Author Topic: venturi shape? (Read 8802 times)

dave siegler · « **on:** October 05, 2012, 04:08:23 AM »

to the guys that make your own venturi's.

1) Do you taper the bore?

2) Is the shape taper important?

3) If so what tooling do you use?

Just got a lathe, trying to make some venturi's for OS engines.

Tim Wescott · « **Reply #1 on:** October 05, 2012, 08:24:04 AM »

1: Yes

2: Hey, I'm a hobbyist machinist, but I'm not an engine expert. I've seen a lot of good engines that have venturis with perfectly round holes, though.

3: For the first one or two I used a small boring bar and my compound slide to make two or three different angles, then I wrapped sandpaper around a pencil and smoothed things out. Later, I got fancy and ground a custom venturi-taper-boring tool. It's just a piece of 1/4" square tool steel with a swoopy curve ground into it on the cutting side, then relieved on the bottom to a half-circle so it'll clear the hole that it's making.

Bootlegger · « **Reply #2 on:** October 05, 2012, 09:12:23 AM »

I have many that the venturi is a straight hole in engines from 25 size to 46 size and I can't see any difference in the way that run.
The expert's might be able to "shead" more light on the subject tho...

Tim Wescott · « **Reply #3 on:** October 05, 2012, 10:19:53 AM »

It's kind of misleading to say that they're venturis with straight holes: a venturi needs a spot where the area necks down in order to work; in the "conventional" CL venturi that's provided by that big honkin' spray bar sticking into the middle of things.

I know that some experts prefer a venturi where the tube necks down and the fuel is fed through holes in the side (is "sprinkler" the right term for this? it seems to be, but I can't get the visuals.). In that case you do need the tube to reduce in diameter, at least if you want to be able to draw fuel.

Andrew Tinsley · « **Reply #4 on:** October 05, 2012, 10:27:39 AM »

Hello,
A straight hole works just fine. As long as the air is moving rapidly past the NVA, then there will be a low pressure area there and the fuel will be forced through the spraybar hole by normal atmospheric pressure.
A tapered or bell shaped venturi should be a little better in theory, as the air will experience a higher velocity. I doubt anyone could tell the difference in performance. Mind you I remember reading about a series of experiments where there was a definite claimed improvement in power output? Maybe small differences in profile do make a difference in team race applications or speed. I would be very interested if anyone can shed some light on the theory behind venturis, in terms of engine running improvements.

Regards,

Andrew.

Andrew Tinsley · « **Reply #5 on:** October 05, 2012, 10:32:06 AM »

Hi Tim,
I am pretty certain that even a sprinkler system would work with just a straight hole . The important thing is that there is air moving with respect to the holes. Any air moving will exert a lower pressure than atmospheric and hence fuel will still be forced out of a sprinkler system even without a restriction. The hole is in fact the restriction!

Regards,

Andrew.

Geoff Goodworth · « **Reply #6 on:** October 05, 2012, 07:55:43 PM »

I would look for a small tapered reamer.

Drill the bore to the size you want and then flare the venturi both sides of the spraybar location leaving a small parallel section slightly wider than the spraybar diameter.

RandySmith · « **Reply #7 on:** October 05, 2012, 09:03:04 PM »

HI
The venturi needs to be a flared bell shape at the top,or at least angled so the top is the largest inside diameter, and transitioning to the smallest point where either the feed hole is, for true venturis ,or smallest at the point where the spray bar goes thru it, I call the later a restrictor, there is also a variation on true venturies called a sprinkler, and other that is a fuel post. The most power will be available with a true venturi. You can make all of them work. Do not make a straight tube, that is the worst of all of the induction devices for getting fuel air mix into a model engine, It is less critical on a restrictor than the others. Since you are making these for an OS engine, it will be a restrictor, you can machine a tube with thick walls so you can machine in a flare at the top, then below where the spray bar goes thru, you can machine a short taper that tapers outwards , somewhere between 5 and 15 degrees wall taper is OK.

The way a real venturi works is by pressure drop by velocity change by the venturi, so taking this into consideration, it can be proven that:
-decreasing area = increasing velocity
-increasing velocity = decreasing pressure
The decreasing pressure is the reason fuel flows in from the tank, which has higher pressure, this is why you have to have the angles/flares/curves cut into the true venturie to work as it should.
This is a quick and simple answer, there are many volumes of text about this if you are really interested.
Regards
Randy

GregArdill · « **Reply #8 on:** October 06, 2012, 04:21:22 AM »

A lot depends on the use you're putting the OS to, is it for aerobatics? If so the final venturi shape is not so critical as you are not really looking for peak power, but good fuel draw and reliable starting and sufficient power to drive the desired propellor size.

You can make a good venturi by first drilling into your bar stock with an centre drill of a suitable angle. I have used 120 degs with some success. Once you complete this step bore to the desired throat size, use either a drill or a boring bar.

Then IF YOU REALLY WANT TO IMPRESS YOURSELF, make a scraper by grinding a triangular jewellers file and while the lathe is turning moderate revs give your venturi a very nice bell curve with this scraper.

It looks a million dollars, but does it work any better for this last step?

Have fun with the lathe.

Greg

dave siegler · « **Reply #9 on:** October 06, 2012, 06:13:15 AM »

thanks all, I know how an venturi works, but wondered if the needle through the middle was enough diameter restriction to work. If it isn't enough power, I could drll the hole bigger.

I can easily open up the input side and crank side a little, and I will have to get or make a tool to cut a taper at least.

I'll try to cut a 1 step taper and smooth the transistions. Is that overkill, or underkill?

I took all the math classes, not any shop so a metal lathe is a new tool to me, but how a venturi operates is not.

The application is general sport flying mostly. I need to make a few for some speed limit combat, where on pressure it may not matter.

RandySmith · « **Reply #10 on:** October 06, 2012, 09:24:09 AM »

Quote from: dave siegler on October 06, 2012, 06:13:15 AM

thanks all, I know how an venturi works, but wondered if the needle through the middle was enough diameter restriction to work. If it isn't enough power, I could drll the hole bigger.

I can easily open up the input side and crank side a little, and I will have to get or make a tool to cut a taper at least.

I'll try to cut a 1 step taper and smooth the transistions. Is that overkill, or underkill?

