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Author Topic: venturi taper  (Read 7283 times)

Offline Bootlegger

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venturi taper
« on: October 09, 2014, 02:14:52 PM »

 What does the taper in the venturi opening have to do with the way the engine preforms,
 is it necessary?
 I have packed some with J B Weld and then drilled them to the size that I wanted, and left them flat, no taper, does this reduce/hurt the performance and how/why?

  Hope that I made myself clear in these questions, and if they are in the wrong place feel free to move them...

  Thanks a lot...
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Offline goozgog

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Re: venturi taper
« Reply #1 on: October 09, 2014, 02:41:43 PM »
Hey Bootlegger,
                      This doesn't really answer your question
but machining the taper in a venturi is easy. What I do is
grind a regular old wood workers spade bit to the taper
I'm trying to match and then feed it into the venturi which
is already drilled to the opening I want. I've done this on
a lathe and on a small drill press. Let me know if you want
to see a picture.

Cheers!
Keith Morgan

Offline Tim Wescott

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Re: venturi taper
« Reply #2 on: October 09, 2014, 03:18:25 PM »
Hey Keith -- are your really 110, or, since you're from Canada, is that your age in kilometers and not miles?

Gil:

There's a lot of dead straight venturis out there, on engines that work just fine.  I suspect that tapering the venturi, or bell-mouthing it a bit, helps, but I certainly don't think it's necessary.

That having been said, when I make a venturi I taper it.  I really do it more for looks than for any other reason.  You can do it Keith's way, or if you have a compound slide on your lathe and a small enough boring tool, you can just set the compound over and bore a tapered hole.
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Offline Andrew Hathaway

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Re: venturi taper
« Reply #3 on: October 09, 2014, 03:52:27 PM »
It seems this topic has come up before, and if I remember correctly, a few of those who would know, stated that a proper venturi should be much more effective and run better.  On the other hand, based on purely subjective experience, I've had venturis with textbook venturi contours, and others that were basically parallel walled restrictors, which all seemed to run fine.  As far as I can tell, in the real world, it doesn't seem to matter too much.  I would imagine if a person were looking for maximum performance or efficiency, it might matter.  

Offline Dick Pacini

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Re: venturi taper
« Reply #4 on: October 09, 2014, 04:43:26 PM »
A center drill makes a nice taper.
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Offline RandySmith

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Re: venturi taper
« Reply #5 on: October 09, 2014, 06:16:57 PM »
What does the taper in the venturi opening have to do with the way the engine preforms,
 is it necessary?
 I have packed some with J B Weld and then drilled them to the size that I wanted, and left them flat, no taper, does this reduce/hurt the performance and how/why?

  Hope that I made myself clear in these questions, and if they are in the wrong place feel free to move them...

  Thanks a lot...

True ventuires with the proper tapers and angles run much better, produce more horsepower, and cycle better, a straight tube is awful when compared to a venturie, now if you are thinking about a restrictor , such as the ones in an OS LA engine, they too are better with the tapers and angles, but nit nearly as much. If you use a straight tube, you normally run a larger diameter  hole to get the same power.
Scott Bair was first I knew of , to run HP and torque curve test on many different stunt engines and induction system.( Peter Chinn also did many of these, but did not test venturies vs ? ) ie things like true venturies vs. restrictors vs. tubes. vs spinklers . Several years later I ran the same type test on many engines and included pipe engines and other hi RPM motors. Same thing happened, true venturies produced the best power.

Randy
« Last Edit: October 09, 2014, 10:44:51 PM by RandySmith »

Offline Chris Wilson

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Re: venturi taper
« Reply #6 on: October 10, 2014, 05:19:26 AM »
Problem is that Giovanni Venturi did not ever consider a pulsed system as found in a two stroke engine wit its constant boundary layer stripping and reattachment.

