Ironflight

Ironflight

VAF Moderator / Line Boy
Mentor
This may be news to those still building their first RV’s, but those flying will know what I am talking about. One of the great mathematical mysteries of RV flying is brought about by the baffles in the fuel tanks – they cause fuel to trickle from the outer bays to the inner at a rate slower than you can pour fuel into the tank neck. So if you fill the tank quickly to the top, then stop putting in gas, the level goes down, and you have to top it off. Got the picture?

OK, so here is the question before the floor - what is the quickest way to a full tank?

a) Fill quickly to the top, then trickle it in to match the flow through the baffles
b) Fill at a slow rate equal to the flow through the baffles
c) Fill at some rate in between that depends on how empty the tank is to start
d) None of the above – please tell us your method!

Mathematical, physical, and/or engineering proofs will be accepted, don’t look at anyone else’s paper, and no attempts to bribe the proctor – I have no idea what the answer is anyway!

Just what wanders through my mind as I stand in the evening sun at the fuel pump….
 
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I use one of two methods:

1) Fast fill till the tank is almost full, then continue at a slower rate until the tank is full. Switch tanks and repeat.

2) Fast fill until the fuel in the first tank is at the filler neck, then walk over to the second tank, fast fill it to the top and continue at a slower rate as the fuel moves into the inboard tank bays. I finish topping off the second tank, then walk back over and top off the first tank.

Sometimes I overfill a tank and watch liquid $$ pour across a wing. That makes me unhappy.
 
Not being a engineer in the normal sense, but being through the rigors of a a few Microsoft interviews, I'd have to vote for A.

Once you reach fill up the inboard most bays, you'll be applying the most pressure to flood the other bays. Then keeping up with the flow, you top off until there is no more room for fuel.

I should go grab a fuel tank rib.. I wonder if you were to place "tooling holes" in the top of the ribs if that would allow for air to escape back to the fuel cap faster, and therefor letting you fill up even faster.

Okay, enough talk I have a plane to build.
 
Fueling

My choice is "C" for my RV. Seems to work best for me. It takes fuel fairly fast at first, then I slow it down as it fills. At todays fuel prices an ounce is worth $0.10, so don't waste it!

We old "retarded" f-rts have to be frugal ya know!;)
 
OK, here is yet another way to do it.

Point the hose towards the inboard rib, not the outside baffle.

Fill the tank until it is full and and then go put fuel in the other tank.

When that tank is full, go back to the first tank and top it off and then return to the second tank and do the same.
 
I know your pain!

Paul,
Timewise, it is probably a wash to do a slow or fast fill. If you go slow, it will take longer to get the tank full, if you go fast it will get to the top sooner but then you have to wait.
My tanks hold 28 gal per side, and I think that it takes even longer to fill the additional bay. Therefore, I will fill one side, then go fill the other side and let the fuel settle. You end up making a few trips back and forth, but what the heck as my legs probably needed stretching anyway!
Anyone who has ever flown a Twin Commander (eeek! I just went OT) can tell you about having to wait for the fuel to settle from the one filler port! There are multiple tanks in each wing, plus the fuselage tank, so it took a while to get a full bag. And then, throw in a nice summer TSM adding pouring rain while you stand on a slippery ladder! Oh, the humanity!
 
While you guys are trying figure out how to fill the tanks, I'm taxing to the runway. I only fly 3.5 hour between fillups anyway. After that long I need to GO!
 
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Hmmm. I don't seem to have that problem, at least with the various pumps I've used over the years. There are some pumps out there that really put out a ridiculous flow rate, but I tend to throttle those back to keep spray from coming back out in any case. Maybe the -6 is different with regard to venting.

With regard to your question, I vote for A.
 
Ironflight said:
Just what wanders through my mind as I stand in the evening sun at the fuel pump….
Click to expand...

Paul, have you considered building another plane to keep your mind busy? :D

Here are a few questions that I got in an email today which might get you pondering other things in life as well if you need some relief from your fuel issue.

1. Why do croutons come in a sealed bag?
2. Do Lipton Tea employees take coffee breaks?
3. What hair color do they put on the drivers licenses of bald men?
4. If a cow laughs will milk come out of her nose?
5. What ever happened to Preparations A thru G?

So as you can see there are also deep thinkers walking around in the general public who Van has yet to reach with advertisements. From the way our hobby appears to be growing I'd say he is getting the attention of a great number of them.

