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

[STONE]

The Sonex wing chord (Mean Aerodynamic Chord or MAC--distance from leading to trailing edge) is 54". The 20%-32% utility CG limits correspond to 10.8" and 17.3" aft of the wing's leading edge (54" * 20% and 54" * 32%). You could use the leading edge as your 0 datum, but most people (everyone?) follow the example in the flight manual: http://www.sonexaircraft.com/support/manuals.html where the 0 datum is the tip of the spinner and is defined in the manual as being 53" ahead of the wing's leading edge. Putting the datum at or beyond the spinner makes it so you never have to mix negative and positive datum locations.

In this case, the forward and aft limits are 63.8" and 70.3" (10.8" + 53", 17.3" + 53") aft of the tip of the spinner.

To complicate things slightly, I doubt most people plumb bob from the tip of the spinner. They probably either use the leading edge of the wings or more likely the forward corners of the aluminum fuselage sides and add an offset to get the distances relative to the spinner (the fuselage corners are also defined in the manual, but I don't have my manual with me now, but it was the most convenient measurement for me to use).

Thank you very much Chris.
Are the data you posted referred to the Sonex-B?

Mario

[Bryan Cotton]

Thank you very much Chris.
Are the data you posted referred to the Sonex-B?

Mario

Welcome Mario! If you check out Chris's blog or posts here, you will see he has an A model. The B will have the same datums. Check out this graphic from Sonex:


If you do a non-standard engine you may need to figure out an updated datum.

[Skippydiesel]

I stand to be corrected - Datum point is just the point at/from which all measurements are taken/rferede back to.
You do not have to use the Sonex recommended tip of spinner (especially if not using the Sonex spinner).
In fact most homebuilts use the leading edge of the wing, in recognitions that there can be diffrent engined/propeller/spinner combinations.

[Bryan Cotton]

I stand to be corrected - Datum point is just the point at/from which all measurements are taken/rferede back to.
You do not have to use the Sonex recommended tip of spinner (especially if not using the Sonex spinner).
In fact most homebuilts use the leading edge of the wing, in recognitions that there can be diffrent engined/propeller/spinner combinations.

You can absolutely set any datum you want. The key thing is to make sure you have the correct percentage of MAC (mean aerodynamic chord) so you are in balance. And under gross of course.

If I had a Sonex with an engine sticking way out, I'd still use the stock location as the datum. Why confuse things? Also it was an older post but we absolutely used plumb bobs off the spinner, wing leading edges, and so on. I used to be responsible for W&B at our Sikorsky facility in NY.