My Thermal Dart was built from a kit.  It is a larger version of the Delta Dart or AMA Cub and is a very simple kit to build.  It is fairly easy to get flights of over a minute, and in that time it can go far.  The Kit is available  and cheap        (www.sigmfg.com/products/sig-thermal-dart-kit).  The Thermal Dart has a 24 inch wing span.  Mine weighed 24 grams.

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Wing Area is 76 square inches.  Length from the propeller hook to the rear hook for the rubber motor is 12 inches.  Propeller diameter is 7 inches.
Rubber used was a 14 inch loop ( 2 strands ) of 1/4 inch wide rubber or the same length of 2 loops ( 4 strands ) of 1/8 inch wide rubber, weighing = approx. 5 grams
I wound the motor with 800 turns ( approx. 30 turns per inch ) to get a flight of 45 seconds with nice climb.