Module 8 IT Carlow 2020/21 Basic Aerodynamics

Part-66 Module 8. Basic Aerodynamics 1. Physics of the Atmosphere

1. Physics of the Atmosphere In order to understand the way in which the air affects
the flight of an aircraft, it is necessary to know
International Standard Atmosphere (ISA), application something about the properties of the air itself. The
to aerodynamics. earth is encased in a thin layer of gases which insulate
2. Aerodynamics us from the devastating effect of the sun's energy and
which supports life. This layer of gases, called our
Airflow around a body; atmosphere, surrounds the earth to a depth of about
five hundred miles. 78% Nitrogen, 21% Oxygen and the
Boundary layer, laminar and turbulent flow, free
remaining 1% is made up of Water Vapour, Carbon
stream flow, relative airflow, upwash and downwash,
Dioxide, Hydrogen, Helium and traces of Argon, Neon
vortices, stagnation;
and Krypton.
The terms: camber, chord, mean aerodynamic chord,
These gases are compressible and in the lower levels
profile (parasite) drag, induced drag, centre of
are pressed down upon by all of the air above. So,
pressure, angle of attack, wash in and washout,
pressure and density are increased. At sea level under
fineness ratio, wing shape and aspect ratio;
standard conditions, the weight of the entire column of
Thrust, Weight, Aerodynamic Resultant; air creates a pressure of about 14.7 psi or 1013 millibar
(mb). About one half of the total air in the atmosphere
Generation of Lift and Drag: Angle of Attack, Lift is below 18,000 feet. The atmosphere is divided up into
coefficient, Drag coefficient, polar curve, stall; different layers ...

Aerofoil contamination including ice, snow, frost.

3. Theory of Flight

Relationship between lift, weight, thrust and drag;

Glide ratio;

Steady state flights, performance;

Theory of the turn;

Influence of load factor: stall, flight envelope and

structural limitations;

Lift augmentation.

4. Flight Stability and Dynamics

Longitudinal, lateral and directional stability (active and

passive).

5. High Speed Flight*

Figure 1
6. Rotary Wing Aerodynamics*

*Denotes non EASA topics

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TROPOSPHERE layers of the atmosphere. We’ll look at air density first.

What is density?
The lowest level of the atmosphere extends from the
surface upwards to the stratosphere. Its depth varies The density of a substance is its mass per unit volume.
daily from about 28,000ft (8km) at the poles to The symbol most often used for density is ρ (the lower
54,000ft (16km) at the equator. Temperature case Greek letter rho). Mathematically, density is
decreases at an environmental lapse rate of about 2°C defined as mass divided by volume:
for every 1000ft increase (6.50C per km). It contains
water vapour that causes clouds and vertical air
currents and accounts for the weather. At the top of
this layer, known as the Tropopause, is where the
temperature remains steady at -56.5°C, hence the where ρ is the density, m is the mass, and V is the
'pause' in the name. It marks the foot of the volume.
Stratosphere.
Air is a gas and is thus compressible. If you consider an
STRATOSPHERE imaginary cardboard box having a volume of one cubic
metre and we were to fill it with air molecules then it
Above the tropopause, the air is extremely thin. Since would not be unreasonable to expect that if you then
there is no water vapour in the stratosphere, there is filled it with compressed air you would have more air
no weather. The temperature gradually increases to molecules in the box than before. The air molecule
just below 0°C, where it holds steady again in the does have mass so the more of them you have in the
Stratopause. The temperature rises in the box the greater the mass of air it would contain. At sea
stratosphere because the ozone layer there absorbs level the mass of air in our box would be 1.225kg.
the UV rays from the sun. With the rays absorbed, it Density is mass divided by volume so the density of our
causes the air particles to move faster, therefore, air at sea level is 1.225kg/m3.
raising the temperature.
Compare this to the mass of cubic metre of water, a
MESOSPHERE AND THERMOSPHERE metric tonne, 1000 kg/m3, nearly 800 times as much.
Yet it is this very property of air, its density, which
The upper two of the four temperature classified
makes all flight possible, or perhaps we should say
regions have no bearing on aircraft operations except
airborne flight possible, because this does not apply to
for the layers of ionised atmosphere within them that
rockets.
exists at altitudes of 80km. These layers of ionised
atmosphere are collectively known as the ionosphere. The balloon, the airship, the kite, the parachute, and
The ionosphere affects the transmission of radio waves the aircraft, are supported in the air by forces which are
sent out from aircraft and from the earth below. The entirely dependent on its density; the lower the
presence of charged particles in this region, will density, the more difficult flight becomes. For all of
profoundly affect the propagation of electromagnetic them, flight becomes impossible in a vacuum.
radiation of long wavelengths (radio and radar). Some
radio waves are absorbed by the ionosphere and are When air is compressed a greater amount can occupy
reflected back to earth, others pass through to outer a given volume; i.e. the mass, and therefore the
space. density, has increased. If the mass of the air in a given
volume is reduced then the density will reduce. It
DENSITY/AIR DENSITY should be noted that, if the temperature remains
constant, density is directly proportional to pressure,
It is now useful to consider just why air pressure, i.e. if the pressure is halved, so is the density, and vice
temperature and density changes occur in the different versa. When air is heated it expands, if allowed, so that
a smaller mass will occupy a given volume, therefore

