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Author Topic: Engine and venturi question..  (Read 1290 times)

Offline jim gilmore

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Engine and venturi question..
« on: July 16, 2009, 05:11:25 PM »
Just to be clear. why do we change venturis in engines. Why would you go smaller and when or why would you go bigger ?
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Dave Adamisin

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Re: Engine and venturi question..
« Reply #1 on: July 17, 2009, 07:35:46 AM »
Roughly:

Larger and smaller id control power, fuel economy, intersection burps (Tiger 60). There are ancelary effects attached to both directions.

Longer and shorter help you find a sweet spot in the airflow resulting from the design of the front of the plane such as large spinners and deep cowls.

The object to the venturi is simple: to duct air into the engine and maintain control of that airflow.

Great question.
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Offline James C. Johnson

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Re: Engine and venturi question..
« Reply #2 on: July 17, 2009, 08:26:31 AM »


Dennis... was this the word you were wanting to use? "ancelary effects".....

adjective
1.    subordinate; subsidiary.
2.    auxiliary; assisting.

Adj.   1.   ancillary - furnishing added support; "an ancillary pump"; "an adjutant discipline to forms of mysticism"; "The mind and emotions are auxiliary to each other"

I THINK THIS IS A COOL WORD !!  Forms of mysticism!!! Awe yes .. the RC engine converted to Stunt.

Jim
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Dave Adamisin

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Re: Engine and venturi question..
« Reply #3 on: July 17, 2009, 08:55:05 AM »
H Jim. My spelling is always suspect. I spent 36 years of my usefull lifee wit engenears. I am of course Dave.  H^^ I only want to be Dennis..... He won't let me.
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Offline James C. Johnson

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Re: Engine and venturi question..
« Reply #4 on: July 17, 2009, 09:47:51 AM »


Dain Bramage from too much castor fumes !! I didn't even look who posted closely... there are too many Adamisins we have Big Art to thank for that !!

JJ
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Dave Adamisin

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Re: Engine and venturi question..
« Reply #5 on: July 17, 2009, 10:32:21 AM »
Quote from: James C. Johnson on July 17, 2009, 09:47:51 AM

Dain Bramage from too much castor fumes !! I didn't even look who posted closely... there are too many Adamisins we have Big Art to thank for that !!

JJ

And the twinkle in Betty's eye.......... mw~
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Offline RandySmith

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Re: Engine and venturi question..
« Reply #6 on: July 17, 2009, 11:05:24 AM »
Quote from: Dave Adamisin on July 17, 2009, 08:55:05 AM
H Jim. My spelling is always suspect. I spent 36 years of my usefull lifee wit engenears. I am of course Dave.  H^^ I only want to be Dennis..... He won't let me.

 LL~ Now that was funny!    All  this free  advice  and  entertainment too!  Great deal and worth evry penny ya  paid for it   ;D
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Offline jim gilmore

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Re: Engine and venturi question..
« Reply #7 on: July 17, 2009, 11:29:55 AM »
Wow, all this entertainment from my question.
So does that mean I could use the venturi size to help control speed some ?
Considering using a .015 rather than an .09 because it's already got c/l setup for it.
Prop size should be anywhere from 6-8 inch for size I want.
Just not looking for a 70 MPH airplane.
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Offline RandySmith

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Re: Engine and venturi question..
« Reply #8 on: July 17, 2009, 01:59:03 PM »
Quote from: jim gilmore on July 17, 2009, 11:29:55 AM
Wow, all this entertainment from my question.
So does that mean I could use the venturi size to help control speed some ?
Considering using a .015 rather than an .09 because it's already got c/l setup for it.
Prop size should be anywhere from 6-8 inch for size I want.
Just not looking for a 70 MPH airplane.

Hi Jim

This is what I suggest doing:
The needle setting controls how you (and the engine )want the engine to run, do not use this as a speed control

prop pitch controls the speed of the airplane, so the plane flies where it is happy


Venturie size controls the power and break of the engine IE, having it deliver power when you want it and in the right amounts. Nitro percent and head shims also are tuning tools for power delivery.

Remember optimum power delivery when you need it in the pattern is what you seek



So in a brief, set the needle where the motor is happy, set the prop pitch where the plane is happy with its speed, use the nitro, venturie size, head shims, etc  where you are happy with the way the motor delivers power at all points in the pattern.

