2 AimTo introduce basic aerodynamic theory to a sufficient level to be able to understand chapter 6 of ATC BAK
3 Objectives State the how lift is produced State the lift formula and its proportional valuesName the control surfaces and the axis around which the aircraft rotatesShow arrangement of forces during S & LState the meaning of, and factors affecting stability
4 P+V=C 1. Lift Bernoulli's Theorem “In the streamlined flow of an ideal fluid, the sum of all the energies remains constant”At low airspeeds air acts like a fluid therefore we can say…Static pressure + Dynamic pressure = ConstantDynamic pressure is caused by movement of an object therefore we can say…Pressure (pressure energy) + Velocity (Kinetic energy) = ConstantP+V=C
5 1. Lift Bernoulli's Theorem P+V=C P+V=C P+V=C To prove the theory we can look at a venturi. A venturi is a converging, diverging duct As air flows through a venturi it’s speed increases. Since energy is being conserved it’s pressure decreases. As it passes into the divergent duct pressure increases, velocity decreasesIf we look at the shape of the bottom half of the venturi it looks like the top of our wingP+V=CP+V=CP+V=C
6 1. Lift Bernoulli's Theorem As air flows over the top surface of an aerofoil it is accelerated, therefore pressure is…ReducedThe pressure difference between the low pressure on the top of the wing and relatively higher pressure on the bottom of the wing creates an aerodynamic force that we call Lift
7 2. Lift FormulaThe factors that affect the aerodynamic force (Lift) produced by our aircraft can be seen in the lift formulaL = CL . 1/2.ρ.V2 . SWhere:CL - Co-efficient of liftρ (Rho) – Free stream air densityV – True airspeed (TAS)S – Plan view wing surface area
8 2. Lift Formula CL - Co-Efficient of lift L = CL . 1/2.ρ.V2 . S CL refers to the lifting ability of the wingIts made up of a number of factors including:Angle of Attack (AoA)CamberAspect RatioSurface conditionL = CL . 1/2.ρ.V2 . S
9 2. Lift Formula Angle of Attack L = CL . 1/2.ρ.V2 . S Is defined as the angle between the relative airflow (RAF) and chord line of an aerofoilLiftChord LineL.E.AoAT.E.RAFAs AoA increases lift increasesL = CL . 1/2.ρ.V2 . S
10 2. Lift Formula Camber L = CL . 1/2.ρ.V2 . S Mean Camber is the curvature of a line drawn equidistant between the upper and lower surfaces of the wingLiftChord LineLine of mean camberAoARAFAs camber increases lift increasesL = CL . 1/2.ρ.V2 . S
11 2. Lift Formula Camber L = CL . 1/2.ρ.V2 . S High camber aerofoils can be found on aircraft that require high lift at low airspeedsMedium camber (general purpose) aerofoils can be found on light training aircraftLow camber aerofoils can be found on aircraft that travel at high airspeedsL = CL . 1/2.ρ.V2 . S
12 2. Lift Formula Aspect Ratio L = CL . 1/2.ρ.V2 . S Aspect ratio is the ratio of wing span to chordIts is measured by:As Aspect Ratio increases lift increasesHigh aspect ratio wings can be seen on gliding aircraftLight training aircraft typically have medium Aspect Ratio wingsLow aspect ratio wings can be seen on aerobatic aircraftL = CL . 1/2.ρ.V2 . S
13 2. Lift Formula ρ - Air density L = CL . 1/2.ρ.V2 . S Ambient density of the free stream air (air not being disturbed by the passage of the aircraft)If density is increased, lift will increaseL = CL . 1/2.ρ.V2 . S
14 2. Lift Formula V - True Airspeed (TAS) L = CL . 1/2.ρ.V2 . S The aerodynamic force produced is directly proportional to the airspeed squaredThe faster the airspeed, the more lift producedL = CL . 1/2.ρ.V2 . S
15 3. Control Surfaces S - Plan surface area L = CL . 1/2.ρ.V2 . S The size of wing area is directly proportional to the aerodynamic force producedA larger wing area, will interact with a larger volume of air and therefore produce more liftL = CL . 1/2.ρ.V2 . S
16 3. Control Surfaces Control Surfaces The three primary control surfaces are:AileronsAilerons
17 3. Control Surfaces Control Surfaces The three primary control surfaces are:Elevator
18 3. Control Surfaces Control Surfaces The three primary control surfaces are:Rudder
19 3. Control SurfacesThe control surfaces work by deflecting the trailing edge of the surface, increasing the effective AoA, therefore increasing…LiftLiftRAFChord LineChord LineChord LineRAFLiftRAF
20 3. Control Surfaces The three axes the aircraft rotates about are: 1. Lateral axis - PitchLateral axis
21 3. Control Surfaces The three axes the aircraft rotates about are: Longitudinal axis - RollLongitudinal axis
22 3. Control Surfaces The three axes the aircraft rotates about are: 3. Normal axis - YawNormal axis
23 4. Forces in Straight and Level Straight and level flight is defined as flight at a constant heading altitude and power setting with the aircraft in balance.In this state, the motion will not change. Therefore, it must have a constant ...velocity.Constant velocity means nil acceleration therefore all forces must be in a state of ...equilibrium.
