Van's Air Force
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Fuel System Design Reference
- Thread starter DanH
- Start date
Dan
I realize this may not fully answer your question. There can be no standard line size with all the system variables in play.
Good info in case of a future search on fuel system design.
https://www.faa.gov/regulations_pol...raft/amt_airframe_handbook/media/ama_ch14.pdf
https://www.lmengines.com/fuel-line-calculators/
http://www.designatedengineer.com/FuelLineSizing.html
GM
I realize this may not fully answer your question. There can be no standard line size with all the system variables in play.
Good info in case of a future search on fuel system design.
https://www.faa.gov/regulations_pol...raft/amt_airframe_handbook/media/ama_ch14.pdf
https://www.lmengines.com/fuel-line-calculators/
http://www.designatedengineer.com/FuelLineSizing.html
GM
The only reference book I could find when I was designing my RV-9A fuel system is Aircraft Fuel Systems by Roy Langton.
https://www.amazon.com/Aircraft-Fuel-Systems-Roy-Langton/dp/0470057084
However, it is concerned primarily with large commercial and military aircraft and after reviewing it, I decided not to buy a copy and cannot recommend it for light aircraft.
https://www.amazon.com/Aircraft-Fuel-Systems-Roy-Langton/dp/0470057084
However, it is concerned primarily with large commercial and military aircraft and after reviewing it, I decided not to buy a copy and cannot recommend it for light aircraft.
Dan, we have typically left that to the engineers that designed the accessories. Since most of the fuel systems we plumb use -6 AN fittings and were originally used with 303 hose (5/16 .312 ID) we use the teflon equivalent.
I'll bet Don Rivera has a guideline.
Tom
I'll bet Don Rivera has a guideline.
Tom
Fluid Mechanics
I dusted off my old fluid mechanics formulas when first looking at the fuel system, since I could not believe that those tiny -6 hoses and tubes would be enough to get me to 20 gph, but clearly they are. I almost went entirely with -8 just because.
Calculations of pressure drop in a complex system with a changing head, and all the other variables introduced by a moving machine are well beyond what was covered back in the '80s when I picked up my degree.
I also looked for some reference material but could not find it, so I went with a very standard fuel system with return, which I only use when using the AFP purge valve.
I'm still convinced that the best solution would be a pump in each tank, with one of those sock filters, exactly as done in the auto world.
Diamond has an interesting system with a transfer pump, but I could not find any technical details on the design.
I'm surprised no aero engineering students have published a comparison of GA fuel systems, and perhaps some suggestions for improvements.
I dusted off my old fluid mechanics formulas when first looking at the fuel system, since I could not believe that those tiny -6 hoses and tubes would be enough to get me to 20 gph, but clearly they are. I almost went entirely with -8 just because.
Calculations of pressure drop in a complex system with a changing head, and all the other variables introduced by a moving machine are well beyond what was covered back in the '80s when I picked up my degree.
I also looked for some reference material but could not find it, so I went with a very standard fuel system with return, which I only use when using the AFP purge valve.
I'm still convinced that the best solution would be a pump in each tank, with one of those sock filters, exactly as done in the auto world.
Diamond has an interesting system with a transfer pump, but I could not find any technical details on the design.
I'm surprised no aero engineering students have published a comparison of GA fuel systems, and perhaps some suggestions for improvements.
rv7boy
Forum Peruser
Dan, I'm thinking most GA fuel systems are designed with the TLAR method.
This is another acronym that I learned at NASA, of all places.
TLAR=> That Looks About Right!
P.S. Surely, there's a rule of thumb as to "flow rate divided by a critical cross-sectional area yielding a limiting velocity" calculation that is used by somebody. Maybe the Reno racers could help. Maybe even Don Garlits (I know, nitromethane vs. avfuel) but it's a start. He concealed his actual fuel lines within the rails of his dragsters.
This is another acronym that I learned at NASA, of all places.
TLAR=> That Looks About Right!
P.S. Surely, there's a rule of thumb as to "flow rate divided by a critical cross-sectional area yielding a limiting velocity" calculation that is used by somebody. Maybe the Reno racers could help. Maybe even Don Garlits (I know, nitromethane vs. avfuel) but it's a start. He concealed his actual fuel lines within the rails of his dragsters.