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the density will have decreased, assuming that the We can also express this pressure in millibars, a millibar
pressure remains constant. being one thousandth of a bar (0.1kPa). Expressed in
imperial measurement this would be around
The density of gases is governed by the following rules: 14.71b/in2. The question is, why the pressure?
- Pressure, when applied to a fluid such as air is defined
• Density is directly proportional to pressure (at as force per unit area. Due to the free flowing nature of
constant temperature) - density increases as fluids, the force exerted by a fluid is always at right
pressure increases angles to any surface in contact with it. The force
exerted on a unit area of any such surface is defined as
• Density is indirectly proportional to
the fluid's pressure.
temperature – density decreases as
temperature increases Pressure = Force/Area
Therefore Remove the sides of our imaginary cubic metre box and
now imagine the cubic metre of air to be a cube of foam
• Air at high altitudes with low pressure is less
rubber. We now stack further similar volume cubes one
dense than air at low altitudes with higher
on top of the other until we reach the top of the known
pressure
atmosphere. Can you see that the cube at the bottom
• A mass of hot air is less dense than a mass of
of the column is supporting all the cubes above it and
cold air.
will thus be squeezed by the gravitational force of
The density of dry air can be calculated using the ideal them, it will be under pressure. The base of the cube is
gas law, expressed as a function of temperature and one square metre so if you divide the force in to this
pressure: area you will have the pressure. If you now look at a
cube halfway up the column, it is not supporting the
weight of quite so many cubes, half in fact. So, it will
not be squeezed quite to the extent as the cube at the
bottom. At the top of the column the top cube is
The decreasing air temperature up to the tropopause supporting nothing at all and will only experience its
reduces the rate at which the air density decreases with own gravitational pull, not squeezed at all. That should
height. The density will decrease at a more rapid rate explain why we have atmospheric pressure and why it
with increases in height above the tropopause because decreases as we gain altitude.
the air temperature will then be relatively constant at
an average -56.50C up to an altitude of about 20km
(12.4 miles or 65,617ft). The rate of density reduction
will then increase as it is affected by the decrease in
atmospheric pressure only.

The loss of lift from aerofoils and the thrust from

engines follows a similar rate change as both depend
on air density for performance.

PRESSURE/ATMOSPHERIC PRESSURE

You may wonder why air has such a mass when you
cannot even see it. The reason is that at sea level it is
compressed under a pressure of about 100kPa, often
referred to as one bar or one Atmosphere. Figure 2

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If you now imagine that we adjust the dimensions of

the squeezed cube at the bottom back to one cubic
metre without altering the pressure in it there will be
more foam rubber in it than in the cubic metre at the
top of the column. Hence the density of the column
reduces with increasing altitude.

MEASUREMENT OF PRESSURE

The apparatus for measuring atmospheric pressure is

called a mercury barometer. This is not suitable for use
on an aircraft though. A glass tube open at one end,
and closed at the other is filled with mercury. The open
Figure 4
end is sealed temporarily and then submerged into a
small container partly filled with mercury, after which The pressure gauges encountered in everyday use
the end is unsealed. This allows the mercury in the tube only measure pressures above atmospheric pressure.
to descend, leaving a torricellian vacuum at the top of A tyre pressure gauge for example may read zero
the tube. Some of the mercury flows into the container when lying on a bench but the workshop it is in may
while a portion of it remains in the tube. have an atmospheric pressure of 15psi. This pressure
does not register on the gauge. This type of gauge
The weight of the atmosphere pressing on the mercury
reads what is called Gauge Pressure. For example, if
in the open container exactly balances the weight of
we now check the air pressure in a tyre and observe
the mercury in the tube, which has no atmospheric
the gauge reads 30psi it is reading the gauge pressure
pressure pushing down on it due to the vacuum in the
only and that is 30psi higher than atmospheric
top of the tube. As the pressure of the surrounding air
pressure. If you were to add the prevailing
decreases or increases, the mercury column lowers or
atmospheric pressure of 15psi to the gauge reading
rises correspondingly.
you would conclude that the actual pressure should
At sea level the height of the mercury in the tube be 45psi. This figure is referred to as the Absolute
measures the actual pressure of the atmosphere. Pressure.

