Low wing fuel system vrs high wing

[GravityHater]

Anyone have a picture (not a diagram) of a jet pump? I have always wanted to handle or at least see one.

 

[DGdaPilot]

There's no splitting hairs. Pumps move fluid, true enough. But the output side of the pump is under pressure, and fluid is being moved to a location. Pumps usually don't draw from a source, they push to a source.

This is important. It's for this reason that many low pressure pumps and engine driven pumps don't have the ability to suction feed, or can do so for only a short period of time. These pumps are life limited on suction feed. On a suction feed operation, the pump is "pulling." However many of these systems, indeed most all turbine installations, need to have fuel fed to them under pressure. The purpose of the pump is to push fuel somewhere, under pressure, at a higher pressure than it's taken in. Boost pumps are there to boost fuel pressure and maintain a line pressure. Often these pumps are submerged, and are being fed by a resorvoir or tank which surrounds the pump. On more complex aircraft that utilize jet pumps, this is handled a little differently, but many aircraft still use submersible pumps.

These pumps draw off surrounding fluid, or a feed to the fluid, which often arrives under it's own head pressure from tank pressure or the weight of the fluid. From here it pumps, or pushes the fluid under pressure to a destination. That may be another customer pump, or an accessory/engine pump, such as an engine driven low pressure pump.

In many cases, the purpose of this pump isn't just to push fuel to the engine, resorvoir, header tank, or other pump, it's there to push with pressure, or to pressurize the fuel lines to prevent vapor lock and pump cavitation.

The output pressure of the fuel pump, by the nature and general definition of a pump, is higher than the inlet, which may have little or no pressure at all. The pump is there to push fuel to a new destination. Some pumps pull, most push.

The reason for the pump is the determining factor, as well as it's location in the system.

Suction pumps, on the other hand, serve the primary purpose of pulling fluid from one location and then providing motive force to get it some place else. Their primary purpose is generally to draw fluid from a source, as opposed to supplying it under pressure to another source. In a typical oil system, the engine driven oil pump serves as a pressure pump to force oil under pressure to the various bearings, ports, and galleys in the engine. Scavenge pumps retrieve this oil, sucking or pulling it from collection points, and returning it to supply the pressure pump via resorvoirs, oil coolers, and other interim stages or devices.

Hydraulic pumps push fluid under pressure to accumulators, actuators, and hydraulic lines, accessories, etc. The purpose of the pump is not to pull fluid through the system, and system customer functions are not actuated by fluid being pulled through the system. These are actuated by fluid being pushed through the system under pressure, typically either 1,500 lbs or 3,000 lbs.

Weather the system pulls or pushes fluid, and pumps do both, is important to it's proper operation, and the understanding thereof.

I hope you feel better after writing all that "blah, blah, blah" in order to showcase a moot point.

 

[avbug]

It's a critical point, the knowledge and understanding of which may save your life some day. It did mine.

Then again, I can only assume that you're the PFM kind of guy, and that's the extent of your desire to understand the systems of the equipment you fly.

 

[A Squared]

Perhaps the engine driven fuel pump on most low wing single engine airplanes does not have enough suction to pull from both tanks.

Good point. engine driven fuel pump not strong enough to push or pull from both tanks

Whoa, not so fast there. It require less effort to feed from 2 tanks at once compared to one tank, assuming the tubing has the same diameter.

Take one of those little plastic coffee stirrers that's like a small straw. and suck some water through it. Now take two and suck water through both at the same time. Which will be easier and quicker to suck up a mouthful of water? one straw or two?

 

[DGdaPilot]

It's a critical point, the knowledge and understanding of which may save your life some day. It did mine.

Then again, I can only assume that you're the PFM kind of guy, and that's the extent of your desire to understand the systems of the equipment you fly.

I'm not about to get into a pissing match, but good ASSumption. Yeah, after restoring a car, working on tractors all the time growing up, and having an in-depth knowledge of aircraft systems I've been led to the conclusion of PFM. Riiiiight. Go back to turning some more nuts on your Polish Camel.

