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RV-4 Fuel Selector Questions

riobison

Well Known Member
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
 
Fuel selector

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.
 
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?
 
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.
 
High wings

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
 
Your Cardinal

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!
 
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:
  1. Start on lowest quantity fuel tank
  2. Prior run-ups switch to other (higher) tank
  3. Use Fuel Pump for takeoff and until at a safe height (I am amazed how many do not use the Fuel Pump!)
  4. 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.
 
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.
 
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Engine out landing

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.
 
I had an engine quit on me this past August. O-320, rv4. Switched tanks, turned on pump. Looked for field.

Did you try carburetter heat? The boost pump should have enough dry lifting capacity to handle any air in the lines and get you running in a few seconds.

Glad you got to walk away from it!
 
I agree with Sid, I had an instructor show me that in a 150, and it is scary how far (long) engine will run on whats in pipes, carb,etc.
 
I agree with Sid, I had an instructor show me that in a 150, and it is scary how far (long) engine will run on whats in pipes, carb,etc.

Some things are better left verbally teaching. The technique of placing the fuel selector to OFF to prove a point is dangerous and has cost lives in some cases where instructors forgot to change the selector to both before takeoff. However, SID is spot on. Pick the tank before you start engines (or after landing the flight before) and don't touch it until you are airborne.
 
Enough fuel remains in the lines to get airborne

Years ago, I had a Titan Tornado II and under the direction of someone whom I placed a lot of trust in his experience because of my ignorance of experimental aircraft at the time, I installed an electric fuel pump he provided and insisted I install. Unbeknownst to me at the time, the fuel pump was from a motor vehicle and was not "free flowing."

Long story short; for my first maiden voyage, I started the Rotax 912, tightened the sea tbelts, (which prevented me from reaching the panel) taxied to the runway, did my run-up, and took off. 200' above the ground the engine stopped. Subsequently, I successfully made an off field landing by threading the plane through a myriad of perk holes in a subdivision under construction.

In hindsight, if I would have turned the electric pump on, the engine wouldn't have starved for fuel for that flight. But that fuel pump could've surely posed a problem in the future.

My ground operations before launching was anywhere between 5-10 minutes so, like stated before, one would be amazed at how long an engine will run with no fuel being supplied from the tank(s).
 
Fuel Selector

My brother and I have an Andair fuel selector valve in our -4. We have a 160 HP, 8.5 compression ratio engine. We use 93 octane or 100LL in the left tank only and 100LL in the right tank only. Anytime we are in the pattern we are on the right tank, so we are using 100LL on take off or landing. We have the same arrangement on our -7A with 180 HP.
 
Interesting reading.

I have an RV 4 with the standard 120 litre wing tanks with an axillary 55 litre tank on the back seat (not happy with this, see my other post inquiring about seat tank)

I usually fly 1.5 hours over open water then another 3.3 (depends on conditions) and end up with required reserve fuel.

I start off on axillary, time it @ 35 liters per hour then switch to left or right main tank, because I never trust fuel gages l again time it and change tanks and look at ETA.

I usually get there with plenty of reserve fuel, like I said l plan my trip when looking have a good tail wind in front of a high pressure system.

But.....
I have often wondered if l ran the axillary tank out and the engine coughed, would l be quick enough to change the selection valve over to mains, could l restart without going swimming?

I've never tried it, anyone got into this situation and exactly what happened?

Thanks in advance.
 
I had a 9 gallon aux tank made that I keep behind the rear seat. Yes you can do that. If you keep an eye on fuel pressure you should see a drop before the engine coughs and can switch over. Even if it coughs, the engine will pick back up within a second or two once resupplied with fuel. I turn the electric pump on before switching tanks in all cases and let it go for 30-60 seconds afterwards.
 
Thanks Slice.

That makes good sense, with the fuel pump, mine is a fuel injection system, so if l hit the primer it should do the same thing, unless the fan stopped and l flooded it with an attempted hot start.

Still would like to hear from someone else who has ran it out to find out how they went.
Cheers.
 
Still would like to hear from someone else who has ran it out to find out how they went.
Cheers.

I have. I usually do when going on a long trip to maximize range. The prop will windmill and engine will surge back to life within a few seconds of switching to a tank with fuel. I recommend caution in doing so though unless you have options to land in case you happen to have a selector or feed issue(blocked vent, etc) after switching. The fuel pressure surging I mentioned above starts to happen about a minute before you run the tank dry unless you're at an unusual pitch angle at the time. So unless I happen to miss it, that's when I try to switch over now.
 