Figure 5

As far as the tyre is concerned we would only be

interested in the gauge pressure because that gives
us an indication of the difference between the tyre's
Figure 3 internal pressure and the pressure already present in
the workshop. So, we need to be clear that the figures

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we quote for atmospheric pressures are absolute

pressure figures. In the example given above, the
figure of 15psi quoted for atmospheric pressure is the
pressure above the zero pressure of a perfect
vacuum. A useful formula for you to remember is:

Absolute Pressure = Gauge Pressure + Atmospheric

Pressure

How do we measure an absolute pressure in the room

you are sitting in. You will have to use a Barometer,
as this is the only instrument capable of measuring
absolute pressure. Examiners like this question. You
may have seen a Fortin barometer in someone's
house being used to check the air pressure to indicate
weather. Aircraft use an aneroid barometer to
measure absolute pressure and thus altitude.

Figure 7

The air temperature on a standard day at sea level is

defined as 15°C. Moving upwards through the lower
layers of the atmosphere there is the gradual drop in
temperature. The reason for the falling off of
temperature is that the radiant heat direct from the
sun passes through the atmosphere without heating
the air and raising the temperature.

The earth, however, absorbs the heat, the

temperature of the earth rises and the air in contact
Figure 6
with it absorbs some of this heat. As already
There is one final point to be made. The atmospheric mentioned, in still air the temperature falls off with
pressures that we record are for still or static air only. height at a rate of about 2°C for every 1000 ft. This
Moving air will have something called dynamic rate does not change until about 35,089ft (11,000m).
pressure as well and that is over and above the figure
we are after. So, atmospheric pressures are absolute, MEASUREMENT OF AIR TEMPERATURE
Static pressures.
If you recall, there were two measurements for static
AIR TEMPERATURE air pressure, absolute and gauge. We have a similar
set up for temperature. If the temperature in the
Temperature is commonly measured in degrees room you are in is 150C this merely indicates that the
celsius or Fahrenheit. However, both of these scales room is 150C above a nominated zero figure based on
own zero point does not represent the absolute zero the freezing point of pure water. It is not a true
temperature. The Rankine system is an absolute temperature reading. The lowest value for heat is a
system using the Fahrenheit scale. (Increment of 10C point where all atomic particle motion ceases. Simply,
= 10K) no heat at all! This point is known as Absolute Zero.
This relates to Celsius scale reading of -273.15°C.

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There is obviously a need for an absolute responded in the opposite direction. For example,
temperature scale to deal with this and that is known when Boyle increased the pressure on a gas sample
as the Kelvin scale. the volume would decrease. Mathematically, PV =
constant value if the gas is behaving as an Ideal Gas.
Using Kelvin, absolute zero equates to -273.150C so
the nominated point where pure water freezes in
absolute terms will be 273.15K and the 150C in our
example will become 288.15K. Be aware of the
temperature scale in use when describing
atmospheric temperatures. You can easily convert
Celsius readings to Kelvin by adding 273.15 to the
Celsius reading. A final point here is that America
frequently uses the Fahrenheit scale instead of
Celsius so they have a different absolute temperature
scale.
Figure 8
GAS LAWS

We have seen how pressure can affect density but

A practical mathematical expression of Boyle's
temperature can do this too. As the air gets colder it
findings is as follows:
will contract. This gives more molecules per cubic
metre. Density will rise. If the air gets warmer it will P1V1=P2V2
expand, density will fall. When temperature is
working in concert with pressure we have to be
careful when considering the density change. For
example, as altitude increases from sea level, the air A practical application illustrating Boyles Law would
temperature falls but so also does the air pressure. All be the action of a syringe. When we draw fluids into
the decreasing temperature does is reduce the rate at a syringe, we increase the volume inside the syringe,
which the air density falls. Above 36,090ft, the air this correspondingly decreases the pressure on the
temperature remains constant so, the air density inside where the pressure on the outside of the
reduces at a faster rate under the sole influence of syringe is greater and forces fluid into the syringe. If
the reducing atmospheric pressure. we reverse the action and push the plunger in on the
syringe we are decreasing the volume on the inside
BOYLES LAW which will increase the pressure inside making the
In the 1700's a number of people pressure greater than on the outside and fluids are
investigated gas behaviour in the forced out.
laboratory. Robert Boyle A more life dependent example of Boyles Law is the
investigated the relationship action of the diaphragm of our body. This is a muscle
between the volume of a dry ideal that is located just below the lungs. When we inhale
gas and its pressure. Since there are the diaphragm moves downward allowing the lungs
four variables that can be altered in a gas sample, in to increase their volume. This decreases the pressure
order to investigate how one variable will affect inside the lungs so that the pressure is less than the
another, all other variables must be held constant or outer pressure. This results in forcing air into the
fixed. Boyle fixed the amount of gas and its lungs. When we exhale the diaphragm moves upward
temperature during his investigation. He found that and decreases the volume of the lungs. This increases
when he manipulated the pressure that the volume the pressure inside the lungs above the pressure on

