Aircraft Engineering, Journal

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American Academy of Political and Social Science

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Aircraft EngineeringAuthor(s): T. P. WrightSource: The Annals of the American Academy of Political and Social Science, Vol. 131, Aviation (

May, 1927), pp. 27-33

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Aircraft

Engineering

By

T. P.

WRIGHT,

B.S.

Chief

Engineer,

Airplane

Division,

Curtiss

Aeroplane

and Motor Co. Inc.

BY

aircraft

is

meant

any weight-

carrying

device

or

structure

de-

signed

to

be

supported

by

the

air.

There

are two classes

of

aircraft,

heavier-than-air

and

lighter-than-air,

the

former

obtaining

its lift

from

dynamic

air

pressure,

the

latter

from

buoyancy

due

to

displacement

of

air

by

a lighter gas. The present article will

deal

entirely

with

heavier-than-air-

craft

or

airplanes.

GENERAL

CONSIDERATIONS

AND

DEFINITIONS

It seems

desirable,

before

describing

the

engineering

methods used in

design-

ing

an

airplane

today,

to

review

briefly

the

stages

of

development

of

aviation,

noting particularly the engineering

progress

made

during

each

stage.

In

order

to fix

clearly

in mind

the

status

of

the

development

during

each

period

discussed,

it

is well

to

consider the

meaning

of

such

general

terms as

art,

science,

and

engineering.

Whereas

mechanical

art

implies

a

prac-

tical

application

of

knowledge,

science

refers

to

an

exact and

systematic

state-

ment of

knowledge.

Art

always

re-

lates

to

something

to

be

done,

science

to

something

to

be

known.

In

engineer-

ing

is included

the

basic

idea

of

execut-

ing

or

managing

a

construction

or

design.

It

will be

seen

that at

a rela-

tively early

date

a considerable amount

of

scientific information

pertaining

to

aviation

was

brought

together,

and

that

experiments

were

conducted,

some

by

scientific and some

by

rule-of-thumb

methods. It has not

been,

however,

until

very

recently,

that

engineering

methods have been

applied

uniformly

to

airplane

design

and

construction.

This

of course

is true

of the

develop-

ment

of

any

art,

as a

considerable

period

of research

and

of

attainment

of

practical

experience

is

necessary

before

the

systematic

methods

of an

engineer-

ing organization

can be

applied.

STAGES OF DEVELOPMENT

The

stages

of

development

of

avia-

tion

may

be

conveniently grouped

into

three

periods,

namely,

the

period

before

the

war,

the

period

during

and

im-

mediately following

the

war,

and the

period

thereafter.

There

seem

to be two

distinct

sub-

divisions

of

the

stage

of

development

prior

to

the

war,

the

first

commencing

about 1890 and lasting until 1903, and

the

second

extending

from 1903 to

1914.

During

the

earlier

period

there

were

at

work two

quite

distinct

types

of

men.

One

type

is

represented by

Professor

Langley

seeking knowledge

of

aerodynamics

by truly

scientific

methods of research.

By

means of a

whirling

arm,

Professor

Langley

ob-

tained

and measured the

aerodynamic

forces on flat

plates

mounted at the end

of the

arm.

From

data

thus

obtained,

an

airplane

was

designed

and con-

structed,

which succeeded in

lifting

its

own

weight.

Unfortunate circum-

stances not

pertaining directly

to the

airplane,

apparently

prevented

him

from

seeing

his machine with

power

carry

a

man

into the

air. There was

another

type

of

investigator

at work

during

this

period represented

by

Lillienthal in

Germany,

and Chanute

and

the

Wright

Brothers

in

this Coun-

try.

The efforts of .these men

were

27






THE

ANNALS

OF THE

AMERICAN

ACADEMY

directed

to

gliders.

Great numbers

of

gliding

flights

were

made,

usually

culminating

in

a

disaster. These

acci-

dents

were due

principally

to

depend-

ence for lateral control on the slight

effect

of

shifting

the

center of

gravity,

accomplished by moving

the

legs

of the

operator.

The

first to

solve

this funda-

mental

problem

of

flight

were the

Wright

Brothers.

