Archives: Theory Courses

12.4.6 Explain how air density varies with altitude within the atmosphere.

12.4.6. Explain how air density varies with altitude within the atmosphere.

Density of air is measured by how many molecules are present in any given volume
Near the earths surface the density is higher than at greater altitudes. It reduces rapidly at lower levels and more slowly at higher levels.

Section 12.4 – Flash Cards

Aerohire - Flash Cards

Welcome to your Personalised Deck

of Study Cards

CardsStatus
18Yet to View
18
Section 12.4

Enjoy your study!

What is the International Standard Atmosphere (ISA).

What is the
International Standard Atmosphere (ISA).

Hypothetical set of atmospheric conditions which represents an average of the conditions experience worldwide.
We use as a yard stick / measuring stick to work out how our aircraft will performance in varying atmosphere conditions

Yard stick / Measuring sticks are …
Pressure Altitude.
Density Altitude.
By describing Pressure and Density as Altitude, it far more understandable to most pilot than as many hPa’s or kgs/m3. 

At sea level 
Pressure: 1013.2hPa (hectopascals)
Temperature: +15 degrees C
Density: 1.225kg/m3

temperature reduction with
an increase in Altitude
approximate temperature lapse rate up to the tropopause 

is 2C/1000ft (1.98c/1000ft)

Altitude Temperature
S/l 15c  (S/L Temp-(2 x 1000ft))
1,000ft 13c  = (15-(2×1))
 2,000ft  11c  = (15-(2×2))
 5,000ft  5c  =(15-(2×5))
 10,000ft  -5c  = (15-(2×10))

Air Density.

Density
Note Images

Density of air is measured by how many molecules are present in any given volume

High Density, more molecules in a given area. eg. a metre2

The more dense the atmosphere is the better your wings and engine work on  your aircraft

 [vfr_Pic p1=”density_high.png”]

Low Density, less Molecules in a given area.

 [vfr_Pic p1=”density_low.png”]
Now if we put some molecules into a big tube,
you can see that the molecules at the top of the tube
will be pushing down on the molecules below them,

thus the molecules at the bottom of the tube will be pushed together the closest.
thus are more densely packed.

This is the same as our earth’s  Atmosphere.

Density
Checking out the the Grey boxes,
The one that the bottom contains more molecules
 thus has more Density

and then grey box at the top with less molecules
thus has less Density.

So Density Decrease (along with aircraft performance) with an increase in Altitude. 

Temperature and Pressure

To easily work out what the temperature is you need to understand the effect that pressurising or creating a vacuum has on temperature

A bicycle tyre  pump or a tyre compressor they all heat up hence if air in compressed(pressure Increased) the air temperature will increase.


If you create a vacuum like inside your carburettor e.g. carb icing the temperature will decrease. 

So how to work out the pressure difference?

This is really simple just have a look at the distance between the molecules.

If the molecules are close together the pressure is high 

if that further apart the pressure is lower.

So what you notice by studying our column of air,

 is the pressure is higher thus temperature is greater at the lower levels.

 At the top of the column the pressure is lower and also the temperature is the coldest.


So Pressure Decreases with an increase in Altitude
and 
So dose Temperature Decrease with an increase in Altitude

 [vfr_Pic p1=”density_column.png”]

12.4 The Atmosphere

12.4 Atmosphere
Subject Key Points

12.4.2. Name the principal gases

which constitute the atmosphere.

Oxygen – 21%

Nitrogen – 78%
1% – others. neon, argon, CO2 etc 
12.4.4. Define air density. 
Density of air is measured by how many molecules are present in any given volume

Near the earth’s surface the density is higher than at greater altitudes 
12.4.6. Explain how air density varies with altitude within the atmosphere. 
Density of air is measured by how many molecules are present in any given volume

Near the earth’s surface the density is higher than at greater altitudes. It reduces rapidly at lower levels and more slowly at higher levels.
12.4.8. State the relationship between pressure/temperature and the density of an air mass. 
Pressure and temperature affect the density of any parcel of air.

High temperature and low pressure will result in low density.
Low temperature and high pressure will result in high density. 
12.4.10. Describe how pressure, temperature and density normally vary within the atmosphere.

The pressure, temperature and density will normally all decrease with increasing altitude

Pressure decreases rapidly at lower levels and more slowly at higher levels.
Temperature decreases at a constant rate up to the troposphere.
Density
decreases rapidly at lower levels and more slowly at higher levels.
(image) 
12.4.12. Explain the basis for the International Standard Atmosphere (ISA) – hypothetical set of atmospheric conditions which represents an average of the conditions experience worldwide

Sea level pressure = 1013.2hPa (hectopascals)
Sea level temperature +15 degrees C
Sea level density 1.225kg/m3 

Lapse
rates 

12.4.14. State the ISA sea level pressure and temperature conditions. 
ISA Sealevel pressure = 1013.2hPa

ISA Sealevel temperature = +15deg C
12.4.16. State the approximate temperature lapse rate up to the tropopause. 
The ISA temperature lapse rate up to the Troposhere is 1.98degC per 1000ft. 

