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How to prevent low-wing fuel system to draw air?

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Lucas Delgado

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Oct 10, 2019
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Hello, I'm designing the fuel system for a low wing aircraft. There is one tank located inside each wing. As locating a header tank is almost imposible (there is no posible place lower than the wing tanks) I can't get to a solution to prevent the fuel pump of drawing air during climb or an uncordinated turn or a slip.
I mentioned climb, because the lower point of the tank is the front part so the fuel pipe will be also in the front.
These are the dimensions of the tank https://imgur.com/HIoIWVz
 

Dan Thomas

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You have no area in the fuselage belly lower than the tank?

You'll unport in a climb with the fuel outlet so far forward. You'd end up with an unacceptable unusable fuel level. Unusable fuel is that amount defined as the fuel that won't flow to the engine when the airplane is in some critical attitude, like a steep climb. In most airplanes it's a small fraction of the total capacity. It's NOT, as some people think, just the fuel in the fuel lines and so on.

Large airplanes use surge tanks. The inboard aft corner of the tank is walled off and a well-fitted hinged flapper over a hole in the wall lets fuel in but not out, and the pump and outlet are in that section. That keeps fuel flowing in slips or turbulence, but it wouldn't save your hide in a climb with your forward outlet. Not enough fuel. The surge tank is only going to be at the level of the fuel in the rest of the tank, and low fuel equals low surge tank.

Low wing airplanes present plenty of fuel flow and venting issues. Pumping is necessary, both with an engine-driven pump and a standby. Fuel cannot be drawn from both tanks at once. You can't have outlets at both front and back as in a high-wing airplane, as any chance the pump can get air means that no fuel will flow. Venting of multiple joined tanks in one wing gets interesting, to keep pressures equal and prevent overboard flow.
 

pictsidhe

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Either put the outlet in a best conprimise position, or use multiple pumps to a header tank. Simple and light pulse diaphragm pump on each suction point to an internal header tank with a conical bottom feeding the engine would be one way round this.
 

Lucas Delgado

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Oct 10, 2019
Messages
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You have no area in the fuselage belly lower than the tank?

You'll unport in a climb with the fuel outlet so far forward. You'd end up with an unacceptable unusable fuel level. Unusable fuel is that amount defined as the fuel that won't flow to the engine when the airplane is in some critical attitude, like a steep climb. In most airplanes it's a small fraction of the total capacity. It's NOT, as some people think, just the fuel in the fuel lines and so on.

Large airplanes use surge tanks. The inboard aft corner of the tank is walled off and a well-fitted hinged flapper over a hole in the wall lets fuel in but not out, and the pump and outlet are in that section. That keeps fuel flowing in slips or turbulence, but it wouldn't save your hide in a climb with your forward outlet. Not enough fuel. The surge tank is only going to be at the level of the fuel in the rest of the tank, and low fuel equals low surge tank.

Low wing airplanes present plenty of fuel flow and venting issues. Pumping is necessary, both with an engine-driven pump and a standby. Fuel cannot be drawn from both tanks at once. You can't have outlets at both front and back as in a high-wing airplane, as any chance the pump can get air means that no fuel will flow. Venting of multiple joined tanks in one wing gets interesting, to keep pressures equal and prevent overboard flow.
I've been thinking about, and the best solution would be to make it a flat tank lossing some capacity. The fuel port would be located in the back part of the tank. An intermediate small tank (arround 1 gallon) may be located in each wing to keep a fuel reserve during descent. With a 5° dihedral angle, Do you think I'll have problems with slips if I don't use this small tank?
 

Lucas Delgado

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Oct 10, 2019
Messages
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Either put the outlet in a best conprimise position, or use multiple pumps to a header tank. Simple and light pulse diaphragm pump on each suction point to an internal header tank with a conical bottom feeding the engine would be one way round this.
I don't have much physical space to locate a header tank but that's one possibility
 
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Dan Thomas

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I've been thinking about, and the best solution would be to make it a flat tank lossing some capacity. The fuel port would be located in the back part of the tank. An intermediate small tank (arround 1 gallon) may be located in each wing to keep a fuel reserve during descent. With a 5° dihedral angle, Do you think I'll have problems with slips if I don't use this small tank?
Five degrees dihedral is a lot.

