I have a very old copy of the David Faser sailplane analysis tool that used the early Selig airfoil information along with some other designing formulas to help predict some of a sailplane's performance. One of the parameters that is in the printouts is a listing of the stabilizer Cl as a function of the speed. When the CG was set back near the neutral point you could see the Cl for the stab go from slightly positive to slightly negative and you could move the 0 point around with a few minor changes to the design. The numbers in these cases was always very small. This is also why for a properley "close to neutral" trimmed model there is no real advantage to using an upright or inverted lifting stab. As I recall the Cl is positive at low speeds and negative at high speed.

There was also some info I read somewhere that said that at low Reynolds numbers the higher aspect ratio planforms stall earlier. The outcome was advice to always have the aspect ratio of the stabilizer be about .5 to .8 of the wing. That ensures the stab chord is sufficienct to prevent an early stall compared to the wing.


With the lift coefficients being so small in a well trimmed design the actual planform becomes more one of style. Only one item changes this. I've seen information about the turbulence of control surfaces that extend right to the tips. When deflected they iduce vortices that create drag. Having a small fixed portion of the main surface outboard of the movable surface acts like an end plate to reduce this vortex formation. There is obviously some turbulence created by the angled break but it's apparently a smaller issue compared to the votices. This is why a lot of high speed designs have the tips that are full chord. The outer fixed portion needs to be about 3 times the chord of the control surface for this to properley dampen the vortices. This is shown on the diagram of the Avionik surfaces.