I am currently building my 2nd 102 inch bud nosen trainer-the plans call for 60 size engine which is too small-I am also adding flaps to it and making it a tail dragger. The big question ? my choices in a powerplant- a os 91 fx or a os120 surpass-any ideas and thoughts would be appreciated-thanks[img]i/expressions/face-icon-small-happy.gif[/img]

Well the .61 size engines will fly it the way the designer meant it to fly.... [img]i/expressions/face-icon-small-wink.gif[/img]

A .72 4-stroke gives about the same useable power as a .61 2-stroke... so a .91 4-stroke would be a reasonable increase. The lower RPM, larger prop will give FAR superieor pull at low airspeeds too. (14X4 or 14X6 is what I use on my .91 4-stroke)

Really... its a matter of personal preference. what type performance are you after? Unlimited vertical climb (definitely non-scale) get the 1.20 4-stroke. It'll swing a nearly scale size prop.

I found someone else asking the same question in a different forum. They say a 60 pulls it quite nice and one guy said a .75 2-stroke is ideal. Here's the link to that thread....

http://fly.hiwaay.net/~dknight/rcpr/...ages/1846.html

You say this is the second one of these models you have built. Was the power on the first one adequate or did it struggle to stay airborne. If the first one was OK why change? If it was underpowered step up a couple of notches. You should be able to find the answer to your question by looking back to the first airplane.