I'm building a +30 year old Bud Nosen Citabria kit, 105". I have a copy of the plans and instructions but cannot find what the engine thrust angle should be. Anyone know what it is?

The plans show the thrust line and incidence but not the engine thrust angle.

I will be using a 25cc Homelite so will not use the engine mount on the plans. Will mount engine directly to firewall.

I don't know the offical thrust number for the Nosen Citabria, but my experience suggests 2-3 degrees would be a good starting point. That said after you fly the beast it's easy enough to determine if more or less is required...

I have a new in box one, but the thrust can be figured out easy, draw a line from the elevator center line down the fuse, draw a line down the center of the engine side view, if bolth are equal, then 0 degrees, if not, make them intersect and use a plastic degree finder to figure it out. 2-3 degrees of down thust is normal, but on this big a model it should almost fly the same with or without down thrust.

Ok, 2-3* of down thrust is what I remember is typical. Don't you need a couple degrees of right thrust too?

Thrust in either circumstance depends on several things, airfoil, thrust line realitive to center of gravity and more; p-factor is pimary for engine side thrust. There is NO magic number, just typical results that very with every model and power/prop combination. Side thrust compensates for P-factor while down thrust helps balance out the need to re-trim the model as a result of power/thrust changes.

Start with two degrees down and side and design your engine mount and or a engine mounting plate to readily accept thrust change/revisions.

Michael,
I got some info from another modeller who built one of these and he used 3* down and 3* right thrust. I'll give that a try.

The engine will be bolted directly to the firewall. I'll add hardwood wedges to get the angle needed.

The thing is tho, the fuse isn't real square. The guy who started it didn't get it square. I tried the best I could to square it up, it's better but... not perfect. The tail is about 3/4" to the starboard side from center. With the fuse is just over 4' long at this stage, that's mabey a degree off. I'll make sure to get the thrust line and incedence correct so it will fly half straight!

The first photo below is how I received the kit. I rebuilt parts of the fuse but the plywood parts were epoxied so could not redo those sections.

The next two photos show progress so far.

"The tail is about 3/4" to the starboard side from center. With the fuse is just over 4' long at this stage, that's mabey a degree off"

Are you suggesting the vertical fin is not in line with the datum line as viewed from a top view? If this is the case you’re suggesting the fin is 3/4" offset to the right, and is the fin inline with the datum line or at an angle front and aft respective of the datum line? Some aircraft design the vertical fin to actually be several degrees offset to compensate for P-factor in lieu of corrective thrust at the engine and or firewall.

If the firewall is out of square and it’s in the direction of the anticipated side/down thrust requirements and or corrections it may have been built as such intentionally…

Darn, I'm gettin' a headache!

I ain't suggesting nuttin, there ain't no vertical fin on it yet. If the v-stab is installed inline with the fuse it will yaw the plane left. Sorry, got my port & starboard mixed up. I plan to install the v-stab so it is parallel to the thurst line. That should take out the angle of the tail.

I doubt if he ment to build it this way, especially since it would counter any right thrust in the engine.