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the outside of the lungs so that gases are forced out CHARLES LAW
of the lungs. Of course, all of this is totally automatic
and we take this important cycle which is performed Jacques Charles investigated the
hundreds of times a day for granted until we receive relationship between the Volume
a sharp blow to that region that briefly paralyzes the of a gas and how it changes with
diaphragm muscle. We say the wind was knocked out temperature. He noted that the
of us, but Boyles Law was not allowed to function. volume of a gas increased with the
temperature. Charles's Law states that the volume of
A practical application of this is the following a given amount of dry ideal gas is directly
question: proportional to the Kelvin Temperature provided the
amount of gas and the pressure remain fixed. When
If 50 ml of oxygen gas is compressed from 20 bar of we plot the Volume of a gas against the Kelvin
pressure to 40 bar of pressure, what is the new temperature it forms a straight line. The
volume at constant temperature? mathematical statement is that the V/T = a constant.
Simply set up a data table to identify the relevant For two sets of conditions the following is a maths
variables. First, let's rewrite the above question; statement of Charles's Law:
identifying the variables:
V1/T1 = V2/T2
If 50 mL (V1) of oxygen gas is compressed from 20 bar
(P1) of pressure to 40 bar (P2) of pressure, what is the An example of Charles's Law would be what happens
new volume (V2) at constant temperature? when a hot air balloon has air heated. The air expands
and fills the balloon. It becomes less dense (lighter
Plug the above values into the equation P1 V1 = P2 V2
than ambient air surrounding it). Of course, other
20 bar X 50 ml = 40 bar X V2 physical principles cause the balloon to rise against
the gravitational force. As the air inside the balloon
Solving for V2, we get 25 ml as our answer. expands the balloon gets bigger and displaces more
air. The displaced air produces a buoyant force that
Here is one for you to try:
counters the gravitational force and causes the
If a gas sample in a balloon had a volume of 100 ml balloon to rise.
and a pressure of 3 bar was compressed to a pressure
of 10 bar, what would be its volume? Assume the
temperature remains fixed.

Figure 9

So, given a problem:

A gas occupies a volume of 100 mL at 300 K (All

temperatures HAVE to be converted to Kelvins). At
what temperature will the gas have a volume of
200ml?

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Let's rewrite that, identifying the variables: A gas cylinder containing explosive hydrogen gas has
a pressure of 50 bar at a temperature of 300 K. The
A gas occupies a volume of 100 ml (V1) at 300 K (T1). cylinder can withstand a pressure of 500 bar before it
At what temperature (T2) will the gas have a volume bursts, causing a building-flattening explosion. What
of 200 ml (V2)? is the maximum temperature the cylinder can
Setting up the problem, we have: withstand before bursting?

100 ml/300K = 200 ml/T2 Let's rewrite this, identifying the variables:

After cross-multiplying to solve for T2, we get 600 K. A gas cylinder containing explosive hydrogen gas has
a pressure of 50 bar (P 1) at a temperature of 300 K (T
Now it is your tum: If the volume of a gas sample is 1). The cylinder can withstand a pressure of 500 bar
500 ml at 250C what will be its volume at 500C. (P2) before it bursts, causing a building-flattening
explosion. What is the maximum temperature the
cylinder can withstand before bursting?

Plugging in the known variables into the following

mathematical expression

50 bar/ 300 K = 500 bar/ T2

50 bar * ( T2) = (500 bar)* (300 K)

T2 = (500 bar)* (300 K)/ 50 bar = 3000 K

we find the answer to be 3000 K.

GAY LUSSACS LAW

Gay-Lussac investigated the

relationship between the
Pressure of a gas and its
temperature. At constant
Volume, the pressure of a gas
sample is directly proportional to
the Kelvin Temperature. The
relationship is similar to the Volume-Temperature
relationship (Charles's Law). The mathematical
statement is as follows:

P1/T1 = P2/T2
Figure 10
An example of Gay-Lussac application is an autoclave.
This is a chamber that sterilizes

medical equipment.

Suppose we have the following problem:

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Now lets try a problem: The mathematical expression for the Combined Gas
Problem: If the pressure of a chamber was 100 bar at Law is as follows:
600K, what will be the temperature of the chamber if
the pressure was raised to 600 bar? P1 V1 / T1 = P2 V2 / T2

Given the problem:

A gas occupies a volume of 20 L at a pressure of 5 atm

and a temperature of 500K. What will the volume be
if both the pressure is raised to 10 atm and
temperature is changed to 250K?

Let's rewrite to put in the variables:

A gas occupies a volume of 20 L (V1) at a pressure of

5 atm (P1) and a temperature of 500K (T1). What will
the volume (V2) be if both the pressure is raised to 10
atm (P2) and temperature is changed to 250K (T2)?

Substituting into the equation

P1 V1 / T1 = P2 V2 / T2
COMBINED GAS LAW we get:
With Boyles, Charles, and Gay Lusaac's Law there are
(5 atm ) (20 liter) / 500K = (10 atm) ( V2) / 250K
only two variables that are allowed to change. The
other two variables were held fixed or constant. This Solving for V2,
is rather unrealistic since in most cases a sample of
gas will be under the influence of all three of the other (5 atm) (20 L) (250 K) / ( 10 atm) (500 K) = V2 = 5 L.
variables changing. When this happens we are
Now let's see if you can do one:
dealing with the Combined Gas Law.
A gas occupies a volume of 200 litres at a pressure of
2 atm and a temperature of 300 K. What will be the
volume if both the pressure is raised to 10 atm and
the temperature is changed to 1000 K?

