Center of Gravity & Percentage of Mean Aeronautical Chord (MAC)

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Jason Calloway

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Hoping that someone in this forum has some experience with CG calculations as a percentage of the MAC. I'm a GA pilot but am starting to fly some in a SkyLeader 600. The W&B calcs are based upon % MAC values and I'm not accustomed to using MAC. I have the formula from the POH but am trying to make sense of the values and what they actually represent. Specifically, the two values with the question marks above them in the snapshot. These values are metric if that adds any context.

Thanks in advance for any insight that can be offered.


1694017896900.png
 
It looks like those ?? Numbers are just constants changing the reference to the MAC

I have to admit I’ve not seen CG referenced to MAC before, but it’s just a reference point.
 
it looks like 74.7 (cm?) could be the offset between whatever is the datum for your moments and i would have thought that .0785 scales from cm(?) to % MAC but somehow the magnitude doesn't seem right.

What are the units for distances? What is the range of %MAC that you need to be inside.
 
Initial Fix & Capt. Geoffrey Thorpe; Thank you both for your feedback - sometimes it just takes a fresh set of eyes to notice some things that should be obvious. The 74.7 (mm) converts to 2.941 (inches) and is in fact the offset between the datum and the leading edge MAC. That makes sense now and get's me one step closer to fully understanding the formula. Now if I can only figure out what the 0.0785 represents. Not sure that I understand what you mean by "scales from cm (or in this case) mm to % MAC.
 
The MAC is the average chord of your wing - 1.5ish meters - guess.
So, (assuming 1.5) 30% of the MAC would be .5 meters or 500 mm
To get from mm (the output of the stuff inside the "( )" you need to multiply by something to convert from mm to %mac - but the magnitude seems off if you want to end up witha number between 25 and 30 percent (just to pull numbers out of a damp poorly lit place). If your cg limits are given as .25 to .3 then it's not really "percent" but the .078 seems to make more sense.
 
ugh.
1/.078 is 12.8... 1.28 meters for a MAC could be reasonable, but something seems off by a factor of 10 here - not sure what I'm missing. Do they give any hints as to units and what is the range of %MAC that you have to meet (Cg limits)?
 
I have enough information to confirm that the units are (mm) and I know that the MAC is 1273.99 mm so you spot on with your presumption on the MAC. The POH says that the CG limits are 20% to 30% MAC.
 
100/MAC= 100/1273,99= .0785

So this is essentially the CG value divided by the MAC which would give you % MAC. Sound correct to you Capt. Thorpe?
 
You can convert the CG datum to a more meaningful location, like leading edge or firewall.
 
Jason, I think the best explication of the entire MAC thing is in the Gleim ATP study guides.....
 
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