Another CG Question
 

I used to know the answer to this question but I am getting old and don't remember.

I do remember that a forward CG is less fuel efficient but gives a more stable platform and an aft CG gives a less stable platform but is more fuel efficient.

Question: Why does planning your CG near your aft limit give better fuel burns?

Wes Afra

Aft CofG requires an upload on the tailplane rather than a download. With a download, the wings must produce extra lift to compensate, which increases drag.

As CG is aft, means it is closer to the point where the lift force is created. So the nose down moment is smaller (moment=force x distance). In an airliner design, the tail should always create a nose up moment about the CG to overcome the nose down moment created by lift. In another word, the total moment should be equal to zero, so result is a balanced flight (second part of Newton`s first law).

Now, as the nose down moment is smaller, the required nose up moment by tail needs to be smaller as well. So this requires less downward force created by tail.

For balanced flight, it should be
L=W+Ftail (First part of Newton`s first law).

As Ftail is smaller, L needs to be smaller as well. So if L is less, required AOA is less. If AOA is less drag is less. If Drag is less, fuel burn is less

L=Required Lift (upward)
W=Weight of the airplane (downward)
Ftail= Tail created force (downward)

http://www.faa.gov/library/manuals/a...H-8083-25A.pdf

Scroll down to around page 4-38... it's a pretty big file, so might take a while to open.

Should tell you everything you want to know, and possibly a lot more.

I was away on a fight....thanks for the help, guys.

Wes Afra

Rainboe:

I hear this upload on the stab every now and then. Could you give me a commercial aeroplane of standard configuration i.e. stab at the back that really does use a lifting stab?

I think people are mislead by the trim terms nose up and nose down, but as far as I'm aware there are no aeroplanes with lifting tails in commercial operation due to stability requirements.