Slope of Lift Curve

In calculating the slope of the lift curve, the effective aspect ratio of the surface must first be estimated. Since this can be influenced by the end-plating effects of the tail boom and a horizontal stabilizer, the estimating process must account for these components. The method of reference 8.1 makes use of the following equation:

where the three factors; (A. R.K/A. R.K+B), (A. R.V+B+H/A. K.V+B), and KH are given as functions of geometric parameters in Figure 8.19. As an illustration, the example helicopter parameters are:

Sv = 33 bv=U <>=4.25

2rx — 1.5 SH= IS x = 2.5 Ztf = o

The resultant values of the factors from Figure 8.19 are:


and the resultant effective aspect ratio is:

A. R.Krff=3.2

Factor accounting for relative size of horizontal and vertical tails

Horizontal tail area Vertical tail area, measured from fuselage centerline Vertical tail span, measured from fuselage centerline Vertical distance between horizontal surface root chord and fuselage centerline, positive for surface below fuselage cpnterline

Source: Hoak, “USAF Stability and Control Datcom,” I960.

Once the effective aspect ratio has been determined, the corresponding slope of the lift curve, a„ can be found from Figure 8.6. For the example helicopter with a half-chord sweep of 27°, the slope is 3.0 per radian.

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