fuel system question

[paid4training]

What is motive flow as it relates to a jet fuel system....thanks!

 

[atlcrashpad]

Push throttles forward... Fuel burn goes up... Pull'em back .. fuel burn goes down... dat's all ya needs ta knows.... Bouyyyyy....

 

[Fedora]

What is motive flow as it relates to a jet fuel system

Kind of a vague question there.
Most jet engines use gravity and low pressure tank boost pumps (15-30 psi typical) to get the fuel to the engine where high pressure centrifical engine pumps provide fuel to the engine fuel controler at volumes up to 150 percent of engine requirements at all power settings. Pretty crude but that's basically it. Google up accurate info on any particular engine or just jet engine design basics.
Good Luck

 

[rightseatjocky]

Laughing, man, I love this guy.
In a sense he is totally correct. Ok for real though.
You have fuel pumps that make the fuel move. Once you get the engines running you don't need them any more and motive flow makes all the fuel go either from the center tank to the wing tanks to the collector tanks to the engines. It really is very simple. The fcu monitors all this and as you know when you have any fuel in the center tank, the monitoring of the wing tanks go down to 94%, the fuel will automatically transfer to the wings and replenish them. If there is a 200 lb imbalance, there is an automatic transfer from wing to wing. Once, the imbalance is transfered to 50 lbs over, it will automatically stop.
I hope that helps. If not pm me and I will help you more. I have a check ride on the 6th so I am pretty up on this right now.

 

[flyboyike]

What is motive flow as it relates to a jet fuel system....thanks!

You don't mention aircraft type, but the principle is this: the fuel is sucked out of the tanks by the engine driven pumps, with all other pumps helping the EDPs. The big issue is that should you lose an EDP, you'll lose the respective engine, because none of the other pumps can compensate.

At least, that's the case on the CRJ.

 

[TOOL CRIB]

Motive fuel works with fuel the same way the wings work with air. Create low pressure and suck it in through movement of the fuel or air.

 

[Tarzan]

Put hose in water, pull water through the hose uphill, once it comes out, it will continue to flow on it's own.

 

[Terry Hunter]

Quoting from The Turbine Pilot's Manual (an excellent pick-up by the way):

One other type of pumping action is commonly found in turbine aircraft fuel systems. Motive flow refers to the use of small venturi-type ports, which are used to draw fuel into collection lines. It is desirable to collect fuel from several places in each tank in order to prevent unporting and to back up any clogged pickups. Rather than installing lots of mechanically driven pumps all over the thank, these venturi devices, or jet pumps, draw fuel into the lines by creating low-pressure areas in the passing fuel moving through the lines. Therefore, jet pumps cannot pump fuel by themselves. Rather, they act as localized secondary pumps (sometimes called scavenge pumps), effectively powered by the main pump on the line. (Usually it's the low-pressure engine driven pump or an electric auxiliary pump.)

Thank you for asking the question, I was wondering about this too...

 

[Terry Hunter]

Put hose in water, pull water through the hose uphill, once it comes out, it will continue to flow on it's own.

I think this is called siphoning?

Upon further review, I looked at the CRJ200 fuel schematic, and agree with an earlier post that when the engine driven pump is up and running (i.e. the engine is running), then some of the fuel flow is sent back to the collector tank to help "push" new fuel into the fuel feed line.

The only analogy I can think of would be to take a high pressure garden hose, and spray it along the trough of a roof gutter. The force of the flow not only pushes the fluid toward the destination, but also sucks fluid from the surrounding area (low pressure) along with it.

 

[Cardinal]

Thank you Terry and toolcrib, that is precisely correct. As to all the other vague and downright incorrect answers, yikes. We don't have to build the aircraft but a general understanding of the physical principles at work would be nice.

 

[mamba20]

You know how lift is created right? Pretty much the same principle.

 

[georgeavia]

Here go’s nothing…

You have electric fuel pumps, engine-driven fuel pumps, and then there are pumps that work of the venturi principle. Those pumps that work of the venturi principle require fuel “under pressure” to flow through them to function properly. That fuel flow “under pressure” is called motive flow.

 

[Papa Woody]

Motive flow is a continuously circulating line of fuel from the engine, back to the tanks, then back to the engine. The flow through the line is maintained by the engine-driven pump.

A venturi-type opening in the line as it transits the fuel tank is called a "jet pump", but is a passive device: merely an opening in the line. The venturi shape draws in more fuel from the tank without any mechanical devices, and keeps the line supply volume and pressure adequate for the engine.

Motive flow is used to avoid the need for any mechanical pump in the fuel tank. This reduces the risk of having electricity and fuel mixing in the wrong place.

 

[dbchandler1]

its like a carbuerator..............ah forget it

 

[Crizz]

I cant believe my eyes reading some of the answers you guys posted... were you all serious?

Papa Woody's explanation is spot on.

 

[Say Again Over]

Are those jet pumps AC or DC?

 

[SpauldingSmails]

What is motive flow as it relates to a jet fuel system....thanks!

I'm surprised they didn't teach you that in the training you paid for.

 

[Cardinal]

Are those jet pumps AC or DC?

LOL. I think they run off the 1.21 jigawatt essential buss. But only when the TRU capacitor is fluxing.

 

[Jack Mehoff]

Motive flow? its fuel moving from the wing tanks to collector tanks to the engines in a continous flow.... But thats not important right now. Striker you've got to land this plane!

 

[Andy Neill]

They everyday experience example I would give when discussing motive flow was the way liquid fertilizer or nutrients or herbicides would be applied to a lawn. You connect the hose to the top of a jar holding the fluid to be applied. As the water goes through a venturi in the top of the jar, the fluid is sucked out to go out with the water onto the lawn.

Same with motive flow with the jar being fuel tanks and the water hose being the line from the engine driven fuel pump.