When fueling have you ever used one of those pumps that has a trigger that is very hard to pull and when you finally are able to gently finesse and squeeze to get a trickle coming out the pump trys to dump several gallons into your tank in about 1/2 of a second? We have one locally and I don't even try to top off with it.
 
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Paul needs a hug

Dude, you need a hobby or another mind diversion.

Remove fuel cap. Put fuel in. When full, put cap back on. Reel up hose. Reel up Static line. Collect receipt. Go burn previously purchased fuel. Repeat as necessary.

Seriously, who really cares how long it takes. In the scheme of life you might save a minute or two. Now lets get back to debating primers or wondering why we weren't inundated with post of all the new alternative engines that were released at Oshkosh etc......
 
My vote...choices a) and b) are the same, and choice c) is really a rewording of a) and b).

Choice d)...(none of the above) should really read "all of the above".

The ultimate flow rate to fill ALL the tank bays will depend on the size of the inner-tank port holes and how much of these passageways are blocked by pro-seal (or whatever). Once the fuel level in any bay passes all the tank rib openings, the governing factor becomes the fuel vent openings / notches at the top of each rib. As a volume of fuel is added to a tank (or between rib bays), an equal volume of air must be allowed to escape from the same areas as being filled. The limiting factor here is the (small?) holes in the ribs.

So...for your particular a/c, what ever the flow rate is through these holes, that will be the maximum fill rate for your tank. You can fill 'em slower, but you won't be able to fill 'em faster. Therefore, a, b, and c seem to be the correct answers.

As an exercise for the reader...which is the faster way to fill an ice cube tray?
Fill each compartment in sequence, or tilt the tray and let the water flow, waterfall fashion, from one compartment to the next? For me, it seems that the waterfall approach is the quickest.
 
noelf,

Now THAT's the kind of analysis that keeps engineer's minds busy in the morning! I came to roughly the same conclusions on the first approximation, but then the desire to refine the answer creapt in....what is the affect of head pressure from the outer bay? (If you keep the outer bay full, does it force fuel through the lower baffle holes more quickly?) How does the air flow rate out of the tank vent relieve the need for air to flow through the little holes in the inner bays? What is the airspeed and carrying capability of a swallow, and is that an English, or African swallow? Oh, wait a minute....;)

Paul
 
captainron said:
Tell the line guy to "fill 'er up"!
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Negative, Ghostrider! I've seen too many ramp rats drag hoses over the paint and pour fuel down the top of the wing. Those guys aren't getting near my RV.

Any time I fuel where there's a truck, I always ask if I can pump the fuel. They are usually pretty surpised but very willing to let me do the work.

As an exercise for the reader...which is the faster way to fill an ice cube tray?
Fill each compartment in sequence, or tilt the tray and let the water flow, waterfall fashion, from one compartment to the next? For me, it seems that the waterfall approach is the quickest.
Click to expand...

Do they still make ice cube trays? I haven't seen one in years. I'd be surprised if many kids today have ever seen one.

Karl
 
RV8N said:
Negative, Ghostrider! I've seen too many ramp rats drag hoses over the paint and pour fuel down the top of the wing. Those guys aren't getting near my RV.
Karl
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I used to have my cargo Aerostar parked next to a cargo MU-2, and when the fuel truck pulled up, the guy would unreel the hose and throw the nozzle up on the MU-2's wing. Then he would get the ladder from the back of the truck and continue fueling. What makes you think that your lowly RV should be treated any better than the Mighty Mitsubishi?:eek:
 
Ironflight said:
What is the airspeed and carrying capability of a swallow, and is that an English, or African swallow?
Click to expand...

What? A swallow carrying a coconut?
It could grip it by the husk!
It's not a question of where he grips it! It's a simple question of weight ratios! A five ounce bird could not carry a one pound coconut.
 
Secret Government Employee

Don't tell anyone but I think that Paul is exhibiting signs of working for the government. Who knows, he might of even been a 20% of the 20% rule.

Do you think he'll be responsible for the new Slow Fueling Law to prevent petrol wasting and environment control by reducing spillage?
 
OK,....

The leakage rate at the bottom of the rib will have no impact on the overall fill rate. At the fuel reaches the level of the center hole of the rib is will rapidly spill into the next bay (faster then the fuel hose will deliver the fuel).
The process is most restricted by the air at the top of each bay needing to escape up into the next bay. This restriction will progress out to the outer bay where you are fueling the plane. The fuel vent line has not effect on this escape of air, as much more air will be escaping through the fuel filler opening.

