Which way should the spraybar hole be positioned in a needle valve?

[frank mccune]

      Hi All:

      This seems like a very simple  answer can be found for this question but I am getting different opinions.

     When a round ball is propelled through the air, the point of lowest pressure is directly behind the ball.  This is where the most turbulance and vacuum is formed.  However, the latest fad in positioning the n.v is to have the orfice at a right angle to the air flow.  This is said to be the point of maximun "fuel draw". Is this correst?  I see the spray bar is round as is a round ball.  Also, the point for most drag for a bullet despite its shape, is directly behind the base of the bullet.

      Perhaps I am confusing two things.  I will have to wait until after hunting season to do a test with a spraybar, vacuum source and a method for measuring the ability to draw a liquid through a fuel line.

     I think that this has been done before and the test showed that the maximum fuel draw was obtained when the hole in the spray bar was placed at a right angle to the airflow. Anybody have an idea as to where this test was done?

                                                                               Tia,

                                                                               Frank

   

[BillLee]

The article on this topic which I published back in the early 80's can be found at http://www.nclra.org/TechTopics/NeedleValvePlacement/NeedleValveAlignment.pdf I believe that Frank Williams repeated the test later and published in Stunt News as well.

Yes, close to 90 degrees. Not quite, though. (The devil is in the details!)

[Serge_Krauss]

Thanks, Bill, for that link - Bernoulli at work. I wonder though about the dispersion of the fuel and its admission to a rotating crank opening. I wonder whether the hole position on the spray bar have any noticable effect on that - probably little if any?

[Igor Burger]

This was already here. There are two distinct effects, Bernoulli and friction. I remember that someone even posted some caculator, I think Howard? Simply minimal pressure in perfect word without viscosity will be as Mr. Bernolli says on place where air flow is highest. Means on place of minimal crossection, means hole should be at 90 deg to air stream. However we have viscosity. It makes pressure on open side of long pipe higher then on its end where wacuum pump soaks, does not matter that the crossection is constant. So yes, also hole down the ventury works well :- ))

[Trostle]

And there is this---

This was in an article in Flying Models years ago by Maynar Hill.

[RknRusty]

I know this little Cox is not the engines you guys have in mind, but I tried different spray hole placements when we had our annual coxengineforum Tach race last Winter... a good way to break the monotony of the cold and gray days. I built an .049 engine with a Tee Dee cylinder and a Killer Bee backplate which has a very wide open venturi. Popular opinion was that 15 deg. offset from straight down the throat always produced the highest RPM and easiest starting. So that's the way I put mine together. I can't find my threads from back then with my RPM numbers, but I tried it 15deg., 90deg., and straight down the hole, and straight in was about 1000 RPMs faster than any other position... as opposed to popular opinion.

I got whipped by a tanked Babe Bee with a Black Widow cylinder by 2,120 RPM.
Rusty

[frank mccune]

     Hi All:

     Thanks for the replies! Keep the ideas coming.

      I am unable get the Hill article to open large enough to enable me to read it.  Is there a way that I may enlarge the article?

      The idea that the maximum fuel draw is not at "rear" of the airflow is a new one on me.  Of course it does not take much to dazzle me! Lol  Perhaps the friction of the air moving over the spraybar at 90 degrees has a bit more effect on the liquid fuel than the vacuum formed at the rear of the spray bar.

                                                                                          Stay well,

                                                                                           Frank
   

     
                                                                               

[RknRusty]

     Hi All:

     Thanks for the replies! Keep the ideas coming.

      I am unable get the Hill article to open large enough to enable me to read it.  Is there a way that I may enlarge the article?

      The idea that the maximum fuel draw is not at "rear" of the airflow is a new one on me.  Of course it does not take much to dazzle me! Lol  Perhaps the friction of the air moving over the spraybar at 90 degrees has a bit more effect on the liquid fuel than the vacuum formed at the rear of the spray bar.

                                                                                          Stay well,

                                                                                           Frank
   

     
                                                                               

I couldn't read it either.

I'm not exactly wording this like a physics professor, but it seems to me that a hole directly(180deg.) behind the spraybar would distort the incoming airflow and cause it to not create the smooth stream that would form a vacuum around an otherwise smooth surface(a surface with no hole). Possibly even having the effect of trying to force the fuel back or hold it in the spraybar. The picture of the hole at 30deg. or so, and even my 15 deg. idea Seems to me like it would work better. But it just didn't with my one experiment. No telling what else might have been affecting it though.

