ERECTION AND ALIGNMENT.
The accurate erection and alignment or truing up of the aeroplane, is
a cogent factor in ensuring that the best performance is obtained,
and it is almost platitudinous to emphasize the fact that a machine
incorrectly aligned gives inferior results in flight, entails greater
attention on the part of the pilot, and may possibly seriously
interfere with the general stability of the aeroplane. The degree of
precision attained in the manufacture of the various components is
reflected in the ease or otherwise with which the complete assembly
is aligned; indeed, accuracy of erection is impossible without the
close observance of limits and general trueness in the production of
the different parts. For this reason the erection of the principal
components is surveyed as a necessary preliminary to a consideration of
their assembly in the complete structure.
Accurate Part Production.
In the production of the various struts, longerons and fittings of
the fuselage, the wing spars, compression and interplane struts of
the planes, the utmost accuracy must be observed. Although tolerances
are permissible with regard to the overall dimensions of the struts,
spars, longerons, etc., the lengths particularly of the fuselage
struts should be absolutely correct to drawing. The bad effects of a
strut, say 1 millimetre short, are not restricted to the particular
component of which it forms a part, but are noticeable in one way or
another in the complete structure. Similarly the ends of struts which
are required to be square should be dead square, and those which are
cut to a bevel should correspond with the correct angle. The result of
the slightest discrepancy in this respect becomes speedily apparent
when the defective struts are assembled, as the tension of the bracing
wires will result in the strut becoming bowed or bent, this being
due to the bedding down of the strut end in the socket or clip. It
is also advisable to trim the ends in a machine after being sawn to
something approaching the correct length, and the practice of sawing
to dead length should not be permitted. The surface of a sawn strut
end is formed of a number of more or less ragged fibres, which in
position in the machine and under pressure of the bracing wires tend
to gradually flatten down, this resulting in slack wires and loss of
alignment. Absolute accuracy and uniformity of part production can only
be obtained by the use of jigs, preferably of metal, and some form of
jig should certainly be used for cutting the various struts to length.
Referring again to the necessity of the strut ends being of the correct
angle, it is surprising to note the effect of the smallest inaccuracy.
The writer has frequently noticed fuselage struts considerably out
of straight, the grain of the timber being sometimes advanced as the
reason. However, the removal of the defective strut always resulted
in its return to a straight condition. It should be realized that the
effect of an initially bent strut is a reduction of strength, and as
this may prove a source of danger, it is in itself sufficient reason
for the rigid observance of length limits.
Drilling of Bolt Holes.
Of equal importance is the drilling of the various bolt holes for the
attachment of the fittings. It is not always advisable to drill the
holes in the spars and longerons before the fittings are applied, but
in numerous instances this is possible, and where interchangeability is
an important consideration it is imperative. The practice of setting
out the positions of the various holes from a drawing and then drilling
with a hand brace, is a procedure only justified when a small number
of machines of a certain type are to be produced, and ought by now
to be obsolete. Under such a system no two spars would be exactly
the same, as owing to the influence of grain in the wood, the drill
or bit always tends to “run” from the correct angle. Viewed from the
aspect of quantity production such a practice is very deficient. It
is only by the use of metal drilling jigs of suitable design that
anything approaching absolute accuracy is possible. Such jigs should
not only locate the hole, but should also form a guide for the drill.
In the attachment of the fittings to a properly jig-drilled spar, it
should not be necessary to again drill through, although this often
occurs. Where this is done, there is a distinct possibility of the
brace not being held true, which means that the hole becomes larger
than necessary and not infrequently oval in shape. An additional bad
point is the impossibility of detecting such a fault after the fitting
is bolted on, and it may not be realized until a noticeably slack
wire in the complete machine indicates the movement of the fitting.
In the foregoing, absolute accuracy in the various fittings has been
assumed, but unfortunately in practice almost the reverse is true.
Variation generally occurs in built- or bent-up fittings, and is
usually the result of jigs of either incorrect or bad design. Where
the variation includes a hole out of position, the use of this fitting
on a previously drilled wood part is only possible by the bad practice
of drilling through with the results explained above. It will thus be
realized that the uniformity and accuracy of component production is
only attainable by the utmost precision in the manufacture of both wood
and metal parts.
Locking of Bolts.
Throughout the complete machine it is necessary to lock the nuts of
the bolts, to prevent their gradual loosening under the vibration of
the engine, and different methods of accomplishing this are in use.