In other words strict theory will never really describe what empirical testing proves.
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Offline Lauri Malila

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Re: venturi taper
« Reply #7 on: October 10, 2014, 10:17:50 AM »
Please make a simple calculation how much air goes through your engine engine during a 6 minute flight. Then think again the importance of good flow dynamics :)

Lauri

Offline Chris Wilson

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Re: venturi taper
« Reply #8 on: October 11, 2014, 04:13:53 AM »
Hi Chris. I don't think the IC engine had even been invented at the time. But, if it takes only 4 or so degrees of time, then it still happens. 9,400 to 22,000 plus rpms  is as close to a steady flow of air as if it were in fact a steady flow of air. Hi energy will always go towards lower energy. Hi pressure goes towards low pressure. Flat tires, bursting balloons, heat towards less heat, low crank case pressure less than ambient pressure, etc. It works. Simple physics. D>K
Hi Ty,
       not quite sure what you mean by "But, if it takes only 4 or so degrees of time, then it still happens."

But while I am certain that the average flow is forward in a two stroke engine you would have to wonder what a valve does that could be more closed than open in terms of degrees would do to a usually considered constant air flow.

Cheers.
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Offline Lauri Malila

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Re: venturi taper
« Reply #9 on: October 11, 2014, 10:08:46 AM »
Ty,

What do you exactly mean by "just a few degrees"? Crank intake duration?
A good average value for intake duration is 180 degrees, that's a half turn of crank.
That means that the amount of air that goes throught venturi per stroke equals the engine displacement.

L

Offline Chris Wilson

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Re: venturi taper
« Reply #10 on: October 11, 2014, 04:28:31 PM »
Ty,

What do you exactly mean by "just a few degrees"? Crank intake duration?
A good average value for intake duration is 180 degrees, that's a half turn of crank.
That means that the amount of air that goes throught venturi per stroke equals the engine displacement.

L
Hi Lauri,
          this is getting really off topic but are you sure that in a 'dirty' system like a two stroke that it clears out all of the previous volume?
Because that would make the system 100% efficient!

I got no real answers here only the observation that the more you look at a 'simple' engine the more complex it becomes.

Thanks.
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Offline phil c

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Re: venturi taper
« Reply #11 on: October 11, 2014, 05:08:31 PM »
I think I'll go with Scott Bair's article in Stunt News in the mid 1990's(?).  A well-designed venturi with a side spigot fuel feed gave approx. 8% more torque across the board.  Both the shape of the intake opening, and the taper that opens back up to the intake valve have to be correctly shaped.  Air actually flows into the intake from more than 180deg from all sides into the narrowest point.  The taper to the intake valve lets the air recover from the  pressure drop back to full atmospheric pressure for maximum torque.  Fortunately for stunt most planes use wildly over-sized engines so the venturi isn't often the most critical issue in regulating the engine run.  But if you can't or don't want to fit in a bigger engine take a look at the straight-sided intake for a plane that is a little underpowered.
phil Cartier

Offline WLGeorge

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Re: venturi taper
« Reply #12 on: October 11, 2014, 09:31:54 PM »
I have a wee bit of practical experience with venturis but not the engine type we use. My experience was with rather large cooling towers. We replaced a lot of straight sided fan ducts with tapered "Venturi" type.  Improved performance/cooling/ airflow anywhere from 5 to 15 percent.  My vote is taper the intakes. 

Offline Lauri Malila

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Re: venturi taper
« Reply #13 on: October 12, 2014, 04:27:34 AM »
Hi Lauri,
          this is getting really off topic but are you sure that in a 'dirty' system like a two stroke that it clears out all of the previous volume?
Because that would make the system 100% efficient!

I got no real answers here only the observation that the more you look at a 'simple' engine the more complex it becomes.

Thanks.

Chris,

Yes this may go outside the original topic. But when pondering 2-stroke engines, you should not concentrate on single details. You are so correct saying how simple it looks but in reality there is very complex interaction between all those simple details going on when engine is running. The magic word is harmony.
I am quite sure that the amount of air going in is not far from calculated value. Of course there are some losses but not so big as elsewhere in the system.


