747 C of G?

[Lump Jockey]

Just wondering, on an "average" flight, where, physically, would the C of G be on the 747-400. (ie: between the wings, aft of wings, forward of w......?) Thanks for any reply!

[Notso Fantastic]

Well Lumpie, you do have some interesting questions in your few posts! Care to enlighten us what the basis for these is? You're not..........writing a book are you?

The C of G has to be:
1- In front of the rearmost wheel axle (but not too far or the elevators will not be able to rotate the fuselage into the takeoff position)
2- Located near the centre of lift of the wing (to the rear of it in fact)
3- The 747 can stay upright if the rearmost body landing gears not do extend, but only just. There is extreme danger on the ground of toppling rearwards.

So if you look at the wing landing gear rearmost axle, that is near enough where the C of G is!

[Intruder]

Around 20% MAC.

If you can find out where the mean aerodynamic chord is, go 20% of the chord aft of its foremost point.

[18-Wheeler]

In flight, I've seen the trim sitting at around the '4' mark on the trim indicator, and that's around the 20% MAC mentioned above, perhaps a little further back though, say, 23% odd ... ?

[Lump Jockey]

thanks guys, and "notso fantastic", it's not a book i'm writing; i'd just like to get a little more info on certain items that interest me! i love to rack the brains of you guys given half the chance, love to be a pilot or anything in the trade come to that! good luck to you all, and keep replying please, really appreciate the answers!:)

[Intruder]

18-Wheeler:

I said "around" 20% MAC because I have seen anywhere from 15% to 27% on takeoff (744F). Also, the cruise CG figure in the FMC defaults to 20%, so it's as good an "average" figure as any.

[Tinstaafl]

Notso.

Are you sure about the CoG being behind the CoP, and not ahead of it? Does the B747's tailplane produce an upforce in flight or is relaxed stability used?

'Conventional' aerodynamics usually revolves around CoG arranged to be ahead of CoP to produce a pitch down couple, balanced by the Thrust-Drag couple + Tailplane downforce acting to produce a pitch up moment.

[18-Wheeler]

No worries, Intruder. ;)

I took another look last yesterday and the trim was sitting on 4.5 units for most of the flight. I guess with a little bit of looking in the charts we could come up with a more accurate figure?

[Flight Detent]

Hi Guys,
After looking around about this subject awhile ago, I worked out that the CofG of a Classic '74 is at the #3 door, and moves fore and aft about the width of that door.

Cheers

[Dan Winterland]

I recently noted the trim settings on a 744 trip in response to a question posted on this forum. The take off position was close to the six mark (this was with about 110 tonnes of fuel - so full wings with none in the centre tank or tail tank) This equates to neutral elevator trim, and this is typical.

Boeing state that the 744's fuel management logic keeps the elevator trim position close to the optimum minimum drag cruise setting. After reacing top of climb, most of the trip was flown at about the 4 to 4.5 setting, thus backing up 18 Wheeler's observation. This equates to a slightly forward C of G thus requiring a slight upforce from the tailplane which is the economical and most aerodynamicly efficient configuration. This is backed up by the aircraft being relatively sensitive in pitch when being flown manually at altitude.

I think 18 Wheeler flys the classic, so I don't hold much has changed with the introduction of the 744, other than the fuel management is now automatic.

But I think it's safe to say that the C of G range is pretty big.

[Dan Winterland]

Oops - I should have said 'slightly aft C of G requiring an up-force from the tailplane'. Thanks for the correction.

[Notso Fantastic]

Tinstaafl- <<Are you sure about the CoG being behind the CoP, and not ahead of it? Does the B747's tailplane produce an upforce in flight or is relaxed stability used? >>. Look, I'm dredging up 33 year old aerodynamics here. I always understood that you should have the CofG somewhere on (for relaxed stability) or behind the centre of lift (so the tailplane provides a small amount of lift). You do not want the tailplane providing downthrust as this increases the lift and drag on the wing. Of course all this predates Airbus rewriting aerodynamics! (Now watch me get shot to pieces!)

[18-Wheeler]

I had another look in-flight yesterday, and the trim was at 4.4 units. With the weight we were at, that makes the CofG at about 22.5% MAC. It would no vary a fair bit from there depending on the position of the load & fuel, etc.