Tip: Electric fuel pump setup

[frankh]

As mentioned in the system description at the last annual, I did change my pump inlet filters to the sintered bronze style supplied by Summit Racing. I did this in case any extra water carryover with the ETOH caused the paper elements to swell.

It is generally not considered a good idea to use paper fuel filters for this very reason, but as I had two independant filters I did not worry about it until forced to run ETOH blended mogas.

System still running flawlessly.

Frank
IO360 now featuring 10% ETOH

[rocketbob]

Frank, with the relief line plumbed in after the pump, there's enough backpressure there to get 25psi at the relief valve? I would have thought there would have to be some restriction in the relief line back to the tank. Evidently those Napa pumps put out quite a bit of volume.

The only thing I would be concerned about is what happens if you're empty on one tank, and you're running on the other and the pump quits. Most twins with complex fuel systems have cross-feed pumps, but I'm all for being simple, so I wouldn't want that.

I like the idea of eliminating the selector and no mechanical pump, but going this route would require some additional electrical system redundancy and complexity which I'm not crazy about either.

[airguy]

Originally Posted by rocketbob

The only thing I would be concerned about is what happens if you're empty on one tank, and you're running on the other and the pump quits.

The same thing would happen here as would happen if you had a standard fuel system, were empty on one tank, running on the other, and the MECHANICAL fuel pump quits. No advantage there...

I'm all for playing "what if" to find gremlins before they bite you - but reality needs to check into the game occasionally. If you're still in the air with one tank completely empty, it better be because you're already on final and screaming at yourself for being stupid and allowing your fuel state to get this low. If you switch tanks every 30 minutes and run one empty, you've got (at most) 30 minutes of fuel remaining. If you're not on the ground (or on short approach) at that point, you're screwing up in a serious way. This is a good point to remember that the FAR's specify 30 minutes fuel reserve PLANNED for daytime VFR, taking into account wind and weather for the entire route at the time of departure.

If you've got more than 30 minutes fuel in one tank, and you're empty on the other, you're either on a maintenance, ferry, or test flight - or you're practicing really really bad flying habits, in which case no level of redundancy is adequate.

[frankh]

Thanks for your question Bob,

The pumps on the bench put out about 35GPH at 35psi. This means the pressure drop through the return to the tank is negligible.

I see 35 psi on the left tank and 40 on the right...Of course when they both run together I see 40psi. The difference is in the return valves as i have swapped from one side to the other and the pressure change goes with the valves. No big deal as the AF system is good between 25 and 90 psi (from memory). The pressure is measured just aft of the firewall.

You have hit on the one downside of the system and that is you have to keep some fuel in each tank....So I never suck a tank dry...But even with 3 gallons you still have half an hour of flying on a single tank.

Typically I only get down to 3 gallons in each tank when arriving back home with plenty of airports to refuel at on the way.

The fuel pumps are very reliable too so lets say you foolishly run a tank dry..The chances of the other pump failing during that last 30 minutes of flight on one pump is statitically unlikely..But I wouldn't do it...


As to electrical complexity..Certainly you have to do what your comfortable with. On the RV it was easy because its an IFR bird so building redundancy into the electrical system was a no brainer..I have an SD8 backup alternator and the dual buss architecture from Bob Knuckolls..

For the VFR Zodiac I simply added a second battery that gave half an hour of flight by itself and charging from the main alternator via a diode....It worked well and was bone simple.

[rocketbob]

Originally Posted by airguy

The same thing would happen here as would happen if you had a standard fuel system, were empty on one tank, running on the other, and the MECHANICAL fuel pump quits. No advantage there...

No that's not correct. In the standard system you have a selector, a mechanical pump, and and electric boost pump. If the mechanical pump quits, you just turn on the boost pump which will pump thru the mechanical pump, and with the selector, you can switch to either tank.

In this setup, there is no selector, no mechanical pump, only pumps at the tanks. If the pump quits on one side, effectively the fuel in that tank becomes unusable. Unless you had crossfeed pumps which will pump fuel to the side with a good pump.

[airguy]

Originally Posted by rocketbob

No that's not correct. In the standard system you have a selector, a mechanical pump, and and electric boost pump. If the mechanical pump quits, you just turn on the boost pump which will pump thru the mechanical pump, and with the selector, you can switch to either tank.

Valid point, I concede to you. I also still say you should not find yourself in a situation with one tank dry as a normal course of flight. If you change tanks every 30 minutes, then you should be on the ground or very close to it by the time one tank is running dry. If you don't change tanks every 30 minutes, you are running a fuel imbalance between the tanks which could be very inconvenient in the event of an unexpected crosswind landing - not to mention the possibility of unporting your one and only operable fuel pickup in uncoordinated flight.

[rocketbob]

Originally Posted by frankh

T
The fuel pumps are very reliable too so lets say you foolishly run a tank dry..The chances of the other pump failing during that last 30 minutes of flight on one pump is statitically unlikely..But I wouldn't do it...

I agree the likelihood of this being a problem is low, and I've had a mechanical pump failure and no electric pump failures, which makes me believe extra complexity to cover this situation is probably not warrranted. In my rocket since I now have dual electronic ignitions I am going to go with dual batteries, dual master switches and contactors, but with only one alternator. I've designed a unique OV protection circuit to work with an internally-regulated alternator, that is stone simple, but I need to test it first with a failed alternator turning on my test setup in my lathe. With a crappy old alternator

[rocketbob]

Originally Posted by airguy

Valid point, I concede to you. I also still say you should not find yourself in a situation with one tank dry as a normal course of flight. If you change tanks every 30 minutes, then you should be on the ground or very close to it by the time one tank is running dry. If you don't change tanks every 30 minutes, you are running a fuel imbalance between the tanks which could be very inconvenient in the event of an unexpected crosswind landing.

While I agree with you, for me I've had two occasions where weather and after hours fuel availability have dictated pushing things a bit, of course reluctantly and with great concern. I just always think about that Murphy guy in those situations, which would make a so-so situation much worse.

[frankh]

Originally Posted by rocketbob

. In my rocket since I now have dual electronic ignitions I am going to go with dual batteries, dual master switches and contactors, but with only one alternator. I've designed a unique OV protection circuit to work with an internally-regulated alternator, that is stone simple, but I need to test it first with a failed alternator turning on my test setup in my lathe. With a crappy old alternator

Actually you have all the extra electrical complexity you need to run the wingroot system right there....Just wire one of the pumps to run from your second battery..Easy

[rocketbob]

I'm mulling changing things over. I'm actually using a PS-5C pressure carb on my Rocket, for a number of reasons which I wont go into here, and I can buy low cost Facet pumps that will deliver 11.5 PSI, so I wouldn't have to run expensive pumps or relief valves. Simple and cheap, the more I think about it the better I like it...