Externals 1fhgfh

8/18/2019 Externals 1fhgfh

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April 10th 20

Discussion Session with our Aero-children

UNIT 1

Easy Na ?

Definitions are easy to mug up !ut do it properly without any mista"e

if you "now the correct unit $ust mention since e%ery e&aminer lo%es

as" such definitions li"e 'ressure( )elocity( Total and Static propertie

li"e temperature( pressure etc* then +ift ,oefficient( Drag ,oefficien

'itching oment coefficient for .D Aerofoils / !ut not to forget the othtwo moment coeffs* 0olling and awing oment coeff for 2inite 3ings45

es get the proper formulae also where%er re6uired*

Then comes the definition of Aerofoil related terms* There is som

confusion in my /?5 note a!out Aerodynamic ,entre and ,entre

'ressure* And please draw any s"etch using properly sharpened soft 7

or 87 pencil and la!el e%ery important item* Treat the diagrams wi

ade6uate sincerity N9 sa! "uchh chaltai hai philosophy 4

2irst recall what is the net effect of airflow past a .D aerofoil* Don:t forg

to distinguish !etween the upper and lower part

anifests as two positi%e effects ;

Static pressure distri!ution on surface which is along the loc

normal to surface at the point of action < p = f/&>,( angle attac"( chord !ased 0eynolds num!er( ach no* if compressi!le

3all shear Stress on surface along the local tangent to surface

the point of action < τ = f/&>,( angle of attac"( chord !ase

0eynolds num!er( ach no* if compressi!le*


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At any point on the surface these two forces may !e resol%ed along th

flow and across the flow direction / tangential for shear stress an

normal for pressure5* 7ut the point of application of these forces als

%ary along the aerofoil surface* So the net effect of these surface force

at different points is e6ui%alent to net force along/@5 the flow /Drag5( n

force normal /5 to flow direction /+ift5 and a couple a!out the a&

normal /5 to the .D aerofoil plane which physically allows the pitching

the foil as nose up and nose down mode depending on the sign of th

moment* This is called the 'itching oment*

'ractice for a gi%en geometry and oncoming flow %ector how to e&pres,l( ,d and ,m in terms of integration of distri!ution of p and τ on th

foil surface* These are non-dimensionalised !y the corresponding inl

dynamic head < B*Cρ). and the aerofoil chord length*

'i Theorem should simply !e memoried and chec" deduction

Anderson pp F - G to show all force or moment coefficients as functio

of 0e and a'ractice E&ample 1*F and 1*C Anderson pp* F1-F

The ne&t issue is 'itot Tu!e and )enturimeter or 9rifice 'lates*9perating principles must !e a!solutely clear* ,hec" whether you ca

compute the flow rate or %elocity if the pressure drop across the )entu

or 9rifice 'late is gi%en* Don:t ignore the 'itot tu!e Huestion done

Internals .* 9ne should draw neat s"etches and clearly e&plain th

location of the holes and also that the total head tu!e must !e along th%elocity %ector direction for correct measurement* If not( th

measurement is incorrect# multiple holes around the outermost tu!e pic

up the a%erage static pressure* 7e sure that gi%en the readings you ca

compute the flow %elocity correctly* Same is the case with )enturimeter


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'ractice E&ample 1*1 p . ( E&ample* 1*. p B Anderson

,orrect Definition of Centre of Pressure ;

0ead p . Anderson and realie the ,entre of 'ressure is located on th

chord at such a point that the resultant forces normal to the cho

produces the same moment created !y the forces around the leadin

edge* 2ig* 1*.C and E& 1* on p Anderson are 6uite self e&planator

Jeep an Eagle:s Eye to 2ig* 1*F p 1 Anderson what one presents

e&plain the different flow regimes*

,orrect Definition of Aerodynamic CentreIt is the point on the aerofoil a!out which the aerodynamically generate

moment is independent of angle of attac"*See Anderson pp G-FB along with e&ample E& F* on p FB(

Anderson* 3hat does this say ?

Assume the Aerodynamic ,entre at & = &ac whereas the lift +( drag D an

'itching oment &>,=B*.C a!out the point &>,= B*.C * Now if &>, = &ac>, is the aerodynamic centre location in non-dimension

coordinate /&!ar ac5( the !alance of moment a!out the point &>,=B*.C isac = +/, &!ar ac-B*.C,5 K c>F

Non dimensionalisation di%ing each term !y B*Cρ).Sc gi%es

/ S is the spanwise length of the aerofoil5 ,m(ac = ,l/&!ar ac-B*.C5 K ,m(B*.C,Differentiating !oth sides wrt angle of attac" α

d,m(ac >dα = d,l >dα/&!ar ac-B*.C5K d,m(c>F >dα

If pitching moment a!out Aerodyn* ,entre is independent of α

d,m(ac >dα=B hence &!ar ac=-mB>aB K B*.C


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where d,l >dα= aB and d,m(c>F >dα=mBSo once the liftline slope aB and the slope of the moment a!out 6uart

chord mB is "nown( it is possi!le to find the Aerodynamic ,ent

coordinate*

3hile wor"ing out the pro!lem on e%aluation of forces and moments f

8ypersonic 2low / rd Internal pro!lem5 with gi%en ,p )s Theta* Now ,

is non ero only in two sectors out of four( !ut when we find the force

and moments( 2orce along @ = ,p /0 dθ5 cos θ( so one needs to put th

%alue of ,p and integrate o%er the two arcs on which p is non-ero an

that integration computes the total drag force*and the area to !

considered will !e the pro$ected area .0*+ This was uni%ersally wrong

the Internals * All of you missed this cos θ due to inclination of the lin

of action of the pressure along with the cos θ already present as part of

distri!ution*

So L** can I !e confident now a!out your promise of not missing

single mar" in UNIT 1*?

A +9UD E LL**ES LLLLLLLLL*4

+et us now shift to UNIT . L**

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