This graph confirms the stall speed corresponding to a forward CG position is higher than the stall speed for a CG at 26%MAC. The difference can reach 1.5 kts.
Note : For conventional aircraft (A300, A300-600 and A310), the reference stall speed, Vsmin, is based on a load factor inferior to 1g. This gives a stall speed that is lower than the stall speed at 1g. All operating speeds are expressed as functions of this speed. For Fly by Wire aircraft (A319, A320, A321, A330 and A340), the airworthiness authorities have reconsidered the definition of stall speed: the stall speed is based on a load factor equal to 1g. It is called Vs1g. Fly by Wire aircraft have a low-speed protection feature (alpha limit) that the flight crew cannot override. Airworthiness authorities have agreed that a factor of 0.94 represents the relationship between Vs1g and Vsmin : Vsmin = 0.94 Vs1g.
A. GENERALITIES
3.2. Impact on takeoff performance
3.2.1. Optimized takeoff distance or takeoff weight
The aircraft operating speeds are referenced to the stalling speed. At takeoff: V2 ≥ 1.2Vs (1.13Vs1g )
Now, the V2 value directly impacts the takeoff distance (or the takeoff weight).
Vs
V2
TOD
So for a given TOD: the lower the stall speed (Vs) value, the greater the takeoff weight (TOW).
The further aft the CG, the smaller the TOD (the greater the TOW).
. Influence on TOD
A340 CONF 3, AC OFF, AI OFF, OAT 15°C, Zp 0ft TOW = 250 000 kg
Forward CG at
CG 26%MAC
TOD (m) 3241.5 3164.8
.TOD = 76.7m = 2.42%
.This example shows that, for a given TOW, the aircraft will need a longer runway if its CG is forward.
3.2.2. Takeoff roll
An aft CG position gives the aircraft a nose-up attitude that helps the rotation. On the contrary, a
forward CG position leads to a nose-heavy situation and a difficult rotation.
Taking into account no other limitation (gear strength, VMU…),
The further aft the CG, the better the takeoff roll performances.
3.2.3. Takeoff climb
An aft CG position gives the aircraft a nose-up attitude that helps the aircraft climb. On the
contrary, a forward CG position leads to a nose-heavy situation and a difficult climb.
Taking into account no other limitation (gear strength, VMU…),
The further aft the CG, the better the takeoff climb performances.
. Influence on Climb Gradient
A340 CONF 3, AC OFF, AI OFF, OAT 15°C, Zp 0ft TOW = 250 000 kg
WEIGHT AND
BALANCE
ENGINEERING
.The aircraft climb is facilitated with an aft CG position.
WEIGHT AND
BALANCE
A. GENERALITIES
. Influence on TOW
A340 CONF 3, AC OFF, AI OFF, OAT 15°C, Zp 0ft
The difference in 2nd segment gradient value may not seem significant.
So instead of calculating the difference in gradient for the same take-off weight, it is interesting to
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