What is typical max aft CG limit in terms of % MAC, for full size flapped stunter ?
Edit: Composed while Steve was posting his response.Allan-
The most agreed upon approximation I've found is Ted Fancher's rule of thumb for flapped stunters, which suggests that the c.g. be placed at a percent of the wing's MAC equal to the horizontal tail area's percent of the wing area. So the c.g. for a flapped stunter with a tail area 25% of the wing area would have it's c.g. at .25 MAC. I don't know how far, and under what conditions, one could exceed this percent, but assume that as flap area and deflection diminish, the c.g. must move forward so that it's in the area of 15% - 19% MAC without flaps.
I disagree with some of what has otherwise been posted. For instance, adding an inch of tail arm would only move the aircraft's neutral point (NP) back a full inch if the tail area were over twice the wing's and if were 100% efficient. For the "typical" stunt proportioned model described below, an inch extension would only move the NP a small fraction of an inch along a 10" chord. To add a tail to the combat wing, you'd naturally need several inches of tail length to begin with and then add to that.
The neutral point for the whole plane is not easy to compute accurately, since it depends on several things pertaining to the horizontal tail's area, position, and efficiency. I've seen several methods on the internet and in recent texts, each with it's own set of approximations or assumptions. You can do a search here and on SSW Forum and find a lot on this. These computations seem to be very approximate for us without CFD.
Anyway, because the tail is not as efficient as the wing, the neutral point of an entire aircraft with a "normal" stunt-proportioned stab/elevator is noticeably, but not extremely rearward of the wing's N.P. If you're interested, the
RC Aircraft Proving Ground site has a calculator that does all the work for you. Just go here and put in your model's dimensions:
http://www.geistware.com/rcmodeling/cg_super_calc.htm(Remember though that their method has some simplifying assumptions too)
For instance, I chose proportions of a typical modern stunter, minus the tapers. I used constant chord wing (10") and tail (5") of spans 50" and 25" respectively. These give 500 in
2 wing with a 25% tail. Using a tail-volume coeficient of .5, I went ahead and computed a tail length (between neutral points of wing and tail) of 20". That gave a distance between Wing and tail leading edges of 21.25". I just plugged these values into their blanks, clicked on their "button", and got my answers:
NP of the entire plane is 4.66" behind the wing's leading edge, or at 46.6% MAC. If you put in trial-and-error static margin figures to get the c.g. at 25% MAC, as in Ted's rule of thumb, you'll find a static margin of 21.55%, which is very much greater than full-sized aircraft, RC, or FF models. That seems to be a characteristic of CL controlled models and has been posted here before.
A simpler computation that assumes 75% tail efficiency and just weights the wing and tail moments by area gives a NP at 53.75% MAC for the same dimensions. Increasing the tail length by 1.0" moves the NP, computed this way, only .15" rearward. Using equal wing and tail areas (500 in
2 each) and the same tail lengths gives a change in NP position of 3/8". That would be 1/2" for a 100% efficient tail.
You can plug these numbers in at RC Aircraft Proving Ground and instantaneously get their presumably slightly more refined answers. No math is involved - for you. Just measure a plane and plug in the measurements. The answer comes out. Have fun - or else!
SK