UNIT - VII

UNIT - VIIUNIT - VII

Aerodynamic Characteristics of Aerodynamic Characteristics of Airfoils and wingsAirfoils and wings

Aerodynamic forces and Moments Aerodynamic forces and Moments coefficientscoefficients

Aerodynamic forcesAerodynamic forces

Aerodynamic forces exerted by airflow comes from only two sourcesAerodynamic forces exerted by airflow comes from only two sources Pressure, p, distribution on surfacePressure, p, distribution on surface

Acts normal to surfaceActs normal to surface

Shear stress, tw, (friction) on surfaceShear stress, tw, (friction) on surface Acts tangentially to surfaceActs tangentially to surface

Pressure and shear are in units of force per unit area (N/m2)Pressure and shear are in units of force per unit area (N/m2) Net unbalance creates an aerodynamic forceNet unbalance creates an aerodynamic force

““No matter how complex the flow field, and no matter how complex No matter how complex the flow field, and no matter how complex the shape of the body, the only way nature has of communicating an the shape of the body, the only way nature has of communicating an aerodynamic force to a solid object or surface is through the pressure and aerodynamic force to a solid object or surface is through the pressure and shear stress distributions that exist on the surface.”shear stress distributions that exist on the surface.”

““The pressure and shear stress distributions are the two hands of The pressure and shear stress distributions are the two hands of nature that reach out and grab the body, exerting a force on the body – the nature that reach out and grab the body, exerting a force on the body – the aerodynamic force”aerodynamic force”

Aerodynamic forces and moments are due toAerodynamic forces and moments are due to Pressure distributionPressure distribution Shear stress distributionShear stress distribution

NomenclatureNomenclature R R resultant force resultant force L L liftlift D D dragdrag N N normal forcenormal force A A Axial forceAxial force

Relative Wind: Direction of V∞Relative Wind: Direction of V∞ We used subscript ∞ to indicate far upstream conditionsWe used subscript ∞ to indicate far upstream conditions

Angle of Attack, a: Angle between relative wind (V∞) and chord lineAngle of Attack, a: Angle between relative wind (V∞) and chord line

Total aerodynamic force, Total aerodynamic force, RR, can be resolved into two force , can be resolved into two force componentscomponents Lift, Lift, LL: Component of aerodynamic force perpendicular to relative wind: Component of aerodynamic force perpendicular to relative wind Drag, Drag, DD: Component of aerodynamic force parallel to relative wind: Component of aerodynamic force parallel to relative wind

Center of Pressure: It is that point on an airfoil (or body) about which Center of Pressure: It is that point on an airfoil (or body) about which the aerodynamic moment is zerothe aerodynamic moment is zero

Aerodynamic Center: It is that point on an airfoil (or body) about Aerodynamic Center: It is that point on an airfoil (or body) about which the aerodynamically generated moment is independent of which the aerodynamically generated moment is independent of angle of attackangle of attack

Moments About Leading Edge , quarter Chord point and about Moments About Leading Edge , quarter Chord point and about center of pressure center of pressure

Center of Pressure: It is that point on an airfoil (or body) Center of Pressure: It is that point on an airfoil (or body) about which the aerodynamic moment is zeroabout which the aerodynamic moment is zero Thin Airfoil Theory:Thin Airfoil Theory:

Symmetric Airfoil:Symmetric Airfoil: Cambered Airfoil:Cambered Airfoil:

Aerodynamic Center: It is that point on an airfoil (or Aerodynamic Center: It is that point on an airfoil (or body) about which the aerodynamically generated body) about which the aerodynamically generated moment is independent of angle of attackmoment is independent of angle of attack Thin Airfoil Theory:Thin Airfoil Theory:

Symmetric Airfoil:Symmetric Airfoil: Cambered Airfoil:Cambered Airfoil:

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4

AAc

cx

cx

lcp

cp

4

4

..

..

cx

cx

CA

CA

Drag PolarDrag Polar

Drag Polar : Drag Polar : both the Cl and Cd are the both the Cl and Cd are the functions of angle of attack thus the drag functions of angle of attack thus the drag coefficient is that it depends on lift coefficient coefficient is that it depends on lift coefficient and the typical variations on Cl and Cd is and the typical variations on Cl and Cd is called as Drag polarcalled as Drag polar

The Drag polar is very important in The Drag polar is very important in performance analysis of aircrafts. For an performance analysis of aircrafts. For an aircraft with symmetrical airfoil sections the aircraft with symmetrical airfoil sections the drag polar is given as drag polar is given as CCD D = C= CDDOO+KC+KCLL22

Lift Curve SlopeLift Curve Slope

Lift Curve SlopeLift Curve Slope

Lift Curve for a Normal Wing and NACA 63415 WingLift Curve for a Normal Wing and NACA 63415 Wing

Lift curve for High aspect ratio wing and Delta wing AircraftLift curve for High aspect ratio wing and Delta wing Aircraft

Lift curve for a straight wing and Swept wing Lift curve for a straight wing and Swept wing AircraftAircraft

