Basic Aerodynamic Theory Flashcards
Total/pitot pressure
= static pressure + dynamic pressure
Freestream static pressure
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
Dynamic pressure
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
Dynamic energy equation
1/2 p V^2
P = freestream air density
V = velocity of airstream
ASI principle of operation
- 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
ASI density error
ASI is calibrated at ISA sea level density this causes the differences in airspeed indications
IAS
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)
CAS
Calibrated airspeed - is IAS corrected for position error (position error is the incorrect sensing of static + pitot pressure due to a/c attitude)
EAS
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)
TAS
True airspeed - is the actual speed of the a/c
Chord
Is the distance between the leading and trailing edge measured along the chord line
Chord line
Is the straight line that joins the leading and trailing edge
Leading edge
Is the edge facing into the airstream
Trailing edge
Is the edge at the downstream side
Aerofoil thickness
Is the depth of the aerofoil
T/C ratio
Thickness/chord ratio is the max thickness of the aerofoil expressed as a percentage of the chord
Camber
The curvature line drawn equidistant between the upper + lower surface
Aerofoil shape (+ classes of aero foil)
Aerofoil shape gives different characteristics, classes include:
- High lift (large thickness)
- General purpose (standard thickness)
- High speed (small thickness)
Aerofoil angle of attack
Is the angle between the chordline and the RAF
RAF
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.
Bernouli’s theorem
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
Streamline flow
- 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
Turbulent / seperated flow in the wake
Occurs if airflow is required to change its direction too abruptly as the flow will breakdown and become turbulent / unpredictable.
Venturi
- 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)
Describe the streamline airflow around an aerofoil
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
Pressure distribution around an aerofoil at low angles of attack
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
Pressure distribution around an aerofoil as angle of attack is increased
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
Pressure envelope
The collective areas of high and low pressure on diagrams of streamline flow around an aerofoil
Upwash
Refers to airflow turning upward ahead of the aerofoil
Effect on upwash and downwash as AOA increases
Both Increase with increasing angle of attack
How is upwash generated
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
Downwash
Refers the airflow passing the aerofoil and turning downward
How is downwash generated
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)
Pressure distribution around an aerofoil as angle of attack is increased beyond 15°
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
TR
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
What components can TR be divided into
Lift and drag
- Lift component is at a right angle to RAF
- Drag component is opposite and parallel to RAF
What happens to TR as AoA is increased
- 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