 

[mtrv]

Whoa, not so fast there. It require less effort to feed from 2 tanks at once compared to one tank, assuming the tubing has the same diameter.

Take one of those little plastic coffee stirrers that's like a small straw. and suck some water through it. Now take two and suck water through both at the same time. Which will be easier and quicker to suck up a mouthful of water? one straw or two?

Now, take two seperate glasses to simulate two tanks, dump the water out of one, and see what happens. No flow at all.

Neither the Cessna high wing, nor the Piper low wing feeds fuel to the pump from two tank lines. Cessna feeds fuel from both tanks to the fuel selector valve, then a single header tank, then the pump,etc. Piper routes each tank supply to the selector valve, then one line to the aux. pump.

 

[mtrv]

I'm not about to get into a pissing match, but good ASSumption. Yeah, after restoring a car, working on tractors all the time growing up, and having an in-depth knowledge of aircraft systems I've been led to the conclusion of PFM. Riiiiight. Go back to turning some more nuts on your Polish Camel.

Fuel pumps for carbed engines as well as fuel injected engines are rated for output in psi. Less for the carburetor, and more psi for fuel injection. All rated on push and not pull.

Why not be an honest CFI, and admit that your explaination tends to suck.

 

[rfeathe1]

low wing fuel systems

The commander 112 is a low wing and has a "both" position on the fuel selector. At least the one that I flew did.

I've heard of Commanders having a "BOTH" position. One owner/pilot I talked to said most experienced AC pilots don't use the BOTH position, especially with a low tank.

Avbug--does the Dromader have a L&R position too?

 

[A Squared]

Now, take two seperate glasses to simulate two tanks, dump the water out of one, and see what happens. No flow at all.

Right. I wasn't commenting on running a tank dry. You apparently missed that.

Neither the Cessna high wing, nor the Piper low wing feeds fuel to the pump from two tank lines.

Didn't say that they did. They all feed the selector valve with 2 lines, though. It will be easier to pump fuel thru say 2-3 ft lines running to the selector valve, and one 4 ft line running from the valve to the pump than one line of the same diameter all the way. It will only be slightly easier, but my point is that it won't be harder, as has been suggested.

 

[DGdaPilot]

Fuel pumps for carbed engines as well as fuel injected engines are rated for output in psi. Less for the carburetor, and more psi for fuel injection. All rated on push and not pull.

Why not be an honest CFI, and admit that your explaination tends to suck.

And that push would not be there if there was not a pull to begin with. Ummm....waitress....check please. Now answer this one: Who came first, the chicken or the egg?

 

[VNugget]

And that pull wouldn't be there if there was not a push to begin with.

Everybody is making this so much more complicated than it needs to be... the bottom line is a pump moves fluid, by pulling it in one side and pushing it out the other. Easy cheesy.

 

[Bernoulli]

So any other comments on why low wings fuel selectors rarely ever have a BOTH selection while gravity fed high wings almost always have a "BOTH"...only two points made so far.
#1. Because if one tank get too low air could be pushed or pulled by the pump(your pick) into the system causing fuel starvation.
#2. a pump may not have enough power to draw from both tanks. (you'd think if this was the case though, a stronger pump would be used???)

 

[A Squared]

#2. a pump may not have enough power to draw from both tanks. (you'd think if this was the case though, a stronger pump would be used???)

No. Again (this is the second time), it takes *LESS* force to draw from two tanks than one, not *MORE*.

 

[Bernoulli]

OK... Then we're down to only one main reason... the emptier tank could have air drawn into the ignition syystem causing fuel starvation when you still actually have fuel. Is that the final answer?

 

[erj-145mech]

OK... Then we're down to only one main reason... the emptier tank could have air drawn into the ignition syystem causing fuel starvation when you still actually have fuel. Is that the final answer?

Ignition system doesn't play into this equasion. Unco-ordinated flight will also unport the tank.