Maximum Lift CoefficientMaximum Lift Coefficient

Lift coefficient changes with addition of control surfacesLift coefficient changes with addition of control surfaces

Lift coefficient changes with increase in aspect ratio and Lift coefficient changes with increase in aspect ratio and also Angle of attack can be decreasedalso Angle of attack can be decreased

Lift coefficient increasing with maximum thickness Lift coefficient increasing with maximum thickness

Variation of Variation of CCll, , CCDD, L/D with AOA, L/D with AOA

Increase in lift Increase in lift coefficient with coefficient with different control different control surfaces or high surfaces or high lift deviceslift devices

Minimum drag coefficientMinimum drag coefficient

To obtain minimum drag coefficient To obtain minimum drag coefficient 1) we have to streamline the body1) we have to streamline the body

2) Increasing the sweep angle2) Increasing the sweep angle

Drag coefficient data obtained for a rectangular wing and swept back Drag coefficient data obtained for a rectangular wing and swept back wing on wing tunnel and lifting line numerical methodswing on wing tunnel and lifting line numerical methods

Min drag coefficient Min drag coefficient for wing section and for wing section and complete airplanecomplete airplane

It gives the maximum possible glide angleIt gives the maximum possible glide angle

Max values of L/D ratios Max values of L/D ratios

Effect of Airfoil and wing geometryEffect of Airfoil and wing geometry

Airfoil NomenclatureAirfoil Nomenclature

Reynolds No, Boundary Layer Transition and surface Reynolds No, Boundary Layer Transition and surface roughnessroughness

NACA Conventional AirfoilsNACA Conventional Airfoils

Laminar Flow AirfoilsLaminar Flow Airfoils

An airfoil designed for minimum An airfoil designed for minimum drag and uninterrupted flow of the boundary layer is called a laminar airfoillaminar airfoil..

Supercritical Airfoils:Supercritical Airfoils:

Designed to delay and reduce transonic drag Designed to delay and reduce transonic drag rise, due to both strong normal shock and rise, due to both strong normal shock and shock-induced boundary layer separationshock-induced boundary layer separation

Whitchomb supercritical airfoilsWhitchomb supercritical airfoils

Drag Reduction And lift Augmentation Drag Reduction And lift Augmentation MethodsMethods

Many theories have been developed on how a wing generates lift. The most common Many theories have been developed on how a wing generates lift. The most common one is the one is the “Longer Path Theory”“Longer Path Theory”. .

This theory describes how the shape of the aerofoil produces a pressure difference This theory describes how the shape of the aerofoil produces a pressure difference which generates lift. As the aerofoil is designed in such a way that its upper surface is which generates lift. As the aerofoil is designed in such a way that its upper surface is longer than the bottom, and because the molecules that hit the leading edge must longer than the bottom, and because the molecules that hit the leading edge must meet again at the trailing edge, the ones that travel on the upper surface do so with meet again at the trailing edge, the ones that travel on the upper surface do so with greater velocity than the lower greater velocity than the lower

Flap systemFlap system Leading edge devicesLeading edge devices Multi element airfoilsMulti element airfoils Circulation controlCirculation control Laminar flow controlLaminar flow control wingletswinglets

Flap systemsFlap systems

Flap is an element attached to the aileron Flap is an element attached to the aileron of the wing sectionof the wing section

It is always possible to reduce stall speed It is always possible to reduce stall speed by increasing wing area by increasing wing area

Different types of flap systemDifferent types of flap system

Flaps change the airfoil pressure Flaps change the airfoil pressure distribution, increasing the camber of the distribution, increasing the camber of the airfoil and allowing more of the lift to be airfoil and allowing more of the lift to be carried over the rear portion of the section carried over the rear portion of the section

Leading Edge Leading Edge DevicesDevices

Leading Edge DevicesLeading Edge Devices Leading edge devices such as nose flaps, Leading edge devices such as nose flaps,

Kruger flaps, and slats reduce the pressure peak Kruger flaps, and slats reduce the pressure peak near the nose by changing the nose camber. near the nose by changing the nose camber. Slots and slats permit a new boundary layer to Slots and slats permit a new boundary layer to start on the main wing portion, eliminating the start on the main wing portion, eliminating the detrimental effect of the initial adverse gradient.detrimental effect of the initial adverse gradient.

A Wing with slats and FlapsA Wing with slats and Flaps

Multi Element AirfoilsMulti Element Airfoils

Circulation Control Circulation Control

Circulation Control Wing technology is one of Circulation Control Wing technology is one of the most important potential applications of the the most important potential applications of the Coanda Effect.Coanda Effect.The objective is to replace the lift devices on the The objective is to replace the lift devices on the leading and trailing edges of a wing by use of leading and trailing edges of a wing by use of Coanda Surfaces and slot blowing instead.Coanda Surfaces and slot blowing instead.

Laminar Flow ControlLaminar Flow Control

WingletsWinglets

WingletsWinglets