Fuel system testing and unporting tests.

Nev

Nev

Quick-build 4-place B #25
Yesterday I spent 2 hours at 8000 ft trying different fuel scenarios.
Firstly I ran the Left tank dry by running only from the left tank. It took quite some time to run the fuel level down as I was running LOP.

The engine first surged several times with approximately 5-8 liters in the tank, so I think it was unporting at this stage. I then selected the right tank, and the engine restarted within around 5 seconds with the fuel pump off.

At this stage I could still see fuel in the left sight gauge, so I repeated the same process a number of times. Each time I was surprised at how long the engine would run for before stopping again. I was also able to keep it running by putting the aircraft out of balance (and flooding the ports )such that the fuel pickups were flooded. Eventually this was no longer possible and at that point I could no longer start the engine from the the left tank. One point that was clear is that with little fuel in the tank, being out of balance away from the ports very quickly unported the tank and stopped the engine.

During this time on several occasions I selected BOTH, and observed that the engine restarted each time in around 5 seconds and ran well. Each time the engine stopped it was preceded by the fuel pressure dropping approximately 10 seconds beforehand and as a result I've now set the low fuel pressure alarm to come on at 20 psi, which just gives enough warning to change tanks. The fuel pressure could also be seen rising just before the engine restarted.

Once the left tank was completely dry, I selected BOTH again, the engine restarted, and I flew for approximately 45 minutes at a cruise power setting. Then I put the aircraft well out of balance and flew for 5 minutes before repeating the out of balance to the opposite side. The engine continued to run normally. At various times I turned the electric fuel pump on, but there was no difference.

Finally, I descended to 2000ft (still on BOTH, with the Left tank empty). I then set climb power and climbed through several thousand feet with a pitch attitude of 10-15°. The engine continued to run normally. I landed with 30 liters in the right tank, and the left tank empty. I then refuelled and noted that the left tank took 103.6 liters to fill.

This exercise has given me alot more confidence on my aircraft to run in the BOTH position, even with a large fuel imbalance. It has also made me reluctant to select an individual tank when it has a low fuel quantity. When running LOP my fuel burn is around 37-42 LPH, so 30 mins reserve is only 20 liters, or 10 aside. I'm still getting my head around this, but I would think that on my aircraft the BOTH position would be safest as Bob intended.

I'm posting this here as another data point regarding possible unporting of tanks in other threads. Bear in mind that I'm not a test pilot, and all of our aircraft have differences. Mine is a Bravo 4 place model, with an IO540, electric fuel pump, no return lines, and a fuel selector with L,R, BOTH, OFF.

I've attached a short video showing one part where I had the left tank selected before it was completely dry, and I was flying out of balance. After the engine surges several times, then stops, I selected the BOTH position and it restarts with the fuel pump off after about 5 seconds.

Incidentally the monitor sitting on the dashboard is a carbon monoxide monitor, which I've now installed permanently on the right wing root panel. It works much better than the card type, has an audio alarm, and a digital readout. It costs NZD$53, which is a cheap insurance in my books and I've learnt that the cards don't often register a low but continuous CO level. As a result I've also installed a small gurney flap ahead of the gascolator fuel drain which has cut the CO ingress a lot when running ROP. When LOP the levels are zero.

Here a link to the video showing testing of my fuel system, and my experience with carbon monoxide.
 

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Very interesting Nev. Thanks for taking the time to methodically look at this. Real world testing is worth a lot. Granted - this was just your plane with the way it was built and its fuel system set up.

Your courage also needs to be commended. I could never be up there and killing my engine repeatedly. My heart would likely fail itself from high blood pressure - or at the very least I would need a change of pants & underwear. Mark
 
Thanks you Nev for this. Your tests were much more extensive than mine in my Patrol which were posted here sometime ago and Jared included in a Beartracks issue. I, like others, did not have the nerve to take it to engine stopping several times, but did enough to convince myself that unporting one tank would not create fuel starvation to the engine when running on both. Your tests further increase confidence for me as well.
 
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Wow! That takes some serious huevos Nev! You're in the Bob Hoover club now!

