CONTROL SYSTEMS.
The mechanism by which the aeroplane is controlled in flight forms
the connecting link between the pilot and machine, and constitutes
a vitally important and somewhat vulnerable item of the complete
structure.
Main Principles.
The control of all modern aeroplanes is effected in a lateral direction
by small planes hinged to the rear spar of the outer ends of the
wings, and known as “ailerons”; in a longitudinal or “fore-and-aft”
direction by the elevator planes; and for steering by the rudder.
Although these functions are alluded to separately, they are more
often than not combined in their actions. The correct proportion of
the controlling surfaces is an important factor in determining the
ease or otherwise with which a machine can be handled in flight,
and faults in this direction are responsible for the terms “heavy”
or “stiff” on the controls being applied to a machine. The use of
subsidiary flaps or ailerons for lateral control is a comparatively
modern innovation. At one time it was usual to warp the entire plane,
or in some cases the outer section only, and although the principle
is the same--that of forming a negative or positive surface to the
line of flight--structural considerations are wholly in favour of
ailerons. With warping, the whole plane is subjected to continuous
torsional movement, and to obtain this some of the trussing wires have
necessarily to be arranged as control wires, the result being that the
plane curvature loses its uniformity, and the whole girder system
of the planes is less efficient under load than if the wires were
permanently fixed; and the latter item is only possible with aileron
control. Although it is usual to attach ailerons to both top and bottom
planes of a biplane, there are occasions when sufficient control can be
obtained with ailerons to the upper plane only, usually when the span
of this plane is greater than that of the bottom.
Control by Inherent Stability.
With machines of the inherent stability class the lateral control
is effected by additional means, the planes being designed to
automatically right the effects of gusts. This element of inherent
stability is obtained by suitably grading the camber and incidence of
the wings, until at the wing tips the chord of the plane section forms
a negative angle to the line of flight. Although this arrangement is
undoubtedly of value, especially for the touring machine of moderate
power, its chief fault lies in the relatively slow righting movements,
which, although of no great consequence at a reasonable altitude,
becomes a source of danger when alighting, and certainly entail the use
of ailerons, or warp, to counteract it. The type was well exemplified
in this country by the Handley-Page monoplane and biplane, while in
Germany it achieved great popularity, surviving in some makes until
the latter part of 1916. In the matter of control-surface design it is
interesting to note the contrast between the preferences of English
and German designers. In almost all German machines the ailerons,
elevators, and rudder are balanced, _i.e._ surface is disposed each
side of the hinge-axis, this applying to the small Albatross scouts
and to the large machines of the Gotha class; while in this country
few examples of this practice occur. The reason for the balancing of
controls lies in the desire to reduce the manual strain on the pilot to
a minimum; and it appears that with large machines balanced surfaces
will be imperative. Several automatic controls have been produced,
the most notable perhaps being the Sperry gyroscopic, this being a
combination of servo-motor and gyroscope. This apparatus has been well
tried.
So far as the arrangement of the control surfaces is concerned, little
variation occurs, which condition has obtained from the early days of
aviation, but the mechanism governing or directing these movements
varied at one time considerably, and although in this country one type
of control is used, there are still instances of the use of widely
different systems. In former days the practice of individual makers
fitting different controls resulted in some arrangements being in exact
contradistinction to others, which not infrequently meant, to a pilot
taking on a new type, the unlearning of a great deal which practice had
rendered instinctive.
The Instinctive Principle.
All modern controls are based on the instinctive principle, _i.e._
the movements of the control lever coincide in direction with the
promptings of natural instinct. Thus, to change the course of a
machine flying level into an upward one, the column is pulled towards
the pilot, and for descent, the reverse, while to correct a bank,
the column is moved in a direction opposed to that of the bank. For
steering, a foot-bar is employed, so arranged that for a turn to the
left the left foot is pushed forward, and the reverse for a right turn.
On one well known machine of former days, the foot-bar actuated the
lateral control, which is sufficient indication of the great diversity
of opinion then existing.
Vertical Column Control.
[Illustration: FIG. 100.--Arrangement of vertical column control.]
