Chapter 6: stability and control
Figure 6.12 Important Velocities and Angles for Longitudinal Stability Analysis
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Figure 6.12 Important Velocities and Angles for Longitudinal Stability Analysis In Figure 6.12, the aircraft angle of attack is measured between the aircraft reference line and the free stream velocity vector,  . For simplicity in this analysis, the aircraft reference line is chosen to coincide with the zero lift line of the wing and fuselage (a refence line such that when it is alligned with the freestream velocity, the wing and fuselage together produce zero lift). As a further simplification, the contribution of the horizontal tail lift to the whole aircraft lift (but not the tail’s contribution to the moment) will be ignored. With these assumptions L = 0 = 0 and a = . At the horizontal tail, the local flow velocity vector is the vector sum of the free stream velocity and the downwash velocity, Vi. The angle between the freestream velocity and the local velocity at the tail is the downwash angle, . The angle of attack of the horizontal tail (stabilator in this case) is labeled t . It is defined as the angle between the horizontal tail chord line and the local velocity vector. The angle between the horizontal tail chord line and the aircraft reference line is called the tail incidence angle and is given the symbol it.
The geometry of Figure 6.6 was used in Section 6.3 in the longitudinal trim analysis. For that analysis, it was required that the moments about the aircraft’s center of gravity sum to zero. The same geometry is used to determine CMo , except that the forces and moments are written in terms of non-dimensional coefficients, and they do not necessarily sum to zero. The expression for CMo is obtained by dividing (6.1) by  , where  is the reference chord length of the wing:
 (6.4)
The following definitions are made:  (6.5)
so that (6.4) becomes:  (6.6)
The ratio defined in (6.5) which was given the symbol VH is called the Directory: ~jps7 -> Aircraft%20Design%20Resources Aircraft%20Design%20Resources -> Systems in Transportation: The case of the Airline Industry Download 2.68 Mb. Share with your friends:
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