I tested my tanks in the Left, Right and Both position at 3/4 full at 5,000. That was about as brave as I could get! Though highly unlikely, I imagined a vapor lock / hot start scenario while trying to maintain best glide and looking for a patch of grass.
 
Good work Nev.

I’d like to point out that this appears to be an IO with an electric and mechanical pump. These results do not apply to those running a carb and gravity, and as Mark said, could be very specific to this airplane even if you do have pumps.
 
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This is really interesting Nev. I’ve heard about injected Bearhawk engines stopping due to Unporting. My understanding is that a fuel pump will rather suck air than fuel if it’s given the choice. Whenever I’ve run a tank out intentionally (haven’t done it in a Bearhawk) on an injected engine, the drop in fuel pressure and change in engine running has given ample time change tanks without the engine stopping. That would seem to be consistent with your results.

My question is then, how or what is different with the Bearhawks that have unported and stopped?

Given that I’m in the process of changing my B Model from a carbureted engine (that did stop but nothing to do with lack of fuel) to injected I’m really keen to understand what creates the risk and what can be done to mitigate it.
 
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Thanks Schu. Yep that's correct, actually exactly why I did the tests to see if having a pump(s) in the fuel system changed the outcome.
 
Bissetg said:
My question is then, how or what is different with the Bearhawks that have unported and stopped?
Click to expand...

At this stage Grant, I don't think it is peculiar to Bearhawks, and I'm no longer convinced that it is confined to fuel injected engines (with a pump sucking fuel).

PaulSA touched on this in another thread that I didn't understand at the time with regards to whether or not the system will suck air with a pump running. Thinking about it now, with an injected engine, both pumps are forward of the gascolator, and so as long as you have sufficient head of pressure feeding to the gascolator, I think the pump inlet will remain flooded. When I did the fuel flow tests before flying, even a single tank was capable of a flow rate well in excess of the full power fuel demand, so my pump inlets are always flooded (the gascolator would always remain full) and therefore there is no tendency to suck fuel (or air).

There are two situations where this could change though - a blockage in any part of the system up line from the aux pump that slows the flow rate (for instance a partially blocked finger strainer etc), and unporting one tank when the other tank is empty (turbulence or out of balance scenario).

Even with a small amount of fuel in each tank, an out of balance situation would unport one side and flood the opposite, so no issue. But as soon as a tank is empty and the other tank is low enough that it can unport, then I think an out of balance situation that uncovered the ports would cause a stoppage.

Yaw Stability
This was why the yaw stability had my full attention and hence the related thread.

Incidentally, my ailerons had a fairly significant reflex in them initially that gave light and responsive roll control. Two days ago I reduced the reflex on both ailerons by 1.5 turns of the rod end bearings on each side. The roll response is now slightly heavier (but has a nice feel to it), but the yaw appears to be slightly more stable at a forward CG - something I wasn't expecting. I've also changed the rudder pedal springs to what Budd Davidson suggested (5lbs/inch). This also helped a small amount. My rudder trim tab now gives an in trim speed of around 110kts. At lower speeds I need to apply light right rudder (during climb for instance) and at higher speeds a small amount of left rudder.

I think that perhaps a combination of all these factors has improved the yaw stability, and combined with gaining flight time on type, it's much easier now to remain in balance. I've been videoing a number of flights and when I play back the video I notice now that it's nearly always in balance.

So for someone with significant experience or currency in similar aircraft this probably wouldn't be an issue and you'd naturally tend to fly in balance all the time without thinking about it, even if the aircraft was less stable in yaw. But for myself and perhaps one or two others like me either through being less current or new to aircraft that require you to be more proactive to keep the aircraft in balance, it could be very easy to fly out of balance. Particularly if the individual aircraft itself was less stable in yaw during initial flights as you fine-tune the rigging, as mine seemed to be for the first 20 hours.

This could potentially lead to an out of balance fuel situation, run one tank dry, and unport the remaining tank. An easy way to avoid this is just to carry more fuel until the aircraft rigging has been fine-tuned and keep the aircraft balanced.
 
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Thanks Nev, your logic seems sound. It would be good to hear from someone who has experienced the problem to see if they can corroborate your proposition. Keep up the good work.
 
