M

mbriese

Member
I am building an RV-7a with a Superior IO-360 8.5:1 with AFP, Dual Lightspeed ignitions, 2 alternators, etc. Since I plan on runinng mogas, I really like Frankh idea of dual Electric fuel pumps in the wing roots with no mechanical pump, and plan on doing this. The one concern I would like to address is the situation where you have one failed pump and no longer have access to the fuel in that tank. (Thinking of Alaska flying where the nearest airport may not be near enough in this situation).

Anyway, I have return lines plumbed into the fuel tanks several bays in and near the rear. What do you think of running a line with a reversible fuel pump between the supplys (or the returns) to transfer the fuel from one tank to the other? Is there even a reversablie pump that won't flow when switched off even available? I would prefer transfering between the suply lines if there aren't any issues to minimze unusable fuel. The pumping from the return should still get most of it I would think. Pro's Cons, Issues?

Thanks,

Matt
 
A couple of ideas

The obvious choice is to put both the fuel pumps next to each other on the floor where the AFP boost pump goes and stick with the Vans boiler selector valve.

Its not quite as good from a vapour lock perspective but the 3/8ths lines are relatively large so the pressure drop on the suction side is minimal. If your primary goal is not to loose access to the fuel then this would be a reasonable compromise.

Its not quite as user friendly as you don't have the elegance of switching tanks by simply switching pumps..You may well get away with only needing a single relief valve as well.

I would still use a filter for each pump however.

I don't know of a reversible pump personally.

The only return you'll need is a 1/4" line to dump pirge fuel which is only required for hot starts.

Of course remember to plumb each pump from a sepearte circuit..no chageover switches or any other single point of failure.

Frank
 
I personally don't like the idea...

...of having all my eggs in one basket. In the event of a serious electrical failure..(burning wires, etc), there's no redundancy and your engine quitting makes a bad situation even worse.

I can't recall ever flying a certified airplane that does not have a mechanical fuel pump, other than high-wing Cessnas, including my turbine Air Tractor. The electrical pumps have always been there for backup and/or priming for startups.

As barren as Alaska is, I'd seriously reconsider.

My .02,
 
Pierre, I've had two mechanical pump failures, have a friend with a rocket that's had three pump failures, and zero electrical failures between the two of us. There is not a single automobile in production today with a mechanical fuel pump, for one big reason: vapor lock. Many garage builts have dual electronic ignitions, so why not fuel pumps..?

In my rocket I have two 40185 cube pumps in series, in each wingroot (total of four pumps that draw 1.5A apiece), and each pair in the wingroots puts out 23 psi. If one fails it still leaves 13 psi, enough to run a FI system. For a total of $200 in pumps I have twice the redunancy of a mechanical/electric pump combo, at 1/8th the cost.
 
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If the fuel stops flowing...

rocketbob said:
Pierre, I've had two mechanical pump failures, have a friend with a rocket that's had three pump failures, and zero electrical failures between the two of us. There is not a single automobile in production today with a mechanical fuel pump, for one big reason: vapor lock. Many garage builts have dual electronic ignitions, so why not fuel pumps..?

In my rocket I have two 40185 cube pumps in series, in each wingroot (total of four pumps that draw 1.5A apiece), and each pair in the wingroots puts out 23 psi. If one fails it still leaves 13 psi, enough to run a FI system. For a total of $200 in pumps I have twice the redunancy of a mechanical/electric pump combo, at 1/8th the cost.
Click to expand...

to your car engine, you just pull off the side of the road and call AAA.

Kent
 
kentb said:
to your car engine, you just pull off the side of the road and call AAA.

Kent
Click to expand...

The truth is there are so many single point failure points in current fuel systems, that there's really no point in arguing the merits of multiple electric fuel pumps being less redundant. Case in point: a friend of mine built an RV-8A and last year and on takeoff, one of the fuel lines from the divider cracked and broke. He barely made it back to the airport, and had fuel literally squirting on a hot cylinder.
 
mbriese said:
I am building an RV-7a with a Superior IO-360 8.5:1 with AFP, Dual Lightspeed ignitions, 2 alternators, etc. Since I plan on runinng mogas, I really like Frankh idea of dual Electric fuel pumps in the wing roots with no mechanical pump, and plan on doing this. The one concern I would like to address is the situation where you have one failed pump and no longer have access to the fuel in that tank. (Thinking of Alaska flying where the nearest airport may not be near enough in this situation).

