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Permalink Reply by John Austin on October 15, 2025 at 4:24pm

The datum can be anywhere on the length of the airframe that you want to put it.  If you're using a worksheet that already has some of the moment arms measured and filled-in that used the leading edge of the slat as a datum, then any subsequent measurements of additional moment arms will have to use the same datum.  So, if this is the case with your worksheet, then yes, if you have to measure from a wing leading edge, the difference in distance to the original datum will have to be accounted for/corrected in any moment arms you measure.  Otherwise, set the datum anywhere you want but all new measurements for moment arms will have to be taken.

John

N750A

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Permalink Reply by Jim and Amelia Isaacs on October 15, 2025 at 6:44pm

Several threads about this topic in the 701 forum. That’s the good news. The bad news? Well, read through them and you’ll know 😀. Anyway here is one of them. CG datum without slats

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Permalink Reply by James H Skaggs on October 16, 2025 at 8:17am

Yeah I gathered as much. Zenith's answer seems to be your guess is as good as ours. To their credit, they don't recommend straying too far away from the original design. The biggest concern I have is the relation to the MAC which is with slats 56.2".

without slats is 51.5. That works fine whenever I'm looking for the final 20 to 35%. 

When it comes to finding the CG and moments to get there, the datum location effects everything quite significantly. As I said before the difference is using the leading edge of the wings gets me a an empty CG of 3.75".

If I use the original datum point given from the slats I get 11" which is still at the forward edge of the limits.

In any case my CG is forward and I'll be looking for ways to push it back. Maybe I can move the battery to the rear or add ballasts to the tail. The thing is I need to have the sheet for W&B filled out correctly for the AW inspection. Thinking I'll just fill one out for each datum and ask him what he wants me to use.

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Permalink Reply by Matt Pope on Tuesday

Hi James, 

It doesn't really matter where the empty CG is - the CG limits are for flying, with the pilot/passenger and min/max fuel, which will move the CG back. 

Also, if you move the datum, you also have to move the CG limits, because the limits are actual points on the wing, not measurements from the datum - the datum is just an arbitrary point to take measurements from. If you move the datum back 4.75" then you need to move the limits forward the same amount so they stay at the same points on the wing. Does that make sense?

Maybe you already understand all that, but it's not clear from your question.

I think the DAR isn't going to care what datum you choose, because you could choose a datum ten feet in front of the spinner and if you do all the calculations correctly then the CG will still end up in the same place (where the actual CG of the aircraft is). What he will care about is your CG limits, and whether or not you're going to be within them when you're flying. 

Because your wing is an experimental design without recommended CG limits from Zenith, you'll need to be able to justify where you set the limits. 

The most important CG limit for safety of flight is the aft limit, because if the CG is too far back then the aircraft will be squirrely and you might not be able to recover from a stall. Without slats, the center of lift will (in theory) move backwards slightly, so the aft CG limit could also be moved backwards, but because we don't have any flight test data to support moving it back, it's not a good idea to do so and the DAR might balk at your reasoning if you can't back it up with data. If you use the same aft limit as the one recommended by Zenith for a wing with slats (500 mm back from where the slats would be, or 930 mm forward from the trailing edge of the flaperons), you'll be erring on the safe side, and you can say to the DAR that you know the aft limit could probably be moved a bit farther back because of the aerodynamic changes, but that you're going to stick with the original aft limit because it's safer. That should make him happy. 

The forward limit will also move backward (in theory), but it's less critical, because unlike a CG that's too far back, exceeding the forward limit by a few inches isn't going to make you fall out of the sky. (It will make your airplane inefficient and increase your stall speed though, and you might run out of elevator authority at low speeds.) You could place the forward CG limit 280 mm (11") back from the leading edge of the wing, but that might be excessively conservative, and if your aircraft really is nose heavy then you might have trouble keeping it within limits. 

For the purposes of the DAR inspection, you could keep the aft limit where it is (the conservative thing to do) and move the forward limit back 4.75" (also the conservative thing to do). Then he won't be able to fault your reasoning. You would end up with a significantly narrower CG range than normal, but unless you can point to other people who have gotten certificates of airworthiness with other CG limits, or you plan to do real test pilot stuff and define the limits yourself during phase 1, that might be the only thing the DAR will accept. If possible, I would try to ask him before the inspection to get his take and see what he wants.

Please let us know how it goes!

Cheers

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Permalink Reply by Matt Pope on Tuesday

Here's a diagram to illustrate what I mean.

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