22.6.6
With respect to aerofoils, describe the meanings of the following terms: section, leading edge, trailing edge, chord, chord line, thickness, thickness/chord ratio, camber.

Having an understanding of the terms used when describing aerofoils helps us because we know the correct terminology. This will make understanding later principles of the wing much simpler.

Below is a diagram explaining most of the important terms. The only one not described is thickness/chord ratio. This is simply the ratio of the maximum thickness of the wing to the total chord. 

Thickness/Chord.

                        

22.6.4
Explain the principle of airspeed indication, and indicate the relationship between indicated, calibrated, equivalent and true airspeeds (IAS, CAS, EAS, and TAS).

Airspeed is one of the most important concepts in aviation. We must understand what it is and the various types of airspeed as well. 

Airspeed indication is a measurement of dynamic pressure entering the Pitot tube.

                      

Static pressure is fed into the case of the instrument which cancels out the effect of static pressure in the Pitot tube. This means Indicated airspeed is shown on the instrument.

Airspeed indicators are calibrated at ISA sea-level density conditions. It is only under these conditions that they will accurately indicate the actual airspeed (that is, the true airspeed or TAS) of the aircraft through the air. When the ambient (freestream) air density differs from standard sea-level conditions the indicated airspeed will be different from the actual airspeed. This difference (sometimes called density error) and other errors which arise in the airspeed indication system has given rise to a number of different terms for airspeed.

Indicated Airspeed (IAS)

IAS is the reading on the airspeed indicator (ASI). There may be some differences between the indications registered by individual ASI’s, but these instrument errors are usually so small they can be ignored.

Calibrated Airspeed (CAS)

CAS is IAS corrected for pressure (or position) error. Pressure error arises mainly from incorrectly sensing the total and static pressure when the aircraft is in different flight attitudes. When pressure error correction (PEC) cards are displayed in the cockpit, they include the instrument error correction for the particular ASI. In practice the PEC at normal cruise speed is so small and can under most circumstances be ignored

Equivalent Airspeed (EAS)

Most ASI’s are designed to measure dynamic pressure (1/2ρV²) on the assumption that air is incompressible. Air is compressible and as speed is increased the pitot tube will increasingly register higher pressure that it should be cause air becomes compressed. When CAS is corrected for the compressibility error becomes known as EAS

True Airspeed (TAS)

TAS may be obtained by dividing the EAS by the square root of the relative density i.e. the prevailing density as compared with the standard sea-level density. If the prevailing density sea level happened to be standard the relative density would be 1 and EAS would equal TAS. Flying at 40,000 ft under standard conditions where the relative density is one quarter of the sea-level value, the EAS would be half the TAS (the square root of ¼ is ½). In practice TAS is usually calculated by using a navigation computer.

22.6.2
Describe the terms freestream static pressure, dynamic pressure (including the term .V2) and total (or pitot) pressure.

These are important ideas that are used throughout our flying career. We have to understand the principles involved and apply them correctly to make full use of them.

Freestream static pressure is a term used in aerodynamics of the prevailing atmospheric pressure. The symbol ρ∞ is used to denote it. The term free stream indicates the air conditions that exist well ahead of a body moving through the air, as yet unaffected by it’s passage. The freestream static pressure decreases with altitude.

                                           

When a solid body is moving that air pressure surrounding it will no longer be even. The surface pressures experienced by those areas facing into the airstream will be increased above the freeestream value, whereas the pressures to the side and the rear will generally be reduced. The differences in the pressure experienced are related to the kinetic(or Dynamic) energy the air has because it is moving and the extra pressure that it is capable of exerting as a result. This is called the Dynamic pressure.

               

Any solid body that is moving has kinetic energy. This is calculated by:

 

                                                 kinetic energy = 1/2mV²

Air also has kinetic energy when it is moving, normally referred to in aerodynamics as dynamic energy. The mass of air is measured by it’s density. The symbol for density it the Greek letter ρ, pronounced RHO. If we substitute density for mass in the above equation, we can calculate that amount of dynamic energy in a moving mass of air.

