No "BOTH" positions allowed on RV's

[Jordan1976]

Originally Posted by Canadian_JOY

I'm building a Glasair Sportsman - a high-wing aircraft with fuel in both wings and gravity feed to the engine.

The default design from the factory has the fuel lines from L and R tanks coming to a Tee, then to a simple On/Off selector. Great! Simple, cheap, easy, light... Except that some aircraft would develop significant fuel imbalances because the fuel tank vents could produce different air pressures in the tank, essentially force-feeding fuel from one tank while the other tank remained pretty much full. Not good!

As a result of the fuel imbalances experienced in the field the standard fit now is a 4-position valve (L-Both-R-OFF) which allows us to even out fuel burn if we need to.

Yes, but given the fuel design of the Sportsman, you can drain one tank completely dry and it will still pull from the other one unless your pressure imbalance is about 1.5 PSI or more, which is darn near impossible to achieve in a sub 150 knot airplane

So the left-right selector is for the annoyance of a heavy wing imbalance, and the pilot discomfort over seeing 1 gallon in one tank and 6 in the other, not the chance of an engine actually stopping due to the imbalance.

I have personally been in a Sportsman on "BOTH" with 0 gallons in one tank and between 6 and 2 in the other over half an hour with no issues. It's a functionally safe fuel system with only an ON-OFF selector.

[2johns]

Originally Posted by Bevan

it is my understanding that a L R fuel valve with no "BOTH" position still has one of you place the selector half way between the L and R. I has to be this way so that when switching tanks, the 2nd tank kicks in before the former tank is cut off. If so, be sure to place the fuel selector all the way to one position or the other and be aware of consequences for failing to do so on a low wing aircraft.

My opinions only. YMMV

Bevan

You are exactly right. I have a spare fuel valve and checked this out... Three dedents - Left, right, off. If you stop between left and right, it feeds from both tanks.

[Canadian_JOY]

Jordan - I was merely pointing out that, even in an aircraft which should be reasonably ideal for the use of a "Both" selection, it's still not necessarily ideal, and that the lessons learned through the preponderance of experience in certain aircraft types should be heeded when making your fuel system.

[Blain]

We mostly build the plane WE want. But barring a catastrophe, our aircraft will eventually end up in the hands of someone not intimate with its construction. Shouldn't there be some responsibility for meeting acceptable standards?

I've heard/read various versions of the accident but we know the loss of John Denver was related to non standard placement of the fuel valve. What I haven't read is what that builder went through afterwards.

Just sayin......

[jrs14855]

What did Burt Rutan go thru afterwards??

[rockwoodrv9]

Originally Posted by jrs14855

What did Burt Rutan go thru afterwards??

This was a case of not following the plans. Burt did NOT build John's plane or design the fuel switch location where it was installed on the plane. I am sure Burt had lots of grief over the loss of John, but no more than Van would have if someone did not follow the plans with a modification that later turned fatal.

[Captain Avgas]

Originally Posted by vic syracuse

Please, please, please don't do this. I was performing a prebuy on an RV-8 today and for the second time I found a fuel valve with a BOTH position. Huge NO-NO!

Vic, it may be possible that the "BOTH" position is not plumbed. Did you ask the owner if he actually gets fuel out of the "BOTH" port. It may just be a capped port. In my aircraft I have the same valve but the centre port is capped and labelled "AUX". It is available only for a temporary ferry tank.

Mind you, if the central port is indeed capped it is certainly misleading and dangerous for it to be labelled "BOTH" (for obvious reasons).

[vic syracuse]

Originally Posted by Captain Avgas

Vic, it may be possible that the "BOTH" position is not plumbed. Did you ask the owner if he actually gets fuel out of the "BOTH" port. It may just be a capped port. In my aircraft I have the same valve but the centre port is capped and labelled "AUX". It is available only for a temporary ferry tank.

Mind you, if the central port is indeed capped it is certainly misleading and dangerous for it to be labelled "BOTH" (for obvious reasons).

An "AUX" input port is different than a "BOTH" position on the selector. The BOTH position is an internal configuration that allows the tanks to feed simultaneously.

Vic

[Paragon]

Perhaps someone can clarify for my benefit how the Beech Skipper fuel system differs from that of an RV.

The low wing Skipper training aircraft only had a BOTH or OFF position. It did have left and right fuel gages, plus a low fuel alarm and annunciator light, which I believe was supposed to go off at 3 gallons remaining.

This system completely removed the issue of fuel L-R switching from student pilot training in the aircraft, which could be viewed as either good (student pilot never loses engine on solo cross country due to failure to switch tanks), or bad (student pilot never learns fuel tank management). Never had an issue with left or right heavy wing that I recall.

I'm particularly interested in how it worked, such that the empty tank sucking issue described here never occurred. Perhaps just a low tube cross-connecting both tanks, such that they always had essentially the same level?

I'm a fan of the simplest possible systems in an aircraft, so it could be something useful in an RV it if can be made to work reliably as in the Skipper.

-Paragon
Cincinnati, OH

[Toobuilder]

IIRC, The Skipper has a small header tank in the lower fuselage that acts as an accumulator that damps out most of the problems of unporting a single tank.