This winter when it's time to build my next tach race engine, I'll fiddle with it again and post back about it. We usually start talking about it in November over there on the forum.

Rusty

[Dennis Toth]

Frank,
A few years back Frank McMillian did an article for Stunt News about the position of the spraybar hole in the venturi. The basic result was that positioning the SB hole just below the 90 deg point (more or less just out of sight) gave 10% - 15% more vacuum (measured with a u tube monometer) then the straight down position. The reasoning being that the air velocity between the venturi side wall and the spraybar is higher then the airflow around the round spraybar. If you were in a free airstream then the opposite would be the case.

Now positioning forward or rearward is another discussion. I have tried it both way and aside from changing the effective height of the tank both worked about the same.

Best,      DennisT

[Chris Wilson]

I can understand that a critical 15º below the horizon may give the best result (as in the most fuel draw in a perfect bench mounted world ) but what bothers me is that Stunt rarely requires the 'best' or the 'most tuned' of anything but rather requires consistency across a far broader range of conditions that present themselves in the field.

[Air Ministry .]

They found it had to aim back , for restarts . in some NZ LA / FP 25 raceing class . so they said .

[Larry Renger]

Reality trumps theory every time! See what works for you. Or run muffler pressure into a uniflo tank and forget about the question.

[Bootlegger]

  Many years ago Marvin Denny told me that he sets his spray bar at 5 o'clock or 7 o clock pointing at the prop, now the difference in the clock  is due to the orientation of the needle valve, in other words is the needle on the inside or the outside of the circle.

  I have been using this same set up on all my engines ever since..  And they all run well.

  Of course your mileage may vary...

[Howard Rush]

Many years ago Marvin Denny told me that he sets his spray bar at 5 o'clock or 7 o clock...

That's fine if you have time, but what if you're at a contest and your spraybar comes loose in the middle of the day?

[Dick Pacini]

     
      I am unable get the Hill article to open large enough to enable me to read it.  Is there a way that I may enlarge the article?

Ctrl +++ as many times as needed.

[Trostle]

     Hi All:
           I am unable get the Hill article to open large enough to enable me to read it.  Is there a way that I may enlarge the article?
                                                                                                Stay well,
                                                                                           Frank     

Sorry for the quality of the copied article from FM magazine.

Two points:

1.  The diagram shows the relative pressure  of the air flow at locations around the spraybar.  Minimum pressure is at approximately 120^o from the top of the spraybar as shown.

2.  The caption at the bottom of the diagram reads:

Fig. 3:  This sketch shows the distribution of static pressure on the circumference of a spray bar in a venturi.   The fuel outlet hole (C) is shown at the position of maximum fuel "draw."  The spray hole should not be put at "D" (bottom, facing the crankshaft) for maximum fuel draw.  The pressure here is not as low as at "C" where the airflow has maximum velocity.  The dotted line is a polar plot of the pressure distribution. If you put the spray hole at "F", no fuel will flow.  If your engine goes lean at mediium throttle, rotate the spray hole up towards "A" about 10 to 20 degrees.  Fix it so that it moves into higher velocity air as you move to medium throttle.


Keith

[Tim Wescott]

      The idea that the maximum fuel draw is not at "rear" of the airflow is a new one on me.  Of course it does not take much to dazzle me! Lol  Perhaps the friction of the air moving over the spraybar at 90 degrees has a bit more effect on the liquid fuel than the vacuum formed at the rear of the spray bar.

If the airflow were perfectly laminar and frictionless the best suction would be with the hole crossways to the airflow, and the point directly back from the airflow wouldn't be any better than one pointing directly forward.

The reality that trumps that theory is that the airflow is turbulent.  I don't know the theory for why the best position, but it doesn't matter because what works does so well enough for me, and I have other things to worry about.

[Howard Rush]

Maynard's article looks like it might be based on pressure distribution on a cylinder.  I can't tell by looking whether it's that or experimental data.  I'd go with the experimental data. 

I did some venturi experiments.  I forget the numbers, but my impression was that most fuel draw comes from plugging the hole.  Does the extra draw when the spraybar hole is sideways come from a venturi effect or from the viscous fuel obstructing the hole?  For stunt it probably doesn't matter.  For Bill's racing application, I think the next step would be to put a sealed, rotating spraybar on an engine, then rotate it and see what effect spraybar orientation has on RPM.

