Airplane Design and Construction

AIRPLANE

5M?

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&

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'AIRPLANE

DESIGN

AND

CONSTRUCTION

BY

OTTORINO

POMIL'IO

CONSULTING AERONAUTICAL ENGINEER FOR THE POMILIOBROTHERS CORPORATION

FIRST EDITION

McGRAW-HILL

BOOK

COMPANY,

INC.

239

WEST

39TH STREET.

NEW

YORK

LONDON:

HILL

PUBLISHING

CO., LTD. 6&8BOUVERIEST.,E.C.

MCGRAW-HILL BOOK _COMPANY, INC.

INTRODUCTION

By

far the

_major

_part of _experimental

work

in

aero-dynamics

has been conducted in

_Europe

rather than in

America, where the feat of _flying in a heavierthan air

ma-chine

was

first_{accomplished.} This

book

presentsin_greater

detail than has hitherto_{been attempted}in this _countrythe

application of _aerodynamic research conducted abroad to

practical airplane design.

The

airplaneindustryis

now

_shiftingfrom_{the design}

and

construction of _{military types} of craft to that of pleasure

and

commercial types.

The

publication of this

book

at

this time _{is, therefore, opportune,}

and

it should go far

toward

_replacing

_by

scientific _procedure

_many

of the "cut

and

_try"

methods

now

used.

_Employment

of the data

presented should enable designers to save both time

and

expense.

The

arrangement, presentationof_{subjectmatter,}

and

explanation of the derivation of _working formulae,

together with the assumptions

upon

which they are based,

and

_{consequently their limitations, are} such that the

book

lends itselftouseas atext in technical schools

and

colleges.

The

dedication of this

volume

to Wilbur

and

Orville

Wright

is at once _appropriate

and

significant; appropriate,

in that it is _{a tangible expression} of the keen _appreciation

of the author for _{the great}

work

of these

two

brothers;

and

significant, inthatit isareturn, intheformofarational analysis of

_many

ofthe_problemsrelating to airplanedesign

and

_operation, on the part of the productofanolder

civili-zation to the product of the new, as asort of _recompense

for the daring, courage

and

inventive genius which

made

human

flight possible.

J. S.

MACGREGOR.

NEW

YORK, 1919.

CONTENTS

PAQB

INTRODUCTION vii

PART

I

Structure of _{the Airplane}

CHAPTER

I. The Wings 1

II. TheControl Surfaces 19

III. The _Fuselage 37

IV. _{The Landing Gear} 44

V. The_Engine 51

VI. ThePropeller 72

PART

II

VII. Elementsof_Aerodynamics 87

VIII. The Glide 102

IX. _FlyingwithPowerOn 115

X. Stabilityand Maneuverability 134

XI. _Flyingin the Wind 151

PART

III

XII. ProblemsofEfficiency . 161

XIII. The _Speed . 167

XIV. The _Climbing 188

XV. GreatLoadsand LongFlights 204

PART

IV

Design of _{the Airplane}

XVI. Materials 221

XVII. Planningthe Project 261

XVIII. Static_Analysisof Main Planesand ControlSurfaces

....

276

XIX. Static _AnalysisofFuselage,Landing Gear andPropeller. . . 324

XX. Determination of_{the Flying Characteristics} 358

XXI. Sand Tests Weighing Flight Tests . . \ , . . 379

INDEX. . - 401

ACKNOWLEDGMENT

The

author desires to _express his sincere thanks to Mrs.

Lester

Morton

Savell forher valuable assistance in matters

pertaining to English

and

to

Mr.

Garibaldi_JosephPiccione

for his _{intelligent assistance} in _{drawing the diagrams.}

0. P.

AIRPLANE

DESIGN

AND

CONSTRUCTION

PART

I

STRUCTURE

OF

THE

AIRPLANE

CHAPTER

I

THE WINGS

While for _birds,

and

in _general for all animals of the _air, wings serve to insure both sustentation

and

propulsion,

those of _{the airplane are} used _{solely to provide the}

_means

of _{sustaining the}

machine

in the air.

The phenomenon

of sustentation is _{easily explained.}

A

body moving

through the air _{produces, because} of its

mo-tion, a disturbance of the _{atmosphere which} is

more

or less

pronounced and

complex in character. In the final

analy-sis, this disturbance is reduced to the formation of zones

of positive

and

negative pressures.

The

resultant of these

pressures

may

then be classified into its three

_components

:

1. Vertical or _sustaining force, calledLift,

2. Horizontal

_component

_parallel

and

_{opposite the} line

of _flight, called _Drag,

and

3. Horizontal

_component

_{perpendicular} to the line of

flight, calledLateralDrift.

The

vertical

_component

_may

be positive or negative.

An

_example of _{the negative}

component

is found in the

elevator used for the climbing

maneuver

of

an

airplane,

as will be

shown

later.

The

horizontal

_component

_parallel to the line of _flight,

is _{always negative;} i.e., it tends to retard the motion of

the body. "Conservation of _energy"1

is the principle

underlying this

_phenomenon.

The

horizontal

_component

_{perpendicular} to the line of flight is called the force of "drift," because it tends to

make

the

_body

drift

from

the line of _flight. This

compo-nent, generally not existing in normal flight, is of _great

importance in the directional

maneuvers

of airplanes.

For a

_body

_having a plane of

_symmetry

_{and moving}

through space so that the line _{of flight} is contained in that plane, the force of drift is zero

and

_{the only}

_components

acting are the lift

and

the drag.

Observations

made

of birds' _wings

and

results based

upon

the experiences of _{experimenters} in _aeronautics, have

demonstrated the _possibility of _devising surfaces of such

form that

_by

_properly

_moving

them

_through the air _they

create reactions, of whichthe vertical

component

has a far greater

magnitude

than the horizontal.