Thanks again Slice, l think I'll try this in the circuit area of my strip and see how it goes.

I had thought of fitting an electric fuel pump on the axillary tank (with in line valve) and just keep topping up the main tank in use.

This would allow the axillary tank to be drained then operate as normal.

I use the extra range to get to (and out) remote areas in central Australia to prospect for gold, so the RV 4 with its low stall and no wheel pants has got all the room for my gear for a few days in the field.
I wonder if anyone over there is doing anything like this, would be good to hear from them.
Thanks again Slice.
 
I had thought of fitting an electric fuel pump on the axillary tank (with in line valve) and just keep topping up the main tank in use.

This would allow the axillary tank to be drained then operate as normal.

It is an option. Van's makes a 4 position fuel selector so I opted to feed direct to the engine thus eliminating the need for an additional pump. It also allows me to start feeding from the aux not long after takeoff and being to shift the CG forward again.

Definitely several ways to go about it. Good luck!
 
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
Tim you are thinking correctly. The one basic rule of thumb is have boost pump on and selector to tank with fuel. Period. Switch as you like otherwise. I am NOT a fan of switching tank back and forth right before take-off. I switch to keep tanks fairly balanced on ground or air... and select the proper tank. As long as there is no big difference in fuel level or imbalance either tank, either tank is proper. I don't adhere to absolute fullest tank.

One way to minimize switching:
1) Select "proper tank", typically fullest.
2) start, taxi and run up, do not switch tank
3) boost pump on, take off
4) Airborne (with sufficient altitude) turn boost pump off.
5) As required or desired to keep balance turn boost on and select opposite tank, boost pump off.

I searched the internet for Piper checklist. Piper being low wing is closest thing to the RV. I saw a wide variation.
One says start on fullest. Then before takeoff "select proper tank". I saw "PROPER tank" not fullest several times.
What does that mean? If the tank difference is small use either tank. This is my technique.

Example L tank 65% and R tank 55% entering pattern on R tank. Switching from R tank you were are on last 30 minutes to L tank make sense for 10%?
Now if you screwed up en-route, R tank was 10% and L tank 50% by all means switch to L tank. I would do that on initial descent not pattern.

Unrelated but the Ted Smith Aerostar (Later Piper Aerostar twin engine, mid wing) I flew had wing and aux tanks. It was complicated. Fuel balance and not running the Aux tank dry before wing tanks was critical. The wings were skinny and you could un-port fuel pickup if too low in double cross feed (bypass Aux tank). If you ran the Aux dry before the wing tank you could also starve the engine. There were a few accidents due to fuel mismanagement. RV' have one engine, one selector, one pump and two tanks. Not that hard. :)
 
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One advantage of having two separate tanks is when you need to sideslip to lose height.
If you are feeding from both tanks you may have no problem, but if your high wing tank is low on fuel you could have starvation.
Even worse is if you have the two separate tanks and run on the low wing tank. I think that put a Bonanza down a couple of weeks ago. He was too high, slipped to get on the glide slope and the engine failed, causing him to undershoot.
I start after I have observed fuel pressure from the pump on one tank, then turn over to the other tank, taxi and do engine run up with pump off, turn pump on and take off. that proves mechanical and auxiliary pump are working on my carbed engine.
 
One advantage of having two separate tanks is when you need to sideslip to lose height.

If you are feeding from both tanks you may have no problem, but if your high wing tank is low on fuel you could have starvation.
Even worse is if you have the two separate tanks and run on the low wing tank. I think that put a Bonanza down a couple of weeks ago. He was too high, slipped to get on the glide slope and the engine failed, causing him to undershoot.

I start after I have observed fuel pressure from the pump on one tank, then turn over to the other tank, taxi and do engine run up with pump off, turn pump on and take off. that proves mechanical and auxiliary pump are working on my carbed engine.
Good point. I wounder what level of fuel (emptiness) and slip would un-port the pickup in an RV. Does anyone one to flight test that?
 
