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I don't think this has been addressed and I got to wondering about the fuel tanks on the Robbie.

 

Ok, so you have a main tank that holds 20 gallons and an auxillary that holds 10. You perfectionists out there know the capacities are off a little, but I'm trying to keep it simple so the math'll be easier. So when the tanks are full, you have 30 gallons total. During the flight, the gas level from the aux tank drops at the same rate as the main due to the transfer pipe. Now then, after a ten gallon burn, the levels in the tanks are now 15 main and 5 aux. After another ten gallon burn, the main has 10 and the aux has 0.

 

Does this seem to be a reasonable assumption? Might the volume remaining in the tanks be non-porportional ie: 12 main and 8 aux after a ten gallon burn?

 

I was thinking about this whilst performing lateral weight and balance calculations.

 

Later

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I don't think this has been addressed and I got to wondering about the fuel tanks on the Robbie.

 

Ok, so you have a main tank that holds 20 gallons and an auxillary that holds 10. You perfectionists out there know the capacities are off a little, but I'm trying to keep it simple so the math'll be easier. So when the tanks are full, you have 30 gallons total. During the flight, the gas level from the aux tank drops at the same rate as the main due to the transfer pipe. Now then, after a ten gallon burn, the levels in the tanks are now 15 main and 5 aux. After another ten gallon burn, the main has 10 and the aux has 0.

 

Does this seem to be a reasonable assumption? Might the volume remaining in the tanks be non-porportional ie: 12 main and 8 aux after a ten gallon burn?

 

I was thinking about this whilst performing lateral weight and balance calculations.

 

Later

 

 

 

 

EDITED~

Edited by AndrewT
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Any two tanks that share a common pipe in the bottom and that contains a liquid inside will equalize the level of the liquid inside the tanks. This means if the liquid-level is 4 inches below the top in the main, it will be 4 inches below the top in the aux regardless of the size of the tanks (if the top of the main and aux is at the same level to begin with).

To find the fuel remaining you have to know the volum of the inside of both tanks and calculate it from there. Since the tanks are not square this is a too complex calculation to start. You wanted to keep it simple, right?

Edited by flyby_heli
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OK, so I have the levels figured out, that wasn't the problem.

 

I'm asking if the volume in the tanks are going to drain at a proportional rate, ie: If the main uses 5 gallons in a certain amount of time, will the aux use the same amount of fuel in the same amount of time? In other words, will the rate of usage in both tanks be relatively the same?

 

Later

Edited by Witch
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I'm asking if the volume in the tanks are going to drain at a proportional rate, ie: If the main uses 5 gallons in a certain amount of time, will the aux use the same amount of fuel in the same amount of time? In other words, will the rate of usage in both tanks be relatively the same?

 

They will until the main reaches aprox 1/2 tank. The crossover tube comes out of the bottom of the aux tank, but its higher than the main tank so the aux tank will empty before the main. If you fill the main only, and leave it set for a while, it will transfer into the aux, and vice-versa. If you take off with both tanks full, the gauges will both show the same until the Main is around 1/2 tank. The aux will empty, then the main will empty and the little red light will come on.

 

fuelsystem.pdf

 

Clark B)

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They will until the main reaches aprox 1/2 tank.

This is true only if the insides of the tanks are of an identical size, which is hard to tell from the drawing. If the main tank is wider or deeper, (like on the S300) the fuel burn will not be the same in the aux as in the main. How much different it will be depends, again, on the difference in size of the inside of the tank.

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You guys are gonna love me.

 

I was asking around the school and others were giving me answers that had nothing to do with the question. That is until I asked Mike.

 

He was telling me that there's a formula in the POH. We went-a-searchin' fer it-complete with search dogs-and could not find any reference to it. He said he'll look for it in his POH.

 

Anyway, the formula he gave me is:

AUX tank=(both-6.6)5/11

 

I plugged this into my W&B calculator and it seems to be accurate as long as one has 6.6 gal total fuel or more.

 

So there you have it. The tanks drain at a disporportional rate and I forgot what I was going to say.

 

If anybody can find this, or similar, formula for the fuel in the POH, let me know.

 

Later

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You guys are gonna love me.

 

I was asking around the school and others were giving me answers that had nothing to do with the question. That is until I asked Mike.

 

He was telling me that there's a formula in the POH. We went-a-searchin' fer it-complete with search dogs-and could not find any reference to it. He said he'll look for it in his POH.

 

Anyway, the formula he gave me is:

AUX tank=(both-6.6)5/11

 

I plugged this into my W&B calculator and it seems to be accurate as long as one has 6.6 gal total fuel or more.

 

So there you have it. The tanks drain at a disporportional rate and I forgot what I was going to say.

 

If anybody can find this, or similar, formula for the fuel in the POH, let me know.

 

Later

 

Witch, just do it the lazy man's way ( my way). Use the excel WB sheet, it has the calculation already built into it..if you don't have this, let me know and I will email it to you.

