Weight Shifting

[JordanD]

It might be a little combination of my fatigue from studying or general slowness, but I'm having a hard time with some of the weight shift formulas in the PHAK, mainly just the one that deals with determining how much weight should be shifted to bring the CG within limits.

Aircraft total weight 7,800 lbs
CG station 81.5 in
Aft CG limit 80.5 in

Determine how much cargo must be shifted from the aft cargo compart-ment at station 150 to the forward cargo compartment at station 30 to move the CG to exactly the aft limit

Solution:
Weight to be shifted = New CG
Total Weight = Distance weight is shifted

Weight to be shifted = 1.0in
7,800 lbs = 120in

Weight to be shifted = 65 lbs

So far I get that we need to move the CG 1 inch forward to bring it within limits, and that we're moving the weight 120in from station 150 to station 130. But I'm still not quite getting how they arrived at 65 lbs. Help?

 

[JustinS]

Well. I'm not sure exactly at which point you don't understand. Did the PHAK give those values, and you don't understand how they got 65 from that equation? Or do you not understand how they figured out what values they plugged in?

I'm assuming it's the later question. If you solve the equation you get obviously Weight to be shifted 65 lbs. New CG I'm guessing is (current CG station - CG station desired) Total weight 7,800 lbs and distance weight shifted like you said the weight is being moved from station 150 to station 30 making distance weight is shifted 120 inches.

Did you happen to look at the weight and balance section of my instructor notebook I sent you? I was very detailed about several different weight shift problems you could encounter. Hope this helps. If not tell me how far off I am in helping...

 

[JordanD]

Did the PHAK give those values, and you don't understand how they got 65 from that equation?

This

 

[JustinS]

This

So in this equation:

Weight to be shifted = 1.0in
7,800 lbs 120in

Weight to be shifted X 120 in. = 1.0 in X 7800 lbs

Weight to be shifted X 120 = 7800

Weight to be shifted = 7800/120

Weight to be shifted = 65 lbs

 

[JordanD]

Makes sense now, I was pretty exhausted when I was trying to learn it and it wasn't making sense. (Der.) Anyone know of some sample problems somewhere I could try? The ones in the written prep I have don't really cover this specific scenario. Maybe some where some weight needs to be added to bring the CG back within limits?

 

[JustinS]

Have you looked at my Flight Instructor Notebook I sent you? It's got a whole weight and balance section, and it's one of the better sections that I detailed weight & balance to Instructor PTS.

 

[JordanD]

Yeah, just looking for some sample problems so I can get to the application part of this.