The O.S. LA 46 comes with a remote NVA.  Replacing that with a more typical NVA mounted in the venturi is popular.  Other than the expected decrease in adjustment lag, will the more typical NVA show any other benefits over the remote?

thanks,

Peter

   The lag is an illusion, confirmation bias. The pressure changes at the venturi occur at the speed of sound in fuel, say, 5000 feet/second., at an inch and a half extra length, that's about 25 microseconds. It's not how long it takes a particular fuel molecule to make its way from the restriction at the needle, to the venturi.

     Brett

   When my son Sean was younger and still prop shy, the last run of the OS FP engines came with rear mounted needle valves. They used the NVA from one of their carburetors for this and they worked very well, better than the current O-ring style that the LA series engines have. A lot of the combat guys like these NVAs to use on the remotes that they use on combat models. It came with a metal bracket the was multi positional and easy to mount it to get the needle out of the way of an inverted landing. In working with this, I found that the only difference was that you need to choke it more on a dead cold start, and then tank location is important for a clean shut off to avoid it burping off and on for countless laps. Getting the back end of the tank cocked out is the main thing. I don't remember plastic or metal making much difference. As far as adjusting the needle, it was more or less the same and I don't recall any difference or difficulty. out of the box, the engines had smaller diameter tubing from the needle to the spray bar and thinking that may make some kind of difference, I always use that, thinking that what goes down that tubing is the actual mixture and is much less than what's coming from the tank and might draw better. The spray bar is a smaller diameter than a typical needle valve assemble spray bar, so that affects the engine run and fuel economy if you are using a random venturi. I don't remember the diameter but is significant because there is no hole and threads in it for a needle on the other side, so check your venturi.
  Type at you later,
   Dan McEntee

A few of my LA's are on profile models w/ metal uniflow tanks where the fuel pickup is on the bottom of the tank. With the remote NVA setup, the fuel tubing has to go up, over, and down to the spray bar. This produces a high point where air bubbles can accumulate. With a conventional front NVA, the fuel tubing has a short and efficient run to the spray bar with no high point.

My experience is that with the rear NVA on a profile model, the needle is sometimes hard to set on the ground due to intermittent air bubbles. But this does not happen with the front NVA.

But once in the air, I have not seen any difference in flight/engine performance with either setup. I have switched to  front NVA on my LA's and never looked back. Engines run great!

    It runs fine either way, it's mostly a matter of preference. One of the defects of the 25LA is that since the mount for the needle is built into the backplate, so on a profile, if crash inverted, it will break the corner of the backplate off. The 20FP RN and the 46LA have it on a separate bracket so you just bend it. When I ran the "OS Remote Needle Kit" on the Fox, I did the same thing.

    Aside from the crashing problem I have seen only one functional issue with the 25LA rear needle- the taper is steeper so one click is a pretty big change. I still prefer it to the infinite adjustment needles, at least for this application.
 
    Brett

   

Thanks for the comments.  Confirmation bias is real, especially with IC control line.

The plane in question, which has a typical uniflow tank, suffers from poor shut off at end of flight, surging for several laps.  The tank centerline is zero-zero, as well, and exhibits 0.3 sec slower inverted lap times, as expected.  Getting a good needle setting was difficult.

Current plan:

1. Change NVA to local.
2. Raise tank 3/16".
3. Cant rear of tank.

Thanks for the comments.  Confirmation bias is real, especially with IC control line.

The plane in question, which has a typical uniflow tank, suffers from poor shut off at end of flight, surging for several laps.  The tank centerline is zero-zero, as well, and exhibits 0.3 sec slower inverted lap times, as expected.  Getting a good needle setting was difficult.

Current plan:

1. Change NVA to local.
2. Raise tank 3/16".
3. Cant rear of tank.

I'd expect 3/16" might be a wee bit too much, BUT I like to hit my adjustments a bit on the HARD side, so I'll know it DID SOMETHING. And then, I'll back it up a little. If you think you'll just "sneak up on it", then you'll burn a lot of fuel finding out that you should have been more aggressive with it. My theory. Often wrong.

As for shimming out the rear of the tank...it won't hurt, BUT you might find that moving the leadouts forward is what's really needed anyway. Just sayin'.  Steve

So, choke the engine, prop pulled through with right hand, left thumb on venturi, hold left thumb on venturi, put glow ignitor on with other left hand...now I see where I went wrong!  Steve

Set up a field shop, drilled the case and tested the "venturi local" Super Tigre NVA.  Number 22 drill bit, btw.

- No changes to tank.
- The three laps of surging at cut-off time went away.  Got a clean, consistent cut-off.
- The three tenths of a second upright vs. inverted lap times difference went away.
- Needle was a challenge to set.  Flights varied from 7 second lap times to 4-1/2 seconds, depending on NV setting.

Overall, felt the rig was tractable and plan no further changes.

thanks for the consultation,

Peter

Set up a field shop, drilled the case and tested the "venturi local" Super Tigre NVA.  Number 22 drill bit, btw.

- No changes to tank.
- The three laps of surging at cut-off time went away.  Got a clean, consistent cut-off.
- The three tenths of a second upright vs. inverted lap times difference went away.
- Needle was a challenge to set.  Flights varied from 7 second lap times to 4-1/2 seconds, depending on NV setting.

Overall, felt the rig was tractable and plan no further changes.

thanks for the consultation,

Peter

       Did you change venturi sizes? Or use the same venturi for both set ups? The spray bar for the rear needle valve is consiuderably smaller that a ST spray bar, so in effect you reduced the choke area, and got better fuel draw over all. The LA.46 comes with a smallish venturi to start with I think, like in the .250" to 2.60" range? Check the pinned thread on LA 46 set ups at the top of the section and see what is posted up there. If it was what came with it for both set ups, the bigger spray bar just gave it better fuel draw.

     Type at you later,
     Dan McEntee

I perfer to meter the fuel as close to the discharge hole as possible.

The thing for me was harder priming with the remote needle.  Dan kinda touches on this.  With the remote, I'd cover the venturi and flip it through until I get fuel on my fingertip.  Then, between removing my finger and attaching the starter battery, it seemed like the prime went away.  I'd have to prime a couple of times to get a reliable flip.  When I replaced the remote with a venturi NVA I had no problem with the primes.

Mark