Hi Bob

I seem to be struggling with this question which states, "An Echo aeroplane is loaded so that its total moment index is 14.1 units inside the aft limit. The maximum amount of cargo that may now be shifted from the wings to the rear compartment?" Im just confused as to work out how much you have to shift if you arent given a gross weight? If you could help that would be great.

This is one of those questions where you need to think about just what is being asked rather than pulling out a solution method and trying to make it fit the problem.

First the question says that the present aircraft loading puts the CG at a position 14.1 IU inside the aft limit. The gross weight actually is irrelevant as we would be 14.1 IU inside the aft limit regardless of what the gross weight might be. So forget about the gross weight.

Then the question asks you how much baggage can you transfer from the wing lockers to the rear compartment. Caveat - when we have done the sums, we need to keep in mind to check that there is enough cargo available in the wing lockers to move and enough capacity in the rear compartment to put it there. If that information isn't given explicitly, one should put a comment with the answer to show that you are considering those things.

So, what the problem now becomes is to figure out a weight to shift so that the IU change due to the shifted weight is +14.1. This is the maximum you can shift and stay within the envelope limits. Caveat - if the previous check showed that you either didn't have enough cargo available to move or enough capacity in the rear compartment, that would further limit the amount which could be moved.

Now you can do the sums either by using one of the various formulae or doing a simple longhand calculation. Your pick as to which way to go.

So, give it another try and see what you come up with.

(Caveat - I am presuming that you have given all the relevant information in your post).

Hi John

Thankyou for that I looked over my notes and found the formulae so I went 14.1x10,000/1450 which gave me the correct answer.

cheers Sam

Well done. However, the important things are

(a) do you understand why you used that formula ? (It is just the longhand method tidied up into a simple formula)

(b) do you understand why gross weight is not relevant to the problem ?

(c) do you understand that having a constant IU from the aft limit is not the same as having a constant arm from the aft limit ?

(c) can you do the sum longhand to demonstrate that you do, indeed, know what you are doing ?

(d) did you check (or comment) on available/maximum cargo to move ?

(e) I trust that you rounded the quantity to be moved down to demonstrate that you understand which side of the aft limit is conservative and which is not for this sort of problem ?