you will need the 180 since most 120 size planes will weigh around 14lbs. my u-can-do weighs only 10.3 lbs and the engine puts out about 14lbs of thrust, thats what my fish scale said anyway. but i am currently building a Balsa Nova 120 and it will get a 160 or bigger engine,. did anyone read about the new 58cc motor from MVVS at the Morris Hobbies web site ? its said to have 9.6 Hp at only 3.75 lbs, that about what the 1.8 weighs but almost has twice the power.