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Author Topic: Strange results from venturi size change  (Read 1442 times)

Offline Kim Mortimore

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Strange results from venturi size change
« on: April 18, 2012, 06:43:16 PM »

I have a Smoothie (not ARF) with an LA40 that was running about 5:30 on 4 oz of 10% fuel, which was ok when I was flying it in Old Time, but now I would like to extend the run time for obvious reasons.  Before I do the surgery needed to install a larger tank, I thought I would try a smaller venturi to see how much effect it would have on run time.  Plenty of power to spare.

So I switched the .282 stock venturi for a .255.  Sure enough, it extended the run time, but that wasn't the only effect.  It changed the run from a very steady wet 2 with a brief leanout at the end to the kind of run you get with a conventionally vented tank (the tank is uniflow)---gradually leaner as the flight progresses until the thing is screaming for the last couple of laps.  I checked the fuel lines, filter, tank for leaks, etc.  Tried again, same result.  Switched from muffler pressure to open vent.  No cigar.  So I switched to a .272.  Run time is now about 6:15, still with some gradual leanout (too much, about .3 sec/lap from beginning to end).  The degree of leanout appears to be a function of venturi diameter.   ???   I will go back to the .282 and install a larger tank.

Changing venturi size is a common trim technique as we know.  I have never heard of anyone else having this problem.  Any thoughts? Thanks.
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Kim Mortimore
Santa Clara, CA

Offline Chris Wilson

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Re: Strange results from venturi size change
« Reply #1 on: April 18, 2012, 07:47:11 PM »
Kim, with the change of venturi insert are you eclipsing any of the jet?

Or indeed shifting its position or rotation of it along the way?

I believe that rotation especially (as in having the jet face upwards) can give this exact effect.
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MAAA AUS 73427

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Offline RandySmith

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Re: Strange results from venturi size change
« Reply #2 on: April 18, 2012, 08:01:07 PM »
This could be telling you that heat could be the problem, You are asking the motor to run longer on less oil going thru the engine per minute.
You may could try to up the oil content, and add pitch to the prop so you can back off the needle just a tiny bit. See if that makes the engine go toward the more steady run.
but .3 tenths from start to finish isn't to bad, many like to have the engine running 2 to 3 tenths faster at the end.

All in all though ,I think you have the best solution add some capacity, That is most always better, I use 4.5 ounces to do a pattern with a FOX 35, 4.5 ounces for a FP or LA 40 is about right

Randy
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Offline Chris Wilson

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Re: Strange results from venturi size change
« Reply #3 on: April 18, 2012, 08:55:33 PM »
Randy, just trying to resolve that logic in my thick head.

Sure the engine is being told to run longer with less oil per minute but it is also with less heat producing combustibles per minute, less air per minute and has more minutes in which to cool.

I just can't see that slightly throttling down an engine that uses a know working fuel mix could cause it overheat since the percentage of oil is constant regardless of consumption.

You got me mate!
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MAAA AUS 73427

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Offline RandySmith

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Re: Strange results from venturi size change
« Reply #4 on: April 18, 2012, 09:13:06 PM »
Quote from: Chris Wilson on April 18, 2012, 08:55:33 PM
Randy, just trying to resolve that logic in my thick head.

Sure the engine is being told to run longer with less oil per minute but it is also with less heat producing combustibles per minute, less air per minute and has more minutes in which to cool.

I just can't see that slightly throttling down an engine that uses a know working fuel mix could cause it overheat since the percentage of oil is constant regardless of consumption.

You got me mate!

Hi Chris

Not really, most every time you put in a smaller venturie the motor will run leaner, leaner is hotter, so it has ..more.. heat not less. Also if your going the same speed with the same prop you have the same combustion s per minute..it is just a leaner mixture which produces more heat. That is why I suggested a increase in pitch so you could make it a tad more rich on the mixture.
Also going from a 282 to a 255 size venturie is  not a slight change, that is a  HUGE change is venturie size.

Just more thoughts for you

Randy
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Offline proparc

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Re: Strange results from venturi size change
« Reply #5 on: April 19, 2012, 09:40:43 AM »
Quote from: Kim Mortimore on April 18, 2012, 06:43:16 PM

So I switched the .282 stock venturi for a .255.     So I switched to a .272.  Run time is now about 6:15, still with some gradual leanout (too much, about .3 sec/lap from beginning to end). 
 