Figure 11

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Gas Laws Questions reducing the rate of evaporation of moisture from the
skin.
1) Charles' Law deals with what quantities?

2) If 5.0g of an ideal gas occupies 9.2L at

Standard temperature and pressure (STP).
What volume would it occupy at 120°C?

3) 10cm3 of a gas has a temp of 200°k. If the

temp is increased to 300°k, what will be the
new volume?

4) According to Charles' law which of the

following happens when the temperature of
a gas increases, as the pressure remains
The maximum amount of water vapour that the air
constant?
can hold depends entirely on the temperature; the
higher the temperature of air, the more water vapour
5) A gas is collected and found to fill 2.85L at
it can absorb. By itself, water vapour weighs
25°C. What will be its volume at standard
approximately five-eighths less than an equal volume
temperature?
of perfectly dry air.

6) What is the proper equation for Charles' law? Therefore, when air contains 5 parts of water vapour
and 95 parts of perfectly dry air, it is not as heavy as
7) A 250cm3 sample of neon is collected at air containing no moisture. This is because water is
44.0°C. Assuming the pressure remains composed of hydrogen which has an atomic mass of
constant, what would be the volume of the
1 (an extremely light gas) which replaces nitrogen
neon at standard temp?
which has an atomic mass of 14, i.e. nitrogen is 14
8) A closed gas system initially has volume and times heavier than hydrogen.
temperature of 6.42L and 779K with the
pressure unknown. If the same closed system Assuming that the temperature remains the same,
has values of 1.98 bar, 1.89L and minus 400C the density of the air will vary with the humidity,
(-400C), what was the initial pressure? which in turn affects the pressure.

9) A closed gas system initially has pressure and On humid days, the density is less than it is on dry
temperature of 8.2 bar and 5770C with the days; hence the pressure on a humid day is less than
volume unknown. If the same closed system that on a dry day. As the air becomes more humid, its
has values of 4.5 bar, 8.09L and 3200C, what density decreases. The higher the temperature, the
was the initial volume in L?
greater amount of water vapour the air can hold. The
amount of water vapour in the air can be measured
HUMIDITY by using a hygrometer. Air is allowed to flow across a
'wet bulb' thermometer and a 'dry bulb'
Humidity is the amount of water vapour in the air. thermometer. The wet bulb has some damp cotton or
Water vapour is the gaseous state of water and is cloth wrapped around the bulb. If the air is essentially
invisible. Humidity indicates the likelihood of dry, the water in the wet material around the wet
precipitation, dew, or fog. Higher humidity reduces bulb can evaporate, causing a local drop in
the effectiveness of sweating in cooling the body by temperature and the wet bulb will read a lower
temperature than the dry bulb thermometer.

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much as air. This means that in high humidity

conditions the density of the air is less than that of dry
air.

Dewpoint is the temperature at which air reaches a

state where it can hold no more water. When the
dewpoint is reached, the air contains 100 percent of
the moisture it can hold at that temperature, and is
said to be saturated. If the temperature drops below
the dewpoint, condensation occurs.

VISCOSITY

An important property of air in so far as it affects

flight is its viscosity. This means the tendency of one
Figure 12
layer of air to move with the layer next to it; it is
The difference in temperature between the two rather similar to the property of friction between
thermometers is compared with values on a chart to solids. The viscosity of a fluid or gas is a measure of its
find the percent of relative humidity. This resistance to change of shape or movement.
measurement is the ratio of the amount of water the
air will hold at this particular temperature.

For practical work in aviation, temperature and dew

point are used more often than relative humidity to
measure the amount of water vapour in the air. The
dew point is the temperature to which a body of air
must be lowered before the water vapour condenses
and becomes a liquid. Water vapour is invisible and
once the dew point is reached, it condenses and
becomes visible, or in another way, appears as steam
or a cloud.
Air has very low viscosity (64 times less viscous than
Absolute humidity refers to the actual amount of water), but its viscosity has a powerful effect, for
water vapour in a mixture of air and water. The example the drag force on an aircraft. The viscosity of
amount of water vapour the air can hold varies with air is affected by temperature but not density.
air temperature. The higher the air temperature the
Surprisingly, air becomes more viscous as the
more water vapour the air can hold.
temperature increases, unlike liquids which become
Relative Humidity is the ratio between the amount of less viscous.
moisture in the air to the amount that would be
For example - when oil is heated the molecules move
present if the air were saturated. For example, a
away from each other enabling ease of movement;
relative humidity of 75 percent means that the air is
when the molecules in air are heated they move
holding 75 percent of the total water vapour it is
closer together and become more erratic which
capable of holding.
resists movement. The colder the air, the less viscous
Relative humidity has a dramatic effect on aeroplane it becomes, and the resistance to change of shape is
performance because of its effect on air density. In reduced.
equal volumes, water vapour weighs 52 percent as