Yes, better to have a flatter-bottomed tank than to have to carry around a bunch of fuel you can't use or rely on being available when you need it.

Most wing tanks conform to the airfoil, so are fairly flat-bottomed, and are also in a position to favor an aft outlet since the wing's angle of incidence is positive. Why would your tank have its lowest section so far forward?

A one-gallon wing tank for descent might not help much. In a descent, as the fuel in the main tank moves forward, the fuel from that small tank would want to drain back into the main tank, and if you had to abort the landing there might be more air than fuel in that tank just when you need the fuel the most. That's why big airplanes use surge boxes with their big flapper valves that let fuel in quick and don't let it out much at all.

Study the fuel systems from other low-wing airplanes to get an idea of what's involved. You want to get it right. Fuel starvation is a fairly common cause of early homebuilt flights, and it's usually due to poor system design.
 

pictsidhe

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I don't have much physical space to locate a header tank but that's one possibility
Your header tank would go inside your fuel tank. It wouldn't need to be large. Its outlet feeds the engine, diaphagm pumps pump fuel/air into it. A hole in the top of rhe header lets it fill or overflow as required.

Edit, added 3000 words, the quick way!

20191011_170313.jpg 20191011_170349.jpg20191011_170355.jpg
 
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wsimpso1

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I had flat tanks, aft pickup from each, surge doors, and concerns over unporting in slips. Sound familiar? We talked this over pretty thoroughly in the attached thread.

https://www.homebuiltairplanes.com/forums/threads/transfer-pumps-and-fuel-system-configuration.30820/

I ended up going with a surge tank that I pump to from the wing tanks, with vent/overflow back to the source tank via duplex valve and Facet pumps. We even talked about several ways to run redundant pumps. Since I wanted more fuel capacity on board and some time to get landed should the wing tanks become unavailable, my header tank is 9 gallons. Yours does not need to be so big to work as a surge tank and cover short term unporting. But if you make it that large, you can put in a flop tube and make it your aerobatic tank.

Bill
 

radfordc

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Why does the header tank have to be below the wing tanks? Put the header tank where it fits and pump fuel from the wing tanks to the header as needed. A Facet electric pump controlled by a cockpit switch or float switch in the tank maintains the header tank at the correct level. If the Facet pump unports occasionally it's not a problem.
 

Lucas Delgado

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Oct 10, 2019
Messages
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Five degrees dihedral is a lot.

Yes, better to have a flatter-bottomed tank than to have to carry around a bunch of fuel you can't use or rely on being available when you need it.

Most wing tanks conform to the airfoil, so are fairly flat-bottomed, and are also in a position to favor an aft outlet since the wing's angle of incidence is positive. Why would your tank have its lowest section so far forward?

A one-gallon wing tank for descent might not help much. In a descent, as the fuel in the main tank moves forward, the fuel from that small tank would want to drain back into the main tank, and if you had to abort the landing there might be more air than fuel in that tank just when you need the fuel the most. That's why big airplanes use surge boxes with their big flapper valves that let fuel in quick and don't let it out much at all.

Study the fuel systems from other low-wing airplanes to get an idea of what's involved. You want to get it right. Fuel starvation is a fairly common cause of early homebuilt flights, and it's usually due to poor system design.
Unfortunately the tank must fit into a wing already build with some special ribs with that shape, so I can't change it, what I'll probably do is to flatten the tank but some space inside the wing would be wasted. As the small tank concerns, it's suppossed to have a check valve, would that help?
I've been searching for the fuel system in the piper tomahawk which is very similar to my plane, I could find a fuel system diagram but I wasn't able to find any photo of the tank itself
 

BoKu

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...As locating a header tank is almost imposible (there is no posible place lower than the wing tanks)...
Maybe you could have a header tank higher than the wing tank, fed from a scavenge pump that draws from either (but not both) of the wing tanks. It would serve as a reservoir to ensure fuel availability to the engine even if the main tank inlet becomes temporarily unported. The header tank might have a return line at the top through a two-row selector valve so that any overflow is returned to the selected main tank.
 

