Spraybar Hole Orientation - Single Hole Type

I'm sure there's plenty of science and theory to support the correct positioning of the spraybar hole. And probably some horror stories from trying to apply some of this "science," as well.

1) Within the context of the SINGLE HOLE in the SPRAYBAR - Where should the hole be pointing?

2) Where do you get your most consistent runs.

3) Any power advantages pointing the spraybar hole one way or the other.

The most consistent runs I get are with the spraybar hole pointed straight down....that is, perpendicular to the crankshaft (and NOT straight down the venturi bore.)

Someone did some tests and published the results in Stunt news. I also seem to remember that the answer was 20 degrees past horizontal. Don't remember for sure I don't have fuel draw problems with a bladder.

>Someone did some tests and published the results in Stunt
>news. I also seem to remember that the answer was 20
>degrees past horizontal.

It was Frank Williams. He used a ST case driven by an electric motor, a venturi and a manometer to measure highest draw coming from the spray bar.

I have found that, especially for the 4 stroke, the hole in the spraybar, while having less effect than the fuel viscosity, does appear to have some effect. The effect being that the engine just runs more steady in the manuevers.

I split the difference between vertical and horizontal. I believe Frank said 20 degrees, so I use 22.5 degrees (close enough) from BDC.

Ray,

As Steve recalled, there was an article in STUNT NEWS on this a few years back.

Basics: think about the flow down the venturii. Venturii sit a quite a range of angles from one engine type to another. Use the airflow direction to "define" the terms horizontal or vertical. "Vertical" is along the flow direction; "horizontal" is at right angles to it.

You should not be able to see the "jet" (fuel hole) when you look down the venturii. JUST past visible is better than having it at the spraybar's "bottom" relative to the airflow. Mark the hex side of the spraybar in line with the jet so you know where it is.

Haven't heard of or obseved any differences from rotating it to the prop side or the cylinder side. Shouldn't matter. ...but maybe it does. At any rate, it is a good start point.

This is supposed to put the jet in the best fuel draw position -- may not increase power, but sure helps get dependable needle settings.

As it was explained to me - maximum airflow is right where the narrowest point of the venturi is just reached. And this is where the fuel should enter. And I've heard that it should be 5 degrees below horizontal. But is this one and the same point?

Believe you're refering to the same article in Stunt News I read.

One of the reasons I kicked this out for whatever discussion it brings is that a friend of mine tried the orientation per the magazine article and ....well....to say the least - he did not concur at all! He got flameouts at certain and critical parts of maneuvers.

This is a very interesting topic. Perhaps someone can explain the following behavior:

How does position of the spraybar whole affect how hard an engine breaks on the inside versus the outside loops?

When I was trimming my Twister last year, I got it to the point where it would fly at the same speed in level and inverted BUT... It would break harder on the outside loops versus the inside loops. A pilot at the field suggested I rotate the spraybar. I did and the engine started breaking the same way on inside AND outside loops.

Hi Steve; Where did you have the spray bar to begin with, and where did you end up with it to make it run right? I seem to be having the same problem with my Fox 59 on my Viking. Thanks;

Holy Cow!

A Fox .59 on a Viking! Oughtta be able to take off straight up and haul the pilot up with it! Hasn't it shaken the nose apart already?

The Viking had really 40s-50s fuselage construction. Weak. And all my Fox .59s shake like crazy. Maybe Randy can persuade Henry to produce new cranks?

Sorry for all the !!!s

The way I mount spraybars is such that the whole is pointing down at the same angle as the ventury shaft. I think the rule is to rotate the spraybar so that the whole is pointing more backwards in order to get less break on the outsides. Rotate the spraybar more forward to get less break on the insides. This is assuming you get same lap times inverted and level before trying this

I've got to be honest with you, I am having hard time understanding why that would make a difference. I wounldn't recommend it to anyone until I can understand why things behave that way.