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Lots depends on the pressures/ head available. Generally speaking, -6 will easily feed anything in the RV world. I've run 5/16 ID lines on methanol burning race cars flowing enough fuel to be equivalent to about 780hp on gasoline.
I know of some other cars plumbed with -6 making nearly 1000hp on gasoline. That's around 1.5 GPM.
Check out page E3 here from Parker: https://www.parker.com/literature/H...atalog 4400 PDF Files/Section_E_Technical.pdf
-6 falls into the safe range for suction lines at our typical fuel flow rates.
I know of some other cars plumbed with -6 making nearly 1000hp on gasoline. That's around 1.5 GPM.
Check out page E3 here from Parker: https://www.parker.com/literature/H...atalog 4400 PDF Files/Section_E_Technical.pdf
-6 falls into the safe range for suction lines at our typical fuel flow rates.
I've rapidly adopted that very conclusion.
Interesting calculator. I've asked if it is available in Excel, as I'd like to see the underlying math. Still, as you say, it's not going to help with fittings and filters and other local restrictions.
What is the ID of a -6 teflon hose? And yes, you would win that bet.
If I were installing EFI, I'd be looking hard at push systems right now. It largely eliminates vapor formation issues, and flow rates can be very high even with -6 line.
That particular page doesn't reference the fluid. I think it's valid for hydraulic oil, but probably not avgas. Huge difference in both viscosity and vapor pressure. Not a big deal on the pressure side, but the suction side is another story. All gasoline systems develop vapor bubbles if you suck 'em hard enough, while lube oil has a vapor pressure near zero.
Nice find however! Chart E4 is identified as hydraulic oil at 20 centistokes, and illustrates why our oil cooler lines need to be -8 or larger. Check out the drops across the 8 gal/min line. The drop would oppose vernatherm closure.
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rmartingt
Well Known Member
I chewed over the idea of pumps in the tanks, but it brought up a few complications. The return fuel would still require a selector valve to make sure it's going to the correct tank. A pump failure leaves you with an unusable tank, unless you put two pumps per side. And switching tanks would be a multi-step process--switch return fuel, switch new side on, switch previous side off. One could possibly rig something up so moving the selector switched the pumps as well, or use a solenoid valve and rig it to a fuel pump switch, but then that leads down a road of more complexity.
In the end, I decided that current EFI practice--duplex valve, dual pumps immediately after the selector, and minimize suction losses before the pump--seems to be working well and I hadn't heard of anyone having issues with that setup. It minimizes fuel management tasks and is close to "standard" operating practice.
Yup, all good reasons why we recommend the standard and well proven layout.
Any pump that pushes out say 1 GPM must also pull that much in. Barring a bunch of tight bends or 90 degree fittings before the inlet and having the fuel level no more than 4 inches below the pump inlet, -6 will work on most any RV application running 100LL or summer blend mogas. I'd simply avoid using winter blend mogas in the summer or flying above 10,000 on hot days with summer blend mogas. These are a bit scary no matter what fuel system layout and line sizes you use.
The Parker chart actually seems reasonable with gasoline too if you throw in some standard numbers and line sizes.
Quite a bit of personal experience doing exactly that myself - and you're right, it can get a bit sketchy - and it's resulted in a few modifications of my fuel system to get it to work reliably.
I don't recommend it as a path for others to follow. The people who need answers to those questions don't need to follow that path - and the people already following that path don't need to ask those questions.
I hear you. EFI, dual inline electric pumps. Can you run both pumps at the same time, sucking through the fuel selector and inline filter?
We have many dozens of folks doing just that on 100LL and maybe some doing it on mogas. I'd personally just use one pump at a time with mogas on a warmish day though. That's worked fine for me for a number of years now although I rarely fly when it's over 27C or so.
I'll be doing some fuel system component testing in June on a new project. I'll try to gather some info on flow rates and temperatures and see if I can induce some vapor at the pump inlet using winter gas on a warm day. Would be good to know how much margin we have. I do have a fuel tester from Petersen I can run some samples through to know where we're at.
Ok, Bob makes good points, and empirical evidence says it's possible, given good hardware layout, to pull some unknown high volume of 100LL to a pair of parallel pumps running at the same time. For now, cancel my statement suggesting push-only systems for the very high flow application.
