Why 38 mph? -- Good point!
I guess the first question was: how fast can I get in 200 feet and still be able to control the airplane ? With a 12 in chord, the only controllable variable I had to increase Re was speed, correct?!
Then my tortuous reasoning was:
1. FX.61 for max static thrust would require, probably, a 13X4 propeler;
2. FX.61 with 13x4 will turn @10-12k (?);
3. A 13x4 moves 4in fwd for each revolution, theorically. @10k, that's 38 mph.
4. Trainers do it (TO) around 24 mph (ref. Andy Lennon's book) - I'd probably be able to control that...;
5. Competitors from previous indicated they were working with Re around 250 ~ 300k.

Honestly, I never challenged that first assumption...it just seemed I was in the ballpark...and, you know, I want to lift as much as I...ehr...the airplane can!
Sometimes we want to believe stuff, you know, and we build reasons to support it...human factors, right?
There's even data for the S1223 @ 300k, etc. I wanted so hard to believe it...

Actually, I've been reading about aerodynamics for a while, but the idea of competing in SAE Aerodesign is new...and the airplane is in the conceptual design phase yet

And this forum is all I ever asked for!!! Thank you all for sharing!!!
I'm learning a lot!!! Rgds. Souza.