Ch 2 - Air, Atmosphere and Speed

Question 1

Amount of Aerodynamic Force created is closely linked to the…

Answer 1

Air mass flow over the object
Greater near sea level (increased density)
- Depends on density of air and airflow speed

Question 2

Air Density Decreases if:

Answer 2

• Air Pressure Reduces
• Temperature increases
• Humidity Increases

Question 3

Air Density =

Answer 3

P (rho) = Pressure/ Temperature

Question 4

Characteristics of the Atmosphere

Answer 4

• Particles are constantly moving - exerting a force which is felt as static pressure
• Air molecules have a mass and thus exert a downward weight force

Question 5

Static Pressure =

Answer 5

= Force/Area = N/m(squared)

Question 6

Temperature

Answer 6

The kinetic energy of the vibrating molecules

• When the particles move more rapidly, they occupy more space = Less Dense

0C = 273K =32F

Question 7

Viscosity

Answer 7

The stickiness of a fluid - The difficulty of flowing past each other

-> Behaviour of Air -> Drag

Question 8

Humidity

Answer 8

Atmosphere always contains water vapour

Water vapour is lighter than completely dry air
- Air Density reduces when humidity increases

= Less performance for aircraft when density reduces

Question 9

Origin of Aerodynamic Force

Answer 9

Aircraft have to do work on air to oppose gravity
- Aerodynamic Force (F=ma)

To quantify force, we need to know airspeed and density in which we are flying

Question 10

Kinetic Energy

Answer 10

Ek = 1/2mv^2

Air has kinetic energy
- Exerts dynamic pressure on an aircraft

Question 11

Dynamic Pressure

Answer 11

q= 1/2 (rho) V^2

Rho = density of air (static pressure)
V = True Air Speed

Airspeed indicator is a Dynamic Pressure Gauge

Question 12

Dynamic Pressure

Answer 12

Can become very large

• > Increases if Air density increases
• > Increases rapidly if TAS increases (square law)

Question 13

Total Pressure

Answer 13

Total Pressure = Static Pressure (rho) + Dynamic Pressure (q)

Question 14

Isolating and Measuring Dynamic Pressure

Answer 14

• Anchored flexible strip fully exposed to airflow
• Surrounded by static pressure
• When moving, q builds up on one side, bending the flexible strip
• Calibrated to show q

Question 15

The Air Speed Indicator (ASI)

Answer 15

Enclosed flexible capsule via

• Pitot Tube which points into airflow
• Capsule surrounded by static pressure
• Expansion of Capsule = Dynamic Pressure

Question 16

IAS

Answer 16

The ASI shows the IAS
IAS = An approximation of q
Subject to:
-> Instrument Error
-> Pressure (Position) Error 
-> Compressibility Error 

Not Accurate

Question 17

CAS

Answer 17

Calibrated Air Speed

= The IAS corrected for instrument error and predictable pressure errors (which are usually small)

Errors removed on transport jets by Air Data Computer

Question 18

EAS

Answer 18

Equivalent Air Speed

=CAS corrected for compressibility (which at high speeds can cause big differences between IAS, CAS and EAS)

Only used on Dreamliner at the moment

Question 19

TAS

Answer 19

True Air Speed

The actual speed of the aircraft through the air (Not GS)

EAS = TAS (square root) (rho/MSL Density P0)
-> Density taken into account
At higher altitudes, EAS/CAS/EAS are all less than TAS

• IAS can exceed TAS when air Density is greater than 1.225kg/m^3
• Close to the surface
• High pressure, cold day