RE: the best prop dle30 piper 1/4 ?
18*18 will need very low revs to slow down. The more pitch you add, the faster the airframe tries to go.
I like an 8" pitch on my planes to get enough airspeed for knife edge loops. 6" provides a lower landing speed. 10" or above means speed-speed! The big YS engines can generate a very low idle, but a gasser needs say 2000RPM, a 10"+ prop at those revs means a high landing speed.