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☺ Physics & Aerodynamics ☺
FRICTION
Friction is a part of every day life
...
It occurs when two surfaces rub together and may be a desired effect (good
tyre traction) or an unwanted side-effect (friction in bearings)
...
In
other situations considerable investment is made in attempting to reduce the friction: bearings, pistons
in cylinders etc, using smooth surfaces, lubrication etc
...
Friction also causes wear as surfaces rub together, affecting performance
...
When two such surfaces are presented to each other it is analogous to a relief m-ap of one
mountain range being turned over and placed on another
...
When one surface is drawn over the other not only are these cold
welds broken initially, but considerable ripping and tearing of these microscopic peaks occur during
the subsequent motion
...
They have been
greatly magnified to show how friction occurs
...
Other surfaces brought
into contact may not cold weld together but will still produce heat as a result of deformation of the
peaks during motion
...

-1-

☺ Physics & Aerodynamics ☺

The four most quoted laws are:
1
...

2
...

3
...

4
...

This last law is true for a wide range of speeds provided that any increase in speed is not sufficient to
produce enough heat to cause a change in the surface composition
...
Steel on ice (ice
skating) behave very differently from rubber on concrete (tyres on the road) or metal on metal
(bearings etc)
...

-2-

☺ Physics & Aerodynamics ☺
So, for example, provided that the composition of the materials remains unchanged rubber on tarmac
will always give the same ratio
...
This constant is known as the ‘coefficient of sliding
friction’ and is given the Greek symbol μ (mu)
...

The lower the value of μ the lower are the frictional forces involved
...

MATERIALS

COEFFICIENT OF FRICTION μ

Metal on metal (dry)

0
...
15

Metal on Metal (lubricated)

As low as 0
...
5 – 0
...
Consider each of the following cases, where the
body is moving at a constant velocity
...

(Note that for simplicity, the four main forces are considered as acting through a single point in the
body)
...


-4-

☺ Physics & Aerodynamics ☺

Forces parallel to the incline are:

F = Wsinθ
Forces normal to the incline are:

∴ R N = Wcosθ
Again since

μ=

μ=

F
RN

W sin θ
or tanθ
W cos θ

-5-

☺ Physics & Aerodynamics ☺
Rolling Friction
If a vehicle is to be moved, driven by its wheels, then the vehicle will only move if there is a friction
force between the wheels and the surface on which they rest
...
Without this, the wheel would simply spin as occurs on an icy surface
...
In the first (figure 6) the wheel is being pulled by means of a
tow rope in the direction of the applied force
...

In the second example, the wheel is being driven by a torque ‘T’ applied to the axle
...
If the wheel radius is ‘r’ then for equilibrium T =
F x r
...
It is not a resistance to the motion of the vehicle
...
If brakes get wet or contaminated with oil then the coefficient drops and the brakes can become
next to useless
...

Tyres also need a high coefficient of friction consistent with good tyre life
...
A good depth of tread
helps reduce the problem of aquaplaning
...
Oils,
greases and air (under pressure) are used to help keep sliding surfaces separate and reduce the
coefficient
...
When torque loading nut and bolts, if the
threads are lubricated, then the effective torque on the thread is increased for any given torque wrench
setting
...

FLUID DYNAMICS
The term ‘fluid’ refers to both the liquid and gaseous state of any substance
...
In any fluid there is an internal friction force present between each layer of
molecules as they slide over one another
...
This is due to the
molecules being further apart following expansion, which in turn, allows them to move past each other
with greater ease
...
The higher the viscosity the more the fluid will act in a
sluggish and slow manner
...
Too high a value of
viscosity will produce a high resistance to flow
...

Viscosity usually changes with temperature; the higher the temperature the lower the viscosity and the
oil flows easier
...

-7-

☺ Physics & Aerodynamics ☺
For engines ‘viscostatic’ engine oils are now designed to have nearly the same viscosity over a wide
range of engine operating temperatures
...