On my -4 I have the Johanson tip tanks, they are controlled by an Andair 5 port valve each with it's own line to the valve. I start off on an either main wing tank for T.O. & climb. I then switch to the other wing tank for about 5 min. to make sure it feeds properly. I like to taxi & TO on the climb tank. I will not switch prior to T.O.
Then I go to an tip tank and drain 50% from it then go to the other tip and run it till empty. Then I go back to the first tip and empty it. then I go back to the takeoff & climb tank and empty it.

With 4 tanks on an long trip, I want to know where all my fuel is. I don't need to be guessing where it is. I then run the last tank to my destination and reserve time. a few gallons left in each of the other 3 tanks on an long trip will not work for me.

I have done this with every A/C i have owned. In my -4 It will hold 50 gallons when topped off. With the O-360 with TBI and good altitude @ LOP it has good range. 95% of my flying is with the tips empty for short flights. With my Dynon EMIS I know what I have at all times. when the engine quits, I switch tanks, boost pump on, and keep on going. If I have an PAX, I advise them what is abut to happen. all of my A/C have had fuel totalizers.

Once I empty an tank in flight, the next time I do about 3 or 4 times of running the engine on about 3 seconds each time to purge the air from each tank I ran to empty. I also next flight T.O. on the tank I landed on as there is no air in the lines. The TBI unlike the carb. has no fuel float bowl. I'm the only person to fly my A.C.
 
This is an excellent thread

I just upgraded my EFIS, and along with that my checklists. (With my new configuration some stuff just doesn't apply anymore.) I spent a lot of time thinking about fuel and what to do on engine start, taxi, run up, and take off. I have a carburated O-360. I came to the conclusion that the ONLY reason to switch tanks would be if the tank I landed on was less than 1/4 tank, and I would probably get gas if that was the case. If the engine was running fine when I landed, why would I want to change anything? So... I leave the tank selector alone until reaching altitude, then switch tanks every 30 minutes to keep them relatively balanced.
 
When the electric pump is running, is the carburetor float and needle valve the only control over the fuel? When I bought N12HR the electric pump appeared to flood the carburetor. For that reason I take off without the electric boost pump running. N12HR has a fuel pressure gauge and I can monitor the pressure. I plan to work with the fuel system more since I have a great deal of trouble starting the engine even using the primer.
 
When the electric pump is running, is the carburetor float and needle valve the only control over the fuel? When I bought N12HR the electric pump appeared to flood the carburetor. For that reason I take off without the electric boost pump running. N12HR has a fuel pressure gauge and I can monitor the pressure. I plan to work with the fuel system more since I have a great deal of trouble starting the engine even using the primer.

Not to get entirely off subject, but this sounds what I went through over the last 5 years. If the carb is flooding, do not pass go, do not collect $200, go straight to carb replacement with a factory Marvel carb. I'd have saved so many headaches and close calls. YMMV, but the factory reman carb has been great for me
 
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I'm a big fan of the KISS philosophy. FWIW, I ALWAYS sump and remain on the same tank for start, taxi and takeoff. As soon as I have reached a safe altitude, prior to turning off the fuel pump, I switch tanks. This confirms that the fuel in the newly selected tank is either useable or not before I am well into a X-Country where diversion options may be limited and while I am still within close proximity of the airport. If the fuel in the newly selected tank is not useable for whatever reason I know two things. 1) The tank I just selected from was working and I have enough in it to get back. From then on, I switch back and forth on a 30 minute interval timer. Keeps things in balance and I know that for every tank switch, I have burned about 3.75 gals from the respective tank.
 
When the electric pump is running, is the carburetor float and needle valve the only control over the fuel? When I bought N12HR the electric pump appeared to flood the carburetor. For that reason I take off without the electric boost pump running. N12HR has a fuel pressure gauge and I can monitor the pressure. I plan to work with the fuel system more since I have a great deal of trouble starting the engine even using the primer.
As Fritz said make sure Carb is operating properly... To answer your question YES the needle on the float bowl meters and shuts fuel off to not over flow the bowl when full. There has been a long history for float issue which cause them to not float and thus not shut off and meter fuel into float bowl. Make sure you check out latest service letter/AD on floats. They went from many iterations of float designs and materials. metal, plastic, composite, hollow, foam filled, back and forth.

If your AUX pump pressure is too high this could be an issue. It only takes a few PSI to feed the Carb. When you check pressure, make sure it is right at Carb INLET after the Aux and Engine driven pumps. Link below is a fun read.

http://www.kellyaerospace.com/articles/Accessory_AMT.pdf
 
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