Needle Valve / Spraybar Location

[Rick Campbell]

(Asking this for my friend . . . I’m all e-power now.)

His full-fuse, inverted engine model has the typical spraybar location with the fuel line connection nipple on the outboard side of the model. This puts the needle valve uncomfortably close to the muffler. Changing to a different style muffler is not an option. He’d like to turn the spraybar around so the needle valve is now on the outboard side away from the muffler, and the fuel line connection on the inboard side. Is there any downside to doing this? Any effect on the way the engine will run? 

[Dan McEntee]

(Asking this for my friend . . . I’m all e-power now.)

His full-fuse, inverted engine model has the typical spraybar location with the fuel line connection nipple on the outboard side of the model. This puts the needle valve uncomfortably close to the muffler. Changing to a different style muffler is not an option. He’d like to turn the spraybar around so the needle valve is now on the outboard side away from the muffler, and the fuel line connection on the inboard side. Is there any downside to doing this? Any effect on the way the engine will run?

    I have seen it done and it worked OK, but not the optimum set up. The fuel line length increases, and it has to go against centrifugal force as it passes behind the engine and then has to go forward to the inlet. It can almost double the fuel line length. I would work out something for setting the needle valve with out burning your fingers. I would not make it longer, as the vibration for the engine will make the end dance all over the place. If it's a ST needle, you can make an adjust like some speed guys use made from a length of brass tube, and has a slot on one side of the end running lengthwise for 1/4" or so. the slot picks up the bent leg of the needle and lets you turn it. Put some sort of handle on the other end. this takes some practice and thought on how you do it so you don't stick the adjuster in the spinning prop!! Another option is a rear mounted needle valve. If this is an OS engine, they made a plugged spray bar for this. If you can find the parts from an older FP R/C carb, or even a FP rea needle valve set up, that puts your fingers in a better place. The draw back here is that the newer style needles like that come with the LA series engines have really crappy O-rings and they can leak air. Applying some heavy grease in that area helps with that, but the problem is eliminated with the older, all metal NVAs. Slow combat guys like using these as they give a better setting. The other draw back is learning how much to choke the engine for starting and setting up the tank for a quick, clean shut off when empty. If it's not right it just keeps surging near the end and if flying a contest flight could put you over the time limit. Having the back end of the tank shimmed out helps here also. I set up several airplanes for the all metal rear needle valve when my son Sean was younger and still needle shy. It makes no difference how the engine runs if everything else if right. It's tricky because your mixture is being set at the back of the engine instead of right there at the venturi/crankshaft pot. So he has some options, he just needs to think about how he wants to go about it and what might work best for him.
  Type at you later,
   Dan McEntee

[John Skukalek]

I have a Nobler with a 46 LA. Last summer I moved the fuel supply line side of the NVA to the inboard side to get the needle away from the muffler. I couldn't get a good stunt run. The engine would lean out significantly after launch every time.

[Colin McRae]

I have a 46LA on a Vector (same basic engine arrangement as the Nobler). My NVA is on the outboard side (opposite the muffler). My engine runs great. No issues.

I also use a clunk tank (and run muffler pressure) on the Vector where the fuel connection on the tank is on the centerline of the model. So, my fuel tubing length to the NVA would be the same overall length no matter what side the NVA was on.

[Dennis Toth]

Rick,
I have done this on all my models. The trick to keep the fuel line as short as possible is to build a tank with the pickup line that exits the tank front on the inside. This keeps the total fuel line reasonably close to the other approach and you can use 5/32" tubing in the tank instead of 1/8" to reduce the loss. Other option is use a RC clunk tank it has the pickup exiting in the center of the tank so fuel line length is the same for either setup. I've done it both ways and never had any issues.

Best,    DennisT

[John Skukalek]

Colin it looks like you have muffler pressure. My uniflow line was open to the air; no pressure. I wonder if the muffler pressure makes the difference? Does your engine lean out after take off at all?

Dennis do you use muffler pressure?

Thanks guys

[Dennis Toth]

John,
I have used muffler pressure but prefer open uniflow with the vent off the inboard side of the fuse about 3/16" - 1/4". This gives me the most consistent runs even with 4 strokes on conventional venting.

Best,  DennisT

[Colin McRae]

Colin it looks like you have muffler pressure. My uniflow line was open to the air; no pressure. I wonder if the muffler pressure makes the difference? Does your engine lean out after take off at all?

Dennis do you use muffler pressure?

Thanks guys

I run muffler pressure on all of my OS LA engines (15,25,40, and 46 sizes). Always have. And all with uniflow design tanks. Engines run consistent throughout the entire flight. And the engine rpm does go up a bit right after launch (maybe 100 rpm or so).

Just like the owner's manual says. I find peak rpm on the ground, then open the needle around 30 deg or so just enough to hear the rpm drop a bit. Definitely 2-stroke. Not anywhere close to 4-2-4 operation. The LA's run best this way in my experience.

[Brett Buck]

Colin it looks like you have muffler pressure. My uniflow line was open to the air; no pressure. I wonder if the muffler pressure makes the difference? Does your engine lean out after take off at all?

   I am curious, why would you expect it to "lean out" after takeoff?  Normal stunt engines "richen up" after launch, due to the reduced load as it builds airspeed.

    I note that we are using "lean" and "rich" to mean "sounds the same as if you tweaked the needle on the ground". Typically, the fuel supply pressure goes down substantially once you take off and start flying in a circle, presumably also it actually goes leaner in terms of the actual fuel mixture. The reduced load is a stronger effect, so it runs more towards 4-stroking even though the actual mixture is leaner.

     Brett

[Lauri Malila]

..

[John Skukalek]

Brett, thank you for your comment. Maybe you can help me understand what happened in my case. When I moved the fuel supply line to the inboard side of the engine/nva in order to get the adjustment side of the needle away from the muffler the engine would lean out after take off.
I was thinking that this caused the fuel to have to overcome centrifical force to reach the nva therefore slowing the fuel  flow. Can you shed some light on this please?

Thanks

[John Park]

Rick,
I have done this on all my models. The trick to keep the fuel line as short as possible is to build a tank with the pickup line that exits the tank front on the inside. This keeps the total fuel line reasonably close to the other approach and you can use 5/32" tubing in the tank instead of 1/8" to reduce the loss. Other option is use a RC clunk tank it has the pickup exiting in the center of the tank so fuel line length is the same for either setup. I've done it both ways and never had any issues.

Best,    DennisT

The metal tank solution is exactly what I've done on a few models over the years - easy enough as I always make my own tanks.  Like you, I've experienced no issues with this arrangement.