Kevin, Glad to see you have REAL weights so far and that's an important step. I assume you took them tail up, ie inflight attitude.
Figuring out your engine location to meet your desired CG is not too hard. I'd recommend you set your firewall as the datum; is isn't going to move. Ever. And I guess the leading edge of the stub wing works as well. Your airplane so your choice. * Get your aircraft landing gear arms measured while in an inflight attitude; * Figure your pilot/ passenger arm, the fuel arm(s) and anything else you want on your W&B sheet as Don mentioned below Now solve for CG where; * all your known arms times their respective weights summed * minus your known engine weight times variable arm Y (it's negative because it is FORWARD of the firewall or wing leading edge datum) gives you the loaded CG point you want For example and THESE ARE JUST GUESSES SO DO THE WORK & MATH YOURSELF! (Put in your own numbers to come up with your solution) Your empty AC weight is 331# where 276# is main wheels & 55# is the tailwheel Multiply those by their respective arms to get your positive AC structure moment Add your pilot moment (you're skinny, 145# times the arm) Add your fuel moment ((guessing fuel capacity @ 14G times 6#/ gal) times the arm) So now you have AC + pilot + fuel for an empty weight & moment Now you know that the engine weight is 182# at a starting NEGATIVE arm of -11.5" If this doesn't work out, change the engine (mount) arm to come up with what you DO want. Does this make sense? John Bouyea N133RM KR-2S - imported, fixed & flying <http://www.bouyea.net/cur_proj/N133RM> www.bouyea.net/cur_proj/N133RM OR81/ Hillsboro, OR -----Original Message----- Subject: Re: KR> Engine mounts Kevin: Going by numbers only could cause a miss in CG location for your mount moment. I would ensure the craft is done firewall back including wings and place sandbags in seats to get accurate estimated flight weight . Don Taylor-monoplane and KR-2 builder On Mon, Sep 21, 2020 at 10:02 AM Kevin Stolhammer via KRnet < > So, my KR 2 (less engine) wieghed in at 331 (276 on mains, 55 on t/w). My > engine weighs 182 (center of mass 11.5" forward of mounts). Can anyone > point me to a specific formula to calculate engine placement? I have my > jig ready to weld the mount, just trying to dial in the spacing from FW to > Eng Mnt. > Thanks, > Kevin
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