The flow of fluids is often considered to be of two main forms, the first is a flow in which all the
molecules travel in parallel layers (called laminar flow), much like the pages of a book, with no
interchange between layers occurring
...
The slowest moving layer being
next to the solid surface with which it is in contact
...
It is a complete random chaotic motion,
in which particle motions are continuously varying in an unpredictable manner
...
For example when air flows over the aircraft
in flight drag is reduced by laminar flow and lift is improved with laminar flow over the wing surfaces
...
This may be visualised by
reference to figure 8, where fluid is leaving a pipe of cross- sectional area a (m2), with a velocity of v
(m/s)
...

Since the rate of flow ‘Q’ is defined as the volume passing by in one second, then:
Q = av m3/s
If the flow is subsequently directed through a tapered tube as shown in figure 9, then the quantity
entering must (in the case of liquids, assuming no compression) be the same as that leaving
...

Compressibility
Compressibility occurs in all fluids at all pressures, but only under very high pressures are liquids
noticeably compressed
...
However, gases are easily
compressed as well as being affected by temperature changes
...
However when a body moves through air at low speeds, including low speed flight, the
amount of compression is so small that for most calculations the air is considered to act as if i is
incompressible
...

BERNOILLI’S EQUATION
It can be said that, unless work is done on a fluid when it moves from one point to another the total
energy level at any point on its journey remains constant
...
With fluid flow, three types of energy are usually considered
...

(a) Pressure energy
...
But in physics it has to be considered
...
This is energy stored in a moving body
...
This is the energy a body possesses by virtue of its position above some datum
...
Applying Bernoulli’s equation
Total energy at entry = Total energy at exit [pressure energy + kinetic energy + potential energy] 1 =
[pressure energy + kinetic energy + potential energy]2
1
Pm 1
+ mv12 + mgh1 = 2 + mv2 2 + mgh2
ρ1 2
ρ2 2

Pm
1

Ignoring losses between 1 and 2
Since mass is common throughout the equation:

Multiplying throughout by ρ (rho) (assuming that p remains constant)
...
The term ‘P’ is often referred to as ‘static pressure’, and
1/2 pv2 as ‘dynamic pressure’
...
(h1 = h2)
Now to satisfy continuity, the velocity of the flow must increase if the area reduces
...
For the total energy at any point in the system to be the
same, it follows that the static pressure at exit, P2 must be less than that at entry P1
...

The Venturi Tube
The venturi tube is a practical application of Bernoulli’s equation
...
The venturi has a reduction in cross sectional area from the mouth
of the tube to the throat, with a gradual increase in cross section from the throat to the outlet designed
to avoid turbulence
...
(With gases
the manometer tubes are replaced by u-tubes often containing mercury)
...

Figure 10 shows a diagram of the venturi tube
...


Relative Density
Before leaving this section on fluids one further unit has to be known - Specific Gravity or Relative
Density
...
Relative density, as its
name suggests, is the ratio of the density of a substance to some standard density
...
For a gas, the standard density may be that of air
or that of hydrogen, although for gases the term is little used
...

The density of pure water is 1000 kg/m3, thus, given that aluminium has a density of 2700 kg/m3 its
relative density is 2
...
Similarly ice having a density 920 kg/m3 will have a relative density of 0
...


- 12 -

☺ Physics & Aerodynamics ☺
Temperature
In an earlier section we considered that all matter was made up of molecules which were in constant
motion and thus possessed kinetic energy
...
If it were possible to measure this kinetic energy then a direct measurement of
temperature could be made
...
The thermometer is an instrument that
measures the increase of molecular kinetic energy in terms of the expansion of either mercury or
alcohol
...
This bulb is
filled with either mercury or coloured alcohol before the small end of the tube is sealed
...

The height of the liquid column is an indication of the temperature
...
The distance between them is then divided into a number of divisions
called ‘degrees’
...
A Swedish
astronomer who, in 1742 used melting ice and boiling water s the two ‘fixed points with 100 divisions
between them, each division being one degree Celsius (I° Under normal conditions ice melts at 00C
and under standard atmosphere pressure of 76Ommof mercury, pure water boils at 100°C
...
This
scale was named after Gabriel Daniel Fahrenheit (1686- 1736) a German physicist and instrument
maker, who invented the alcohol thermometer and the mercury thermometer
...