Excellent. I'd suggest testing with the fuel at 40C (104F) minimum, pulling through an Andair selector, a filter, and a bunch of right angle fittings, at two feet of head. 40C is a realistic sun-soaked fuel temperature across the South (sunny day tank upper surface temperatures are higher), and it puts you at the measurement standard for 100LL vapor pressure. Six inches of head is pretty close to real, two feet is margin.
F1Boss
Well Known Member
Maybe!
I read that a 1/2? system has 30% of the flow restriction that a like-designed 3/8? setup has.
That being said, I know one TSIO550 is using a hot-rodded standard rotary type pump to push/pull ~70GPH into that engine. That same plane uses 1/4? as the return, as the idle pressure is around 25PSI, so the idle return is OK at that pressure and flow. I would think that return flow at max power is also well under control with the 1/4? return - I?ll bet that pump is close to max flow & pressure, and it could be augmented by the boost pump at that level. Ross might know better numbers, as this plane uses his full system.
I?m not sure where you are going with this particular thread, but 3/8? will do for at least 320HP/32GPH - maybe a bit more if you keep the 90deg fittings in their box. If you?re looking at 350Hp up to 700hp, I would say go with the 1/2? setup, and depending on the choice of pumps, you might need a 3/8? return (elec pumps).
Hot fuel? Dunno - have not run into a problem in any engines I have run - including the pair of 1700hp ones I flew for a while..magnificent noise from those beasties!
I?ll have definitive info once I get my next project up and running - till then I?m using the TLAR method, and hoping the mix is rich enough for the 1st lap around the home drome.
I read that a 1/2? system has 30% of the flow restriction that a like-designed 3/8? setup has.
That being said, I know one TSIO550 is using a hot-rodded standard rotary type pump to push/pull ~70GPH into that engine. That same plane uses 1/4? as the return, as the idle pressure is around 25PSI, so the idle return is OK at that pressure and flow. I would think that return flow at max power is also well under control with the 1/4? return - I?ll bet that pump is close to max flow & pressure, and it could be augmented by the boost pump at that level. Ross might know better numbers, as this plane uses his full system.
I?m not sure where you are going with this particular thread, but 3/8? will do for at least 320HP/32GPH - maybe a bit more if you keep the 90deg fittings in their box. If you?re looking at 350Hp up to 700hp, I would say go with the 1/2? setup, and depending on the choice of pumps, you might need a 3/8? return (elec pumps).
Hot fuel? Dunno - have not run into a problem in any engines I have run - including the pair of 1700hp ones I flew for a while..magnificent noise from those beasties!
I?ll have definitive info once I get my next project up and running - till then I?m using the TLAR method, and hoping the mix is rich enough for the 1st lap around the home drome.
rmartingt
Well Known Member
Did some more thinking on how one could maybe get a push system in place without making it too complicated.
Basic idea would be to have the pump in the tank or wing root and put the regulator right there, dumping the "return" right back into the tank*. The single fuel line coming into the airplane would then carry high pressure fuel to the center of the airplane, where your selector would normally be. However, each side would have a check valve somewhere between the regulator and the tee/manifold at the center.
Tank switching would be accomplished by turning one side on and then the other side off.
The plumbing would be very simple with this setup. You won't have hot fuel returning to the tank from the engine compartment, but if the pumps are on it'll be under pressure so I wouldn't expect any problems with it vaporizing in the lines.
The trick would be dealing with a pump failure. As noted above, with a single pump failed you lose access to about half your fuel and wind up with a gradual increase in imbalance. I'm also not sure if you could run both pumps simultaneously. You could fit dual pumps on each side, with a small increase in plumbing complexity and a tight fit space-wise.
You could also fit a small transfer pump and/or valve to move fuel from side to side, but that's also complexity and opens up an avenue to cause yourself more problems than you're solving. I've heard many stories of jet crews getting themselves in trouble leaving an intertank valve open, and the small imbalance on one side becomes a big imbalance on the other...
* I think this is how newer cars do it--they regulate pressure at the tank/pump so there's not a return line. But I'm not a car guy so I could be wrong.