Offline John Stiles

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Re: venturi taper
« Reply #14 on: October 13, 2014, 02:05:59 AM »
What does the taper in the venturi opening have to do with the way the engine preforms,
 is it necessary?
 I have packed some with J B Weld and then drilled them to the size that I wanted, and left them flat, no taper, does this reduce/hurt the performance and how/why?

  Hope that I made myself clear in these questions, and if they are in the wrong place feel free to move them...

  Thanks a lot...
I always thought the taper was basically to force the fuel and oxygen immulsion. Which helps get a cleaner burn. I remember diddling with an old Triumph M/C which had Amal carbs, with straight pipes. Everyone behind me said I was wasting a lot of fuel. I switched to 55mm mikunis that had a more pronounced taper and stopped the raw gas out the pipes. Not only that, but my economy was better. I'm not sure this drawing is totally correct....but it might help you envision what's going on:
John Stiles             Tulip, Ar.

Offline Scott Richlen

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Re: venturi taper
« Reply #15 on: October 13, 2014, 07:05:37 AM »
So, how could I install a side spigot fuel feed on an LA-46?  I think it would be great to get 8% more torque!

Offline Andrew Tinsley

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Re: venturi taper
« Reply #16 on: October 13, 2014, 09:22:29 AM »
Hm, not sure about the constant flow. When the shaft valve closes you will get a reflected wave formed by the column of air/fuel that is travelling down the venturi and then bouncing back when it hits the closed shaft valve. If you are clever, you can probably tune the inlet tract by altering its length. Once the reflected pulse comes out, then it is followed by an inrush of fresh air, if the shaft valve then opens as this plug of air arrives, you will get a slight supercharging effect. Maybe not much compared to the depression in the engine as the piston rises, but in speed applications maybe worthwhile.
I seem to remember someone experimenting with venturi lengths, maybe for some other reason, or maybe to get a litte extra filling?

Andrew.
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Offline Brian Hampton

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Re: venturi taper
« Reply #17 on: October 13, 2014, 08:31:49 PM »
The taper to the intake valve lets the air recover from the pressure drop back to full atmospheric pressure for maximum torque.
I recall reading somewhere that the taper below the spray bar should be no more than a 7 degree angle (14 degrees included angle). With a larger angle the flow can separate from the walls and cause turbulence which reduces the pressure recovery. Ideally the bottom edge of the venturi should blend in as closely as possible to the casting for the highest pressure recovery. A reasonably large bellmouth entry (similar to the diagram above) gives a smooth flow into the venturi with the least amount of turbulence.

Offline George

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Re: venturi taper
« Reply #18 on: October 13, 2014, 09:39:50 PM »
I seem to remember someone experimenting with venturi lengths, maybe for some other reason, or maybe to get a litte extra filling?

Andrew.
 

One case was in the late fifties when some speed flyers were trying to get a "ram" effect with a tuned intake. For the RPM of that time the intake length needed to be about three feet to work so interest was abandoned.

George
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Offline Chris Wilson

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Re: venturi taper
« Reply #19 on: October 19, 2014, 08:31:29 PM »
Chris,

Yes this may go outside the original topic. But when pondering 2-stroke engines, you should not concentrate on single details. You are so correct saying how simple it looks but in reality there is very complex interaction between all those simple details going on when engine is running. The magic word is harmony.
I am quite sure that the amount of air going in is not far from calculated value. Of course there are some losses but not so big as elsewhere in the system.


Hi Laurie,
           I really can't see that the amount of fluid inducted each stroke will ever equal the swept displacement of the engine.

Sure if you had a Wide Open Throttle with a ram air intake as found on a speed model (suggested above) you might approach it as a theoretical maximum but in stunt use we are far from that with maximum performance way down the list in favour of a more 'draggy' fuel draw.

Are you suggesting that no matter what size venturi (or indeed any gas induction passage) you use that they all will allow an almost 100% fluid draw?

