Fuel Flow Sensor on Rotax 912s

[jiott]

I am installing the Dynon Skyview in my SS7 and was getting ready to install the optional fuel flow sensor in the fuel line on the engine. The only reasonable place to put it is in the fuel line coming out of the fuel pump just before the tee to the two carbs. Then I noticed that the fuel bypass orifice fitting is part of that tee, which sends a small amount of fuel back to the header tank. In other words, my sensor would be measuring total fuel flow INCLUDING the bypass flow. This would make the fuel consumption reading quite inaccurate. Short of completely reworking the very nicely done Rotax fuel line plumbing, I am not sure what to do.

Surely others with fuel flow sensors have run into this-what did you do?

Is it possible to program the Dynon Skyview to compensate (ignore) for the bypassing fuel? If the fuel pressure remains fairly constant from idle to full throttle then the bypassing fuel thru a fixed orifice will also remain constant (my engineering background tells me this). This would allow a constant flow rate to be subtracted from the total flow thru the sensor. Does anyone know if this is possible to program in the Skyview? Do any of you flying the Rotax 912s know if the fuel pressure remains constant thru the RPM range?

Any other ideas would be appreciated. Maybe John McBean will chime in-I know they have installed Skyview in several SLSA's.

Jim

[Russell320]

That return fuel orifice is so small, I doubt that you would notice much difference.The orifice is the size of an idle jet on a motorcycle. This is not a fuel injection system where large amounts of unused fuel is returned to the tank. The Rotax return line is used mostly for hot start issues where as fuel is bled off after shut down to avoid an over pressure on the carb float needle valve. It also allows a little bit of new cooler fuel to be introduced for cooling of pooled stagnate fuel during start. Also, I'm not sure putting the fuel flow sensor where you describe is a good idea. They don't like to vibrate or the heat. Maybe you could find a place behind the firewall. How about in the tunnel?

I just installed the fuel return line and my hot start issues have gone away. I also have a fuel flow sensor and I have not noticed any difference as of yet.

Hope that helps...

[cap01]

Russ , what did you use for an orifice and did you go all the way back to the header ? I haven't had any hot start problems yet but I can see where it's a possibility with the pressures I get after shutdown .

[Russell320]

I live in Phoenix, and it gets warm around here:eek:. During the cooler months it's not an issue. But, when it's hot, I've had starting problems. I rerouted my fuel lines so as to keep them as high off of the engine as possible. I made my fuel distribution manifold from brass fittings. Inside the return line fitting, I drilled, tapped and soldered a pilot jet from a motorcycle carb that was the size of the Rotax return line pilot jet (Rotax Parts Manual page 21-4 item 17). From the manifold I ran my return line all the way back to the right tank (header tank vent), not the header tank behind the seat. My reason was that when I turn the fuel off, I don't want any fuel finding it's way back to the engine not matter how small the orifice is. I also installed a one way check valve in the return line(off ebay cheap). Make sure if you use a motorcycle pilot jet that you use the proper size. The two carb makers measure their jets differently.

All of the Rotax parts in their fuel distribution manifold would cost a zillion dollars. My setup cost considerably less and is readily available locally.

Hope that helps!!!

[kitfox2009]

Hi Guys
With my 912UL in a smooth cowl Vixen I found the fuel pressure (measured between the fuel pump and the Tee) would increase to 11 pounds about 25 minutes after shutdown on a hot day. Always had to wait for cool down before starting.
I installed the return line back to the gascolator and also put a strip of silver insulating material under this line. Line is also firesleeved.
The material is automotive heat shield for wrapping the starter on race cars. I have also used the same material fastened with zap straps under each cab bowel.
This mod may sound a bit "Micky Mouse" but it works!!
Pressure runs at 4.7 lbs constantly and bleeds off instantly on shut down.
Cheers
Don

[cap01]

Thanks for the ideas , I can make some plans for winter projects

[kmach]

Hi,
I have a fuel flow sensor hooked up to the dynon d120/d100 ems/efis.

you can set the " K factor" in the ems setup menus to correspond to the

actual fuel burn , some trial and error but it is accurate once complete.

On my plane the fuel flow sensor is after the facet electric pump, when the

facet pump is on , I get very high inaccurate fuel flow reading. but, it is used

as a backup pump and not on unless a failure of the mechanical pump occurs.

[jdmcbean]

Jim,
We do not install fuel flow in the S-LSA aircraft.. To do so correctly would require two senders one, on the inlet and one on the return. Fuel flow sendors are expensive.

If you ignore the return line then a logical location is in the main line from the header tank to the fuel valve.. This keeps the sender level and is an easy install.

Unfortunately this would only give you flow to the engine not fuel usage of the engine.

[jiott]

Since I already bought it I think I will install it where John suggests and then fiddle with the "K" factor to correct for the bypass flow.

Jim

[Sacha]

Jim,
We do not install fuel flow in the S-LSA aircraft.. To do so correctly would require two senders one, on the inlet and one on the return. Fuel flow sendors are expensive.

If you ignore the return line then a logical location is in the main line from the header tank to the fuel valve.. This keeps the sender level and is an easy install.

Unfortunately this would only give you flow to the engine not fuel usage of the engine.

Hi, this is my first post on this forum. I'm rebuilding a Kitfox IV which I plan to fly in Sicily with a friend of mine. Here is my question:
I'm currently installing a FloScan 201 and thought of installing it as follows: i have an ACS gascolator, which has IN and OUT attachments, as well as a smaller IN that they say can be used for priming). My idea is to connect the OUT to the fuel pump, and the return fuel line from the engine to the priming attachment of the gascolator. The FloScan is mounted upstream of the gascolator, that way it only measures fuel which actually gets consumed by the engine.

What do you think of this idea? Is it potentially problematic in any way? I'd appreciate your thoughts on this.

[mr bill]

Do you mean between the gascolator and the engine?

[Sacha]

Do you mean between the gascolator and the engine?
I mean between the fuel tank and the gascolator

[Sacha]

I mean between the fuel tank and the gascolator
So basically I guess I'm asking: is it ok to have the fuel return line feed into the gascolator instead of e.g. back into the fuel tank?

[kitfox2009]

Hi Sacha
I have my return line plumbed to the third fitting on the gascolator. But I do not have a fuel flow meter. I believe if you install the sender in the line anywhere before the gascolator you will have to "adjust" somehow for the fuel volume that continually circulates.
Cheers
Don

[jrevens]

Hi Sacha
I have my return line plumbed to the third fitting on the gascolator. But I do not have a fuel flow meter. I believe if you install the sender in the line anywhere before the gascolator you will have to "adjust" somehow for the fuel volume that continually circulates.
Cheers
Don

It seems to me that the fuel that "continually circulates" with the system plumbed as Sacha described, will all be downstream of the sender. The sender will only be measuring the total fuel flow consumed by the engine - not that which is going around & around downstream of the sender. It should work fine. If the return line was emptying upstream of the sender it would be a different matter.

[Sacha]

Thanks everybody for your input. The point of this solution for me was to make sure that the fuel sensor is only measuring actual fuel consumed but being a novice at aircraft building, I wanted do double check there were no known problems with using the third fitting on the gascolator in this way.