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Aerodynamicorcesan Moments

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The aerodynamic forces and

moments on the bod are dueto only two basic sources

1. Pressure distribution overthe body surface

2. Shear stress distribution

over the body surface

No matter how complex the flow field, and no matter

has of communicating an aerodynamic force to a solid

object or surface is through the pressure and shear

stressdistributions that exist on the surface.

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Both pressure P and shear stress

havedimensions of force per unit area

PressurePacts normal to the surface and shear

stress acts tangential to the surface Shear stress is due to tugging action on the

sur ace w c s ue o r c on e ween e o yand the air

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To get theResultant Aerodynamic Force Rand

Moment Mon the body, the net effect of the P

and distributions should be integrated over

the complete surface

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Resultant Aerodynamic ForceTotal Aerodynamic Force

Sum of Pressure and ShearLift

Airfoil

Drag

v

V

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Rresultant force

L lift rag

N normal forceA Axial force

V- free stream velocityC - is the chord

-

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There are 3 axes-systems:

Body

Therefore the resultant aerodynamic force Rcould be split into components in the body oraerodynamic axes systems, as shown in figure

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The resultant force R is split into two components

L = Lift = Component of R perpendicular to

relative windD = Drag = component of R parallel to relative

w n

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Again the resultant force R is split into two components

N = Normal force = component of R perpendicular tochord c

= =

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e geometr c re at ons etween t ese two sets

of components can be written as

sincos ANL =

cossin AND +=

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Calculations of Forces & Moments

-

The pressure and shear stress on the upper surface

are denoted by Pu and uat a distance of Sufrom leadingedge along the body

m ar y lan lare t e correspon ng quant t es

on the lower surface

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At a given point the pressure is normal to the

surface and is oriented at an angle relative to the

perpen cu arShear stress is tangential to the surface and is

oriented at the same angle relative to horizontal

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Consider an elemental surface area ds

The total normal force N and the total axial force A dueto the pressure and shear stress on the elemental area ds

The primes on N and A denote force per unit span

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By integrating the above equations from L.E to T.E

get moment a out t e ea ng e ge per un t span

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Location of center of pressure

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