Drag Parameters

In the equation for the drag of the vertical stabilizer, there are three new terms that must be evaluated; 6,; Cn, and АД. . The first two also occurred in the equation for the drag of the horizontal stabilizer and can be found by the same methods using Figures 8.17 and Figure 8.19. For the vertical stabilizer of the example helicopter, the required parameters have already been calculated for use in determining the slope of the lift curve. They are:

Xv= .21

A. R.^cff= 3.2

From Figure 8.17:

6, = .01

and from Figure 6.30 of Chapter 6, at a Reynolds number of 2 x 106 corresponding to a flight speed of 150 knots:

Cn = 0.0064

u0

The vertical stabilizer and tail rotor have a mutual interference sometimes called biplane effect, which results in the combination having a slightly higher induced drag than if they were isolated components producing the same side force and thrust. Using the methods derived for biplanes in reference 8.17, an equation for the additional interference drag can be written in helicopter terms:

where KlM is the interference factor, which is a function of the tail rotor radius, the span of the vertical stabilizer, and the separation distance between the two as shown in Figure 8.22. For the purposes of this estimate, the vertical stabilizer force, Yv, can be assumed to be the value calculated assuming no tail rotor interference effects; thus:

Qm у jv_

It V It

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