Using propriatory venturi

[Maurice Bishop]

In an effort to reduce the runaway experienced by the traditional RC venturi, I recently purchased a couple of these:

http://www.justengines.unseen.org/jenven.htm

...and had very mixed results - fuel not drawing consistently resulting in variable revs, cut out following loops.

Anyone have experience of using propriatory venturi's?

TX in advance   

Maurice

[Steve Holt]

I had one on an ASP (Magnum) 36 and on a profile it worked great.  The venturi area was adjustable a bit by screwing the needle valve assembly into the flow stream a bit.  Because it put the needle valve so high relative to the mounts, I replaced it with a conventional venturi for upright and inverted installations.  I'm planning to go back to the JENN venturi when I put the engine back on a profile.
Steve

[Bill Little]

I would like to see the article again that fairly much proved that the height of the venturi off the motor mounts actually had very little, to nothing, to do with tank placement.  Instead it has something to do with crankcase volume,. or some such.  A classic example was a ST G-51 which comes stock with a NVA above the case in the venturi.  When this was switched to a NVA mounted through the case, nothing needed to be changed regarding tank height.

I do not claim to know the *whys and wherefores* of this, but the article had empirical proof that disclaimed the traditional theory of NVA placement being the determining factor in tank height.   It did not say that the tank would need to be moved up or down, just that where the NVA is positioned is NOT the actual determining factor for this adjustment as was traditionally believed.

[RandySmith]

I would like to see the article again that fairly much proved that the height of the venturi off the motor mounts actually had very little, to nothing, to do with tank placement.  Instead it has something to do with crankcase volume,. or some such.  A classic example was a ST G-51 which comes stock with a NVA above the case in the venturi.  When this was switched to a NVA mounted through the case, nothing needed to be changed regarding tank height.

I do not claim to know the *whys and wherefores* of this, but the article had empirical proof that disclaimed the traditional theory of NVA placement being the determining factor in tank height.   It did not say that the tank would need to be moved up or down, just that where the NVA is positioned is NOT the actual determining factor for this adjustment as was traditionally believed.

Hi Bill

I would like to see that too, because it isn't true...  I have several engines with movable venturies..that is  you can, at an instant, raise or lower the venturi, thus raising or lowering the point where the fuel enters.
When you raise the venturie the tank ...has.. to go the same way to run even
when you lower the venturie the tank  needs to be  lowered to keep the run the same..

The big advantage of this is that you never need to lower or raise the tank in the plane, you just adjust the ventuie to get the same run inverted and normal flight. Scott Bair used these in most all of His StuntFires, I also have the same in my Stunfire, and have made many of these for Kaz Minato and a few other flyers. Billy has one in one of the Stunt ships of mine that is on loan to him.

I think this question came up because of the perceived difference in NVA location between the stock location of a ST 46 and ST 60 as opposed the same  motors that had been drilled thru the center for using restrictors. At a glance one would think the drilled case was lower, it really isn't as it appears. The location of the spray bar is not that important, what is important is where the fuel enter to the atmosphere inside of the venturi. 
When you look at the 2 setups the NVAs are about 1/8 to 3/16  apart , however  the True venturi is angled downward and when the fuel gets to the point where it enters the open flow, it is lower than the spray bar.
Others have shown that you can get a small, but noticeable change in tank run by rotating the spray bar around to a more higher location.
 So it seems this isn't at all crystal clear.

Regards
Randy

Regards
Randy

[Bill Little]

Thanks, Randy.

Maybe it was all about the ST's and their NVA location, I don't know.  If I see it/find it, I will save it and post it.

Like I said, all I remember is that in the article a ST G51 was used, IIRC, and the tank position set, then the stock NVA positioning was changed to a through the case NVA like you mention.  No differences were seen according to the article. I sure as heck don't know.

Bill

[James Lee]

I have a dim memory of a note in one of Al Rabe's articles about a ST 46 that he had slotted the case and venturi to move it up and down.   Me memory is that he said made no difference....   
I've had some set ups that seemed to be very critical about tank height and others that didn't much care....   go figure
Later
Jim

[don Burke]

Tank movement inboard and outboard is used to fine tune the leanest practical needle setting for racing.  This is true for both upright, inverted and side mounted engines.  It probably has not as much effect in PA due to the usually richer NVA setting.  Although tank up and down does have an effect for PA manuevers.