QUESTION :

Why did both physicists use the boiling point and freezing point of water as their
fixed points? (2 mins)

ANSWER:

These points are readily reproducible in a simple laboratory
...


- 13 -

☺ Physics & Aerodynamics ☺
On occasions it is necessary to make conversions between the two scales, these may be done using the
following equations
...

William Thomson Kelvin (Lord Kelvin) (1824 - 1907) showed that a temperature scale could be
devised that was independent of the properties of any individual substance
...
15°C (generally
considered as minus 273°C, see Charles Law)
...
The
size of each division (each degree) on the Kelvin scale was made the same as that on the Celsius scale,
so that 0°C equals 273K and 100°C equals 373K
...

So zero K (OK or minus 273°C) is the absolute zero temperature below which it is not possible to fall
further
...

It is interesting to note that this temperature has not actually been reached but scientists have come
close to it
...

Heat
Heat is energy in transit, ie energy on the move
...
Consider two bodies (figure 12) at different
temperatures
...
This heat flow will continue until a
state of equilibrium is reached, when both bodies will have the same amount of heat (be at the same
temperature)
...
For a 1°C rise in temperature water would require 4
...
46kJ
...

For aluminium this value is O
...
39OkJ/kg°C, which indicates that
copper requires less heat energy to reach the required temperature than aluminium
...

With gases, two values of specific heat capacity are considered; this is because the heat absorbed
depends on the conditions existing at the time
...
The specific heat capacity of a gas is thus defined as ‘the heat required to raise
1kg of gas at constant pressure (or constant volume) through 1°C’
...
72kJ/kg°C

For oxygen

cp = O
...
66kJ/kg°C

Heat Transfer
Heat transfer, or energy in transit, takes place in three ways:
* By conduction
...

* By radiation
...
As
heat reaches the surface molecules of a body, they increase their vibration
...
In this way the vibration is transmitted through
the body
...
Solids are, therefore, better
conductors of heat than liquids and liquids better than gases
...


Convection
...
Molecules in
contact with a ‘hot’ body absorb energy and then move off
...


A good example of convection is when air contacts a hot water radiator
...
This will
move around the room making it feel warm
...
In the last two forms of heat transfer, direct contact between the bodies involved was
needed
...
In fact, radiation does not
need a medium for heat energy transfer to occur and will travel through a vacuum
...
The energy is transmitted in the
form of electromagnetic waves traveling at 299,800 km/s in empty space
...
For example the air in a room
adjacent to an open fire receives heat energy by means of radiation and conduction, becoming less
dense it rises and moves around the room transmitting heat to surfaces it comes into contact with by
convection and conduction
...

The First Law of Thermodynamics
In a previous section the conservation of energy was stated as ‘energy cannot be created or destroyed
but only changed from one form to another’
...
But before stating this
law, it is necessary to define the concepts used, first the concept of the ‘System’
...
The
region outside the boundary is known as the surroundings
...
The system might be a person, a house or an aircraft or a
container of gas
...
Outputs include:
heat; noise; people; gases; waste material etc
...


A Closed System is one in which no transfer of anything (light, heat, noise, mass etc) occurs across the
boundary (into the system or out of it) even though the boundary may expand or contract
...
This will either compress the gas or allow the gas to expand (figure 16) ie no mass or heat
transfers across the boundary
...

- 17 -

☺ Physics & Aerodynamics ☺

It is important, for this section, to remember that both work and heat are energies in transit, they
cannot be stored
...
Since in our example, the cylinder is perfectly lagged, there is no heat flow into or out of the gas,
the intrinsic energy has therefore occurred as a result of the work done on the gas by the piston
...


Since the boundary is fixed no work can be done on the gas so if heat is added then the pressure and
temperature of the gas rises as does the intrinsic energy of the gas
...
This means that it is always the change of intrinsic
energy that is considered
...


- 18 -

☺ Physics & Aerodynamics ☺
In a closed system that is conducted through a non-flow process, during which heat ‘Q’ flows into the
system and work ‘W’ flows from the system, the First Law of Thermodynamics states, “The difference
between the sum of the heat flowing into a closed system, and the work flowing from the system, is
equal to the increase in the internal energy of the system”
...

