tony0707

HI
Performance is always a POWER to WEIGHT RATIO !
All my info will be for two cycle glo power,building and flying RC for 21 years
My advice would be to put as much power into your airframe as is "REASONABLE "
For a 40 size airframe i have learned not to go any higher than most 51-53 cc engines ,like the ST 51 a lot
Engine weight being the biggest factor ,more weight than that -you will be adding tail weight to balance and kill the wing loading
With to much wing loading the plane will take off fine ,fly in the air fine-,but will be difficult to control nicely on final and landing,kills the glide
If you do not perform a good landing ,it ruins the flight in my opinion
I have used 60 and 75 size engines in 40 size airframes,they drop out like lead ballons on landing-,learned the hard way here reduces the fun of the flight
For 60 size airframes my favorite to date is an OS 75 AX ( prior was a 75 ASP-with an os 7D carb ) halls my 60 stik to unlimited vertical (all my planes do unlimited verticalmy goal some will roll going up )
I have an OS 91 FX in my 60 Ultrasport the 91 fx is a bored out 61 and the same weight as the 61,so that works great (all of the bored out engines designs work well for the same reason ,lite )
I fly it just above 1/2 throttle and have the rest of the power to pull me out or thru anything I desire
Power will get you out of trouble and prevent some kinds of stalls -,take off for example -,-getting out of the lawn
Do keep in mind if adding more power to an airframe there are a few things to consider
One is the additional fuel the larger engine will require-,therefore a large tank
Two you will need additional prop clearence to turn a bigger prop,taller landing gear
Three you will need to reinforce the fire wall,the wing center section ,go to a bigger bigger motor mount-,all considered before the build starts
I have put an OS 1.08 FSR into my 60 stik -,as i saw that combo ,go unlimited vertical ,slide backwards and go vertical again
That was about 17 years ago just when the very first fellows at my field were starting to do 3 D stuff
I did go to a 75 ASP at that time and enjoyed the plane a lot more,especially on landing
Those are my been there done that TIPS from many years of experimintation with increasing power
Just like to ad nothing worse than flying a plane that has marginal power ,either from a smaller engine OR a poor engine adjustment (bad tune )
Your plane will ONLY FLY as well as your engine is running-,so you need to learn tune well
ALWAYS use an engine on the high side of the kits instructions ,or a bit more
Fly the new plane if it does not perform to you liking you can change the engine,shorten the wing,increase the wing size,change the landing gear- ETC
On an average it takes me 5-10 flights to work out the BUGS after the maiden flight and to get my planes to do what i require
Best Regards TONY