They

obtained

lateral

control

by

warping

the

wings

on

either

side,

thus

securing

a counteract-

ing

force

by

increasing

or

decreasing

the

lift on

one

side

or the

other

of

their

machine, as necessitated by the atti-

tude

assumed

after

striking

a

current

of

air.

The first human

flight

in a

power

operated

airplane

was made

by

Orville

Wright

on

December

17,

1903. It

is

thus seen

that this

first

subdivision

of

the

first

stage

of

development

of the

art

closes

with

the attainment of

flight,

accompanied by

the

accumulation

of

a

certain

amount

of

scientific

knowledge,

some gained through laboratory and

some

through gliding experiments.

The

next

period,

from

1903 to

1914,

is characterized

particularly

by

the

rapid

development

of

an

adjunct

of

the

airplane,

the

gasoline

engine. Only

slow

progress

was

made

in the

develop-

ment of the

airplane

itself.

Flights

of

greater speed

and

longer

duration were

achieved

and a

number

of

improve-

ments made. The outstanding flyers

of

this

period

were the

Wrights,

Curtiss,

who

was

the

first to

fly

from

and

light

on

the

water, Bleriot,

Santos Dumont

and

Farman.

Their

contribution

to

the

art was of

the nature

of

invention.

There

was,

however,

another

type

of

men

at

work

who were

developing

and

using

the

basic

equipment

used

in

aerodynamic

research,

the

wind tunnel.

Experiments of vast scientific value

were

carried out

in France

by

Eiffel,

in

Italy

by

Crocco,

in

Germany

by

Prandl,

and

in

England

at

the

National

Physi-

cal

Laboratory.

The

standard

mathe-

matical

equations

of

motion

of

a

rigid

body

were

applied

to

the

disturbed

motions of an

airplane

by Bryan

in

England, culminating

in his

book

pub-

lished in 1911. No essential changes

in

the

theory

have

since been found

necessary.

This

period

of

develop-

ment

may

be

considered as

closing

with

the

commencement

of

the

war.

The

art had

advanced

to

such

an

extent

that

it

may

be

considered that the

mathematical

theory

was

established;

scientific

experimental

equipment

was

being

used,

and

flights

were

being

made. There was, however, strictly

speaking, yet

no

airplane engineering.

The

next

stage

of

development

in-

cludes the

years

of

the War

and the

two

or

three

years

just

following

it.

The

advance

of

course

was

by

leaps

and

bounds.

However,

the

progress,

both

in

quantity

and

quality

required,

was

greater

than could

be attained on a

sound

engineering

basis.

In

conse-

quence there were many design failures,

only

discovered as

such after

very

large

quantities

of units

or

completed

ma-

chines

were constructed.

Engineering

organizations

were

gotten

together,

and

functioned

to as

great

an extent as

was

possible

under

the

conditions.

The

machines

produced

were, however,

more

of the nature

of

designs by

indi-

viduals

than

by

systematically

func-

tioning engineering

groups. It may,

however,

be

said

that it was

during

this

period

of

rapid

advance

that

airplane

engineering

commenced.

Certainly

the

science of

aeronautics

was well

advanced.

The latest

stage

of

development

catl

also

be divided

into

two

periods,

from

1921

to

1925,

and

from

1925

to the

present

time.

The

former

period

was

characterized in the

industry

by

the

successive failure

of over

half

of

the

companies

existant at

the

close

of

the

war.

There were

endless

controversies

and

investigations.

In

the

companies

28






AIRCRAFT ENGINEERING

which

were able

to

exist,

only

slight

progress

could

be

made

in

engineering,

as in

all but

one or two cases the

engi-

neering

organizations

were

disbanded.

Some advance in the art was made,

however,

notably

in

the

development

of

high

speed

machines.

During

this

period

speeds

were

increased

roughly

'from

150 to 250

miles

per

hour.

