Airfoil

AIRFOIL 

The concept of the wing and vertical and the horizontal tail was discovered by George Cayley in 1799. Consider the wing of aircraft, the cross-sectional shape obtained by the intersection of the wing with the perpendicular plane is called Airfoil. The thickness distribution of the airfoil essentially controls the lift and the moment characteristics of the airfoil.

NOMENCLATURE OF AIRFOIL 

MEAN CAMBER LINE: It is (which divides the airfoil into two equal parts) if we plot the points that lie half the way between the upper and the lower surface we obtain a curve.

LEADING EDGE and TRAILING EDGE: The most forward and the rearward point of a mean camber line is the Leading Edge and Trailing Edge respectively

CHORD LINE: The straight line connecting the leading and trailing edge is called a Chord Line of an airfoil.

CAMBER: The maximum distance between the mean camber line and chord line, measured perpendicular is called as Camber.

CHORD: The precise distance from the leading to trailing edge measured along the chord line simply designated the Chord (also written as c) of an airfoil.

                                               figure A

When the airfoil is inclined with relative wind another term angle of attack is defined.

ANGLE OF ATTACK: The angle between the chord line and the relative wind is called Angle of Attack (α)

The angle of attack is written as (α)

Relative wind is written as V∞

V∞ is the free –stream velocity of the air far up-stream of air. The direction of V∞ is defined as the RELATIVE WIND (V∞)

TYPES OF AIRFOIL 

1. CAMBERED AIRFOIL

The airfoil in which we have camber 
When α=0, there is still a positive value of lift coefficient (Cl) this means there is still some lift even when the airfoil is at zero angles of attack of the flow. This is due to the positive camber of the airfoil. The value of α when the lift is zero is defined as the ZERO-LIFT ANGLE OF ATTACK (αL=0)
The positive camber of the airfoil means airfoil is not symmetric it has some camber.

                                                    figure 1

As we can see in the above in diagram the lift curve doesn’t pass through the origin which means that if α =o so lift is not equal to 0 

2. SYMMETRICAL AIRFOIL

 Symmetrical airfoil is an airfoil in which mean camber line and the chord line are the same i.e. there is no camber in symmetric airfoil

                                                figure 1a 

As we can see in the figure the curve of the symmetrical airfoil pass through the origin that means that when α =0 there is no lift produced by the airfoil

If we see both the diagram properly as we increase α beyond a certain value of Cl  there is a rapid decrease in Cl at high α, the airfoil is called as STALLED

CAUSE OF STALLED

Stalled is caused due to flow separation on the upper surface of the airfoil as shown in the figure as the stall airfoil means nearly no lift, this phenomenon of airfoil stall is very critical importance in aeroplane design

How to read NACA serires

case 1  

   4 digits NACA series 

   NACA MPXX 

 M= max camber  (M/100)

 P= chordwise position of max camber(P/10)

 XX= max section thickness(XX/100)

example 
        NACA 2412 

           M=2 

           P=4

        XX=12

max camber is 2/100=0.02

chordwise position of max camber is 0.4

max section thickness  is 12/100=0.12

case 2 

5 digits NACA series 

NACA  C_LPPXX 

 C_L  = Lift coefficient ( C_L*1.5)

  PP= chordwise position of max camber(PP/2)

  XX= maximum thickness (XX/100) 

example  

             NACA 12018 

                           C_L=1

                        PP= 20
                        XX=18
   the lift coefficient is 1*1.5=1.5
   chored wise position of max camber is 20/2=10
   maximum thickness is 18/100=0.18