Cheers.
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Offline Douglas Ames

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Re: venturi taper
« Reply #20 on: October 19, 2014, 08:51:59 PM »
Then you have to factor in the airflow disturbance to the venturi's entrance caused by the prop's turbulent slipstream, not to mention the flow around the spraybar.
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Offline Chris Wilson

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Re: venturi taper
« Reply #21 on: October 20, 2014, 02:50:04 PM »
Just be careful about labelling 'spit back's' cause soley from valve closing.

With a low revving 4 stroking Fox the non firing stroke would severely impinge induction.

As would any misfire.
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Offline Steve Helmick

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Re: venturi taper
« Reply #22 on: October 20, 2014, 06:39:49 PM »
So, how could I install a side spigot fuel feed on an LA-46?  I think it would be great to get 8% more torque!

The Tower .40 NV Assy. removed from the throttle and screwed into the side of a suitably fabricated venturi would work nicely. I'm not sure if OS made anything with such a NV/Spraybar setup, but possibly.  D>K Steve
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Online Brett Buck

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Re: venturi taper
« Reply #23 on: October 20, 2014, 08:21:34 PM »
I think I'll go with Scott Bair's article in Stunt News in the mid 1990's(?).  A well-designed venturi with a side spigot fuel feed gave approx. 8% more torque across the board.  Both the shape of the intake opening, and the taper that opens back up to the intake valve have to be correctly shaped.  Air actually flows into the intake from more than 180deg from all sides into the narrowest point. 

  I don't dispute Scott's observations or experiment. But getting more torque is not at all important. Getting it to run consistently in flight is much more important, and I think a "true venturi" or anything that relies on laminar flow is about the worst way to get that. I think you want as turbulent a flow as you can get within reason and you absolutely do not want the fuel injected right along the sidewall.

     Brett

Offline RandySmith

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Re: venturi taper
« Reply #24 on: October 22, 2014, 09:32:59 PM »
 I don't dispute Scott's observations or experiment. But getting more torque is not at all important. Getting it to run consistently in flight is much more important, and I think a "true venturi" or anything that relies on laminar flow is about the worst way to get that. I think you want as turbulent a flow as you can get within reason and you absolutely do not want the fuel injected right along the sidewall.

     Brett

A true venturie, made correctly , NOT drilled out and destroyed, is the best way to get a really good run, and they do not dribble when used properly with the correct angles, they may dribble when they have been drilled straight thru, and ruined.. When used and made right there is a white spray coming out of the venturie going across the middle.
 I ran /tested many more engines after Scott did his findings and used most every induction system you will ever see. The true venturie worked very well. And they work very very well all over the world in 1000s of motors.
 All sorts of STs, OPS, AERO TIGERS, MAGNUMS, PAs, etc.. are running with 1000s of my venturies in them from the over 6000 motors I have done. The OS , FSR clones Fp LAs have side fuel post as this fits those better and it is very hard to make true venturies in these also work very well.
 The side post system I setup for Evos  works well too.
I have personally, as many many others have, ran unmolested True venturies back to back to back against fuel post and plain restrictors and found the best. however like so many things opinions vary.
 I also can tell you for a fact that 100s of ventuires, maybe many hundreds have been ruined  by drilling and not cutting the proper angles back into them, and in most cases they got drilled too large, and the only way to fix that is by sticking a tube from the side  (fuel post), this increases the suction and makes the area smaller, as it needs to be.
There are many others who have done these type experiments and found true venturies work really well.

Randy
« Last Edit: October 23, 2014, 11:39:53 AM by RandySmith »

Offline Chris Wilson

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Re: venturi taper
« Reply #25 on: October 23, 2014, 05:00:36 AM »
Why would a true Venturi rely on laminar flow?

From I have observed about these is that the fluid stream wants to funnel to the centre of the throat where the velocity is highest and pressure the lowest.
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Offline Andrew Tinsley

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Re: venturi taper
« Reply #26 on: October 23, 2014, 01:41:50 PM »
Hi Randy,
Maybe you would like to define the various fuel atomisation systems that you talked off. Many people don't know a spigot venturi from Adam. Nor a true venturi come to that. Just helps to clarify the discussion.

