Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
AFT CoG: Why More Fuel Efficient?  
User currently offlineJER757 From United Kingdom, joined Jun 2006, 350 posts, RR: 0
Posted (6 years 5 months 3 weeks 3 days 5 hours ago) and read 7885 times:

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!

Gale force fog... don't you love it?
2 replies: All unread, jump to last
User currently offlineTdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 80
Reply 1, posted (6 years 5 months 3 weeks 3 days 3 hours ago) and read 7859 times:

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.


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 7034 posts, RR: 53
Reply 2, posted (6 years 5 months 3 weeks 3 days 2 hours ago) and read 7856 times:

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.

Always keep your number of landings equal to your number of take-offs
Top Of Page
Forum Index

Reply To This Topic AFT CoG: Why More Fuel Efficient?
No username? Sign up now!

Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)

Similar topics:More similar topics...
More Prop Blades = A More Fuel Efficient Q400? posted Wed Jul 5 2006 14:29:34 by NWDC10
Which Is More Fuel Efficient? posted Thu Sep 9 2004 05:11:58 by PacificWestern
Airbus A319 CJ - How Much More Fuel Capacity? posted Sun May 10 2009 13:45:31 by Mozart
Most Fuel Efficient Large Passenger Transport? posted Fri Jun 13 2008 13:28:20 by Africawings
Wing-to-Fuselage Fairings: Why No Fuel Tanks? posted Tue Nov 13 2007 04:22:40 by Faro
BAe 146/ RJ Vs 737 Classic Which Burn More Fuel? posted Tue Jan 2 2007 00:04:00 by 747400sp
RR Engines Burn More Fuel Than GE. posted Fri Jul 14 2006 01:01:14 by BOE773
Why Is Fuel Coming Out The Wing? posted Sun Oct 23 2005 13:49:51 by Wrighbrothers
CO 757 Fleet Carry More Fuel? posted Sun Oct 31 2004 07:11:00 by FlewGSW
Why Better Fuel Economy At Higher Altitude? posted Mon Sep 6 2004 03:59:29 by Yhmfan

Sponsor Message:
Printer friendly format