24 4. Forces in Straight and Level There are four main forces acting in S & L flight:LIFTTHRUSTDRAGWEIGHT
25 4. Forces in Straight and Level These forces do not act from the same pointLift - Is produced by the wings and acts upwards through the centre of pressure.LIFTWeight - Acts straight down through the centre of gravity to the centre of the earth.Thrust - Is provided by the engine through the propeller.CoGDRAGTHRUSTCoPDrag - Is the resistance to motion felt by all bodies within the atmosphere.WEIGHT
26 4. Forces in Straight and Level Because the forces are not acting from the same point they create a coupleA couple is two equal and opposite forces acting about a pivot point creating a torque or turning momentThe two couple’s generate opposing pitching momentsLIFTL / W Couple = Nose DOWN momentT / D Couple = Nose UP momentDRAGTHRUSTWEIGHT
27 4. Forces in Straight and Level We said that the forces must be in equilibrium, therefore:LIFT =WEIGHT(L / W Couple)THRUST =DRAG(T / D Couple)For the aircraft to fly S & L the nose down moment must equal...the nose up moment.
28 4. Forces in Straight and Level WEIGHTLIFTDRAGTHRUSTIf the moments are not equal, the tailplane makes up the difference.In a correctly loaded aircraft the tail plane will create a small force…DownwardsForceThe forces are now in equilibrium
29 5. StabilityStability describes the properties of a body’s motion after it is displaced from equilibrium.Static Stability refers to the initial reaction of a body after being displaced from a position of equilibriumDynamic Stability becomes apparent after a body shows static stability. It refers to the subsequent motion of the disturbed body
30 5. StabilityPositive Static Stability refers to a body which will return to equilibrium after being displaced.
31 5. StabilityNeutral Static Stability refers to a body which will maintain its displacement from equilibrium.
32 5. StabilityNegative Static Stability refers to a body which will increase its displacement from equilibrium.
33 5. StabilityTo have dynamic stability the body must have Static StabilityIf an aircraft has Positive Static and Positive Dynamic Stability it will eventually return to its original attitude without pilot inputPositive static stabilityPositive Dynamic StabilityReturns to original attitudeAircraft is disturbedOriginal attitude
34 5. StabilityIf an aircraft has Positive Static and Neutral Dynamic Stability it will oscillate about the original attitude without pilot inputPositive static stabilityNeutral Dynamic StabilityAircraft is disturbedOriginal attitudeWill continue to oscillate about original attitude
35 5. StabilityIf an aircraft has Positive Static and Negative Dynamic Stability it will diverge from its original attitude without pilot inputPositive static stabilityDiverges from original attitudeAircraft is disturbedOriginal attitudeNegative Dynamic Stability
36 5. Stability Longitudinal stability Stability along the longitudinal axisStability about the lateral axisDesign features that affect longitudinal stability include longitudinal dihedral4˚2˚
37 5. Stability Lateral stability Stability along the lateral axis Stability about the longitudinal axisDesign features that affect lateral stability include tail/fin surface area, wing position, pendulum effect and lateral dihedral
38 5. Stability Directional stability Stability along the normal axis Stability about the normal axisMost important factor affecting is surface area aft of CoG