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There are several fuel system threads, and people reporting their fuel flow tests. I came to the conclusion that stock fuel system with 3/8 lines are totally compliant for 200 hp, marginal for much more than that. An MFI injected Lyc 540 doesn't flow any more fuel than a carburated one. I don't see any difference. The big difference is with Conti or EFI injection where they flow excess fuel, and return fuel to the main tanks. If they return to a header or "collector" tank (not my term, but from a Boeing engineer), the fuel flow from the mains is the same. 3/8 lines are not big enough if you return to the mains in those cases. Either increase the size of the fuel lines, or return to a header tank. That was my choice. Whee chose 1/2 inch lines, which also works.

Bob's fuel system is designed to be gravity fed, not suction fed. If gravity isn't enough, I think more gravity is better than more suction. I think a lot have gotten away with a 540 BH with 3/8 lines, no problem. Fuel pumps or not. But I don't think their is a lot of headroom there, although the prototype BH5 has 320-ish hp and 3/8 lines.

Nev, awesome info. Keep it coming. My flying experience, and lack thereof in taildraggers, is similar to yours so I am learning from yours.
 
That's a very good point about the extra fuel Flow required for a system with return lines. I wonder if there's a link there.
 
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I have come around to seeing that (on our aircraft due to gravity feed) an unported feed line will not empty there is gravity feed of fuel from other sources, and my concern about an unported tank this was largely in error. A Piper Cherokee with an inferior suction feed are different animals. My only concern still remains with some of the EFI systems, but that is diminished too, so I'd rather keep quiet and read. My prior concerns stirred things up I know.

What I put out in this forum was not science and cant be easily be retracted.

Again I echo what others have said that Nev's testing is science, and I like data and science
 
My test, reported earlier, where I intentionally unported a tank was with an EFII system and 3/8 lines that returns excess fuel. I concluded that it was not a problem. With one side unported and fuel selector on both there was no loss of fuel pressure.
 
Bcone1381 said:
I have come around to seeing that (on our aircraft due to gravity feed) an unported feed line will not empty there is gravity feed of fuel from other sources, and my concern about an unported tank this was largely in error. A Piper Cherokee with an inferior suction feed are different animals. My only concern still remains with some of the EFI systems, but that is diminished too, so I'd rather keep quiet and read. My prior concerns stirred things up I know.

What I put out in this forum was not science and cant be easily be retracted.

Again I echo what others have said that Nev's testing is science, and I like data and science
Click to expand...

Brooks, I also shared the same concerns as you before. One of the things I really enjoy about this forum is the discussion of sometimes opposing or contradictory ideas, and I usually learn the most when I'm proven wrong !

My own tests proved me wrong (happily) - for my particular installation. Mark has also mentioned to me that some fuel systems operate at pressures of around 80psi. This is important because mine operates at 25 psi on the engine driven pump, and 32 psi with the electric pump. So what I have learned is applicable to my installation with no return lines and a system pressure of up to 32 psi. For installations with higher pressures and different configurations perhaps further testing would be prudent before operating with lower fuel quantities to see if they can indeed suck air if the demand exceeds supply.
 
Nev, I do think the issue is always the "the demand exceeds supply". Some times just rough air/turbulence can unport a tank's fuel outlets. Not to mention slipping or skidding. Mark
 
My 2 cents worth is that this is a really important issue. Confidence in your power plant is everything in the terrain we operate in. I’ve had real reservations about the added weight and complexity of going from carb to injected and I’ve seen the “Unporting power loss” as a significant negative for the injected system. The positives of economy etc are undeniable but one doesn’t want to take that at a reliability cost. Unless I’m mistaken from what has gone before on this thread and others, the IO 360 200HP should be fine with the standard B model plumbing unported or not so long as there is fuel available to the gascolater. Therefore I can discount the unported power loss as an additional risk. Please tell me if I’ve got that wrong.
 
Bissetg said:
My 2 cents worth is that this is a really important issue. Confidence in your power plant is everything in the terrain we operate in. I’ve had real reservations about the added weight and complexity of going from carb to injected and I’ve seen the “Unporting power loss” as a significant negative for the injected system. The positives of economy etc are undeniable but one doesn’t want to take that at a reliability cost. Unless I’m mistaken from what has gone before on this thread and others, the IO 360 200HP should be fine with the standard B model plumbing unported or not so long as there is fuel available to the gascolater. Therefore I can discount the unported power loss as an additional risk. Please tell me if I’ve got that wrong.
Click to expand...

