TwoDimensional Airfoils Aerostudents（TwoDimensional翼型Aerostudents）.pdf

Two-Dimensional Airfoils 1 Definitions There are various ways to describe an airfoil. The NACA-terminology is a well-known standard, which defines the following airfoil properties. The mean camber line is the line formed by the points halfway between the upper and lower surfaces of the airfoil. The most forward and rearward points of the airfoil are the leading edge and the trailing edge, respectively. The straight line connecting the leading and trailing edges is the chord line. The length of the chord line is defined as the chord c. The maximum distance between the chord line and the camber line is called the camber. If the camber is 0, then the airfoil is called symmetric. And finally, the thickness is the distance between the upper and lower surfaces of the airfoil. In this chapter we will be looking at 2-dimensional airfoils. We’re interested in finding c , the lift l coefficient per unit length. At low angles of attack α, the value of cl varies linearly with α. The dcl lift slope a0 is the ratio of them, so a0 = dα . If α gets too high, this relation doesn’t hold, since stall will occur. The maximum value of cl is denoted by cl,max . This value determines the minimum velocity of an aircraft. The value of α when cl = 0 is called the zero-lift angle of attack and is denoted by αL=0 . 2 Vortex sheets In the last chapter we treated the source panel method. We put a lot of sources on a sheet. We can also put a lot of vortices on a curve s. Let’s define γ = γ (s) as the strength of the vortex sheet per unit length along s. The velocity potential at some point P can then be determined, using dφ = −γ ds θ ⇒ φ = −