WARNING; loooong meandering post to follow...
Been watching this thread with interest. My fundamental question is how do you define "best"; Highest lift? Lowest drag? Blazing corner while flying like a trainer in level flight? Only Muhammad Ali could "float like a butterfly AND sting like a bee", so choose wisely! We have seen credible stunt performance with airfoils ranging from 1/4" thick flat plates (the designs from down under) to really thick semi-symetrical airfoils with huge flaps (Rabe Sea Fury) to laminar flow sections to... you name it. While it is possible to design a BAD airfoil, I think there are other elements of the aircraft design that overhelm the contribution of the airfoil. I want to talk about a couple of those details.
GAP SEALS
THANK YOU BRETT for the shout out, presumably about hinge gap seals. My history on seals is this. When a preceeding design fell short of expectations, I started over with the first Eclipse (1980) with a full airfoil flaps. My 7:1 aspect ratio wing used a dead stock NACA 0018 airfoil at the root and a NACA 0021 at the tip modified with the high point moved forward to 25%. Of course the correct way to build flaps into an airfoil is to use a rolling hinge pocket per Keith Trostle and his excellent Ta-152 & Eagle designs. However, I figured the structural details were way out of my league. I resorted to a refined version of a simple hingeline. I limited the bevel on the flap to allow for a max of 30 degrees movement. I assumed a hinge gap and then put the absolute minimum bevel (i.e, less than 30 degrees) on the LE of the flap that would allow 30 degrees deflection The resulting surface gap was actually smaller than many "normal" airplanes I saw on the flight line. I also used a thickish full airfoil on the stab/elevator which ended up about .5" thick at the hingeline, and used the same minimum bevel philosophy there too. The resulting airplane was a complete dog - it would barely manage a 45 degreee round loop. I immediately gave up on the airplane, my brother wanted to mess with it so he took it home. The NEXT day he invited me to fly it again. The performance was now astounding. The difference was taped-over hingelines. The next Eclipse had the seals designed & built in, and I have used them in every airplane I have built since. BTW, I mentioned the hinge gap seals but no one was listening.
It must have been early 1982 Ted Fancher was in town on a flight layover and we visited. Ted had recently crashed his new airplane and he said the only thing worse than crashing the new airplane was he had to go back to an older airplane that he hated. I mentioned my experience with the hinge gap seals - with the usual caveat "what have you got to lose?" Several weeks later Ted won his first NATs with the gap seals on his formerly doggy airplane. Along with the performance gain, Ted mentioned that he had removed other trim elements that he had previously used to get the bird to fly less-bad (it had never really flown well before). In short, the seals gave him a brand new airplane and he started over on trimming. This just in: Ted is an EXCELLENT builder, but even then it seems the gap seals overcame any inconsistencies he may have had in the hinge gaps, prompting the opinion that gap seals should be used all the time for just that reason. Ted mentioned the effect gap seals had, and between his NATs Championship & MA column (where he generously mentioned where he heard the idea) everybody was listening now!
Real world experience seems to range from "didn't notice a change" to game changer (Ted & I). I can rationalize some possible ideas why, but really do not care. I like the way David Fitzgerald once summed it up: gap seals have all the advantages of adding tail weight with none of the disadvantages. I just use the seals and move on to bigger issues.
WING TO FLAP AIRFLOW
Regarding the discussion here on getting a smooth airflow on the top surface when the flaps are deflected. No doubt, my simple flap on the full airfoil without gap seals was nearly the worst scenario possible. Al Rabe's approach certainly looks like the best approach - but what ANGLE of flap deflection do you design for? Logically it seems prudent to design for the best airflow at max deflection. However, please note: a airfoil designed for a smooth airflow path with 45 degrees flap will be SUB-OPTIMIZED for every flap deflection less than 45 degrees. I typically set up my airplanes for 30 degrees or so max deflection, and I never fly up against the stops, so to me designing for a smooth transistion with 15-20 degrees deflection makes the most sense. I also use slow controls, with CG a bit aft (I think) from what most folks like. Brett has a funny story about checking the CG on one of my airplanes...
Another building detail may be more important than the airfoil shape: how should we deal with airflow jumping the hinge gap? It is fashionable to make the front edge of the hingeline (the trailing edge of wing or stabilizer) squared off, and put all the bevel for surface movement on the leading edge of the flaps or elevator. I suggest this approach is incorrect, for the same reason that using a simple hinge on a thick flap is incorrect. The hard edge on the trailing edge enables/promotes airflow separation - that we then want to re-attach the airflow on the top of the deflected flap. I suggest (and my last several airplanes have exhibited) that the trailing edges should be rounded - to enable a smoother transition from the wing, across the gap, and onto the flap. Yup, I am using 1950's philosphy (plus seals) on hingelines! However this approach, combined with Rabe inspired airfoils with the proper design point on flap deflection, and gap sealed flaps, SHOULD result in the smoothest airflow transition. Then do it again on the tail.
A NEW IDEA
My flying buddy Wesley Dick researched flap design and learned that the double slotted flaps should yield more lift with a lower drag penalty than a simple flap. His personal contribution was to design a simple and robust mechanism that would allow the flaps to deflect in either direction. The double slotted approach actually tries to take advantage of the gaps to create more lift. Wes built two of his "Velvet" designs with normal flaps, then 2 with the double slotted flaps. From the outside of the circle the two airplanes with the double slotted flaps seem to fly better than the two with the simple flaps. There are several new Velvets under construction so we will get some more/new opinions on the merits of double slotted flaps. Then SOMEONE is going to try it for the elevator too.
Of course the trailing edges of the flaps and elevators should be as thin and sharp as possible, but that is another discussion..!