                                              dynamic energy = 1/2ρV²

                                                           ρ = density

                                              V = velocity of the airstream

If this moving mass of air is stopped by a solid body and bought completely to rest, the dynamic energy it contains is converted to pressure energy. For this reason, the pressure energy that arises is called dynamic pressure and it is exerted on the body in addition to the prevailing static system.

                                             dynamic pressure = 1/2ρV²

The term 1/2ρV²  therefore stands for ‘the additional pressure imposed when air of a certain density moving at a given velocity is bought completely to rest’. It is also used in a more general way to describe the amount of dynamic energy contained in a moving airstream.

Very little of the air moving past an aircraft in flight is bought completely to rest. The term for dynamic pressure (1/2ρV²) is nevertheless very important; all aerodynamic forces are proportional to it.

Dynamic Pressure is utilized in the measurement of airspeed. A small amount of air is bought to rest in a forward facing tube called the Pitot Tube. The pressure that is present inside the tube is called total (or pitot) pressure and it comprises the dynamic pressure caused by bringing the moving air to a rest plus the freestream static pressure.

 

                Total (or pitot) pressure = free stream static pressure + Dynamic pressure

                                                        =ρ∞ + 1/2ρV²

ρ∞ =Standard Sea Level Air Density

So to measure airspeed we need to work out the difference between total pressure and static pressure.

                Total pressure (dynamic + static) – static  = dynamic pressure

                which can be written as (ρ∞ + 1/2ρV² ) – ρ∞ = 1/2ρV² 

The airspeed indicator (ASI) is simply a dynamic pressure gauge that is calibrated to read airspeed (knots)

            

 

24.4.16
Explain the term viscosity, when related to air.

When air moves across a perfectly flat plane surface, the layer of molecules in immediate contact with that surface will be bought to a standstill (or nearly so) because of it’s viscocity (or stickiness). Successive layers of ait above that very bottom layer will be slowed down by decreasing amounts until the point is reached where the effect of viscocity is no longer felt.

24.4.14
Explain the meaning of density altitude (DA) and, in broad terms, the effect of pressure, temperature and humidity on DA and thus aerodynamic and engine performance.

Density altitude is the altitude in ISA that has the same air density as the actual altitude

Pressure temperature and humidity all have an effect on density altitude.

Density altitude has a direct effect on aircraft performance, both aerodynamically and in terms of engine power

A high temperature will indicate low density and poorer performance

A low temperature will indicate high density and better performance

A high pressure will indicate high density and better performance 

A low pressure will indicate a low density and poorer performance

24.4.12
Describe the approximate altitude bands in which atmospheric pressure and density are reduced to 75, 50 and 25% of their normal sea level values.

75% sea level pressure    8-10,000 feet

50% sea level pressure   18-22,000 feet

25% sea level pressure   34-41,000 feet

24.4.10
State the ISA sea level pressure and temperature conditions, and the approximate lapse rates in the lower atmosphere.

24.4.8
Describe the International Standard Atmosphere.

ISA The International Standard Atmosphere is a set of known conditions on which aircraft performance is based

Pressure 1013.2hPa

Temperature +15° C

Density 1.225kg/m³ 

24.4.6
Describe the normal changes in pressure, temperature and density with increased altitude.

Inc the lower part of the atmosphere, called the troposphere pressure, density and (normally) temperature decrease with altitude

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”]

Describe the effect of temperature, pressure and humidity on air density. (24.4.4)

Air Density: The amount of matter/mass in a given volume.


The Effect of ...... on Density


Effect Of ..
 Change
Mass 
Comment




Temperature
 Increase 
Less /m3




 Decrease 
More /m3



Pressure
 Increase 
Less /m3




 Decrease 
More /m3



Humidity
 Increase 
Less /m3




 Decrease 
More /m3








Why do we need to have an understanding of the atmosphere:
To have an understanding of the physical properties of the
atmosphere and how this impacts to aircraft performance.