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THE INTERNATIONAL STANDARD ATMOSPHERE existing at 40° latitude north, and is represented in
the following table:
Since the temperature, pressure, and density of the
atmosphere change from place to place and from day Note, as the temperature remains constant, the air
to day it is necessary to have a standard set of density is directly proportional to the air pressure.
conditions with observed readings that can be
compared, which facilitates the correction of aircraft The standard sea level conditions are:
and engine performance calculations for any non- Pressure at zero altitude
standard conditions. The set of standard conditions is
usually known as the International Standard • 1'bar' or 1 'atmosphere'
Atmosphere (ISA). In the aircraft industry this is
• 29.92 inHg -inches of mercury or 760mmHg - mm
known as the ICAO Standard Atmosphere. ICAO
of mercury
stands for international Civil Aviation Organisation. If
the performance of an aircraft is computed, either • 14.69 psi - pounds per square inch
through flight tests or wind tunnel tests, some
standard reference condition must be determined • 1013.25 mb - millibars
first in order to compare results with those of similar
• 101.32 kPa - kiloPascals or 1013.2 hPa -
tests.
hectoPascals
The conditions in the atmosphere vary continuously,
Temperature at zero altitude
and it is generally not possible to obtain exactly the
same set of conditions on two different days or even • 15°C or 288°K or 59°F
on two successive flights. Accordingly, there must be
set up a group of standard conditions that may be Air Density at zero altitude
used arbitrarily for reference.
• 1.225 kg/m3

Gravity at zero altitude

• 32.174 ft/sec2

• 9.80665m/s2

Actual pressure varies at any given point in the

atmosphere. On a standard day, at sea level, pressure
will be 1013mb. On non-standard days, pressure at
sea level will vary considerably above or below this
figure. In Ireland, it can be as low as 960mb during the
winter and as high as 1040mb during the summer. As
altimeters are used to measure pressure and relay
height/altitude to the pilot, can you see why some
international rules had to be devised in order to avoid
air traffic collisions?

Table 1

The set of standard conditions presently used are

known as the International Standard Atmosphere
(ISA) This approximates the average conditions

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ALTIMETRY BASICS QNH refers to the aircraft's height above sea level.
When this is used, the altimeter will display the height
One of the more obvious problems to be overcome above the sea. When the aircraft is parked at the
when we try to relate atmospheric conditions to airfield at this setting, the altimeter will display the
flying an airplane is that of knowing at what precise elevation of the airfield above sea level. The air traffic
altitude we are. The instrument used to establish this control will advise the pilot to set the altimeter to a
is the altimeter. An altimeter can measure height reading that is the airfield barometric pressure
above just about any chosen reference point. It is of adjusted to that of local sea level.
little comfort to us if we are all using any convenient
reference by individual choice. The problem is Above a point known as the transition altitude,
overcome by using three established references. normally around 5,000ft in Ireland, higher in the US,
QFE, QNH and Flight Level (QNE). This involves setting all aircraft set their altimeters to a setting of 1013.2
the altimeter to read height above one of the chosen millibars. This is called Flight Level (QNE). This is in
references in cooperation with the local air traffic effect the barometric pressure existing at ISA mean
advisory service. This is done by adjusting the sea level. It may not reflect the actual sea level
altimeter to an advised barometric pressure setting pressure for the day, it is in fact an ISA figure. This
that is displayed in a small window in the instrument. ensures that all aircraft operating above the
The reading is shown in millibars in Ireland/UK but transition altitude are using the same setting and thus
some American operators like to use inches of there will be no confusion in establishing separation
mercury (Hg). There is a proposal to alter this to altitudes. If you hear that an aircraft is at Flight Level
Hectopascals in the future. For our purposes we will 80 for example, you will know that it is 8,000ft above
use millibars. mean sea level ISA. Though the code is QNE it is more
often than not just referred to as Flight Level. All
pilots put this setting in as a routine above transition
altitude and not necessarily under instruction from air
traffic control.

The Transition Level (lowest flight level above the

transition altitude) depends on the local QNH, which
can be different throughout the country.

below 977 977 to 994 995 to 1012 1013 and above

Figure 13 FL75 FL70 FL65 FL60

QFE refers to the aircraft's height above the airfield. PRESSURE ALTITUDE
When this is used, the altimeter will display the height
above the airfield and will thus read zero feet when This is the altitude in the standard atmosphere
the aircraft is parked on the airfield. The air traffic corresponding to a particular value of air pressure.
controller will pass the barometric pressure at the The aircraft altimeter is essentially a sensitive
airfield elevation to the aircraft and the pilot will set barometer calibrated to indicate altitude in the
the altimeter accordingly. Sometimes the term QFE standard atmosphere. With the altimeter of an
Threshold is used. This is the barometric pressure at aircraft set to 1013mb, the dial will indicate the
the airfield elevation converted to that existing at the number of feet above or below a level where 1013mb
approach end of the airfield's runway. The altimeter exists; not necessarily above or below sea level,
will show height above the threshold and will read unless standard day conditions exist. In general, the
zero feet on touch down. altimeter will indicate the altitude at which the
existing pressure would be considered standard
http://www.youtube.com/watch?v=-Dvsh-udkJQ pressure.

