23 Up might not do it
Trim tab angles aren't mentioned in the builders manual. They probably should be, as flight control surfaces are of great importance to the proper flying of any airplane.
Trim authority required is directly related to airspeed and CG. The lower the airspeed and/or the more forward your CG, the more authority you need. Authority can be had with either more trim tab area, more tab angle, or both. So, to trim at a normal approach speed requires lots more tab angle (authority) than it would to trim, say, for 100kt. And to trim all the way into the flare takes LOTS of tab angle.
Being a practicing aero engineer, I calculated the tab angles I'd need to trim my RV4 into the flare assuming fairly standard CG and GW values. I came up with the need for about 45 degrees of NOSE UP trim movement to do that. I settled with 35 on my RV4 which got me down to a minimum trimmed airspeed of about 60 mph at a forward CG (pilot and fuel only). Since I normally approached at 70, that was enough. Incidentally, I could actually trim right into the flare with only 35 degrees tab angle if I had a heavy weight passenger in the back (about 210 pounds in the back seat). This made my RV4 nearly neutral in terms of static stablility. So the lesson here is that your CG location will also affect how much trim authority you'll need. More tail heavy airplanes need less trim authority to fly at slower speeds. They also will feel less pitch stable, so there is a tradeoff for this.
IMO, I'd increase the tab angle on your project as 20-23 probably won't be enough to trim the airplane for a normal approach. I believe its important for a pilot to be able to trim an airplane at normal approach speeds, and 23 degrees probably won't cut it. Try to get it to 30 as a practical minimum.
Best Regards,
Bill Wightman
OKC, OK
