ORIGINAL: bcatone
in responce to "all day dan", the plans and the parts are designed so that the firewall has no right or down thrust..But the plans do show that the engine needs both right and down thrust. On the plans the engine shown is a glow engine on typical motor mount with the right and down thrust achevied by spacers behind the motor mount..Since that engine mounts between the motor mount rails, like any glow engine it presents no problem. Yes, if I had forseen this I would have engineered a way to put the proper right and down thrush in the firewall. In responce to the "trigonometry idea" it wasn't my best subject...I will just put in the down thrush and leave off the right and problem solved other than having to hold in left rudder on roll out...
It isn't that hard to add right and down trust and no trig needed. You just need to know that 1/4" tip offset on a 14" prop is equal to 1 degree. Do the following:
Get a stick or prop that's 14" long and install it on the engine. The length of the stick (14") is important for the trig to work
Measure distance from the fin post to each stick tip. Make the measurement from fin post tocenter of the engine shaft as you can eyeball. In other words, make sure your tape measure is reasonably square to prop circle
Now start adding washers to the firewall side of the stand off to achieve the desired degrees of right thrust (3/4" differential in your measurements for 3 degrees)
Once happy with the measurements, cut the right stand offs accordingly to remove the washers.
3 degrees is substantial right thrust. Ideally you want to remount the stand offs so that the shaft of the engine is centered on the fuse spinner ring
For down thrust, make yourself a stick protractor that mounts on the engine shaft (drill a hole for the shaft).The stick should be about the thickness of your prop at the hub.
Define a center line onthe stick's side.
Mark 3-4 degree marks using a regular protractor on the side of the stick, with the center of the protractor on center of the shaft hole you drilled earlier.
Pin a thread with a small weight on the end, from the center
Place the model horizontal on your bench and level it
Install the stick protractor and see where the thread settles out
Detrmine how many washer you will need to get the down thrust.
Cut the bottom 2 stand offs the appropriate amount.
3 degrees down is substantial downthrust. Ideally you want to remount the stand offs on the firewall to center the shaft of your fuse spinner ring. The relocation of the stand offs should be done once for both right and down. It is considerable
Once happy, sand the front of the stand offs to provide a flat mounting surface for the engine's lugs