Undoubtedly the best form of lock is by the use of a castellated nut
and split pin. By this method one can readily ascertain whether or no
a bolt is locked, while by the withdrawal of the split pin the bolt
may be taken out. A disadvantage is that its use entails considerable
drilling, so that a modification consists of fitting castellated nuts
to all bolts liable to removal for minor adjustments; while elsewhere
the threaded portion of the bolt is left a little longer than the nut,
and then riveted over. Although this reduces labour, it is a somewhat
destructive method; and it is also difficult to determine the adequacy
of the riveting. Another method consists of filing the bolt end flush
with the nut, and then centre punching three or four dots in the joint
between nut and bolt.
This method is neat, the removal of a bolt is easily effected, and the
fact that it has been used in the construction of some fast scouting
biplanes is proof of its effectiveness.
Other systems include the use of two nuts, of a single nut soldered
to the bolt end, and the various patent lock-washers, which in this
country are not greatly in vogue. The practice of re-running down the
threads of bolts to ensure ease in the application of the nut is not
to be recommended--that is, indiscriminately done. Unless the die is
properly adjusted there is a possibility of too much thread being
taken off; the result, an extremely slack nut, being detrimental to
general reliability. The durability of an aeroplane in service is
dependent upon the good workmanship effected in the smallest and most
insignificant detail. Moreover, it should be remembered that the
absence of a split pin may eventually result in disaster.
Truing of Main Planes.
The planes or sections of a machine of the straight-wing type, as
distinct from a machine possessing arrow-shaped or retreating wings,
should, when erected on the fuselage, form a straight line from tip to
tip. This feature is dependent upon (1) the trueness of the planes,
and (2) the alignment of the attachments on the fuselage, the latter
being considered under the fuselage heading. To ensure that the plane
is quite square, it should be checked previous to covering by diagonal
measurements on the wing spars, these being taken from accurate set
positions such as are provided by the wing-root attachments and the
interplane strut fittings. Should a difference in the diagonals exist,
this can easily be rectified by a slight adjustment of the turnbuckles
incorporated in the internal plane wiring. As the ribs of the plane
are built up beforehand, and checked for correct contour by pattern,
little variation should occur in the camber. A point where differences
may occur is between the front spar and the leading edge, as the nose
formers are generally inserted during the assembling of the plane. For
the detection of faults in this direction the template illustrated by
Fig. 34 in Chapter IV. is of great utility.
Fabric Covering of Planes.
The evenness and correct tautness of the fabric covering contributes
largely to the trueness of the plane. Should the covering be stretched
unevenly or too tightly, the application of the dope will cause
distortion of the framework, which can only be obviated by re-covering.
The bad effects of this is more noticeable with regard to the ailerons,
elevators and rudder, which, being of very slender construction, are
more liable to deformation. Twisted or warped control surfaces should
never be used, as such surfaces not only offer increased resistance,
but also interfere with the balance of the machine in flight.
Fuselage Erection.
As the fuselage constitutes the nucleus of the aeroplane, accuracy of
alignment in this component is essential, and the degree of accuracy
obtained in the complete erection depends largely on the correctness
or otherwise of the fuselage. In different individual designs the
methods employed for the construction of the body will be found to vary
considerably. The process of erection adopted in many instances is to
assemble the sides first, upon a table or bench upon which the correct
disposition of the various parts have previously been set out. The
wires are adjusted until the sides conform to the setting out, which
are then packed up on a pair of trestles and the cross-struts attached.
It now remains to align the body so that it is perfectly symmetrical
in plan; and this is accomplished by marking the centre of each
cross-strut, preferably before insertion in the fuselage, and then
adjusting the plan-wires until a cord stretched from the stern-post to
the nose covers each centre line. The cross or sectional bracing-wires
are then tensioned until each diagonal coincides absolutely in length.
This procedure answers very well for a small fuselage of simple
construction, and of the wire-braced fabric-covered type; but where
the forward portion is covered with ply-wood, and the top rails of the
body are horizontal, viewed in side elevation, it is usual to true up
on a bench. This consists of a wooden structure built up of strong
sides, with legs at short intervals, the whole being well braced. The
top surface, on which the body lays, is composed of boards placed
wherever a plan-strut occurs. The bench should be rigidly fixed to a
concrete floor, the top planed until it is level both longitudinally
and transversely, and a centre line marked on each board, while these
lines, checked with a fine steel wire stretched from end to end, should
be in exact agreement with it. The fuselage, having been previously
assembled, with the wires inserted and the plan struts accurately
centred, is placed on it in an inverted position. All wires should be
then slacked off, and the top, which is now underneath, should be wired
until the centre on each strut coincides with the centres on the bench.
