Aeroplane Construction

AEROPLANE

CONSTRUCTION

AEROPLANE

CONSTRUCTION

A

Handbook

on the various

Methods

and

Details

_of

Construction

_employed

in

the _Building

_of

_Aeroplanes

BY

SYDNEY

CAMM

' _* II

ASSOCIATEFELLOW ROYAL AERONAUTICALSOCIETY

LONDON

CROSBY

LOCKWOOD

AND

SON

7,

STATIONERS'

HALL

COURT,

LUDGATE

HILL

WILLIAM CLOWESANDSONS. LIMITED, LONDONANDBECCLEe.

PBEFACE

THE

articles

embodied

with

other

matter

in this

_book,

were intended

as a

_{broad survey}

of the _principles

and

details of

modern

_aeroplane construction,

concerning

which

there is a noticeable deficit

_amongst

_existing

aeronautical literature.

They

were

written at a

time

when

_specific references

to

modern

British aircraft

were

forbidden,

and

although

from

a

_comparative

_point of

view

this is to

be

_regretted,

the details

and methods

dealt

with

are, in the author's

opinion, representative of those

most

generally

used

in

machines

of _{present-day design.} It is

_hoped

tthat the

book

will _appeal

not

_only

to those

_engaged

on

the

manufacture, but

also to those

concerned with

the uses ofaircraft.

TABLE

OF

CONTENTS

CHAPTER

I. PAGE INTRODUCTION . 1

CHAPTER

II. MATERIALS . . . . ' . .

...

. . 6

CHAPTER

III.

SPARS AND STRUTS . . . . 18

CHAPTER

IV.

PLANE CONSTRUCTION . . '.. 30

CHAPTER

V.

DETAILS OF PLANE CONSTRUCTION ,

_..;

...

40

CHAPTER

VI.

INTERPLANE STRUT CONNECTIONS . . .

.51

CHAPTER

VII. WING-TRUSSING SYSTEMS . . , -,', -. ' 59

CHAPTER

VIII. FUSELAGE CONSTRUCTION

67-CHAPTER

IX. FUSELAGE FITTINGS .

CHAPTER

X. PAGE UNDERCARRIAGE TYPES 86

CHAPTER

XI. UNDERCARRIAGE DETAILS ! . 93

CHAPTER

XII. CONTROL SYSTEMS 101

CHAPTER

XIII.

WIRES AND CONNECTIONS . 108

CHAPTER

XIV.

ENGINE MOUNTINGS . 116

CHAPTER

XV.

ERECTION AND ALIGNMENT 123

AEROPLANE

CONSTEUCTION

CHAPTER

I.

INTRODUCTION.

THE

_purpose of this book is to _give

some

indication of the

principles

and methods

of construction of

modern

aeroplanes, as distinct from those considerations _{pertaining purely} to design, although occasional references to various elementary

principles of _aerodynamics have been found _necessary to

illustrate the

_why

and wherefore of certain constructional

details.

To

_many

_{the aeroplane} is a structure of _appalling

flimsi-ness, yet the principle which it _exemplifies, that of _obtaining

the

maximum

strength for a

minimum

of _weight, constitutes

a _problem of which the _solving is not _{only an unceasing}

labour, but one

demanding

the observance of the _best engi-neering procedure.

The

whole future of aviation,

commer-cially or otherwise,

may

be said to be indissolubly

bound up

with the _developmentof _efficiency;

and

whether this is to be

attained in _improvements in _{aerodynamical qualities,}

_by

the discovery of a material giving a greatly enhanced strength to

weight ratio, or

by

progress in the arrangement of the various

members

of the complete structure of the _aeroplane, is a

matter

_{upon which some}

_diversity of _opinion exists.

How-ever, itis certain thatthe _{very great developments} of the last

few years are clue

more

to refinements in _{design rather}than

construction;

and

it is questionable whether the

construc-tional

work

of the

modern

aeroplane has developed equally with _{design, so} that, even taking for _{granted the oft-repeated,}

but very doubtful, statement that

we

are _approaching the limitations 01 _ilengu, there irf _{certainly plenty} of _scope for

experiment

and improvement

in the constructional _principles

ofthe

modern

_aeroplane.

Standardization

of Details.

Whatever

_may

be said for the standardization of _aeroplane

types, a

scheme

which should effect a _{considerable saving} in labour

and

material, and which offers chances of success,

would consist in the standardization of metal fittings and

wood components

generally, for in this direction there is cer-tainly great need for _{improvement. Taking} as _{a hypothesis} the various

makes

of _scouting machines,

we

find _hardly

_any

two details the same. This

means

that if in this _country

there are six firms _{producing machines} of their

own

_design (these figures, of course, being purely suppositionary), there

will be six sets of detail _drawings, six sets of _{jigs, templates,}

and _press tools,

and

sundry special

machine

tools. There

seems no valid reason

_why

_many

of the _fittings for all

machines withincertain dimensions should not be of standard design,

and

a brief review of the various details which could be standardized _{without detracting} in the least from

aero-dynamical efficiency willindicate the extent to whichthe con-serving oflabour could be carried. In the construction of the fuselage, the clips fastening the longerons and cross struts could _easily be of one design, whatever the

make

of the

machine. At _present

we

find

some

_clips are bent

up

from a

stamping

and

attached to _{the longeron without the} drilling

of the latter;

some

built

_up

from various parts, such as washer-plates, duralumin pressings, and bolted through the

longeron; while

some

combine advantages

and

others the disadvantages of both. In

some

cases _{the longerons} of _spruce

are _{spindled out}for _lightening; inothers nospindling occurs;

while in a few instances _hickory or _ash, with or without channelling, is used. There are _{the interplane} strut attach-ments, stern-post fittings, control-surface hinges, and

under-carriage attachments, all _{showing great variations,} and inall of which_{the design could bebrought}within reasonable limits.

INTRODUCTION

3

As

_indicating

how

_unnecessary a_good deal of thevariation is,

one

_may

instance the fact that for the _swaged streamline, or R.A.F. wires, there are at least three different terminals in

use. _Although

more

difficult of achievement, there is _scope

for

_improvement

in the different _arrangements for the fixed

gun

mounting, while a standard instrument board would

benefitthe pilot.

