Just got to the point of fitting the cowl to my Ultra RC YAK. Placed the engine at the proper distance from the firewall (125MM), fitted the cowl and found the prop will rub the cowl. My question is what would be the best way to reposition the engine?

1) use spacers behind the motor mount against the firewall

2) slot the engine mount and slide the engine forward

3) anything you guys can suggest

TIA
Gary

Can the cowl be pushed on any further? Plus does the plane balance with the engine at this location moving it forward on the mount more could make it nose heavy.

Assuming you drilled the cowl, Throw a few washers behind it. Or get a new engine mount

I haven't drilled the cowl, just prefitted and found the problem. Looks like I will be buying a new motor mount and repositioning.

As noted above - before you do anything you should do
a preliminary balance with all components placed in the
plane and then make you decision on mounting the
motor. You want to avoid adding ANY dead weight
if you can.
Good Luck
Bruce

Of the three methods you mention, #1 is by far the best. If you slot the holes for the engine mount screws, there's always the possibility they will loosen and allow the engine to shift. I guess #3 is the same as #1 in this case, because that is what I would suggest. The fit must be pretty close as it is, just determine how much more you need, and cut a square of hobby ply that will be a little larger all around the engine mount / firewall, drill it to match the holes and glue it in place. I am assuming you have blind nuts on the backside of the firewall, you might need a bit longer mounting screws. This will be just as strong as the original, the weight shift should be minimal, something you can compensate by moving the battery or receiver.

Of course, if you haven't cut the cowl, or drilled the mounting screws, you could alsways just set the cowl back a bit more, as long as it will still fit. I'm guessing that the amount of spacr you need is less than a 1/4 inch, anyway?

Thanks all. After further review the ref has discovered the person building this plane is ARF challenged. I didn't take into account the spinner backplate which will kick the prop out far enough to clear the cowl. Leeson learned once again.