I took all the math classes, not any shop so a metal lathe is a new tool to me, but how a venturi operates is not.

The application is general sport flying mostly. I need to make a few for some speed limit combat, where on pressure it may not matter.

Hi Dave

YOu will need to size it properly for the application, generally the larger the hole, the less fuel draw, smaller holes give more fuel draw, example for sport and stunt flying, on an OS 40 you would use a range of 260 to 290 on the restrictor size with a stock .157 spraybar (this is the size of the ST,PA, and older OS bars) , if you use the newer, small OS spraybar you would use a 240 to .265 size . The size of the hole affects how the engine breaks, or comes on when the plane leaves level flight.
When running them on bladder you can use a much larger size as you need not worry about fuel draw at all.

Randy

Andrew Tinsley · « **Reply #11 on:** October 06, 2012, 09:53:04 AM »

Hello Randy.
As far as I am aware Bernoulli's theory states that a moving mass of gas exerts less pressure than the same gas that is stationary. If I am correct in this interpretation, then a sprinkler system will work if the venturi is simply a parallel bore. There seems to be a misconception amongst some folk, that the gasflow shows a reduced pressure, only if it is being accelerated. I am quite interested in whether bell shaped venturis with either sprinkler systems or conventional NVAs show improved fuel draw or any other advantage.
I would imagine that a any venturi system that creates a very turbulent flow would promote excellent fuel / air mixing and would be much preferred to any system where the flow was more laminar in nature.

Regards,

Andrew.

sleepy gomez · « **Reply #12 on:** October 07, 2012, 12:50:34 PM »

I first bore the minimum size hole for the venturi I want. Then using a wood paddle bit I grind the sides of it to the taper and/or bell mouth shape I want. Using this I plunge to a depth close to the minimum hole size. By turning the workpiece around I put a little taper on the bottom side. This can give a quite true venturi. It can also be made in a drill press. Is this better than the straight hole? I don't know but I feel better doing it this way.

RandySmith · « **Reply #13 on:** October 07, 2012, 01:32:02 PM »

Quote from: Andrew Tinsley on October 06, 2012, 09:53:04 AM

Hello Randy.
As far as I am aware Bernoulli's theory states that a moving mass of gas exerts less pressure than the same gas that is stationary. If I am correct in this interpretation, then a sprinkler system will work if the venturi is simply a parallel bore. There seems to be a misconception amongst some folk, that the gasflow shows a reduced pressure, only if it is being accelerated. I am quite interested in whether bell shaped venturis with either sprinkler systems or conventional NVAs show improved fuel draw or any other advantage.
I would imagine that a any venturi system that creates a very turbulent flow would promote excellent fuel / air mixing and would be much preferred to any system where the flow was more laminar in nature.

Regards,

Andrew.

Hi Andrew

i have made 1000s of these ,in most every shape you can think of, A tube works crappy, the fuel just dribbles into the inside of the venturie, if at all, it is not very efficient at all. Look at a venturie on airplanes, there is a reason why they are shaped that way, it works much much better if you have a large intake angled or curve to reduce the diameter, at the smallest diameter just before the size opens up again the velocity will be the greatest, just as you would want, this gives the lowest pressure, not so with a tube. It really isn't the bell curve shape that makes it work, it is the difference in diameter that is the most important, the curve just helps you with air intake.
I have tested many dozen types of venturi,restrictor,fuel post type of systems, and it is clear what works better.
AS far as the many people who are confused about "There seems to be a misconception amongst some folk, that the gasflow shows a reduced pressure, only if it is being accelerated." I don;t think there is much confusion about when you accelerate the air more , instead of less, you get a better pressure reduction to do the job, and with air forced thru a smaller are of the venturie, you get better fuel draw than when it is going thru a larger area.
I also think there is a reason why engine mfg. do not use tubes in their motors, unless you are using a bladder or forced fuel feed, they simply do not work well.
But you are very welcome to run tubes, or test them and let us know how they work.

ps if you run tubes, it would be best to use a restrictor with a spray bar going thru the middle, instead of a sprinkler, the vortex effect will help greatly with giving you fuel draw.

Regards
Randy

rustler · « **Reply #14 on:** October 07, 2012, 01:40:47 PM »

Where does the S.T. C/L venturi shape stand in all this? A sharpish curve leading to a parallel section, which suddenly, almost a step, reduces to a minimum dia throat, at the 6-hole sprinkler point? I always seem to get good results with these.

RandySmith · « **Reply #15 on:** October 07, 2012, 02:13:42 PM »

You can see the pressure drop on the diagram, this is not the case with a sprinkler tube

Chris Wilson · « **Reply #16 on:** October 07, 2012, 04:35:53 PM »

Just wondering about the pulsed nature of the flow in IC engines.

Lab tests only go so far as to look at steady flow and make no account of the rapid back flow model engines must deal with - hence most successful systems having 'length' or distance between the shuttering valve of the crankshaft port and the spray bar.

Flow is not as consistent as we would like to believe.

phil c · « **Reply #17 on:** October 07, 2012, 05:48:28 PM »

Randy's little picture shows the key characteristic of a good venturi design. Since we pull air from essentially and infinite box, the outer bore should be a smooth, rounded radius feeding into the intake taper. A square cut off tube causes a loss of air pressure ahead of the needle valve, leaving less pressure drop to suck in the fuel. The narrow neck drops the pressure smoothly for maximum fuel draw. The key element is the narrow neck transitions smoothly into a straight taper. The straight taper, usually 7 deg., allows the air to recover pressure. This results in minimum loss of airflow and leaves the air at nearly atmospheric pressure for maximum horsepower as it goes into the venturi.

There was an article in stunt news years ago by Scott Bair(?). Being in the small engine design business he had the equipment to measure the actual power output. If I remember correctly he found a really good venturi gave an ST 46 8% greater torque and lower fuel consumption. Nowadays though, everyone just puts in a bigger engine.

dave siegler · « **Reply #18 on:** October 07, 2012, 07:42:05 PM »

will this work to cut the taper?

http://www.harborfreight.com/3-piece-stepless-drill-bits-66463.html

Or do I have to grind a tool?