[Eric Viglione]

That's fine if you have time, but what if you're at a contest and your spraybar comes loose in the middle of the day?

Then you just re-mount it quick and make sure you can't see the hole and optimal's aside, it'll prolly run just fine. If you are prepared like a good boyscout, you have your spraybar flange scribed and your case scribed, and you just align the scribe marks and re-tighten it. 

[Howard Rush]

Without waiting until 5:00?

[Eric Viglione]

Without waiting until 5:00?

It's always 5:00 somewhere...

[BillLee]

Maynard's article looks like it might be based on pressure distribution on a cylinder.  I can't tell by looking whether it's that or experimental data.  I'd go with the experimental data. 

I did some venturi experiments.  I forget the numbers, but my impression was that most fuel draw comes from plugging the hole.  Does the extra draw when the spraybar hole is sideways come from a venturi effect or from the viscous fuel obstructing the hole?  For stunt it probably doesn't matter.  For Bill's racing application, I think the next step would be to put a sealed, rotating spraybar on an engine, then rotate it and see what effect spraybar orientation has on RPM.

Without actually having DONE that experiment, I can only guess ......

But since I HAVE done the experiment using a manometer to measure the effect of hole placement, I would GUESS that I would see the engine richen as the "sweet spot" is found.

Just may have to do that one of these days.  

[Howard Rush]

Never mind.  You did the experiment without fuel.  What I said was stupid.

[Jim Thomerson]

I flew my Fox stunt 35 powered clockwise OTS Demon through a pattern and all went as it was supposed to. Wiping down the airplane I saw the hole in the ST spraybar looking up at me.  As we all know, a Fox stunt 35 will run well under conditions where a lesser engine wouldn't even start.

With a single hole spraybar, if you can't see the hole, the engine should run fine.

[RknRusty]

I flew my Fox stunt 35 powered clockwise OTS Demon through a pattern...

Jim, is that the one you accidentally built clockwise? I remember getting a chuckle out of that knowing how easily I could have done something like that, all the while being so meticulous.

Rusty

[phil c]

This works fine for a standard needle valve.  I've found, by trial(and mostly error) that another factor comes into play with a spigot(side-mounted that doesn't go all the way through the venturi).  Where the fuel feed comes in, relative to the crankshaft, can affect fuel feed dramatically, especially on side mounted engines(profile).  I haven't tried testing on an upright or inverted engine.

On a profile, feeding the fuel in from the engine's left side(viewing from the back) makes the fuel feed go significantly lean on inside maneuvers and rich on outsides.  It can be enough to kill the motor, particularly if the venturi is on the large side.

Moving the fuel feed around, close to the engine centerline, fore or aft, takes care of the problem.  The engine runs steadily and predictably.  Feeding the fuel from the right side also causes some mixture changes, but not nearly as much as the left side.

I won't speculate on what happens that causes this.  But being forgetful, I've inadvertently done the experiment at least five times by accident.  Using a conventional needle valve, positioning the fuel feed hole markedly off center can also cause similar problems on a profile plane.

[Lauri Malila]

 Phil,

It is also possible to confuse 2 things that cause speeding up in outsides when engine is mounted horizontally:

 -Fuel accumulation in the bottom of crankcase. If you have a possibility to run the engine in both directions, you will notice that this asymmetry reverses when you change the direction of engine rotation. It's kind of funny that often the best running symmetry is not reached by the most symmetric engine position.
 -At least what I have found, a minimal a running asymmetry caused by non-symmetrical fuel supply to venturi. If venturi design is good, there is no issues.

Lauri

[John Cralley]

       I am unable get the Hill article to open large enough to enable me to read it.  Is there a way that I may enlarge the article?
Stay well, Frank 

Nothing to do with spray bars (I'm all electric these days) but, for a Windows PC, you can zoom in and out by using the Clt key with the plus or minus keys. To quickly go back to normal size use Clt with zero and it will pop back. I have no idea what works for zoom on an Apple computer. 

[phil c]

Lauri, I'm sure other things can cause the problem too(Fox 35 for example).  Maybe I make poor venturis.  But the setup used a pinch bar to hold the venturi, so it was simple to rotate the venturi between flights.  Fuel feed on the left, the motor coughed, rotate it to get the fuel feed near the centerline it ran fine, rotate it back and it coughed again.

[Bootlegger]

  Howard Rush, the 5 o'clock and 7 o'clock ONLY work with an analog watch, cancel if using a digital watch...

