Weight & Balance

[rizzz]

I'll be doing my W&B this weekend and I've run into a question about the mean aerodynamic cord on a Sonex, which I understand starts on the leading edge of the wing and ends on the trailing edge.
However, given my unconventional engine the tip of my spinner which will be my datum point might be slightly forward compared to the 53" in the manual, hence I need to measure this again to correctly position my MAC and then calculate the corresponding 20-32% MAC positions.
However, which leading edge do I measure to? On the Sonex one wing is positioned slightly forward of the other isn't it? (Can't remember which is the forward one).
Should I measure both and get the average position? (Which would make sense given it's called MEAN aerodynamic cord).

[DCASonex]

Average of two leading edges sounds reasonable. I have a 5/8' prop spacer and simply established the datum point for my plane as 5/8" behind the tip of the spinner. That keeps all other reference dimensions same as Sonex handbook. Should be able to do same (with different dimension) for yours.

David A.

[Rynoth]

Good question! I just watched Joe Norris' W&B video and he doesn't mention using a specific wing or combining the wings, though he does use the right wing when demonstrating how to measure (which was probably just convenient to the camera angle.) I don't think the leading edges are offset by much more than about 1/8 - 1/4" (the left wing is forward of the right wing.)

About 6 minutes in:

http://bcove.me/lf0ipgpx

[fjdoug]

I left the datum where it is and re-drew the spinner, my spinner is 1/2" behind the datum.

[rizzz]

Response from Sonex, "It does not matter":

It won’t matter which leading edge you measure from as the difference is .125” at the most and that is level of accuracy beyond what matters. However, if you really want to get down to the nitty gritty just for the sake of the math, you could measure both and average. Just know that the it won’t matter as nothing on the Sonex is that critical.

Good to know, that explains why Joe does not mention this either in his EAA W&B videos.

[rizzz]

I did my Weight and Balance yesterday, here's how she turned out:

Left Main: 143 kg (315.2 lbs)
Right Main: 142 kg (313.0 lbs)
Tail Wheel: 17 kg (37.5 lbs)

Total: 302 kg (665.7 lbs)

Empty CG: 22.3% MAC
Most adverse forward CG: 22.7% MAC
Most adverse aft CG: 31.8% MAC