SAWE Report - Cal Poly San Luis Obispo

difference in neutral point and center of gravity greater. This difference means the aircraft is
actually more stable in a supersonic cruise. The fact that the center of gravity is so far forward in
relation to the neutral point causes the aircraft to pitch down. More trim is required which causes
drag. This phenomenon is known as Mach tuck.
It is because of this that the weight and balance of the aircraft must be closely in synch with the
control system. Trim drag will be minimized and controllability will be enhanced with
completely integrated systems.
Canting the horizontals in a v-tail configuration was investigated in an attempt to shape the
empennage in a stealthy manner. The effective area of the vertical and horizontal are functions of
the square of the cosine of the cant angle. These effects are reflected in Figure 10.2.
Figure 10.2 - Horizontal Area Required for Static Stability with Cant Angle
It can be seen from the plot that as the cant angle increases the total planform area of the
horizontal must increase to maintain the nominally desired static stability of 5%. Five percent
was chosen because at this stage in the sizing it was uncertain what the dynamic characteristics
of the aircraft would be. Attempting to maintain a minimally statically stable aircraft would ease
the job of control system design if future needs warrant one. Angles up to 30° were looked at
because it would be unwise from an RCS point of view to approach a 90° angle created by larger
cants near 45°. Beyond 45° the trend would be the same; however the horizontal would drive the
area instead of the vertical.
This plot shows that only 118 ft 2 of horizontal area is required to maintain the desired static
margin. This is far off from the historical class I method and by initial inspection appears small.
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