The side wires are then tensioned until the stern post is vertical,
or until various fixed points, such as wing-spar attachments, are in
agreement with points marked on the bench and squared or lined up,
and also until the longerons are touching every board. The sectional
wires are then tightened and adjusted so that each diagonal is of the
same length; and this will ensure the centre lines on the cross-struts
connecting the bottom rails being plumb or vertical over the centre
lines of the cross-struts connecting the top rails. Where the top rails
of the fuselage are not parallel to the line of flight, but slope down
towards the tail, it would be necessary, if the bench method is used,
to construct it so that the boards conform to the slope. With the
wire-braced fuselage minor adjustments to the wing-spar attachments,
which predetermine the angle of incidence of the main planes, can be
subsequently made. A type of fuselage which precludes this operation,
and which demands extreme accuracy in construction, is that in which
the bracing of the forward portion is effected by three-ply, all wiring
in a vertical dimension being eliminated, this system being described
in Chapter VIII. and illustrated by Fig. 66.
With this construction points such as the wing spar attachments are
fixed, and cannot be altered after the fuselage is built, so that
meticulous care must be taken in the setting of the short wing spars
across the body, or the fittings to which the wing roots are anchored.
Where a joint occurs in the fuselage it is usual to build the tail
separate from the front portion, and occasionally the two sections are
trued up independently. This does not give such good results as when
the two portions, although separately built, are joined together and
trued up complete.
Checking of Fuselage.
To check the fuselage for alignment it should be placed on a pair of
trestles, one underneath the forward undercarriage strut fixing and the
other under a vertical strut a short distance from the stern post. The
body should then be levelled up longitudinally by a straightedge placed
on two short straightedges of exactly similar widths, one being placed
at the front and the other towards the tail. It should then be packed
up on the trestles until the top longerons are dead level across. At
this point, if the body is in correct alignment, the engine-bearers
would be level both longitudinally and transversely, the incidence
of the main spar attachments should be correct and the stern post
perfectly vertical in all directions. Other tests should include the
placing of a straightedge at the nose, and another placed at the points
where struts occur, should, when sighted across the top edges, be “out
of wind,” that is in agreement. A point which should be carefully
levelled is that portion of the fuselage towards the stern post to
which is attached the fixed tail plane. Any inaccuracy here will result
in the tail being twisted in relation to the main planes. Each fitting
or attachment should also be equidistant from the stern post, and the
effect of variation here will be evidenced by the tail plane being
out of square with the centre line of the fuselage. Where the type of
machine is such that the engine is supported on bearers of wood, it
is usual to drill the holes for the accommodation of the holding-down
bolts to jig before the bearer is built in the structure. In this case
care should be taken to ensure that the corresponding bolt holes in
each bearer are square with the centre line. Any deviation will result
in the axis of the engine forming an angle with the centre line.
Alignment of Complete Machine.
In this connection it will be better to consider the alignment of a
type of machine in common use: a tractor-biplane in which the upper
plane is composed of two outer planes and a centre section, and the
lower plane in two sections, each abutting against the side of the
fuselage, this arrangement being shown in front elevation by Fig. 122.
The first operation is the levelling of the fuselage transversely by
placing the level across the engine bearers, and the attachment of
the centre section, which is mounted upon four struts which have been
previously cut to dead length and tested by jig. This, considered in
front elevation, should be centrally placed over the body, and this
is assured by adjustments in the wires A-A1. This can be checked by
dropping a plumb-line from the centre plane spar ends and measuring the
distance from the line to the side of the body, the distances on either
side should, of course, coincide. The next point is to brace the outer
sections to the correct dihedral. One method of accomplishing this,
as shown by Fig. 122, is by the use of a dihedral board, this being
prepared perfectly straight on one edge, the other being tapered to
the desired angle. The wires are then adjusted until the straightedge
is level. Another method is to use an ordinary straightedge placed
along the top surface of the plane, the angle being measured with a
protractor or clinometer, the latter instrument being most accurate. To
check the dihedral a line can be stretched between points immediately
above the top interplane struts on each side and then measuring to the
centre section, but it would be difficult to detect differences in the
angles of each wing. With regard to the undercarriage, the distances
between lines dropped from the fuselage sides and the wheel centres
should coincide.
Alignment of Machine in Side Elevation.
Considering the side elevation, Fig. 123, alignment here is concerned
with the incidence of the main planes, the distance forward of the top
plane from the lower plane or stagger, and the level of the engine
bearers in relation to the top longerons of the fuselage.