Mathematically

Q

=

W

In this case, the First Law of Thermodynamics is stated as “when a closed system has passed through a
complete cycle, the sum of the heat energy taken in across the boundary from the surroundings is
equal to the work delivered from the system to the surroundings”
...
This law is based on the conservation of energy
principle which in turn is observed from natural events
...
This is because there must always be losses in a system as no
machine is perfect or has 100% thermal efficiency
...
An alternative statement of the Second Law may be “Heat, cannot of itself, move from
one body to a hotter body”
...


- 19 -

☺ Physics & Aerodynamics ☺
GASES
Properties of a Gas
A gas is made up of a large number of molecules which move about freely at various speeds in straight
lines, constantly in motion, and only being deflected from when in collision with other molecules or
the sides of the container to which they are confined
...
When confined in a
container the molecules strike the walls of the container and the sum of these minute forces produces
the effect known as pressure
...
Compare this to the definition of a liquid where the substance
takes up the shape but not necessarily the volume and the definition of a solid where it takes, up
neither the shape nor the volume
...
In the study of gases three variables are considered, pressure, volume
and temperature
...
given mass of gas a change made to any one of these results in a change to the
other two
...
The first relationship (Boyle’s Law) was discovered by Robert Boyle (British chemist 16271691)
...


Graphically this means that graph results, which passes by plotting values of P against , a straight line
through the origin (figure 18)
...

This means, for example, that if the volume is halved the pressure is doubled and the knowledge of
this relationship provides a way of calculating the new conditions of pressure and volume given the
original conditions
...
In (a) the piston is out and the gas is at a
certain volume (V1) and temperature (T1) and pressure (P1)
...

QUESTION:

How can the piston be pushed in without raising the temperature? (2 mins)

ANSWER:

Slowly
...
Try operating a
bicycle pump quickly with your finger over the air outlet end, the pump get-s hot
...
- (When a gas is compressed so that its temperature remains the same it is
called an Isothermal compression
...
)

So the piston in figure 19 is pushed in slowly, the temperature will remain constant but the pressure will
rise
...
It is important to note, at
this stage, that all values of pressure must be absolute, ie values taken from a complete vacuum (take the
gauge pressure and add 14
...


- 21 -

☺ Physics & Aerodynamics ☺
The second relationship for a gas was made by Jacques Charles (French physicist), who in 1787 studied
the relationship between the volume and temperature of a gas, whilst keeping the pressure of the gas
constant
...


The graphical representation of this relationship is shown in figure 20
...
It followed that the reverse was possible, a reduction of 1/273rd of the original
volume for every 1°C fall in temperature
...
15°C
...


It is only from this new origin that volume is directly proportional to the temperature, in other words,
values of temperature must be in absolute units (K)
...
If heat is applied to the cylinder then to keep the pressure
constant the piston must be moved out
...


As with Boyle’s Law,

To put °C into absolute temperature values (K) add 273°C
...
e

20°C = 20 + 273 = 293K

With reference to figure 22, it is of passing interest that the gas would have no volume at OK, or in other
words it would disappear
...

What we do know at these temperatures is that some conductors have zero resistance to current flow
...
Boyle’s Law and Charles’ Law are brought together and
called the Combined Gas Laws
...
The
result is that

PV
= a constant
...
If required, it can be introduced into the equation thus:

This new ‘constant’ is given the symbol ‘R’ and is known as the Characteristic Gas Constant
...
The value of ‘R’,
the Characteristic Gas constant, depends on the nature of the gas
...
287 kJ/kgK and for oxygen is 0
...
Hydrogen is 4
...

Specific Heats of a gas
Earlier in this unit, mention was made of the fact that a gas had two values of specific heat capacity
...
The
reason is that the expanding gas will do work and this output of energy must be balanced by an
additional input of energy in the form of heat
...
Different amounts of expansion will give different values of specific
heat, however, only two situations are generally considered
...


and
(ii)

The gas pressure is kept constant whilst the gas is allowed to expand
...