Note for me this is more about how to run the lathe

Tim Wescott · « **Reply #19 on:** October 07, 2012, 08:09:26 PM »

Those would do it.

What lathe do you have, and what sort of toolpost does it have? It's not a bad investment to get a a boring tool holder and one or two (or a whole set of) boring tools -- that way you can drill the venturi through a bit small, then bore it out to whatever diameter you want. Done right, a bored hole has a nicer finish than a drilled hole. Then, when the straight hole is made, it's easy to set the compound slide for a ten degree angle or so and bore the taper.

dave siegler · « **Reply #20 on:** October 07, 2012, 09:47:30 PM »

It is a sherline 4400. It came with only a rocker tool post, and nothing else.

I had to buy a chuck for the tailstock and a parting tool and holder.

It doesn't have a compount slide, the headstock can turn. I guess I will have to buy a toolpost that will take round tooling and a set of boring bars.

Tim Wescott · « **Reply #21 on:** October 07, 2012, 09:53:43 PM »

Check the Sherline site, and see what they have for boring tools. You're kind of stuck with whatever itty bitty stuff fits your itty bitty lathe: most machine tool supply places seem to think that a 3/8" boring bar is "tiny" -- but for your lathe, that's humongous.

Andrew Tinsley · « **Reply #22 on:** October 08, 2012, 03:46:21 AM »

Hello Randy,
I wasn't advocating a parrallel bore as the best venturi, far from it! I was making the point that it works and indeed works quite well considering. I had a sprinkler system on a parrallel bore for quite some time (laziness!) and it didn't appear to make a happeny's worth of difference in terms of performance. I later gave it some fancy curves and it did then improve fuel economy.
I wouldn't argue with your experimental results, except to state that Bernoulli's theory simply says that the reduction in pressure is proportional to the flow velocity. There are people around (and have just posted!) that can't seem to get their head around a simple scientific fact like that. The fuel draw is simply proportional to the difference in pressure. It doesn't matter how that difference in pressure was created.
There is a lot more to venturi design than has been stated by any contributor so far. Perhaps one of the most pertinent questions was raised by Chris Wilson. He stated that the system is NOT one of freeflow, it is one of oscillating columns of air. It is this area that I would like to see analysed. No one in the model community has come even near a decent venturi theory. Perhaps one should be looking at full size practice to get this. Does anyone know of a suitable treatment that has been made in full size practice? I find the subject fascinating and would like to see some rigooous mathematical treatment, rather than empiracle results.

Andrew Tinsley · « **Reply #23 on:** October 08, 2012, 03:49:24 AM »

Apologies for the many typos in the above, just got carried away!

Regards,

Andrew.

Lauri Malila · « **Reply #24 on:** October 08, 2012, 04:33:21 AM »

Another question is, that is the strongest vacuum the best.
I made true venturi's as per Randy's instructions, 7 degr. taper before fuel injection, and 11 degr. taper after.
I made identical venturi's (all important measurements similar within 1/100mm), some with a fuel post that enters 1mm into the bore, and some without it. To get the same ground rpm, differece between those two is about 1/2 turn of needle (needle more closed with fuel post venturi).
Still, my opinion is that the engine runs nicer without fuel post. It's a small difference but very noticeable. L

RandySmith · « **Reply #25 on:** October 08, 2012, 09:27:28 AM »

" I wouldn't argue with your experimental results, except to state that Bernoulli's theory simply says that the reduction in pressure is proportional to the flow velocity. There are people around (and have just posted!) that can't seem to get their head around a simple scientific fact like that. The fuel draw is simply proportional to the difference in pressure. It doesn't matter how that difference in pressure was created."

Hi Andrew

I was not saying that you were advocating the use of a tube, and Yes the reason the venturis with curves and angles work better, is because of Bernoulli's theory, the fact that the venturi gets smaller with a convergence cone, then an expanding cone is the reason the velocity is highest at the point between, the way you get that smaller point is by using angles and curves to make this exact shape. I still, can tell you that when you drill one of these into a tube, they work horrible, and the fuel draw is down. They also work much better when the fuel hole is at or just below the point where the smallest point is.
Look at the diagram and you will see the pressure difference, notice the largest difference is at the smallest point, and the angles are important because they help the air intake, and then help the recovery at the other end.
When you say the fuel draw is proportional to the difference in pressure, it is clear the venturi shape does this far better than a tube. I can attest to this as I have used both and there is a huge difference in the performance of each , in a venturi.

Regards
Randy

dave siegler · « **Reply #26 on:** October 08, 2012, 10:37:37 AM »

Ok i have the parameters now.

The OS ventui I have has a minor diamater somehere just under 17/64. (1/4 drill fits but the 17/64 does not)

It looks like, based on what I just read, I should use about 7deg taper on the upstream and 11deg on the downstream.

Small boring bars are one way to cut the taper. But small ones look to me are 1/4 inch, not much room.

Before I go out and spend another $50 on this lathe for tools posts and boring bars, what method do you use to cut the taper?

Tim Wescott · « **Reply #27 on:** October 08, 2012, 11:29:11 AM »

Quote from: dave siegler on October 08, 2012, 10:37:37 AM

It looks like, based on what I just read, I should use about 7deg taper on the upstream and 11deg on the downstream.

Small boring bars are one way to cut the taper. But small ones look to me are 1/4 inch, not much room.

Before I go out and spend another $50 on this lathe for tools posts and boring bars, what method do you use to cut the taper?

You can get smaller boring bars than that. Depending on how insane you are about spending lots of money, and on getting tooling that'll shatter if you look at it cross-eyed. Look at McMaster, Enco (http://www.use-enco.com), Wholesale Tools, and, of course, the Sherline site.

The method that I use to cut the taper is to use a compound slide. But that's not what your lathe provides for cutting tapers. You have a lathe head that can be rotated. So the way that you cut a taper is to rotate the lathe head clockwise the desired amount, then make a straight cut into your slanted work piece.

Just a comment before you go spending money on tooling: Venturis are really easy to make. The hardest part is drilling the cross-hole straight and true, and that ain't bad. These things could be training pieces for budding machinists. So it doesn't hurt to make one that you know isn't going to be perfect. In fact, you should plan on your first parts not being perfect anyway, because as a beginner you're going to be a wellspring of Dumb Mistakes (at least, I still am, and I've been doing hobbyist-level machining for years).