Too big a change bro. Normally you change in .010 increments. Depending on where you started .280-.270-260-250 etc. More oil bro! One thing I have seen in later years, is that our guys seem to be influenced by the RC boys and run these motors TOO HARD!

RC guys tend to treat their motors like a junkyard guard dog. We need to treat our motors like an AKC puppy home for the first time. Do like Randy say's, (he has a Fox 35 on pipe LOL) and you’ll be happy.
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Milton "Proparc" Graham

Offline phil c

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Re: Strange results from venturi size change
« Reply #6 on: April 19, 2012, 01:36:47 PM »
What size prop?  The Smoothie is a small, light plane.  I use an LA 40 in a Brodak Original Smoothie with a 10/4 Top Flite or Zinger prop.  1/4 in. venturi with an OS NVA.  It four cycles as about 10,300 rpm. for a full tank(about 3.75 oz.).  Similar fuel, 10% nitro, 11/11 castor/synthetic oil.
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phil Cartier

Offline Kim Mortimore

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Re: Strange results from venturi size change
« Reply #7 on: April 20, 2012, 10:34:06 AM »

Randy,
Thanks for the info.  I had never thought in terms of different fuel/air mix for different venturis when engine/prop/RPM are the same.  Hmmm, interesting.  Over time I think it's easy to starting taking SH for granted and forget what an opportunity it is to conveniently (for both sides) get advice from people with your level of experience and expertise.  Very cool.

I will test props this weekend with the .272 before switching back to the .282.  BTW, the .255 comes from the FP20.  I know about quite a few guys using this venturi in the LA46 without any problem, usually with an extra head shim, which I take it would reduce any tendency to overheat, along with the compression.  And of course the 46 is not the 40.  I just now remembered a friend who had an LA40 that used to overheat with the stock venturi, normal prop (don't remember which), also in a Smoothie, just by coincidence. 

Phil,
I wish my Smoothie were a light plane!  46 oz.  Betcha either swapped out some of the kit wood or built it from plans.  If you're getting a full pattern out of 3.75 oz., then you're in a different time zone from me.  The prop is a 10 x ~3.75, 3-bladed MA. 

Randy,
Phil has been posting on the subject of running smaller diameter props, same pitch, higher RPM than is typical among the tribe, including an LA46 with 10x4 four-stroking at 12K rpm in the pinned LA46 setup topic at the top of this engine forum.  Any comments on this approach pro or con?  (I'm definitely not trying to start anything here other than constructive info exchange.  I don't recall ever seeing either of y'all two go postal and squeeze off a burst.)  LL~ LL~ LL~

Thanks a bunch,
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Kim Mortimore
Santa Clara, CA

Offline RandySmith

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Re: Strange results from venturi size change
« Reply #8 on: April 20, 2012, 11:03:38 AM »
Hi Kim

First off if I were to do this I would not have changed what you did, You were using 10% nitro, and needed a few more laps to finish the pattern, I would have dropped to either 7.5 or 5% nitro fuel, that will give you more time. You may or may not have to go to a little more pitch on the prop, that is one way to do this easy.
Another way people do this is to use a lower pitch prop and run higher RPM. Some run a 11 x 4 TF prop at about 10,800 to 11,00 RPMs in a wet 2 stroke.
Others add a head shim, this adds time to your run also.
If none of the above works, I would then ,at this point switch to a "slightly" smaller venturie, like the 272 or 275. This will also give extra run time without choking down the engine too much.

I will point out that the solution I would have used is just to add more fuel capacity, this would make the above a moot point and the problem would have gone away.

So you have many things to try if you want and see how these work for you

Randy
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Offline RandySmith

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Re: Strange results from venturi size change
« Reply #9 on: April 20, 2012, 11:17:14 AM »
"Randy,
Phil has been posting on the subject of running smaller diameter props, same pitch, higher RPM than is typical among the tribe, including an LA46 with 10x4 four-stroking at 12K rpm in the pinned LA46 setup topic at the top of this engine forum.  Any comments on this approach pro or con?  (I'm definitely not trying to start anything here other than constructive info exchange.  I don't recall ever seeing either of y'all two go postal and squeeze off a burst.)  Layingdown Layingdown Layingdown

Thanks a bunch,"



Hi Kim

That is a setup you can try and see how it works in your plane and if the fuel burn is OK, be careful to not get the needle setting where you get any sagging at any point in the pattern, you could also use less nitro if needed , You may have plently of power running the High RPM so the lower nitro fuel may work perfect.

Randy
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