THEORY OF FLIGHT 351

weight per unit volume. It has m<'ltual adherence for

adjacent particles, which is called viscosity. It follows
that with weight and viscosity, air must be classed as a
THEORY OF FLIGHT fluid. This, at first, may be hard for the student to realize.
He can come to a better appreciation of this fact by test­
Introduction ing the heavy "feel" that the air has as he puts his hand
out of the window of a fast movirlg automobile.
Possibly from the beginning of time, one of man's
greatest ambitions has been to be able to fly like a bird, . Surrounding the earth to a depth of about 40,000 feet
to transport himself from place to place through the air. is a layer of air where temperature decreases about
Unfortunately we have not been born with wings. We 3 of. per ] ,000 feet, and this is called the t1'oposphe1'e.
have, therefore, had to use our own ingenuity to create Outward from this, where temperature does not decrease
mechanical devices that would do for us what wings do with altitude so far as is known, is what is called the
for birds. Man, being a naturaliy dissatisfied being, has stratospher'e, It is in this stratosphere that all future
improved upon design to such an extent that airplanes long distance high speed flights are proposed. Much
are now flying several hundred m.p.h., whereas birds greater progress than has yet been made will be possible
average from 40 to 100 m.p.h. These developments have when the complications due to reduced pressure and
not conie overnight. In spit2 of this, they have come so temperature, at which the human body could not iive if
rapidly that the average person on the street seems to exposed to them, have been solved.
think that there is some strong mysterious force that
keeps an airplane in the air, instead of the practical ap­ The Air.
plication of a fe,v of the simplest and oldest laws we
have. The air is a mixture
of gases, which has
weight and pressure,
Atmosphere. ~NITR06EN .
and flows like water.
Before commencing to study what makes the airplane One cubic foot of air CARSON
fly we must consider the medium in which it flies-the at sea level weighs DIOXIDE ETC.

air. People are too apt to forget all about this important 1% oz. (Fig. 1).
JsOXYGfN
substance, possibly because they have never known a The weight of a two
lack of it, but it is important that something be known hundred mile column
about it, and its behaviour under all conditions. Air is a of ah: resting on the Fig. 1.
mixture of gases and has mass, and thus under specific surface of the earth, creates a pressure of 14.7 pounds
conditions of temperature and pressure, density or to the square inch, (Fig. 2).
350
 (2) Any change of motion is proportional to the force applied,
The Principles of Flight.
and takes place in the dircction of the straight line in which
The flight of the force acts.
the airplane (or (3) To every action there is an equal and opposite reaction.
heavier than air
Pascul's Law of Fluid Pressure.
machine), is due
(1) Pressure exerted on a mass of fluid filling a closed vessel,
to the function- is transmitted undeminished in all directions, and acts with
ing of several the same force on equal surfaces in a direction at right
basic nat u r a I angles to them.
laws. As these (2) The pressure against
are studied the a solid moving through
LOWERED
Fig. 2. student will see a fluid, is proportional
to the square of their PRESSURE
that there is nothing more mysterious about flight, thall relative velocity.
there is about the wind blowing, or water flowing in a
It can be seen from
stream.
the above laws, that the
speed of a boat or air­
Newton's Laws of Motion. craft is directly de­
(1) A body will continue in its state of rest, or uniform in a pendent on the amount
straight line, unless it is compelled by force to change that of power obtainable,
state.
and to get even small
increases in speed will Fig'. 4.-Suction.
take more power than one might first think reasonable.
Bernouilli's Theorem.
\ As the velocity of a fluid in motion increases, the pres­
sure exerted by the fluid grows proportionately less.
From Bernouilli's theorem, it can be seen that if we
have a velocity change we have a pressure change, and
we know from previous study that all pressures tend to
equalize. Hence it follows that the force exerted by the
Fig. 3. equalization of pressures, can vary in direct proportion
to the change of pressure. It is on this simple law that
The turbulent airflow behind a moving car, is like the agitated flight is largely dependent.
flow of water in he wakc of a boat-.
 - _. _... - - .......- ..-,, ..-..,.-
"- - - --

