Center of Gravity and Mean Aeronautical Chord (MAC)

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Jason Calloway

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I posted this in the light sport forum but am posting here in hopes of reaching a broader audience. 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.

1694018114758.png
 
I don’t know the airplane, but I assume it doesn’t have a rectangular wing.

The CG envelope is between two percentages of wing chord, generally speaking. If the wing isn’t rectangular, the manufacturer has to figure out where that chord starts and ends, since it’s in different places throughout the span of the wing. One way to do that is to mathematically create an equivalent, rectangular, wing, with a Mean (average) Aerodynamic Chord on which to place the CG envelope.

The formula for %MAC then becomes something like (CG-LEMAC)(100)/(TEMAC-LEMAC)

So wherever the CG is, determine how far behind the Leading Edge of MAC (LEMAC) it is, divide that by the length of the chord of MAC, which is the distance between the leading and trailing edges of MAC (LEMAC and TEMAC), and since it’s looking for a percentage, multiply by 100.

Looks like most of the formula in your example is calculating CG (all the stuff you have marked as “moment”), then it subtracts LEMAC (the 74,7) and multiplies by a factor that is probably (TEMAC-LEMAC)/100.
 
MauleSkinner - Thank you! Very helpful reply. The LEMAC is 74,7 and the MAC is 1273,99 which would make the TEMAC (1273,99 - 74,7)/100 = 11,99. What you suggested makes sense to me but doesn't jive with the 0,0785 value. I'll have to keep digging on this one.
 
Perfect! Thank you again MauleSkinner. I appreciate your assistance on this.
 
If you are using ForeFlight, perimeters of the SkyLeader 600 W&B is already defined.

In most of AOPA recommended apps, including FF, you can switch to %MAC, helping to avoid the trap of conversion error; you know the SM to NM kind of trap we have to look out for on the written.

AOPA WB apps.
 
Interesting - thank you for the heads up. I use FF but my primary airplane doesn't rely upon %MAC calcs. I'll look and see if I can set up W&B profiles for %MAC for a specific airplane and not all of them. I found one of several SkyLeader 600 profiles but the one I chose didn't have any parameters available.
 
Jason Calloway said:
Interesting - thank you for the heads up. I use FF but my primary airplane doesn't rely upon %MAC calcs. I'll look and see if I can set up W&B profiles for %MAC for a specific airplane and not all of them. I found one of several SkyLeader 600 profiles but the one I chose didn't have any parameters available.
Click to expand...
Yeah, had the same problem in FF with my Bushcat. Still, using the POH as reference, it’s not too difficult to enter a custom WB profile. Just a little tedious (designing the envelope is the chalkenge. The secret is to work in cg pairs: for empty weight, forward cg, aft cg. Then move up a weight. If you enter all your forward cg limits for each weight, then the aft, you’ll get some wild cg envelopes. FF has a guide in document dedicated to W&B).

But once you complete the entry, you can shift weight & explore various loading & range scenarios with ease.

AFAIK, if you switch on %MAC, that is specific to each profile, not global. Plus, you get a graph.
 
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