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rvaitor87

Well Known Member
I have a question for all of you who know more than I do. I am currently working on a -4, we installed another engine and mount due to engine problems, and when working out the w&b, we find that the builder used a 50" datum line, my book calls for a 60". Also the limits are further forward. The aircraft was built in the 80's.
Question, did they use a 50" datum line in the early years? If they didn't why did the builder? A call to Vans didn't really give an answer, but I need to know.

Thanks to all
 
I used the wing leading edge instead of some imaginary location. Easier to take measurements. I also rounded to nearest 1/10th of an inch instead of 1/100th.
 
I agree with Wayne.

I always use the wing leading edge. It never changes.

Besides, on my RV-6, the landing gear is exactly in line with the leading edge. Makes math easy. Lots of zeros in the ciphering.

Only reason for using an imaginary datum is to avoid negative numbers. And if you can't deal with negative numbers, maybe you shouldn't be doing W&B calculations anyway.
 
It's not that you can't do negative numbers, but why make it harder than you have to? The datum on all the military aircraft I have worked on is often several feet in front of the airplane... Makes it very easy.

I convert all my airplanes to the datum at the tip of the spinner. Makes it really easy (and I used to do W&B for a living).
 
I thought about using the spinner too. There is a chance that someone in the future may replace prop/spinner, so settled on wing leading edges.
 
W&B

A great thanks to all who replied! I will use the 50" that was used before. It just goes to prove, if you want the RIGHT answer, just go the the forums!
And Mel, negative numbers do not bother me,,, just look at my savings balance!:D
 
Just remember that despite the method of expression used, whether it?s total moment, distance from a datum, % MAC, or other, the relationship between the center of gravity and the center of lift is what you?re ultimately solving for. You can change the datum of any airplane to the tail post or move it out 20 feet in front of the airplane; you can convert the expression to ?total moment? or % MAC and be just as accurate in the end.

It really comes down to whatever method you can understand and use reliably.
 
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