The ultra rc yak will balance with the 115. I know of one flying and with a DA 100 they have their batteries and equipment all the way at the back of the hatch for balance, so it shouldnt be hard to balance with the BME. Just know when you mount the engine, have at LEAST 1" from the front of the cowling to the spinner backplate, dont butt the prop right up to the front of the cowling as this makes the prop less effecient. If you have to move the motor out to 1.5" of space that will probably be even better and help balance if necessary. Look at the full scale yaks and sukhois, there is a reason the prop and spinner is out in front of the cowling by a good bit.