[john e. holliday]

I am far from being an expert on any of this, but, was told on profile planes on suction it depended on the where the vent line was located on the tank as well as the pick line for the fuel.  I know Bill Lee has done experiments on setting the spray bar hole for getting the most flow of fuel into the venturi.  It was quite a gadget.  Maybe we can get him to post the thing again.

[Bill Little]

So it seems this isn't at all crystal clear.

Regards
Randy

Clear as mud to me! 

A question of similar note.

On my Vulcan (before I took it to Huntersville) I had muffler pressure hooked up, and flying here, the engine broke prefect all around the maneuvers.  As you know, when i got it to Huntersville, it was breaking hard inverted and not coming back soon enough.  When I got home, I realized I had the muffler line off. (since I never run muffler pressure, I didn't even notice!)  How did muffler pressure make that much difference, or was it something else?

Thanks
Bill

[Dave Denison]

Randy.


   Great explanation of the S.Tiger venturi problem.  A favorite question I ask and you can answer it better than anyone: what is the "correct/perfered" position of the hole in the spray bar? Some say straight down, while others say 9:00 or 3:00.

   I've attached photos of a venturi assy. for my LA 40, for reference.  Look forward to your comments.

       REGARDS.    DAVE

[Jim Thomerson]

My experience has been that the opening of the uniflow in the tank needs to be near the vertical elevation of the hole in the spraybar.  I say near because my techniques are sloppy enough that I cannot say exactly.

[Paul Smith]

Looks like a nice venturi.  In my experience, screwing the "remote" NVA in and out really does have a significant effect on suction.  So that works.

The price is a bit spendy for me 18 pounds = $35 US for a venturi & NVA.

[don Burke]

For the "best suction position" for the NVA.  If I remember right, Bill connected a U-tube water manometer to the NV inlet.  Then a vacuum cleaner on the muffler/exhaust.  With the vacuum running rotated the NVA until the lowest pressure (most suction) showed on the manometer.  Varied with each installation, I think.

A water manometer is just a piece of plastic tubing in a U shape partially filled with water.  One vertical end is open to atmosphere, the other to attached to the NVA.  A scale is mounted behind the open end side so that elevation changes in the water column can be seen when the vacuum is run.  The u-tube needs to be below the NVA so that the water doesn't have a chance to be sucked out.

[Steve Helmick]

Both Ted Fancher and Al Rabe wrote of the same findings, regarding moving the spraybar up/down the venturi. I believe both were in their columns in AAM > MA.

My one interesting experience was changing my Humongus from MACS Muffler to a Big Art Tube muffler, along with more pitch, and finding that I needed to change the tank height.  Same fuel, same .46LA, no other mods. I also had to change tank height on a stunter changed from ST .60 to DS .60bb (a ST clone). I don't know what the sekrit is, but it isn't the spraybar height, IMO.    Steve

[RandySmith]

Randy.


   Great explanation of the S.Tiger venturi problem.  A favorite question I ask and you can answer it better than anyone: what is the "correct/perfered" position of the hole in the spray bar? Some say straight down, while others say 9:00 or 3:00.

   I've attached photos of a venturi assy. for my LA 40, for reference.  Look forward to your comments.

       REGARDS.    DAVE

Hi Dave
I hardly ever use the type of restrictor you are showing, when ever possible I use a true venturie,on engines that do not have enough material behind the venturie  I use a hybrid venturie-fuel post type of arrangement.
The restrictor you are showing causes some tank problems with inverted engine arrangements, But not so much with profiles.
True venturies also don't have the power lose of most restrictors and in my finding even are much easier to choke and prime motors.
I will try to post a few pictures later.
On the question about what position to set the  spray bar in a restrictors, I would have to say  none of the above. I have not tested extensively but from the test I have done I found that either 7 O'Clock or 5 O'Clock position to be best

Regards
Randy

Regards
Randy