Anyway, I have return lines plumbed into the fuel tanks several bays in and near the rear. What do you think of running a line with a reversible fuel pump between the supplys (or the returns) to transfer the fuel from one tank to the other? Is there even a reversablie pump that won't flow when switched off even available? I would prefer transfering between the suply lines if there aren't any issues to minimze unusable fuel. The pumping from the return should still get most of it I would think. Pro's Cons, Issues?

Thanks,

Matt
Click to expand...

I'm building an RV9A with an 0-320-H2AD, EFI w/integrated electronic ignition. No mechanical pump. 2 fuel pumps on engine side of the firewall, automatic pump 2 activation upon loss of fuel pressure.Andair duplex valve w/return to tank I'm drawing from. I would most definitely have crossfeed capability if you use wingroot or in tank pumps. Here's the scenario: You're flying along, merrily switching tanks.....but you get distracted, run one tank low before you realize your error. So you switch pumps to the fuller tank--pump fails--now you have no way to get to that fuel if you have no crossfeed. I'd look at some certified designs with crossfeeds and try to get some ideas on how it has been done. May be as simple as a line between the pump inlets with a valve in it to gain access to the fuel in the opposite tank.
 
Believe me that will never happen

You will never get so distracted that you forget to switch a tank before you run it dry. Well lets put it this way ..I would never forget to switch unless I fell asleep with the A/P in cruise...:)

fuel management is a critical (and simple) task..If you forget to switch tanks and your running that low anyway you need to have a serious conversation with yourself about being a pilot!

Fuel pumps on the engine side of the firewall is really not the best place from a Vapour lock perspective.

I have nearly 900 hours on wingrrot pump systems without a single issue, ever.

Frank
 
And

Your also working in the double failure scenario..I,e you fail to switch pumps AND the good pump happens to fail just as you need it.

I would argue the combined risk (basically the riskof the two independant events multiplied together) is probably lower than a single crankshaft failure..I.e Its really just not a risk worth considering..In a single engine airplane at least.

Oh..I just remembered I did have a pump failure when i got the worm sucked up into the filter that cut supply from the pump..I chose to fly home but it would have not been that hard to land and clean out the filter either.

Frank
 
Yes but

kentb said:
to your car engine, you just pull off the side of the road and call AAA.

Kent
Click to expand...

i think this is Missing Rocket Bob's point..I.e if the electric pumps are more relaible than the mechanical pumps anyway..then what reliability have you lost??

I.e you should be having to do less pulling over to the side of the road in the first place.

Frank
 
Proper planning

Thanks all for your ideas. I did not want to re-open the debate about no mechanical pump. I for one am completly comfortable with only redundnant electrical pumps. I believe the benifets when it comes to vapor lock and mogas far out weigh the risk in this senerio. I can see why many would not be comfortable with this setup however, and I would venture so say many probably stick to 100LL as well. I think it is easier to write of vapor lock as something that wont happen to me, then it is to become comfortable with an electrically dependant engine (redundant as it is). If I were not going to be using mogas, then I might stay with the "tried and true" standard setup. But I do not believe it is best way just because everyone has done it that way forever. There is always room for improvement.

Anyway, I want to keep is as simple as possiple, and with proper flight planning and fuel management I think most trips can be done safely without the x-feed capability. (If you are switching tanks every 30 min as I do, you should have plenty in the other tank to get you back where you came from if the failure happens early in the flight. If a failure were to happen toward the end of the flight, you should be able to reach your destination with a little proper previous planning. And under most conditions there are usually alternates in between.)

Unless there is an simple way to add x-feed capability with the pumps in the wing roots (I don't really care for all those fuel line connection in the cockipt), I will go with frank's setup and take a little more care in planning a Max duration flight.