Also,

although

not

involved

in the

engineer-

ing

progress

of

the

art,

this

period

saw

the commencement

of

commercial

avia-

tion in

this

country,

the most

note-

worthy

feature

being

the establishment

and successful operation of the Air

Mail. The

lack

of a definite

policy

on

the

part

of the

government

with

regard

to

the

industry,

received

a

great

deal

of

attention,

and

after several

Congres-

sional

committees

had

investigated

the

situation,

a

group

was

selected

by

the

President

to

report

fully

the conditions

found

after

making

a

thorough

search

for

facts. This was

the Morrow

Board,

whose report in December 1925 may be

considered as one of the most

important

documents of

American

aviation.

Sound recommendations

were made on

practically

all

phases

of

the

subject.

The

subsequent

adoption by Congress

of the

greater part

of

the recommenda-

tion

gave

new life

to

the

industry

and

new

impetus

to

progress

in

the art.

The

latest

period

of this

stage

of

development

may

therefore be con-

sidered

as

beginning just

after

the

Morrow

Board

report

was

published.

Civil aviation

started

to

be

a

reality;

military

aviation

policy

was estab-

lished,

and

the

industry

was

in

a

healthy

condition. This

permitted

the

continuance

and formation of

airplane

engineering

groups,

comparable

in

organization

and in methods

used

to

those of the more

firmly

established

industries.

Thus

in

the

present

period

theory

and

practice

are

being

co-

ordinated so

that

aeronautics

may

be

now

considered

both

an art

and a

science,

with

designs

produced

with

advance assurance of

success,

because

of the

systematic

methods

employed

by

the

Engineering Departments

of

the

various companies. It has been said

that

progress

in

engineering

science,

like

changes

in the

sphere

of

political

organization,

may

be

by

evolution or

by

revolution.

In

general,

during

the

earlier

stages

of

development

described

above,

the

latter

type

held.

It

is

believed that the

present

stage

may

be

characterized as a

period

of

evolution.

There

is

being

made a

scientific

analysis

of experience gained, reducing the

lessons

to

engineering

terms.

It

is

a

period

of

patient spade

work,

much

needed in aviation.

PRESENT STATE

OF THE ART

Airplane

Engineering

Airplane Engineering

is

perhaps

unique

in that it includes

within itself

practically

all other

recognized

branches

of

engineering.

Although,

of

course,

no

branch

of

engineering

when

dealt

with in

practice

can

be found

to

be

isolated

and out of contact

of some

sort

with

other

arts and

sciences,

it

appears

that

in

Airplane Engineering,

more

than in other

cases,

a

considerable

number

of

distinct

branches

of

engi-

neering

assume

a

major

and

equal

importance.

Naval

Architecture

For the

general

method

of

attack

of

new

problems,

the

airplane

engineer

borrows from

the naval

architect. All

engineers

base their

procedure,

or

should do

so,

on the

experience

of others

in their

field,

yet

it is

perhaps

more

true

in

naval

architecture

than in

other

branches,

that new

design

is made

to

follow

closely

a

precedent

of former

practice;

with

only

sufficient

improve-

ment

to warrant

the

new

structure.

Aside

from

the

general

method

of

attack

there

are three

or

four

specific

29






THE ANNALS

OF THE

AMERICAN ACADEMY

problems

which

are

common

to

the

naval

architect

and

the

airplane

engi-

neer,

and which are

treated

in

much the

same manner.

These include

prob-

lems of

weight

with its distribution, the

design

of

propellers,

the determinations

by

model

test

of the

dynamic

factors

entering

into conditions

of

equilibrium,

and

the

determination

by

an

entirely

different

type

of

model test

of

the

stability

and

general

behavior

of

floats

or

hulls

on

the

water,

used

in

designing

seaplanes

and

flying

boats.

Without

doubt,

weight

and

weight

distribution,

or

balance,

are of more

importance

in

airplane

design

than in

any

other

branch

of

engineering.

The

very

nature

of

the

duty

which an air-

plane

performs,

the

lifting

of

objects

into the

air,

indicates

that

the

strictest

attention must

be

paid

to

weight

economy.

It

has

been said

that

a new

design

can

be

judged

a success

or

failure

immediately,

when after

completion,

it

is

placed

on the

scales. A

measure

of

the

efficiency

of

an

airplane

is

the

ratio

of

the

useful

or

disposable

load carried

to

the

gross weight.

