Those who have clocked flying speeds of 20 mph will know that it is all but impossible to do with wingloadings much over 10 oz ft you can wash out wash in wash up -whatever makes no difference once you attempt speeds where required low wing loadings are impossible to produce
My Seniorita which weighs 4 lbs and has 800 plus squares -on a low aspect ratio wing -will barely stay controllable at 20 mph.
My ERATIX weighs 3.75 lbs and has well over 700 squares on a very low aspect very numb thichk wing and yet -it's min flying speed is about the same

My 450 sq in flat foam wing ONE LB 3D model will fly controllably UNDER 20 mph
note: it has a 1/4"thick flat wing an no coupled flaps
yet it easily flys the slowest-controllably
Why?
it ain't the airfoil -it's the weight that gets you at low speeds.
Scale speeds would work on th DC3 if it were a flat foam electric weighing a few pounds
you could even add a scale airfoil -if the weight stayed the same but in this case- weight is everything.