Lucas Delgado

Member
Joined
Oct 10, 2019
Messages
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I had flat tanks, aft pickup from each, surge doors, and concerns over unporting in slips. Sound familiar? We talked this over pretty thoroughly in the attached thread.

https://www.homebuiltairplanes.com/forums/threads/transfer-pumps-and-fuel-system-configuration.30820/

I ended up going with a surge tank that I pump to from the wing tanks, with vent/overflow back to the source tank via duplex valve and Facet pumps. We even talked about several ways to run redundant pumps. Since I wanted more fuel capacity on board and some time to get landed should the wing tanks become unavailable, my header tank is 9 gallons. Yours does not need to be so big to work as a surge tank and cover short term unporting. But if you make it that large, you can put in a flop tube and make it your aerobatic tank.

Bill
I don't really like the idea of a higher-than-wings header tank, that unporting you mention, it's a risk in intentional slips, right? or may I encounter an unporting during a non so well cordinated turn? about the surge doors, how did you build or plan to build them? I'm afraid of flapper valves may fail.
 

Dan Thomas

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Messages
5,546
Maybe you could have a header tank higher than the wing tank, fed from a scavenge pump that draws from either (but not both) of the wing tanks. It would serve as a reservoir to ensure fuel availability to the engine even if the main tank inlet becomes temporarily unported. The header tank might have a return line at the top through a two-row selector valve so that any overflow is returned to the selected main tank.
That way works but then you're totally reliant on the scavenge (tank) pump, since the main tank won't flow into the header without it. Low-wing airplanes with a pump in the belly or in each tank also have the engine-driven pump that can draw the fuel under most conditions with the tank/belly/aux pump turned off. All such pumps are electrically-driven and so you're electrical system now becomes unacceptably critical, and electrical systems are prone to failure. The mechanical pump on the engine keeps running even if the electrics quit, so systems are designed so that it can keep the fuel moving to the engine even if the other pumps don't work.

Aux pumps are there to prime injected engines and to take over if the engine-driven pump fails. One shouldn't rely on them unless there's no place for an engine-driven pump (as on some auto conversions). Large airplanes (airliners) rely on them, but they have multiple electrical systems and multiple generators.
 

wsimpso1

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I don't really like the idea of a higher-than-wings header tank, that unporting you mention, it's a risk in intentional slips, right? or may I encounter an unporting during a non so well cordinated turn? about the surge doors, how did you build or plan to build them? I'm afraid of flapper valves may fail.
A little etiquette here. Please have some respect for the folks who would volunteer their time and knowledge to help you. Apply at least as much care and thought in preparing your questions as you would like for the responses. Low effort questions tend to get low effort answers, at least in a volunteer situation... This is a big topic, and requires some organized thinking just to respond adequately.

I am not talking huge efforts in editing or long paragraphs. Single topics per sentence, complete sentences, and brevity while avoiding compound questions would go a long ways towards inspiring the effort to help with thoughtful answers, pictures, construction details, etc.

Billski
 

Lucas Delgado

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Oct 10, 2019
Messages
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A little etiquette here. Please have some respect for the folks who would volunteer their time and knowledge to help you. Apply at least as much care and thought in preparing your questions as you would like for the responses. Low effort questions tend to get low effort answers, at least in a volunteer situation... This is a big topic, and requires some organized thinking just to respond adequately.

I am not talking huge efforts in editing or long paragraphs. Single topics per sentence, complete sentences, and brevity while avoiding compound questions would go a long ways towards inspiring the effort to help with thoughtful answers, pictures, construction details, etc.

Billski
Maybe I haven't made myself clear, what I meant when I talked about the idea of the header tank. It's what Dan suggustes, that would make me electrical dependent.

I've been reading the threath you linked, but I would like to know why you changed to a header tank.

I'm completely grateful with all the awnsers that are helping me to figure out the most adequate system for my aircraft. If it seems different, I'm truly sorry. I'll make an effort to make better questions

Lucas
 

wsimpso1

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Messages
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Location
Saline Michigan
Surge tanks and headers tanks are common pieces of fuel systems, and pumping to them is a common way to keep them filled. When kept reasonably full, they will not unport unless you go negative g.

