Roger with the stab tilt. Interesting isn't it.

I thought a graph or two might help some with the direction and amount of tail loads involved in the typical symmetrical aerobatic model. Remember graphs are our friends. Don’t hold my feet to the fire with absolute numbers. I hope the plots are readable. I take no credit for mis types, mistakes and the like. This is just some representative numbers to show trends.

Definitions

dH is the horizontal tail deflection in degrees. The convention is that + is the deflection to cause an up load on the tail.
Alpha symbol is angle of attack in degrees.
CL is the lift coefficient. Up is positive.
CM is the pitching moment coefficient. Nose up is positive.

Think of CL as lift, Cm as the pitching moment.

These are curves that might come out of a wind tunnel. Looking at the CL verses angle of attack plot, for the zero tail deflection case, we see that the line goes through the 0-0 point of the axes. CL gets larger at high alphas and goes to large negative values at low alphas. This is what you expect.

The CL vs Cm curve for the zero tail deflection case goes through 0-0 and has for positive CL values a negative Cm (airplane nose down). This indicates that the airplane is stable and a small disturbance in angle of attack will return to the trimmed point.

Now for the tail deflections. Looking at the CL verses alpha plot for + values of alpha and + dH at each angle of attack the increment is up and the overall lift is positive as long as alpha is positive. When the dH is – the increment is down but the overall lift is still positive. Of course the trend is reverses at – alphas.

Looking at the CL vs Cm plot and for + values of dH the moments are negative which is airplane nose down. For – values of dH the moments are positive which is airplane nose up. Each line of dH data is parallel to the dH=0 line so the airplane remains stable but will go nose up or down as commanded.

The curves will shift as airfoil/wing and airplane-body lift and pitching moment characteristics change.

So what do the plots finally show?? An airplane flying in the upper right hand quadrant of the CL vs alpha plot will have tail loads that are up. The exception is for transients at the beginning of maneuvers at low angles of attack. Once the maneuver has obtained steady state conditions (with our models this is really rapid) the loads are up on the tail. Fly upside down and you are in the lower left quadrant.