Given that, you may just want to bang out a straight-bore venturi or two, just to get some mileage and to give yourself the satisfaction of having a motor that runs on a part that you made. Later, if you want to come closer to perfection, you can build a venturi to Randy's specifications, and see if you're a good enough engine operator to notice any difference.

Whoops! Rotate the lathe head counter-clockwise. Or, better yet, think about the problem and rotate the head the direction that makes things work!

L0U CRANE · « **Reply #28 on:** October 08, 2012, 01:52:38 PM »

A key point is being overlooked?

The diagam in #15 is correct, of course, but notice:

Pressure at the inlet is just about ambient (include any motion-speed ram effects.) The velocity and pressure changes occur within a defined length of plumbing flow.Theoretically, the same volume of "air" should pass any point along the length in the same time. An obstruction that slows the velocity, at the neck, will also slow the rate of air entering at the inlet - considering just theoretical smooth flow through a venturii.

If the diameter at the inlet is twice the diameter at the 'neck,' inlet area is four times 'neck' area. Ideally, velocity through the neck must be four times inlet velocity to keep the same volume of flow at all points.

Bernoulli's statement is for smooth, steady flow conditions: pressure varies inversely with the square of velocity change between two reference locations in a defined condition.

P1/P2 = V2^2/V1^2

This is what I didn't see in the discussion so far - that the velocity change, from "ambient" to neck minimum area pressure, depends on the change in area between those locations in the intake flow path.. ...Presuming the same volume per unit of time. Say, for example, we have a .40 cu in engine turning 12,000 RPM... that's 200 rotations per second, right?

200 X .40 = 80 cu in per second - if all were perfect (it isn't) - that could be the flow rate to fill the cylinder completely each revolution. Of course, that never happens - too much else involved.

But, let's just say, ok? If inlet diameter is .5", its area is just under 0.2 sq in. Velocity to pass 80 cu in per second would be around 400 "/sec or 33.33... '/sec.

If the choke min diam is .25, its area is about .05 sq in (gee whiz! that's 1/4th of inlet area!) So, I guess the needed velocity would be something like 1600 "/sec, or 133.33... '/sec.

P1 is atmospheric, so long as we're simplifying wildly - say 14.7 psi at the entry with velocity of 33.33'/sec.

The equation becomes: P2/P1 = V1^2/V2^2 ---Solve for P2 --- P2 = V1^2 /(P1 *V2^2)

Works out to something like P2 = 1.1 psi. IF the venturii flow were perfect, smooth, and all presumptions of ideal conditions were present.

Also, consider that the shaft intake port is only open about half a revolution, so the flow in that brief time would need to average twice as fast.

Air has mass: inertia and momentum affect how well the charge gets through the shaft port.

MACH drag rise type of resistance to flow can get in play, too.

And these don't consider the rebound wave in the intake tract when the shaft valve slams shut.

Makes it all sound impossible? Nah, despite the aerodynamicist's opinion, THIS bumblebee does fly...

We make do with a lot of losses along the way, but we still have plenty left after them...

L0U CRANE · « **Reply #29 on:** October 08, 2012, 02:05:48 PM »

Oh, yeah... Venturii shape...

I seem to recall, in Gordon Jennngs' Two-Stroke Tuners' Handbook, discussion of plumbing friction losses in long runs of tubing...

Motorcycles in the 1970's had complex bundle of snakes exhaust headers, and some had exotic ductwork to the carbs.

I think it was in regard to his tuned pipe section and formula, that "pipe friction losses" occurred in long runs of parallel sided tube, and that tapering open the diameter by something like 3° helps.

So, for our shorter, but very fast flowing venturii sections, a taper might be preferable to a straight-sided tube...

Tim Wescott · « **Reply #30 on:** October 08, 2012, 02:21:38 PM »

Quote from: L0U CRANE on October 08, 2012, 02:05:48 PM

So, for our shorter, but very fast flowing venturii sections, a taper might be preferable to a straight-sided tube...

OTOH, a straight-sided tube that flows worse than a similar-sized tapered venturi will just act like the opening's smaller, and you'd compensate with a slightly bigger opening.

Tim Wescott · « **Reply #31 on:** October 08, 2012, 02:26:06 PM »

D'oh. This is the kind of thing that I remember when I'm standing in front of my lathe whacking out parts, but not when I'm bloviating on the fora:

Another way to taper (and to get nifty bell-shaped tapers), is to just stair-step it freehand: cut out a couple of thou in diameter, then go in however far in length, then back and forth, etc. When you're done making a sorta-nice-but-ugly bellmouth, wrap some sandpaper around a dowel, put on a few drops of oil, and sand it all smooth as the lathe spins. If you use ever-finer grits you can get a satin-smooth finish that shows barely any tooling or sandpaper marks.

RandySmith · « **Reply #32 on:** October 08, 2012, 02:49:58 PM »

Quote from: L0U CRANE on October 08, 2012, 01:52:38 PM

A key point is being overlooked?

The diagam in #15 is correct, of course, but notice:

Pressure at the inlet is just about ambient (include any motion-speed ram effects.) The velocity and pressure changes occur within a defined length of plumbing flow.Theoretically, the same volume of "air" should pass any point along the length in the same time. An obstruction that slows the velocity, at the neck, will also slow the rate of air entering at the inlet - considering just theoretical smooth flow through a venturii.

If the diameter at the inlet is twice the diameter at the 'neck,' inlet area is four times 'neck' area. Ideally, velocity through the neck must be four times inlet velocity to keep the same volume of flow at all points.

Bernoulli's statement is for smooth, steady flow conditions: pressure varies inversely with the square of velocity change between two reference locations in a defined condition.

P1/P2 = V2^2/V1^2

This is what I didn't see in the discussion so far - that the velocity change, from "ambient" to neck minimum area pressure, depends on the change in area between those locations in the intake flow path.. ...Presuming the same volume per unit of time. Say, for example, we have a .40 cu in engine turning 12,000 RPM... that's 200 rotations per second, right?

200 X .40 = 80 cu in per second - if all were perfect (it isn't) - that could be the flow rate to fill the cylinder completely each revolution. Of course, that never happens - too much else involved.