If we have a glass of water, and we put one end of a to any shape from which lift can be created when moved
drinking straw in it and draw, a simple action takes place. through the air), and to the velocity through which it is
The pressure in the straw is reduced, and atmospheric passing through the air or the airflow over it. It can be
pressure forces liquid up the straw. The pressure re­ readily seen that air, being a fluid, will offer resistance to
duction in the straw is quite low, but it can be seen that this moving body and this resistance is known as Drag.
the action is quite large. An aircraft has been so designed Drag is the price ,ve pay for lift, and every effort must be
that the wings have made by the designer to keep it at a minimum.
a definite shape . It
Airfoil.
can be seen in the
diagrams that fol­ In the previous paragraph we used the term airfoil
low that as the air (aerofoil), but very little has been said about it. Byex­
strikes the leading perimenting, it has been found that various shapes give
edge of the wing.
the velocity on the
top surface is in­
creased and the
FLIGHT PATH +- ­ - - -- -- ----
pressure reduced
Fig. 5. (B ern 0 u i I Ii's
Bernoulli's Principle (Atomize r). theory). While thE> l,,-\
pressure of the under surface is increased, the velocity
decreases due to the angle which the wing meets Fig. 6.
the air. Now we have a condition of unequal pressure, A lin e d rawn from the leading t o th e tra iling edge of the airfoil is
the Chol'd Line. The aerodynamic r ea : tion acts through an
and in the action of equalization, a force acting at right imag inary point in the airfoil called th e Centre of Pressure. Th e
angles to the airflow is generated. A simple proof of how Relative Airfl ow is the direction fr om which the air strikes the
this works can be seen by holding a flat piece of paper so airfoil a s it moves along its Flight Path and the angle between
\ this dire ction and the Chord Line is th e Angle of Attack. (See
that it bends downward. If you blow over the top, you Fig. 6).
will notice that the trailing edge rises. In the same way
different amounts of lift (aerodynamic re3.ction) for the
roofs from barns and buildings are lifted during a severe
same amount of drag. Remembering that the drag in­
wind storm.
creases as the square of the velocity, it can be readily
Lift. seen that there will be an airfoil section for each indi­
This force that is generated, as mentioned above, is vidual aircraft; for example an aircraft used for carry­
termed lift or wing generation, and it varies accord­ ing heavy loads at a comparative ~ y low speed, will use a
ingly to the size and shape of airfoil (the name given wing section that would create too much drag for a
 356 DEFENCE TRAINING THEORY OF FLIGHT 357
•
racing machine. A racing or fighting aircraft, where the surface of the wing. Further increasing of the angle
speed is of utmost importance, will use a wing section of attack causes the air to no longer flow smoothly over
that will make that particular machine as fast and the surface, and it breaks up into eddies or burbles. The
manoeuvrable as possible. (See Figs. 7, 8, 9 and 10). wing is no\", said to be stalled, and this angle is known as
the burble point, or the point of stall. When this eon­
~-~ c·~ dition is reached the drag becomes excessive, and the lift
falls off very rapidly as the angle of attack is increased.
Fig. 8.
Fig. 7. Good Lift. Averag'e Drag.
High Lift. High Drag. Low Speed. (Trainers, Trans­
Low Speed. (Birds, Gliders.) ports. )

c =-==--=--=­ c= Fig. 10.

--­ c

Fig. 9.
Symetrical Section. Very Low
Low Lift. Low Drag. Drag:.
High Speed. (Fighters, etc.). Any Speed. (Tail Surfaces,
Fuselage.)
Airflow. Fig.11.
The airflow about an airfoil at different angles of attack. A and
When the air strikes the leading edge of the wing it B i'how small angles, C is the angle of maximum lift. and in D the
divides, and part goes above and part goes below. In airfoil is stalled. .
order that the air passing above the airfoil may join up Some airfoil sections start to stall rather slowly, and
\\lith the air that has just left the under surface, its vel­ give the pilot plenty of warning, as the airplane shud­
ocity must increase, and therefore the pressure it exerts ders and starts to sink in a nose dive attitude. This pai'­
\ on the upper surface is reduced. Thus, as the pressure On ticular condition is classed as a mild, or gentle stall.
top of the wing becomes less, the normal or higher pres­ When the opposite is true, and the burbling sets ill \vith­
sure against the lower surface pushes the wing upwards, out any warning, we say the wing or aircraft has an
this push being knovvn as wing reaction or lift. As the abrupt stall characteristic. If at the time when the air­
angle between the wing and the airflow is increased, the craft is stalling, a yavving couple is introduced, (as will
reaction becomes greater. be explained later) the aircraft will spin.
It can be seen from the illustration (Fg. 11) that at
Drag.
the increased angles something is happening to the air
near the trailing edge, it is starting to break away from Drag is the resistance of a body to its mo\'ement
through the air. There are three types:
358 DEFENCE TRAINING THEORY OF FLIGHT 359

