model airplane engine question

[scattermaster]

Hello,
Lurker here.
I want to convert an RC engine with a carburetor to a control line engine with a spray bar and remote needle valve. (it's an OS 25 FX) (I'm setting up for 75 mph combat)
My question is, how can I know what size hole should be in the side of the spray bar?
Other places on line suggest installing one from another mfg and for a much larger engine.
For example use an Enya 40-60 spray bar.
My concern is that the larger engine would use more fuel and need a larger hole. When installed on my engine the fuel might not "spray" (does it need to be atomized or made into a mist per say?)
Are there any modellers here that might know?
thx,
Jim

 

[lohring]

I suggest that you consider machining a bell mouth inlet eo fit where the carb does and holding it in with a Brodak needle valve assembly. It's used on 25 and 40 engines. I think most needle assemblies are adjustable enough to cover several engine sizes.

Lohring Miller

 

[WOB]

Hello,
Lurker here.
I want to convert an RC engine with a carburetor to a control line engine with a spray bar and remote needle valve. (it's an OS 25 FX) (I'm setting up for 75 mph combat)
My question is, how can I know what size hole should be in the side of the spray bar?
Other places on line suggest installing one from another mfg and for a much larger engine.
For example use an Enya 40-60 spray bar.
My concern is that the larger engine would use more fuel and need a larger hole. When installed on my engine the fuel might not "spray" (does it need to be atomized or made into a mist per say?)
Are there any modellers here that might know?
thx,
Jim

Ask your question here: http://adriansmodelaeroengines.com/wordpress/?p=71#comment-761

It might take a few days to get an answer, but there is no other site like this one.
WOB

 

[Neil Lickfold]

So you just need a spray bar with no needle on the front. If you make the cross hole 1mm , ( the one letting the fuel into the venturi) and the hole down the centre of the spray bar 1.5 mm it will work fine. For the remote needle valve , almost any needle valve will work for the remote aspect of it. The remote spraybar will want to have a 1mm outlet hole as a minimum. You can buy a remote needle valve assembly that can be mounted onto the engine mount or onto the centre bearer. I make a small remote block for F3D pylon model, it uses the Nelson Needle valve assembly and has 3 mounting holes for a fire wall, but could be mounted onto the bearer or engine mount.
Neil

 

[scattermaster]

thx Neil.
I have been using a Jett remote needle on my 424's and even my Club 40's but those motors come set up that way.
I want to set up my slow combat models the same way. Just been getting conflicting opinions about the spray bar.
Most of the ones that I've seen recommended aren't even available anymore. I can't believe OS doesn't have this motor in a CL version. It's a good sport motor.
I don't need an F2D motor for this event...

 

[scattermaster]

I took Neil's recommendation on hole sizing and made my own spray bar yesterday. Took about 3 hrs, but I work slow...
Came out pretty nice.
Thx,
Jim

 

[petertha]

Late to your post but you can buy spray bars. Its more a matter of ensuring same casing hole size.
https://www.planethobby.com/products/20102

I converted a Jett 40 Q500 remote needle valve assembly for use in a piped/racing 0.21 Novarossi (came stock with spray bar above). The NVA is capable of shutting fuel completely off so its basically the big faucent that feeds your spray bar orifice. It has fine threads & good needle valve shape so perfectly suitable for complete mixture control.
http://www.dubjett.com/accy2015.html#needle

Side note - the stock venturi for this engine which fit in the inlet casting hole had an annular ring with 4 or 6? radial holes dumping into the venturi throat. Curious thing, the section area of all these holes combined is much more than the single hole in the spray bar. It looks cool & theoretically more atomized but I wonder. Once you dial in correct A/F mixture with needle valve in the end didn't add to performance. Once practical downside to the annular ports over single hole is it doesn't care for high or sudden G as much as the single hole spray bar. Sp on the bench about the same. In the air the single hole spray bar is better, at least for racing apps.

 

[Neil Lickfold]

If you are using a bladder, the fuel can be added directly into the back of the crankcase, and not go through the venturi at all. It is still the same performance. Well done on a nice looking spraybar as well.

 

[scattermaster]

Neil,
"added directly into the back of the crankcase" ??
I've never heard of that. In the old days we used crankcase pressure from a tap in the back plate on our rat race planes.
As far as I know, the F2D guys are using bladders and going in through the venturi area just like I'm planning. I wonder why?

 

[Neil Lickfold]

It maybe that on prime, you can see what goes into the venturi for the prime amount and you can't if it is a direct feed into the case. There was no power gain for direct into the case over adding at the venturi.

 

[scattermaster]

yeah,
It seems like starting would be easier when you can see what's happening at the venturi.
Not as apt to flood.