NATURAL STABILITY
AND
THE PARACHUTE PRINCIPLE
IN AEROPLANES
BY
W. LEMAITRE
_Hon. Sec., Aeroplane Building and Flying Society_
WITH 34 ILLUSTRATIONS
LONDON
E. & F. N. SPON, LTD., 57 HAYMARKET
New York
SPON & CHAMBERLAIN, 123 LIBERTY STREET
1911
CONTENTS
PAGE
FRONTISPIECE iv
PREFACE ix
CHAPTER
I. THE IMPORTANCE OF STABILITY 11
II. SPEED AS A MEANS OF STABILITY 14
III. THE LOW CENTRE OF GRAVITY 17
IV. SHORT SPAN AND AREA 28
V. VARIABLE SPEED AND THE PARACHUTE PRINCIPLE 36
VI. THE DESIGN WHICH FULFILS THE CONDITIONS 39
PREFACE
Since there is nothing new under the sun, it is useless to pretend
that there is anything new in the design here advocated or the
theories advanced. Both are rather the result of a commonsense
consideration of the different points of all flying machines, natural
and artificial, and an endeavour to select from the great number of
good points those which seem most likely to blend together into a
practical machine. The conclusions reached are the result of a quite
independent investigation, carried on over three years by means of
numberless experiments, and the writer has endeavoured to make no
single statement which he cannot by some experiment amply prove.
NATURAL STABILITY IN AEROPLANES
CHAPTER I.
THE IMPORTANCE OF STABILITY.
In considering the whole question of aviation, it becomes evident that
the one point to strive for at the present juncture is stability. If
we are ever to have a practical flying machine, that is, a machine
which we can use as we do a yacht, a motor car, or a bicycle, it must
be one that we can trust to keep its balance by reason of the natural
forces embodied in it, and without any effort of control on the part
of the pilot. It may be objected that a bicycle does not do this, and
this is true, but, on the other hand, the upsetting of a bicycle is a
very small matter, whereas the tilting of an aeroplane mostly means
sudden death to its occupant, and it is probable that if the same
consequences followed the tilting of a bicycle, bicycles would soon
have been made with four wheels.
At present aeroplanes are the most unstable of all things. The least
gust, the least shifting of weight, the slightest difference in the
density of the strata of the supporting air, and the machine sways,
and if not instantly corrected by the pilot the sway becomes a tilt,
the tilt a dive, and the rest is silence. The first aeroplanes, the
Wrights’ for instance, were so unstable that twenty minutes in one of
them was as much as the most iron-nerved man could stand, the
continual strain being too exhausting to keep up for any length of
time. By throwing out extensions and outriggers in all directions we
have altered that to a certain extent, but only to an extent--we have
not yet got rid of it. The monoplane is probably the most unstable, as
might be expected from its smaller surface, but the bi-plane runs it
pretty closely.
And the difficulty seems to arise chiefly from the fact that the
machines are built round the propeller. In the case of a yacht or a
car, the machine is built first and the propelling means is fitted on
as an auxiliary. The consequence is that an aeroplane which is safe
enough while the propeller is exerting a tractive force of some 250
lbs., becomes, the moment this power is for any reason stopped, merely
a shapeless construction at the mercy of the wind and the force of
gravitation. It is true that most machines may be made to glide if the
pilot is clever enough and quick enough to steer them into the proper
gliding angle, but the machine that will naturally and by reason of
its design assume its proper gliding angle when the propelling force
is withdrawn, has not yet been built.
Such a machine would have “Natural Stability.”
It will be recognized that this natural stability, which depends on
the design of the machine, is something entirely different from
“automatic stability” of which there are many systems, all having this
one defect; that, depending upon working devices, movable planes,
gyroscopes, compensating balancers, pendulums, etc., they are all
liable to go wrong and refuse to act the moment a sudden strain makes
their perfect action most important.
Considering that the propeller is the only means the aeroplane has of
keeping in the air at all, the question arises: Is it possible to
design a machine that will be stable to the extent of descending
safely when the propeller stops, and that will yet be a good and
speedy flyer?
That is the problem we have to solve.