But, let's just say, ok? If inlet diameter is .5", its area is just under 0.2 sq in. Velocity to pass 80 cu in per second would be around 400 "/sec or 33.33... '/sec.

If the choke min diam is .25, its area is about .05 sq in (gee whiz! that's 1/4th of inlet area!) So, I guess the needed velocity would be something like 1600 "/sec, or 133.33... '/sec.

P1 is atmospheric, so long as we're simplifying wildly - say 14.7 psi at the entry with velocity of 33.33'/sec.

The equation becomes: P2/P1 = V1^2/V2^2 ---Solve for P2 --- P2 = V1^2 /(P1 *V2^2)

Works out to something like P2 = 1.1 psi. IF the venturii flow were perfect, smooth, and all presumptions of ideal conditions were present.

Also, consider that the shaft intake port is only open about half a revolution, so the flow in that brief time would need to average twice as fast.

Air has mass: inertia and momentum affect how well the charge gets through the shaft port.

MACH drag rise type of resistance to flow can get in play, too.

And these don't consider the rebound wave in the intake tract when the shaft valve slams shut.

Makes it all sound impossible? Nah, despite the aerodynamicist's opinion, THIS bumblebee does fly...

We make do with a lot of losses along the way, but we still have plenty left after them...

Hi Lou

I did not overlook these items you mentioned because there is little to nothing that can be done with them, however ,All the things you wrote above that affect the flow thru a venturi, also effect the flow thru a restrictor, or a tube , or any of the venturi variants like fuel post,spinklers ..etc.. If you make or buy a manometer vacuum Air suction pressure meter , and use it to test all of these things, including a tubed, you will find that the true venturi does it best.
There is more, as you say, to all of this, and the number of differant induction system you can come up with are staggering, I make true venturis, or the classic shape venturi when I can on all my engines, They do perform the best in all of my test.
Also Scott Bair did a very in depth look , and study into this, he found this also to be true.
In engines That I could not make true venturis for, because they could not accept them, because of whatever reason, I made true ventuis and added fuel post into them.
The OS Max engines are such motors, as they do not have the material to add a passage way next to the venturi, What we did was to make the venturi and add a OS Max 10 spraybar/fuel post combo to one side threaded into the venturi.
You could then use the venturi and could also have some control over size by rotating the threaded NVA in deeper or Shallower, thus making the effective hole size larger or smaller.
I used these on many of the OPS and OS engine I made for myself and sold to many others.
If I were to have to use a sprinkler tube as Andrew spoke about, it would have multi fuel post tube sticking out into the slipstream.
This was the only way I could get such a device to work as I would need it to. I discounted them early on because of the end of flight burping problem that I did not care for, the ditch that is machined all the way around the outside of the venturi for all the 360 degree sprinkler holes, would be a large area, it would fill with fuel and air during the last part of the flight and the engines would burp off and on for several laps sometimes before cutting off. I did use some of these after blocking off the holes around the venturi and just using either 1 or 2 of them that were opened slightly. And also quite frankly because they did not work as well as the classic venturis I was making with 1 fuel feedhole.

Randy

Chris Wilson · « **Reply #33 on:** October 08, 2012, 03:30:30 PM »

Just for a change of pace here, hows about using Delron instead of alloy?

Shapes just fine - no fancy tools needed.

Lauri Malila · « **Reply #34 on:** October 08, 2012, 03:33:13 PM »

Randy,

In the system that I use, it's very easy to make experiments with differend amount of sprinkler holes. Could you give some tips regarding the size of holes, please? At the moment I use 0,8mm (1/32") hole when there is no fuel post, and if there is a post, it's been a tube with 0,8mm hole & 1,0mm outer dia. The post enters 1.0mm into the bore.
Venturi outside dia is 8,1mm, and a good i.d. seems to be 5mm +/- 0,20mm.
So, if I try with several holes, what would be your recommendation, regarding amount & diameter?

Lauri

Chris Wilson · « **Reply #35 on:** October 08, 2012, 04:13:58 PM »

Quote from: Andrew Tinsley on October 08, 2012, 03:46:21 AM

Perhaps one of the most pertinent questions was raised by Chris Wilson. He stated that the system is NOT one of freeflow, it is one of oscillating columns of air. It is this area that I would like to see analysed. No one in the model community has come even near a decent venturi theory. Perhaps one should be looking at full size practice to get this. Does anyone know of a suitable treatment that has been made in full size practice? I find the subject fascinating and would like to see some rigooous mathematical treatment, rather than empiracle results.

Bingo!

Bernoulli's theorem assumes constant flow in a gravitationless environment - in other words there is no outside influences acting upon the flow.

I am sure that the theory is of immense help in understanding what is going on in general but I too would like to see a hi G pulsed system analyzed a bit better.

Steve Helmick · « **Reply #36 on:** October 08, 2012, 06:48:41 PM »

For most of us, and Dave specifically, being a neubie to machine tools...do yourself a huge favor and buy Delrin rod, and don't bother with aluminum. They may tell you that Acetal is the Generic and it's the same, but it is NOT, so do not settle. Tell them that your customer wants real Delrin, and wants it black. It looks cool, and that may also mean that it's really Delrin. You can buy a 10' length of 5/8" (I find this is a good size for OS and Magnum engines) for about $10. Stay away from Nylons, and especially UHMW...nasty crap, too soft to cut without a deadly sharp cutter. Delrin can be filed and sanded, if you have to, but like all plastics, when it gets warm, it grows, so let the part cool before you measure and test for fit!

My process is to turn the section to diameter and length that will go into the case, then cut it off on my bandsaw with a bit of excess, and then chuck on the finished diameter in the 3-jaw. "Parting" and "Trepanning" are things I learned to avoid if at all possible, in my 37 years as a machinist. Both processes tend to make loud, unexpected noises. Be sure to allow for the fitting of a neoprene O-ring under the shoulder of the venturi/restrictor. Then I face the top end to the length I want, and "center drill" the end with a single flute countersink, drill the bore to size (in two steps, about a .221" drill followed by the final size), and shape the "bell" with a #11 Xacto. Adjust the lathe's rpm to as high as you can without getting chattering of the #11 "turning tool"! You're really scraping the shape into the bore, after roughing it with the countersink. Don't ever buy a countersink with more than one flute...they'd tend to chatter more. When drilling Delrin, try to keep the rpm down and the feed high as possible...keeps the plastic cooler, so gives a better finish. You might benefit from drilling in more steps, due to the smaller size of your lathe.