[Chris Wilson]

 -Fuel accumulation in the bottom of crankcase........

Hi Lauri,
              I think that fuel accumulation in the channel of the ports is far more likely - how would the lower crank volume hold anything unless there is a blank niche or trapped space somewhere.

Cheers.

[Lauri Malila]

Then tell me why the asymmetry reverses when I reverse the running direction? Our channels (scavenging) are very small and with various tests, I have more or less eliminated the possibility that port channels cause it.
Because we have piston scavenging, I even made a piston with a plenum chamber in it, I thought it would even out pressure differences inside the differend sides of carter, caused by rotor effect of crankshaft. It had no effect, just added mass in the piston and caused big risks.

[Igor Burger]

Os LA was rich in positive Gs if mounted inverted and rich in negative Gs when mounted outboard ... so I mounted it 45 degrees and I got perfect neutral run ... I expect it will be rich also in negative Gs with reversed rotation ... looks like crank shaft or conrod cause it

[Lauri Malila]

Igor,

My starting point was:

 -With "normal" prop direction, it goes rich in outside turns.
 -With pusher prop direction, it goes lean in outside turns.

Engine is mounted horizontally, cylinder outboard.

By reducing engine load and improving burning efficiency I managed to make the difference small enough, now I prefer a pusher prop and a very small speed-up in outsides. But it's still annoying.

In my new model, cylinder will be tilted 40 degrees down from horizontal (no, not the new electric model.) L

[Chris Wilson]

Then tell me why the asymmetry reverses when I reverse the running direction?

Because the ports that 'see' the approaching crank rotation first get supercharged via the gas swirl - in other words the 'leading' ports get more fuel/air charge that the 'trailing' ports.

Reverse the crank direction and you reverse that effect, and since a horizontal rear exhaust engine is completely symmetrical about the thrust line in all ways except for any crank rotation effects it must be that variable that changes things.

[Igor Burger]

Yes Lauri, it is exactly what I discovered also ... inverted installation was not so perfect and outboard also ... 45 degrees installation canceled both effects and it worked perfectly ... I would sa it was my best powertrain (I had two such models) ... I do not know, why, but I was not so happy with larger motors :-))

[Lauri Malila]

Chris,

It is possible in some cases but not in this. We use full piston scavenging and that more or less eliminates the flow differences. Of course I thought about that possibility and that's why I built the plenum chamber piston.
Also, I have tried blocking all channels on either side of cylinder. That screws up the efficiency of scavenging so there was not much power. But the model was enough flyable to find out that it had no effect on the running symmetry
issue.
Also, i tried to even out the pressure difference of opposite channels by connecting them with a horizontal channel. If you look carefully, it can be seen in my previour picture.
Also, another indication of fuel accumulation is that rpm change happens only in an aggressive change of G, like from an intersection of inside/outside loop. Not much change when the model turns from level/inverted flight.
Before my new model with a better engine position is ready, the solution is simply a smaller prop/higher rpm. Deep down, I think that the answer is in harmony between venturi/bottom end volume/scavenging size. It means lots of work. L

[Igor Burger]

nooo ... it is not so much work, the secret is to replace liquid fuel to some polymerized stuff :-P

[Lauri Malila]

A solid fuel rocket?

[Igor Burger]

nooo ... I say polymerized ... with lot of lithium 

[Brett Buck]

Yes Lauri, it is exactly what I discovered also ... inverted installation was not so perfect and outboard also ... 45 degrees installation canceled both effects and it worked perfectly ... I would sa it was my best powertrain (I had two such models) ... I do not know, why, but I was not so happy with larger motors :-))

   This has been a problem with schneurle engines in particular since pretty much day one. In fact it was the bane of our existence when trying to run schneurle motors without a pipe. You still hear it happening at times - listen from the intersection of the square 8 through the outside loop, 95% of the time you will hear it speed up.

 That's why I run a RO-Jett vice the alternatives. It does this far less than any other engine I have tried - nearly undetectable even when you are listening/feeling for it to happen. I don't want to go into the details I have been told or discovered myself over the years as they could arguably be considered proprietary, but I will note that the cast-case version doesn't work the same as the bar-stock version as far as this effect is concerned.

  Just about everything Dave/Ted/I have done over the years with engines has been to get rid of this effect. It's why we have gone "off the farm" on engine setups.