Thus, a surface _capable of _{developing high values} of lift

with small values of drag is called _{a wing.}

In actual _{practice, as}will be

shown

further on in a

more

detailed _studyof _{aerodynamicalprinciples (Chapter 7),} the

value of the ratio

_^

varies from 15 to 23. This

means

that _wings

_may

be _{built, which,} for _every 23 Ib. of load

carried, offer a resistance to motion of but 1 Ib. It is

natural, then, that designers direct all efforts

toward

in-creasingthe

g^

ratio,whichisusedto define the_efficiency

of _{the wing.} Three factors influence such _efficiency:

the profile of the wing section,

theratio of the _{wing span} to its _depth or chord _(called the _{Aspect Ratio),}

and

1

This principle statesthat energy can beneither created nor_destroyed.

If the horizontal _componentwere _{positive,perpetual motion} would ensue,

sinceitwouldbe_{necessary only}tofurnish theinitialforceto setthebodyin

motion. _{The body would}thencontinueinitspath withoutfurther

THE WINGS

3

the relative position of the _{wings (in multiplane}

ma-chines).

The

_{profile of}a

_wing

section is its _{major section at right}

angles to the span of _{the wing.} Because of the _simplicity

of

modern

construction, wings are _generally built with

Back

FIG. 1,

a constant section _throughout the _span. In the early

days of aeronautics, however,

many

types of wings were built with a variable

wing

section, but the aerodynamical advantages derived

from

their use were never sufficient to compensate for the _complicated construction required.

In the _profile of _{a wing, there} are the following distinct

elements (Fig. 1): leading edge, back,

bottom

and

trailing

edge.

The

proper use of these elements

makes

it _possible

to _{obtain the highest values}of the

j^ ratio, as well as to

vary the Lift'coefficient _accordingto the load tobe carried

per square foot of

_wing

surface.

Lineof

FIG. 2.

The

_angle between the

wing

chord

and

the line of flight,

called_{the angle} of incidence of the

wing

(Fig. 2),

may

vary between greater or smaller limits.

As

a result, the

distri-bution

and

value of the positive

and

negative pressures

will _vary,

and

_give differentvalues of Lift,

Drag and

_^

The

lawsofvariationofthesefactors arerather complicated

and

cannot be _expressed

_{by means}

of formulae. It is

4

AIRPLANE

illustrated in _Figs. 3

and

4. These illustrate the laws of

variation for the values of the Lift,

Drag

and

^

coeffi-cients for

two

typesof aerofoils,which, althoughhavingthe

same

lengths of chord, differ in other elements.

It is

now

_necessary to introduce a

new

_{factor, namely,}

the speed or velocity of translation of_{the wing.}

All aerodynamical

phenomena,

when

considered with

respect to speed, follow the general law that the intensity ofthe

_phenomenon

increasesnotin_proportionto the speed, but to the square of the speed. This is accounted for

_by

the fact that for _{redoubled speed not only} is the velocity

of _impact of air molecules against the

body moving

in the

air _redoubled, but so also is the

number

of molecules that

are struck

_by

the body. Consequently it is seen that the

intensity of the

phenomenon

is _quadrupled.

Assuming

a _{wing with}

an

area of

A

_square feet, the

fol-lowing general equations

may

be written:

L

=

X

X A X

V*\

D

=

d

X A

X

V

2

J

where

L

=

total Lift for area

A

in

_pounds

D

=

total

_Drag

for area

A

in

_pounds

V

_speed of translation in miles per hour

(m.p.h.).

In_practice it isconvenient to refer the coefficients X

and

5 to the velocity of 100 m.p.h.,

whence

the equation (1)

becomes 7 \2 1 If

A

.

=

1 sq. ft.,

and

V

=

100 m.p.h., then Li

=

X (3)

that is, X is the load in

pounds

carried

by

a

wing

with an

THE

WINGS

8 A. 1.7535 1.50.30 1.25.25 1.00.20 0.75.15 0.50:10 0.2505 25 22.5 17.5 15 12.5 10 7.5

-3-2-1

I

2S4-567&9

Degrees FIG. 3. -3 -2

2345678

Degrees*. Fia. 4.

6

AIRPLANE

and

5 the

head

resistance in

pounds

for a _{wing with an}

area of 1 _sq. ft.

and moving

at a velocity of 100 m.p.h.

Knowing

X

and

5,

by

usingequation (2) the valuesof

L

and

D

may

be found for

any

area or

_any

speed. Also, the

ratio - is _equal to _7. which is obtained

_by

dividing the

L

5

D

equation

by

the

D

_equation.

Now,

the coefficients X

and

8

may

assume

an entire

series of varied values

by

changing the angle of incidence

of the wings. Figs. 3

and

4

show

the laws of variation

of X, 5

and -

for

two

different types of _wings to which

we

will refer as

_wing

No. 1

and wing

No. 2.

An

examination of the _diagrams isinstructive because it

shows

how

it is _possible to build wings which

may

have

totally different values of Lift, the speed being the

same

for

both wings. For example, at an _angle of incidence of

3,

wing No. 1 _{gives X}

=

11.8,while wing No. 2 gives X

=

17.6;

in other words, with equal speeds, wing No. 2 carries

a load 49 _{per cent, greater} than wing No. 1.

The

laws ofvariationof X

and

5

_{depend upon}

the several

elementsof_{the wing, namely,}the leadingedge, top,

bottom

and

trailing edge. Letus consider separately the function

of each of these elements:

Actually, the function ofthe leading edgeis to_penetrate

the air

and

to deviate it into

two

_streams, one which will

pass along the top

and

the other which will pass along the

bottom

of _{the wing.} In order to obtain a _good efficiency

it is _{necessary that} this _penetration be

made

with as little

disturbanceas_{possible, in} order to_preventeddies. Eddies

give rise to considerable

head

resistance

and

are therefore

great consumers of energy. For that reason, the leading

edge should be designed with the

same

criterions as those

adopted in _{the design} of turbine blades. _Figs. 5

and

6

show

the

_phenomenon

schematically.

Due

to inertia,

THE

WINGS

7

rectilinear _path, thus _producing a _{negative pressure} or

vacuum

on

_top of_{the wing.} This _{negative pressure}exerts a centripetal force

on

the air_{molecules, tending} to deflect

FIG. 5. Loadingedgeof_goodefficiency.

their _path

downward

so as to _{flow along the top curvature}

ofthe wing.

