performance and ops

[Gasket18]

Hi all, been a while. This is going to be a bit of long one sorry. Onto last subject now I sat exam last week failed miserably. Did feel a little under prepared but i had surgery the day after and wasnt sure f how long till id feel like studying again. Anyway a big struggle for me is figuring out how much weight can be added to stay within c of g. I can move weight around ok, but can't work out how to add other than the long process of guess a weight calculate it out and keep trying. Also when fuel weight is outside of whats listed ,I'd like to see if I'm calculating correct so here goes a sample q
pilot weighs 95kg take off fuel is 15o litres land fuel is 60 litres how much baggage can be added.i can work out zero fuel weight. now most aft is 150.6 itres so to me 150 will be most aft. i can calculate the weight of 150 litres but to work out arm do you go 150.6 - 135(which is the 2 surrounding options shown) is 15.6 so is the arm diff 3564-3541/15.6 x .6 gives .8 so i made the arm 3563. then 60 litre weight is in the book so i had most forward and most aft arms then how do i calculate how much weight i can add. I can move weight around with the dist cof g x auw/ diff in arms but i cant remember how to calculate how much weight can be added cheers folks
hopefully what im trying to ask and explain makes sense.

[Wannabe60Driver]

The key point is that there’s nothing exponential in CoG calcs- it’s all linear. Throw 100kg at any given point and you’ll get four times the shift as if you threw 25kg at that point.

With that in mind, you can throw more than necessary at any given point in order to get it outside the box, then join your answer with a straight line to your point inside the allowable box. Where that line crosses the edge of the box will give you a number that corresponds close enough to one of the available multi-choice answers.

I was lucky to figure this out on the fly when I did rotary ops/planning but when I did the same exam for fixed-wing it was interesting to see it’s a technique Bob Tait teaches. No surprises as Bob was a high school maths teacher.

[Gasket18]

ah i do recall seeing something somewhere about drawing a line between 2 points but cant think how it worked. Also in these prac questions there are no multiple choice answers they just asked for how much baggage could be loaded

[Wannabe60Driver]

Sometimes there’ll be a white box needing an answer typed in, otherwise a multi-choice.

Re the exaggeration technique, it’s just a matter of throwing a number you know to be too big to be correct but that allows an accurate measurement of how wrong you are.

For example: How many kg can you add to the one of the seat rows or baggage compartment before you’re outside the CoG limits? Just work out your point with zero kg at that point, then work it out with a silly number- say 100, 200, or 300kg- whatever is necessary to get the CoG outside the acceptable limits box. Plot both CoGs (zero and whatever other number you chose) and draw a line between the two. Whatever value is at the point where the line crosses the box is your answer.

CASA have tolerances for white box questions. Get close enough and you’ll get it right.

[Gasket18]

cheers for the replies. Greatly appreciated. I did end up remembering something like work out the zero weight, plot it, then minus the weight on offer plot it. Draw a line from the 2 until you hit c of g limits go across to weight and the difference in zero weight and what you read is approx how much you can place in the compartment
I'll just keep going over diff things and hopefully be ready to resit exam as soon as I'm able to drive again

[Wannabe60Driver]

Hot tip: see if you can borrow the Bob-Tait book re how it’s done in the fixed-wing world. It will clarify a lot and like the best sources, it won’t teach you what to think, but HOW to think.