- 24 -

☺ Physics & Aerodynamics ☺
Work Done by an Expanding Gas
Consider a piston and cylinder arrangement shown in figure 23 below, in which 1 kg of gas is contained
at temperature TK
...
Now let the temperature of the gas increase by 1K
...
Let L metres be the
movement of the piston
...

For the initial volume

PV1

=

RT

and

for the final volume

PV2

=

R(T + 1)

Since

Work done

=

P(V2 -V1)

or

PV2 - PV1

=

R(T + 1) RT

or

RT+R-RT

Then by substitution,

Work done

Therefore Work done (W) = R (the characteristic gas constant)
- 25 -

☺ Physics & Aerodynamics ☺
In this arrangement, we are considering the raising of the temperature of the gas by 1K, increasing its
molecular energy and this is represented by cv for 1kg of gas
...
The internal gas engine (internal combustion engine), operates on a
cycle, the purpose being to obtain useful work
...

Such a cycle is shown below, represented on a pressure - volume diagram
...
As the piston moves from position 1 to 2 the gas is compressed, shown by a
rise in pressure as the volume decreases
...
From 3
to 4 the gas is shown as it expands driving the piston back down the cylinder, and resulting in a fall in
pressure
...
This diagram represents an ideal
cycle, the actual cycle is more banana shaped
...
From 3 to 4, during the expansion stroke, work is done by the gas and the area
below this curve to the base line represents the work done
...


When a gas is expanded or compressed in an engine cylinder it pvn = c where c is a constant (figure 25)
...
However two are of particular
importance, this occurs when ‘n’ equals either 1 or δ, where δ is the ratio of the specific heats cp and cv
ie when δ =

cp
cv

- 27 -

☺ Physics & Aerodynamics ☺

When n = 1, the expansion is called an Isothermal expansion, and is one in which the temperature of the
gas remains constant throughout the process
...
Pushing the bicycle pump handle in slowly, with the air outlet blocked
...

When n = 6, the expansion is called an Adiabatic expansion and is one in which no heat flow takes place
across the boundaries of the system
...
This is called an adiabatic compression)
...
Again
at the end of the expansion stroke, at bottom dead centre (BDC), the sudden fall in pressure and heat loss
occurs with little or no movement of the piston
...
Because of this the cycle is known as the constant volume
cycle
...


- 28 -

☺ Physics & Aerodynamics ☺
With reference to figure 27
...
Between 2 and 3, at
constant volume the pressure rises, as heat is taken in
...

For many engines the thermal efficiency ii is used for comparison
...

The Constant Pressure Cycle
This cycle is the ideal for the closed cycle gas turbine engine
...


Between 1 and 2 work input to the compressor occurs
...
Between 2 and 3 heat is
supplied from the heater (burners with the energy supplied by the fuel)
...
Between 4 and 1 heat is rejected in the cooler
...


- 29 -

☺ Physics & Aerodynamics ☺

As the P-V diagram shows, the heat supply and heat rejection processes occur at constant pressure,
hence the name given to this cycle
...

P
1

For both the constant volume and constant pressure cycle the working substance for an ideal cycle is air,
and y for air is constant and equal to 1
...

The Air Standard Cycle
This cycle is one in which the working fluid is considered as a fixed mass of air that undergoes a full
cycle as an ideal gas, that is, one that obeys the gas laws
...

To produce the rise in pressure at the combustion stage, heat is added at constant volume to the air
...
The air standard cycle is
a true thermodynamic cycle
...


- 30 -

☺ Physics & Aerodynamics ☺
THE HEAT ENGINE
The heat engine is a system which operates on a complete cycle and develops a net work output from a
supply of heat
...

The term ‘sink’, receiver or cold body refers to a reservoir to which heat may be rejected by the working
substance
...

Figure 30 shows a heat engine in diagrammatic form
...

THE HEAT PUMP
The heat pump operates on a cycle that works in the reverse direction to that of the heat engine
...
Figure 31 shows this in diagrammatic form
...


- 32 -

☺ Physics & Aerodynamics ☺
A Typical Cooling System
In a standard refrigerator system the refrigerant travels within a closed cycle in a sealed system of pipes
and components (figure 32)
...

