Weight and balance questions

[Shabazza]

Hi. I've a question regarding the weight and balance popup (Ground services and pre-flight)

The IXEG 737 allows setup of the ZFW and fuel distribution via that popup.
And this popup also allows the input of a desired % CG position.

So, I was actually a bit puzzled why you should "select" a specific % CG position which then gets translated to a TRIM setting.
and why this % CG setting is completely detached from the actual fuel distribution in the other dialog.

Isn't the % CG determined automatically by your weight distribution (mostly cargo and fuel)? So the TRIM value has to be calculated to be appropriate for that given % CG?
I find it weird that I can simply adjust the desired % CG directly myself, without changing the fuel distribution and without IT changing the fuel distribution.
It's kinda backwards.
I mean, I don't have to care about a specific % CG, as long as it does not exceed my available TRIM range, right?
My expectation would have been to juggle the fuel around in it's compartments and get the resulting % CG displayed and the matching TRIM setting.
But changing fuel distribution does not affect the CG or vice versa in this aircraft.

Is this some simulation simplification? Am I thinking too complicated? Will those values be connected at some point in development?

[Litjan]

Hi Shabazza,

you are right in that the fuel load does affect the CG. As does the payload.

We - for the IXEG - try to simulate what the pilot does...which is regularly not determining how the payload is distributed, pax seated etc. The pilot simply gets a loadsheet which shows him the CG for takeoff (with the fuel being at take-off fuel level) and the CG for the zero-fuel-weight (which would be where the CG is after all fuel is burned). He (double-)checks wether this is "in limit", then enters the takeoff CG into the FMS and sets the trim accordingly. Thats it.

So when you fly the IXEG 737, imagine that the "ground handling" guy gives you those figures (as a printed loadsheet). You simply punch in the numbers.

I understand what you are saying - if you have a certain CG and NOW change the fuel, the CG should change...but think of it this way: You have an aircraft with at CG of 23% MAC at 100pax and 6.0tons fuel. Now you decide to take on another 2 tons - so now its 8.0. CG should change...but it doesn´t because suddently 20 pax from the back move to the front - voila, CG stays the same - even though fuel changed!

What I am saying is that everything influences CG. Ultimately the pilot gets the CG for takeoff - and in our simulation you get to determine the CG yourself. It is not "calculated" by simulating fuel and weight distribution. Just like in X-Plane, you simply set it where you think it is. If you want to, you can use the real load+trim sheet for a 737-300 and figure the CG out for yourself, of course.

Cheers, Jan

 

[Shabazza]

Thanks for the detailed answer.
I already assumed that. So I totally understand what you are saying.
My only "grudge" with this is, that I can change my fuel distribution between 3 tanks and nothing changes.
If you can just ignore the distribution and the only thing that counts is the total FOB, then personally, I would not even bother giving the pilot 3 sliders.
Just to make clear that it doesn't matter for the simulation.

But I can cope with the current system of course. It's just looking like it's more than it actually is. That threw me off a bit.
(I like to fiddle with all aspects of the aircraft. Even things like number of PAX, fuel distribution, or even cabin heat.... So that's why my expectations were not matching. ;-) )

I don't want to fly with 20%CG all day every day. So I'll look up/calculate the real numbers for my daily loadout.
Thx again.

[Litjan]

Well, there is a certain fueling schedule to be adhered to (wing tanks first, then center tank) - and when fueling automatically, this is what you should see.

However, we want to give people the chance to also experiment with non-standard fueling setup, or to see what happens when they don´t use the fuel in a standard way. You could theoretically use up the wing fuel (or one side of it) first. Or you may want to try to remedy an imbalance between the wing tanks, and to set that up it helps to set each level individually.

Cheers, Jan

 

[mmerelles]

20 hours ago, Shabazza said:

I don't want to fly with 20%CG all day every day. So I'll look up/calculate the real numbers for my daily loadout.
Thx again.

 

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