RE: What is the physics behind 150+ mph slope soarers without engines?
For some time the world record speeds for model gliders where higher than for powered models. Problem was twofold: at speed the propeller was actually slowing the airplane down because the engine could not turn it fast enough to give positive thrust; and, there were questions about how the Russians timed speed runs. I think all that has been sorted out. Model Aviation gives all the world records once or twice a year. There are a number of world records not held by USA folks.