Zenith Aircraft Builders and Flyers
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Question about W&B on a 701
I am trying to do my weight and balance figures on my 701. I have built it without slats. On the worksheet it calls for the datum to be taken from the leading edge of the slats I do not have. Should I take measurements from the leading edge of my wings instead or should I add 4.75 inches to that number and use that as my datum.
If I try to use the new datum from just the wing, all of my calculations put my CG way forward. Like empty CG is coming out 3.75" when its supposed to be between 11" and 20". Has anyone else run into this? Should I be using the same datum without slats? Help please. Very near inspection for airworthiness.
Thanks
Replies to This Discussion
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Permalink Reply by John Austin
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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
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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
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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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