Thanks,

Andrew.
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Offline RC Storick

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Re: venturi taper
« Reply #27 on: October 23, 2014, 05:45:39 PM »
Hi Randy,
Maybe you would like to define the various fuel atomisation systems that you talked off. Many people don't know a spigot venturi from Adam. Nor a true venturi come to that. Just helps to clarify the discussion.

Thanks,

Andrew.

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Offline Chris Wilson

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Re: venturi taper
« Reply #28 on: October 23, 2014, 09:00:43 PM »
Hi Randy,
Maybe you would like to define the various fuel atomisation systems that you talked off. Many people don't know a spigot venturi from Adam. Nor a true venturi come to that. Just helps to clarify the discussion.

Thanks,

Andrew.

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Then go to design, then venturi's.
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Offline RandySmith

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Re: venturi taper
« Reply #29 on: October 24, 2014, 12:17:56 PM »
Hi Randy,
Maybe you would like to define the various fuel atomisation systems that you talked off. Many people don't know a spigot venturi from Adam. Nor a true venturi come to that. Just helps to clarify the discussion.

Thanks,

Andrew.

Hi Andrew

I will post a couple off the site Chris listed then try soon to take pictures of ones I make here, The venturie is posted above, plus anyone who has seen the more than  10,000 I have made know what they look like
Below is Restrictor, like you find in OS and  some ST  FOX engines.  then a  Fuel post venturie, I made these many years ago for OS OPS and some other engines that were hard To make true venturies for. Lauri has also posted some of the ones he makes in the engine forum.
I have also made many sets of venturies that have aluminum bodies and delrin inserts that you can just pull in and out to change size in about 10 seconds, these are also ajustable up and down, so it makes tank ajustments very easy, if your within about an 8th of an inch, they have 2 small o-rings just above and below the feedhole, and seal against the aluminum wall of the base. I modeled these from ones that Scott Bair made so he could adjust the fuel flow in his Stuntfires, which had the tanks permanyly glued in. Then made the different size sets from there, KAZ Minato used to run ones I made for him years ago, and now has a shop in Japan making similar ones for PAs.

Online Brett Buck

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Re: venturi taper
« Reply #30 on: October 24, 2014, 12:46:58 PM »
A true venturie, made correctly , NOT drilled out and destroyed, is the best way to get a really good run, and they do not dribble when used properly with the correct angles, they may dribble when they have been drilled straight thru, and ruined.. When used and made right there is a white spray coming out of the venturie going across the middle.

  I can see where Dave gets his spelling hints...

   All I can say is that this IS NOT consistent with my experience, and when we started running spigot venturis the grotesque inside/outside speed differences we had with the PA40 and 61 went away almost completely. And don't forget that we ran it stock, first, for a long time. We clued in after Billy managed to tame that with, hey, how about that, a spigot venturi, first discovered by us when David found out about it at the WC in Germany.

    The "dribble hole" does indeed "dribble" and fail to atomize sometimes, Dave and I have both seen it happen on stock, out-of-the-box, "trueest of the true" Randy-blessed venturis, on the test stand. A drip forms, builds, then drops off, engine goes "blurp" for a moment. If it makes you feel any better, the original Jett venturi with the .100 diameter dribble hole did it far worse. That's why I asked for a smaller hole, which helped it considerably. The problem was cured forever with a spigot venturi, which curiously happened to fit perfectly in the hole size I requested.

    I think the effect is moderated enough in the midwest and southeast (which definitely does seem to be different) that you have convinced yourself there is no issue. Although I heard several of your buddies with very similar issues, and they also had the tell-tale surging on the ground of the unsealed and leaking  case/spraybar interface. All denied that it happened afterwards, of course.


    Brett

Offline RandySmith

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Re: venturi taper
« Reply #31 on: October 24, 2014, 01:59:37 PM »
 I can see where Dave gets his spelling hints...

   All I can say is that this IS NOT consistent with my experience, and when we started running spigot venturis the grotesque inside/outside speed differences we had with the PA40 and 61 went away almost completely. And don't forget that we ran it stock, first, for a long time. We clued in after Billy managed to tame that with, hey, how about that, a spigot venturi, first discovered by us when David found out about it at the WC in Germany.

