POF 1 Flashcards
Principles of Flight
Aerofoil (definition)
A body with a cross-sectional area shaped to produce an aerodynamic force.
Angle of Attack (definition)
The angle between the chord line and the relative airflow. = alpha
Chord line (definition)
The straight line joining the centre’s of curvature of the leading and trailing edges.
Chord (definition)
The distance from the leading edge to the trailing edge along the chord line.
Aspect Ratio (equation)
Aspect Ratio = Wing Span ÷ Mean Chord
Mean camber line (definition)
The line equidistant between upper and lower surfaces of the wing. This line is a measure of the wings camber.
Camber (definition)
A measure of the wings camber along the chord.
Symmetrical Aerofoil (definition)
An aerofoil with the same camber on the upper and lower surfaces.
Streamline (definition)
A line showing the direction of the airflow.
Rigging angle or angle of incidence (definition)
The angle between the chord line and the fore and aft datum line of the aircraft.
Dihedral/Anhedral (definition)
The wing sloping either upwards or downwards from the fuselage.
Pressure Envelope (definition)
The line showing the pressure around the aerofoil above or below ambient pressure.
Total reaction (definition)
The resultant of all the aerodynamic forces acting on the wing or aerofoil.
Centre of Pressure (definition)
The point at which the total reaction is assumed to act.
Lift
(definition + equation)
The component of the total reaction perpendicular to the flight path.
Lift = CL 1/2 rho V^2S
CL is coefficient of lift, rho is air density, V is free stream velocity, S is wing area
Drag (definition + equation)
The component of the total reaction parallel to the flight path. Drag = C_D 1/2 rho V^2 S
Taper (definition)
The decrease in chord from wing root to tip.
Washout (definition)
The decrease in rigging angle from wing root to wing tip, often used to prevent wing tip stalling.
Wing area (definition)
The area of the aircraft’s wing S = Span × Mean chord
Wing loading (equation)
Wing loading = Aircraft weight ÷ Wing area
Load factor (equation)
Load Factor (n) = Lift ÷ Aircraft Weight Often referred to as g.
Air Density (definition)
The mass of air per unit volume. Symbol rho 1.225k225kgs/m^3 at sea level
Bernoulli’s Theorem
(definition + equation)
Bernoulli’s theorem states that in a steady streamlined flow of an inconpressible fluid, the sum of the energies is constant.
Pressure Energy + Kinetic Energy = Constant
Static Pressure + Dynamic Pressure = Constant
P + 1/2 rho V^2 = constant
Explain the theory of airflow/pressure through a venturi?
As the airflow accelerates through the throat of the venturi the pressure will reduce - denoted by the streamlines converging. The lower half of the venturi is shaped like a low speed aerofoil and it is the difference in pressure due to the accelerated air that creates lift.
Stagnation point (definition)
Point at which the airflow is at rest/stagnant. Occurs close to the leading edge of the aerofoil.
How do seperation point and CoP move with AoA?
As AoA increases:
Seperation point starts close to the TE and moves forward towards the LE.
CoP moves forward towards the LE until the critical angle when the wing stalls and the CoP moves rearward (explains nose up).
Relative Airflow (definition)
Airflow remote from the aircraft and unaffected by it’s presence.
What 4 things vary the coefficient of lift?
- Angle of Attack 2. Aerofoil shape (camber and LE radius) 3. Wing plan form 4. Condition of the surface
Draw the Coefficient of lift / AoA Curve
Add image
How does LE radius affect C_L?
Sharp LE radius gives a more abrupt stall at a lower angle of attack. Lower C_L max.
How does Camber affect C_L?
More Camber gives a higher CL for a given AoA, so more lift produced but a lower stalling angle. Aerofoils with camber produce some lift even at zero degrees degrees AoA.
How does Aspect Ratio affect C_L?
The downwash behind the wing effectively reduces the AoA. A Higher aspect ratio, such as a glider, will have less downwash than a lower aspect ratio wing.
Higher A/R gives a higher C_L and lower stalling angle.
How does sweepback affect C_L?
Sweepback acts in the same way as lowering aspect ratio, increasing downwash and therefore increasing the stalling angle.
How does surface condition affect C_L?
Roughness causes the airflow to seperate =
rough wing gives less lift
What is the Flat Plate Effect?
When the aerofoil stalls it can be considered at a flat plate. Lift produced is from the combined effect of stagnation pressure and flow deflection.
Draw the Total Drag Tree.
How does Zero Lift Drag Vary?
Zero lift drag increases with the square of the speed.
Explain Surface Friction Drag?
Air is viscous and as the aircraft moves through the air particles collect on the surface and get dragged through.
Surface Friction Drag varies with:
1) the total surface area of the aircraft
2) the viscosity of the air
3) the thickness of the boundary layer
Reduced by designing/manufacturing smooth surfaces.
Explain Form Drag?
Form drag depends on the shape of the body moving through the air flow. It can be reduced by streamlining the shape.
Explain Interference Drag?
The boundary layers of all the parts on the aircraft interfere with each other creating a larger boundary layer at the junctions.
Reduced by adding fairings.
How does Lift Dependent Drag vary?
