Flow Scan location?

[almarsh]

can anyone tell me why i see alot of rvs in paticular rv6a with the flow scan 201-b or any other fuel metering device that is on the suction side of a engine driven fuel pump im told and shown in a&p text books that this is suppose to be inline from outbound side of fuel pump to carb. this is the faa approved method for mechanics im shown and told any comments would be helpful

thanks allen

[647jc]

I can't speak for others but my DYNON installation manual states

Placement of the fuel flow sender relative to other items in the fuel system like fuel pumps is left to the builder. The manufacturer of the fuel flow sender does not make strong recommendations on this point. It is not uncommon, though, to place the sender downstream of any auxiliary electric boost pumps but upstream of the engine driven fuel pump.

My sender is installed on the FW after the gascolator but before the engine driven fuel pump (carbureted engine) and it works very well in that position.

[noelf]

...it is a compromise of multiple recommendations that are at odds with each other, and also the builder's implementation of the final solution.

As an example, the fuel selector is within reach of the pilot. The gascolator is desired to be at the low point of the fuel delivery system, which sort of places it at at the firewall (I know some folks have eliminated the gascolator, and some used two gascolators, one in each wing root).

The flow scan optimal location is after (down stream) of the fuel filter so that the filter can filter out any crud before it has a chance to enter the fuel flow transducer and either jam the "pinwheel" or cause a permanent obstruction to the fuel flow.

It would also be desirable to mount the flow scan just prior the the carb (or equivalent point in a fuel injected application), as this is the ultimate point that is independent of boost pump or engine driven pump flow / pressure fluctuations. But it may not be physically possible to mount the flow transducer in that optimal location. Fuel pressure pulse fluctuations from the boost pump and/or engine driven pump can lead to some inaccuracies in the spin up / spin down of the fuel flow pinwheel as the flow changes. Therefore, manufacturers like to specify straight runs for input and output connections, away from the filters and pumps.

Another "factor" to be concerned with is that the input and output ports on the flow scan transducer are most likely smaller than any of the fuel hoses that you will plumb the system with. In a "high" fuel flow demand scenario, this "could" look like a reduction point for the fuel flow, can cause a slight suction effect, and the end result can be the formation of "fuel vapor bubbles". If this happens, you could end up with "vapor lock". Following the transducer installation instructions helps prevent the formation of the bubbles, and if some small bubbles do form (for whatever reason), they are passed through the transducer and on to the carb (float bowl takes care of these small bubble occurrences).

The computers that totalizes the flow transducer information have a "K" factor that is used to "modify" the value of flow information that you (as the installer) can change (if needed) to accurately correlate the flow scan consumption value with the amount you actually pump from the gas pump.

So, with all this said, it becomes your task to find a solution (or compromise) that delivers the required amount of fuel to the engine (priority 1), and allows you to monitor what the fuel flow is.

[PIN 37]

There is another thread here somewhere that has quite a bit of information and recommendations for the location of fuel flow transducers. I recently read that thread and talked to Airflow Performance, and am now in the process of moving my Floscan 201 from after the boost pump, (but before the mechanical pump) to a position that is between the fuel servo (injected engine) and the flow divider. It is worth reading the other thread, but I just cant remember where it is right now:

[Bevan]

PIN37,

exactly where do you plan on putting/mounting/configuring the floscan in the new position?

Bevan

[Camillo]

There was a TIP thread from Brantel on EI "red cube" location. I copied it. He installed it on a bracket on the firewall. Fuel from engine drive fuel pump and from red cube to carburetor. This needs one more piece of 3/8 fuel line manufactured. Can't find the thread.

[PIN 37]

I plan, at this stage, to put it in the -4 line from the fuel servo (throttle body) to the fuel divider and it will be located under the front left cylinder, then the hose will continue on up between the cylinders. I cant see any other place t to put it at this stage and it is where AP recommend.

[prkaye]

I also used the red cube. Positioned it on the left side of the firewall, towards the very bottom. The red cube is the first thing upstream of the carb (i.e. it is plumbed between the fuel pump and the carb). I had to buy one extra fuel line from Vans (one of their stock sizes worked ok).
The only think I'm not crazy about in my installation is that I had to have angled fittings on both sides... one 90 degree and one 45 degree. The red cube doc says it is OK to do this. Because the way the geometry of my installation worked out the outlet of the red cube faces outboard, and the hose has to bend around to come back to the carb. But I expect it will work fine this way.

[Radomir]

Mine's between the fuel servo and the flow divider (per AFP's suggestion).. and it seems to work just fine. it's really steady reading (used to be very jumpy where I had it before when I had a carb)..

[L.Adamson]

RV6A, left sidewall in front of gear tower , just past the electric aux. pump.
Works great, very accurate, and is only a bit off when the aux. pump is running. Many 6's have installed it this way. To be honest, I don't even want it FWF with fire sleeve and every thing else packed around it. It's also a carb, and doesn't jump around. Just nice and steady.

L.Adamson --- RV6A