Obviously, any

overweight

in the

structure

reduces

by

an

exactly

equal

amount

the useful

load

for

which

the

machine was

designed.

The

essential

nature

of

weight

economy

must

be

constantly

before the

airplane

engineer.

An

airplane

must

not

only

be

light,

it

must

also

be

stable. This

character-

istic is

dependent

on

the location

of

the

center

of

gravity,

or

on

weight

distribu-

tion

along

the

longitudinal

axis. There-

fore,

when

considering

any

detail

of

the

design

which

calculations

or

actual

weighing

shows to

depart

in

weight

from

the

original

estimate,

due

account

must

be

given

to

the effect on

balance

as

well as

on

reduction

in

useful load

involved.

In

the

design

of the

screw

propeller

the

airplane

engineer

and

the

naval

architect

are on

common

ground.

Although

the

effect,

quantitatively,

of

the

various elements

of

design,

are

vastly

different in

the

case

of

a

propeller

designed

for

use in

air as

compared

to

one for use in

water,

nevertheless the

same

problems

do exist. It is

quite

interesting

to note that it is

only

quite

recently

that air screw

designers

have

come

to realize that

better

results can

be obtained

by modifying

slightly

a

previously

constructed

successful

de-

sign,

rather than

by

attempting

to

effect a

completely

new

design,

based

on

theoretical

considerations,

to cover

the new conditions. This fact

has,

of

course,

long

been

known

and

followed

by

the

naval architect.

Before

proceeding

with the

construc-

tion of a new

design

of

ship,

the

naval

architect

assures himself

that the

com-

pleted

vessel

will be

satisfactory

on

the

water

from

the

standpoint

of

stability

in

waves,

by testing

a

model

in

a

towing

basin.

The.

airplane designer

uses

the

same

equipment

in

connection with

float and

hull

design

for

seaplanes,

and

analogous

equipment

for

determining

stability

and

controllability

character-

istics of his

machine in

the air. The

latter

equipment

consists

of a wind

tunnel,

in which all forces

and

moments

on

an

accurately

constructed

model

are

determined

for the

various

attitudes

which the

airplane may

assume. An

air

stream of

known

velocity

is

forced

past

the

model which is

mounted

on

an

extremely

accurate balance.

From the

results of

such model

tests

and with a

knowledge

of

the

laws

of

dynamic

similarity,

the

aeronautical

engineer

can

predict,

with

an

accuracy

astonish-

ing

to

the

laymen,

the characteristics

and

performance

of

the full

size

air-

plane.

Aeronautical

Engineering

Aeronautical

engineering

is a

new

branch,

peculiar

to

the

design

of air-

planes.

In

the

present

instant

it is

30






AIRCRAFT

NGINEERING

defined as

covering

the

limited

field

of

aerodynamics

although

it

is

frequently

and

perhaps

more

correctly

made

synonomous with airplane engineering.

The

aeronautical

engineer

deals with

the

forces

produced

on

solid bodies

by

air in

motion.

His

particular

field

in

airplane

design

is, therefore,

air

resist-

ance,

stability,

and

controllability,

all

of which are determined

both

analyti-

cally

and

experimentally.

The

wind

tunnel,

described

above,

is the

equip-

ment

of

the

aeronautical

engineer.

Civil

Engineering

In matters of

structural

design,

the

methods of the

civil

engineer

are

closely

followed.

They

are, however,

ex-

tended

and refined to

an extent

seldom,

if

ever,

required

in

bridge

or

building

construction. This refinement of

struc-

tural

analysis

is necessitated

by

the

extreme

importance

of

weight

saving,

above referredto, and by the equal or

greater

requirement

of

absolute

struc-

tural

safety.

Each

detail,

as

well as

the

main

structural

members,

must be

carefully

analyzed

for

strength

and

weight;

the

two

factors

interact

through-

out

the

design.

The

problems

en-

countered

are

more

novel and

varying

in

character,

and,

therefore,

require

closer

study

than is

usually

necessary

in

the structural analyses involved in the

older branches of

structural

engineer-

ing,

such as

the

design

of

bridges

or

buildings.

