For example on an MD-88, is the center of gravity on the back of the plane because the empennage, wings and engines are located there? So they would have to put most of the cargo on the back of the plane?


Yes, the wing is positioned so that the center of lift is near the center of gravity. The MD-80 is very tail heavy due to the aft mounting of the engines. I have actually personally seen the nose of an MD-80 lifted off the ground by a jet bridge, and it didn't even hurt the door that was supporting the weight.

This chart is from the MD-80 weight and balance manual, which can be found on google. You can see that the center of gravity (CG) must be within -2.8% to 33.4% of the Mean Aerodynamic Chord (MAC) which is an approximation of average wing chord. So basically the CG must be in the forward 1/3 of the average wing chord, typically the center of lift is around 25% of chord. On the MD-80 the leading edge of the MAC is at body station 885.547, which is behind the actual LE of the wing at the fuselage.

enter image description here

The reason you want to place the CG near the center of lift is to create the smallest pitch moment possible between the CG and CL. The pitching moment created by the difference in the CG and CL location must be reacted out by the horizontal stabilizer. The more moment, the larger the stabilizer must be and the more inefficient the aircraft. Here is an illustration (stolen from this question How does a commercial airliner measure its weight/mass?):

enter image description here

Here is the picture of the MD-80 doing the wheelie:

enter image description here

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  • $\begingroup$ Thanks for noting that your charts and drawings were "stolen from Google". However, siting the exact location they were "stolen" from is expected (it's nice to know whether they were actually stolen, or just borrowed). Otherwise, nice answer! $\endgroup$ – FreeMan Feb 17 '16 at 15:00
  • $\begingroup$ The chart is from the MD-80 weight and balance manual, as stated, technically the weight and balance manual is proprietary but can be easily from multiple legit sources on google. The drawing is from here: aviation.stackexchange.com/questions/16307/…, I'll update the answer. The picture was taken by myself. $\endgroup$ – OSUZorba Feb 17 '16 at 15:57
  • $\begingroup$ @OSUZorba What I don't understand is that if the heaviest part of the plane is on the back, where does the personnel have to put the baggage? On the nose to make a counterweight? $\endgroup$ – kepler22b Feb 17 '16 at 20:09
  • $\begingroup$ The plane was manufactured to be balanced around the desired center of gravity. It is the pilot (and ground crew's) responsibility to ensure cargo (including the self-loading kind) is distributed appropriately to maintain said CoG. $\endgroup$ – FreeMan Feb 17 '16 at 21:16
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    $\begingroup$ @kepler22b basically what FreeMan said. There are a lot of variables. For example on a short flight without much fuel they may have to load it more tail heavy. On a long flight with a lot of fuel, they may have to load nose heavy. $\endgroup$ – OSUZorba Feb 18 '16 at 2:53

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