Confused about centre of gravity envelope?

[EZY_FR]

I know that the centre of gravity must remain in front of the aerodynamic centre at all times to ensure longitudinal stability. However, I have found myself confused with the centre of gravity envelopes that aircraft designers formulate during the design process. Page 20 in this document (http://fly.shanghai-air.com/flyiis/h...37_800_AFM.pdf) shows the centre of gravity envelope for the 737-800 in terms of % of Mean Aerodynamic Chord (MAC). However, I know that it is normally assumed that the aerodynamic centre is always at 25% MAC, so according to this diagram, it is possible for the aircraft cg to move aft of the ac. Am I looking at this diagram correctly?

[Genghis the Engineer]

You are.

The whole business about always being in front of 25% MAC is a gross simplification designed to allow FIs and PPLs to cope with some much more complex material than they really need to know about. Put differently, it's untrue.

The aeroplane has been tested to demonstrate a safe set of CG values, you're given those safe limits, after that it's just arithmetic.

G

[Tinwacker]

EZY

Thanks for the AFM it makes a good read for a non B737 person
and good to review the C of G envelope with the varing weights and thrusts.
Do I also read it correctly that dependant upon the aircraft weight you could move the CG to 36%, the edge of the envelope.
OK for a test flight

TW