Low wing fuel system vrs high wing

[Bernoulli]

Anyone know why the fuel selectors on single engine low wing airplanes never have a "BOTH" selection. It's always left or right which begs for human error. Just curious because high wing gravity fed planes have BOTH... why can't low wings have the selector bring both fuel tank lines into a manifold that is pulled by a machanical fuel pump / elec boost pump? Even new low wing singles have not come out with a both fuel selector. What is it about the low wing that restricts this capability. It can't be just that they are not gravity fed like high wing.... there must be some other design factor that limits this ability. Anyone who thinks they know please chime in...Thanks in advance for all constructive answers.

 

[gkrangers]

Good question.

 

[rfeathe1]

If a low wing aircraft had a "both" position, when a tank runs dry on "both" the pump would suck air from the empty tank.

 

[avbug]

My Dromader is low wing, and draws from both tanks.

 

[mtrv]

My Dromader is low wing, and draws from both tanks.

Whatever a Dromader is, does it have a header tank?

As a general rule with low wings, a both selector is not the norm. Things happen, such as sucking air as mentioned, or cross-feeding in an un-level airplane, that results in an overfull fuel tank that get's fuel vented overboard.

 

[avbug]

Whatever a dromader is? Good gravy.

Not really a header tank, though this varies from airplane to airplane. More of a collector or common point between the two feed-in lines.

A good rule of thumb with the airplane is don't go below 1/4 tanks, if there's any way of really identifying where that might be in the airplane, then drawing air from one tank or the other isn't usually an issue.

The type design did have several engine failures this summer from pilots who drew one tank or the other down below 1/4 on one side, and flamed out, resulting in several forced landings and a really spectacular long distance glide back to the airport, in one case.

In the event everything else goes wrong, there is a wobble pump which is supposed to supply the engine until the pilot's arm gives out. Of course, that doesn't leave him with a hand to fly with...

 

[freightdog227]

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

 

[Bernoulli]

So the final answer for almost all low wing GA aircraft is because if one tank gets too low the pump will suck air? Good answer. Any other reasons. It just seems like the aircraft manufacturer would want to create the most safe aircraft possible... realizing that eventually human error will cause far more engine failures because pilots forget to switch tanks than with an aircraft that feeds off both at the same time. So if some low wing aircraft do have a "both" selector, why would companies like Cirrus who strive for safety (going so far as parachute protection and automatic prop adjustment) not create a fuel system with BOTH. I still think theres more to it. Thanks for the comments so far... keep em coming.

 

[StrykerFL]

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

To answer your question about the Cirrus:

The prop in the Cirrus is not so much automatic as it is mechanical. The prop govener control cable is connected directly to the throttle. There is no way to adjust the prop other than changing the power setting. It is designed more or less to reduce the workload on the pilot and make power adjustments easier. Most cruise settings in the Cirrus have the prop usually between 2400 and 2600 rpms. I'm not too sure about the safety aspect of it.

As for changing the tanks, there is a scheduler that can is set on both of the GARMIN 430's onboard to remind the pilot every 30 minutes to switch tanks. It is relatively idiot proof.

 

[Bernoulli]

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 and pump could suck air from a low tank. Those are all very good answers. I just thought of one too...possibly...Perhaps when you are not gravity feeding your fuel (high wing) but are drawing it, or sucking it from a low wing the quantities from either tank could be drawn or sucked unevenly resulting in the first good answer of drawing air from the empty tank. Any other ideas... keep em coming. Much appreciated.

 

[DGdaPilot]

Good point. engine driven fuel pump not strong enough to push or pull from both tanks and pump could suck air from a low tank.

FYI: fuel pumps never push, they can only pull the fluid.

 

[avbug]

That is incorrect. A fuel pump draws, or "pulls" fluid to the pump. From the pressure side of the pump, it's all "push." Pumps are used to not only draw fluid and "push" it somewhere, but to "push" it under pressure or boost pressure to a useful value. The "pull" or inlet side of the pump is the low pressure side. The pump output or "push" side of the pump is the pressure side and it is very much pushing the fuel.

 

[GravityHater]

Anyone know why the fuel selectors on single engine low wing airplanes never have a "BOTH" selection. It's always left or right which begs for human error

My humble opinion is that you have it backwards.
Would you rather have the engine stumble, sucking fumes when you had the selector in the 'both' position...................or in 'right'/'left' position? The human-error thing can bite a lot harder with that danged 'both' position!
C'mon, we are pilots. We can manage fuel.

 

[DGdaPilot]

That is incorrect. A fuel pump draws, or "pulls" fluid to the pump. From the pressure side of the pump, it's all "push." Pumps are used to not only draw fluid and "push" it somewhere, but to "push" it under pressure or boost pressure to a useful value. The "pull" or inlet side of the pump is the low pressure side. The pump output or "push" side of the pump is the pressure side and it is very much pushing the fuel.

Ummm...close. That push is ONLY there because of the associated pulling action. You cannot have a one-ended pump and expect it to push a fluid. There has to also be a pull to create that. Therefore, pumps pull, not push. We're splitting hairs here, ya know?

 

[avbug]

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.

 

[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.