Gascolator Placement

For years I have wondered why the recommendation is to place the gascolator at the lowest point in the fuel system. I can not see how it makes any difference what so ever. If you have a scientific explanation, please share...

My Cessna 170 (tailwheel airplane) gascolator on the firewall is higher than the fuel selector valve when the airplane is in the 3-point attitude, and it seems to work fine, though I know of several folks who have installed a drain on the fuel selector too.

pa38112 said:

For years I have wondered why the recommendation is to place the gascolator at the lowest point in the fuel system. I can not see how it makes any difference what so ever. If you have a scientific explanation, please share...

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I agree. My inspector just wanted to see it was installed. I have one in each wing root. I think it should be somewhere where it is easily checked (for water). In-flight attitude is usually different than sitting on the ground attitude so which low point, if any, are we to use?

The tank sump needs to be at the low point (when aircraft is sitting on the ground) of the tank for obvious (to me) reasons. Maybe some people get this mixed up.

My understanding anyway. Use at your own risk.

Bevan

The answer is that water seeks the lowest point and in flight, water can be suspended in tiny blobs within your fuel. The goal is to have it collect in the Gascolator and not in the carb bowl.

N941WR said:

The answer is that water seeks the lowest point and in flight, water can be suspended in tiny blobs within your fuel. The goal is to have it collect in the Gascolator and not in the carb bowl.

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My contention is that the gascolator will do this equally well mounted on a 6' pole above the plane. Maybe in a high-wing gravity fed system with no fuel selector valve there could be an advantage to having it at the low point; but only if the fuel pick-up is lower than the fuel tank sump, which I don't think exists. Otherwise the gascolator should do it's job equally well no mater where it is mounted. There is nowhere in the fuel system to introduce water other than the fuel tanks, and in an RV the fuel must go uphill before it goes to the engine or the gascolator.

pa38112 said:

For years I have wondered why the recommendation is to place the gascolator at the lowest point in the fuel system. I can not see how it makes any difference what so ever. If you have a scientific explanation, please share...

Click to expand...

The gascolator is not a mechanical water/fuel separator (like you have with a cyclone separator). If mounted at the lowest point in the fuel system, water (being heavier than fuel) will work it's way down to the gascolator and collect as the output of the gascolator is above the bowl. The gascolator is simply a collection point for gravity driven water.

A gascolator with fuel running through it with entrained water will remove little to none of the entrained water.

A gascolator in an RV makes little sense other than the filter element meets fuel system requirements. In other words for RVs the gascolator is simply a fuel filter.

Carl

pa38112 said:

My contention is that the gascolator will do this equally well mounted on a 6' pole above the plane. Maybe in a high-wing gravity fed system with no fuel selector valve there could be an advantage to having it at the low point; but only if the fuel pick-up is lower than the fuel tank sump, which I don't think exists. Otherwise the gascolator should do it's job equally well no mater where it is mounted. There is nowhere in the fuel system to introduce water other than the fuel tanks, and in an RV the fuel must go uphill before it goes to the engine or the gascolator.

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Agree 100%. The fuel lines are too small in diameter and at too flat of a pitch for water to separate from the fuel and flow downhill to the gascolator. The gascolator is where there is a space in which the flow slows enough for fuel and water to potentially separate by gravity. Also, the gascolator has a screen which is a nice filter.

pa38112 said:

My contention is that the gascolator will do this equally well mounted on a 6' pole above the plane. Maybe in a high-wing gravity fed system with no fuel selector valve there could be an advantage to having it at the low point; but only if the fuel pick-up is lower than the fuel tank sump, which I don't think exists. Otherwise the gascolator should do it's job equally well no mater where it is mounted. There is nowhere in the fuel system to introduce water other than the fuel tanks, and in an RV the fuel must go uphill before it goes to the engine or the gascolator.

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+2

People often confuse active and passive separation principals. Tank sumps are passive separators, as they only remove the water after gravity and time do the separation work; Clearly the drain valve must be at the low point to dump the water as gravity will take it there. A gacolator is more of an active / hybrid separator. It still uses gravity to separate, but it is doing so on a moving mass. As Kyle mentions, the mini-sump in the gascolator acts as a reservoir. New gas from upstream is mixed in and gas is pulled from the top to go downstream. The reservoir concept allows a chance for the water to settle to the bottom where surface tension will help hold the larger water mass together and gravity will keep it at the bottom. The larger the water droplet entering the gascolator, the more likely it will quickly sink to the bottom. The smaller the droplet, the less likely; however, the small droplet are also less likely to cause problems downstream. The screen also helps. Gas will flow through the screen easier than the water will, again due to surface tension.

In the end, it uses gravity to do it's job, but only at the component level. The gascolator is no more or less efficient at any given height in the system.

In theory, if the fuel in your lines was loaded with water, having the gascolator at a low point would help. However, reality says that the fuel in your lines can't realistically be loaded with water, as the engine wouldn't be running long enough to pull it there with that much water in it, excepting the case where the engine just quit from water contamination.

My thoughts.

If nothing else; having the gascolator at the bottom of the firewall provides a realistic and clean location to access it and drain it. For that reason i'm not sure i'd want it anywhere else.

Dbro172 said:

If nothing else; having the gascolator at the bottom of the firewall provides a realistic and clean location to access it and drain it. For that reason i'm not sure i'd want it anywhere else.

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How and how often do you drain the gascolator? I was told at an Oshkosh workshop it should be done before each flight, like a fuel sump. I'm building a RV-7A and it seems that with the gascolator mounted on the firewall, one would have to either reach up through the lower cowl exhaust opening--very awkward at best--or have a special access door in the cowl for this purpose.

cjohngraham said:

How and how often do you drain the gascolator? I was told at an Oshkosh workshop it should be done before each flight, like a fuel sump. I'm building a RV-7A and it seems that with the gascolator mounted on the firewall, one would have to either reach up through the lower cowl exhaust opening--very awkward at best--or have a special access door in the cowl for this purpose.

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I cut a 1" diameter hole in my cowl for gascolator drain access. The fuel pump must be on to drain any fuel from my gascolator.

If I had a do-over, I would not install a gascolator.

The best place for a gascolator is on the manufacturer's warehouse shelf collecting dust!

I never drain mine, but I do clean it about every 50 to 100 hours, and there is usually a small amount of very fine particulate.