 

Ok here it is. If D13 equals the total fuel, then =((D13-4.425)*0.585)+4.425 will give you that portion of fuel in the main tank and =((D13-4.425)*0.415) will give you the aux. Seems to work !!

 

Goldy

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You know, I might have that Excel somewhere. I have the Excel thingy but it's the "viewer", so I can't get into it or anything. That's why I made a simple one with Quattro Pro. Corel rocks and Bill doesn't get money from me for that software.:D

 

I think I can use that in the W&B computator. I'll try it.

 

I just had a stupid idea; if I could find that Visual Basic program somewnere and have the time, I might be able to write a stand alone program for weight and balance for the Robbie and Schweizer and maybe others. Yeah right! Like I don't have enough projects to take care of around the house.

 

Later

 

I got to looking closer at this formula and noticed that for the main tank, for some reason you're subtracting 4.425 from the total and then adding it back in. Wouldn't that just cancel it out completely? Might it make sense to express it as; Total*0.585=main?

 

Or am I missing something? Mathematicians speak up.

 

Later

Edited by Witch
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You know, I might have that Excel somewhere. I have the Excel thingy but it's the "viewer", so I can't get into it or anything. That's why I made a simple one with Quattro Pro. Corel rocks and Bill doesn't get money from me for that software.:D

 

Later

 

Hey-- I use open office, its free open source and has all the ms office programs.

 

www.openoffice.org

 

 

 

I tried the excel viewer, it kept crashing.

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Witch... I thought you were wanting to keep this simple! <_ src="%7B___base_url___%7D/uploads/emoticons/default_biggrin.png" alt=":D"> :D

 

(Total fuel -4.425)*0.415)=aux fuel quantity

Hmmmm, that's interesting. I've used that Excel Wt and Bal before, but I guess I never paid any attention to that formula.

But here is my "Old School" way of doing it, and it works out about the same.

Go to the airport, turn on the master, look at the fuel gauges, they read 1/2 tank each. Open fuel caps and visually confirm that they look to be aprox 1/2 tanks. Main ~19.5 less 1/2 ~9.5 gal. Aux ~9 less 1/2 = 4.5 gal. 4.5+9.5=14. 14/9=1.5 hours to 0 fuel. 4.5*6=27 lb., 9.5*6=57lb or 84lb total. Fill in the spaces on the Wt and Bal form, do some simple math on your $1.00 calculator, and your done, go fly.

The spreadsheet formula takes 14 gal and shows 10 in the main, 4 in the Aux, and 84 lb. total fuel wt. Granted, its much faster, and more accurate, but what if your out in the jungle filling your machine with 5 gal cans, no computer, no spreadsheet, just you, the natives and your trusty calculator watch?

 

Hey, I learned something out of it, I don't remember ever seeing a formula in the POH for calculating the fuel in each tank, but there will be a formula in mine now. ;)

 

BTW- the spreadsheet we are referring to is in the Free downloads section of VR

 

Fly Safe

Clark B)

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I agree about Open Office. It's at least as good as M$ Office, opens, edits, and saves any Office files, plus its own formats, and it's freeware. Highly recommended.

 

No, you're not just taking 4.425 out and then putting it back in. There is manipulation happening. Try plugging in some numbers, and you'll see what happens.

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Anyway, the formula he gave me is:

AUX tank=(both-6.6)5/11

So to keep it in simple man terms: The aux tank burns 5/11 or 45.45% of total fuel burn (means the main burns 6/11 or 54.54%) until you have a total of 6.6 Gal left, in which case the Aux is empty and all the fuel is in the main.

 

Ok here it is. If D13 equals the total fuel, then =((D13-4.425)*0.585)+4.425 will give you that portion of fuel in the main tank and =((D13-4.425)*0.415) will give you the aux. Seems to work !!

Same thing here, but different numbers: Aux burns 41.5% of total fuel burn (means the main burns 58.5%) until you have 4.425 Gal left, which is all in the main.

 

So, anyone know why the difference? Different models maybe?

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I got to wonderin' about that too.

 

Later

 

I also got to wonderin' about the fuel amount in the main when the aux is empty. I mean there's got to be more than 6.6 or 4.425 gallons left in the main. I mean there must be more than 40 or 26 minutes left after the aux runs dry. Then again, I'm probably missing something.

 

Later

Edited by Witch
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Since the tanks crossfeed through the bottom of the smallest tank, as seen in the drawing posted before, it doesn't really matter how much fuel you have left when the aux runs dry. You don't have a fuel selector or anything so 6.6 Gallons is 6.6 Gallons no matter what tank it is in.

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For what it's worth, our rule of thumb method is to take the total fuel weight and multiply by .65 for the main and .35 for the aux and go with that. I know there was a graph of actual weights vs. total weight posted at Quantum at one point, but I've never looked to use it as the lateral weight has never been anywhere close to the limits. No idea where that list came from either.

 

a 65/35 split seems to work out ok and be just as accurate as the Robbie gauges. Think of it in terms of significant digits, the gauge isn't going to be accurate to the half gallon, so your balance won't be that accurate either. YMMV

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