Flow Scan location?

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

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.

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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.

As with most items aircraft related...

...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.

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:

Where?

PIN37,

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

Bevan

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.

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.

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.

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)..

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

Radomir any chance of a picture?

Sorry.. don't have any photos.. it was a relatively recent change and I just forgot to take some pics of it... I have vertical induction and the sender is connected with shortest -4 hose that AFP could make.. then from there it goes into the flow divider that's located between cylinders 1 and 3.

PS. You'll need the "non-standard" nipple fittings... The AN816-4-4 (steel).. it's the one with the 1/4NPT threads on one end...

flow scan

mounting position is clearly stated in text in a&p hardbacked book that states that the floscan not be hard mounted nor should be on suction side of engine driven fuel pump thier must be a reason to make this statement in text to mechanics on install procedure the ap i spoke to has built an rv8 and ive seen where he put his it is suspended inline on the exit side of fuel pump and before entry into carb. i asked him why he did this he showed me documentation in book that states just this but i guess as kit builders some of us are testing new limits on things. fact is as rv builders some of us will let the rest know at what point in time if any that its ok to mount elsewhere. sensor should be pressure driven not suction driven quote unquote.

Almarsh, I can't remember that EI installation manual does not suggest mounting it on firewall. I will check it again.

Edit: checked. FP-5/FP-5L installation manual, p. 17, covers the firewall mount.
Manual, at p. 5, in addition to straight fittings, suggests both 45° and 90° fittings.
My only concern is which kind is used with our RV's hoses: should be -6D, is it?

Edit: this was Brantel thread I was talking about:
http://www.vansairforce.com/community/showthread.php?t=44424&highlight=fuel+flow+install

almarsh said:

mounting position is clearly stated in text in a&p hardbacked book that states that the floscan not be hard mounted nor should be on suction side of engine driven fuel pump thier must be a reason to make this statement in text to mechanics on install procedure the ap i spoke to has built an rv8 and ive seen where he put his it is suspended inline on the exit side of fuel pump and before entry into carb. i asked him why he did this he showed me documentation in book that states just this but i guess as kit builders some of us are testing new limits on things. fact is as rv builders some of us will let the rest know at what point in time if any that its ok to mount elsewhere. sensor should be pressure driven not suction driven quote unquote.

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We sometimes test new limits, but in this case, it's been tested for many years. Even a well known DAR on this list, has his mounted in the cockpit side, as I do. His has worked successfully for a long time. In fact, even the instructions for my fuel scanner have the inside mount as an option.

L.Adamson --- RV6A

I have flow scan installed after the boost pump but before the engine driven pump and have been getting accurate and steady readings. Flow scan document had some installation limitation which was mostly in regards to having a certain distance (inches) of straight line before and after but nothing that fuel need to be pushed or pulled.

At least two other friends have them in the same location with no issues.

Is there anybody out there that has mounted the flowscan (on a fuel injected engine) in the hose downstream from the mechanical fuel pump and before the fuel servo (throttle body). If so was there any issues? was accuracy affected by any pulsing that might have come from the pump?

Summary + info we had:

Dynon's advice contradicts FloScan's quite clearly

Some say do not put sensors etc. on the engine (vibration likely to fatigue fittings / sensor to a fairly terminal in this case leak ). If you have the knowledge / technique /risk profile to contradict this, it will give the best readings.

If you put it between the electric pump and mach pump, either in the cockpit or on the firewall it will work fine when the elec pump is off. With the elec pump on, it will likely overread (ours is there and does by 30% say?).

This is only an issue if you use the FF to totalise to a high degree of accuracy. If you just use it to lean, you are unliklely to be running the fuel pump and leaning at the same time. If you roughly totalise, it will be slightly higher for the short period the pump was running.

Andy

I guess I am so interested in accuracy because I had a flowscan and fuel totalizer in my C150 (Texas taildragger) that was so accurate it made the fuel gauges redundant. Mind you I have always thought the fuel gauges in an aircraft were only there to fill up the holes in the dash panel. I could never understand why automotive gauges were so accurate and aircraft gauges were abismal.

I have mine in the tunnel half way between the fuel select valve and electrical fuel pump (carb setup). Yes, when the electric fuel pump is on the FF reading is too high. However, I am one of those who do not turn on electrical fuel pump often. In the past 700+ hrs, my total fuel reading has been very accurate (<<1 gallon difference when top off the tank). I am sure that if I had the electric fuel pump on all the time the reading will be useless. Not suggesting anybody to put the sensor there but just to add a data point.


Ted

However, I am one of those who do not turn on electrical fuel pump often

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But I trust you always do <1000', say

Not enough time to make a difference on total fuel flow.

I have mine setup between the electric pump and the mechanical pump and ran test today. The difference between the boost pump on and off is 12%. Overall mine has been very accurate, within one gallon in each fill up which usually is in the neighborhood of 30 gallons. That is about 96% accuracy and also I am never sure how full my tank is as I don't fill it exactly to the neck. I actually use this is my gauge which I trust it more then my float tub gauge in each tank.

This problem for me has been an absolute pain, I originally had my flow scan mounted in the tunnel, after the boost pump and before the mechanical pump.After reading a previous post I decided to move it to the engine bay after the mech pump. But on my engine with horizontal induction it was not possible to put it in the recommended position between the fuel servo and the flow divider, so I thought I would put it in the second best place as recommended, that is, in the line from the mech pump to the fuel servo. But this also proved to be also extremely difficult. In the end I decided to put it back were it started, in the cabin. Safety was the main consideration here.

The other reason for moving was that it was reported that readings were inaccurate with the boost pump on, but I have to ask why this would be so? A flow meter measures flow and as such the transducer would not know what pressure it was operating at (and within reason wouldn't care). On reflection, I would say that if the readings go up with the boost pump on, it is because more fuel is flowing hence no inaccuracy. Thoughts on this please

PIN 37 said:

This problem for me has been an absolute pain, I originally had my flow scan mounted in the tunnel, after the boost pump and before the mechanical pump.After reading a previous post I decided to move it to the engine bay after the mech pump. But on my engine with horizontal induction it was not possible to put it in the recommended position between the fuel servo and the flow divider, so I thought I would put it in the second best place as recommended, that is, in the line from the mech pump to the fuel servo. But this also proved to be also extremely difficult. In the end I decided to put it back were it started, in the cabin. Safety was the main consideration here.

The other reason for moving was that it was reported that readings were inaccurate with the boost pump on, but I have to ask why this would be so? A flow meter measures flow and as such the transducer would not know what pressure it was operating at (and within reason wouldn't care). On reflection, I would say that if the readings go up with the boost pump on, it is because more fuel is flowing hence no inaccuracy. Thoughts on this please

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I know that I prefer mine in the cabin. My 6A doesn't have the tunnel, and it's mounted against the left wall, after the boost pump. While the reading will increase a bit, while the boost pump is on, overall, it's very accurate within a few tenths of a gallon on every fillup. I've never left the boost pump on to see how it all evens out. Many others have done the same with good success.

L.Adamson

Prior to the FWF cleanup I'm now doing on the -8, the transducer was mounted just ahead of the engine driven fuel pump, zip tied to the engine mount, with about 6 feet of hose between the firewall and pump. Despite the messy installation, it was plenty accurate. It is now mounted in the same relative position (between the boost and engine driven pumps), but on the firewall, inside the fuselage directly in front of the left rudder pedal. It is also connected with stainless hard lines. Much lighter install and keeps the transducer out of the heat. I expect the same high performance, and much better longevity with the new setup.