A

dynamic

equilibrium is_therebyestablished

between

_{the negative pressure}

and

the centrifugal force of

FIG. G. Leadingedgeof_poorefficiency.

the various molecules (Fig. 7). It is obvious, then, that

the topcurvature hasa_pronouncedinfluence_{not only}

upon

theintensity ofthe

vacuum,

but also

on

thelaw of_negative

pressure distribution along its _{entire length.}

POSITIVE PRESSURE.

FIG. 7.

The

air deviated below the wing tends instead, also due

to inertia, to condense, thus producing a positive pressure

the

bottom

curvature. Becauseof this_changeinthe direc-tion of velocities, a _{centrifugal force} is _{developed which} is

in

_dynamic

_equilibriumwiththe_{positivepressureproduced}

(Fig. 7).

Curves showing the laws of distribution of the positive

and

_{negative pressures} are _{given in Fig.} 8.

The

resultant

FIG. 8.

of these pressuresrepresents the value -T-- It will be noted

that _{the portion} of the sustentation due to the

vacuum

aboveis

much

_greaterthanthat

due

tothe_{positivepressure}

below. In the case under consideration, it is 2.9 times

greater,

and

equal to_{74 per} cent, ofthe total Lift.

There-fore, the study of _{the top curvature}

must

be given

more

careful consideration than that ofthe

bottom

_curvature, as

a wing is not at all defined

_by

the

bottom

curvature alone.

THE

WINGS

9 toincrease both_{the convexity} of_{the top}

and

_{the concavity}

of the

bottom

of _{the wing, thereby increasing} the

intensi-ties of _{the negative}

and

_{positive pressures.}

The

_{trailing edge} also has its _{bearing on the}

efficiency. Its _shape

must

be such as to _{straighten out the} air

stream-liness

when

the air leaves _{the wing, affecting} a _smooth,

gradual decrease in the _negative

and

_{positive pressures}

FIG. 9. Trailingedgeofgoodefficiency.

until their difference

becomes

zero. In this _manner, the formation of a

wake

or eddies behind _{the wing,} with the resulting losses of energy, is avoided _(Figs. 9

and

10).

In_brief,for

_good

_wing

efficiency,it is_{primarily necessary}

for_{the leading}

and

_{trailingedges}tobeof_{a design}whichwill

avoid the formation of eddies,

and

in order to obtain a

higher value of the Lift coefficient X _{the top} and'

bottom

curvatures

must

be increased.

FIG. 10. Trailingedgeofpoor efficiency.

From

the foregoing it is _easy to understand the

impor-Of

tance oftheratio

^; that is, the relation between the span

S

and

the chord

C

of a wing.

Considering thefrontviewofa_wingsurface,Fig. 1 1,which

represents a section parallel to the leadingedge,

and

shows

the

mean

negative

and

positive pressure curves for the top

AIRPLANE

the central _{part the curves are represented}

_by

lines _parallel

to _{the wing,} at the _wing tips

A

and

B, theysuffer serious

disruption,forattheendofthewing a shortcircuitbetween the compression

and

depression occurs. This is

due

to the

air under _{pressure rushing} toward the

vacuum

zone, thus establishing an airflux (the so-calledmarginallosses), with

the result thatat the

_wing

tips the average pressure curves

come

together,

and

the Lift is decreased considerably,

thus lowering the value X of _{the wing.} It is _necessary to

Negative Pressure.

Positive Pres&ure.

fir c"~~

FIG. 11.

reduce the _importance of this

_phenomenon

to a

_minimum,

which is done

_by

increasing the ratio of the _span to the

(Cf\r*)

W

Assume, as it is sometimes done in _practice, that the

disruption in the average curves due to _marginal losses

extends for a distance

AC

and

_BD,

_equal to the chord of

the wing;

and

also that the _diagram is _{modified according}

toalinearlaw. Thisis_equivalentto_assuminga decreasein

the Lift measured

_by

the triangles AA'C',

AA"C",

BB'D'

and

BB"

D".

The

same

result is obtained as _though the

averageXremainedconstant

and

theliftingsurface were

re-duced

_by

the

amount

c2, which

means

that thetotal surface

would

be reduced

_by

sXc

c2

. If the product

sXc

is _kept

constant

_by

increasing s

and

_diminishingc_{correspondingly,}

the_importanceofthetermcis_greatlydecreased.

The

lossis

expressed

by

-

=

-> that_is,

_by

the inverse of the ratio

THE

WINGS

11

span

chord So it is seen that

by

increasing the ratio c> the

average value of the

coeffi-cient of Lift is _increased,

and

it is therefore _advantageous

tobuild _{wingsof large spread.}

In_practice, however, there is

a limit

beyond

which this

ad-vantage

becomes

a

minimum,

and

there are also static

and

structural_problemstobe con-sidered which limit the value

of the ratio

c In

modern

FIG. 12.

machines, this value varies

from

5 to 12,

and

even more.

In _{biplanes, triplanes}

and

multiplanes,another very

im-portant problem is_presented;

that of the

mutual

interference of each _plane

_upon

the

others. Inviewoftheclose

arrange-ment

of the surfaces necessitated

_by

structural considerations,

and

the

high values of their _negative

and

positive pressures of air, a

conflic-tion of air flow is formed over the

entire _wing surface, with the result

that the value of the Lift coefficient

is lowered. Figs. 12

and

13

illus-trate this

_phenomenon

for _{a biplane}

and

triplane respectively. In the case of the biplane, the following

effects ensue:

1. Decrease in

vacuum

on top of

lower plane,

and

2. Decrease in _positive pressures

on bottom

of _upper plane.

FIG. 13. Triplane system. _j

_n

_{^he cage Qf}

^

_e

triplane, the losses arestill_greater, dueto

CONSTRUCTION

1. Decrease in

vacuum

on

_top of

bottom

plane,

2. Decrease in _{positive pressures}

on

bottom

of inter-mediate plane,

3. Decrease in

vacuum

on_top ofintermediateplane,

and

4. Decrease in _{positive pressures}

on bottom

of _upper plane.