Mechanical

Engineering

The

mechanical

engineer

is found

in

an

airplane

engineering organization

in

the

design

staff,

where he

lays

out

and

designs

not

only

the

general

arrange-

ment of the complete machine, but also

the

detail

parts

and

mechanisms.

The

number

and

diversity

of

parts

involved

in

an

airplane

are

frequently

not

real-

ized.

Essentially

an

airplane

consists

of

a

body,

the

functions

of

which

are to

house the

crew,

passengers,

cargo

and

equipment;

a

supporting

wing

struc-

ture;

stabilizing

and

control

surfaces,

with necessary control mechanism;

landing

gear

for

alighting

on land

or

water

(or

both)

and the

power

plant

consisting

of

engine

and

propeller.

All

require

for

proper

designing

the

attention of the mechanical

en-

gineer.

Materials

Engineering

Under this

heading

is included

the

metallurgical and chemical engineering

branches.

Alloys

of steel and

alumi-

num,

requiring

the attention

of

men

versed

in

the above

sciences,

are

used

extensively

in

airplane

construction.

In

addition,

there are a

great

number of

other

special

materials

used

including

wood,

fabric,

dope,

paint,

and

miscel-

laneous non-ferrous

metals.

When

consideration

is

given

to

the ever

present need of weight saving, it can be

seen that

highly

trained

men must have

cognizance

of the

special problems

arising

from

the use of

materials

of

such

a

diversified

character.

General

Engineering

Problems

Under

this

heading

fall

the

problems,

common

to

all

engineering

concerns,

such as

drafting,

estimating,

inspection

and standardization. The latter con-

sideration is

one

requiring

constant

attention,

in order

that

there

may

be

attained

a

proper

balance

between

reduced costs

of

construction

on

the one

hand,

and

advancement

of

the art on

the

other.

The

above

outline,

indicating

the

branches

of

engineering

involved in an

airplane

engineering

department,

will

immediately suggest the prime neces-

sity

of

organization,

and

the

establish-

ment of

systematic

methods

of

proced-

ure

in

producing

new

designs.

Being

new

and

difficult,

the

field is

of ne-

cessity

inspiring.

81






THE ANNALS

OF THE

AMERICAN

ACADEMY

EQUIPMENT

The vast

expense

involved

in

produc-

ing

a new

design

makes

it

absolutely

essential,

that insofar

as is

humanly

possible,

all chance

of

failure

be

elim-

inated. This makes

necessary

the

availability

to

the

airplane

engineer-

ing

organization

of

proper

equipment.

The

principle

items

of

equipment

neces-

sary

are the

following:

1. Wind

tunnel for

predetermining

aerodynamic

characteristics.

2. Testing machines for determining

strength

of

materials

and

parts.

3.

Static test

equipment

for

testing

structural assemblies.

4. Model

basin

for

predetermining

hydrodynamic

characteristics

of

floats.

5.

Whirl test

rig

for

testing

pro-

pellers.

6.

Flying

field and full

flight

test

equipment.

All items

should

be

immediately

available

for

use.

The

expense

in-

volved

is,

however,

more

than

any

company

now existent

can

afford

to

set

aside

for

this

purpose.

Nevertheless,

it is

extremely

desirable that the first

three

and

last items

be

directly

avail-

able,

and

operated

by

the

staff of

the

designing organization. At present all

the

large

companies

are

equipped

with

items 2

and

3;

several with item

6;

one

or

two with item

1;

and none

with

items 4

and

5,

in

which cases it

is

neces-

sary

to

have

recourse to

government

owned

and

operated

equipment.

In

order to

establish

airplane

engi-

neering

on

a

scientific

basis,

good

equipment

must

be

available.

It

is

to

be hoped that all major companies will

gradually

equip

themselves

with the

items

above listed.

The ideal

toward

which

airplane

engineering

should

de-

velop,

is

the creation of

designs

through

a

systematic

procedure, by

a

group

of

guided experts

using properly

function-

ing equipment.

RESEARCH

Although

a

great

amount of aero-

nautical

research,

both

along

the lines

of

applied

science and

pure

science,

has

been

accomplished

and

recorded,

there

is

still

a

practically

limitless

field

ahead.

