Haha mate no problems, glad I'm on the ball, always a good sign picking up the professional's mistakes

Cheers

Hi rich

Just wondeing if the shift to weight formula also works with the charlie loading system or does it just work with the bravo loading system

chers

Hi Mark,

The formula works for all aircraft types. It is based on the physical principle of calculating moments:

Moment = Weight x Arm

and is a manipulation of a derived formula:

w/W = D/d

where:
w = the weight to shift
W = the weight of the aircraft
D = the distance the aircraft's centre of gravity is to be moved
d = the distance the load is moved i.e. the difference in arms.

If we rearrange that second equation to solve for w we get the familiar formula for shifting weight:

w = (W x D)/d

The same formula applies when adding or subtracting weight too. The only difference is the value d to be used: it would now be the distance between the arm of the compartment where you are adding/removing the load and the desired position of the CofG.

I stumbled across a derivation of the weight to shift formula at the AVCANADA.ca website which saved me a heap of typing. Here it is for your edification



That is just for fun and not needed for the exam.

Cheers,

Rich

Hi Rich

Thanks so much for your help mate

Kind Regards

One problem with the Alpha and Bravo loading systems is that you don't know exactly where the aft limit is as a distance from the datum. They don't give you that information so you have to plot it on the envelope. So you cannot use a formula or flow chart [which is exactly the same calculation as the formula], because you don't know how far the centre of gravity has to be moved. The Charlie loading system does give you the aft limit as 3004mm from the datum, so you can use a formula or flow chart to solve its aft limit problems.

In all loading systems I know of, the forward limit varies with gross weight, so you can't use a formula to solve weight to add or subtract problems on the forward limit.

Bob

Hi bob and Richard

Thanks so much for the advice guys, makes sense bob as to why it wouldn't work with the other loading systems

Kind Regards

Hi bob and Richard

Thanks so much for the advice guys, makes sense bob as to why it wouldn't work with the other loading systems, also does the shift to weight only work when the fuel has already been calculated, another words fixed fuel

Kind Regards

Hi Mark,

You can use that formula for adjusting the ZFW or the all-up weight distribution by shifting, adding or subtracting load. Note though, you cannot use the formula to calculate weight-to-add problems with the forward limit in the Echo since the forward limit changes with weight. That is a topic for CPL Performance though.

Which brings us to the excellent point Bob made about about the formulas not being usable in the Alpha and Bravo in the exam. In these systems you are stuck using the graphs. Having said that, just for fun you can still get a good estimate of the aft limit in the Bravo system by taking some readings off the graph and calculating the CofG position indirectly.

e.g.
At 2200 lbs the moment index at the aft limit is 204 thousand inch pounds. Working out the arm we get 204 x 1000 / 2200 = 92.7 inches.
At 1920 lbs the moment index is 128 --> Aft limit of Cof G is about 92.7
At 1620 lbs the moment index is 120 --> Aft limit of the CofG is about 92.6

If you test some other values you'll see they all sit around the same value depending on how fussy you are with your measurements. This is not precise enough for the exam of course but in real life it would give you an idea just how far out of the approximate aft limit you are. You could theoretically then use the formula to give you a ball park figure how much weight needs to be shifted or added. You could then double check the result on the graph to be sure.

Of course, you could just draw lines and be done with it

As Bob said, the formulas are not much use in the exam for Alpha and Bravo but this is one of those neat tricks that might win you a beer at the Aero-club bar

Cheers,

Rich

Hi Rich

Thanks so much, appreciate all the help your giving me and overall to the forum, can't thank you enough on how much we all learn of the forum, thanks RIch and Bob

Kind Regards