So, if I built a 1/8 scale champ it should weigh about 100 lbs. but cruise at about 12 mph. Pretty crazy. A 1/8th scale champ that weighs even 5 lbs. will likely fly way to fast to appear scale due to high wing loading. The ultra light electric planes really do look scale on a really calm day but add a 5-7mph wind and they look quite un-scale due to the response to turbulence.
Anyway with my current fleet of 2 scale planes I only use full throttle on some takeoffs and rarely fly over 1/3 throttle in full flight.
Maybe I can find a happy medium at about 27-30 oz. for a 53" span.....
In the late 1990's I built a 1/4 scale Piper Cub from a kit. That model weighed in at 16 pounds, with 38 Nicad cells for battery power. It looked to just hang around in the sky while flying, but ground checks showed that it was actually flying at about 45 Miles Per Hour.
I ran your numbers of a 53 inch wingspan, and about 600 square inches through www.motocalc.com
, playing with the motor/battery to get 30 ounces model weight. Motocalc predicts that your stalling speed with the 30 ounce weight will be about 13 Mph, so your flying speed would likely be in the range of 25-30 MPH.
So if you tried to fly that model in even 5 MPH winds, it might be a real handfull. I'm by no means an expert in aerodynamics but something called Renolds Effect has a lot to do with scaling down a full sized airplane down to a model size. What it means is if your full size airplane flys at 80 MPH, a 1/4 scale model might not stay in the air at 20 MPH.