    The "dribble hole" does indeed "dribble" and fail to atomize sometimes, Dave and I have both seen it happen on stock, out-of-the-box, "trueest of the true" Randy-blessed venturis, on the test stand. A drip forms, builds, then drops off, engine goes "blurp" for a moment. If it makes you feel any better, the original Jett venturi with the .100 diameter dribble hole did it far worse. That's why I asked for a smaller hole, which helped it considerably. The problem was cured forever with a spigot venturi, which curiously happened to fit perfectly in the hole size I requested.

    I think the effect is moderated enough in the midwest and southeast (which definitely does seem to be different) that you have convinced yourself there is no issue. Although I heard several of your buddies with very similar issues, and they also had the tell-tale surging on the ground of the unsealed and leaking  case/spraybar interface. All denied that it happened afterwards, of course.


    Brett

HI Brett ,I don't doubt you have seen this, that is why I did so many test on my bench, and asked MANY people from all over the US and the World to check if they have it, The last person that checked this for me was Bruce Perry when he was breaking in his latest PAs, He saw nothing at any time except a white high velocity spray going toward the center of the venturie. There was never a surge or a dribble, It is very hard to even see the hole when looking at a running engine, . If you see the hole in the side, and see a dribble build below the hole, then you have a venturie that has been drilled out and has a straight section in the middle, or you have one that was made wrong, or a mistake. No less than 2 dozen people have helped me in researching this.
 I do not take anything that you or David say lightly, or blow it off. Which is why I took that seriously and did so much checking/testing.

As far as the side seal, this is a problem with ANY engine, and most every time I see or hear this is when the spraybar has been removed, put back in with debris between the flat hex of the bar, and the side of the case,This happens when they compress the venturie, force the bar thru, nick off some al. or delrin debris with the threads, and install the nut with the debris still in the thread, it then goes toward the case and ends up being trapped between the nut and the case. I have seen this many dozens of times with many engines, STs, OS, TTs, Aero Tigers, PAs and others.

 I never claimed that does not exist, it does, and makes for inconsistent runs. However I, personally have had ZERO problems with this . I am not sure who you are saying denies that exist, it does, and I have covered that above.

 I have NEVER sealed a bar with any PA engines. others... not so much.  I don't think it is just "my buddies" that have had this happen, It happens to many people, and I have also seen big problems caused by the sloppy application of sealant on many types of engines, especially PA , ST and Thunder Tigers.
by the way Billy won a WC with a non spigot true venturie, I was there and looked, so has other people, plus NATs, so obviously they both work. And there have been many top 5 flyers use and test both, that are using true venturies now. This is really not an issue to fight over, use what you like, its all fine by me, I have no problems with that at all.
You should send one of the dribble holes venturies to me for testing, so I can see what went wrong.
and by the way, problems with fuel post are in play too, like not being able to tell the size, never being able to make the excat same ones, having the brass post come out and go thru the motor (is have seen that happen over a dozen times) and having to send them back to me for a new PS crank or sometimes case. There are other but you  get my point.

I will give one caution here on dribble venturies I have seen, they had the fuel feedhole drilled out much larger to use a fuel post (piece of brass tube, or eyelet) then when it was removed they tried to use it as is, the larger hole (way too big) would NOT  feed correctly and  did  dribble into the engine.

regards
Randy
« Last Edit: October 24, 2014, 03:37:08 PM by RandySmith »

Offline Lauri Malila

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Re: venturi taper
« Reply #32 on: October 24, 2014, 02:55:41 PM »
Hi.

This is the true venturi geometry that Randy suggested me some years ago. It works very well.
At the moment I use 4,3...4,5mm (about 0,17") dia venturi in my .77 engine. Fuel consumption is about 2,8oz/6 1/2 min flight. With higher consumption I'd use slightly bigger spigot diameter. I use 2 radial spigots that are 60 degrees apart.
Also, I'm quite anal about eliminating all possible leaks.