Basic idea would be to have the pump in the tank or wing root and put the regulator right there, dumping the "return" right back into the tank*. The single fuel line coming into the airplane would then carry high pressure fuel to the center of the airplane, where your selector would normally be. However, each side would have a check valve somewhere between the regulator and the tee/manifold at the center.
Tank switching would be accomplished by turning one side on and then the other side off.
The plumbing would be very simple with this setup. You won't have hot fuel returning to the tank from the engine compartment, but if the pumps are on it'll be under pressure so I wouldn't expect any problems with it vaporizing in the lines.
The trick would be dealing with a pump failure. As noted above, with a single pump failed you lose access to about half your fuel and wind up with a gradual increase in imbalance. I'm also not sure if you could run both pumps simultaneously. You could fit dual pumps on each side, with a small increase in plumbing complexity and a tight fit space-wise.
You could also fit a small transfer pump and/or valve to move fuel from side to side, but that's also complexity and opens up an avenue to cause yourself more problems than you're solving. I've heard many stories of jet crews getting themselves in trouble leaving an intertank valve open, and the small imbalance on one side becomes a big imbalance on the other...
* I think this is how newer cars do it--they regulate pressure at the tank/pump so there's not a return line. But I'm not a car guy so I could be wrong.
Just what it says...locate design references.
My mentor taught me that until you quantified a thing, you didn't really understand it. I'd like more insight into the physical mechanics of why some systems work, and others don't.
For example, we all know that 90 degree fittings can cause problems. So, on the suction side, how do we determine the flow rate above which a certain size 90 starts forming vapor bubbles?
Here's the thing. It is a complex problem, apparently an interplay of temperature, pressure, and Reynolds number, plus the fluid is a mixture of compounds with different vapor pressures. There may not be any easy way to nail down a figure, but I'll probably learn a few things while chasing the question.
On most EFI systems the orifice size on the pressure regulator valve is usually around 3/16. That's usually capable of regulation of the system pressure to around the 45 psi range at flow rates between 1 and 1.5 GPM. The return line, if relatively short, could be on the order of -4. Longer runs with more bends and fittings should be -6.
As I said before, I know of one Reno racer using a huge Waldon pump plumbed with -6 working just fine pushing enough fuel for over 600hp with the ADI fuel contributing another 175 hp or so through a separate system and I've done multiple auto race systems running similar flow rates with -5 plumbing (no 90 degree fitting on the inlet sides of course).
Of note, the Walbro pumps have quite a small inlet fitting and this is probably the biggest restriction to suction limitations on them. The cross sectional ID area of two of these (twin pumps) is about the same as a single -6 fitting.
Once the fuel is pressurized, vapor lock issues are pretty much no concern downstream any more.
As I said before, I know of one Reno racer using a huge Waldon pump plumbed with -6 working just fine pushing enough fuel for over 600hp with the ADI fuel contributing another 175 hp or so through a separate system and I've done multiple auto race systems running similar flow rates with -5 plumbing (no 90 degree fitting on the inlet sides of course).
Of note, the Walbro pumps have quite a small inlet fitting and this is probably the biggest restriction to suction limitations on them. The cross sectional ID area of two of these (twin pumps) is about the same as a single -6 fitting.
Once the fuel is pressurized, vapor lock issues are pretty much no concern downstream any more.
Toobuilder
Well Known Member
Admitedly not guided by hard engineering data, my fuel system for the Rocket simply tries to follow the best design elements available within the confines of the Rocket structure. For example, the SDS dual fuel pump module is as low as possible without punching through the OML of the fuselage; I have no 90 fittings on the suction side; there is no flex hose on the suction side; and I even changed the fuel tank pickup to a straight fitting with a bent tube to eliminate that 90.
High altitude, high temps and mogas are in my future so I want to stack the deck in my favor before first flight. Will it work? Not sure, but thats what a disciplined flight test series is supposed to verify.
I have my fuel pump module wired with one pump providing continuous duty, while the other is connected to my old BOOST switch (on the throttle, in my case). I have done fuel flow checks with one and both pumps running and there seems to be no adverse effects on the ground. The single pump provides plenty of flow to feed the engine, and both increases that flow about 40%. That tells me that the suction side is working harder with both pumps running, so its worth considering that running 2 pumps for takeoff might actually increase your chances of a fuel starvation event? Sure, you are pretty safe from a pump "failure", but does that help you if you pull so hard that you cavitate both on a hot day departure? Not trying to be alarmist - just thinking out loud.
