RV-4 Fuel Selector Questions

riobison · #1 11-08-2012, 07:16 PM

A couple of questions on the fuel selector. Why is there not a "Both" setting on the RV's at least to use until you need to change tanks to balance out the fuel flow?

In addition, on my pre-flight checklist, it has me starting the engine on the R tank, do the run up on the left and then switch back to the R tank. To me this is a lot of changing of the tanks prior to take off and is asking for trouble.

Why not just function the selector, turn it to the fullest tank and leave it there until safely in the air at altitude and then start changing tanks as needed?

Thanks

Tim

fixnflyguy · #2 11-08-2012, 07:47 PM

A "both" position doesnt work on a pressure feed system, as starvation in one tank would cause air to be drawn into the system. Its OK in a gravity system, where there is no need to pump from the tank. The checklist you have is simply ones perception of checking fuel feed from both tanks...its not a nessesary function. I start,taxi and take-off on the same tank (usually the last one a flew on),and seldom ever select the "off" position. Just my preference, as it is the least amount of change from the prior flight where all was working fine.

riobison · #3 11-08-2012, 11:06 PM

Hello Bill

I'm with you on the least amount of change prior to a flight when all was working well. I don't even like going past the "off" detent in flight when changing tanks.

Pressure feed?

Even high wings such as the 172 are fed with a mechanical driven fuel pump. My Cardinal had an electric pump that I had to use for takeoff and landing so they could as well suck air on an empty tank. But I'm thinking that maybe the Cardinal's at least had the wing tanks interconnect so maybe there would be some flow from the fuller tank to the lower tank when on both?

Has anyone interconnected their tanks and gone with a "Both" setting?

Steve Sampson · #4 11-09-2012, 01:08 AM

Tim, by having two separate tanks you have two completely separate sources of fuel. If they were interconnected and you had a leak on one you could loose all your fuel.

The process you describe before takeoff proves both sources are working, no blockages, fuel not water. Surely better to find this out on the ground than in the air.

The engine wont stutter when you switch through the off position since there is fuel in the lines and air gets purged if bubbles introduced.

The procedure you are doing is tried and tested. I would stick with it.

foka4 · #5 11-09-2012, 07:27 AM

Just to put a finer point on things:

The reason high wing airplanes have a "both" setting is that fuel flows downhill. If one tank empties before the other, the remaining fuel from the other tank will gravity-feed and displace the air in the lines. So air is essentially never drawn down the fuel lines to the selector until you're really out of gas.

On a low wing aircraft, the fuel must be pulled uphill from the tanks to the selector and pump. With a "both" setting, as soon as one tank unports, it will start feeding air into the system. The fuel in the other tank will not be able to displace the air, since it nothing is driving the remaining fuel uphill against gravity.

M

riobison · #6 11-09-2012, 10:02 AM

Makes sense

Thanks

fixnflyguy · #7 11-09-2012, 07:51 PM

It had a pump because it was injected, so it needed an engine driven pump for normal operation(higher pressure than a carb on gravity),and the electric was a back-up in the event there was failure during landing. The engine in my RV-4 is an O320H2AD removed from a C-172, and it never had a pump on it because it was carburated, and gravity feed was all it needed. Fuel systems are simple, and we can over complicate them, so I keep mine simple. Just remember if you are switching tanks on run-up/taxi, and you have a carburetor, it will take a couple minutes before you know if you truely have feed,as the bowl holds enough for longer than you think...long taxi times arent always a bad thing!

Andy Hill · #8 12-09-2013, 01:21 AM

Quote:

In addition, on my pre-flight checklist, it has me starting the engine on the R tank, do the run up on the left and then switch back to the R tank. To me this is a lot of changing of the tanks prior to take off and is asking for trouble.

Agree with the "trouble" aspect. There is quite a quantity of fuel in the engine components / pipework. Certainly in an FI engine, it will run for maybe even 30s before failing. As such, as an instructor / if asked for advice, I would suggest that whichever tank you do your "run-ups" on do not change fuel selector now until at a safe height.

There was a fatal accident (PA28) in the UK in the last 2 years, and a forced landing by a well known VAF contributor, both caused by a change in fuel selection. In each case something in the fuel selection system broke (unknown to the pilot) leaving the fuel at off. In the PA28 case this got him to ~200' after takeoff before the engine failed.

My understanding is that the Vans fuel valve has to go via "Off" to change between L & R. A good reason why, if you need one, to fit a better valve

The Andair valves for RVs have no "Both" for reasons as stated above. However, if you position the valve (non detent) between L & R it will act as both. Not suggesting you use it, but it does mean that switching between L&R does not go via Off.

As an aside, I was taught and practice/teach/advise:

Start on lowest quantity fuel tank
Prior run-ups switch to other (higher) tank
Use Fuel Pump for takeoff and until at a safe height (I am amazed how many do not use the Fuel Pump!)
Swap Tanks / turn off Fuel Pump at a height where you have a good time to diagnose and setup for forced landing. NB if either fail, it may be some seconds before the engine fails - and may take some time to sort and get engine going again.

This ensures both tanks have been checked prior takeoff, and you takeoff on the higher quantity tank.

Sid Lambert · #9 12-09-2013, 06:15 PM

Every carb guy on this forum needs to run their engine on the ground with fuel selector 'off' to see just how long it will run. It will scare you... You could taxi out, run up and start to takeoff before it dies in most cases.

I select the tank before I start the engine and don't touch it for at least 30 minutes.

rus23 · #10 03-19-2015, 05:28 PM

I had an engine quit on me this past August. O-320, rv4. Switched tanks, turned on pump. Looked for field. Had plenty of fuel when leaving home. I am wondering if the pump air locked and couldn't draw any fuel from other tank. I was only 1200 agl when engine stopped. Gave it 15 seconds or so to come back and ended up In a soybean field upside down. Major damage to aircraft but I walked away.