Thank you all,
Matt
 
have you ever run the engine with only 1 pump running?

rocketbob said:
Pierre, I've had two mechanical pump failures, have a friend with a rocket that's had three pump failures, and zero electrical failures between the two of us. There is not a single automobile in production today with a mechanical fuel pump, for one big reason: vapor lock. Many garage builts have dual electronic ignitions, so why not fuel pumps..?

In my rocket I have two 40185 cube pumps in series, in each wingroot (total of four pumps that draw 1.5A apiece), and each pair in the wingroots puts out 23 psi. If one fails it still leaves 13 psi, enough to run a FI system. For a total of $200 in pumps I have twice the redunancy of a mechanical/electric pump combo, at 1/8th the cost.
Click to expand...

Bob,

I thought that 13psi would be a little low for FI (AFP setup) Have you tested it running at this pressure with a single pump? Any noticable symptoms, etc. If the engine does run fine on a single pump, I think this would be the way to go, and you would have half the electric draw as well at 3 amps vs 6-7 for the other.

Do you have pic of your install?

Matt
 
I have a problem with comparing aircraft electrical systems with auto electrical systems. Everyone talks about never having an electric fuel pump fail in a car. How many times have you had an alternator failure in a modern car? How many alternator failures have you read about in experimental aircraft. Not quite apples to apples.
 
Aaaaah, experimental aviation at its best

...reminds me of so many Subie guys who just had to have a "modern" engine.
They now fly Lycosauruses...

To each his own,
 
I may have missed Bob's point, but....

frankh said:
i think this is Missing Rocket Bob's point..I.e if the electric pumps are more relaible than the mechanical pumps anyway..then what reliability have you lost??

I.e you should be having to do less pulling over to the side of the road in the first place.

Frank
Click to expand...

You may have missed mine.

Which is that comparing car and airplane are not the best approach to airplane safety.

When designing a car the engineer for safety, but fuel supply is not considered safety critical. I would venture to guess that cost is more important issue in using electrical fuel pumps.

Now I am not saying that mechanical is better the electrical system, but the sighting that they do it in cars means that we should do it in airplanes is a bad conclusion.

Kent
 
why not?

And actually Mel brings up a good point..Cars electrical systems are probably statistically more reliable than in airplanes..Hmm Why is this?

Seems there are two potential issues

1) Planes that are OLD and have components on them from the 1940's it seems..Not relavent here.

2) Experimental aircraft have unqualified people wiring them up..And I think this is valid point..I don't honestly buy the idea that stupendously reliable components suddenly become unreliable in airplanes. That goes for alternators and fuel pumps for that matter.

i do think that maybe the whole system is potentially less reliable cus the builder made a mistake..gee maybe I made a mistake. This is statistically more likely than say Toyota making a mistake.

Then again the AFP fuel pump is an automotive pump..they certainly don't make their own..They even sell a dual electric pump setup if I remember correctly.

So I really don't see an inherent risk in using electric pumps..Indeed the apparent failure rate see,ms higher for mechanical ones.

One other thought is that a fuel system is exactly that a system...Unless you understand how these systems work.from tank vent to FI servo then once again there is another variable. Hopefully we have engineered most of those issues out on this forum however.

Frank
 
mbriese said:
Bob,

I thought that 13psi would be a little low for FI (AFP setup) Have you tested it running at this pressure with a single pump? Any noticable symptoms, etc. If the engine does run fine on a single pump, I think this would be the way to go, and you would have half the electric draw as well at 3 amps vs 6-7 for the other.

Do you have pic of your install?

Matt
Click to expand...

I've got a pressure carb on the rocket and have a relief valve in each wing root set for 13 psi, so the 23 psi in my rocket is a bit overkill, since the PS-5C only needs 13 psi. I believe (correct me if I am wrong) but the AFP system will run down to 9 psi. The rocket isn't flying yet, so I have no actual data to provide, other than I've tested the setup on the bench with a calibrated digital pressure gauge, and have tested the pressure at 25gph.

