Help for newbie with aircraft.cfg file please.....

Hi there! I've searched the Forum for posts relating to Centre of Gravity and Contact Point values - but can't find the answer - and, yes, I probably wouldn't understand it if I saw it anyway.... 
So, here's the problem - I've made and exported a very simple model into FSX. It looks fine in SLEW - i.e., it sits with its wheels on the deck O.K. But when I go back into the sim (press 'Y') the bl**dy thing does the Dance of Death and disappears half way into the ground.
Any pointers to tutorials or posts most gratefully received - thanks for sharing my frustration!
Cheers - Dai (EGFE). 8-)

So, here's the problem - I've made and exported a very simple model into FSX. It looks fine in SLEW - i.e., it sits with its wheels on the deck O.K. But when I go back into the sim (press 'Y') the bl**dy thing does the Dance of Death and disappears half way into the ground.

Any pointers to tutorials or posts most gratefully received - thanks for sharing my frustration!
Cheers - Dai (EGFE). 8-)