
the old course and the new, because of the
inertia and momentum, just as when gust
turns the plane sidewise it side slips.
The result is that the effective angle of
incidence of the planes which is their angle
to direction of motion, is less than normal.
This means decreased lift. But the tail sur
face, having less angle than the main
planes to start with, loses more of its angle
than the main planes with this crab-wise
nosing downwards. So the tail now having
less lift tends to fall more quickly than the
nose, and the plane rights again, although.
of course, at lower altitude.
Nothing has as yet been said about those
small parts of plane which enable it to fly
At all-the ailerons or flaps or warping sur
faces, whichever scheme is used. Probably