[mlwynn]

I am in the final stages of getting the cowling installed. When I first set it up, some years ago, I allowed a little for engine sag as others have described. My engine has been on the mount for about four years (really slow build) and I expect is about as settled as it is going to get.

I was reading through the forum on spacing for the spinner and came upon a post where a gentleman suggested getting the engine perfectly aligned with the fuselage, i.e. the thrustline parallel to the fuselage centerline and waterline and then fitting the cowling to the engine. The notion is that you maximize efficiency this way. Then, if there is any sag, you shim the engine mounts to keep the engine properly aligned. His was an RV 6.

As I look at my engine mount, I get the impression that a little down thrust and right thrust may be built into the mount design. We used to do that with radio control airplanes. I assume the design issues are similar with real airplanes. My question is, does anyone know the design parameters of the RV 8 and if they specify thrust offset? Would going back and trying to get the engine perfectly aligned make any significant difference in my aircraft's performance?

To change the engine alignment, I would assume I put shims or washers between the engine mount and the firewall. Is this correct? A couple of posts seemed to suggest that they are somehow on the rubber motor mounts. Seems like that would be quite a lot more challenging to get right.

At any rate, I am trying to figure out if this is an issue worth spending a bunch of time on or if I should just accept my current alignment and finish up.

Michael Wynn
RV 8 Finishing
San Ramon, CA

 

[aerhed]

The engine is intentionally offset and the cowl should be fitted to compensate. Don't try to shim for alignment. The shimming for sag you hear about is to put large area washers between the rubber mount and the engine ears. You still may sag a little after running, but probably not much.

 

[Bob Axsom]

NO I would not change the alignment

I had written out a long rationale but you know from your R/C days I'm sure that the engine and prop produce undesirable rolling and turning forces. I believe it is best to deal with these at the source with thrust line orientation rather than ignore them and compensate aerodynamically later. I believe Van has determined an acceptable thrust line to do this for his aircraft and to remove this with no functional verification of proper effect would be a mistake.

Bob Axsom