Methods

of

Manufacture,

It is _fairly well

known

that the output of

some

firms is

considerably better than others, although themachines are of

the

same

design. Although a good

many

factors

_may

con-tribute to this _result, it seems _fairly certain that in

some

cases the

methods

of manufacture

must

be _superior, which

calls for

some

system of _{standardizing the} broad _principles

pertaining to _{manufacturing} procedure.

Under

this

arrange-ment

a

much

better estimate of _{probable output could} be

made. It is also _necessary

_by

the fact that

some

firms have

been _{developed through the exigencies} of war,

and

not as a result of

_any

_{great manufacturing} ability, whereas in peace

time _{the spur} of _competition would force _{the adoption} of the

most _rapid

methods

of _production.

The

creation of a central or universal office for _{the design} of the various _jigs used in the manufacture of _{aircraft,with power} to decide _{the process}

of _{manufacture, although} a

somewhat

far-reaching reform,

would certainly eliminate a

number

of useless experiments

made

_by

the individual constructors,

and

would also _greatly

improve the interchangeability of the various _components.

Inaddition, fresh firms to the aviation industry would be at

once _{acquainted with} the _general

methods

of manufacture,

which

should be of considerable assistance in _expediting

initial _output. Of course, this system would tend rather to destroy individual initiative, in that

much

that is

now

leftto the skill

and

_experience of the

workman

would be

predeter-mined, althoughthis would be

more

_{than compensated} for

_by

the increased benefits _accruingto the State. _{Jigs designed} to

produce the

same work

in different works often differ in

rate of _production.

As

an instance, in

some

works elaborate

benches are _{considered necessary}for the erection of _fuselages,

while in others a _pair of trestles suffices.

With

this _system

of unified _{manufacturing procedure} extreme _regard would have to be _{paid, in the design} of various _jigs

and

_fixtures, to adaptability for modifications in _design. Otherwise the various alterations

which

are

bound

to occur wouldresult in

an

_{unnecessary expenditure} on fresh _jigs. It is

somewhat

unfortunate that in _{the general design} of an _aeroplane, in

numerous

cases, far toolittle _regard is _paid to considerations

of easeof manufacture,

and

this is _{frequently responsible} for

the

_many

_changes in _design after a contract has been started.

Under

anideal _system of _{standardization, the requirements} of

manufacture would necessitate consideration in _{the design} of

the constructional details.

Metal

Construction,

The

_question of the aircraftmaterials of the future is not

so

much

a _problem as amatterof _gradual evolution. In view

of _{the dwindling supplies} of suitable _timber, it _certainly seems

more

than _{probable that}

some

formof metal construction will

one day constitute thestructure of _{the aeroplane.}

The

manu-facture of the various_componentsin

wood

does notnecessitate

an extensive plant, the labour necessary is _{comparatively}

cheap

and

easily available,

and

moreover tHe transitory nature

of the whole business,

and

the ease with which essential

changes in _type can be

made

_{without the wholesale scrapping}

of the _expensive jigs associated with the use of steel, all

strengthen the case in favour of wood.

The

conclusion of

hostilities would introduce another state of _affairs, and it is

conceivable that _{the various types} will then be standardized

fordifferent _purposes,

which

_may

necessitate the _greater use

of steel. _Certainly the _advantage of steel would be better realized under

some

_system of standardized design, but this unfortunately is not possible while present conditions obtain.

The

advantages of metal as a material considered briefly,are

thatit _permits of _design to close limits without the allowance

INTRODUCTION

5 the _great variation in the _strengths of wood, manufacturing

procedure wouldbe expedited,whileone can reasonably expect a _{greater degree} of _precision in the finished _{machine, due} largely to the increased facilities for accurate manufacture of

components which metalaffords. Itis _{quite possible,}ofcourse, given a uniform grade of steel, to design to extremely close limits without fear of _collapse; but the

human

factor in the

shape of fitter, welder, or _operator introduces the

unknown

element,

and

one for which

some

allowance

must

_always be

made.

One

cannot assert that

_any

_very decided indication

exists of a trend in

modern

design towards metalconstruction,

and

it is _{quite possible} that this will not arrive until it is

rendered imperative by reason of the scarcity of timber.

The

precise composition of the metal is rather a controversial matter,

some

authorities _favouring steel,

and

others

some

alloy of aluminium, suchas, for instance, "duralumin."

The

production of a suitable alloy constitutes a real problem

and

one

upon

whichthe _Advisory

Committee

for Aeronautics have

already

made

investigations

and

experiments.

A

disadvantage with steel is_{that, although} it is _{quite possible toproduce, say,}

a _{fuselage entirely} of this material to withstand _easily the greatest stress encountered in flying, such a structure, owing

to the thin nature of the various _{components, would} suffer

damage

through shocks induced byrolling over rough ground,

and also _byhandling. In addition the effects of

crystalliza-tionwould _require

some

considerable _study. These

and

other

reasonsindicate that an _{alloy of} aluminium,

which

for _{a given} weight would be considerably

more

rigid than steel, offers possibilities as a material. It might prove advantageous to

combine both metals, using steel for the

more

highly stressed parts,such as,forinstance,

wing

spar attachments, inter-plane bracinglugs, and indeed

any

part

where

the load to be carried

is one induced

by

tension.

The

foregoing is indicative of

some

of the

more

_important directions inwhich

improvement and

developmentarepossible,

and certainly

ample

scope yet exists for the attention of the student, or indeed

any

one interested in the future of the aviation industry.

CONSTRUCTION

CHAPTER

II.

MATEEIALS.

SEEINGthat

wood

constitutes the material for the _{greater part}

of the structure of _{the aeroplane,} that is _{with very few} excep-tions,

some

noteson the characteristics and qualities of those

woods

most

_commonly

used

_may

_prove of interest.

The

choice

of a suitable

wood

for aircraft construction is a matter of

some

difficulty, engendered bythevariety of considerations of

whichat least

some

observance is essential.