I've come this same conclusion too Grant, for a Bendix system with no return lines and fuel pressure of up to 32 psi - with the caveat that we keep reserves to a sensible level and fly in balance. This morning I decided to put faith in my engine and fuel system, and went flying over some of the aforementioned inhospitable terrain, and I thoroughly enjoyed the flight !
 
Good stuff Nev, pleased to hear the yaw dampers are performing as advertised! I agree the ball in the middle and 10L a side should make everything continue to work.
 
Great stuff Nev! I went through a very similar testing method with my airplane. Personally I have no qualms with running a tank dry, it is standard procedure in some airplanes to extend range. I still use the technique in my BH.

The unporting deal varies pretty widely depending on the system. There is lots of empirical data about unporting with a low wing airplane but that doesn’t correlate to high wing airplanes.

Via google you can find a Cub in AK that experience a partial power loss due to unporting. That plane had an EFI system with no header tank and fuels lines sized such that a single fuel line did not have the capacity to keep the pump inlet flooded with fuel. A couple other Cub types had the same issue. The commonalities were EFI systems with no header tank.

My airplane doesn’t exhibit any unporting issue unless I force it. Because of how the Continental FI works the engine surges (runs, quits, runs, quits…) rather than just looses power and sputters like and EFI system does.

Because of how the FI systems work I’d SWAG that the Bendix system is the most tolerant of unporting to the point of it being a nonissue.
 
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Lots of fuel system discussion here. It's important to note that there are at least 3 types of fuel delivery systems into the engine (carb, mechanical FI, and electronic FI) and at least 5 different ways of getting the fuel from the tanks to those delivery systems (gravity fed, electric pump, returnless, return excess to tank, header tank, etc).

You can mix and match a bit of that. You can run carbs with low pressure fuel pumps (I know of a few), you can run mechanical FI with no return and pumps, if you are running EFI you almost certainly need a header tank, etc.

Here are two notable threads on fuel system design:

https://bearhawkforums.com/forum/be...uel-flow-discussion-moved-from-float-mounting
https://bearhawkforums.com/forum/sa...b-about-fuel-tanks-on-systems-with-fuel-pumps

Here is my post in the second thread trying to explain all of the various options and why it's important to understand all of the variables before deciding what will work for you:

https://bearhawkforums.com/forum/sa...t-fuel-tanks-on-systems-with-fuel-pumps/page2

Lastly, while many bearhawks have similar fuel systems, due to materials, bends, and until the B model, hard points to mount to, they are all a little different, that means the best way to know your airplane is safe is to do exactly what Nev did, TEST!!!!

Everybody should get some altitude over an airport and run a tank empty. If you want to delay until you have 100 hours on the airplane because you don't feel like you can manage a deadstick landing, that's understandable, but flying for 100's of hours and never validating that the fuel system does what it's designed to do seems even more risky to me.
 
"if you are running EFI you almost certainly need a header tank, etc."

I do not agree. The EFII system in my Patrol has no header tank and the gravity feed to the electric pump provides enough fuel to maintain constant good fuel pressure.
 
Gents, thanks for your insight and explanations of the differences in systems and requirements, much appreciated.
 
Bissetg said:
....... I’ve seen the “Unporting power loss” as a significant negative for the injected system. .
Click to expand...

I re-read the links provided in Post #22 and would like to submit this viewpoint for consideration.

The tone of the issue is

"Some mystery exists regarding fuel delivery in a few airframes in specific configurations. Tank venting and fuel selector configuration leads the speculation and discussion. The mystery has not yet been solved with scientific data."

I'm installing a Bendix FI system with the standard Bob Barrows fuel system and believe there is data that shows the system design to be prudent and well tested. It seems prudent to cross vent the L and R tanks and I will if my design is not complex or onerous. I will test my fuel system during the flight test phase to the same high standards that NEV did.

Resiliency happens when we investigate and expose all possible lurking issues.
 
