I have no experience with gas engine installation..My AT-6 kit made no provisions for 3 degree right and 3 degree down on the engine. The firewall installation was designed to be at 0 degrees and left up to the builder to mount the engine with the proper thrush.. I am using stand-offs..if I place washers between the firewall and the stand-off I can get either the down thrush or the right thrush. But to do both at the same time seems to put the motor backplate in a bind when tighting the engine bolts down...I don't want to break off one of the ears on the motor...Please tell a novice to gas the best way to get the 3 degrees both to the right and down at the same time..thanks ...my engine is a 26cc size

bc, why do you want to put in right and down thrust? Is that what is shown on the plans? If not, use the thrust that is built in to the firewall. Dan.

You could use trigonometry to calculate the spacers needed. Use the horizontal mount bolt distance for calculating side thrust, the vertical distance for downthrust.

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...

bc.............the right offset is to compensate for the tendency for the airplane to want to go left on take-off due to the force of the propeller. You don't want to hold left rudder or you will be in trouble..........if you are not building in the right offset, then you will have to hold in right rudder.

Bc, do the plans show any incedences built in to the horizontal stab and the wing? Dan.

I assume we're talking an ARF here? If so, most ARF's need a bit of additional strength in the firewall area anyway, so I make a thrust wedge from a piece of birch plywood. This goes between the standoffs and the existing firewall and gives a perfectly flat surface for the standoffs to mount to. I usually glue this to the existing firewall and motor box for additional strength.

I usually start out with ply that is 1/4" or so thick and tack glue it to the end of a 2x4 or something similar. Then support the 2x4 at the proper angle and run the ply through a table saw or even a bandsaw. When done, I true up the cut surface with a sanding block, pop the ply off the 2x4 and clean up the second surface. Takes longer to describe the process than to do it. Just one method and I'm sure there are dozens more.

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

Well Matt has explained the way it used to be done when no other options were available.

Three degrees right and three degrees down is a BUNCH, are you sure? Your concerns about stressing the mounting ears on the engine are valid and if it really does require that much offset, Truckracer has a pretty good idea.

As for setting or checking those angles myself, I use the electronic protractor from the Horizon incidence measuring kit and it's really easy. Just turn the fuselage on it's side for the right angle and leave it upright for the down angle. Compare the first former in the fuselage (F1 in full scale terms) to the prop flange and adjust as necessary.

http://www.horizonhobby.com/products...e-meter-HAN192

here try this calculator, it used to be on the website, but now it is floating around the web.
http://www.gsal.org/tools/offset_calc.htm

Here's the old Ernst Thrust plate, real cheap. Dunno if the large size here will accomodate your 26cc.

http://www3.towerhobbies.com/cgi-bin...?&I=LXE953&P=7