The

fundamental

principleofaircraft construction, thatofobtaining the

maximum

strength for a

minimum

of weight, affords one standpoint

from which a _particular

wood

_may

be regarded, but this does

not constitute in itself a sufficient reason for its choice. Of

almost equal importance are such considerations as_{the length}

and

size of the balks obtainable from the log, the total stock

available, therelative straightness of grain

and

freedom

from

knots as well as the_durability ofthewood.

Variable

_Qualities of

Wood.

The

choice is _{additionally complicated by the very great}

variation found in _{the strength}

and

characteristicsof trees of exactlythe

same

species,

and

alsoof different portions cutfrom

the

same

tree.

The

nature of the site

_upon

which a tree is

grown

exercises a

marked

influence _upon its _properties, while as a _{general rule,} it

_may

be taken that the _greater

number

of

annual growth rings per inch, thegreater the strength. It is

also _{a general} rule that

_up

to certain diameters, the timber contained in that _partof the tree the greatest distance from the_pith, or centre, is the stronger.

MATERIALS

7

The wood

obtained from the base of atree is heavier than

that at the _top,

and

one finds the influence of this in the necessity for balancing

and

alternating thedifferent laminae of

air-screws before gluing.

Shrinkage.

Another_point,

and

one which is _intimately concerned with

the _{proper seasoning} of timber, is the

amount

of moisture contained in _{a specimen,}

and

this latter _pointis of

some

con-siderable _importance,as _{not only}is a _large

amount

ofmoisture

detrimental to _{the strength values} of the timber, but it also renders useless

_any

_{attempt at precision} of _workmanship. It isthis _{very point}of _shrinkage,

which

constitutes the_greatest

barto the achievement of a

measure

of

_component

standard-ization,

and

it is also one of the

most

serious disabilities of

wood

as a material for aircraft construction. It is

now

necessary in the production of finished _{parts to}

make

some

allowancefor resultant _shrinkage, which is a matter of

guess-work,

and

only practicable

where

some

time will elapse

between the _finishing of the _part

and

its erection in the

complete machine.

Under

present conditions,

more

often

than not _{the parts} are assembled almost _{immediately they} are made, which

means

that noallowance over the actual size is _possible, this _being due to the various _fittings

which

in the majority of machines are of set dimensions

and

_clip or

surroundthematerial.

As

_{a natural sequence shrinkage occurs subsequent} to the

attachment of the _fitting, followed _by looseness

and

loss of

alignment in the structure. Until the _{proper period} for

seasoning can elapse, between the cuttingof the tree

and

its

conversion into _{aeroplane parts,} it is difficult to see

how

this

disabilitycan be obviated, although latterly

some

considerable

advances have been

made

withartificial methods of _seasoning.

The

_{prejudice against} kiln _drying is founded on the belief

that the strength ofthetimber is _reduced,

and

that extraneous defects are induced.

A

method

which is a distinct

improve-ment

on _{those systems, using superheated} steam

and

hot _air,

system, steam under very low compression is _constantly

cir-culated _{through the timber, drying being} effected _{by a gradual} reductionin the _humidity of_{the atmosphere.}

Unreliability of

Tabulated

Tests,

The

various tables which exist _indicating the _strength, weight,

and

characteristics ofvarious woods areof_very doubt-ful _utility, in

some

cases _fallacious,

and

in _nearlyall cases far

too _specific.

The

_foregoing enumeration of

some

ofthe varia-tions _existing with

wood

will indicate the

enormous

_difficulty

of _{obtaining with}

_any

exactitudea result _{representative}of the species of

wood

tested,

and

which could be regarded as reliable

data for the calculation of _stresses, or for _{general design.}

The

moisture content of _timber, an _extremely variable quantity, greatly affects the figures relating to _{the strength}

and

_weight of _{timber, so that tables indicating} the _properties

of woods _{should include the percentage} of moisture contained

in the _examples tested. _Again, certain woods _possessing

relatively high strength values, are frequently short-grained

and

brittle, and therefore not so suitable as other woods of

lower strength values, but of_{greater elasticity}

and

_resiliency.

WOODS

IN USE. Silver

_Spruce.

The wood

most _extensively used for the

main

items of

construction is silver spruce, or Sitka spruce, found in great quantities in BritishColumbia. Experience has proved this

wood

_{pre-eminently} suitable for _aeroplane construction, its

strength-weight ratio is _{particularly good,} it can be (at least

until _recently) obtained in _{long lengths}

_up

to 80ft., and, moreover, is _{particularly straight grained}

and

free from knots

and

other defects. There are otherwoods _{possessing higher}

strength qualities, but in

most

cases their value is greatly diminished _by reason of _{the greater weight,} and that _{only a} limited _portion straightofgrain

and

free from knots is

obtain-able.

The

weight of _{Sitka spruce} varies from 26 to 33 Ibs.

MATEEIALS

9

figure, a good average specimen fairly dry would weigh about

28Ibs. _per cubic foot.

Some

_impression of the extent to

which it enters into the construction of _{the aeroplane}will be gathered if the _{components usually} of _spruce are detailed.

For

the

main

spars of _{the planes spruce} is _{almost universally}

used, as here great strength for the least weight is of extreme

importance, while a consideration almost as important is the necessityof agood average length, straight grained

and

free

from defects. It is also usedfor thewebs

and

_flanges of the

wing

ribs, the leading

and

trailing edges

and wing

structure generally.

The

longerons or rails of the _fuselage of

_many

machines are _spruce, although in this instance ash and

hickory are used to a moderate extent.

The

growing practice

is to

make

_{the front portion} of the _fuselage of ash, as this is

subject to the greater stress, while the tail _portion is of

spruce; but in a

number

of cases thelatter material is used

throughout.

The

cross struts of the _{fuselage are invariably} of spruce, as well as such items as _inter-plane

and

under-carriage struts

and

streamline fairings.

Virginia

Spruce.

Thisis of _{a lower weight per cubic} foot than _{Sitka spruce,}

but does not possess such a _{good strength} value, cannot be obtained in such _{large pieces,}

and

is _{generally subject} to small _knots, which limit the _{straight-grained lengths} procurable.

Itis _{distinguishable} from _{Sitka spruce by} its whitenessof colour

and

_general closeness of _grain.

