Fuel flow issue

Jimboscr · #1 10-18-2014, 04:37 AM

Today was the second test flight of the aircraft and a fuel flow/ tank issue has come up. First, I have to tell you that I have an Andair fuel selector that has L and R, Off and also a Both setting. I was using the both setting when it became obvious that the left tank was being used at a faster rate than the right tank. However switching to the right tank resulted in a steady flow from the right tank. So I found myself wondering why, and after a lot of hangar flying with some of the others at the airfield we had a lot of possible options, from faulty fuel selectors to fuel line problems etc.

Finally a call to a very experienced engineer seems to have brought an answer. It seems this has happened to a couple of other types of aircraft in the past ( not RVs) The fuel vents on the RV8 are under the fuselage pointing forward, and this pressurises the tanks. This is o.k. in operations when you are using the tanks alternately. However when using a Both setting it seems that the vent that is the most square to the airflow will pressurise that tank more and as a result it will use fuel from the more pressurised tank ahead of the other tank.

The solution is to square up the fuel vent so they are equal, and slice them at 45deg to stop a pressure "bubble" at the inlet so both tanks pressurise to the same amount, thus allowing the tanks to flow equally. The other solution is to not use the Both setting.

I am wondering if anyone else has had this sort of problem? I have looked in the forums and can't seem to find anything. Some feed back from those experienced engineers would be appreciated.

Cheers

Jim

DanH · #2 10-18-2014, 05:59 AM

There is no reliable way to use a "both" setting with an RV. First tank to reach empty will stop fuel flow from the other. Use "left" and "right" only.

Walt · #3 10-18-2014, 06:30 AM

Quote:

Originally Posted by DanH

There is no reliable way to use a "both" setting with an RV. First tank to reach empty will stop fuel flow from the other. Use "left" and "right" only.

Agree! I wouldn't let an RV thru a condition inspection with a 'Both" fuel selector.

BlackhawkSP · #4 10-18-2014, 06:47 AM

I'd bet that if you check with Van's that that type of valve is a no-no in any RV. The Left, Right, Both valve arrangement is for high wing aircraft. Andair valves are sweet, but you have to get the correct one for the application.

sailvi767 · #5 10-18-2014, 07:36 AM

Quote:

Originally Posted by Jimboscr

Today was the second test flight of the aircraft and a fuel flow/ tank issue has come up. First, I have to tell you that I have an Andair fuel selector that has L and R, Off and also a Both setting. I was using the both setting when it became obvious that the left tank was being used at a faster rate than the right tank. However switching to the right tank resulted in a steady flow from the right tank. So I found myself wondering why, and after a lot of hangar flying with some of the others at the airfield we had a lot of possible options, from faulty fuel selectors to fuel line problems etc.

Finally a call to a very experienced engineer seems to have brought an answer. It seems this has happened to a couple of other types of aircraft in the past ( not RVs) The fuel vents on the RV8 are under the fuselage pointing forward, and this pressurises the tanks. This is o.k. in operations when you are using the tanks alternately. However when using a Both setting it seems that the vent that is the most square to the airflow will pressurise that tank more and as a result it will use fuel from the more pressurised tank ahead of the other tank.

The solution is to square up the fuel vent so they are equal, and slice them at 45deg to stop a pressure "bubble" at the inlet so both tanks pressurise to the same amount, thus allowing the tanks to flow equally. The other solution is to not use the Both setting.

I am wondering if anyone else has had this sort of problem? I have looked in the forums and can't seem to find anything. Some feed back from those experienced engineers would be appreciated.

Cheers

Jim

I would not use both on a low wing aircraft. It's not considered a acceptable practice. I would label the both position inop and be done with the issue.
George