Regards
Randy
« Last Edit: July 17, 2009, 03:12:57 PM by RandySmith »
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Dave Adamisin

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Re: Engine and venturi question..
« Reply #9 on: July 17, 2009, 02:54:26 PM »
Quote from: RandySmith on July 17, 2009, 01:59:03 PM
Hi Jim

This is what I suggest doing:
The needle setting controls how you (and the engine )want the engine to run, do not use this as a speed control

prop pitch controls the speed of the airplane, so the plane flies where it is happy


Venturie size controls the power and break of the engine IE, having it deliver power when you want it and in the right amounts. Nitro percent and head shims also are tuning tools for power delivery.

Remember optimum power delivery when you need it in the pattern is what you seek

Regards
Randy

Hmmmmm all the ancelary stuff showing up.........
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Offline Frank Sheridan

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Re: Engine and venturi question..
« Reply #10 on: July 17, 2009, 03:41:20 PM »
On a similar note - On a typical stunt motor, operating with a given spray bar and needle setting what happens to the running characteristics if one were to enlarge the orifice of the spraybar inside the venturi (where the fuel atomizes) by say, 10% and then an additional 10%? I have had engines with single orifice and double orifice spraybars. Without changing the venturi size and shape, does one type of spraybar atomize the fuel more efficiently than the other with the same needle setting?
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Dave Adamisin

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Re: Engine and venturi question..
« Reply #11 on: July 18, 2009, 10:03:36 AM »
Hi Frank. A question that begs a few more before we can discuss it (I think)

1. By typical set-up do you mean spraybar through the vent in the airstream?
2. If that's true as we open up the holes do we keep the needle taper the same?
3. By atomization do you mean how big the fuel droplets are?
4. How do we measure it?
5. What is/are the deliverables?
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Offline RandySmith

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Re: Engine and venturi question..
« Reply #12 on: July 18, 2009, 10:39:08 AM »
Quote from: Frank Sheridan on July 17, 2009, 03:41:20 PM
On a similar note - On a typical stunt motor, operating with a given spray bar and needle setting what happens to the running characteristics if one were to enlarge the orifice of the spraybar inside the venturi (where the fuel atomizes) by say, 10% and then an additional 10%? I have had engines with single orifice and double orifice spraybars. Without changing the venturi size and shape, does one type of spraybar atomize the fuel more efficiently than the other with the same needle setting?

Hi Frank

Typically the fuel is mostly atomize when it enters the venturie (or in the case of a bar thru the center.a restrictor)instead of inside the spraybar, I have heard of a few people claiming good things by opening or adding more holes in the Spray bar in restrictors. Also heard of opening the orifice hole inside of the bar. I have done this more than a dozen times and have never seen any increase in anything good. As long as the spraybar will flow enough fuel for the engine if you drill the orifice hole larger you just have to turn the needle in more to get the "mixture" back where it needs to be.
More holes in the bar using a restrictor doesn't seem to do anything, except if you have it positioned wrong it will  hinder fuel flow.

Regards
Randy
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Offline Frank Sheridan

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Re: Engine and venturi question..
« Reply #13 on: July 18, 2009, 12:27:07 PM »
In my youth (a few decades ago) I broke a needle valve off in a crash on a Fox 35. When I went to the hobby shop to get a replacement, I was given an assembly that differed from the original, but the only difference that was visible was the number of exit holes on the spray bar. I was flying Flite Streaks at the time and always ran a wet 2 cycle setting. I could tell no change in the run as long as the bar was oriented properly in the venturi, but I always wondered why they were made differently. I even watched a guy run an engine on a test stand with no needle valve assembly by feeding it with an eyedropper... All right ,I admit it.. It was me. Not the smartest thing I've ever done, but I was a lot younger at the time. I guess a suction fed engine is more sensitive to mixture than method of delivery.
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Offline Jim Thomerson

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Re: Engine and venturi question..
« Reply #14 on: July 18, 2009, 01:12:03 PM »
I once had a K&B 4011 which would wind up in maneuvers.  I restricted the venture and solved the problem.  As said above, there are lots of different things one can fiddle with that affect how an engine runs. 
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