Lots of airplanes use header tanks very successfully including all of the two-seat Lancair models.

The very high reliability way to pump is a pair of Facet pumps of appropriate size and with integral check valves plumbed in parallel to fill the tank. Yes, it makes you electricity dependent. More on that below.

The pumps fill the header tank with the feed to the top. The vent/overflow is also attached to the top of the tank. Several schemes for vent/return exist:
  • A common one is to plumb the return to one tank, and you always draw that one down before the others to prevent dumping fuel overboard;
  • Another scheme is to use a duplex valve to select tank to be drawn from and simultaneously run vent/return back to the selected tank. There are certified and experimental only duplex valves out there;
The engine then draws fuel through a finger screen low in the header tank. You might even be able to make this a gravity feed system from here.

In this combination of high fill and return points plus low point to draw for the engine, the tank will not back feed to the wing tanks, and its fuel will be available even if feed from the wings is interrupted. 45-60 minutes in this tank gives you a lot of time to get onto an airport when the level starts dropping.

Power for the pumps might seem questionable, but there are lots of Long Ez's running for many years with a single Facet pump, no backup. These pumps are pretty reliable. As I said above I am running a pair of them in parallel. You have to buy the versions with internal check valves to allow running one at a time and keep them from running fuel in a circle.

Power reliability is another issue. I recommend buying and reading Bob Nuckoll's AeroElectric Connection. He has a bunch of information about airplane electricity and fault tolerant systems already designed. If you are electrically dependent, you would be very wise to have redundant batteries or redundant alternators or maybe even both. I like his Z-14, but then I plan to operate IFR. Dual batteries, dual alternators, cross feed between buses. I am also running EFII, so I need redundant electricity. I would wire one pump to one bus, the other pump to the other bus, and connect them so you can select both simultaneously. I suppose you could still put in a wobble pump to keep the header filled after you lose both alternators and run down both batteries.

My issue with unporting - long thin tanks could have fuel flow away from the pickups. How? Slips and uncoordinated turns both happen. Slips can easily get more than five degrees of bank and start fuel away from the pickup. Inadvertant uncoordinated flight with more than five degrees of bank error, I suppose it could happen, but I expect it would be rare. I expect slipping turns to be way more likely to unport. All deliberate slips are power off and likely to last no more than 30 seconds at a time, so you only need small amounts of fuel for them.

A partial answer to unport worries in slips is slosh doors. My design is a simple top hinged aluminum door with extruded aircraft hinge at the top, an aluminum frame that is attached with flush pulled rivets. The tab with the hole in it allows a bent wire to be inserted through a hole (normally filled with a pipe plug) to exercise the door for confirmation it is still there and working. There is a stop tab on each to keep the door from flopping too far open. This scheme traps about a gallon, which should keep an idling engine fed for a lot longer than you will hold a slip even with the door leaking fuel the whole time. The below photo shows one of them. The picture was taken with the wing inverted before we installed the bottom skin.

upload_2019-10-13_10-19-40.png

Even with slosh doors, the finger screen has to stay covered to draw from that tank. If you only have these wing tanks, minimum usable fuel the amount that will just barely not unport in a full rudder slip or slipping turn. Could be quite a bit. Ultimately I designed and am building a header tank that is pumped to full and then recirculates fuel with reliable redundant pumps and electricity. Now the engine side fuel flow is not interrupted when the wing tank unports for a couple seconds, and when you go wings level, it resumes pumping until the level gets to the screen and starts to draw air. My unusable fuel that way will be more like a gallon with another couple quarts unusable in the header tank. Yeah the slosh doors and their intermediate bulkheads are now moot, but they are sealed in the wings, so they are staying.

Billski
 
Last edited:

Lucas Delgado

Member
Joined
Oct 10, 2019
Messages
8
Surge tanks and headers tanks are common pieces of fuel systems, and pumping to them is a common way to keep them filled. When kept reasonably full, they will not unport unless you go negative g.

Lots of airplanes use header tanks very successfully including all of the two-seat Lancair models.