Now the technique that will result in the fastest full tank would be as follows:

1. Insert hose into tank and point towards the root of the wing.
2. Pull trigger firmly to maximise fuel delivery.
3. Place mouth over fuel filler opening and suck lightly (don't suck to much as you might collapse your tank).
4. When your fuel level reaches your mouth, release trigger and quit sucking.

Oh yes, Paul, make sure you don't swallow, regardless of whether you are in England or Africa.:eek:

Kent
 
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Trapped air

kentb said:
The process is most restricted by the air at the top of each bay needing to escape up into the next bay.
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That's what I was thinking. The trapped air needs to escape, and if many builders were like me, we diligently prosealed the flange portion of each rib. I don't think you will see any air passing through there.

What is left is the fuel vent bushing. There is some void between the rib hole and the bushing, as well between the bushing and the vent line. This will allow some air to escape to the next bay.

In retrospect, I should maybe have drilled out a 3/16" hole near the top of each fuel tank rib web to allow for an air escape path.

I suspect that people who do not have problems filling their tanks were not as 'diligent' prosealing as the rest of us. :eek:
 
As a Quickbuilder who never saw the inside of his tanks (except through the access hole), maybe I need to go and look at tank drawings so i am better armed for this discussion....:)

Oh, and if you think a question like this is bad, you should see me torture Flight Controllers on a midnight shift when all is going well and I am trying to keep the team alert....;)
 
I think ya all are talking about the fill on the tanks being slow. I remember a friend of mine, that has an 8, stateing this very thing while I was first starting to build my tanks. My solution was to take the very top hole, vent hole, in every rib of the tank, except the one closest to the fuel pick up, want that section to stay full of fuel, and opened it to I think 3/8" maybe bigger. My thought was to give some good ventalation between the ribs for fuel to go into the each section.
 
5 a side, go fly. Takes three minutes. <G> I never leave the St. Louis sectional anyway... <BG>
 
At the self-serve, I usually pull the trigger to squeeze out an extra pint before I flip the activate lever.

Then I'll use a medium trigger pull, pause, let baffle flow drain, continue...

When I shut down & my receipt prints, I do another trigger pull to squeeze the last half pint at no charge.

I'd like to think I get an extra bonus of LL.

I'm new at all this LL pumping as it wasn't an issue before Florida starting mandating ethanol a few months ago.
 
Paul's in Cahoots with somebody?

I can see it now. As I am slowly working my way through the punch list and can finally start to see daylight at the end of the tunnel, you guys start this discussion thinking all of us guys still building, will buy the new whiz bang single point refueling system that you are about to announce for sale. We'll all be delayed another 2 months trying to out do each other to be the first one to have bragging rights on who can fuel the fastest. I'm on to you guys. Hang it up... the wallets empty. :)
 
Ironflight said:
As a Quickbuilder who never saw the inside of his tanks (except through the access hole), maybe I need to go and look at tank drawings so i am better armed for this discussion....:)
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I don't think you should be allowed to ask any questions regarding tanks period if you didn't build them...humph...quick build indeed. :D I recommend after filling the tanks you stir the contents just like you did with the pot when you started this thread;)
 
The issue is mostly moot for me these days. 95% of my flying is local and for that I generally put 8-10 galls in a side trying to avoid the $100 purchase threshold as much as possible.

Just a mental thing.
 
jbDC9 said:
What? A swallow carrying a coconut?
It could grip it by the husk!
It's not a question of where he grips it! It's a simple question of weight ratios! A five ounce bird could not carry a one pound coconut.
Click to expand...

Is it an African or European swollow?
 
I know how to settle this--someone fill one tank using one method and someone else do the other side using another. I'll provide the plane and run the stopwatch. If you have any doubts, give me a few hours and you can try again. All you have to do is bring the credit card...

Bob Kelly
 
OK, here I thought I was closing in on finishing my build, and now find that to "keep up with the Dyes," as it were, I'm going to have to add a vacuum pump on each wing to help with the fueling so I can get flying faster. Probably also will have to purchase a few very large O-rings to help seal around the fuel nozzles of various sizes when they are inserted into the filler neck.

greg
 
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