The extraction of this heat produces the cooling effect in the refrigerator compartment or provides a cold
air stream for cabin conditioning purposes
...
It should
not get down to its freezing point* and its pressure should be above atmospheric pressure (but not too
high)
...

•

Freezing points of most refrigerants are between -56°C to -155°C (R12)
...


- 33 -

☺ Physics & Aerodynamics ☺
The refrigerant leaves the evaporator as a saturated vapour to be compressed by a pump (piston type on
some systems and centrifugal compressor type on others, electrically or air driven)
...

In the condenser the refrigerant gives off its heat to the cooling medium (usually air) and reverts back to
a saturated liquid
...

The surface heat exchanger on a refrigerator is a radiator attached to the back and the heat exchanger on
an aircraft is located within a duct so as to be cooled by ram air (assisted by an electric fan when the
aircraft is stationary)
...
If the evaporator gets too hot then,
then the gas in the sensing bulb will expand and more refrigerant will be allowed to pass through the
expansion valve
...

After the evaporator the refrigerant reverts to a liquid/vapour mix and the pressure drops
...

Note
...
On
most aircraft systems there is a liquid receiver in the system before the expansion valve, though many
text books do not show this
...

On some systems oil is introduced into the refrigerant just before the compressor for lubrication
purposes
...

Many substances have been used as refrigerants, however, they should have the desired properties of
being non-toxic especially in domestic use in case of leakage
...
In practice almost any liquid can be used (including water), although
cooling results would be poor
...
Refrigerants should be non-flammable although some, such as butane, propane,
ammonia and methyl-chloride, constitute a fire and explosion hazard
...
It is these physical properties that govern the size of the compressor and the system required
...
For small refrigeration units helium may also be
used, with ammonia and lithium bromide for larger units
...


•

Be non toxic, non corrosive and present a low fire and explosion risk
...


•

Specific enthalpy of vaporisation at the low temperature should be as high as possible
...


•

The refrigerant should not react with oil if the compressor is lubricated with oil in the refrigerant
(some systems introduce oil into the refrigerant immediately upstream of the compressor and
remove it with an oil separator immediately after the compressor
...
It works very
similar to the system described above with the condenser being used as the heater
...
In effect the heat pump transfers heat from the medium outside to inside the building with the
condenser getting hot
...

Some electronic equipment on (usually military) aircraft have to be heated at altitude when switched off
so as to maintain a good working temperature, but when switched on will generate too much heat and
need cooling
...


- 35 -

☺ Physics & Aerodynamics ☺
LATENT HEAT
At the start of this unit when molecular structure was considered, two forms of heat were briefly
mentioned, sensible and latent heat
...

When the addition of heat to a body does not cause a rise in temperature then this is called Latent Heat,
(hidden heat)
...

When a substance changes from a solid to a liquid then the latent heat required is known as ‘the latent
heat of fusion’, and is given the symbol Lf
...
For ice at 0°C to be completely melted into
water at 0°C, 335kJ of heat energy for each kilogram is required so, Lf for ice = 335kJ/kg
...
Both these values apply for water at standard
atmospheric pressure (ISA - International Standard Atmosphere at sea level)
...


To determine the amount of latent heat required to produce a change of state, the mass of the substance
involved is multiplied either by Lf, the latent heat of fusion, or the latent heat of vaporisation Lv ie:
For a change of state from solid to liquid, the amount of heat required:
=

Latent heat of fusion x mass of substance

=

Lfm (joules)

For a change of state from liquid to vapour or gas, the amount of heat required:
=

Latent heat of vaporisation x mass of substance

=

Lvm (joules)

- 36 -

☺ Physics & Aerodynamics ☺
Example
Calculate the amount of heat required to completely convert 2kg of ice, initially at -30°C into steam at
100°C (see graph)
...
1 kJ/kg°C

CWATER

=

4
...
1 kJ/kg°C x 2kg x (30°C) = 126kJ

Latent heat required to change the ice to water at 0°C
...

= CWATER x m x ∆t
= 4
...

= LV(WATER) x m
= 2257kJ/kg x 2kg
= 4514kJ

- 37 -

☺ Physics & Aerodynamics ☺
Total heat energy required = (126 + 670 + 840 + 4514)kJ = 6150kJ
Note
...