Norwegian

Spruce.

This

wood

is also

known

as _spruce fir

and

white _deal,

and

is

_grown

_{principally in} North Europe. Selected balks can be obtained to _{weigh no}

more

than 30Ibs. _per cubic foot,

which compares very favourably with silver _spruce. It can be obtained in _{average lengths,} but it is _{subject to the presence} of small hard knots and streaks of resin, althoughthe writer

has seen _consignments with _{very few} knots.

A

material

from the

same

source, and is

more

durable _{than Norwegian} spruce. It is inclined to brittleness

when

_dry, and is heavier

thanwhite deal, weighing about 36Ibs. _{per cubic} foot.

The

recent shortage of silver_spruce has led to the

_employment

of

Norwegian spruce for items such as _fuselage struts, hollow

fairings to tubular struts, the webs

and

flanges of the plane

ribs,

and

generally for those components for which _long

straight-grained lengths arenot absolutely essential.

For

_fuselage struts, where the chief consideration is

stiff-ness, to resist the _bending strain _{produced byinequalities} of

wiring, fittings, etc.,it

_may

_{actually give better results, being}

slightly

more

rigid than silver spruce at least that is the writer's _experience of it. In _{addition, very} little increase in weight would result, as this

wood

can be obtained of almost the

same

weight per cubic foot as silver _spruce.

The

defect usually

met

with in thiswood, of _{knots occurring} at _intervals,

would be of no _{great detriment, the lengths} needed for the

fuselage struts being approximately 3 feet

and

less, and it

would therefore be _{easily possible} to _procure

wood

of this

length free from knots.

The

other items enumerated areof

varyinglengths, which, with care in selection and conversion, could be _arranged for.

The

_{practical application} of this

would be the increased

amount

of silver _spruce available for

such highly stressed items as _{wing spars, interplane} struts,

and

_longerons.

Ash.

This

wood

is one of the most valuable of _{those employed,}

being extremelytough and resilient. There are two varieties

in _{use, English}

and

American, the former being considered

the better material. It is used

tmainly for longerons,

under-carriage struts,

and

for all kinds of bent work. It _possesses

the _qualityof _{being readily} steamed to _{comparatively sharp}

curves, and will retain the bend for a considerable period.

The

_strength

and

characteristicsof _{ash vary greatly} with the climate under which it is _grown,

and

itis also

much

heavier

than _spruce, the _{weight per cubic} foot _{ranging between 40}

MATERIALS

11

greater than 20 ft.,

and

even in lengths

up

to that figure,

continuity of _grain is

somewhat

rare. It is notable that on

various

German

machines, ash in _{conjunction with a species}

of

_mahogany

is used for the laminae ofthe air-screw.

Hickory.

Hickory, aspeciesof walnut,is_{imported from}

New

Zealand

and

America,

and

possesses characteristics similar to those of ash. It is obtainable inabout the

same

lengths as ash, but in the writer's _experience is of_{greater weight.} Its chief _property

is extreme resiliency, which

makes

it _especially suitable for skids,

and

it has also been used to a limited extent for longerons. It is _subject to excessive _warping in _drying, is

not sodurable as ash,

and

_{the great difficultyexperienced} in obtaining straight-grained lengthsis_responsibleforits

_waning

popularity.

Walnut.

This

wood

is almost _entirelydevoted to the

_making

of

air-screws, although the dwindling supplies

and

the very short lengths obtainable haspractically enforced the

employment

of

other woods for this_purpose.

Mahogany.

The

term "

_mahogany

" covers

an

infinite _variety of

woods, possessing widely different characteristics,

many

ofthe species being quite unsuitablefor the requirementsof aircraft

work. That

known

as

Honduras

_mahogany

_{possesses the} best strength values, is of

medium

_{weight, about 35} Ibs. _{per cubic} foot,

and

is in _{general use} for airscrews

and

_seaplane floats.

It has been used on

some

German

machines for such _parts

as rib _webs, but is not _really suitable for _parts of

com-paratively small section,such as_longerons, as itis inclined to

brittleness. It is of particular value for _seaplane floats

and

the hulls of the flying-boat type of _machine, as it is not affected _bywater.

A

defect _peculiar to

Honduras

_mahogany

is the occurrenceof_{irregular fractures across the grain}

known

similar in _appearance to the

Honduras

_variety, a _{species quite}

distinct in _appearance is that

known

as

Cuban

or _Spanish

mahogany,

which is of darker _colour,

and

much

heavier in weight, averaging about 50 Ibs. _{per cubic} foot, which latter

factor _{almost precludes} its use for_aeroplane construction.

Birch.

One

finds _{very few instances}of the use of this

wood

for aeroplane details, although it is used _{fairly extensively} in

America

for air-screw _{construction,} for which it is _only

moderatelysuited. It _{possesses a high} value of _compressive

strength across the _grain, but is

much

affected

_by

climatic changes,

and

does not take glue well. It is useful for bent work,

and

might conceivably be used insteadof ash for small bent

work

details. Its _weightis about 44 Ibs. _{per cubic} foot.

Poplar.

Under

this

name

is included such woods as

American

whitewood, cotton wood, bass wood, etc.

The wood

soldunder one or other of these

names

is _{generally very} soft

and

brittle,

and although of a light nature, weighing about 30 Ibs. _per

cubic foot and less,it is of_verylittle utility for the

work

under

discussion. Ithas been used for

minor

_partssuch as rib _webs,

and fairings to tubular struts.

Oregon

Pine.

The

scarcity of silver sprucehas led to_{the adoption} of the

wood

known

as _{Oregon pine} for most of the _components for

which 'the former

wood

has hitherto been used.

The

term

"

Oregon pine

"

is _appliedtothe _Douglasfir,oneof the largest

ofthefir _{species, a length} of 200 ft. _being an _average. It is

altogether heavier than silver _{spruce, weighing} about 34 Ibs.

per cubicfoot,

and

also differs_{greatly inappearance, possessing}

a reddish-brown grain, with very distinct annual rings. Its strength to_weight ratios_{are practically equal} tothoseof silver

spruce, although in the writer's _experience it has a _tendency towards _brittleness,

and

is not so suitable as _{Sitka spruce} for

MATERIALS

13

wood

it is noticeable thatthe effect of _drying on _freshly

sawn

lengths for _longerons, etc., is the _appearance of " shakes" or cracks, not previously discernible. Its _{appearance generally} isreminiscent of _{pitch pine, for}

which

wood

it is sometimes

substituted in connection with _building.

