how to calculate fuel weight and balance in a robby?

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nuggs
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how to calculate fuel weight and balance in a robby?

Postby nuggs » Tue Jan 21 2020, 03:21

Hey quick question, what do you guys do when calculating the weight and balance of fuel in a Robby? when the tanks are full I know how much is in each tank but if I only put 65 ltrs in the tanks how much will be in each tank? will It be the exact same ratio as when they were full? I have seen some people use a single fuel arm and moment for both tanks when calculating this but don't know if this is the official way? cheers
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Re: how to calculate fuel weight and balance in a robby?

Postby Niko » Tue Jan 21 2020, 06:35

2/3 of total fuel in the Main Tank
1/3 of total fuel in the Aux Tank
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Re: how to calculate fuel weight and balance in a robby?

Postby Chopper Guy » Tue Jan 21 2020, 10:22

Hey Nuggs

If you refer to the POH for the Robbie you're flying, it'll give you a single fuel arm for both tanks when doing a longitudinal calculation.

You'll need to work out exact litres in each tank if your doing a lateral calculation.

Easiest way to do this is to work out how much more fuel the main tank holds than the aux tank, and then divide anything over this by two.

Eg. If the main tank holds 60 litres and the aux tank 40 litres, the main tank will always hold 20 litres before the fuel balances across both tanks (60 - 40 = 20).

So lets say you had 60 litres on board, the main tank would have 40 litres (20 liters + 1/2 of 40) and the aux tank 20 litres (1/2 of 40).

CG
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Re: how to calculate fuel weight and balance in a robby?

Postby nuggs » Tue Jan 21 2020, 10:46

thanks for the replies. I use that ratio too niko but I doubt it is accurate. Hi CG, so a 44 holds 180 lts (115 in main and 65 in aux). so you're saying the main tank will always hold 50 ltrs more than the aux? so with 50 ltrs on board main tank would have 50 ltrs and the aux nothing? or have I misunderstood? thanks!
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Re: how to calculate fuel weight and balance in a robby?

Postby Chopper Guy » Tue Jan 21 2020, 19:02

Nuggs, I'm willing to be corrected but from what I've been taught, your calculations above are correct :)
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Re: how to calculate fuel weight and balance in a robby?

Postby Niko » Wed Jan 22 2020, 02:01

nuggs wrote:thanks for the replies. I use that ratio too niko but I doubt it is accurate. Hi CG, so a 44 holds 180 lts (115 in main and 65 in aux). so you're saying the main tank will always hold 50 ltrs more than the aux? so with 50 ltrs on board main tank would have 50 ltrs and the aux nothing? or have I misunderstood? thanks!


Depends on your definition of correct
It's pragmatically correct; in a way this is how the bulk of the industry do it and is taught by most. It's simple and it's effective.

It's not technically correct; but take comfort in that most what we teach in theory is not either
It is just simple and correct enough to get you the pragmatic answer that you need to say, 'yes I am safe to fly' or 'no, I'm outside of the limits'

If that is not good enough and you want to know what the true ratio is then do the following. Go to your/your flight school's R22/R44 when it's not utilised and with the fuel level at reserves dip the fuel tanks. Record. Then add 5l of fuel. Dip tanks. Record. Then Add 5l... until you reach the full capacity
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homersimpson
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Re: how to calculate fuel weight and balance in a robby?

Postby homersimpson » Wed Jan 22 2020, 02:34

I almost just said, “Why do you want to work out lateral fuel?” Then I had a look. You need two really big guys both on the right and burn to almost zero fuel to get out of cog limits, but it is possible.
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Skid Marks
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Re: how to calculate fuel weight and balance in a robby?

Postby Skid Marks » Wed Jan 22 2020, 23:37

nuggs wrote: I have seen some people use a single fuel arm and moment for both tanks when calculating this but don't know if this is the official way? cheers


Exact wording in the POH states "A longitudinal arm of 104.5 in. may be used for combined main and aux fuel." so I guess that makes it "the official way".

I'd been taught (and consequently teach others) the 1/3 Aux : 2/3 Main method, but will switch to the combined going forward as that is what is stipulated as an acceptable means in the POH.

We all know that it's not 100% spot-on as the Aux gets to 0 at about the time that the Main gets down to 1/4, but if the POH allows it ....
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Re: how to calculate fuel weight and balance in a robby?

Postby Mallard » Thu Jan 23 2020, 06:16

I needed that info for a spreadsheet I was making up a while back... we contacted Robinson but they were not interested in helping.

You can use the total fuel and the combined arm for longitudinal but I needed the lateral to work, so we measured the values similar to when you do a fuel calibration.

I will PM them to you.

:)
Last edited by Mallard on Thu Jan 23 2020, 06:18, edited 1 time in total.
arrrj
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Re: how to calculate fuel weight and balance in a robby?

Postby arrrj » Thu Jan 23 2020, 06:16

In my (short, 11 year) experience, it is almost impossible to get a 44 out of balance. You will be performance limited by weight, which in most situations means you cannot lift it off the ground.

Yes, I know, no calculators being used here, but it is the practical reality.

I have no experience with 22s, but the 66 is very much the same...except the weight limitation almost disappears.

Arrrj
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Re: how to calculate fuel weight and balance in a robby?

Postby Mallard » Thu Jan 23 2020, 06:23

arrrj wrote:In my (short, 11 year) experience, it is almost impossible to get a 44 out of balance. You will be performance limited by weight, which in most situations means you cannot lift it off the ground.

Yes, I know, no calculators being used here, but it is the practical reality.

I have no experience with 22s, but the 66 is very much the same...except the weight limitation almost disappears.

Arrrj


Many of the older R44's with the battery in the nose are surprisingly easy to go out of forward balance CofG... many of ours have ballast on the tail boom.
Lateral is almost impossible unless you also have gear hanging off the sides (pods etc).
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homersimpson
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Re: how to calculate fuel weight and balance in a robby?

Postby homersimpson » Thu Jan 23 2020, 09:40

Not true. Put two 130kg guys in the right hand seats and you’ll end up out of lateral limits while running out of fuel. Unlikely as the situation might be it’s possible.
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Re: how to calculate fuel weight and balance in a robby?

Postby Mallard » Thu Jan 23 2020, 10:45

homersimpson wrote:Not true. Put two 130kg guys in the right hand seats and you’ll end up out of lateral limits while running out of fuel. Unlikely as the situation might be it’s possible.


Correct! quick run thru the calcs for the 10 R44's in our fleet (a mixture of raven1 and 2)

With a 136kg rear right passenger, most of the raven1's go out of lateral (at ZFW) at about a 80kg pilot, the ravenIIs around 95kg pilot.
With a 68kg pilot, and two 136kg passengers in the left two seats, they are all in lateral CofG, but all out of forward CofG at ZFW.
arrrj
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Re: how to calculate fuel weight and balance in a robby?

Postby arrrj » Fri Jan 24 2020, 07:54

Are you guys using butter to squeeze these 136kg blokes into a 44. LOL !

PS - I am pretty sure there is a 130kg seat limit.

Anyway, someone will always have a story of stupidity when loading a machine. V V funny.

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