( a) F onn Drag: (c) Induced Dmg:

(Sometimes called Profile Drag) is due to the actual This occurs at the wing tip, where a swirling vortex of
shape of the body. The better the streamlining of a air is formed by the interference between the low pres­
body the less turbulence is set up in its wake, and the sure air above the airfoil and the high pressure air
less drag it has. (Fig. 12). below, the formation of this swirl absorbing otherwise
useful power.
~ ~
~
5D ~§~
~ ~ ~
~ d ~lJ (d) Pamsite Dmg :
Thi s is exactly what the nam e implies; drag set up by
Fig. 12. the non lifting parts of the aircraft such as the under­
(b) Skin Drag: carriage, fi ying wires, interplane struts, etc.

This is caused by the retardation of the layers of air Centre of Gravity.

in the immediate vicinity of the body, the air next to The point in an aircraft, or any body, through which
the surface tending to stick to it and be dragged along a resultant of all the weight acts (an imaginary point
with it. This particular action takes place in a layer of which can be calculated according to the design of the
air only five to seven thousands of an inch in thickness, aircraft is called its centre of gravity). In normal con­
next to the body, known as the Boundary Layer. Rivet ditions it can not be changed but the shifting of cargo,
heads, seams, and roughness 011 the surface tend to in­ etc., can cause a change in the centre of gravity. (C. of
crease this kind of drag. The drag set up by this action G. See Fig. 14).
is perhaps one of the most important we have, and the

L1FTt<:'1 -1' 1~
exact behaviour of this layer and the control of it is at
present under investigation.
In Fig. 13 the Laminar Theory is illustrated . Note I
that the layers of air are not to scale, each being actually
about one or two thousands of an inch in thickness .
1HRU~1C C"~~ ,,,,:,,,,

~ = -~
, WEIGI-fr

~~
-200MPH=-?e .
o~'oo_
~~ Centre of Lift.
Fig. 14.