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Ed.Meyer How did you test?

In order for that system to be safe you must confirm that at minimal fuel the gravity portion of the system can supply the fuel required for the engine at idle as well as the fuel that the electric pumps put back into the tank otherwise they can suck air.

The only way I can think of to test this is to do a fuel flow test using only gravity at the inlet of the pump then another fuel flow test at the inlet of the tank which measures how much fuel is put back into the tank.

Even if that checks out, you have to be sure that at minimum fuel, and at odd attitudes, the fuel put back in the tank can immediately picked up by the ports to the pump. This is why a header tank is typically recommended, because when you only have a few gallons, you can be sure that the fuel cycled will always be available to the pump without chance of sucking air.
 
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Nev,

Great video, good information. In regards to low fuel and the possibility of unporting a tank and your thoughts about always keeping 45 minutes of fuel on board:

It's always better to have more gas on board, and 45 minutes is a bit, however, given that you know how well your airplane will restart, even going to the 'both' setting, you might consider switching to single tank operation if you ever find yourself low on fuel due to a miscalculation or failure. Switch to one tank, run it dry, when your engine surges or low pressure alarm hits, switch to the second tank. That will ensure you have the most amount of fuel over a tank when landing and may be a safer way to operate if you have less than 8 gallons in the entire airplane for the last 20 minutes of flight.
 
I originally thought I was going to return fuel to the mains so I had a couple of bungs welded on the tanks at the high point, inboard end of the tank. There is only 1 degree of dihedral so it is close enough to the top of the tanks for me, at least I hope.
 
That pretty much sums it up Brooks. I’ve been told recently that one of the cases I couldn’t fathom was due to a mostly blocked finger strainer so I’m feeling a whole lot more comfortable. I don’t have the ability to test and present the results with the alacrity of Nev, but I’ll definitely be putting my fuel system through the Unporting tests.
 
A consideration for testing involving engine stopping that I hadn’t thought about. I’m running an 80 inch composite trailblazer prop, when my engine stopped I was on final at a about 55KIAS. The lack of inertia in the prop meant it just stopped rotating immediately. There was no opportunity to pull the pitch to extend glide, or lower the nose to get more airflow and windmill it.

If you are testing for unporting in an approach configuration of say a prolonged slip at low speed just be aware that you may need to either use the starter or lower the nose to rebuild airspeed to relight if it does stop.
 
Wait, you had an engine stoppage? Did you detail this in a thread I missed? Anything special about your engine or fuel system?
 
Hi Schu, The engine stoppage was on final and related to the engine only not the Bearhawk design or airframe/fuel system. The motor was a zero time since OH carbureted Lycoming O360 200HP, it almost did 18 hours. I’ve replaced it with an injected one out of a Cherokee Arrow.

The engine had not been happy from the get go so I was flying conservative approaches. On final when reducing power it just stopped dead, I was taken aback at the lack of windmilling with the composite prop, during training in the past I’ve pulled mixture on forced landing practice on CPL students and watched the metal props slow down and stop but I’ve never seen one just stop dead like this did. I put it down to the lack of inertia in the composite prop, I may be wrong there but that was what it appeared like. My point is if anyone doing these fuel tests that does actually stop the engine, be aware the prop may stop rotating very quickly and require intervention of either starter or increasing the airspeed to restart it so make sure you’ve got plenty of height and options available that’s all.
 
I agree with schu here. If you're going to run down to min fuel, it's best to run a tank dry and have the remaining 30 mins in one tank.
 
The stationary prop, probably, has less drag than your coarse pitch windmilling prop. It's probably close to the same. So I wouldn't worry too much about that part.
 
Yep agreed, it’s more the time it takes for the relight that’s the issue. Lesson, be careful to source your engine from a reputable supplier!
 
