Higher A340 RTOW with Aft CoG
Joined: May 2004
Posts: 308
Likes: 3
From: Down under land
I don't operate the 340 but would assume it's the basic aerodynamic principle of elevator/stabiliser downforce. The THS would be have less downward deflection with an aft CofG. It is the same reason that you will get better fuel economy with an aft CofG.
Check out 'Getting to grips with fuel economy' --- think you can download it from the Airbus website - else your company should be able to provide you with a copy.
Check out 'Getting to grips with fuel economy' --- think you can download it from the Airbus website - else your company should be able to provide you with a copy.
Joined: Aug 2000
Posts: 324
Likes: 1
From: Hindhead, UK
Its the same on the 747-400. An aft C of G means you can haul more weight and it saves you fuel when airborne.
Purely down to aerodynamics. Very simply
: Stabiliser produces downforce which the wing has to overcome. Stick the weight at the back and the stabiliser has to do less work, which means the wing can produce more net lift.
Recently flew at 744 out of Mexico City. Had to have an aft C of G just to get the aircraft out of there, but it made a substantial difference on the fuel burnt during the cruise.
T'bug
Purely down to aerodynamics. Very simply
: Stabiliser produces downforce which the wing has to overcome. Stick the weight at the back and the stabiliser has to do less work, which means the wing can produce more net lift.Recently flew at 744 out of Mexico City. Had to have an aft C of G just to get the aircraft out of there, but it made a substantial difference on the fuel burnt during the cruise.
T'bug
Joined: Jun 2003
Posts: 156
Likes: 0
From: Madrid
FAA/JAA rules dictate that all take off data must be based on the most unfavorable CG position, FWD limit.
Reduced negative lift from stabalizer (less ANU trim setting) due to aft movement of GC location leads to:
Stall speed reduction and V2 reduction
Reduction in trim drag associated with less THS setting, less ANU.
Reduction in TO limit speeds.
All of the above improves climb capability and reduces TO ground roll.
Taking credit of this advantages, better ground aceleration and improved climb,a B767-300 could increase its RTOW up to 2.0 tons if its CG is located aft of 20%MAC (FWD limit is 11% for a 184 MTOW), "trim drag" is reduced 2%.TO speeds must be increased 2-3 knots.
REGARDS.
Reduced negative lift from stabalizer (less ANU trim setting) due to aft movement of GC location leads to:
Stall speed reduction and V2 reduction
Reduction in trim drag associated with less THS setting, less ANU.
Reduction in TO limit speeds.
All of the above improves climb capability and reduces TO ground roll.
Taking credit of this advantages, better ground aceleration and improved climb,a B767-300 could increase its RTOW up to 2.0 tons if its CG is located aft of 20%MAC (FWD limit is 11% for a 184 MTOW), "trim drag" is reduced 2%.TO speeds must be increased 2-3 knots.
REGARDS.
Joined: Nov 2003
Posts: 496
Likes: 12
From: UK
Back in the mid seventies the 737-200Adv had an "Alternate Center of Gravity" AFM appendix option, whereby if the aircraft was operated with CofG at or aft of 15% MAC you had different performance figures. The benefits were of the order of 600Lbs increase in TOW, 2,000Lbs inc in LW or reduction in distances / speeds.
