I took a more extended look at the very interesting NACA Technical Note No. 1483 and thought I'd add a comment or two. First, I wouldn't know if it is worth the time to code into AH II the precise reaction of an aircraft losing its horizontal tail. But on the supposition that some day it might be, according to NACA there is a lot more at play here than just c.g. and center of lift.
Three load components for the tail
On page 9 at the top of the page is the formula used to calculate total load on the tail at any given moment in flight. There are three main components to the solution, L1, L2, L3, each of which relate to specific aerodynamic issues. According to the explanation of the first paragraph on page 9, only L1 and L2 are involved if there is no pitching angular acceleration.
Steady-level-flight pitch down moment
Straiga points out in his post above that on page 10, below formula (4), we read that the steady-level-flight data shown by figure 20 indicates a force on the tail of -5.2 lbs/sq ft power-on, and -4.72 lbs/sq ft. power-off. From this the Cm or pitching moment coefficient of the airplane less tail is calculated and is -0.0552 power-on and -0.0501 power-off. That means in steady-level-flight, a tail less plane is prone to nose down whether power-on or power-off.
In fact, the L1 component of the total tail load (measured during condition of zero main-wing lift) is shown in the graph of figure 21 on page 48. An aircraft in a zero-lift condition (dive) shows increasing negative load on the horizontal tail as speed increases. So for an aircraft at altitude in a no-lift dive that loses its tail, the nose will want to continue to drop, not rise.
The L2 component of total tail load is shown in the graph of figure 22 on page 49. The graph shows the effect of g load forces on the tail load for varying c.g.s. Presumably the center line of the graph, 30 percent MAC, is something near average for fighters. So, in normal steady flight of 1 g at 30 percent MAC the L2 component shows to be 400 lbs up load. But don't forget, this tail load factor must be added to L1.
Loads L1 and L2 together
The combination of the L1 and L2 loads in steady level flight power-off are shown in the graph of figure 23 on page 50. Print the page and draw in a curve precisely between the solid black curves labeled 0 and 2 to show the tail load for steady flight at 1 g. At about 200 mph the tail load is zero, at 300 we are getting close to -1000 lbs on the tail. And this is even with the c.g pushed far back at 34 percent MAC. Moreover, as mentioned above, power-on would add further nose-down moment, requiring further negative load on the tail to balance the flight.
Flight data shows down load on tail at higher speeds.
Table 1 on page 19 records data during level flight, power-on and power-off.
No speed indication is given, but engine power is in the fourth column "bhp."
The load at the c.g. is given in the sixth column "n c.g. (g)"
The coefficient of lift is given in the seventh column "CL"
Dynamic pressure is given in the eleventh column "q (lb/sq ft)"
The load on the tail is shown in the last three columns.
In all the runs of flights 4, 5, and 13 the load n at c.g. stayed close to 1 g. In every case in those runs where the bhp went above about 500 (indicating a higher speed) the tail load was negative. In those cases, the coefficient of lift "CL" is low, and the dynamic pressure "q" is high, so presumably, even with power off, if the coefficient of lift "CL" is low, and the dynamic pressure "q" is high, the aircraft would seem to be moving at a higher speed. On all those power-off runs we again see a negative load for the tail.
NACA says on page 15 in the third paragraph regarding steady-flight:
"The largest up tail load will occur the center of gravity rearward, a large value of airplane load factor, moderate airspeed, and low altitude with power off."
"The largest down tail load, however, will occur in high-speed, power-on flight at high altitude, with a large negative load factor and with rearward center-of-gravity location."
It seems then, that the current "nose up" attitude assumed by tail less planes is okay for slower speeds and g maneuvers. But for high speed level flight, and dives, the nose apparently should drop on a plane that loses its tail.
Nevertheless, thanks again for a fine product.
Cement1