A typical control of the immensely popular “joy-stick” type is shown
by Fig. 100. This consists of a vertical column pivoted through the
medium of a fork-joint to a rocking shaft. The elevator wires are
taken round pulleys mounted under the seat, and the aileron wires
from a form of bell-crank, flanged and welded to the steel tube. A
disadvantage with this system, in addition to the complication of the
wires, is that lateral movement also affects the elevator, although
the extent of this is of no great moment. It is obvious, although
somewhat paradoxical, that if the elevator is to be depressed by a
forward movement of the column, the control wires will required to
be crossed, _i.e._ the wire running from the base of the tube to the
pulleys will be attached to the arm on the top side of the elevator,
and _vice versâ_. On single-seater machines it is sometimes necessary
for the pilot to have both hands free of the controls, so that it
becomes necessary to install some form of locking device for the
elevator control, there being many simple ways of accomplishing this.
The locking of the control lever fixes the flight path of the machine,
but, of course, lateral equilibrium can be maintained by movements of
the lever sideways, and steering by the rudder bar. The German machines
of the Fokker and Albatross types are both fitted with the single lever
control with a locking arrangement. Another method which achieves the
same purpose consists of bracing the lever in a normal flying position,
with rubber cable or coil springs anchored to various parts of the
fuselage, and although this permits of movement, the control column
always tends to return to the normal position.
Wheel Controls.
[Illustration: FIG. 101.--Arrangement of wheel control.]
While the “joy-stick” type of control is greatly in favour, there are
various forms of wheel control in use. American machines are almost
entirely fitted with wheel controls, and all things considered, it
appears that modern practice is evenly divided between the two types.
The sequence of movements of the wheel type may be varied in a number
of ways, the general arrangement shown by Fig. 101 being typical of an
average system. In this case the hand-wheel is mounted on a central
column, which in turn is rigidly fixed by some form of Tee joint to
a transverse rocking shaft. A sprocket attached to the wheel centre
engages with a short length of chain, which connects to the aileron
control, while the elevator wires are connected to short tillers,
arranged to work on the outer side of the fuselage. With this system
the hand-wheel is rotated for the aileron movements, a fore-and-aft
rocking motion for the elevation, and the rudder is actuated by an
outward movement, with either foot on the rudder bar. A development
designated “three in one” embodies all these movements in the wheel
column, which in this case is pivoted at its base: a to-and-fro motion
in the column for the elevators, sideways for the ailerons, while the
rudder control is effected by the rotation of the wheel. This system is
fitted to a number of American machines, but it is a moot point whether
the rotation of the wheel for warping or steering is quite such an
instinctive action, as the sideways movement of the lever combined with
the movements of the foot on the rudder-bar; in any case, there is just
a suspicion of complication in its working which is undesirable, that
is, for machines intended for popular use.
The “Dep” Control.
[Illustration: FIG. 102.--“Dep” type control.]
The type of control used on the Deperdussin monoplanes of 1910 and
onwards has survived until the present day, and forms a distinctive
arrangement. Its chief attribute is that, compared with other systems,
much greater room and freedom is afforded the pilot, which is evident
by a consideration of the diagrammatic sketch, Fig. 102. The inverted
U-shaped lever is composed of either ash, bent to shape, or steel, or
duralumin tube, the general system of its working being the same as the
wheel control shown by Fig. 101. Incidentally, passing reference may
be made to the fact that the usual close proximity of the compass to
the controls precludes the use of steel in any great quantity for the
construction of the lever, as the various movements adversely affect
the compass readings.
The Wright System.
Another variant of the wheel control is instanced by the Wright system,
this consisting of a general lay out similar to that shown by Fig.
101, but no rudder-bar is fitted. The rudder control is provided by a
small lever, mounted concentric with the wheel, the latter carrying
a rigidly attached sprocket. The hand-lever is also connected to a
sprocket, this running free on the wheel shaft, so that by gripping
both hand-lever and wheel it is possible to operate the ailerons and
rudder simultaneously, this action being a characteristic feature of
all the Wright productions. Although there are many types of control
in use, those described in the foregoing chapter are illustrative of
general practice.