I agree, I had a goldberg eaglet 50 that was very light... it was a delight to fly low and slow... then my cousin rebuilt it with monster slotted flaps to make it even slower... all it did was increase the weight and turn it into a pig with all that drag. It couldn't even be landed with full flaps power off, but the 45 degree approach was kind of cool. Later I built a heavy lift plane with the gottingen 501 and Junkers style flaperons trailing below and behind the wing... it was great without the flaps down, but with the flaps down it was a pain to fly with all the drag. To get the benefit of the flaps you have to fly behind the power curve with substantial power on. That means lots of prop effects, lots of adverse yaw ect... very difficult with an RC. If you want a high lift device that doesn't add weight and is much easier to handle go with leading edge slats. I mounted fixed leading edge slats to a balsa USA stick 40 and I could fly it at 30 degrees AOA... again behind the power curve and fairly tough to do, but much easier then using flaps. Flaps actually lower stall AOA so you end up having to be very careful flying close to stall.... slats on the other hand increase stall AOA and make slow flight much safer and predictable.

Ty