     Near as I can tell, there are two things going on, and you have to do something about both of them to get it right - the engine scavenges different based on the direction of the G loading, and the flow of fuel/air mixture in the venturi changes as the direction of the air changes from maneuvering.

    Another interesting observation os that this effect diminishes as the air gets thinner. What works just dandy in a Midwest summer day can be utterly unworkable here at sea level, to the point that many people have not been willing to admit that this effect even exists. Just like the Fox burp - and I think it is closely related.

    Ted discovered the tilting effect back when he was trying various schneurle engines on the Imitation back in the late 70'/early 80's. He had round RC radial mount that was drilled to permit rotating it to 16 different positions (22.5 degrees). In most cases with inverted mounts, it runs much faster in outsides than insides no matter how you shim the tank. I think Ted told me that he found that his 40FSR did as expected in an inverted mount, ran faster on insides than outsides with the cylinder at 45 degrees right roll, and ran symmetrically at 22.5 degrees right roll.

    The problem with the tilting engine solution is that it isn't the same from engine to engine. One engine might want 22.5 degrees, another might want 45, another might want 90 degrees.

     Brett

[Ted Fancher]

   This has been a problem with schneurle engines in particular since pretty much day one. In fact it was the bane of our existence when trying to run schneurle motors without a pipe. You still hear it happening at times - listen from the intersection of the square 8 through the outside loop, 95% of the time you will hear it speed up.

 That's why I run a RO-Jett vice the alternatives. It does this far less than any other engine I have tried - nearly undetectable even when you are listening/feeling for it to happen. I don't want to go into the details I have been told or discovered myself over the years as they could arguably be considered proprietary, but I will note that the cast-case version doesn't work the same as the bar-stock version as far as this effect is concerned.

  Just about everything Dave/Ted/I have done over the years with engines has been to get rid of this effect. It's why we have gone "off the farm" on engine setups.

     Near as I can tell, there are two things going on, and you have to do something about both of them to get it right - the engine scavenges different based on the direction of the G loading, and the flow of fuel/air mixture in the venturi changes as the direction of the air changes from maneuvering.

    Another interesting observation os that this effect diminishes as the air gets thinner. What works just dandy in a Midwest summer day can be utterly unworkable here at sea level, to the point that many people have not been willing to admit that this effect even exists. Just like the Fox burp - and I think it is closely related.

    Ted discovered the tilting effect back when he was trying various schneurle engines on the Imitation back in the late 70'/early 80's. He had round RC radial mount that was drilled to permit rotating it to 16 different positions (22.5 degrees). In most cases with inverted mounts, it runs much faster in outsides than insides no matter how you shim the tank. I think Ted told me that he found that his 40FSR did as expected in an inverted mount, ran faster on insides than outsides with the cylinder at 45 degrees right roll, and ran symmetrically at 22.5 degrees right roll.

    The problem with the tilting engine solution is that it isn't the same from engine to engine. One engine might want 22.5 degrees, another might want 45, another might want 90 degrees.

     Brett

Brett is, as usual, pretty much on the mark with respect to what we've done with the "Schnerle Effect" over the years.  I can only add a little to his comments about my experiment with the Imitation and the FSRs.

As stated, it ran rich in insides and lean in outsides of figures eights (the worst of it would occur when the second inside was flown and the engine would go dramatically richer--the first inside was pretty much just fine as if one was doing consecutive loops--and then accelerate lean again in the second outside.  The difference between three different engine speeds in the four loops made consistent sizes almost impossible.  Tons of power but much of it unusable due to the inconsistent delivery. 

On the Imitation I first experimented with a sidewinder "profile style" configuration.  The "Schnerle Effect" was absolutely reversed, lean insides and rich outsides.  I then tried it at 45 degrees off inverted and it ran exactly the same as the sidewinder.  I then tried the 22.5 degree from inverted installation and the engine ran perfect!  Problem solved, right?

Well, I was smart enough to try to duplicate it with another FSR and, at 22.5 it ran just like it was inverted...back to the original problem.

At the time my solution was simple.  It was spelled S.T.4.6.  Sure, it wasn't as powerful but boy was it easy to fly figure eights again.

Finally, I do concur with Brett that the Schnerle issue was somewhat of a problem with some large bore tuned pipe installations but the RoJett .61s seemed immune.  Ergo, on the rare occasion I fly nowadays the old Trivial Pursuit has the Ro Jett in it.

Ted