I'm not sure how much torque your lathe will develop...I got the smallest lathe (3") Grizzly sells, a cousin of the Harbor Freight and Mark I Tools lathe. The Grizzly has a longer bed than the HF, but shorter than the Mark I. It's ok, and in some ways, impressive. I always run it in low range and pretty close to full throttle. I have no idea how that works, but puts less load on the motor, which I figure is a plus. The mistake I made was getting a 1/2" drill chuck for the tailstock...3/8" would be plenty big, and take up less bed length. "Stubby" or "screw machine drills" are the way to go, but I already had a full set of jobber length drills in fractional, numeric and alphabetical sizes. All those sizes are handy for me, not so much for the lathe, but for the drill press. I have to drill props out to .386" ("W" drill) for my .46VF (a flaw I should correct this winter).

Steve

RandySmith · « **Reply #37 on:** October 08, 2012, 08:29:13 PM »

Quote from: Lauri Malila on October 08, 2012, 03:33:13 PM

Randy,

In the system that I use, it's very easy to make experiments with differend amount of sprinkler holes. Could you give some tips regarding the size of holes, please? At the moment I use 0,8mm (1/32") hole when there is no fuel post, and if there is a post, it's been a tube with 0,8mm hole & 1,0mm outer dia. The post enters 1.0mm into the bore.
Venturi outside dia is 8,1mm, and a good i.d. seems to be 5mm +/- 0,20mm.
So, if I try with several holes, what would be your recommendation, regarding amount & diameter?

Lauri

Hi Lauri

I would advise to not try that, as I do not care for sprinkler systems at all, However if you must I would suggest that you only use 2 feedholes on one side of the venturi, maybe 25 to 35 degrees off the center of the fuel feed of the system, You need to keep the area on the outside of the venturi that exposes the venturi holes to a minimum. The larger the area, the more air can collect there.
So with that in mind, you could link the two sprinkler holes together with the fuel delivery holes dead in the middle , maybe cut a slot in the venturi to link the two holes with the feedhole the fuel comes from, so , when you look inside you would see only 2 sprinkler holes on one side about 60 degrees or so apart.
You could still use the 1/32 inch tubes for the sprinkler tube feed. Or if you just use drilled feedholes I would use smaller ones by about 80% of the size you have now. As with any sprinkler venturi, the tiny feed holes cause the problem of being very easy to be stopped up by any tiny piece of debris, or Castor snot/slug. If I did it I would just use feedholes instead of the tubes , make sure these are right at or just barely below the point where the 2 angles meet.
If I am not being clear on this , please let me know.
And by all means if you do this please let us know how it went.
Regards
Randy

RandySmith · « **Reply #38 on:** October 08, 2012, 08:32:32 PM »

Hi Dave

I would suggest you do buy Delrin if you are going to consider using any type of hand held tool. Please be very careful doing this, especially with using a Xacto for cutting a part on a lathe.

Randy

Tim Wescott · « **Reply #39 on:** October 08, 2012, 08:41:06 PM »

Quote from: Steve Helmick on October 08, 2012, 06:48:41 PM

For most of us, and Dave specifically, being a neubie to machine tools...do yourself a huge favor and buy Delrin rod, and don't bother with aluminum. They may tell you that Acetal is the Generic and it's the same, but it is NOT, so do not settle. Tell them that your customer wants real Delrin, and wants it black. It looks cool, and that may also mean that it's really Delrin. You can buy a 10' length of 5/8" (I find this is a good size for OS and Magnum engines) for about $10. Stay away from Nylons, and especially UHMW...nasty crap, too soft to cut without a deadly sharp cutter. Delrin can be filed and sanded, if you have to, but like all plastics, when it gets warm, it grows, so let the part cool before you measure and test for fit!

Um, does this mean I have to stop using strips cut off of old plastic cutting boards and turned round?

Chris Wilson · « **Reply #40 on:** October 08, 2012, 09:17:08 PM »

Quote from: Steve Helmick on October 08, 2012, 06:48:41 PM

They may tell you that Acetal is the Generic and it's the same, but it is NOT, so do not settle.

Delrin = Acetal Homopolymer

Tim Wescott · « **Reply #41 on:** October 08, 2012, 09:41:19 PM »

Quote from: Chris Wilson on October 08, 2012, 09:17:08 PM

Delrin = Acetal Homopolymer

Ford = car.
Yugo = car.

And yet, they aren't the same.

RandySmith · « **Reply #42 on:** October 08, 2012, 10:07:50 PM »

Delrin is just a DuPont trade name for the acetal rods ,blocks ,strips ,etc that they sell and make, there are at least 8 other Companies that manufacture and sell this product, they all work very well and are all a thermoplastic known as acetal, or Polyoxymethylene (POM) or polyacetal .
Great stuff, I have machined thousands of feet of the stuff..
Note this is NOT nylon, or any other similar looking material. Nylon does NOT machine well, and is not as stable.

Regards
Randy

Chris Wilson · « **Reply #43 on:** October 08, 2012, 10:39:14 PM »

Quote from: Tim Wescott on October 08, 2012, 09:41:19 PM

Ford = car.
Yugo = car.

And yet, they aren't the same.

Tim, that was less than helpful.

http://www2.dupont.com/Plastics/en_US/Products/Delrin/Delrin.html

Delrin is in fact Acetal Homopolymer, they are the same, please check the above site.

If its not then please explain what it is.

Steve Helmick · « **Reply #44 on:** October 09, 2012, 12:44:04 PM »

I have bought and machined both Delrin and Acetal, and found the real Delrin machined much better, didn't get as hot so didn't swell as much, and ended up with a better finish. Engineers say it's the same, but a machinist will tell you it is not. Acetal isn't bad stuff, it's just not as good as the original, IMHO.

Steve

Tim Wescott · « **Reply #45 on:** October 09, 2012, 01:07:16 PM »

Quote from: Chris Wilson on October 08, 2012, 10:39:14 PM

Tim, that was less than helpful.

http://www2.dupont.com/Plastics/en_US/Products/Delrin/Delrin.html

Delrin is in fact Acetal Homopolymer, they are the same, please check the above site.