It is thus seen

how

undesirable,

from an

aerodynamical

point of _{view, the triplane really} is.

At

the present _time,

however, the triplane is not a

common

_{type of airplane, so}

the discussion here will be limited to the _biplane.

Another important ratio in aeronautics is the unit load

on _{the wings,} or the

number

of

_pounds

carried _{per square}

foot of

_wing

surface. _{Theoretically} this value

_may

_vary between wide limits; for example, for

_wing

No. 2 set at

an

angle of 6

and moving

at _{a speed} of150 miles

an

_{hour, the}

ratio is 51 Ib. _{per sq.} ft. In_{practice, however,} that value

has never been reached. Special racing airplanes have

been built

whose

unit loads were as _high as 13 Ib. _per sq. ft., but the principal disadvantages of such _{high unit}

loads are the resulting highgliding

and

landingspeeds,

and

an _appreciable loss in _{maneuverability.}

For

this reason

designers strive to confine the unit load between the limits of 6

and

8 Ib. _{per sq.} ft.

Consider _{a biplane} with a chord

and

_{gap each} of 6 ft.

with aunit_{load equal} to 8Ib. _{per sq.} ft.

_Keeping

in

mind

what

has been _{previously stated (Fig. 8),} it can be

as-sumed

that the values _{of positive}

and

_{negative pressures} (vacuum) found at _{the top}

and bottom

of_{both wings}

would

be_equalto2Ib. _{per sq.} ft.

and

6 Ib. _persq.ft. _{respectively,}

provided, of course,that the

_{two wing}

surfaces

had no

effect

on each other.

_Now,

if a difference in _pressure of 8 Ib.

per sq. ft. is _{produced between}

two

_points in the air at a

distance of 6 ft.

from

each _other, the air under _pressure

rushingviolently to fill

_up

the

vacuum

will result in a

veri-table _cyclonein _{the intervening space.}

When

a

_wing

is in _motion, condensed

and

rarefied

THE

WINGS

13 8 /t 1.7535 1.50 30 1.25 25 1.0020 0.75 15 0.50 10 0.25 5 25 22.5 20 17.5 15 12,5 i Z 3 4

5-6

7 & . I

23456789

0.75 15 0.50 10 0.25 5 75

-3

-2 -I

AIRPLANE

a certain

_dynamic

_equilibrium ensues. In order to _study the

_phenomenon

more

closely, a fewbriefcomputationswill

be made.

Again consider the _typeof

_wing

curve

whose

characteris-tics are _given in _{Fig. 3,}

and assume

that it isto _{be adopted}

for a _biplane. In such a case, the curves in Fig. 3 are

no

longer applicable

and

new

curves

must

be determined

experimentally, since the aerodynamical behavior of the

wing

shown

in Fig. 3 will _change for_{every one} of the three

following conditions:

1. _{Acting alone, as} for a _monoplane,

2. _Serving as the_upperplane of _{a biplane structure,}

and

3. _Servingas the _{lower plane} of _{a biplane} structure.

Fig. 14gives the characteristics for wing No. 1 _serving as a

lower _plane. Considered as _{an upper plane, the}

aerody-namical curve is _practically the

same

as that in _Fig. 3.

Fig. 15 gives the characteristics of a _{complete biplane}

whose

_upper

and

_{lower planes} are similar.

Compare now

a

_monoplane

_having a_wing surface of 200

sq. ft., possessing the type of

_wing

mentioned above, with

a biplane also _having the

same

_{wing section,}

and whose

planes are each 100 _sq. ft. in area.

Assume

each

machine

to_{carry a load} of 1500 Ib. at _{a speed} of _{100 miles per hour.}

The

_{problem then}isto find the values of the angles of

inci-dence

and

the thrust efforts_requiredto

overcome

the_Drag.

From

the equation

Since

L

== ₁₅₀₀ Ib.

and

A

--= ₂₀₀ sq. ft., then 1500 _ _r 200 :

which value of X _gives, for the

_monoplane

_(Fig. 3),

i

=

1

d

=

0.415

THE

WINGS

15

and

for _{the biplane (Fig. 17),}

i

=

1 45'

d

=

0.450

D

=

_0.450

_X

₂₀₀:

-

90 Ib.

In the case of_{the biplane}

^

isseentobe _{12 per}cent,smaller than in the case of the _monoplane.

The

thrust _required is

8 per cent, greater, therefore 8 per cent,

more

H.P. is

re-quired to

move

the

_wing

surfaces of this _biplane than that

necessaryto

move

asimilar

_wing

inthe

_monoplane

structure.

However,

the final deduction

must

not be

made

that a

bi-plane requires 8 per cent,

_{more power}

than the

_monoplane

of _equal area.

The

_power

absorbed

_by

the _{wing system} is

really onlyabout 25 percent, of the total H.P. _required

_by

the _machine, so that the total loss due to the

_employment

of _{a biplane structure} is _{8 per} cent, of 25 per cent., or 2

per cent.

Of

_{late, the biplane} structure has almost _entirely

sup-planted that of the _{monoplane, due largely to} the _great

superiority,

from

a structural _point of _view, offered

by

a cellular structure over a linear type. For lifting

surfacesof_{equalareas,the biplane takes}

_up

much

less_ground

space

and

is

much

_lighterthanthe _{monoplane. Regarding}

the former, the

r~~g ratio being the same, the span of

the biplane is _only 0.71 _{that required}

_by

the _monoplane.

As

to _weight, it is to be noted that a

_wing

structure

usually consists of

two

or

more

main beams

called _wing

spars, runningparallel tothe span.

Wing

ribs, constructed

to

form

the outline of the

_wing

section, are fitted to the spars.

The

junction of the _wings to the

_body

or fuselage

of a

machine

is

made

_{by means}

of the spars, which are

the

main

_{stress-resisting}

members

of the wing.

The

spars of

_monoplane

_wings are fixed or hinged to the

fuselage

and

braced

by

steel_{cable rigging} (Fig. 16). In the

AIRPLANE DESIGN

lower planes are _{held together}

_by

struts

and

cross bracing,

forming atruss (Fig. 17).