Weight = more weight (lift) INCREASES lift dependent drag
Manouvre = pulling g (more weight) INCREASES lift dependent drag
Speed = increase speed/reduce time over wing REDUCES lift dependent drag
Explain Induced Drag?
When the wing produces lift, there is a pressure gradient between upper and lower surfaces and between the wing root and wing tip. Air flows root to tip, and spills lower to upper - creating vortices.
Explain Increments of Zero Lift Drag?
Angle of attack increases to cater for downwash therefore form drag changes and increases.
Interference drag increases due to boundary layer thickening.
Draw the Drag Curve
Shows the minimum drag speed.
At what angle does the best Lift/Drag ratio occur?
The best Lift/Drag ratio (highest value) occurs at approx 4 degrees AoA and this gives the most efficient cruising AoA.
Why is the wing designed to stall from the root to the tip?
3 reasons
- To induce early buffet over the tail surfaces and warn the pilot of approaching the stall.
- To retain aileron effectiveness up to the stall.
- To avoid large rolling moments (wing drop) which would occur if one wing tip stalled before the other.
List the 4 design features to reduce wing tip stalling?
1) Washout - lower AoA at tip
2) Root spoilers e.g. toblerone spoilers - sharper wing root LE will induce early separation
3) Change of section from root to tip - aerofoil section with more gradual stalling characteristics used at tip
4) Slats and slots - energy from air under the wing redirected ove upper section, used at thw wing tip increases critical angle at tip compared to root
What are the aerodynamic signs of approaching the stall?
- Low and reducing IAS
- High nose attitude
- Less effective controls
- Light buffet
- Audio warning
What are the aerodynamic signs of full stall?
1) heavy buffet
2) nose drop
3) sink (high rate of descent)
4) possible wing drop
Stall speed
(definition)
The speed of a clean aircraft, for a particular weight, throttle closed and flaps up, at which the aircraft can no longer maintain level flight.
What is the lift equation at the stall?
Lift = CLMAX 1/2 rho VB S
where VB is the basic stalling speed
What is the calculation for Manoeuvre Stalling Speed?
Vm = Vb x square root(g)
How does Vb ( basic stalling speed) vary?
1) increased weight/g = higher stalling speed
2) increased angle of bank = higher stalling speed
3) flaps, slats and slots = reduced stalling speed
4) increase thrust = reduced stalling speed
What are the 6 ways to reduce Induced Drag?
1) Winglets - forward component from airflow around wing tip
2) Wing Taper - less lift at tip = reduction in span wise flow
3) Washout - less lift at tip = reduction in span wise flow
4) Camber less lift at tip = reduction in span wise flow
5) Aspect Ratio - high aspect ratio wing has reduced chord for same wing area = airflow less affected
6) Tip tanks/missiles - act as barrier to flow
What is the Standard Stall Recovery Drill?
Apply Full Power
and
Move stick centrally forward until buffet and audio warning stops.
Hold the resulting attitude.
Level the wings if required (after buffet + audio warnings stop).
Ease out of descent.
Why maintain ailerons in a neutral position at the stall?
When an aircraft is flying close to the critical angle of attack any use of aileron could induce stall at the downgoing wing and possible autorotation.
Why is lift augmentation used?
To increase CLMAX which reduces the stalling speed meaning the aircraft can fly safely at lower speed.
Explain how the use of flaps will decrease the stalling speed.
Flaps vary the camber of wing. Lowering either LE or TE flaps will increase camber, increasing CLMAX and lowering the stalling speed.
TE flaps reduce the AoA at which CLMAX occurs.
List 3 methods of augmenting lift
- Slats - which can be either automatic or controlled by the pilot or permanent.
- Flaps which may be either leading edge or trailing edge.
- Boundary layer control using techniques such as vortex generators, blown air to re-energise the boundary layer.
On a graph of CL/AoA show the effect of lowering flap
List 5 types of trailing edge flap
Plain
Slotted
Double slotted
Fowler
Split
Which flight instrument shows the direction of spin?
The turn needle
4 Stages of Spin?
Entry (deliberate from straight and level or inadvertently from a manoeuvre)
Incipient Spin
Full Spin
Recovery
What are the 3 ‘gyros’ acting during the spin?
A - ‘aileron’, rolling gyro around the longitudinal axis (pro spin)
B - ‘body’, fuselage pitching gyro around the lateral axis (anti-spin)
C - total aircraft mass, yawing gyro around the normal axis (affects recovery roll rate)
What’s the significance of the B/A ratio?
B/A > 1 = hard to spin, but easy to recover
B/A < 1 = easy to spin, but hard to recover
How does the aircraft behave in an errect spin?
Roll and yaw in the same direction, pitch up
How does the aircraft behave in an inverted spin?
Roll and yaw in opposite directions, aircraft pitching down
What is the full spin recovery drill?
Check height sufficient for recovery.
Check throttle closed.
Check direction of the turn needle.
Apply and hold full rudder in the opposing direction to the turn needle.
Pause (approx 1 second)
Control column centrally foward (smoothly, ailerons neutral).
When spin stops, centralise the rudder.
Level the wings and recover from the ensuing dive.