If its not then please explain what it is.

I was just backing up what Steve said.

My dad's company made parts from fiberglass-reinforced polyester resin. By your logic, fiberglass mat = fiberglass mat, and polyester = polyester, right?

Nope: if the manufacturer isn't doing their job, then fiberglass mat that is nominally some weight (say 1oz/sq foot -- I can't remember what was standard) can vary tremendously from one side of a roll to the other, and from one spot along the roll to another. We saw as much as 2:1 variation. Furthermore, (unlike, apparently, DuPont), there was never one manufacturer that was consistently the best: we'd test the mat periodically to make sure that the quality was always up to snuff.

Ditto resin: not only are there huge variations in the promised specifications of polyester resin, but, just like mat, the manufacturer's ability to hold to their own specifications would vary. You can get polyester resin that will permanently deform when it gets over 150 degrees F (not C!!), or you can get polyester resin that has to be raised to 450 degrees F (and is smelling and smoking) before it'll hold a new shape.

Ditto cars, in a much better known way.

Just because two things share a generic name doesn't mean that they're the same thing. So when Steve claims that one manufacturer's acetal is better than anyone else's, I'll believe it until I'm convinced otherwise.

Randy Cuberly · « **Reply #46 on:** October 09, 2012, 01:59:39 PM »

Quote from: Steve Helmick on October 09, 2012, 12:44:04 PM

I have bought and machined both Delrin and Acetal, and found the real Delrin machined much better, didn't get as hot so didn't swell as much, and ended up with a better finish. Engineers say it's the same, but a machinist will tell you it is not. Acetal isn't bad stuff, it's just not as good as the original, IMHO. Steve

The difference is process quality control and Dupont does it very well...others maybe less so.
For the above reason at Raytheon we specified "Dupont Delrin" in addition to the Mil-Spec for Delrin. Trust me there is a difference with most vendors.

Randy Cuberly

Lauri Malila · « **Reply #47 on:** January 11, 2013, 04:20:17 AM »

Quote from: RandySmith on October 08, 2012, 08:29:13 PM

...her with the fuel delivery holes dead in the middle , maybe cut a slot in the venturi to link the two holes with the feedhole the fuel comes from, so , when you look inside you would see only 2 sprinkler holes on one side about 60 degrees or so apart.
You could still use the 1/32 inch tubes for the sprinkler tube feed. Or if you just use drilled feedholes I would use smaller ones by about 80% of the size you have now...

Hi Randy.

Like this?
Also, I can rotate the venturi +/- 30 degrees. The idea came from the "Banjo"-type venturi (like in Irvine´s), it doesn not work symmetrically when engine is mounted horizontally. Now I maybe can play with the symmetry a little. Maybe.

Lauri

Jim Kraft · « **Reply #48 on:** January 11, 2013, 08:18:43 AM »

Maybe that is why most of the old ignition engines had straight tubes for intakes. On gasoline if they had real high suction the needle would have to be closed so far that even the smallest particle would plug the spray bar. Or maybe they just did not know any better back then. I usually just wedge a wooden plug between the spary bar and the venturi on the back side to restrict it for running glo fuel. Seems to work OK.

Phil Bare · « **Reply #49 on:** January 11, 2013, 11:49:59 AM »

Quote from: Jim Kraft on January 11, 2013, 08:18:43 AM

Maybe that is why most of the old ignition engines had straight tubes for intakes. On gasoline if they had real high suction the needle would have to be closed so far that even the smallest particle would plug the spray bar. Or maybe they just did not know any better back then. I usually just wedge a wooden plug between the spary bar and the venturi on the back side to restrict it for running glo fuel. Seems to work OK.

Hi Jim, just an observation, the basic definition of a venturi is a tube with a reduction of cross sectional area. It would seem to me that when a spray bar is introduced across a straight tube, we have a reduction of cross sectional area and thus, a venturi.

Howard Rush · « **Reply #50 on:** January 11, 2013, 02:13:41 PM »

I set out to make some actual venturis awhile back that maximized pressure recovery. I used a MEMS pressure sensor and took measurements. It's at http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=98273&mesg_id=98273&listing_type=search . A picture is missing.. It's below.

I think I concluded that stunt "venturis" are just restrictors that let the crankcase vacuum suck fuel. Restriction is no problem for normal stunt engines. The main objective of the stunt venturi is probably to atomize fuel and get a steady run. Listen to Randy and Brett Buck about how to do that. The guy who has done the best experiments is Frank Williams. I hope he'll discuss them here.

Chris Wilson · « **Reply #51 on:** January 14, 2013, 05:21:16 PM »

I consider the entire model engine intake assembly (before the crankshaft) to simply be a fuel/air metering device that has very little atomization capabilities. (And the more you try and chase atomization capabilities the more inconsistent it becomes in a changing stunt environment.)

Witness the fact that one can run an engine on a burst just by priming it with straight fuel, no droplets, no atomization, just a small solid volume of fuel as most of the atomization occurs much later in the exit of the transfer ports - e.g. small ports give high speed and far better flow and atomization just like miniature low pressure fuel injectors.

Thus one should look at how well any stunt venturi system will give a consistent metering of sheared droplets and not bother even considering how fine the resultant spray is going to be.

Lauri Malila · « **Reply #52 on:** January 15, 2013, 02:06:11 AM »

At least our engine is quite sensitive to such things, propably because of low operating temperature. So far I've managed to find clear differences in the way the engine runs with different venturi types.
But I must admit that there is a bigger problem somewhere that makes the engine so sensitive.
It has given me an exellent possibility for interesting experiments, I propably wouldn't notice the differences if the engine was in better thermal balance. L

Brett Buck · « **Reply #53 on:** January 15, 2013, 05:56:46 AM »

Quote from: Chris Wilson on January 14, 2013, 05:21:16 PM

I consider the entire model engine intake assembly (before the crankshaft) to simply be a fuel/air metering device that has very little atomization capabilities. (And the more you try and chase atomization capabilities the more inconsistent it becomes in a changing stunt environment.)