Thanks to those of you that so generously share your knowledge on this forum. Im a newbie a few weeks from getting my Bearhawk 5 kit. I cant share Bearhawk info but will share what little I know. Regarding fuel line size: while building a Lancair IV there was a similar debate that 1/2 inch lines were better. The design was 3/8inch lines and 1/4 inch lines returning the fuel/vapor to the tanks (Continental IO-550 in my case). I stuck with the design but decided to flow test the system by disconnecting at the engine fuel pump and running to a bucket using high boost on the two speed Dukes pump. Those little 3/8 inch lines flowed 120GPH. I also used the electric pump on low boost to run the tanks dry one at a time to determine minimum fuel at loss pump suction. I did this in the hangar simulating level flight and 10 degrees pitch. In each case I drained my tanks afterwards and measured my minimum fuel. I encourage everyone to test their fuel system in the hangar before they do their flying testing. Know the point at which your pump will lose suction. But as Nev’s testing shows if you get uncoordinated your minumum fuel can change. That was some great testing BTW. I did test minimum fuel in flight as well to confirm my ground data. But I must admit I learned the uncoordinated imapct unintentionally. On my first day of flight testing I was being the good test pilot and circling the airport. I thought that was great advice. I just wish they had told me to circle the airport coordinated?. Turns out if you do that long enough the engine will die?. This was the one time in my flying career that I had a sort of out of body experience. I am flying an airplane with 50% fatality rate on engine outs and I just did something stupid to kill the engine. Thankfully some possessed person sitting in my seat turned on the boost pump and swapped tanks and relit before losing 300 ft. Literally I was thinking who the hell did that…because I did it so fast without thinking.

I do want to add one thing though that I believe where 3/8 lines pressure drop might hurt. On high fuel flows at high altitudes with hot fuel, you can lose the engine. Most Lancair IV’s run the electric pump on low boost above 10000 ft. When you couple low atmospheric pressure (of high altitude) with additional pressure loss in the 3/8 inch line along with fuel at a higher temp, vapor bubbles can form and cavitate the engine pump. The boost pump is low enough that the additional head pressure prevents the bubble formation there in a sufficient level to impact its flow. The engine relights within 2 seconds of going to low boost ( if you have not already proactively turned it on). I further discovered if you give the fuel 30 minutes at altitude to cool you can turn the boost pump off. This may or may not be applicable to Bearhawks because most of you likely stay low, but I thought I would share it just in case.
 
I have been reading the threads on fuel systems to research what I would like to put in my Companion QB. I am impressed with the knowledge and the openness of the input different people have shared. The sharing of experience and scientific data has been very helpful. The testing that Nev and others have done has given me confidence in what Bob B”s has designed as a foundation for what I would like to do. I am very grateful to those who have shared and to those who have made this forum possible.

This is what I am considering. I want to use an SDS EFII system on a IO 360 of IO 390. The fuel system would not be changed until past the gascolator. The fuel would flow by gravity from the gascolator to the bottom of a 231 cu-in ( about 1 gal.) header tank then out of a bottom port and 3/8 in line to the SDS pumps. The 3/8 in. return line would go into the top of the header tank. The header tank would be vent to both tanks by 1/4 inch lines T ed into the top fuel gauge port on each main fuel tank. Ross Farnham from SDS specified the size tank and that a tall tank was better than a flat tank. He also suggested that a gascolator was not needed, but I like the having the ability to collect and drain water in the lowest location in the fuel system. After 50+ year of sumpings fuel systems on airplanes I feel more comfortable seeing clean fuel in the sampler.

I would appreciate any feed back, caution, or advise. Thanks
 
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I was helping a neighbor some on an RV10 build that used the complete SDS system. Quality and well thought out system. The components are well machined. In this install the recirc fuel was routed back to the fuel tank. But Ross knows his stuff and if he says a one gallon header tank is adequate, I would trust him. Also dont forget to get Vetterman to weld on the oxygen sensor fitting. You will love the system. We tuned the fuel injection in flight so that virtually all cylinders peaked at the same time.

Definitely is dependent on a solid redundant electrical system. I admit I had anxiety when we removed the mechanical fuel pump. But Ross has many reliable systems out there.
 
I used a 1/2 gallon tank under the right front seat. The lowest part of the fuel system is the bottom of this tank, so I have a drain valve on it to drain water. I deleted the gascolator. I returned fuel to this tank, and vented it to one tank only. I was going to do similar to the sight gauges but was worried about losing fuel out the low wing on uneven terrain.

Not flying yet, but doing final assembly.
 
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