The very high reliability way to pump is a pair of Facet pumps of appropriate size and with integral check valves plumbed in parallel to fill the tank. Yes, it makes you electricity dependent. More on that below.

The pumps fill the header tank with the feed to the top. The vent/overflow is also attached to the top of the tank. Several schemes for vent/return exist:
  • A common one is to plumb the return to one tank, and you always draw that one down before the others to prevent dumping fuel overboard;
  • Another scheme is to use a duplex valve to select tank to be drawn from and simultaneously run vent/return back to the selected tank. There are certified and experimental only duplex valves out there;
The engine then draws fuel through a finger screen low in the header tank. You might even be able to make this a gravity feed system from here.

In this combination of high fill and return points plus low point to draw for the engine, the tank will not back feed to the wing tanks, and its fuel will be available even if feed from the wings is interrupted. 45-60 minutes in this tank gives you a lot of time to get onto an airport when the level starts dropping.

Power for the pumps might seem questionable, but there are lots of Long Ez's running for many years with a single Facet pump, no backup. These pumps are pretty reliable. As I said above I am running a pair of them in parallel. You have to buy the versions with internal check valves to allow running one at a time and keep them from running fuel in a circle.

Power reliability is another issue. I recommend buying and reading Bob Nuckoll's AeroElectric Connection. He has a bunch of information about airplane electricity and fault tolerant systems already designed. If you are electrically dependent, you would be very wise to have redundant batteries or redundant alternators or maybe even both. I like his Z-14, but then I plan to operate IFR. Dual batteries, dual alternators, cross feed between buses. I am also running EFII, so I need redundant electricity. I would wire one pump to one bus, the other pump to the other bus, and connect them so you can select both simultaneously. I suppose you could still put in a wobble pump to keep the header filled after you lose both alternators and run down both batteries.

My issue with unporting - long thin tanks could have fuel flow away from the pickups. How? Slips and uncoordinated turns both happen. Slips can easily get more than five degrees of bank and start fuel away from the pickup. Inadvertant uncoordinated flight with more than five degrees of bank error, I suppose it could happen, but I expect it would be rare. I expect slipping turns to be way more likely to unport. All deliberate slips are power off and likely to last no more than 30 seconds at a time, so you only need small amounts of fuel for them.


A partial answer to unport worries in slips is slosh doors. My design is a simple top hinged aluminum door with extruded aircraft hinge at the top, an aluminum frame that is attached with flush pulled rivets. The tab with the hole in it allows a bent wire to be inserted through a hole (normally filled with a pipe plug) to exercise the door for confirmation it is still there and working. There is a stop tab on each to keep the door from flopping too far open. This scheme traps about a gallon, which should keep an idling engine fed for a lot longer than you will hold a slip even with the door leaking fuel the whole time. The below photo shows one of them. The picture was taken with the wing inverted before we installed the bottom skin.


View attachment 89485


Even with slosh doors, the minimum fuel in the tank has to fully cover the finger screen, so minimum useable fuel will have to be a few inches. So the unusable fuel would be quite a bit, maybe two or three gallons a side. Ultimately I designed and am building a header tank that is pumped to full and then recirculates fuel with reliable redundant pumps and electricity. My unusable fuel per side will be more like a gallon with another couple quarts in the header tank. Yeah the slosh doors and their intermediate bulkheads are now moot, but they are sealed in the wings, so they are staying.


Billski
I've been doing some designs and as you said, using surge tanks let me with a big amount of unusable fuel. The best design I was able to figure out consist of an independent tank next to the wing tank of approximately 1 gallon with a check valve between them. In level flght the unusable fuel is less than 1 gallon, but if I'm climbing it goes down to almost 2 gallons of a 13 gallon-tank. And during a nose down attitude, as the port is in the aft part of the tank, I only have the reserve in that 1 gallon tank which has the port in front and lower part, making almost all the full inside it usable.
That 1 gallon tank should solve the problems of slipping and I believe it would be enought for a descent.

If I go with the header tank I couldn't use a two-battery two-alternator system, as my aircraft is not big (Gross weight of 1400lbs) and I want to keep it as simple as it could be. However, as you say, 45 min of flight time should be enough to make it to an alternative.
 
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