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Question Variable fuel arm M&B calculation

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Heli.Lamb
Posts: 124
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I think I might know the answer but.............Is there a way to account for a fuel arm that changes depending upon how much fuel is in the tank? The aircraft flight manual has a table from 5 to 90 USG in 5 gal increments so we use the corresponding fuel arm for the amount of fuel we have. We have an excel spreadsheet that uses a VLOOKUP to decide what number to use. Is there a way that EVFR could get around this?

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Stewart Buckingham
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Interesting question, Andrew!

As you probably expect me to say, EasyVFR does not at present have any mechanism to handle that situation. However, there are, I suppose, some "workarounds" you could use to set EasyVFR up to give a correct answer - but except the first one I suggest below, they are somewhat laborious to set up or use.

The remembering that when you create an aircraft profile, it is up to you to define the station for the fuel tank, you just have to edit the tank station depending on how much fuel you want to carry, rather than regarding the tank station as being a fixed constant in the CG system. This method would, of course, give the correct answer for whatever your initial departure fuel load is (because you would have set the tank station to match that case), and will also give you teh correct CG for the "empty tank" case (because when the tank is empty, the moment of the fuel is zero because teh mass is zero), but the CG for intermediate fuel states will not be correctly represented. 

Another way would be to tell EasyVFR that the aircraft has two fuel tanks, one at the position teh flight manual says to use for teh first 5 USG, and the other at the position the manual says to use for the final 5 USG. Now if you are only going to have up to 5USG in the aeroplane, you "pretend" it is all in the 0-5 USG tank, and if you have between 85 and 90 USG in teh aeroplane, you pretend it is all in the 85-90 USG tank. But if you have somewhere between 5 and 85 USG aboard, you will need to put some of it in one tank and the rest in the other - and to chose which proportion to put in each tank so that the CG comes out to be the same as teh Flight Manual method calculates. It should not be too difficult to set up your own spreadsheet to tall you how much to put in each tank to get the right answer - but if you have to create and use a spreadsheet for that and use it on every flight, you might as well use your present spreadsheet and not use EasyVFR at all for the W&B! Again, as with teh fort method, the departure fuel state, and teh "tanks empty" fuel state woudl be shown correctly on the W&B diagram, but the intermediate fuel states would not be correctly represented.

I can envisage a final method which woudl be harder to work out and set up at the outset, but which would not need you to do any spreadsheet analysis later for each flight - Let us imagine first you tell EasyVFR that there is one fuel tank (tank #1) with just 5 USG capacity, located at the station that the AFM says is the arm for the first 5 USG. Then tell EasyVFR there is a second fuel tank (tank #2), again with only 5 USG capacity at a different station. This tank is NOT at the station the AFM says to use for a 5-10 USG fuel load, instead it is at a station twice as far removed form tank#1 than the difference in station the AFM shows between that for 0-5 USG and for 5-10 USG. So with 5 USG in each tank, the arm of the fuel will be the average of the two tank stations, and you need to fix the location of tank#2 so that that corresponds with with the AFM calculation method would give you. Then you need to apply a similar process to define a position for tank#3 so that with 5 USG in each of tanks #1, #2, and #3 teh fuel cg is at teh arm teh AFM gives for 10-15 USG fuel load. repeat this procedure to define further tank positions up to the final 85-90 USG tank position. Quite a lot of careful calculation to define the position of so many tanks (though perhaps in practice you might find that doing it in 10 USG steps gives an acceptably accurate answer, bearing in mind that in reality, the AFM method will, in any case, only be correct for one pitch attitude - any pitch attitude variation would redistribute the fuel within the tank and make the actual CG differ from the calculation!). The advantage, though, of this method - if you can be bothered to set it up (and if EasyVFR will let you define a sufficient number of fuel tanks - I know it can do multiple tanks but I've never tested how many it will accommodate) - is that, when if comes to using it, all you need to do is to for example if you want to see the result for say 23 USG, you enter:

5 USG in Tank#1

5 USG in Tank#2

5 USG in Tank#3

5 USG in Tank#4

final 3 USG in Tank#5

In other words, you "fill up the tanks in order" but you do not need to do any pre-calculations to split fuel between tanks; one tank may be part-full but all the lower-number tanks will have their full 5USG , and all the higher number tanks will be empty. In principle this method ought to correctly represent all intermediate fuel states, providing EasyVFR assumes teh fuel is taken the tanks sequentially, rather than assuming fuel is being drawn simultaneously from all tanks at the same rate. That is a question I don't know the answer to!

So something can be done, but whether it is worth doing is debatable!

 

VBR

Stu

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Heli.Lamb
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@deeahbtinternet-com your last option was the best I came up with as well.....sadly, I think just using either a dedicated MB app or a spreadsheet is still going to be batter.

shame

 

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