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Module 8 IT Carlow 2020/21 Basic Aerodynamics

Pressure altitude is the indicated altitude when an that the speed of sound in air will also reduce as
altimeter is set to an agreed baseline pressure setting. altitude increases up to the tropopause. Remember,
it is only the air temperature that affects the speed of
SPEED OF SOUND IN AIR sound in air.
The speed that sound travels in air is 331 m/s at sea MACH NUMBER
level when the air temperature is 00C. Sound travels
four times faster in water and fifteen times faster in
steel. From this you would imagine that air density
would affect the speed of sound in air. Strangely, it
does not. The reason is found in the relationship that where M is the Mach number, v is the velocity of the
the velocity of sound in air has with the ratio of air source relative to the medium, and vsound is the speed
pressure and density. of sound in the medium. So, in aviation, Mach
number relates the true airspeed of an aircraft to the
Speed of sound, a = (dp / dρ)1/2
local speed of sound in air.
If the air pressure were to rise then so also would the
The figure is derived by dividing the aircraft's true air
density. In fact, if you doubled the air pressure, the
speed by the local speed of sound in air. For an
density would also double. The ratio does not alter.
aircraft climbing at constant airspeed the gradual
However, this ratio only holds at constant air
reduction in the value of the speed of sound in air
temperature. If the air temperature were to reduce
means that the aircraft's Mach meter will record a
at constant pressure it would increase the air density
gradual increase in the Mach number displayed. This
without altering its pressure. The ratio then changes.
increase is a true indication of the aircraft's increasing
If we look at the relationship again we would see that
Mach speed.
the speed of sound in air would reduce with a drop in
temperature and increase with a rise in temperature.
Put simply, the Speed of Sound in Air is Proportional
to the Absolute Air Temperature.

Figure 14 – Charting increased mach at higher altitudes for

constant true airspeed.

Another problem encountered because of the change

in air pressure and density is in establishing how fast
an aircraft is flying.

Air temperature falls by 1.98°C for every 1,000ft

increase in altitude up to the tropopause, so it follows

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Module 8 IT Carlow 2020/21 Basic Aerodynamics

What is airspeed? True Airspeed : TAS

Airspeed is the speed of an aircraft relative to the air. This is the actual speed of the air rushing by. The TAS
In other words: How fast is the aircraft moving of an aircraft is the speed of the aircraft relative to the
through its surrounding air. air mass in which it is flying. The true airspeed is
important information for accurate navigation of an
There are several different measures of airspeed. aircraft.
Indicated airspeed (IAS) and true airspeed (TAS) are
the most common ones. TAS is the true measure of aircraft performance in
cruise, thus listed in aircraft specs, manuals,
How to measure it? performance comparisons, pilot reports, and every
It is measured within the flying aircraft with an situation when actual performance needs to be
airspeed indicator. This device is connected to ram air measured. It is the speed normally listed on the flight
pressure from outside the aircraft and compares it to plan, also used in flight planning, before considering
non-moving air pressure outside the aircraft. The ram the effects of wind.
pressure is sampled by a device called a pitot tube, Indicated Airspeed : IAS
carefully located clear of the propeller blast and other
airflow distortions. Quite often there are more than This is the airspeed indicated on the instrument. This
one of these static ports carefully located on the indication is only equal to the true airspeed under
outside of the aircraft. standard conditions.

The Instrument The only speed that really exists is the TAS. The only
speed that is indicated and therefore used to fly the
The primary way of measuring airspeed is through the airplane is the IAS.
measurement of dynamic air-pressure. This pressure
corresponds to a speed relative to the air around the Effects of altitude
airplane. By calibrating a pressure sensor, the
With higher altitude, the pressure decreases and so
airspeed can be displayed on the Airspeed Indicator.
does the temperature. As a result, a higher true
Due to the laws of physics, this pressure also depends airspeed needs to be obtained to result in the same
on density of the air involved. For calibration indicated airspeed. Or the other way around, when
purposes, the air is kept at a standard density. This is climbing at constant indicated airspeed, the true
the density at sea-level in the 'ICAO standard airspeed increases.
atmosphere'.
True airspeed is impossible to measure, but can be
In the 'ICAO standard atmosphere', the air pressure calculated by measuring the IAS, air-pressure and
and temperature at sea level are : 1013.25 hPa and temperature.
15° C. The instrument is calibrated at these standard
conditions, and under these conditions, the indicated
speed will be equal to the actual speed of the airmass
measured. Any deviations from standard conditions
and corrections need be applied.