With this system you eliminate the fuel selector and can run all the pumps at the same time, feeding from both tanks. Also I have a dual-battery setup. As someone else said in another thread, I'm not going to argue over it with anyone because I don't need to argue over anyone about it. :p If I didn't think this setup was the cat's meow I wouldn't be doing it.

Tuesday night I flew in an MD600 helicopter and its got one FADEC...so what happens if the FADEC goes TU...?
 
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On the Metro I used to fly a 1000 yrs. ago, the cross feed was a simple tube with a valve in the middle. You opened the cross feed valve, induced a small side slip to start fuel flowwing and wait until the tanks balanced. If you put a t on the tank side of the pump, then ran a line with a ball valve for x-feed to a t on the tank side of the other pump, I think you could have a simple x-feed system. Or even have a dedicated line for fuel transfer in the tanks themselves. I wouldn't think vapor lock would be a problem due to cool fuel.

Rick Maury
RV 7SB
 
I've been in an RV, in which we had a near total electrical failure. Turns out it was something rather simple, and I'll get to that.

But I sure liked the feeling that the engine was completely dependent of the electrical supply..................considering we were over some pretty rough mountains, and had at least 50 miles to go before the nearest airport. We weren't worried about looking for the nearest mountain dirt logging road because we were not heading downwards too soon!

This is why I feel much better about redundancy. Mechanical fuel pumps and at least one magneto!

After landing, we pulled the cowl, re-adjusted the alternator belt just in case, and fired up the engine. Voltage went to around 14.4 which was excellent. However, once airborne, the voltage again dropped as before. We flew 120 miles back home using minimal electrical devices.............because we could.

On pulling the alternator off for a checkout, it turned out that the connector from the alternator B-lead just wasn't crimped well enough. It had been arching, when ever a higher load was placed on the electrical buss. A fix for less than a dollar solved the problem. But I sure like the thought of a redundant system; even if it's a secondary alternator as well as a second battery............or the mechanical pump.

L.Adamson --- RV6A (flying)
 
check certified aircraft

I believe one or more of the diamond aircraft put two fuel pumps in one tank, and have a transfer pump that moved fuel from the other tank. It might be useful to get a look at one of these aircraft.
 
frankh said:
fuel management is a critical (and simple) task..If you forget to switch tanks and your running that low anyway you need to have a serious conversation with yourself about being a pilot!
Click to expand...

Airplanes crash on a very regular schedule because of fuel exhaustion, but also quite often because of fuel mismanagement. Mismanagement meaning there was still fuel on board but the pilot did something improperly or didn't do something he could have to restore the flow. For this reason I am very against modifications to a fuel system that will raise the level of operating complexity.
I caution builders to consider making a change very carefully, and factor how much increase in safety they think the are adding, against how much they are loosing by making the system more complex.
 
VAlid Point Scott

I had a friend two weeks ago total his C172 by running out of fuel..So yes it does happen.

For this reason adding layers of complication rubs me the wrong way (see conversation about adding a selector combined with WR fuel pumps)

I would however argue that my basic design of wing root electric fuel pumps is in fact simpler to operate than even the standard electro/mechanical setup.

Lets do two scenarios.

Standard system..Things get stressful, suck tank dry, forget to to switch tanks and now your a glider..It has happened I read of a Rotary powered RV that made a forced landing because a flop tub got hung and the pilot forgot to switc tanks.

WR electric fuel pumps...Suck tank dry engine quits...Set both pumps to ON..It doesn't matter the empty tank pump is trying to pump air as the head from the pump pumping fuel will easily overcome it..Engine will estart as normal.

I would argue that throwing a single switch to "ON" that is right in front of your face is easier than looking at fuel gueages (bouncing around in turbulence on an instrument approach) Thinking about what happened, looking DOWN (away from panel), figuring out valve is turned the wrong way and then switching tanks.

Of course assumig one is running a Dynon EMS or similar you do get the audible warning that something is fouled up together with "FUEL PRESSURE ALARM" on your EFIS.

Frank
 
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