Other

Woods.

The

foregoing constitute woods

which

are in fairly general use for _{one purpose} or another, there being, of course, very

many

other varieties,

some

of which

_may

be called into use with the progressofthe_industry. Ofthe_{conifer species,}silver

spruce is _easily the

most

suitable timber for _aeroplane con-struction,

and

one realizes this

more

as the various substitutes are tried.

As

an _{instance,.cypress} is _straight of _{grain with}

no _{very great} increase over the _weight of _{spruce, being} also well

_up

the tableof _strengths. It is,however,

much

too brittle

forthe various

members

of smallsection ofwhich an_aeroplane

is _composed,

and

does not

seem

to have

_any

extensive future

for aircraft work. Another, at one time much-advertised

wood, is _Parang, a species of

mahogany.

It has been reputed

to bend well, but it _certainly does not enter into the

con-struction of

modern

_aeroplanes.

A

_{consignment handled by}

the writer

some

_{years ago}

and

intendedfor _bending,was found

to _{be exceedingly}brittle,

and

although standing a good load, fractured _{almost square} across the _{grain, in} a

manner

known

colloquially in the

workshop

as "carrot-like."

The

latterterm

is indicative of a characteristic

which

_{precludes the use} of

many

woods _possessing other _{physical properties especially}

suitable for aircraft work.

Multi-ply

Wood.

This term is _applied to the sheets of

_{wood composed}

of a

number

of _{thin layers glued together with the grain} reversed.

Asthe_layersare obtained _{by rotating} thetree _against cutters in such a

manner

that a continuous cut is taken from the outside almost to the _centre, it is _{possible to get very great}

widths, which

makes

it _{particularly suitable for}aircraft work.

from _2\> in.

_up

to _-J i

n

-> _consisting of three, five, and seven

layers,althoughthethree-ply variety inthicknesses

up

to _3%in. is

more

_commonly

used. It is

made

_up

in _nearly all _woods,

but those _mostly utilized in _{the aeroplane industry are birch,} ash, poplar,

and

satin-walnut, birch being superior by reason

of its closeness of grain.

Ash

ply-wood in

some

'instances tends towards _brittleness, while poplar, although exceptionally

light,is _verysoft and _{only used} forminor_parts. Satin-walnut

is _very even _{in quality}but is _apt to_warp.

Defects

in

Timber.

Perhaps the

most

common

and

_prolific defect encountered

with the use of timber is _{the presence} of cracks or shakes of

different character,

which

are due to different causes. _{Fig. 1} indicates a _very

common

form,

known

as a "_{heart shake,"}

FIG. 1. Heartshake. FIG.2. Starshake. FIG. 3. _Cupshake.

dividing the timber at the centre; while Fig. 2, a

" star shake," is _{really a}

number

of heart _{shakes diverging} from the centre.

The

_process of _seasoning sometimes results in the separation of the annual rings, forming cup shakes, as

shown

in _Fig. 3. It should be understood that the _presence of

FIG. 4. Twistedgrain.

shakes

_may

render useless an otherwise perfect specimen

of timber, as it _{frequentlyhappens} that in the conversion of

timber so affected _{the usable portions do not permit} of the

sizes_necessaryfor suchitems as _{wing spars} and struts.

The

defect of twisted _grain (Fig. 4) is often found in _ash, and is

MATERIALS

15

and

renders such

wood

of limited utility. Shrinkage affects all timber in _{varying degrees,}

and

its effect on boards due

to their _position in the log is

shown

_{by Fig.} 5, while

Fig. 6 indicates the effect of _{drying on} a _squared-up section. Incidentally one

may

point out that the annual rings, viewed

from theend of the section, should be as straight as possible,

which would obviate to an extent the distortion due to _drying in a

_component

_subsequent to its _finishing. Another defect,

and one

somewhat

difficult to detect, is the _presence of a

brownish _speckled tintin the grain.

Any

evidence of this in

FIG. 5. Shrinkageofboardsdue

to position inlog.

FIG.6. Effectof _drying on a squared-upsection.

a _specimenindicates the_beginning of _decay, and is caused _by insufficient seasoning and lengthy exposure in a _stagnant situation.

Steel.

The

_{greater proportion} of the various _{fittings employed} in the construction of the _aeroplane are built

_up

from sheet nickel steel, usually of a low tensile strength, to permit of

working in a cold state, as, with a higher grade steeJ, the

process of_bending to _template

_by

hand, in

_many

cases a none

too careful _procedure, would result in a considerable

weaken-ing ofthe material at the bend. Inaddition, the operation of

welding, which

now

enters into the construction of a

number

of _fittings, also necessitates a moderate _grade of steel.

A

higherclass of sheet steel, from 35 to 50tons tensile, is used

for _{parts subject to} stress, such as interplane strut-fittings,

wiring-lugs, etc.

As

a higher grade of steel is better from a strength-for-weight point of view,its

_employment

for _bent-up

CONSTRUCTION

clips is desirable, although where such a steel is used it is

almost necessary, if the_{original strength} of the material isto

be retained in the finished _{fitting, to} effect the various bends

in a _machine, in _conjunction with _{bending jigs.} Careful

heat-treatmentafter _bending to _shape is _{an important} factor in_removing the stresses set

_up

_byworking, and in _rendering the structure ofthe material

more

uniform.

Steel

Tube.

Steel, in the form of _tubing of various _sections, enters largely into aeroplane construction,

and

_may

be said to con-tribute _largely to the _efficiency of the structure. It is

now

being used for the different items of _{the undercarriage,} for

struts in the _{fuselage, interplane struts,}

and

in

_many

cases control surfaces, such as the ailerons, elevators, and rudder, are being built of this material _entirely. In the _earlydays of

aviation steel _tubing attained

some

considerable popularity,

many

machines _being built almost _entirely of _tubing; but

difficultiesin its _{manipulation,}and the fact that _veryoften the

methods

of attachment reduced its _{strength considerably,}

gradually ledto the general

employment

of wood.