Figure 15

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Module 8 IT Carlow 2020/21 Basic Aerodynamics

Questions 22 Explain 'Pressure Altitude'

1 State the composition of the atmosphere in MCQ’s

percentage terms.
1. The altitude of the lower Tropopause according to
2 List the 4 ‘sphere’ layers of the atmosphere closest the International Standard Atmosphere is:
to the earth’s surface
a. 18,090ft
3 What is the temperature range of the stratosphere? b. 36,090ft

4 There are 3 (THREE) atmospheric layers ending in ' c. 56,090ft

... pause', why are they called that?

5 Between what heights does the third atmospheric 2. As the humidity of the air increases its density will:
layer exist?
a. increase
6 Why does the majority of weather occur in the b. remain constant
lowest atmospheric layer?
c. decrease
7 Define Pressure.

8 State the ISA Pressure in millibars and psi. 3. The oxygen content of the lower atmosphere is:

9 What does 29.92 inHg actually mean? a. 21% by volume

b. 28% by weight
10 What is the density of dry air at mean sea level?
c. 78% by volume
11 Which is heavier, moist or dry air?

12 State how density varies with pressure. 4. At what altitude does the Stratopause commence
13 State how density varies with temperature. according to the International Standard
Atmosphere?
14 Explain the term 'lapse rate' and state its value.
a. 62,000ft
15 The temperature at an altitude of 4,500ft is -120C, b. 50km
what is the temperature at sea level?
c. 22km
16 The temperature at an altitude of 9,500ft is -16 C, 0

what is the temperature at 1,000ft above sea level?

5. The barometric pressure at sea level ISA is:
17 Explain why water vapour has a different weight
a. 1013.25mb
per volume than dry air.
b. 0.1kPa
18 Explain how a Hygrometer works.
c. 101.325mb
19 Explain 'Dew point'

20 How does the ISA day help to maintain aviation 6. The rate of decrease of air density with increasing
standards around the world? altitude in the Troposphere in comparison to that in
the lower Stratosphere is:
21 What are the operational differences between an
Aneroid and a Mercury barometer? a. faster

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Module 8 IT Carlow 2020/21 Basic Aerodynamics

b. slower 12. The ISA value for the temperature at the

Tropopause is:
c. similar
a. 56.50C

7. A millibar is a unit of: b. -150C

a. air density
c. -56.50C
b. pressure altitude
c. barometric pressure
13. The barometric pressures quoted for the
International Standard atmosphere are:
8. The temperature in the Stratosphere:
a. absolute
a. increases with altitude
b. decreases with altitude b. gauge

c. remains constant c. dynamic

9. As altitude increases from sea level to the

14. The Q codes for barometric pressure corrections
Tropopause the ratio of the percentage volumes of
are:
oxygen to nitrogen in the atmosphere:
a. increases a. QEF, QEN, QFI

b. remains the same b. QNH, QFE, QNE

c. decreases c. QHN, QHG, QIF

10. QNH refers to height above: 15. The term Flight Level 50 means:

a. an airfield a. 50,000ft above ISA sea level

b. sea level b. 5,000ft above airfield elevation

c. a safe approach altitude c. 5,000ft above ISA sea level

11. The percentage of water vapour that can be held 16. The value of the speed of sound in air is:
in the air will:
a. proportional to the absolute air temperature
a. increase with an increase in air temperature
b. inversely proportional to the air temperature
b. decrease with an increase in air temperature
c. proportional to air density
c. be unaffected by air temperature
17. If air temperature remains constant its density
is:

a. inversely proportional to the barometric pressure

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Module 8 IT Carlow 2020/21 Basic Aerodynamics

b. directly proportional to the barometric pressure 22. The instrument used to measure absolute
pressure would be a:
c. remains constant with variation in barometric
pressure a. manometer

b. hydrometer

18. As altitude increases up to the Tropopause c. barometer

temperature:

a. does not decrease at a constant rate 23. At what altitude would the barometric pressure
be half that of sea level pressure under ISA
b. does decrease at a constant rate conditions:
c. increases exponentially a. 18,000ft

b. 33,000ft
19. The indicated airspeed IAS at altitude will be: c. 12,000ft
a. higher than the true air speed

b. the same as the true air speed 24. If an aircraft is climbing at a constant true
c. lower than the true air speed airspeed its Mach No. will:

a. not alter

20. If an aircraft is flying at a given true altitude b. decrease

when the barometric pressure is below the ISA value c. increase
for that altitude the indicated pressure altitude will
be:

a. higher 25. The density of air at sea level ISA conditions is:

b. lower a. 1.056kg/m3

c. the same b. 1.225kg/m3

c. 0.225kg/m3

21. If a gauge pressure of 20psi is observed under

standard ISA sea level conditions, the absolute
pressure is:

a. 5.3psi

b. 34.7psi

c. 14.7psi

18