Quote from Al Rabe .

As far as I know, the 1973 Mustang V was the first stunt ship design taking tail volume calculations into consideration for determining moments and areas. First, I measured all of my airplane’s moments and areas to determine their tail volume and related this to their handling characteristics. This gave me an approximate design goal for the Mustang V. To calculate a tail volume, I use the entire width of the wing including flaps, and emphanage (stab plus elevators) as shown in the plan view to determine the mean aerodynamic chord (MAC) of these surfaces. The MAC is the wing chord location where the areas of the wing, both inboard and outboard of that chord, are equal. The calculation takes into consideration the width of the wing and emphanage MACs, the areas of the wing and emphanage and the distance from the center of gravity to the emphanage, quarter chord This formula tells us that for a given value of tail volume we can vary the area of the tail, or length of the tail moment in our calculations. Specific TVCs can be calculated by varying the chords, moments or areas within the formula. The engineering need for using MAC lies in stability calculations for airplanes with swept wings where the MAC may be a long way from the root chord where the wing enters the fuselage.

Is there a builder who hasn’t considered the possibility that if enough is good, then might not more be better? The problem with adding both tail area and tail moment, or a lot of either, is that it creates very powerful pitching moments, which is another way of saying very sensitive and responsive controls. Strong pitching moments and sensitive controls make airplanes turn easily with small control deflections. Large tail volumes though, don’t work well for semi-scale airplanes because powerful controls limit control deflections. Semi-scale airplanes with reduced wing area depend on airfoils and flaps designed to produce the maximum possible lift, and this only occurs with relatively large control deflections. Practically speaking, I can get the same corner radius and control sensitivity with a different configuration, but need a carefully considered tail volume and larger line spacing at the handle. Tail volume coefficients must be big enough for longitudinal stability and small enough to allow large control deflections for maximum lift. My design TVC is .4.

What about moving the CG back to reduce longitudinal stability a bit for sharp corners with only a twitch of the wrist? This is an example of how builders can make a seemingly minor and logical change which has dramatic effect. Moving the CG back 1/2” has a negligible effect on the tail volume. This would typically be less than .01 or, for example, from .42 to .41. That same 1/2” happens to be a very large reduction in the distance between the center of gravity and the center of lift for a substantial loss of stability. We could reduce elevator sensitivity by making the elevator area or deflection smaller. I can only guess what, if any, desirable effect this reduction in stability and less effective elevators might have.

Area limited airplanes must have bellcrank/flap horn ratios large enough to deflect the flaps. Flap/elevator ratios, elevator area, CG location and tail volume can all be varied, but with results difficult to predict. I stick with relatively forward locations of the CG, and target TVCs, for adequate stability and use approximately 50-50 elevator/stab areas. This puts me in the ball park with enough pitch for a competitive corner, and enough flap deflection for the necessary lift. Performance is fine tuned with trim adjustments.

Personally, I consider tail volume calculations necessary only for the design of stunt ships which depart from conventional values. My goal is to make a grooving airplane with a competitive corner without over sizing moments or areas. The calculated value is useful, but at best, it is only a rough estimate in determining the distance between the wing and tail or an appropriate area for the tail. In any case, when designing the Mustang, I drew the fuselage using the desired outlines and tail moment, and used the formula to determine the area of the tail.

If you are interested, the formula for tail volume coefficient is: area of the tail divided by the area of the wing, times, length of the tail divided by the wing MAC. A value between .4 and .45 is usually enough.

Might pay to skip to the last paragraph .

A few of mine with inadequate LOOKING tailplanes , are inside the formula , and work fine .

Just to throw a spanner in the works , Im thinking Trailing Edge Deflection is more relevant than elevator area .

I like equal or larger elevators , which makes em toey .

Conversly , narrower elevators requier more deflction for the same ' offset ' dimension ;

( Distance above centerline Vs angle of deflection of control surface )

and would be less jittery .

Theres a ton of stuff here ,

https://stunthanger.com/smf/open-forum/m-a-c-t-v-c-aerodynamic-center-and-'tail-moment'-formulas/msg482235/#msg482235and elsewhere , Search :" Tail Volume " .