JER757
Topic Author
Posts: 165
Joined: Thu Jun 01, 2006 7:48 am

AFT CoG: Why More Fuel Efficient?

Mon Aug 17, 2009 11:55 pm

I've been told quite a few times that having a slightly AFT centre of gravity is more fuel efficient then having a 'perfect' or FWD CoG.

Is it simply because the elevator 'lifting' the tail up results in an increased lift, therefore reducing fuel consumption in the cruise?

I am aware that for a CoG right near the limit this is not the case as the trim tabs on the elevator will cause too great an increase in drag.

I'm not sure how well I've explained it... I'm sure someone else can do better!
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tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: AFT CoG: Why More Fuel Efficient?

Tue Aug 18, 2009 2:02 am



Quoting JER757 (Thread starter):
I've been told quite a few times that having a slightly AFT centre of gravity is more fuel efficient then having a 'perfect' or FWD CoG.

The farther aft the CG goes, the less lift the tail needs to generate to balance it. That lowers induced drag. Note: lift = force on the surface perpendicular to the direction of travel. In the case of the tail, the lift vector points down.

Quoting JER757 (Thread starter):
Is it simply because the elevator 'lifting' the tail up results in an increased lift, therefore reducing fuel consumption in the cruise?

On a commercial aircraft, you'll never be in a situation where the tail is actually lifting up because that would mean your CG was behind your CP, which is unstable in pitch. A FBW system can technically deal with this, but I'm not aware of any commercial airplane that does it and I have no idea how (or why) you'd certify it.

The terms "aft" and "fwd" with respect to CG are all relative to the CG envelope. "Aft CG" just means you're near the aft limits of the envelope...the CG is still forward of your CP.

Tom.
 
prebennorholm
Posts: 6561
Joined: Tue Mar 21, 2000 6:25 am

RE: AFT CoG: Why More Fuel Efficient?

Tue Aug 18, 2009 2:29 am

Very well explained in reply #1 by Tdscanuck.

Just to avoid possible confusion, on all commmercial airliners the tailplane always produce negative lift. The more forward the CG, the more negative lift it produces (and the more stable the plane is on the pitch axis).

Producing that negative lift, produces drag. That negative lift has to be lifted by the wing in addition to the actual weight of the plane, which produces even more drag.

Therefore an aft CG (and reduced pitch stability) produces less drag and improves fuel efficiency.

A positively lifting tailplane gives an unstable pitch axis. It is used entirely on the newest military fighter planes where the FBW computers are amended with artificial pitch stability. In very simple terms it works the way that the pitch control computer automatically keeps the plane at 1 G with constant, ultrafast compensations for the instability on the elevator- (or flap-) control. The pilot input then tells the computer to diviate from the 1G.

Artificial stability is - like Tdscanuck also mentions - something which I cannot imagine can be certified on a commercial airliner. It is simply not safe enough, and in addition I would assume that there is a serious passenger comfort issue. Maybe the extremely powerful actuators needed would also be too much of a weight penalty, especially with the redundancies which are mandatory on an airliner.
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