SAWE Report - Cal Poly San Luis Obispo

used to see how the increased weight of a bigger horizontal affects the longitudinal static margin.
This “X-plot”, as it is commonly known, is shown as Figure 10.1.
Figure 10.1 - Longitudinal X-Plot at Mach 0.3
It is notable that only about 110 ft 2 of horizontal area is required to keep the Vendetta neutrally
stable at Mach 0.3. It can be seen that as the tail grows, the CG of the entire configuration shifts
aft. This also shifts the effective neutral point (center of pressure) of the aircraft aft at a faster
rate than the CG shifts aft. A horizontal that is bigger than 100 ft 2 yields a stable aircraft but will
pay the price in trim drag if the aircraft is too stable.
A stable static margin is necessary in flight without the use of a digital flight control margin. The
RFP mandates this as well as adherence to MIL-8785C, the military specification for handling
qualities of aircraft. A statically unstable aircraft would have a tendency to pitch up in a static
level condition. The purpose of the horizontal tail is to apply a force which counteracts this
offending moment. This comes at the price of trim drag, however. As the elevator is deflected,
drag is created and this hurts the overall aircraft performance in cruise. It is because of this that a
neutrally stable or marginally stable (1-3%) aircraft is desired in cruise.
To complicate matters, it can be seen from Figure 6.1 that areas above 110 ft 2 are required for a
Mach number of 0.3. The aerodynamic center (center of pressure) on the wing and most surfaces
propagates aft as the Mach number passes the transonic regime. This shift effectively leaves the
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