Basic Aerodynamic Theory

Question 1

Total/pitot pressure

Answer 1

= static pressure + dynamic pressure

Question 2

Freestream static pressure

Answer 2

Is the prevailing pressure at any point in the atmosphere, it’s conditions exist well ahead of a moving body of air, so when there is no movement between a body and the air, pressure is experienced equally on all surfaces

Question 3

Dynamic pressure

Answer 3

An extra pressure is experienced when there is movement between a solid body and air surrounding it, this is due to kinetic/dynamic energy the air has as a result of the movement

Question 4

Dynamic energy equation

Answer 4

1/2 p V^2

P = freestream air density
V = velocity of airstream

Question 5

ASI principle of operation

Answer 5

• Total/pitot pressure is fed into a flexible diaphragm from the pitot tube and static pressure is fed into the instrument case via the static vent
• total pressure is compared to static pressure so that a reading for dynamic pressure is obtained
• ASI is calibrated to read this dynamic pressure as an airspeed in kts

Question 6

ASI density error

Answer 6

ASI is calibrated at ISA sea level density this causes the differences in airspeed indications

Question 7

IAS

Answer 7

indicated airspeed - is the aerodynamic speed of the a/c, it’s the reading on the ASI (which is effected only negligibly by instrument error)

Question 8

CAS

Answer 8

Calibrated airspeed - is IAS corrected for position error (position error is the incorrect sensing of static + pitot pressure due to a/c attitude)

Question 9

EAS

Answer 9

Equivalent airspeed - is CAS corrected for compressibility error (compressibility error refers to air being compressible so as speed is increased the pitot tube will increasingly register a higher pressure than it should)

Question 10

TAS

Answer 10

True airspeed - is the actual speed of the a/c

Question 11

Chord

Answer 11

Is the distance between the leading and trailing edge measured along the chord line

Question 12

Chord line

Answer 12

Is the straight line that joins the leading and trailing edge

Question 13

Leading edge

Answer 13

Is the edge facing into the airstream

Question 14

Trailing edge

Answer 14

Is the edge at the downstream side

Question 15

Aerofoil thickness

Answer 15

Is the depth of the aerofoil

Question 16

T/C ratio

Answer 16

Thickness/chord ratio is the max thickness of the aerofoil expressed as a percentage of the chord

Question 17

Camber

Answer 17

The curvature line drawn equidistant between the upper + lower surface

Question 18

Aerofoil shape (+ classes of aero foil)

Answer 18

Aerofoil shape gives different characteristics, classes include:
- High lift (large thickness)
- General purpose (standard thickness)
- High speed (small thickness)

Question 19

Aerofoil angle of attack

Answer 19

Is the angle between the chordline and the RAF

Question 20

RAF

Answer 20

Relative airflow is the airflow which is remote from the a/c and is unaffected by its passage through it, the RAF vector is of the same magnitude but opposite direction to the a/c flight path vector at any given moment.

Question 21

Bernouli’s theorem

Answer 21

States that in the streamline flow of an ideal fluid the sum of the energy of position + energy of motion + pressure energy will remain constant

Question 22

Streamline flow

Answer 22

• Is where succeeding particles of air in an airstream follow the same steady and predictable path
• There will be no flow across streamlines, only along them
• Streamline flow can be maintained provided the air particles flowing around/through a body can change direction gradually + smoothly and shapes designed to achieve this are said to be streamlined

Question 23

Turbulent / seperated flow in the wake

Answer 23

Occurs if airflow is required to change its direction too abruptly as the flow will breakdown and become turbulent / unpredictable.

Question 24

Venturi

Answer 24

• Is a practicable application of the Bernoulli theorem
• It is a convergent - divergent duct and when placed in a steady stream of air it causes air to accelerate past the venturi throat and then decelerate back to normal speed through the exit
• Therefore, at the throat a reduction of static pressure is experienced (due to speed being the highest at the throat)

Question 25

Describe the streamline airflow around an aerofoil

Answer 25

An aerofoil acts as a half venturi with air moving over the aerofoil accelerating to pass over the top (increasing dynamic energy and decreasing pressure), whereas air passing below the aerofoil is not deviated from its path meaning there is no change in pressure. This pressure distribution (lower pressure above the aerofoil than below it) generates a force which tends to move the aerofoil towards the lower pressure area

Question 26

Pressure distribution around an aerofoil at low angles of attack

Answer 26

There’s little of the airflow past the aerofoil, ahead of and slightly below the leading edge there’s an area of high pressure (within this area there will be a point on the leading edge called the stagnation point where the flow is brought completely to rest). There’s another area of slightly raised pressure around the trailing edge.

In accordance with Bernoulli’s theorem, there’s a large area of low pressure above the aerofoil and a smaller area of low pressure below the aerofoil

Question 27

Pressure distribution around an aerofoil as angle of attack is increased

Answer 27

As AOA increases the airflow must increasingly deviate from its path and accelerate to follow the contour of the upper surface particularly over the forward part, as a result the upper area of low pressure moves forward and the area of low pressure under the aerofoil disappears. As AOA increases the area of high pressure forward of the leading-edge spreads towards the rear until it covers the lower surface

Question 28

Pressure envelope

Answer 28

The collective areas of high and low pressure on diagrams of streamline flow around an aerofoil

Question 29

Upwash

Answer 29

Refers to airflow turning upward ahead of the aerofoil

Question 30

Effect on upwash and downwash as AOA increases

Answer 30

Both Increase with increasing angle of attack

Question 31

How is upwash generated

Answer 31

By small pressure disturbances transmitted ahead of the aerofoil at the speed of sound, which cause air particles to move towards the areas of lowest pressure

Question 32

Downwash

Answer 32

Refers the airflow passing the aerofoil and turning downward

Question 33

How is downwash generated

Answer 33

Its a consequence of lift production (a mass of air must be moved in a given direction to produce lift force in the opposite direction)

Question 34

Pressure distribution around an aerofoil as angle of attack is increased beyond 15°

Answer 34

The change in direction of the airflow around the leading edge and forward upper surface becomes to abrupt and the airflow can no longer conform. The airflow separates from most of the upper surface and the turbulent wake behind the aerofoil increases greatly. The angle at which this occurs is the stalling/critical angle. When this angle is exceeded the low pressure envelope over the upper surface collapses and becomes unpredictable, pressure below the aerofoil continues to increase with AoA as more of the lower surface is presented towards the oncoming airflow

Question 35

TR

Answer 35

Total reaction - it is the resultant vector of the pressures existing around an aerofoil, it acts through the CP. It indicates the magnitude and direction of the aerodynamic force on the aerofoil

Question 36

What components can TR be divided into

Answer 36

Lift and drag
- Lift component is at a right angle to RAF
- Drag component is opposite and parallel to RAF

Question 37

What happens to TR as AoA is increased

Answer 37

• CP moves gradually forward until the stalling angle is passed, after which it moves rapidly backwards (this is for a cambered aerofoil, for a symmetrical aerofoil there is very little movement of the CP)
• TR magnitude increases and becomes more tilted towards the rear until the stalling angle is reached, after which the magnitude rapidly reduces and becomes more markedly tilted towards the rear