mass+ balance

[muck-savage]

A quick question


when the centre of gravity is aft of the safe range is fuel consumption and range increased or decreased ?

please and thanks

[Keith.Williams.]

A forward C of G produces a nose down pitching moment. In order to maintain pitch attitude, this nose down moment must be balanced by a downward force from the tailplane (or upward force from the canards). Creating these forces produce trim drag, which decreases fuel efficiency and range.

As the C of G moves rearwards towards the neutral point, the nose down pitching moment decreases. This decreases the nose up trim force required, which in turn reduces trim drag and increases fuel efficieny and range.

this process continues until the C of G reaches the neutral point, at which stage the aircraft becomes neutrally stable. In this condition it generates no pitching moments and so requires no pitch trim. The fuel efficiency and range are (in theory) at their maximum at this point, but the handling is dreadful.

If the C of G continues to move aft, the aircraft becomes unstable and generates an increasing nose up pitching moment. This requires an increasing tail up trim force. The generation of this force produces trim drag, so fuel efficinecy and range decrease.

So fuel efficiency and range increase as the C of G moves towards the neutral point, and decrease as the C if G moves away from it. In conventional aircraft the entire C of G range is ahead of the neutral point, so aft C of G movement within authorised limits improves fuel efficiency and rage.

The short (increase or decrease) answer to your question depends on the relationship between the C of G and the neutral point.

[Dick Whittingham]

Aft CG means that the CP/CG couple is nose up, so to maintain trim you have to have an up load, extra lift, on the tail. This gives you better range than a forward CG. Big ships nowadays usually fly with aft CGs, not aft of the safe limit, but getting near there, and accept that the static longitudinal stability will be less.

At height, the dynamic longitudinal stability will also be less, IAS for IAS, and the combination of aft CG and high altitude cruise can make the handling a touch dodgy. The modern fix is to compensate for this by adjusting the control response in the flight control system.

We had a thread on this in Tech Log a while back, and got a reference to a Boeing paper on the subject. I don't have the reference handy, being retired (at last), but ask Ken at BGS for the data. ([email protected])

Dick W