Lauri

Offline Chris Wilson

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Re: venturi taper
« Reply #33 on: November 07, 2014, 01:16:56 AM »
Hi Laurie,
           I really can't see that the amount of fluid inducted each stroke will ever equal the swept displacement of the engine.

Sure if you had a Wide Open Throttle with a ram air intake as found on a speed model (suggested above) you might approach it as a theoretical maximum but in stunt use we are far from that with maximum performance way down the list in favour of a more 'draggy' fuel draw.

Are you suggesting that no matter what size venturi (or indeed any gas induction passage) you use that they all will allow an almost 100% fluid draw?

Cheers.

Just to add a more knowledgeable voice to my own in regards to the above -

A quote form Gorden Cornells book on  "Model Two Stroke Engine Mechanics" concerning primary compression and volumetric efficiencies - 

"With a naturally aspirated system, the maximum volume which can be taken in is limited by the opening and closure of the scavenge ports, and/or by pump pressure falling below atmospheric. This limits total intake volume to around 80% of the swept volume. "

So it appears that about 80% of the engine 'size' is a theoretical maximum for intake volume.
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Offline Motorman

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Re: venturi taper
« Reply #34 on: November 07, 2014, 07:28:31 AM »
"With a naturally aspirated system, the maximum volume which can be taken in is limited by the opening and closure of the scavenge ports, and/or by pump pressure falling below atmospheric. This limits total intake volume to around 80% of the swept volume. "

That's not right. The exhaust leaving the engine creates a depression in the crankcase far below that of pumping pressure. This is why the intake valve opens before the transfer ports close. It's called the cadency effect. Also, on some designs (Fox 35), allot of air comes in the exhaust port before it closes. There is more going on than just swept volume.

MM

Offline Lauri Malila

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Re: venturi taper
« Reply #35 on: November 07, 2014, 01:27:18 PM »
Amount of air/stroke?

That is a subject that regards allmost all the research ever done on two stroke engines. So a complete answer is too much to handle in here.
It depends on so many things, many of which we cannot easily tell.
According to Robbie, it is allways more than half of the capacity, and in a non tuned exhaust situation it can be close to 1 time the displacement (low rpm, effective scavenging, sufficiently high exhaust, efficient venturi).
Modelling gas dynamics, even in seemingly as simple as 2 stroke engine is much more than just the summing up measured  volumes and durations. I think that's what Cornell hasn't got right. His work is based on quite simple and inefficient engines.

Lauri

Offline Chris Wilson

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Re: venturi taper
« Reply #36 on: November 08, 2014, 08:49:55 PM »
More than happy to discuss this further but not inside of a 'Venturi' thread.

Cheers.
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Offline Chris Wilson

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Re: venturi taper
« Reply #37 on: November 20, 2014, 02:44:03 AM »

Just to prod this discussion along further, what happens to those theoretically straight laminar lines when the very close crank opening valves shut for half the operating time?

Dare I venture to say that the those wonderful straight lines become rather turbulent?
MAAA AUS 73427

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Offline Lauri Malila

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Re: venturi taper
« Reply #38 on: November 20, 2014, 03:17:59 PM »
Well, remember that nothing really pushes air through venturi, it is sucked in. So I don't think that much really happens, unless intake opening and closing is way out of harmony.
Stunt engines are quite straightforward with this as usually the volume between spray point and shaft is small. L

Offline Chris Wilson

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Re: venturi taper
« Reply #39 on: November 20, 2014, 10:01:51 PM »
Well, remember that nothing really pushes air through venturi, it is sucked in. So I don't think that much really happens, unless intake opening and closing is way out of harmony.
Stunt engines are quite straightforward with this as usually the volume between spray point and shaft is small. L

With all due respect Laurie I must disagree with the nothing pushes bit.

There is no energy in the most ultimate of low pressures - a vacuum.
Whereas there is great energy to be had from atmospheric pressure.