~ ~ The centre of lift is the point on an aircraft through

Fig. ] 3. which the resultant lift from all lifting surfaces acts.
360 DEFENCE TRAINING
THEORY OF FLIGHT 361
In an ideal aircraft, if it were possible to design one, the
Controls.
C of G and the C of L would coincide. It can be seen that
in movement of the centre of lift (which changes with To manoeuvre an aircraft, to keep it level fore and aft
the angle of attack) will cause the movement depending and on an even keel, suitable control surfaces must be
on which way the C of L is moved. designed. The exact position, size, and shape of the con­
trol surfaces will depend on various things such as the
Straight and I"evel Flight.
type of machine, whether landplane, seaplane, flying
When the aircraft is on the ground it represents a boat, or amphibian. It is the designer's problem through
dead weight, and in order to support it in the air, suf­ careful calculation to find the most suitable position
ficient lift must be generated to offset the weight of the and arrangement. The success of the aircraft will de­
aircraft. To do this we use an engine and airscrew; as pend on its general flying characteristics and its effici­
the engine rotates the airscrew Thrust is developed, ency. The "actual feel" is something that is hard to ex­
causing the airplane to move forward. As the aircraft plain; either an airplane has it or it has not, and the
accelerates, sufficient speed is attained to generate best looking aircraft is not necessarily the most pleasant
enough lift on the wing surface to offset the weight of to fly.
the aircraft, which will then leave the ground. Then we An airplane that is quiet, comfortable to sit in, and
say it is airborne. With the aircraft at a speed safely takes little effort on controls will be one that a pilot will
and efficienttly above the stall, the extra Thrust is used consider to have a good "feel". If the visibility is re­
to keep the aircraft climbing. stricted, the aircraft is noisy and uncomfortable, and has
When an aircraft is in straight and level flight, approx­ to be handled with harsh movements to get reS'ults, it i3
imately seventy per cent of its total horsepower is used. more than likely that its reputation will not be desirable.
The Thrust balances the Drag, the Lift balances gravity
or the weight of the aircraft, and flight may be sus­ Elevators. (Fig.
tained indefinitely, as long as gasoline is supplied and 15. These are
the engine functioning normally. It can be seen from flat-like controls
this that as the throttle is opened, the extra Thrust avail­ behind the tail­
able beyond that required to balance Drag at that speed, plain that move
may be used to climb, and the aircraft will take on a new upward or down­
climbing attitude. Similarly, if the Thrust is reduced ward, as the con­
the speed falls off, and the aircraft coming to a larger trol column is
angle of attack to generate sufficient Lift, may stall. moved fore and
Therefore, we lower the nose and glide, utilizing the aft, the reb y
weight of the aircraft to keep it airborne. forcing the tail
Fig. 15.
up or down, and
 moving the nose about the lateral axis in the looping or Rudder. (Fig. 17).
pitching plane.
This surface is at the end of the tail assembly behind
the fin. It is controlled by the rudder pedals and moves
Ailerons. (Fig. 16). the tail to the left or right, causing a corresponding
These are flap-like surfaces on the training edge of each movement of the nose about the normal axis called
\ving near the tip. As the control column is moved to yawing.
either side, a suitable linkage brings one aileron up and
Stability.
It can be seen that in order to have an aircraft that has
good handling qualities, it should have a certain amount
of stability; that is, the ability to return to its original
position, after being disturbed from level flight. If
features are incorporated in the design of the aircraft to
gi\'e it this characteristic, which functions automatical­
ly, we would say the aircraft had Inh erent Stability.

The Tail Plane. (Fig. 18).

The Tail Plane gives the aircraft longitudinal stability.
Fig'. 16.

the other down. Th 2

up-going ailel'on
causes a loss of lift
\ on that wing, while
the down - going
aileron increases
the lift on its wing. RHATJVE AlRrlO\J
The res u I tin g ~ .. - ,­

action is a move­ Fig. 18.

ment about the
longitudinal axi s, The Centre of Lift is always a little behind the Centre
called rolling. Fig. 17.
of Gravity and the two forces thus form a couple, so
 there is a tendency for the nose of the aircraft to be increased, while it is decreased on the higher wing away
turned downward. Note that in Fig. 18 the tail plane, from the slip, which is also slightly blanketed by the
in level flight, has a negative Angle of Attack to the fuselage. The difference in lift on the two wings roll s
the aircraft back to level flight. (Fig. 19).
Relative Airflow, in this case, the downwash of air from
the wing; therefore, its Lift is downwards to balance the
weight-lift couple. Keel Surface.
If the nose of the aircraft falls, the tail rises and thJ This is the whole side area of the aircraft, including
angle of attack of the tail plane becomes even more the fin and rudder. (See any illustration of a side
negative; a greater downward lift therefore brings the view) .
aircraft .back to level flight. Since there is more surface behind the Centre of
If the nose of the aircraft rises, the tail falls and the Gravity than ahead of it, the aircraft tends to head into
angle of attack of the tail plane becomes less negative or its Relative Airflow, like a weather vane; this is known
even positive; a smaller downward lift or an upward lift as Directional Stability, or Weathercocking.
therefore allows the weight lift couple to lower the nose.
Actually, after any disturbance the tail rises and falls
through successively smaller distance called Harmonic
Oscillations, till level flight is restored.

Lateral Dihedral.
This is the angle between the lateral or sidewise axis
of the airplane, and the wing or main plane.

\
Fig . 19. --­
There is a small loss of lift or efficiency of the main­
RETRACTABLE
LA NDING GEAR
f USELAGE

plane but this is offset by the improvement in stability. Fig. 20.

The reactions of both wings are equal in level flight. Drawing of a Modern Type Airplane showi ng major parts .
When one wing drops it has a more effective lift than the
rai sed wing. Also the aircraft slips toward the down
wing, the angle of attack of the wing towar:d the slip is