For

those familiar with the principles of structures it is

easy to see the great superiorityof the biplane structure

over the

_monoplane

structure in stiffness

and

lightness,

and

the impossibility of

monoplane

structure in _large

machines because of its excessive _weight.

FIG. 16.

Wing

structure is

_{becoming more and more}

uniform for

all_{types of airplanes.}

As

_{already pointed out,} the frame

consists of

two

or

more

spars

on which

the ribs are fitted (Fig. 18).

A

leadingedge

made

of

wood

connects thefront extremities of the ribs, while for the trailing edge a steel

wire or

wood

stripisused.

The

_spars are also _{held together}

by wooden

or steeltube struts

and

steelwire _{cross bracing,}

the function of

which

is to stiffen the

_wing

_{horizontally.}

The

ribis _usuallybuilt

_up

withathinveneer_web, towhich

strengthening flanges are glued

and

nailed or screwed

(Fig. 19).

The

spars are _usuallyof

an

I,[ or

box

section for

lightness (Fig. 20).

The

vertical struts between the _upper

and

lower _wings

THE

WINGS

₁₇

either case, they

must

have a streamline section to reduce toa

minimum

their

head

resistance.

Wood

_{struts are often} hollowed to obtain _lightness.

_Many

_different

systems of

AngleStrut:'

Box.Section ">

EndRib.

End_Fitting-For

ConnectingSpar,, _>g totheFuselage:''"" trIntermediate I"SectionRib... Trailing Edge. Inferior Wing Trussing Strut

Rear_Spar,

FIG. 18.

Interior SteelWireCross Bracing.

SECTION A-B

(ENLARGED)

FIG. 19.

FIG. 20.

attaching the struts

and

cables to the spars are used,

and

some

of the

_many

possible

methods

are

shown

in Fig. 22.

The

_wing

skeleton is covered with linen fabric, attached

AIRPLANE

leading

and

trailingedges. It is then _given

an

application

of special varnish, called "dope,"

which

stretches it

and

makes

it _{air tight.}

The

surface isthenfinishedwith _bright

SECTION

A-B(ENLARGED)

FIG. 21.

FIG. 22.

waterproof varnish,

which

leaves the fabric

smooth

so as

to reduce frictional losses to a

_minimum,

thereby

CHAPTER

II

THE CONTROL

SURFACES

In _{studying the} directional

maneuvers

of

an

_airplane,

reference

must

be

made

toitscenterof_{gravity (C.G.)}

and

to

its three principal axes.

Two

of the axes are contained in

the plane of

_symmetry

of the

machine

while the third is

normal to this plane.

One

of the

two

axes in _{the plane} is

parallel tothe line _{of flight} while the otheris _{perpendicular}

to it.

By

a

known

_{principle of mechanics, every}rotationofthe

machine

about its C.G.

_may

be considered as the resultant

of three distinct _rotations, one about each of the three

principal axes.

On

the other _hand, if three _systems of

control are used, each capable of _producing a rotation of

the airplane about one of its _{principal axes,}

_any

rotation

of the

machine

about its C.G. can _{be brought about} or

prevented.

The

_{principal axis perpendicular} to _{the plane} of

sym-metry, is called _{the pitching} axis. Rotations about that

axis are called _pitching

movements.

The

devices used to

bring about, or prevent a pitching

movement

are called

devices of longitudinal stability.

The

axis _{perpendicular} to the line of _flight, in _{the plane}

of

_symmetry

is called the axis of direction of _flight.

The

devices

which

cause or _prevent

movements

about that axis

are called devices of directional stability.

The

axis parallel to the line of flight is called the rolling axis,

and

the devices causing or preventing rolling

move-ments

are called devices of lateral stability.

Thereare _usually

two

surfaces

which

control longitudinal

stability, one fixed, called the stabilizer or tail plane,

and

the other _movable, called the elevator.

<The

stabilizer or tail _plane isa relatively small surface fixed at the rear end of the fuselage. Its function is, first

20

AIRPLANE

CONSTRUCTION

of all, to offset or even completely invert the

phenomenon

of the inherent instability of curved wings,

and

secondly,

to act as a

_{damper on}

longitudinal or pitching

movements.

The

stabilizer

_may

be of various shapes

and

sections. It

_may

be either lifting or non-lifting, but it

must

_always

satisfy the basic condition that its unit _{loading per} sq. ft.

be lower than that of the principal

wing

surface.

Under

this condition only, will it act as a stabilizer; otherwise it

would add

to the instability of _{the wings.}

As

to_{the proper}dimensionsofthestabilizer, they

depend

on

various factors such as the weight of the _airplane, its

longitudinal

moment

or inertia, its _speed,

and

the distance the stabilizer is set

from

the center of _gravity of the machine. _{Moreover, the proportions} ofthe stabilizerwith respect tothe other partsofthe airplane are also dependent

on

another factor: the type of _airplane. For small, swift

combat

machines whichrequirea high degreeof

maneuvera-bility, the stabilizer will require relatively less surface

than that requiredfor large, heavily loaded machines, such

as thoseused for

_bombing

_operations

and

requiring a

much

lower degree of maneuverability.

The

framework

or skeleton of the stabilizer is _generally

of

wood

or steel _tubing. In _general its _angle of incidence

may

be _adjusted either on the _ground or while in _flight.

However, that incidence

must

never be _greater than the

angle used for the

main wing

surfaces. Its value is

gen-erally 1 to 4 less than thatof _{the wings.}

'C

The

_{elevator or}

movable

_{surface is}

hinged to the rear edge of the _stabilizer,

and

it

_may

be raised or lowered while in

flight.

In normal flight the elevator is set _parallel to the air

flow so that there is no air reaction

on

its faces. If it is

swung upward

or

downward

the air will strikeit, producing

a reaction

whose

direction is

_upward

or

downward

respec-tively, thus tending to set the

machine

for _climbing or descending.