If someone was so inclined, it probably would not be hard to cobble together a recirculating test bench that could easily add branches with ever increasing restriction (lots of 90's) to measure the effect. You could have an adjustable temperature fuel tank to simulate that August mid afternoon departure from PHX, and if really tricky, pull a vaccum on the fuel system to simulate altitude.
High altitude, high temps and mogas are in my future so I want to stack the deck in my favor before first flight. Will it work? Not sure, but thats what a disciplined flight test series is supposed to verify.
I have my fuel pump module wired with one pump providing continuous duty, while the other is connected to my old BOOST switch (on the throttle, in my case). I have done fuel flow checks with one and both pumps running and there seems to be no adverse effects on the ground. The single pump provides plenty of flow to feed the engine, and both increases that flow about 40%. That tells me that the suction side is working harder with both pumps running, so its worth considering that running 2 pumps for takeoff might actually increase your chances of a fuel starvation event? Sure, you are pretty safe from a pump "failure", but does that help you if you pull so hard that you cavitate both on a hot day departure? Not trying to be alarmist - just thinking out loud.
If someone was so inclined, it probably would not be hard to cobble together a recirculating test bench that could easily add branches with ever increasing restriction (lots of 90's) to measure the effect. You could have an adjustable temperature fuel tank to simulate that August mid afternoon departure from PHX, and if really tricky, pull a vaccum on the fuel system to simulate altitude.
I think this thread has brought out some useful discussion. Many people have asked us over the years if they can run both pumps for TO and landing and I've said yes with some caveats (free inlet path, and watch for fuel pressure rise in some cases). Some have asked what I do on my plane and I tell them I just use one pump at a time. I'm not much worried about a pump failure given our long experience with them.
I've been running mogas for about 7 of the 15 years I've been flying my RV. No issues yet plumbed 3/8 hard lines from the tanks to the Andair selector with a 1/4 NPT Tee from the selector that splits off to twin 90 degree fittings dropping down to one Earls 230106 filter before each pump plumbed with -5. Using the standard Walbro GSL 393 pumps sitting on the floor.
IMO, running only one pump at a time on mogas is safer as TooBuilder suggests, at least until we can do some more instrumented testing on a bench setup.
I've been running mogas for about 7 of the 15 years I've been flying my RV. No issues yet plumbed 3/8 hard lines from the tanks to the Andair selector with a 1/4 NPT Tee from the selector that splits off to twin 90 degree fittings dropping down to one Earls 230106 filter before each pump plumbed with -5. Using the standard Walbro GSL 393 pumps sitting on the floor.
IMO, running only one pump at a time on mogas is safer as TooBuilder suggests, at least until we can do some more instrumented testing on a bench setup.
Free flow (pumping to a bucket, so to speak), or pumping back to the tank through the pressure regulator and return system?
A pressure gauge near the inlet to the pumps would tell a lot. Two conditions, engine at idle, WOT at max AOA.
Very few have suffered a sudden engine driven pump failure. However, we use the boost pump for takeoff, approach, and ground hugging, just so we don't have to correctly recognize pump failure and react when the spit hits the fan. The alternative is automatic pump switching, available from your competitor.
Toobuilder
Well Known Member
In my flow test it was "regulated" pressure (45 PSI), but dumping into a bucket. The alternative is to just let the fuel travel around the fuel rail (in my particular case) and back to the tank via the selector valve. With the EFI system like this, there really is not much difference in system flow between the engine at 100% power, idle, or for that matter - off. The "system" is flowing 45 GPH at 45 PSI at all times, whether the engine is feeding off it or not.
BillL
Well Known Member
Not much is done empirically. I think you are looking for a sizing reference? You might consider doing it by Reynolds number, Re. Tubes and hoses might have a different surface roughness though. A teflon lined should be a smooth as a tube, but braided hose would be rougher. Anyway, one of the online calculators eluded to Re as the key, to prevent getting into turbulent flow.
You might look into mathmatica as a calculation tool. Steep learning curve, but very powerful design tool and much easier to use complex, interative types of calculations than linear tools like excel and/or basic.
