I am working on a project where I need to understand flight physics. I am having some trouble getting the whole torque vs power vs thrust thang. I'm basing my project off a existing ultralight, the LightSport Ultra from Free Bird Innovations. Here are the specs: 530lbs gross weight 121 sq ft wing area 70" 2-blade Warp Drive propeller Rotax 447 dual-carb, 2-stroke, 41.6hp engine 850fpm climb rate 55mph cruise speed, 62mph top speed these are the specs for the engine: 41.6hp / 31kW @ 6500rpm 47Nm / 34.7 lb-ft @ 6000rpm 6800rpm max So the way I get it is that the engine spins the shaft, which turns the propeller (assuming direct drive). The faster the propeller turns the more thrust it generates. If you look at the engine above, it generates 34.7 lbft at 6000rpm. Using power = (torque x speed)/5252 you get 39.6hp. How do I know this is enough torque to spin my propeller? How do I know this is enough power to spin the propeller at 6000rpm? Now let's say I drop in a different engine, a Rotax 582 with 65hp @ 6500rpm and 55.3 ft-lb @ 6000rpm and a 6800rpm max. Obviously this motor cannot spin the propeller any faster, because it has the same max rpm as the Rotax 447. But it has more torque, so it can spin a bigger prop? Or a prop with more blades? Basically, I just want to know for a given propeller how much torque and power I need to turn it at a given speed, and what thrust it will give me at that speed. Any help would make me very happy!