The

size of the elevator also _depends on the _weight,

dis-THE

CONTROL SURFACES

21

tance

from

the center of _gravity of the machine; alsothe

typeofairplane

and

theserviceforwhichit isintended

must

be givenconsideration.

_However,

for_quick

and

_responsive

machines the elevator

must

be _{proportionally larger} than

FIG. 23.

for slow machines

endowed

witha _{greater degree} of

stabil-ity. In other words, the

two

proportions vary inversely

as those of the stabilizers. However, this will be

22

AIRPLANE

the

two

devices which are in a certain sense^ completely

opposite.

The

function of the stabilizer is to insure longitudinal

stability, just as its

name

_implies.

The

elevators function instead, is to disturb the equilibrium of the

machine

in

order to _bring about a _change in the

normal

flying.

An

outline of a type of stabilizer

and

elevator _system is

given in Fig. 23.

A

closer study

may

now

be

made

ofthe function ofthese

two

parts of longitudinal stability. First of all,

examina-tionwill be

made

ofthe

mechanism by which

thestabilizer,

when

_properly set, exercises its stabilizing property.

When,

inanairplane,the incidenceofthe

wing

is

_changed

withrespect to the air, through which it isprogressing, the

airreaction will _{not onlyvary} in intensity but also in

loca-tion. If the

new

reaction is such as to _antagonize the deviation, the airplane is said to be stable; otherwise it is

said to be unstable.

Wings

having curved profiles,

when

acting alone, are

un-stable. Laboratory experiments

have shown

that for a wing with a curved profile, the reaction

moves

forward as

the incidenceis _increased,

and

_{vice versa;} thus the reaction

moves

insuch a

_way

as to_aggravatethe disturbance.

The

pointofintersection of theairreaction

on

the

wing

chord is

called the center of _pressure of the

wing

(Fig. 24).

The

location of the center of thrust is _{usually indicated}

_by

the

/p />

ratio

The

curves for X

and

for - as functions of the

c c

angle of incidence for a given

wing

section, are

shown

in

Fig.25.

By

applying thedata

from

thesecurvestoa

_wing

of

5 ft. chord

and

40 ft. _{span, supposing} the normal speed to

be _{100 m.p.h.}

and

the normal _{angle of flight}

_2,

the

_wing

loading will be

L

=

7.3

X

200

=

1460 Ib.

and

it will be in _equilibrium if the center of _gravity of the

load falls at a distance of 40 _per cent, of _{the chord,} or 2

inci-THE CONTROL

SURFACES

23 dence is increased from 2 to

_4,

then the _{sustaining force}

becomes

L

=

₁₀

_X

₂₀₀

-

_{2000 Ib.} Center ofPressure. C - ' FIG. 24. TV, 17.5 15.0 12.5 10.0 7.5 5.0 2.5

X

"c" 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15

-3-2-1

01

23456769

Degrees. . FIG. 25.

and

it will be applied at 37 _per cent, of the chord, or 1.85

ft.

from

_{the leading edge;} this result will then produce

around the center _{of gravity,} a

moment

of

24

AIRPLANE

and

such

moment

will tend to

make

the

machine

nose _up;

that is, it will tend to further increase the angle of

inci-denceofthe wing. Followingthe

same

lineofreasoningfor

acase of decreasein the angleof incidence, it will be found

inthat case that a

moment

is_{originated tending}to

make

the

machine

nose down. _Therefore, the

_wing

in _question is

unstable.

A

practical case will

now

be considered,

where

a

stabil-izer is set behind this _wing,

and

constituted of a surface of 15 sq. ft. (2

X

7.5) set in such a

manner

as to present an

angle of 2 with the line of flight

when

the

wing

in front

presents

an

angle of

+

2.

In normal flight there is

1.

The

_sustainingforce of the

main

wing, equal to

L

s

=

7.3

X

200

=

1460 Ib.

2.

The

center of _pressure of the

main

wing

located at

0.40

X

5

=

2 ft.

from

the leading _edge,

3.

The

sustaining force of _{the elevator equal} to

L

a

=

2

X

15

=

30 Ib.,

and

4.

The

center of _pressure of the elevator located at

0.44

X

2'

=

0.88 ft. from its _{leading edge.}

Suppose

now

that the incidence of the

machine

is

in-creased so that the angle of incidence of the front

_wing

changes

from

+2

to

_+5,

then there is

1.

The

_sustainingforce of the

main wing

_equal to

L.

=

11.30

X

200

=

2260 Ib.

2.

The

center of _pressure of the

_{main wing}

located at

0.355

X

5'

=

1.78 _ft.,

3.

The

sustaining force of _{the elevator equal} to

L

s

=

6.05

X

15

=

91 Ib.,

and

4.

The

center of _pressure of the elevator located at

0.410

X

2'

=

0.82 ft. from its _{leading edge.}

With

thesevalues, thetotalresultant ofthe forces_acting

in each case is _obtained,

and

it is found that while in

nor-mal

flight, the

moment

of total resultant about the e.g. of

THE

CONTROL SURFACES

25 to

_5,

that

moment

becomes

_equal to2351

X

_{(2.71/ 2.44')}

=

₆₄₅ ft. Ib. _tending to

make

the

machine

nose

_down;

that is, tends to prevent the deviation

and

therefore is a

stabilizing

moment

(Fig. 26).

In_analogous

manner

itcanbe

shown

thatifthe incidence

of the

machine

is _{decreased, a}

moment

_tending to _prevent

V0.6Ft

FIG. 26.

that deviation is _developed. It is _obvious, then, that if

the airplane were provided with only a stabilizer

and

with

no

_elevator, it

would

_fly at only one certain angle of

inci-dence, since

any

change in this_angle

would

develop a

stabil-izing

moment

tending to restore the

machine

toits _original

angle.

Thus

the exact function of the elevator is to pro-duce

moments

which will balance the stabilizing

moments

26

AIRPLANE

CONSTRUCTION

due

tothestabilizer. Thiswillallowthe

machine

to

assume

a complete series of angles of _{incidence, enabling} it to

maneuver

for climbing or _descending.

