POF Flashcards
State Bernoulli’s theorem
in a steady streamlined flow of an incompressible fluid, the sum of the energies is constant.
constant = static pressure + dynamic pressure
Constant = p + 1/2ρV ²
ρ = air density
Definition and formula of lift
Definition = Component of the total reaction, perpendicular to the flight path of the aircraft.
Equation = CL 1/2 ρ V² S
CL = co efficient of lift Rho = Air Density V = free stream velocity or true air soeee S = Wing Area
The co efficient of lift varies with:
- Angle of attack
- Aerofoil shape
- Wing plan form (ie top down view)
- Condition of the surface
A blunt leading edge with a large radius gives what stall characteristics?
Well rounded peak and gentle stall at a higher angle of attack
A small radius leading edge gives what stall characteristic?
More abrupt stall at a lower AoA
What effect does more camber have on a CL graph?
More camber gives more CL (so more lift), but a lower stalling angle.
More camber gives a parallel shift left and up of the CL curve. So more lift at a given AoA, but also a reduced stalling angle.
Effect of aspect ratio on CL curve?
Higher aspect ratio (ie longer wings) gives more CL, but lower stalling angle.
Graph pivots about X (AoA) axis (anti clock) with higher aspect ratio.
Effects of rough surface on a wing
Roughness causes airflow to separate from the wing earlier when the AoA is increased.
What is flat plate effect?
When aero foil stalls there is a partial collapse of the Lowe pressure on top of the wing. The contribution of the lower surface remains unchanged and the aerofoik can be considered a flat plate. The lift produced is now as a result of stagnation pressure and flow deflection.
Movement of centre of pressure with AoA?
Total reaction acts through a point called the centre of pressure.
As AoA increases, the centre of pressure moves forward until aerofoil stalls. As it stalls, centre of pressure moves rearward which will then give a nose down pitching moment to the aircraft.
Types of Drag and how it’s reduced.
TOTAL DRAG
Zero Lift Drag ——— Lift Dependent Drag
ZERO LIFT DRAG=
- Surface friction drag - smooth surfaces.
- Form Drag - streamlining.
- Interference drag - fairings.
LIFT DEPENDENT DRAG.
- Increments of zero lift drag
- Induced drag (caused by vortices)
- Winglets
- Wing tapers
- Change of camber
- Wash out
- Aspect ration
- Tip tanks / missiles.
What does surface friction drag vary with and how is it reduced?
- Total surface area of aircraft.
- Viscosity of air.
- Thickness of boundary layer.
Reduced by smooth surfaces.
What effects form drag and how is it reduced?
Shape of the body presented to the airflow.
Reduced by streamlining (optimal ratio is 3-4:1)
What causes interference drag and how is it reduced?
Caused by mixing of separate boundary layers.
Reduced by fairings.
What affects total lift dependent drag?
Reduces with square of airspeed
What causes induced drag and how is it reduced and what does it Change with?
Caused by vortices
Reduced by
- Winglets
- Wing tapers
- Change of camber
- Wash out
- Aspect ration
- Tip tanks / missiles.
Varies with weight, manoeuvre and speed
How does increments of zero lift drag work?
- AoA increases to cater for down wash
- Form drag increases
- Interference drag increases because of thickening boundary layer
What does a drag curve show?
Minimum drag speed
Explain boundary layer
Aircraft moving through air collects and drags particles with it. Layer of air slightly further away from surface will also be pulled along, at slightly slower speed. This continues on until eventually a layer of air, a certain distance from the aircraft isn’t subjected to the dragging force. Layer of air affect by the aircraft is called the boundary layer.
Boundary layer is either laminar or turbulent.
If laminar, the layers of air do not mix and are approx 2mm above surface.
If turbulent, the layers mix and become uniform in velocity and energy but thicken to 20mm. This thickens the boundary layer resulting in more drag being produced.
Explain interference drag.
Total drag of an aircraft is treated than the sum of drag of individual parts. Because the boundary layers of each part interfere with each other and result in a thicker boundary layer at junctions. Reduced by adding fairings.
Explain induced drag.
When wing produces loft there is a pressure difference between upper and lower wing surfaces.
There is also pressure gradient between wing root and wing tip. Air therefore flows from root to tip below the wing. And tip to root above the wing.