>This is a very interesting topic. Perhaps someone can
>explain the following behavior:
>
>How does position of the spraybar whole affect how hard an
>engine breaks on the inside versus the outside loops?
>
>When I was trimming my Twister last year, I got it to the
>point where it would fly at the same speed in level and
>inverted BUT... It would break harder on the outside loops
>versus the inside loops. A pilot at the field suggested I
>rotate the spraybar. I did and the engine started breaking
>the same way on inside AND outside loops.
>
>Steven Yampolsky
>Webmaster for www.control-line.org

Ron B.
F4Fguy

The position of the jet in the spray bar can have a direct effect on fuel draw.if it's in the wrong position it may draw just fine as long you're flying straight and level,but reduced draw wil cause the engine to go leaner earlier.This will have the effect of causing the "break" to come early and hard.

I have no idea why it should be any different turning one way or the other.If I had to guess,I'd say it's probably tank related rather than engine.

>I'm sure there's plenty of science and theory to support the
>correct positioning of the spraybar hole. And probably
>some horror stories from trying to apply some of this
>"science," as well.
>
>1) Within the context of the SINGLE HOLE in the SPRAYBAR -
>Where should the hole be pointing?
>
>2) Where do you get your most consistent runs.
>
>3) Any power advantages pointing the spraybar hole one way
>or the other.
>
>The most consistent runs I get are with the spraybar hole
>pointed straight down....that is, perpendicular to the
>crankshaft (and NOT straight down the venturi bore.)

Ron B.
F4Fguy

Ray:

You're right,there is some science involved,and getting it wrong can,as you've apparently discovered,cause some problems.The mechanics are simple,the spray bar defines the smallest area in the venturi.If we lived in a perfect world,this is where we'd want the jet.In the real world the air passing through has inertia which makes it delay any directional change.This causes the actual highest velocity ,and the lowest pressure to occur somewhat downstream of the dimensional "smallest" point.Technically this point is different for every flow condition.That is,for a given set of conditions,and given engine,the velocity is different in a larger venturi than in a small one.In practice it doesn't matter too much,as long as the jet is near the actual highest velocity point.Straight down is not the right point.Straight across is not the right point.Somewhere in between is.The only way to find out exactly is by testing.I don't recommend it.

Now to get practical.Lou is probably very close to the "best guess"position,if you are running a fairly large venturi.I generally turn the spray bar until the jet is just out of sight when looking down the venturi.The smaller you go in venturi size,the more you need to angle downstream to get maximum fuel draw.With a very small venturi for a particular engine,you may have to go as much as 15 deg. downstream.The only time this becomes really critical is when fuel draw is borderline.

If you have a two jet spraybar,the jets need to be equally spaced in the venturi.Be sure to turn the spraybar through a full 360 deg.while observing the jet position,as some of the "two-holers" have a built in angle which only works one way.The side with the jets closest together goes down.

Hey Ralph; My Viking started out with a Super Cyke on spark for VSC this year. When I got home from there I replaced that with my Fox 59. I had built it to take either engine. I also used ply doublers in the fuse, but otherwise built to F&B plans. The original plan shows several engines of which the (59) is listed. It really does not shake very much in this plane as the fuse is fairley wide and has a lot of bulk in the front end. It does have plenty of power, and I am running low compression and a ventura insert quite a bit smaller than stock. Seems to fly fine with either a 12-5 or 13-6 prop.

That was an excellent article in SN in the mid 90's by Frank Williams. I did some experimenting after I read it. I tried the spray bar hole 90 deg to the flow, and the first flight was marvelous. The second flight was totally inconsistant. So I went back to putting the hole straight down to the flow. Plus or minus 20 deg. I've never taken the time to evaluate straight down vs 5 deg vs 10 deg vs 20 deg.
I did note in Franks article that the very best fuel draw is with an r/c type spray bar. The kind that has a 90 deg angle in it. I have set up several engines with that type of spray bar and they work very well. From now on, I will always use that type of spray bar in my stunt planes.

I have a Magnum 36 with the needle valve to the rear of the venturi/carb riser. Leonard says is a true venturi. When he sent me a venturi & NVA, if I understood his instructions, the true venturi needs the spraybar hole to point the the front, not like in an engine with the nva through the middle of the venturi, which needs to point down. Or am I confused again??