Heat of Combustion
During a chemical reaction chemicals may be formed or broken down into simpler units or elements
...

This heat is known as ‘the heat of reaction’
...

To determine, satisfactorily, the heat of combustion of a fuel an experimental method is often used,
using an apparatus known as the bomb calorimeter
...
Hence the units of ‘Heat of Combustion’ are Joules per kilogram
...

ELEMENT

HEAT OF COMBUSTION (MJ/kg)

Hydrogen

14
...
7

Sulphur

9
...

Note
...
l868joules
...
The air is made up of
approximately 21% oxygen (02) and 78% nitrogen (N) by volume, with the remaining 1% being made
up from other gases
...


- 38 -

☺ Physics & Aerodynamics ☺
Because of these variations and to allow standardisation and calibration of instruments and engine
performance figures etc, a Standard Atmosphere has been devised
...

Pitot static operated instruments can be calibrated using the standard atmosphere and they can be set
for flight using the same parameters
...
Such a standard atmosphere is therefore taken as the reference for these
parameters in free air (excluding those dependent on water vapour)
...
The pressure at sea level is 10 13
...
325kPa)
...
98°C per l000ft up to a height of 36,000ft
where the temperature will remain constant at -56
...
T-he value of g’ (gravity) is given a
uniform value of 9
...
Effectively that means that the temperature falls with
altitude at a rate of about 2°C per l000ft from 15°C at sea level to 36,000ft where it holds almost
steady at -56°C until about 36,000ft where the temperature starts to rise
...
For those working on large aircraft the atmosphere is of interest up to say 60,000ft or so
...

The pressure starts at 10 13mb (14
...
Losing most of its value at the lower altitudes so that at 18,000ft, for example,
the pressure is halved to 506mb
...
If an ordinary pressure gauge is open to
atmosphere it will read zero
...
7 = 44
...
The tyre pressure as measured by the gauge is called gauge pressure
...
7psi absolute
...
Starts at 1
...
Its rate of change is non-linear which means the graph is a curve
and the amount by which it drops changes with height
...
It
the temperature drops density will increase and if the RH increases the density will decrease
...

The RH falls with altitude
...


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☺ Physics & Aerodynamics ☺
RH is the amount of moisture that is in a unit volume (m3) of air compared to the maximum amount it
will hold (ie when it is saturated) - at that temperature
...
(It is not necessarily raining - it is just that the
air cannot accept any more moisture)
...
The wet bulb method uses two temperature thermometers - one that is kept wet by a wick
and one that is kept dry
...
Hygrometers can be “mechanical” or electronic
...
A
similar measure to RH but gives the value as a specific amount, eg 2
...
When air is continuously
cooled there comes a point when a temperature is reached which causes any moisture present to
condense out - this is called the Dew Point
...
25mb

237
...
325kN/m2 (more correctly kPa)

32°F

1O1325kPa
14
...
92inHg (inches of mercury)
760torr*
33
...
2inWG (inches of water gauge)
2116
...

Normal Temperature and Pressure (NTP) at sea level is defined as:
Pressure

Temperature

Density

As for STP above

20°C

1
...
15K

0
...
15K
DEW & FROST

Previously it was stated that one form of heat transfer was by radiation
...

If, at the same time, the air is saturated with moisture then this will condense out to form small droplets
of water on any cooled surfaces
...

In areas of the world such as deserts where the humidity levels are low, dew rarely forms even though
night-time temperatures can get well below freezing
...

Should this point occur below the freezing point of water (0°C) then the water droplets solidify into tiny
ice crystals forming Hoarfrost which covers surfaces with a white soft crystalline layer
...

Fog & Mists
Both are formed by small droplets of water suspended in the air
...
00 1mm, about 1/200th
the thickness of a human hair)
...
The cooling effect causes the moisture to form into
tiny droplets and clouds are formed
...


- 42 -

☺ Physics & Aerodynamics ☺
As with the growth of ice crystals, a fog or mist droplet requires a nuclei on which to condense and these
are often fine dust or salt particles carried in the air
...

When smoke particles combine with fog the result is Smog
...


- 43 -



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