[Illustration: FIG. 122.--Showing use of dihedral board and
spirit-level.
FIG. 123.--Checking of main-plane incidence and stagger.
FIG. 124.--Template for checking incidence.]
The fuselage should be levelled longitudinally by placing the level
on the engine bearers, assuming the engine is not in place. When the
bearers are level, the top longeron should also be level, in any case
the incidence of the plane should only be adjusted in relation to the
engine bearers. To check the stagger, a plumb line should be dropped
from the leading edge of the centre plane, and adjustments made with
the incidence wires from the fuselage to the centre-plane struts, until
the required distance forward from the leading edge of the lower plane
is obtained. The incidence can be tested by a straightedge placed
under the plane and a clinometer, as in Fig. 123, and another device
sometimes used is shown by Fig. 124. This is made of dry wood, the
lengths of the legs to the tops of the spars being obtained from a
drawing of the wing section, and its incidence.
Plan Alignment of Machine.
[Illustration: FIG. 125.--Showing points to check for correct alignment
on plan.]
In the plan view, Fig. 125, the distances AB and AC must be equal,
the same applying to CD and BD. With modern machines external drift
wiring is obsolete, so that discrepancies in these measurements must
be rectified by alterations in the wiring of the fuselage, as it is
inaccuracy at some point in the latter component to which the trouble
may be ascribed. It is at this point that one realizes the need for
precision in the construction of the fuselage. In Fig. 126 is shown
a plan view in which the main plane is very obviously out of square
with the centre line of the body, the amount is not likely to occur in
actual practice, but it has been exaggerated in the drawing. The cause
of this trouble can be traced to the short wing spars in the fuselage,
to which the lower plane is attached, or in other cases to the
fittings, to which the lower plane is anchored, being out of centre,
possibly only an insignificant amount. The lengths of the fuselage wing
spars are also possible causes of trouble. Assuming that the rear spar
is the correct length, and the front spar is over the length, this
would result, when the outer sections were attached, in the latter
sloping backwards, which again emphasizes the need for accurate part
production.
[Illustration: FIG. 126.]
With regard to the tail plane, measurements taken from the extremities
of the back spar to some fixed point forward on the fuselage, to the
strut sockets on the planes, or to the rear wing spar anchorage, as in
Fig. 125, should be equal.
The primary consideration with regard to the rudder and fin is that,
viewed from the rear, they should be perpendicular, which can be
verified by a plumb-line dropped from the top of the rudder-post. In
plan view the fixed fin should correspond with the centre line of the
fuselage although there are exceptions to this rule, notably where the
fin is set over, to neutralize propeller torque, and in this case the
measurements given in the general drawings must be adhered to.
Tension of Wires.
The correct tensioning of wires is a matter upon which some variation
of opinion occurs. Although wires should not be left slack, conversely
they should not be over-tensioned, as this results in the spars, wires,
and struts, being initially stressed before any load due to flight is
applied. In this connection the importance of even or uniform tension
in the wires may be emphasized. The wires in one bay being of greater
tension than those in an adjacent bay, is the frequent cause of bent
or deformed struts. The more extended use of a tautness meter for the
interplane wiring would result in greater uniformity and the more equal
distribution of stresses.
INDEX.
