Summary
I used XFoil to calculate characteristics of this peculiar airfoil for seven elevator positions. The data show that the boundary layer transition jumps abruptly from the stab LE to the crack. A limited test shows that Reynolds number effect is puny.
Method
I used XFoil via Profili Pro. I eyeballed a stab drawing to pick something close to the mean aerodynamic chord of the original Impact stabilizer and elevator. I picked a place where chord was 7 inches, which is close enough. I used Bill Lee's Reynolds number calculator to find the Reynolds number for a 5.3-second lap time on 70-foot lines on a sea level standard day. XFoil rounded it off to 307K. I assumed incompressible flow (Mach = 0) and picked 6 for the XFoil Ncrit value on the advice of real aerodynamicists. I calculated my own airfoil shapes with deflected elevator, rather than using Profili's flap function. XFoil barfed when I tried to replicate the actual flap hinge geometry on wing airfoils, so I prophylactically faired the stab to the elevator with straight lines between tangents to the stab TE and elevator LE or to the point where the LE or TE curve transitioned to a straight line. It appears that XFoil or Profili went on to do additional smoothing. This smoothing may have perverted the airfoil shape some. I entered one point every three degrees around the stab LE and TE and the elevator LE, but no points on the straight line segments between these curves. The spline function may not have felt sufficiently constrained by these straight segments. I did not close the 1/32" elevator TE thickness on the files I entered into Profili. XFoil gave warnings that it did not converge, but went ahead to calculate most conditions.
I have difficulty communicating with Profili, as I do with people. Sometimes it refuses to plot some conditions, often for cause. Many of these refusals seem to be for no aerodynamic reason. The 25-degree elevator plots, for example, are truncated. The plots for elevator deflection up to 10 degrees look pretty complete, but those for steeper deflections are weirder. These are ugly airfoils, so I guess we shouldn't expect much.
Results
Igor Burger and Frank Williams have written about movement of the laminar-turbulent boundary layer transition as a cause for stunt planes "hunting". I was interested in boundary layer transition flying level right side up and upside down and in trying to figure out why the Impact's peculiar configuration of zero stab incidence, a flat stabilizer, and elevator downrig works so well at preventing hunting. In particular, I was interesting in extrapolating its nice characteristics to electric stunt planes. The first attachment shows the airfoil and the elevator positions I tested. Next is lift vs. angle of attack. Behold that the stab probably spends most of its time at negative alpha. Next is moment coefficient (of the stab only, referenced to stab area and chord). Then is transition point. Xtr is the fraction of distance from the LE of the stab to the TE of the elevator. The hinge line is at .6.
Some folks were speculating that a low-aspect-ratio stab would benefit from having a higher Reynolds number than a longer, skinnier stab. I ran the five-degree-elevator (neutral for the Impact) case at three different Reynolds numbers. Lift coefficient and transition plots are shown. Looks like you wouldn't pick a stab chord based on Reynolds number.
These data might be useful if we knew at what alpha or lift coefficient the stab operates in various tricks. Figure that out and let me know.