Section 12.2 – Flash Cards

Aerohire - Flash Cards

Welcome to your Personalised Deck

of Study Cards

CardsStatus
56Yet to View
56
Section 12.2

Enjoy your study!

Inertia – Momentum

Inertia
Is the Latin word for inaction or laziness
 Object Mass is the proportionally the object’s Inertia. 
e.g. how hard it is to get an object moving or to stop / slow downed a moving object. 
Example of Inertia Let’s start off with a scooter (motorbike) it doesn’t take much energy to get the scooter moving
and you don’t have to run into much, let alone a power pole before coming to a stop.
 Scooter has a lot less mass than a train.
Train
on the other hand requires a lot of energy to get moving
and would probably knock over 2 or 3 power poles
before even look like slowing down let alone stopping

Momentum
Formula = Mass x Velocity
 Formula in action 1. Body 10 units of Mass  multiplied by moving at Velocity of 2 units = 20 units of Momentum. 
2. Body 5 units of Mass  multiplied by moving at Velocity of 4 units = 20 units of Momentum. 
The first one had a greater mass and second had a greater velocity
but they both had the same momentum
thus both would be equally difficult to stop.
Example of Momentum A train that has a large mass and a low velocity
compared with a bullet with a small mass and a high velocity
both will be difficult to stop
and both can do considerable damage to anything that tries to stop them.
An object with mass always has inertia,
however a body only has momentum while it is moving,
if body is stationary the momentum is 0.

Useful Velocity Vectors

 

Force – a push or a pull – identified by what it does moving an object out of its state of rest or of uniform motion in a straight line.
Force Vectors – usually drawn as an arrow, indicating a vector’s quantities of both magnitude and
direction 

 Adding VectorsVector-1 is right 6 units+ Vector-2 is right 2 units= resultant of right 8 units

Subtracting VectorsVector-1 is right 8 units+ Vector-2 is left 2 units= resultant of right 6 units
 

Couples – consists of two equal but opposite parallel forces which create a twisting moment about a point between the two force lines.

Components – these are the resolution of a force vector into two components at right angles to
each other. Used to show the amount of a total force acting in a particular direction. 
An Example: Weight and Lift Couple which in balanced by the Tail Plane.

 Couples
  Components – these are the resolution of a force vector into two components at right angles to each other. Used to show the amount of a total force acting in a particular direction.

An Examples: 
Reaction of the Wing, Components
of Lift and Drag
Reaction of the Propeller, Components of Thrust and Torque

 Resolving a Vector into componentscomponent-1component-2Vector

Centripetal Force Explained

By understanding the centripetal force  calculations you will see the big effect that speed has on your turning radius.


Let look at the centripetal force on mass EG no gravity.

( CPF = frac{Mass x Velocity 2 }{Radius} )

( CPF = frac{Mass x Velocity 2 }{Radius} )
Diagram
Mass(m) Increase Mass then there is a an Increase of Force  
Velocity(V2) Increase Velocity then there is an Increase in Force is now Squared.
E.g. Double the Speed with equal 4 time the Force.
100 units Double speed would become 400 units   
Radius(r) Increase the Radius the Force is Decreased.
Double the Radius the Force is halved.
100 unit is now 50 units
Now the centripetal force using weight e.g. in the earth with gravity.
Start with ( CPF = frac{Mass x Velocity 2 }{Radius} )
Replace Mass(m) With ( frac{weight}{g} )
CPF use on an Aircraft in Flight ( CPF = frac{Weight x Velocity 2 }{‘g’ x Radius} )

Force – Work – Power

The  “Power Required” is generally proportional to the “Amount of Fuel” you will use.

Walking up a set of stairs is work done.

If you run up the stairs unfortunately the same work is done, 

however you will require more power e.g energy thus use more fuel.

The Construction of the Power Formula.
Force
Weight Lift Thrust Drag

 
Force = Mass x Acceleration.
 One
Newton is 1kg x (1 Metre / Second /Second )

Sir Isaac Newton

Work 

Work = “Force” x Distance

One  Joule = One Newton x 1 Metre.

 
James Joule
Power

Power  is Work / Time.
(Power = frac{Work}{Time} )
One Watt  One Joule per Second

In short if you want to do work quickly you are going need more power




To make the power formula useful …
we’re going to need to expand the work formula (Power = frac{force x Distance}{Time} )
You may have noticed the distance over time (Ture Air Speed(TAS) = frac{Distance}{Time} )
Which is our true airspeed. ( Power = force x TAS )
Now change force to drag
and we have a Formula that we can use.		 ( Power = drag x TAS )

Even though the initial drag force could be the same, 

the speed at which it is required

this has a big bearing on fuel consumption.

All this is used to explain the difference between 

best endurance and best range speeds.

James Watt