There are also _usually

two

_parts controlling directional

stability;onefixedsurface calledthefinor verticalstabilizer,

and

one

movable

surface called the rudder.

Consider, for example, an airplanein

normal

flight; that

is, with its line of flight coincident with the rolling axis

FIG. 27.

(Fig. 27). In this case there is no force of _drift, but if

for

some

reason the line of flight is no _{longer coincident}

with the _{rolling axis,} a force of drift is _{developed (Fig. 28),}

whose

_point of _application is called center of drift. If

this center is found to lie behind the center of _gravity, the

machine

tends to set itself _against the wind; that is, it

becomes

endowed

with directional _{stability. If, instead,}

the center of drift should fall before the center of gravity,

THE

CONTROL SURFACES

27

turn sharply about at the least deviation from its _normal

course. In _{practice, since} the center of _gravity of an

air-plane is found very close to the front end of the _machine,

the condition of directional _stability is _{easily attained}

_by

the use of a small vertical surface of drift which is set

at the extreme rear of the _fuselage. This surface is called the fin or vertical stabilizer.

There is, however, a type of airplane called the

Canard

FIG. 28.

type, in

which

the

main

wing surfaceisthe one in therear,

(and consequently the _e.g. falls entirely in the rear)

and

in which the _problem of directional stability presents

considerable difficulty. This typeof _airplane, however, is

notused at _{the present} time.

A

machine

provided with only a fin

would

possess good

directional stability, but for that very reason it would be

impossible for the _{airplane to change} its course. For that

28

AIRPLANE

surface, which,

when

properly deviated, will _produce a

balancing

moment

to

overcome

the stabilizing

moment

of

the fin, thus permitting a change in the courseofthedrift.

The phenomenon

may

now

'be studied

more

in detail.

Let us _suppose that the directing rudder is deviated at

an

angle; this deviation will _{then provoke}

on

the rudder a

reaction D' (Fig. 29), which will

have

about the center of

gravity a

moment

D'Xd';

as a result, the airplane will

FIG. 29.

rotate about the axis of direction

and

the line of _{flight will}

no

_longer coincide with the rolling axis; that is,

when

the

airplane starts to drift in its _course, a _{drifting force}

D"

is

originated, whichtends to stabilize,

and

when

D"

X

d"

=

D'

X

d',equilibriumwillbeobtained. _Obviously, then,the

lineof_{flight will}no_longerberectilinear,sincethe

two

forces

D"

and

D' are_unequal,

and

if _transported to the center of

gravity they will _give aresultant

D

=

D"

D

f

other than

THE

CONTROL SURFACES

29

flight

becomes

curvilinear; in fact, a centrifugal force

$

is

then developed which will be in _{equilibrium with the}

re-sultant force ofdriftD.

Then

_equilibrium will be obtained

when

<J>

'

=

Z); as

where

W

istheweightofthe _{airplane, g}theaccelerationdue to gravity,

V

the velocity of the airplane

and

r the radius

of curvature of the line of _flight, therefore

from whichis obtained

.E

'

_X

Z!__E

_X

_!!_

g 'v

D

' g

A

D"

-

D'

From

this_equationitwillbe seen thattoobtainremarkable

maneuverability in turning, the difference

D"

D'

must

have

a_{large value. Or, since}

_

D'

"

d"

it is _{necessary that the center} of_{drift, althoughbeing} inthe

rear of the center _{of gravity,}

must

be not toofar behind it,

and

it is_{necessary that the} rudder belocatedat a

consider-able distance

from

the center of _{gravity. In other words,}

for

_good

_{maneuverability,} an excessive directional _stability

must

not exist.

The

_{foregoing applies to}

what

is called a

flat turn without _banking, which is _analogous to that of a

ship.

The

airplane, however, offers the greatadvantageof

being able to incline itself_laterallywhich greatlyfacilitates turning, as will be

shown

when

reference is

made

to the devices for transversal _stability.

In summarizing the foregoing, it is seen that in addition to the _{fixed surfaces,} stabilizer

and

fin,

whose

functions are

to _{insure longitudinal}

and

directional stability, airplanes

are _{provided with}

movable

_{surfaces, elevator}

and

rudder,

which are intended to _produce

moments

to _oppose the

30

AIRPLANE

better understood that excessive _stability is _contrary to

good maneuverability.

In like _manner, for transversal stability, there are

two

classes of devices opposite in their functions.

Some

are used to insure _stability while others serve to _produce

moments

capable of neutralizing the stabilizing

moments.

Letus consider an airplanein normal flight,

and

suppose

that a gust of

wind

causes the

machine

to

become

inclined

laterally

by

anangle a.

The

_weight

W

and

the airreaction

L

will

have

a resultant

D

n which will tend to

make

the

machine

drift (Fig. 30);thisdrifting

movement

willproduce

a lateral air reaction

D

_{a acting} in the direction _opposite to

D

a.

The

resultant of the lateral

wind

forces _acting

on

the

machine

is

D

_a. If this reaction is such as to

make

with the force

D

a a couple tending to restore the

machine

to its _{original position,} the

machine

is said to be

transver-sally stable; this is the case

shown

in _Fig. 30. If

D

_ahas

the

same

axis as

D

_a, the airplane is said to

have an

indif-ferent transversal stability. If, finally,

D

a

and

D

a form

a couple tendingto_{aggravate the}inclination ofthe_machine,

the latter is said to be transversally unstable.

Consequently, in order to

have an

_{airplane laterally}

stable, conditions

must

be such that the lateral reaction

D

a together with the force

D

_a form a _{stabilizing couple;}

THE

CONTROL SURFACES

31 situatedabove_{the point} ofapplication of force

D

_a, whichis

the center of gravity.

However,

the _couple of lateral

stability

must

not

have an

excessive value, as it

would

decrease the maneuverabilitytosuchan extent as to

make

the

machine

_dangerousto_handle, as will

now

be explained.

It has been explained before

how

a turning action

_may

be obtained

_by

_{merely narrowing} the _rudder,

and

how

FIG. 31.

this cannot be actually

done

in practice since there is a possibility of the

machine

banking while turning.

