C of G and Range

[VFE]

Folks,

Recall from my ATPL's that a rearward C of G increases range, or increases fuel efficiency, but cannot recall the mechanics behind this - could someone enlighten me as my old notes seem to be drawing a blank at the 'moment'!

Moment! ha! Geddit?!

Regards,

VFE.

[Rainboe]

An aft CG reduces the download on the tailplane, or possibly even creates a small lift on it. Any download on the tailplane has to be countered by increased lift on the wing, which increases drag, so aft CG saves fuel.

Of course they could have designed this out altogether, but aeroplanes are left like this just to create another conundrum for generations of aerodynamicists and pilots on interviews to have to answer. The 'A' Level question then becomes: 'what are the problems created by having an aft CG?'

[rhovsquared]

stall speed drops a bit too, 'cuz of less weight on wing

[chornedsnorkack]

Originally Posted by Rainboe

Of course they could have designed this out altogether, but aeroplanes are left like this just to create another conundrum for generations of aerodynamicists and pilots on interviews to have to answer. The 'A' Level question then becomes: 'what are the problems created by having an aft CG?'

The answer being that the pitch stability vanishes.

The pitch stability of all flying designs depends on the angle of attack being lower in the rear and higher in front. It could mean a tailplane at a lower, and often negative angle of attack while the main wing gives lift, or it could mean canards in front at a higher angle of attack than the main wing, or it could mean a reflex airfoil with the trailing edge turning upwards... But in all cases, it means you cannot have the whole system optimized for minimum drag... you have trim drag. And you can save on trim drag by sacrificing your margin of pitch stability.

[VFE]

Thanks for the replies guys!

For a while I'd forgotten just what the pleasure of discovering the answer to a PoF question felt like!

Cheers,

VFE.