Every time there is a pressure change it is the high pressure doing the pushing and wants to equalize and expend itself towards the low pressure.

So when the piston moves it merely gives a time lapse before allows the atmosphere can push and refill the space left behind it.

To sum up, there is no such thing as a 'suck' or low pressure, the effect is merely comparisons between areas differing pressure.

Cheers.
MAAA AUS 73427

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Offline Lauri Malila

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Re: venturi taper
« Reply #40 on: November 22, 2014, 02:35:11 PM »
[quote author=Brett Buck:

   All I can say is that this IS NOT consistent with my experience, and when we started running spigot venturis the grotesque inside/outside speed differences we had with the PA40 and 61 went away almost completely. And don't forget that we ran it stock, first, for a long time. We clued in after Billy managed to tame that with, hey, how about that, a spigot venturi, first discovered by us when David found out about it at the WC in Germany.

     ... The "dribble hole" does indeed "dribble" and fail to atomize sometimes, Dave and I have both seen it happen on stock, out-of-the-box, "trueest of the true" Randy-blessed venturis, on the test stand. A drip forms, builds, then drops off, engine goes "blurp" for a moment. If it makes you feel any better, the original Jett venturi with the .100 diameter dribble hole did it far worse. That's why I asked for a smaller hole, which helped it considerably. The problem was cured forever with a spigot venturi, which curiously happened to fit perfectly in the hole size I requested...

    Brett
[/quote]

OK, there has been lots of discussion about this topic about necessity of fuel spigot. But nobody has mentioned how differend running styles we are talking about. And considering that, it is absolutely clear that David's rich run benefits from the fuel post venturi. I may be wrong but I think that Randy himself prefers a leaner and more dynamic setup. I haven't seen his but I'd think it's not far from how Derek's (Barry) PA is running.
When you raise burning efficiency, you increase stability too and details like fuel spigot become less important. L


Offline RandySmith

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Re: venturi taper
« Reply #41 on: November 22, 2014, 08:21:24 PM »
 " OK, there has been lots of discussion about this topic about necessity of fuel spigot. But nobody has mentioned how differend running styles we are talking about. And considering that, it is absolutely clear that David's rich run benefits from the fuel post venturi. I may be wrong but I think that Randy himself prefers a leaner and more dynamic setup. I haven't seen his but I'd think it's not far from how Derek's (Barry) PA is running.
When you raise burning efficiency, you increase stability too and details like fuel spigot become less important. L  "

Hi Lauri

I can run with a solid 4 cycle everywhere if I choose, or a 2 cycle beep at the tops, or a 2 coming on at 10:00 and off at 2:00  if I  choose, all without any of the problems Brett mentioned, Many many many people are doing this with the PA engines with the  stock venturie. There are some having bad run problems with fuel post venturies, others are having very good runs with them. There are several ways to setup engines that work well. People who tell you that "only" a fuel post will work, are wrong, Even restrictors have been made to work very well , I have engines setup with fuel post, true venturies, and restrictors, that all run very well. I perfer the true venturie because of its many advantages , but many others are just as happy with the other setups. I even have people who drill a hole thru the middle of the case and venturie... making a tube restrictor, and will not use anything else.

Randy

Offline Mark Godfrey

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Re: venturi taper
« Reply #42 on: November 28, 2014, 03:39:33 PM »
I haven't read all of the replies to this post but after 40+ years of modelling this is what I have found.
You don't need a physical taper in your venturi all the time.
If you have a straight sided venturi and you stick a needlevalve through the centre of it, you have effectively introduced a taper.
If you have an annulas type setup in a straight sided venturi you will not get fuel flow unless the fuel flow is pressurised.
If you use the straight sided venturi with the needlevalve through the centre the fuel hole SHOULD face down the hole parallel with the venturi walls.
When you run pressure such as bladder or crankcase then taper is not that critical as the fuel is being forced into the venturi and does not rely on the suction caused by the taper to get the fuel in.
Muffler pressure is not as positive as the above two pressure sources and is used mainly to help with a smooth fuel flow.
 
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