I have a 650mm quadcopter with an all up weight of 4 lbs. 10 ozs. Flight controller: APM 2.5. With 12 x 3.8” props the hover is very stable. Just a little bit of wobble near ground level. The frame is made by Dynam and has very short legs; the props are only 5.25” off the deck. Due to the props close proximity to the ground, prop wash may be the cause of the wobble. When I put 15 x 4” props on the quad it’s very unstable (especially below 3 feet) and difficult to fly. According to calculations the hover RPM with the 12 x 3.8” props is 4500 RPM. The hover RPM’s with the 15 x 4” props is 2900 RPM’s. Originally I thought that the problem was due to undersized motors. I replaced the original NTM 28-30, 750 Kv, 120 Watt motors with NTM 35-42, 1000 Kv, 450 Watt motors. The problem actually seems worse with the larger motors. I would really like to use the 15” props to improve efficiency. My suspicions are: 1. RPM with 15” props too low. 2. Low ground clearance causing too much turbulence. 3. Default gain settings for aileron and elevator channels not compatible with 15” props. Any suggestions?