The

_great

advances lately

made

in the productionofa high-grade

nickel-chrome

steel, with a high ultimate tensile stress, are

respon-sible forits_{present increasing} use.

Aluminium.

The

present use of

aluminium

is restricted to _{the cowling}

of _{the engine,}

and

_occasionally as a _{body covering. Although}

it is_{light in weight,} its _{extremely low strength values render}

it of _very little use for _{other purposes.} It attained

some

measure of _popularity in the _earlydays of _aviation,

particu-larly for the manufactureof different strut-sockets,which were castfrom

aluminium

; but the general bulkiness of the fittings,

in addition to thefact thatit

was

_{generallynecessary}to incor-porate a steel lug to form the wire anchorage, caused it to

gradually fall into disuse.

The

_tendency of

aluminium

to

flake

and

corrode, which is intensified _by the action of salt

MATERIALS

17 attempts have been

made

through various alloys to impart

greater strength to the material,

and

although progress has resulted, the characteristics of

most

of the products are unreliable.

Duralumin.

Of the different _alloys, duralumin is _probably the best,

although one believes that its _qualities are _principally the result of _special heat treatment. Its use is at _present restricted to those _parts not _subjected to

_any

_great tensile

strain. It is _considerably less than half the _weight of steel,

bulk for bulk, and, properly used,

may

effect a considerable saving in weight.

The

fact that it has not achieved the popularity it deserves

_may

be ascribed to the difficulties

experienced in working it, especially for such parts as body

clips,

where

several bends are necessary,

and

to -the rather arbitrary

methods

in use. If _{properly annealed,} no _difficulty

should occurin _{obtaining a reasonably sharp} bend.

The

pro-cess

recommended

_by the

makers

consists in _{heating the}

metal in a muffled furnace to a _temperature of _{approximately}

350 C.,

and

the necessary

work

done as-soon as possible after

cooling.

The

importance of this is due to the fact thatthe process of _{annealing imparts} to the metal a _tendency to

become

brittle with time.

The

writer has often contended

that,where duralumin is _used, it should be with a real desire to _{reduce weight.}

Too

often one sees a _fitting of such lavish

dimensions as _{to entirely nullify}the _advantage of the_lighter metal.

CHAPTER

III.

SPARS

AND

STRUTS.

HAVING

_{thus considered generally the} chief materials of air-craft construction,

we

will _proceed to examine the various types of spars

and

struts in _present use.

The main

spars of

the _wings are _byfar the _{most important} items of_{the complete}

structure,

and

very great careis _always taken to ensure that only the best of materials

_{and workmanship}

are concerned

with their manufacture. _Looking back at the days one usually associates with the aero shows at Olympia,

multi-tudinous

methods

of _building

_wing

_spars can be recalled.

Some

_composed of _three-ply

and

ash; others, less

common,

of

channel steel;

and

a few of steel tubing, either plain or

wood

filled. Various reasons

and

causeshave combined toeliminate these

methods

of construction.

For

instance, the _spar of

channel steel _proved

much

too flexible, although this

charac-teristic

was

no _{great disadvantage} in those _machines employ-ingwing-warping forlateral _control,forwiththis _arrangement

a certain

amount

of _flexibility in the _wing structure is essential.

While

steel tubing is excellent for

_many

details it can _{hardly be} said to be _{really suitable for wing spars,} which

are stressed _essentially as beams.

Now,

the _strength of a

beam

varies as _{the square} of the _depthof the _beam,

and

it is

obvious thatin the case ofacircular steel tube the material is

evenly distributed about the neutral _axis,

and

therefore its

strength in both horizontal and vertical directions is _equal;

although employed as a _strut, this feature becomes of real

value. One, however, still encounters its use on

modern

SPARS

AND

STRUTS

19

made

in_{construction generally} since 1914 has tended _greatly towards a reduction in the

number

of different

methods

em-ployed,

and

this will be realized from a consideration of the

accompanying spar sections which are in _{use to-day} on one

make

of

machine

or another.

Spar

Sections.

The

I section form of _{wing spar,}

shown by

_Fig. 7, is in general use, being spindled from the solid. It is compara-tively easy to _produce, which in a

measure

_explains its

FIG. 7. Solid spar.

popularity,

and

it _{also disposes} the material in _{probably the} best

manner

for the stresses involved.

The

laminated _spar, Fig. 8, is an

improvement

on the solid _{channelled spar}; it is

stronger, willwithstand distortion toa _{greater degree without} injury,

and

the strength is also

more

uniform thanwith the

solid spar.

An

additional point in its favour is that it is

much

easier to _{procure three pieces} of small section timber free from defects than one large piece,which, inview of the increasing scarcity of perfect timber, is

an

_important con-sideration. In order to minimize the riskof_{the glue} between

the laminations failing, the usual practiceis to _copper rivet or bolt the flange portion, while both spars are left solid atthe point of attachment of the interplane strut fittings and wire

anchorages.

The

spar

shown

byFig. 9 is of the hollowbox

variety, chiefly used for machinesof _{large wing} surface, where

FIG. 8. Laminatedspar.

weight reduction is _{an important} factor.

The

two halves of

channel section are _spindled from the solid and glued

to-FIG. 9. Hollow_box-spar.

gether.

The

joint is _{strengthened bythe provision} of small

fillets or _tongues of hard wood,

and

in

some

instances the complete spar is

bound

with glued fabric.

_Comparing

the

SPARS

AND

STRUTS

21 hollow _{spar with} the solid,

and

neglecting the cost factor,

the writer contends that the _advantage is _{indisputably with}

the former.

The

_tendency of the I-section _spar to buckle

laterally is of

much

lesser

moment

in _{a hollow spar} of the type

shown by

Fig. 9, while for a given weight it shows

an

increase in strength,

and

for_{equal strength} it is

much

_lighter.

A

differentversion of_{the hollow spar system} is that indicated

by

Fig. 10, consisting of two channelled _{sections, tongued} together at the joint, the sides being stiffened with _three-ply.

