chapter 3 aerofoils

8/19/2019 Chapter 3 Aerofoils

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Chapter - 3


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AEROFOIL TERMINOLOGY

• Chord: Distance between leading and trailing edges.

• Chord Line: A straight line joining the leading and trailingedges of an aerofoil.

• Mean Camber line. Line joining the leading and trailing edges

of an aerofoil which is equidistant from the upper and lowersurfaces.

(A symmetrical aerooil has no camber!

• Thic"ness#Chord Ratio. he ratio of ma!imum thic"ness to#ean Chord$ e!pressed in percentage of the chord.

• An$le o Attac" (%!. he angle between the chord line and

the relati%e airflow&relati%e wind&free stream flow.


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•An$le o Incidence. The an$le bet&een the chord line andthe hori'ontal datm o the aircrat.(This an$le is i)ed or the &in$!.

•Relati*e Air Flo&. Airlo& that is +arallel to the Fli$ht,ath bt in the o++osite direction.It is close to the air+lane bt its *elocity- +ressre tem+eratre are not aected by the +resence o theair+lane. Its ma$nitde is the TA/.

•Center o ,ressre (C,!. The +oint on the chord linethro$h &hich all the Total Reaction acts.

(Lit is a com+onent o Total Reaction!

•Total Reaction. The resltant o all aerodynamic orcesactin$ on an aerooil.

It acts through the center of pressure.


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he difference of cur%ature between the top andbottom surfaces of the aerofoil with the uppersurface ha%ing a larger cur%ature causes theairflow to tra%el a longer distance than the lowersurface. o reach the trailing edge of the aerofoil atthe same time$ the airflow o%er the aerofoilaccelerates causing a drop in pressure as

compared to the lower surface. The net resltcases +ressre dierence bet&een the to+ the bottom srace &hich reslts in an +&ardorce (Total Reaction! and its *ertical

com+onent is called LIFT.

- he ma0or component of lift is generated b' thene$ati*e +ressre on to+ o the aerooil.


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LIFT 1 CL 23*4s

5RAG 1 C5 23*4s

,itchin$ Moment 1 Cm 23*4s c

In aerooils 6s7 is the +lan area

6c7 is the chord len$th


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LIFT *s. A o A

• Lit increases as the 6A o A7 increases

• As the A o A increases the Center o,ressre mo*es or&ard on to+ srace

• /e+aration ,oint also mo*es or&ard &ith

increase o 6A o A7 on the to+ srace

• E*entally lit cr*e reaches ma)imm

*ale at the stallin$ 6A o A7 i.e. 89:

• Lit alls sddenly &ith any rther

increase in 6A o A7 de to or&ard tra*elo se+aration +oint


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Lit rom/ymmetrical Aerooil

Lit only starts abo*e ;: an$le o attac"


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,arasite 5ra$< Cased by non=lit $eneratin$sraces and body +arts.

Form 5ra$ (,ressre 5ra$!< Cased de tosha+e and si'e o the aircrat and its +arts.

/"in Friction 5ra$< Cased de to rictionallosses bet&een *iscos mo*in$ air and theaircrat srace. (i.e. smooth srace &ill+rodce less /"in Friction 5ra$!.

,roile 5ra$< /"in Friction and Form 5ra$are to$ether "no&n as ,ROFILE 5RAG


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Intererence 5ra$< Cased de tobondary layer

>intererence? at&in$#sela$e-&in$#en$ine- andother 0nctions.

Indced 5ra$< Cased de to lit$eneration.


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Aerooil Lit Moment and Aerodynamic Center

Aerod'namic Center Aerod'namic Center Aerod'namicCenter

Aerodynamic Centre: AC is a fi!ed point on the aerofoil where the %alue of lit indced +itch moment remains constant at

(ormal Angles of Attac" )o-*+,.


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C@ARACTERI/TIC/ OF I5EAL AEROFOIL

@i$h Ma)imm Lit Coeicient

Good Lit#5ra$ Ratio

@i$h Ma)imm ale o CLB# #C5

Lo& Minimm 5ra$ Coeicient

/mall and /table Mo*ement o Centre o ,ressre

/icient 5e+th


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C


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CL C@ARACTERI/TIC/CL D It is the ratio bet&een lit +er nit &in$ area and the dynamic+ressre

CL - s determined e!perimentall' for an aerofoil.

CL / Depends upon bod' shape$ angle of attac" and A0.

CL / s a measure that indicates how efficientl' the wing transformsd'namic pressure into lift.

CL / ncreases with increase in Angle of Attac"

CL (ma)! / #a!imum %alue of CL is reached at about *+ Angle of Attac".

CL / 1educes significantl' abo%e *+, A of A- indicatin$ a >/tall?.

Im+ortant : @i$h lit de*ices increase the camber o the &in$- therebyincreasin$ the CL.

chapter 3 aerofoils

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