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rgmwa

Well Known Member
I'd be interested to hear from those who have installed a separate fuel pump switch. What type of switch did you choose and where did you locate it?

The one or two pictures I've seen show it between the choke and the throttle, which seems like a logical place.

As a side note, I wonder if Vans installed the boost pump due to the apparent less than 100% reliability of the Rotax mechanical pump. Or maybe it was simply considered a good thing to do. Either way, it will be interesting to see if Rotax's new pump performs better.
 
rgmwa said:
I'd be interested to hear from those who have installed a separate fuel pump switch. What type of switch did you choose and where did you locate it?

The one or two pictures I've seen show it between the choke and the throttle, which seems like a logical place.

As a side note, I wonder if Vans installed the boost pump due to the apparent less than 100% reliability of the Rotax mechanical pump. Or maybe it was simply considered a good thing to do. Either way, it will be interesting to see if Rotax's new pump performs better.
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In certified aircraft if you have an engine driven fuel pump and are not gravity feed (high wing aircraft) you must also have an electric fuel pump or an approved wobble pump (they quit using them in the 50's). I can not say what is called for in the funny rules on light sport as they like to keep it a big secret (yes they will sell you a copy of the rules for a lot of money).

In larger aircraft they require the boost pump to be stronger than the main engine driven pump so that if the engine driven fuel pump fails you will still have plenty of fuel to run full power. It would seem that the electric pump that is in the RV-12 is not capable of providing sufficient fuel to keep the engine running correctly, not sure why we would have a pump that can not do the job correctly?

Best regards,
Vern
 
Thread drift...as it relates to more than the fuel pump

Vern, you mentioned something I just stumbled across myself: The ASTM LSA standards are not in the public domain but rather considered proprietary and are both 'managed' and also sold like any other retail property. As a newbie to the LSA (and RV) world, this truly baffled me. My understanding is that the standards were developed thru a lengthy industry/govt. process that was routinely reported on in the various aviation-related publications, and the standards did receive public (governmental) review & approval. Can you - and anyone else is welcome as well - provide a little historical clarity on how the final results ended up being an expensive, private commodity that must be purchased?

I've been meaning to ask AOPA and EAA if their reference libraries include the ASTM LSA standards and, if so, whether a member can at least view them in some fashion. One would think they must have the final (current) version, given their role in the process.

Jack
 
Boost Pump Power

Vern, You make a good point about the RV-12 boost pump being inadequate to run the engine on its own. IIRC, the Facet model in our planes is rated at 4-5 lbs. But with the fuel return line, my indicated fuel pressure (engine off) seldom gets over 2.6 PSI. That's not much margin, especially on a day when the boost is feelin' kinda puny.

I'm sure there was a good reason for selecting this model--maybe the fact that an earlier-model 912 engine-driven pump was SB'ed due to over-pressurization of the carbs. The Facets are pretty inexpensive so I'm trying to figure out a way to plumb in a parallel setup for a switch-operated second boost. Pretty tight down in that tunnel, though...

Jim
 
Switch

Jim,
You can run a switch right off of the wires coming from the electric pump itself and then locate the switch itself anywhere in the plane you wish. I put one in because my engine pump does exactly what the older SB ones did...its internal regulator can't understand additional pump pressure. So I just installed the switch for t/o and landings as a temporary fix until the new pump is available. Of course I am planning on the new pump being perfect.

Location: I ran the wires under the carpet and located the switch on the left side floor near the headset plugs. I already have an A/P button in the hole that Larry used.

Pete
 
I am curious as to your group consensus that the elec. pp. won't allow the 12 to run at full power. This raises the question of: at what level of engine perf. will it support? Be nice to know in case of an in flight engine pump failure.
Thanks.
Dick Seiders
 
Switch is your choice!

I don't think the aux fuel pump was ever designed to keep the engine running in-case of engine driven pump failure. Its only Job is to provide positive fuel flow from the Gas tank to the engine driven fuel pump. Since our tank is level with the engine. If it was a high wing gas tank we would probably not have an aux pump. Having an aux pump switch is just builder preference. In our cars we have fuel pumps in the gas tanks that do the same thing and run continuously with out any problems. These are just my opinions on the subject. A switch is just another thing to have to remember to turn on.;)
 
Bypass valve

The fault is not with the electric pump. It can pump 30 gallons per hour, albeit at low pressure. The carburetors need volume, but not much pressure. When the engine pump fails, part of the fuel escapes out of the weep holes, thus lowering fuel pressure. And air can leak in through those same holes. A bypass valve would allow the full output of the electric pump to be delivered to the carburetors without letting fuel leak out of a defective engine pump and without a pressure drop associated with the engine pump. The disadvantage of a bypass valve is that it adds weight and complexity and another failure point.
Is it possible that enough fuel could leak out of a defective engine driven fuel pump to cause a fire?
Joe Gores
 