The

disposition of the joint in a vertical plane is a distinct

improvement

on the hollow spar previously considered,mainly

in that better resistance to_{a shearing} stress is afforded.

The

_{principle underlying} the construction of the _spar

FIG. 10. Hollow_sparwith stiffenedsides.

FIG. 11. Hollowsparwith multi-ply sides.

shown by

Fig. 11, is that in its manufacture _{the lengths} of

wood

necessary are of small section.

The

sides of this spar are built

_up

with a centre of _{spruce about-J} in. _{thick, to} each

side of which is _glued thin _three-ply, these _being glued,

screwed,

and

bradded to the _flanges.

The

_{wing spar}

shown

in section

_by

Fig. 12 is _unique in thatit_{really constitutes} two

spars placed closed together, the connection being formed by the top

and

bottom flanges of three-ply. This spar

was

used in a

machine

_{with planes} of small chord, but of _{very deep}

section,

and

in which no _{interplaiie wiring occurred,} the

wings functioning as cantilevers. Its chief _advantageis _great

rigidity fora low weight, but such a spar necessitatesa deep

wing

section,

and

is notin _general use.

FIG. 12. Twinboxspar.

Hollow

_Spar

Construction.

The

_advantages of _{the hollow type} of _spar

summarized

are _{(1) greater strength} for _{a givenweight}; (2) it can be

pro-duced from

wood

of small section, arid is therefore a better

manufacturing proposition.

On

the other hand, the strength

of _{a hollow spar} is _greatly

and

almost _{entirely dependent on} the glue used.

Now,

however well the _joint

_may

be _made, the _glue is _susceptible to a

_damp

_atmosphere, and if so affectedis of_{greatly reduced strength,}while _possible deprecia-tion in the _glue due to _{age renders the} life of _{the spar a}

problematic quantity.

Where

the various fittings occur it is

also _necessary to _{place blocks} before _{the spar} is _glued up,

which

is rather an unmechanicaljob.

The

practice of_forming

vertical sides of _{a hollow spar} from _three-ply is not to be

commended,

by reason of the doubtful character of _{the glue}

used in its manufacture. _However, in _spite of these dis-abilities, there is a future for _{hollow spar construction} in the

manufacture ofthe_big commercialmachines of the future, for

with these the question of

maximum

_strength for

minimum

weight, to _{permit the carrying}of the _{greatest possible} useful

load, will be a _primary consideration. This, of _course,

SPAKS

AND

STEUTS

23 Strut Sections.

In the construction of _{the interplane}

and

undercarriage

struts, one does not finda very decided preference for

any

one

particular method, although the interplane strut spindled

from the solid to a streamline section is

common

to

_many

types of

modern

aircraft.

The

strut

shown

in section by Fig. 13 is inuse for both _interplane

and

_{undercarriage} struts.

This consistsof _ordinary round section steel_tubing, to which

is attached atail _{piece or fairing}of wood, this _being

bound

to

FIG. 13. Steeltube strut with fairing

boundon.

FIGS. 14, 15. Inter-planestrutsspindled

fromthesolid.

the tube

_by

_{linen tape} or _{fabric, doped}

and

varnished. This strutis of _{practically equal strength} inboth lateral

and

longi-tudinal directions,

and

from this _point of view is _superior to the solid _spindled strut,

which

is _usually of _{great strength} in the fore

and

aft _direction, but _{always possesses a tendency} to buckle laterally. Fig. 14 indicates _{a hollow plane} strut, n

which the sides of _{spruce are spindled}

from

the solid,

and

glued to a central _{stiffening piece} of ash; while Fig. 15 is

arranged so that a _stiffening

web

is formed in _{the spindling} process.

Owing

to the rather extensive nature of the latter

operation, one does notfind

_many

instances of itsuse.

Where

the hollow

wood

struts _{used are not completely}

bound

with tape orfabric, they should at least be

bound

at intervals with tape or fine twine, as there is _always the _possibility of the gluedjoint failing under the combined attentions of rain and

heat.

A

_type of strut which is

now

_{being widely used} is that of

streamline section steel _tubing,

drawn

or rolled from the

round section. It is _employed for both the _inter-plane and

undercarriage struts, but for the latter has _{not given entirely}

FIG. 16. _{Inter-plane support}frombody. FIG. 17. Sectionof built-upstrut.

satisfactory results, owing to the _tendency to buckle under

extra _{heavylanding shocks.} This would be

more

_pronounced

with a tube of fine section than with one _possessing a bluff

contour; but in

any

case, a strut of parallel section, whatever

the material, is not well suited to withstand sudden shocks. This point is referred to later. _Seeing that _progress is _being

made

_{with the production} of a seamless streamline _tapered strutthis defect should soon _disappear.

In

some

machines _{the top plane} is _supported from the

fuselage

by

struts which are formed _integrally with a hori-zontal _compression

member,

as in _Fig. 16; the section of

SPARS

AND

STRUTS

25

cut to the _shape of the _{complete component,}

and

forms a tie

for _{the spruce layers,} whichare _{jointed at the junction}of the vertical

and

horizontal

members.

Strut Materials.

Referring again to the material _{generally employed} for

struts, i.e. silver spruce, it is _{perhaps necessary} to _explain further the reasons for its _predominance over ash, as on a strength-for-weight ratio the latter

wood

is _slightly the better material.

The

_{points already} detailed,indicate that

an

inter-plane strut is _{stressed essentially in compression,}

and

there-forethe chief characteristic ofash, great tensile strength, is of but secondary importance. Thereis alsothe fact that, for the

same

_{weight, spruce} would be thicker,

and

correspondingly

more

able to resist _collapse. However, in machines of the flying-boat class,

where

the engine is _invariably

mounted

between the four central _planestruts,

and

_consequently sub-jected to an

amount

of vibration varying with the type of

engine used, ash forms the material.

Tapering

of

_Interplane

Struts.

The

correct _shaping of struts _{longitudinally, particularly} those for_interplane use, is _{apparently a rather} controversial subject. Taking the case of _{an untapered} strut, it is evident that the _{greatest stress} will be located at or near thecentre, so that if at this _{point the} section is _{strong enough,} clearly there

must

be an

amount

of _{superfluous material} atthe ends.