At wing tip air spills from lower to upper surface and forms vortices. They intensify during high lift manoeuvres.
The span wise flow along the upper and lower surfaces modifies the whole flow pattern around the wing and results in downwash behind the trailing edge.
Faster = smaller vortices because air less time to be effected by span wise flow. And vice versa.
This extra down wash effectively reduced AoA of the wing and results in a loss of lift. To compensate the AoA must be increased, resulting in increased drag (induced drag).
This increases form drag and causes boundary layer to thicken, which will increase surface friction drag and interference drag.
How to reduce induced drag
- Winglets. Positioned to create an AoA with the airflow around the wing tip. This reaction has a component pointing forward, thus effectively opposing some of the induced drag.
- Aspect ratio. High aspect ratio (with same surface area) will produce same amount of life. But because of reduced chord (length), airflow across the wing will be less affected by span wise flow. Also less of total length of wing is affected by it.
- Wing taper, washout and camber change. Result in less lift at wing tip and less spanwise flow. Tip tanks and missiles act as barrier to spanwise flow.
- Winglets
- Wing taper
- Change of camber / section
- Washout
- Aspect ratio
- Tip tanks / missiles
What affects lift dependent drag?
- Weight. Increase in weight will need more lift if aircraft is flying level and hence require a greater pressure difference between upper and lower surface, this will mean more drag.
- Manoeuvre. When a/c manoeuvres, wing produces more lift and thus more drag.
- Speed. Increase in speed gives the airflow over the wing a shorter time to be affected by spanwise pressure differences and the drag will be reduced. Conversely low speed = increased induced drag.
Stalling speed definition
The speed at which a clean aircraft,
At stated weight
With Throttle closed
Can no longer maintain level flight
Signs AND symptoms of stall
SIGNS Low IAS High nose Ineffectiveness of controls Light buffet Audio warning
SYMPTOMS Heavy buffet Nose drop A/C sinks / descends Possible wing drop
Features to reduce likelihood of wing tip stalling
- Washout (reducing AoA towards wing).
- Root spoiler. (Sharper wing root leading edge will cause early separation).
- Change of section. (More gradual stalling characteristics towards wing tip).
- Slats and slots. (Used in outer section of wing to increase crit angle.
WRCs
(World rally championships)
What is the lift formula
Lift = CLmax 1/2 ρ vb^2 S
S = SA Of wing
Vb = Velocity basic stalling speeed
How does G affect stall speed (give formula)?
More bank gives much higher stall speed.
Vm = Vb √G
e.g. Pulling 4G and Vb = 150kts
New stall speed @ 4 G = 150 x √4
= 150x2
= 300
Vm = stalling speed in manoeuvre
Standard stall recovery drill =
Full power Move stick CENTRALLY forward Wait for audio warner and buffet to stop HOLD resulting attitude (allowing speed to build) Level wings (if required) Gently ease out descent
What is lift augmentation and what are the methods of it?
If CLmax can be increased, the stalling speed will reduce (and a/c can fly safer at a lower speed):
- Slots / slats. Bleeds air from below wing to renenergise flow over upper surface and delay separation.
- Flaps. Increase camber of wing section.
- Boundary layer control. Vortex generators used to reenergise air flow, by using slower moving air near wing surface to mix with faster moving air above boundary layer.
SFB - special forces boat
Effects of lift augmentation on CL max graph.
SLAT / SLOT - extension up curve
TRAILING EDGE FLAP - shifts up and left (parallel)
LEADING EDGE FLAP - shifts right and up
(Trailing trails, leading leads)
Aerofoil def
A body with a cross sectional area shaped to produce an aerodynamic force
AoA def
Angle between chord line and relative air flow.
Chord line def
STRAIGHT* line joining centres of curvature of leading and trailing edge
Chord def
Distance from leading edge to trailing edge along chord line
Aspect ratio
Wing span
—————
Mean chord
OR
Wing Span^2
—————
Wing area
Mean camber line
Line equidistant between upper and lower surfaces of the wing. Measures camber of wing.
Camber
Measure of wing’s curvature along the chord
Symmetrical aerofoil
Aerofoil section with same camber on upper and lower surfaces
