Low wing fuel system vrs high wing

[jafo20]

Do pumps suck or blow?

Some suck, some blow, most do both to varying degrees.

Can you have a "Both" setting in a low wing airplane? Yep, but it's not common.

Here's another thing for everyone to consider:

Why do a lot (but not all) of low-wing airplanes only allow the pilot to enter and exit over the right wing?

 

[jafo20]

O wait, a jet pump only sucks.

 

[jafo20]

I guess I don't have much on my mind today, because I was sitting on the toilet thinking about this fuel tank thing. Whether an airplane has high or low wings, it really wouldn't affect the ability of fuel to reach the pick up points of a tank or to drain from both tanks at even rates. High/Low wings will affect the pressure at which the fuel reaches the engine, requiring or not requiring boost pumps. Also, even though people talk about low wings with "both" settings sucking air when they're run below 1/4 tank, the same issue exists on Cessnas below 1/4 tank when you're uncoordinated or at very low pitch attitudes.

If the tanks are vented properly, the strain on a pump to draw fuel won't vary with the number of tanks available. A pump works the same whether it's drawing from a bathtub or a swimming pool as long as the atmospheric pressure above the fluid is uniform. If we consider the pressure built up from fuel above the fuel pick-up points, then a larger tank (or combination of tanks) will have a higher pressure leaving the pick ups. (Simple model: if you make a hole in a 5 gallon bucket and a similar hole in a drinnking cup, fluid will squirt farther out the hole inthe five gallon bucket.) This, however will be independent wing position.

Is it possible for crossfeeding to be more of an issue in low wing airplanes than high-wing planes? I could be wrong, but I don't think so. If one tank is "Lower" than the other, it shouldn't matter whether the plumbing has to make a U (viewed from the tail) or a straight line from one tank to the other.

This is perplexing. If anyone has a good answer, PM me.