Have to go back a lot of years for this. A test glide for the free flight types was standard during the 30's. A gentle toss to see if the CG was correct. Dead engine of course. If the model had a straight glide and went about 30 feet in a gentle descent it should be O.K. to fly with power. This was standard procedure for the "first" flight of the day every day. To call them a glider was a lot different than making a test glide. Those airplanes had a lot lower wing loading as well. Modern trainers will not be able to do the same thing because of the wing loading and design. All control surfaces would have to be dead neutral and a test glide of 20 feet would be exceptional. If you want a glider get something specifically designed for that purpose. If you want busted props, bent landing gear and a scraped up airplane, test glide your trainer and make sure you have a strong throwing arm.