Aileron and warp control, 101
Aluminium, 16
Anchorage of wires to eyebolts, 55
Ash, 10
Ash, weight of, 10
Axle fairings, 98
Biplane trussing, 62
Birch, 12
Birch, weight of, 12
Brazing and welding, 52
Bridge-type undercarriage suspension, 98
Control systems, 101
Control by inherent stability, 102
Control, vertical column, 103
Control, wheel type, 104
Control, the “Dep,” 106
Control, the “Wright,” 107
Control surfaces, balancing of, 102
Cypress, 13
Drift bracing, 65
Drilling of bolt-holes, 124
Duralumin, 17
Engine mountings, 116
Engine mountings, rotary, 118
Engine mountings, overhung, 119
Engine mountings, stationary, 121
Engine mounting materials, 117
Erection and alignment, 123
Fabric, attachment of, 47
Fabric, sagging of, 40
Fabric, tension of, 48
Fabric, friction blocks, 49
False ribs, 41
Fineness ratio, 26
Fuselage construction, 67
Fuselage covering, 71
Fuselage contours, 75
Fuselage erection, 127
Fuselage fittings, 77
Fuselage struts, 28
Fuselage types, 67
Glue, defects of, in wing spars, 33
Hickory, 11
Interplane struts, tapering of, 26
Interplane strut connections, 51
Leading and trailing edges, 43
Lift wires, anchorage of, 62
Locking of bolts, 125
Longerons, 68
Longeron sections, 69
Longeron, jointing of, 70
Longeron, shaping of, 69
Mahogany, Honduras, 11
Mahogany, Honduras, weight of, 11
Mahogany, Cuban, weight of, 12
Main planes, truing of, 126
Main spars, shaping of, 32
Materials, 6
Maurice Farman strut arrangement, 62
Metal construction, general, 4
Metal wing construction, 45
Methods of manufacture, general, 3
Moisture content of timber, 8
Monoplane bracing, 60
Monocoque fuselage construction, 72
Multi-ply wood, 13
Oleo-pneumatic gear, 95
Oregon pine, 12
Parang, 13
Piano wire bracing, 108
Piano wire connections, 109
Plane construction, 30
Plane construction, details of, 40
Planes, arrangement of, 33
Poplar, 12
Pratt truss, 59
Raked wing tip, efficiency of, 43
Ribs, spacing of, 40
Ribs, types in use, 34
Ribs under compression, 36
Shakes, 14
Shrinkage, 7
Silver spruce, 8
Silver spruce, weight of, 8
Single-strut truss, 63
Spar construction, hollow, 20–22
Spar, laminated, 19
Spar sections, 19
Spars and struts, 18
Spruce fir, 9
Standardization of details, general, 2
Steel, 15
Steel tube, 16
Steel-tube fuselage construction, 84
Stranded cable, 109
Stranded-cable connections, 111
Streamline wires, 113
Streamline wires, attachments, 113
Strut sections, 23
Strut sections, design of, 26
Strut materials, 25
Strut socket, “Wright,” 53
Strut socket, “Cody,” 53
Strut-socket fairings, 58
Suspension springs, steel, 95
Suspension springs, rubber, 97
Tabulated tests, unreliability of, 8
Three-ply fuselage construction, 71
Thunder shakes, 11
Timber, defects in, 14
Trailing edge, wire, 43
Twisted grain, 14
Undercarriage brakes, 99
Undercarriage details, 93
Undercarriage, principles of design, 86
Undercarriage skids, 93
Undercarriage types, 89
Walnut, 11
White deal, 9
Wing baffles, 44
Wing fittings, attachment to spars, 51
Wing-tip construction, 39
Wing-tip details, 38
Wing-trussing systems, 59
Wireless wing structures, 61
Wires and connections, 108
Wires, results of tests on, 109
Wires, relative strengths of, 112
Wires, tension of, 134
Wood, choice of a suitable, 6
Wood, variable qualities of, 6
PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, LONDON AND BECCLES.
Just Published. Over 100 pages. With numerous Diagrams.
Fcap 8vo. Paper wrapper, Price 3s. 6d. net.
The Aircraft
Identification Book
A CONCISE GUIDE TO THE RECOGNITION OF
DIFFERENT TYPES AND MAKES OF ALL KINDS
OF AEROPLANES AND AIRSHIPS
BY
R. BORLASE MATTHEWS
Associate Member of the Institution of Civil Engineers, Member of the
Institution of Electrical Engineers, Fellow of the Royal
Aeronautical Society, Whitworth Exhibitioner,
Author of the “Aviation Pocket-Book,” etc.
AND
G. T. CLARKSON
EXTRACT FROM PREFACE
Just as one make of motor car can be distinguished from another by
noting some characteristic detail, such as the radiator, the trade
mark, the hubs, etc., so can the main types of aeroplanes and airships
be readily differentiated by the recognition of the outstanding
features of each machine. It must be admitted, nevertheless, that it
needs considerable experience to be able definitely to classify detail
modifications of a particular type of aircraft, for the difference is
often very slight. Besides, the machines are frequently only visible
at a great distance, and for but short periods (their speed being
so high--60 to 150 miles per hour). There are, moreover, physical
difficulties to contend with, in the form of clouds, effects of winds,
position of the sun, etc.
It is claimed, however, that this book will enable the reader to
identify any aircraft, provided it can be seen for sufficient time to
observe its main features.
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Transcriber’s Notes
Punctuation, hyphenation, and spelling were made consistent when a
predominant preference was found in the original book; otherwise they
were not changed.
Simple typographical errors were corrected; unbalanced quotation
marks were remedied when the change was obvious, and otherwise left
unbalanced.
Many illustrations in this eBook have been left near where they
originally appeared, even when that was mid-paragraph.
The index was not checked for proper alphabetization or correct page
references.