Now,

when

_{the airplane} "banks," the forces

L

and

W

will admit a lateral resultant

D

a which tends to deviatelaterally the line of flight.

A

centrifugal force

$

is thereby developed,

tending to balance the force

D

a

and

equilibrium willobtain

AIRPLANE

where r is the radius of curvature of the line of flight;

therefore

W

V

2

M?-

X

T

which will _give

r _. w_

x

Z!

g

Da

As

Z)_a

=

IF tan a,

we

obtain 1

F

2 r

=

-tan a

This _equation shows that the turn can be so

much

_sharper

as the _speed is _decreased,

and

_{the angle a} of the

bank

is

increased. This _explains

_why

_pilots desiring to turn sharply,

make

asteep

bank and

at the

same

time nose the

machine

_upward

in order _{to lose speed.}

Now

the angle of

bank

_may

beobtained in

_{two ways}

;

by

operating the rudder or

_by

_{using the} ailerons

which

are

the controls for lateral _stability. In _{using the rudder,}

ithas been observed that the

machine

assumes

an

_angle of

drift. If the force of drift

D

= D"

-

D'

(Fig. 29) passes through the center _{of gravity,} a flat turn _{without banking}

will result. If force

D

_passes below the center of gravity, the airplane will incline itself so as to _produce a resultant

D

a of

L

and

W,

in a direction opposite to force D.

Then

the total force of drift is _equal to

D

D

a. This case is

of

no

_{practical interest,} since it _corresponds to the case of _{lateral instability,} which is to be avoided. If, instead, force

D

_passes above the center _{of gravity,} then _{the angle}

of

bank

a is such that

D

_a is of the

same

direction as D. Therefore, the total force of drift is

D + D

_a.

Now

if force

D

_a

had

its_pointof_{application too}farabove the _{center of gravity,} the result

would

be that witha slight

movement

of the _rudder, a _{strong overturning}

moment

would

_develop which

would

give the

machine

a dangerous

angle of bank. Therefore it is evident that an excessive

THE

CONTROL SURFACES

33

The

ailerons are

two

small

movable

surfaces located at the

wing

ends (Fig. 32). Letus

now

observe

what

_happens

when

they are _operated.

The

ailerons are _{hinged along the axes}

A

A

and

BB'

',

and

are controlled in such a

manner

that

when

one swings

upward

the other swings

downward.

With

this inverse

movement,

the equity of _{the sustaining} force on both the

FIG. 32.

right

and

left _wings, is broken.

Thus

a couple is _brought

into_playwhichtendstorotate the

machine

abouttherolling axis. Since it is _{possible to} operate the ailerons in either direction, the pilot can

bank

his

machine

to the right or

to theleft.

Supposingthat the pilotoperates the ailerons so that the

machine banks

to the _right; let a be the angle of _bank;

then, a force

D

a is _{produced, which,} in a laterally stable

34

AIRPLANE

by

the ailerons.

The

_rapidityofturning,

and

consequently

the mobilityof the machine, will increase in _proportion as

the rapidity ofthe _banking

movement

increases.

Now,

all

other conditions being similar, the rapidity with which the

machine

banksis _proportionaltothedifference ofthe couple

due

tothe actions ofthe ailerons,

and

the coupledue to the

force of drift ; ifthe value of the latteris very large (that

FIG. 33.

is, if

D

ais applied very far above the center of _gravity)

the

maneuver

will be slow. Therefore for _{good mobility}

of the _airplane, the force

D

a

must

not be too far above

the center of gravity.

The

_{foregoing considerations}

show

the close

interdepend-ency existing between the problems of _{directional stability}

and

thoseoftransversal_stability. Itis _{practicallypossible}

FIG. 34.

to control directional _stability

_by

means

ofthe lateral

con-trols,

and

vice versa. For'example, birds possessno

means

ofcontrol for_{directional stability alone,} butuse themotion of their _wings for _changing the direction of their _flight.

To

raisetheforce

D

awithrespecttothe centerof_gravity,

we

_may

either install fins above the_{rolling axis, or,} better

THE CONTROL

SURFACES

35 to thetip ofthe wing, the so-calleddihedral angle (Fig. 33).

The

effectofthis_regulationisthat

when

the

machine

takesan

angleofdrift,the

wing

onthesidetoward whichthemachine

drifts, assumes

an

angle of incidence greater than the

inci-FIG. 35.

dence of _{the opposite wing,} thereby developing a lateral

couple which is favorable to stability.

^The framework

of the ailerons is _usually of _wood, steel

tubingor_pressedsteel

members.

An

outlineof

wood

ailer-ons is _{given in Fig.} 34.

FIG. 36.

Concluding to be _{relatively safe}

and

controllable at the

same

time,

an

airplane

must

be provided withdeviceswhich

will _{produce stabilizing couples} for every deviation from the position of equilibrium; but these couples

must

not be

36

AIRPLANE

CONSTRUCTION

of excessive _magnitude, for the

machine would

then be too slowin its _maneuvers,

and

_{consequently dangerous} in

many

cases.

These

stabilizing couples

must

be of the

same magnitude

as _{the couples}

which

can be produced

by

the controlling devices. In this

manner

the pilot always has control of the

machine and

it will answer readily

and

effectively to his will.

The

_system of control of

_maneuvering

_by

the _pilot

usu-ally consists of a rudder-bar operated

by

the feet,

and

a

hand-controlled vertical stick _(called the _{"joy stick")}

piv-UNBAUANCED RUDDER

IA

BALANCED RUDDER

FIG. 37.

oted on a universal _joint,

moved

forward

and backward

to

lower

and

raise the elevator,

and

from left to right to

move

the ailerons (Figs. 35

and

36).

Balancedruddersarefound

on

some

of the_high-powered

machines, as _{they reduce,} to a slight degree, the muscular

effprt_of the pilot.^

The

effort required to

move

a control surface _Depends

on

the distance h _{(Fig. 37)}

between

the

center of _pressure

C

and

the axis

AB

of rotation. If axis

AB

is

moved

to A'B'', the value of h is reduced to h',

and