It

is the

duty,

for

the

advancement of

the

art,

for

each

company

to

bear

the

burden of its share

of

research work.

At

present

this must

of

necessity

be

confined

to the

realm of applied science,

leaving

investigations

in

pure

science

to

governmental agencies,

such

as

the

Laboratory

Staff

of

the National

Advisory

Committee

for Aeronautics

and

the staffs of the

Universities,

which

have aeronautical

courses

and labora-

tories. This

duty

is

being

realized

more

and

more,

and it is believed that

companies

will

automatically

equip

themselves, both with laboratories and

personnel,

as

the

growth

of

the

industry

permits.

DESIGN PROCEDURE

The

general

function of an

Airplane

Engineering Department

is to

develop

designs

and

produce drawings

by

means

of

which can be

constructed

airplanes

which

are

structurally

sound;

aero-

dynamically stable and manoeuver-

able;

which

possess

the

specific

qualities

and

characteristics

originally

specified;

and which are so

designed

as to be

susceptible

of

construction at a

profit

in

competition

with the

product

of other

concerns. To

attain

this

end,

it is

necessary

to

properly

coordinate

the

work of

the

engineers

who

control

different

phases

of the

design,

as above

described. This co6rdination is, in

practice,

attained

by establishing

a

definite

design

procedure

which

permits

each

expert

to

approve

the

design,

at

the

proper

stage

of

development,

and

for the

features

coming

under his

32






AIRCRAFT

ENGINEERING

cognizance.

The

procedure

believed

best

adapted

for

this

purpose

is

the

following:

1. A discussion by the experts, in confer-

ence,

of the

design

elements

laid down

in

the

specification

requirements,

with

an

agreement

reached

on

the

general

type

of

design

to be

followed.

2. The

development

and

approval

of

a

general

arrangement

drawing; descrip-

tive

specifications;

weight,

balance

and

performance

estimates;

character-

istics

sheets;

and wind

tunnel tests.

3.

The

development

and

approval

of

struc-

tural and assembly arrangementdraw-

ings;

stress

analyses,

strength

tests and

weight

calculations.

4.

Production of

detail

construction draw-

ings.

5.

Construction,

with

engineering

control

of

inspection

and

weight

and

design

changes.

6.

The

superintendence

of final

weighing

of

the finished

airplane

and of

flight

tests.

CONCLUSION

In

the

preceding pages,

an

attempt

has been

made

to

trace the

develop-

ment

of aviation

in

terms of

progress

in

the

scientific and

engineering

field. As

opposed

to the

conditions

that main-

tained

during

the

early stages

of

devel-

opment,

when

the

men

interested

in

aviation

were

either

pure

scientists or

inventors,

it

has

been

shown that

today

the engineering phase of the art is

operative.

With

the art on a

scientific

basis,

the

engineer

can continue

the

progress

in

an

orderly

fashion.

It

is

the job of the airplane engineering

organization

to coordinate

the works of

the

research

laboratory,

the

statistician

and the

engineering expert

in

specific

phases

of the

design,

to the

end

that

a

reasonable

and

useful

machine,

slightly

better than

anything

of its

class

pre-

viously

constructed,

may

come

into

existence. This

indicates,

as

pre-

viously

mentioned,

the most

desirable

form of progress, that is through evolu-

tion as

distinguished

from the

occa-

sional

occurence

of

more

rapid

advance

(not

always

lasting)

brought

about,

to

use

a

political

term,

through

revolu-

tion.

Granting

that

attainment

of

more

rapid

means

of

transportation

of

both

men and

goods

is a measure

of

general

progress

and

prosperity,

cer-

tainly

the

benefits

to

mankind

which

can be envisioned from aviation are

vast

and almost

limitless.

The

prog-

ress

now

being

made in

airplane

engi-

neering

is

extremely

gratifying

to those

deeply

interested

in

its

development,

and

the fact

that

it

now

appears

to

be

on

a sound

basis

of

organization,

com-

parable

with

engineering

in

other

branches

of the

automotive

industry,

bodes well for

the

future

advance

of

the art.

33

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