Engine mount fabrication

[RatherBFlying]

I recently received my engine and fabricated the mount using Eric Newton's manual and Russ Erb's pictures as a guide. I bolted one of the engine mount isolators together an measured it's compressed thickness, then used washers to simulate that thickness when welding the mount together. I leveled the fuselage fore/aft and side to side then used a laser to establish crankshaft to thrust line orientation ensuring the engine was also level. After the mount was complete I bolted the engine up using the actual isolators and found that it was not level, it sagged a little. This makes sense because the weight of the engine compresses the isolators unevenly, the forward isolator is compressed on top mounts and rear isolator compressed on the bottom mounts. I didn't account for this when fabricating the mount, and it doesn't seem that Eric did either - or didn't include that detail.

Is this even an issue? Will the engine "pull" itself up to centerline when generating thrust? I haven't attempted to measure what nose down angle the engine makes relative to the fuselage, it can't be a lot, but it's enough for me to see on a level. If it is an issue can I just stack some washers under the bottom mounts to bring it back up to level?
IMG_9555.jpg

[gregc]

Since neither Bob (in the original Bearhawk Book) nor Eric Newton say anything about accounting for isolator sag when building a mount, I would be surprised if it is a problem. However, it's normal for the engine to sag further over time so if you think it is excessive now, it might be worth further checking. I know that various washers/shims are commonly used in the certified aircraft world to compensate for it.

[RatherBFlying]

That’s what I was hoping gregc, nobody mentioned it so maybe it’s not a big deal?

[alaskabearhawk]

You'll be fine. The main concern is the alignment of the crankshaft centerline with the fuselage centerline in the horizontal plane, i.e. laterally. That needs to be as spot-on as possible. The vertical component is small and a bump of the trim wheel will take care of anything, if you need it at all. If it does become an issue later on you can always add a washer or two like gregc suggested.

[RatherBFlying]

Thanks Paul, that makes sense.

[fairchild1934]

are engine mounts generally made out of traditional cro-mo tubing --- or --- are they made out of something like seamless low carbon alloy for more fatigue resistance ? I thought I remembered reading that some where- or maybe it could have been the engine mount bolts were soft alloy---- cant remember.
Perhaps the whole thing is a false memory implanted by the grey aliens :-)

[RatherBFlying]

Bob calls out 4130 tubing for the engine mount itself, same as the airframe. AFAIK the bolts are standard aviation grade hardware.