If you are good at W&B, I could use your help!

clayfenderstrat

clayfenderstrat

Well-Known Member
This is a problem that I have to study for the final exam, but I am not sure if I am doing it right. Could somebody see if this is correct?

Given:
Weight = 8600 pounds
Center of Gravity = 90"
LEMAC = 95"
TEMAC = 115"

What is the value of the MAC, and where is the center of gravity when expressed as a percent of the MAC?

When I did the calculations, I got that the value of the MAC is 20" and the CG is 25% of the MAC. Is this correct?

Thanks in advance for all the help!
 
Would you verify that the CG and LEMAC numbers are correct? As is, your CG is 5 units forward of the LEMAC, which would be a negative CG.

If the LEMAC is 90 and CG is 95, that's a MAC = TEMAC - LEMAC = 25 units (inches seems impractically small to be lifting 8600 lbs).

The %CG = 100*(CGBS - LEMAC)/(TEMAC - LEMAC) = 20 %MAC
 
So this is what your numbers are depicting...
cg.png

MAC, LEMAC, & TEMAC are somewhat arbitrary numbers chosen by a designer. Normally, all reasonable loading arrangements do not venture fore or aft of LEMAC and TEMAC (LEMAC+MAC).

Using your numbers, yes, the length of the MAC is 20 and the CG is at -25 %MAC, which seems really weird.
 
Thanks for all the help. Yeah, I thought that the numbers were a little wierd too, that is why I asked. I think that they are just arbitrary numbers used for the sake of problem solving. Once again, thanks for the help. I have another one that I haven't looked at yet, so hopefully it will go smoother.....
 
Bernoulli Fan said:
I could go for a Big Mac.
Click to expand...

For those of you who don't know what they mean or are to lazy to Google.

MAC = Mean Aerodynamic Cord
TEMAC = Trailing Edge Mean Aerodynamic Cord
LEMAC = Leading Edge Mean Aerodynamic Cord

No math for me after midnight, I'll help tomorrow.
 
Minuteman said:
So this is what your numbers are depicting...
cg.png
MAC, LEMAC, & TEMAC are somewhat arbitrary numbers chosen by a designer. Normally, all reasonable loading arrangements do not venture fore or aft of LEMAC and TEMAC (LEMAC+MAC).

Using your numbers, yes, the length of the MAC is 20 and the CG is at -25 %MAC, which seems really weird.
Click to expand...

Yes this.

Weight and Balance is my bread and butter, however when dealing with it, I've never messed with TEMAC. What seems to be missing here is a 100% MAC number. 100% MAC being the average of measurements from the LE - TE chords throughout the length of the wing.

The way we calculate % of Mac on a C-130 goes something like this:

CGA - LEMAC

524.7 - 478.4 = 37.3

37.3 / 164.5 (100% MAC) x 100 (often I will divide it by 1% of mac so I can avoid multiplying by 100)

22.7% of MAC for computed aircraft gross weight

I can only *guess* that they want you to divide TEMAC by LEMAC to get the average of 1.2. Maybe I've had a few too much to drink tonight however, I have never seen a negative number come from subtracting a CG by it's LEMAC. Interesting....
 
Here is my other problem:

The CG is located at 29% of the MAC. If the LEMAC is at 550" aft of the datum, and the TEMAC is at 842" aft of the datum, Where is the CG when expressed as a distance from the datum?

Here is what I've got........

842-550=292
292(.29)=84.68

Position of CG from datum: 634.68

Does this look right?
 
Wow, I was really really drunk last night. I can see now that you guys subtract TEMAC to LEMAC, makes sense.

Your problem looks correct, however do you guys utilize any kind of rounding procedure? For us, we round to the nearest 10th so *our* answer would be 634.7
 
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