Hey John, if the elec. pumps exclusive job is to provide pos.flow to eng. pump that might explain why it should run all the time. However, that raises another question. I had the same setup on my RV6A and I only ran the elec. pump on takeoffs and landings for backup and I presume to insure high flow rate. I wonder why the 12 is wired to have the elec. pp. running all the time. Anyone know the reasoning behind that? Save the cost of a switch, but at increased pump wear? Or do the pumps last forever? I know some have said just pull the fuse dummy, but how easy would it be to find it or to pick it up off the floor when they need it in an in flight emergency. Issue raises more questions than it answers.
Just curious.
Dick Seiders
 
dick seiders said:
I wonder why the 12 is wired to have the elec. pp. running all the time. Anyone know the reasoning behind that? Dick Seiders
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Dick - here's a comment from rvbuilder2002 on that same question from a couple of years ago:

Originally Posted by Geico266:
Why is it a good idea to have a fuel boost pump and no way to turn it off? Every RV I have ever been in has a boost pump switch for start up, take off, and flying below 1,000' AGL (pattern). Why not in the RV-12? Why would the -12 be different? Was this an oversite.

Response by rvbuilder2002:
if you look at anything that has been previously published about the primary design criteria for the RV-12, one of the top things was a simple uncomplicated airplane to fly. The RV-12 has about as simple of a fuel system as you can have. Verify there is fuel in the tank, and that the fuel valve is turned on, and you are ready to fly. No tank switching to monitor or aux. pump to remember to turn on and off.
 
do not have to worry about forgetting

I know some have said just pull the fuse dummy, but how easy would it be to find it or to pick it up off the floor when they need it in an in flight emergency.
Click to expand...
The fuse can not be pulled for any length of time because the avionics cooling fans are on the same fuse. Overheating avionics will shorten their life. If one forgets to shut off the master switch while pulling or replacing the fuse, it is possible to accidentally touch the aluminum panel and blow the fuse. Some builders have installed a fuel pump switch. The switch is more convenient than pulling a fuse without the danger of overheating the avionics. However, the switch creates the problem of remembering to turn it on when appropriate. Pilots are forgetful. Just ask an insurance agent how many land each year with the landing gear in the up position.
It will not hurt anything to leave the electric pump running all of the time. Replace it at the same time the engine is overhauled if it gives you peace of mind.
I like how easy it is to fly the RV-12. With my CRS disease, I do not have to worry about forgetting to adjust mixture, or pull on carb heat, or switch fuel tanks, or turn on the fuel pump. If only there were a "Master-Switch-Left-ON" warning.:D
Joe Gores
 
I hope it was not TOO much more pressure, a float type carb will overfill if more than designed pressure exists on the neetle valves.
 
redmark said:
Also, my electric pump failed somewhere between 7-10 hrs. and with concerns over the Rotax pump I replaced it with the next size up. Facet 40106, same GPH & just a little more on the psi rating.
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Not sure I understand you reasoning here. It seems to me that a pump with higher pressure would build up back pressure and have a tendency to fail sooner.
 
If you look back through the posting you will find I had problems with low pressure warnings from the start. Once the first pump failed and I replaced it the low pressure problems stopped and I'm NOT having over pressure problems either. All is fine and it works well for me.

Thanks,
-Ron
 
Pressure relief valve

Someone please correct me if the following is incorrect. I believe the only difference between the Facet 40105 and 40106 is the setting of the internal pressure relief valve. These pumps do not change speed to regulate pressure. The maximum pressure that the pump is capable of producing is limited by its internal pressure relief valve. The relief valve only comes into play if the output is restricted or deadheaded. The actual operating pressure is determined by the fuel system in which the pump is installed. In the RV-12, the fuel system is never deadheaded. Fuel pressure is dropped by the fuel flow used by the engine and by the fuel flow returning to the tank. Any fuel pump, regardless of its volume or pressure rating, will not produce very much pressure if allowed to free flow without any restriction. On the RV-12, the fuel pressure will not change if the fuel pump is replaced with one that has a higher setting of its pressure relief valve. If the fuel pump is replaced with one that pumps a higher volume, then the pressure WILL go up. Higher flow rate causes a higher pressure drop across any restriction, especially the return line orifice.
Opposing viewpoints welcome.
Joe Gores
 
Well, you got it right, but there are lots of variables in your hypothetical setup.
Since the return line is smaller than the delivery line, some pressure will occur solely because of that restruction of course. There is some sort of restrictor in the spider area if I recall correctly as well. A higher pressure rated pump with no increase of capacity flow rate would PROBABLY make no difference at all, since it would never hit the magic number to bypass the pressure relief valve in either circumstance, unless of course there became some restriction of trash or foreign susbstance in the return line, then we would see the pump relief valve come into play..
 
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