By

suitably reducing or tapering the strut from the centre

one can obtain the

same

_degree of _strength for less _weight.

Conversely, for the

same

_weight a

much

_stronger strut is

possible. So it has always appeared to the writer. It is,

however, admittedly possible that unless carefully done, the operation of _tapering a strut

_may

_actually diminish the strength.

One method

of_{tapering, that}of

_making

the

maximum

cross-section at thecentre,

and

from this _{pointdiminishing} in a _{straight line} to _{the ends,} is _{undoubtedly open} to criticism,

and

a

_way

more

_{nearly approximating} to the correct

method

that thefinished contouris_{curvilinear, as in Fig.} 18. In this connection it is _pertinent to _emphasize the _importance of

ensuring that all strut ends are cut tothe correct bevels,and

thisis _{particularly applicable}tothosestrutswhich seat directly ina socket.

The

_{slightest irregularity} will cause considerable distortion

when

assembled under the tension of _{the bracing}

wires, and frequently the writer has seen anostensibly perfect

FIG. 18. _Taperingof_inter-planestruts.

strut

assume

the

most

_hopeless lines _{directly the operation}of

truing

up

is

commenced.

Design

of Strut Sections.

Although, strictly speaking, the design of strut shapes is

outside the _scope of this _book, a few remarks anent the

development of streamline

_may

_emphasize the advances made,

and

alsothe need for careful construction.

The

resistance of

a _bodyis _{generally considered} toincrease as _{the square}of the speed, i.e. double the speed

and

head resistance is doubled,

and

while this is true for a_{moderate range} of _speeds,

experi-ment

has _proved that for_high speeds,exceeding say 100 miles per hour, resistance increases atrather lessthan as _{the square}

of the _{speed. However,} itis certain that the correct _shaping or otherwiseof the struts

and

_{other exposed}

members,

affects

generally the performance in _flight of the _aeroplane.

The

accepted feature of all streamline forms is an _easy curve,

having a_fairly bluffentrance and _{gradually tapering} toa fine edge.

The

ratio of _length to diameter, called the fineness

ratio, varies in

modern

machines, being in

some

instances 3 to 1

and

in others 5 to 1, a good average being 4 to 1. Considering only the point of head resistance, it would be

better to choose a section of _high fineness ratio, but

con-structionally such a strut would buckle sideways under a

SPARS

AND

STRUTS

27

to resist this.

The

strut section used on the earliest aero-planes, such as the

Wright

biplane,

shown

by Fig. 19, is

FIG. 19. FIG. 20.

FIG.21. FIG. 22. FIGS. 19 22. Strutsections.

nothing

more

than a rectangle with the corners rounded off.

Fig. 20 shows adevelopment of _Fig. 19 _consistingof a semi-circular head with a _cone-shaped tail,

which

by

gradual

FIG. 23. Showinginefficiency ofpointedsection inasidewind.

evolution has resulted in the section _{Fig. 21.}

Some

experi-ments

carried out a considerable time _ago

_by

Lieut.-Col. Alec

CONSTRUCTION

Ogilvy, revealed the rather interesting point that a strut

shaped as in Fig. 22 _{gave the}

same

results as a similar strut

taken to afine _edge.

The

reasonsfor the _{non-suitability} ofa sharp-pointed section are _{apparent from} a consideration of

Fig. 23, showing the action of a side windwith the resultant

dead air _region.

Fuselage

Struts.

In _{the general}features of those struts associated with the construction of the _fuselage

and

_nacelle, there is _very little

FIG. 24. Channel-section fuselagestrut.

FIG. 25. Tsection fuselagestrut.

diversity of practice, the majority of constructors favouringa square spruce strut, _{Fig. 24,}channelled out for lightness.

A

defect with this _type of strut is _{the tendency, engendered by}

irregularities in the fittings

and

wiring, to buckle laterally,

although this can be obviated

_by

the provision of a strut of

larger section at the centre and diminishing in width tothe ends.

A

strut _{not nearly} so _{popular but nevertheless} in use

is that indicated _{by Fig. 25, consisting} of _{spruce spindled} to

a

T

section the

web

beingof considerable width at the centre,

It would

seem

that the _piece of

wood

necessary to obtain such

SPARS

AND

STRUTS

29 and from the _standpoint of

_economy

in both labour and

material is not _justified.

The

circular turned

and

_tapered strut noticeable ona

number

ofmachines_{disposes the material}

in _{probably the} best

manner

for the conditions _applicable to

this _{component, although} it necessitates the _provision of

tubular ferrules in the _{fuselage clip.}

On

one

modern machine

the_{fuselage struts are circular,} but of hollow section, built

_up

of two pieces glued together.

An

obsolescent

method

is that

in which the strut is _shaped to _{something approaching} a

streamline section, as the fact that all _{aeroplane bodies are}

CONSTRUCTION

CHAPTER

IV.

PLANE CONSTRUCTION.

OF

the various _components which _comprise the _complete

machine, the wings, aerofoils, or planes, as these items are

variously designated,

may

be said to contribute the greater part of the ultimate success of the _{complete machine.}

The

aerodynamicalproperties of a _wing are

now

_fairly well

deter-mined,

and

have been the _subject of a _great

number

of

experiments, resulting in the clearing

away

of

_many

_hazy

ideas

and

notions, so thatthe actual design ofthe

_wing

section

for machines of _givenpurpose is almost standardized.

From

this it _might be deduced that the methods of construction

were _equally well _determined,

and

_although absolute uni-formity of practice does not exist, the

wing

construction of

most machines is similar, as far as the

main

_assembly is

concerned.

Effects of Standardization.

Incidentally, one

may

point out the detrimental effects of

undue

standardization as _applied to an _industry in its

pre-liminary stages. 'These effectsare well exemplified by certain machines, in which standardization has been studied to an

almost meticulous extent, with the logical result that their

performance is _considerablyinferior to that of other machines

of _contemporarydesign, but in which desirable improvements

are incorporated as they occur. Although at _present one cannotgive actual figures, the averageperformance of

modern