Andair fuel selector question

noelf · #1 01-10-2014, 05:17 AM

A friend just bought a new-to-him RV-7, and has flown the a/c just a few hours. When I looked in the cockpit, I noticed that the Andair fuel selector valve was positioned 1/2 way between the left and right positions. I asked why, and he stated so he could pull fuel from both tanks.

The valve in question has the L-R positions located 90* apart, there is no BOTH position. (My -6A has the L/R 180* apart). Also, he did not know that you could pull up on the "knob" to select the OFF function.

So my question is: do you get the same fuel flow to the engine if the selector is half way between L and R as you would get with the selector in a "proper" L or R position? I know it is not desire able to have a BOTH on our RVs, but I am just curious about the fuel flow through the valve.

cfiidon · #2 01-10-2014, 07:07 AM

Wow... great question. I'll bet the best ones to answer it would be Andair.

Now that you brought it up, it sounds like a great design in that it would avoid the possibility of breaking the shaft or handle part way between left and right tanks, in other words... as soon as one line is partially closed, the other is similarly opened... no "OFF" position between the two.

I'll have to check this out next opportunity.

Don

kevrv7 · #3 01-10-2014, 07:53 AM

Noel..I have the Andair fuel selector and have flown with it for six years. During construction I verified that the Andair design does not shut off one tank before turning the other on. The Vans selector does stop the flow of one tank before the other opens.

HOWEVER... In a low-wing aircraft it is a bad idea to blend two tanks. If one of the tanks is allowed to run empty or the pickup in the tank "un-ports" due to abrupt un-coordinated flight the result is air in the fuel supply. The engine supply will stop because the mechanical or boost pump must always pump fuel up-hill to the engine and air prevents them from pumping.

This is why most low-wing airplanes do not have a Left / Right / and Both position.

Kev
RV-7 N426KF

cfiidon · #4 01-10-2014, 09:09 AM

Lev makes an excellent point. It's good to know how our equipment works, but just because it CAN do something, doesn't mean it should.

I have tip tanks on my Bonanza. I can run off the left or right tanks, or flip one switch and run on both. However, I never do that because if the tanks are full, and you are not flying perfectly level, one tank will "feed" the other and if full, will dump fuel overboard.

I do see however where knowing how it works is useful info. If I had an unbalanced fuel load on the ground I could select a mid position and over time the fuel levels would balance.

Don

longranger · #5 01-10-2014, 09:48 AM

Noel,

Have your friend get two drinking straws, put one in a glass of water, the other outside the glass. Have him try to suck water out of the glass with both straws in his mouth. Then ask him if he still thinks drawing out of BOTH tanks in a low wing airplane is a good idea.

Also, if he didn't now how to operate his fuel valve well enough to select OFF (good idea in the event of a fire), what else doesn't he know about his airplane?

noelf · #6 01-10-2014, 10:55 AM

Yes, I agree with all the above posts. I have been flying with the Andair fuel valve sence day one, but mine is the 180* type. I know how it works. When I was researching Andair many years ago, I talked to THE MAN himself at SnF, where he stated that their design does not inhibit fuel flow when switching from one tank to another...ie, it does not pass through any OFF condition. The only way to turn the fuel flow off is to select OFF. I was just curious what the fuel flow would be if the selector was between the L and R positions. Is the flow somewhat restricted, or is it as it would be if only a L or R was selected.

Yes, I agree with you that the new owner is not very familiar with the design of his a/c and some of the operational ramifications of some of the controls. I know both the seller and buyer and I am somewhat amazed at the lack of pilot checkout /familiarization he received. Or the lack of knowledge the original builder volunteered. The new owner is out of the area right now (snowbird), and when he returns has asked me to sit down with him and go over the entire a/c from an operational viewpoint.

Oh, and the new owner is well versed in the jets he flies in his day job, as well as the piston driven Yak he also flies on weekends.

N941WR · #7 01-10-2014, 06:05 PM

The answer is...

Per Andy a few years back:

The Andair valves does draw from both tanks while in transition from one side to the other.

With the valve on the left tank, it is fully open on the left an 100% closed on the right. At a quarter way to the right tank the left side is restricted to 75% and the right is restricted to 25%. At the midpoint it is 50/50 and 25/75 at 3/4 turn and so forth until it is all the way to the right (0/100).

Andy said it was designed this way so as not to disrupt fuel flow ht high power settings or if the valve jammed you could get home, assuming you have fuel in both tanks.

So yes, you can run on "both" but the law of physics suggest you might want to run on one tank at a time.

Andy Hill · #8 01-11-2014, 01:12 AM

Post an RV-3 (very successful) forced landing (detailed on this forum) I took more interest in this issue. As per the RV-3 technical problem, there was also a PA-28 fatal in the UK recently where the fuel selector was used to change tanks, but seemingly the mechanism took it to "off" and the engine failed shortly after takeoff.

In the UK, we have to measure the fuel flow using the elec pump to ensure it is 150% (I think) of the max engine requirement. You disconnect the pipe at the mech pump entry and pump fuel (with care) into a calibrated container.

I did this recently using an Andair valve, and measured L, R and halfway, and essentially the flow was the same, and well above that required. So although from the post above the flow is less at "both", either the electric pump (AP) and/or the fuel system are more restrictive (RV-8 180HP IO-360 components).

noelf · #9 01-11-2014, 03:06 AM

Thanks for the feedback. Just what I was looking for.

Jesse · #10 01-11-2014, 08:19 AM

I have been working on a -10 that has an Andair selector with a detent and label in the both position. I have told the owner to make a habit of only using it in Both for the top half of the tanks, if that, and only in coordinated cruise flight, but better to leave it only right or left for all of the reasons stated. If he's just buzzing around for an hour with full tanks, then both is acceptable, but still not advisable. There is also the danger of dumping fuel overboard if tanks are full and flight is not fully coordinated.