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exercise 5.12/ 5.13
oudam2326
Topic Author
oudam2326 created the topic: exercise 5.12/ 5.13
hello all.
I have a bit of a question regarding some of the questions in exercise 5.12 and 5.13
regarding question 1 and 2 of 5.12, im confused as to why we dont use the limit of 2680mm as we previously have been doing. i understand that bob has previously answered this question and has suggested keeping the echo chart out while doing this question, however im still confused.
as for 5.13,
in question 7 and 8,
i presumed that we had to use the flow chart to find the requirements, however it seems in the answers that the echo loading chart was used instead. is the question worded a way that im not understanding or am i just doing something wrong with the question.
In execise 5.12, there is a mixture of aft and forward limit problems. If you quickly check the weight and moment combination on the Echo graph, you will see the current loading of the aircraft in relation to the envelope for the question you are working on . This is a vital step in all questions really, especially in the CASA exam when you don't know what type of problem is being presented.
In question 1, we are forward of the forward limt and are shifting weight back to the rear compartment from Row 1. Therefore we must be trying to get the aircraft's CofG back inside the envelope by shifting weight from Row 1 to the rear. We don't use 2680 in this case because we don't need to load the aircraft so the CofG is all the way back at the aft limit. We only need to move enough weight to get back into the envelope. For question 1, calculate the forward limit for the given weight and use the weight to shift formula.
With Q2, you are actually inisde the envelope but the question wants you to move weight from the rear to the nose so you are moving the CofG forward. You must work out how far forward you can move the CofG first. once you have that value you can use the Weight to Shift formula on the problem. We don't use the aft limit (2680) because we are moving the CofG forward, not rearward.
In Exercise 5.13:
Q7 is a forward limit problem and you are changing weight. This is the one situation where you cannot use the flow chart to solve the problem. Adding or subtracting weight on a forward limit problem relies on the "two-point" method to solve it . In other words, plotting two points, one on each side of the envelope boundary and look at where the line crosses into being within limits (see page 5.49 for more details).
Remember:
Flow chart method for all Aft limit problems and for Weight to Shift Forward limit problems.
Two-point Method for adding/subtracting weight on a forward limt problem
Q8 is a weight to remove aft limit problem which means you CAN use the flowchart. The trick here is you first need to add in the two new passengers to row 3 and check the new CofG position on the envelope. If you do that, you will confirm it is indeed an aft limit problem.
Actually, the question already hints at that since if they are asking you to remove weight from the rear compartment you can be failry sure you must be outside the aft limit and are trying to get back inside the envelope again.