Witness the fact that one can run an engine on a burst just by priming it with straight fuel, no droplets, no atomization, just a small solid volume of fuel as most of the atomization occurs much later in the exit of the transfer ports - e.g. small ports give high speed and far better flow and atomization just like miniature low pressure fuel injectors.

Thus one should look at how well any stunt venturi system will give a consistent metering of sheared droplets and not bother even considering how fine the resultant spray is going to be.

   Atomization is absolutely critical to reliable stunt runs. David and I have experimented a lot with both the fuel feed and the air intake over the past 10 or so years, following on to Frank Williams' experiments with spigot venturis. Contrary to the laminar flow or pressure recovery theories, the most reliable and consistent runs come from venturis with the most turbulent flow, and even something like the panty hose diffuser/filter can make a marked difference in the run quality. The first example was the astonishing improvement in the inside/outside speed variation by switching to a spigot venturi just exactly like Frank's article on the topic. If we hadn't solved that I would still be running 40VFs.

   I am pretty convinced that despite the fact that it might flow better on a bench, the very last thing you want is anything like streamlined flow and you certainly do not want the charge flow attaching to the walls and just dribbling into the engine. Sergei Belko's "top intake" engine is intriguing because it allows you to use the entire nose of the airplane as an airbox or plenum. I think that's why the original ST sprinkler venturi worked better than a lot of the later "flush inlet" types, even if you drill out the seat where the sprinkler holes are. There are a few sharp edges right upstream of the holes that certainly break up any notions of laminar flow.

There's a few long threads on this topic in the archives on SSW, but we have gotten smarter since then. And I am sure our mavens for rigor will be down on me for the rampant speculation and anecdotal evidence shown here.

   The other thing that appears to make a huge difference is how the fuel supply path works, and the viscosity and flow characteristics of the fuel itself. The flow rate is pretty slow ON AVERAGE, but it's not constant so the the peak flow rates can apparently be pretty high. This we learned from both Brian Eather's observations on fuel viscosity and the early problems with 4-strokes on overheads. One of the keys, with a remarkable improvement, was switching (at Brad Walker's suggestion at the 2002 NATs, probably originally from Bill Wilson and RC guys) to a clunk tank. That led to looking at what was going on with fuel flow.

   Some of what found on the fuel path is still semi-secret, sorry about that, because I didn't invent it and it has not been published. I am sure that there will be 5 people claiming that they know what we are doing, they originally invented it, and we're stupid, but that's the way it goes sometimes. For the most part, reduce the flow restriction to the extent possible even if it seems pointless and works OK the way it is now. But don't make the spigot itself too big, anything downstream of the needle needs to be small enough to stay wetted all the way across the tube. If it's too big after the fuel is metered, it's possible to get the fuel flowing down one side and air coming in the other side. That's probably OK if it stayed that way all the time and never changed, but it will.

   Brett

Lauri Malila · « **Reply #54 on:** January 15, 2013, 07:44:06 AM »

Brett,

Thank you. Other than pantyhose, I would be interested in other ways to make the flo turbulent. I don't like the panties as the venturi is quite well hidden behind the engine, And I use a syringe with needle for priming.
Should I just make steps inside the venturi before spigot? Or should the inside diameter increase more aggressively right after the spigot/fuel hole. The last option is the standart in team racing engines which are also very sensitive to these issues. L

Avaiojet · « **Reply #55 on:** January 15, 2013, 12:47:35 PM »

Quote from: Lauri Malila on January 15, 2013, 07:44:06 AM

Brett,

Thank you. Other than pantyhose, I would be interested in other ways to make the flo turbulent. I don't like the panties as the venturi is quite well hidden behind the engine, And I use a syringe with needle for priming.
Should I just make steps inside the venturi before spigot? Or should the inside diameter increase more aggressively right after the spigot/fuel hole. The last option is the standart in team racing engines which are also very sensitive to these issues. L

What effect does density altitude have on electrics?

Charles

Brett Buck · « **Reply #56 on:** January 15, 2013, 01:17:31 PM »

Quote from: Lauri Malila on January 15, 2013, 07:44:06 AM

Thank you. Other than pantyhose, I would be interested in other ways to make the flo turbulent. I don't like the panties as the venturi is quite well hidden behind the engine, And I use a syringe with needle for priming.

Use a metal screen instead of a nylon screen, that way you won't get an unsightly run in your venturi.

Brett

Phillip Dunlap · « **Reply #57 on:** January 15, 2013, 01:58:16 PM »

I use a corner rounding tool in the tail stock to obtain a round entry on the venturi. The tool could be mounted in the tool post allowing feed in two directions to blend into the diameter. A 15 degree reamer chucked up in the tail stock can be used to obtain taper. Finally, a tip from John McCollum, an end mill mounted in a tool post makes a very smooth cutting boring bar. I hope I am not repeating something already covered.
Phil

Jim Kraft · « **Reply #58 on:** January 15, 2013, 03:16:57 PM »

That is true Brett, but sometimes you can fix them with a little finger nail polish.

phil c · « **Reply #59 on:** January 16, 2013, 07:31:38 PM »

Another semi on topic tale of the value of knowing what you are working with and why different items that are the "same"(acetal homopolymer) may not work the same.

My brother worked at 3M in the tapes division. They made lots of electrical insulating tape. He mainly did troubleshooting, and got called in because the rolls of tape were blocking(the adhesive was sticking to the next layer) and pulling the adhesive off when the roll was unwound. Not so good. They spent 2 months going over their processing and materials, the line conditions, everything. It turned out that the manufacturer of an antiblocking additive that they put into the plastic tape had changed suppliers for one of the ingredients. The new ingredient was chemically the same, but had a solvent in it from a different supplier. The solvent ended up in the ppm range in the tape, but the change was enough to keep the main antiblocking agent from getting properly lined up on the surface of the tape to do its job of keeping the adhesive from sticking too hard.

So I'm pretty sure that if a machinist says Delrin works better than generic acetal he's speaking the truth. There are about 50 variables between polyoxymethylene and finished, extruded rod. All of the polymerization variables are critical, and hard to control, and can have major effects on the finished product. Things like friction, glass temperature, deformation at various temperatures, how well the internal lubricants are dispersed and dissolved, nearly ad infinitum.