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Chord of a Profile
A chord of any profile is generally defined as an arbitrarily fixed line drawn in the plane of the profile, as illustrated in Figure 1.8. The chord has direction, position, and length. The main requisite is that in each case the chord should be precisely defined, because the chord enters into the constants such as the lift and drag coefficients, which describe the aerodynamic properties of the profile. For the profile shown in Figure 1.8(a), the chord is the line joining the center of the circle at the leading and trailing edges.
For the profile in Figure 1.8(b), the linejoining the center of the circle at the nose and the tip of the tail is the chord. For the profile in Figure 1.8(c), the line joining the tips of leading and trailing edges is the chord.
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(a) Leading and trailing edges are circular arcs. |
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(b) Circular arc leading edge and sharp trailing edge. |
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(c) Faired leading edge and sharp trailing edge. Figure 1.8 Illustration of chord for different shapes of leading and trailing edges. |
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———— Chord c——————— Figure 1.9 Chord of a profile. |
A definition which is convenient is: the chord is the projection of the profile on the double tangent to its lower surface (that is, the tangent which touches the profile at two distinct points), as shown in Figure 1.9. But this definition fails if there is no such double tangent.
