good question. back in the day... at steve snyders drop zone in new jersey we had an old howard dga 13.he replaced that with a stinson mule,which was a one of a kind prototype built by the late dave steves.it was a heavily modified stinson V77. it had all the fabric coverings replaced by metal, the anemic stock 450 pratt and whitney was replaced by a 650,both doors were permanently removed,and the right seat was turned around facing the rear for jumpmastering. this thing climbed at over 1000 ft per minute. noisy as hell.a favorite display was to get as much airspeed as posible ,hold it on the runway till the very end and just stand it on its tail. it would climb to about 300 feet (in just a few seconds)then be leveled off.what a rush!it was sold to perris where it operated till it crashed in 1970.