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VENTURI SIZE vs ENGINE SPEED

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VENTURI SIZE vs ENGINE SPEED

Old 08-04-2005, 08:13 AM
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2BFlying
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Default VENTURI SIZE vs ENGINE SPEED

Hello all

Got a question here. I've been making some venturis out of Delrin and they are working OK.

Will a smaller bore size slow the engine down as far as RPM?

I started with a .304 (metric 7.74mm) and have it on the engine and running. It sounds good but maybe a little too much RPM. I ran almost a tank of fuel thru it and wondered what a smaller venturi size would do.

It only takes a few minutes to turn out another and I can change my bore size easily, the outside dimensions stay the same.

Any suggestions on this?
Old 08-04-2005, 09:50 AM
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Jim Thomerson
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Default RE: VENTURI SIZE vs ENGINE SPEED

Up to a point, the larger the hole, the faster the engine will run, the more fuel it will use, and the less fuel suction it will have. With a large enough hole you will need to run pressure of some kind to get fuel feed. Once you get to the size where the venturi is not the choke point, making it bigger will not help.

Jim
Old 08-04-2005, 10:08 AM
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Default RE: VENTURI SIZE vs ENGINE SPEED

ORIGINAL: 2BFlying
Will a smaller bore size slow the engine down as far as RPM?
That's a bit of a tricky question to answer because it depends on the type of engine and what it's to be used for as well as the size prop it's driving amongst other things.

Too large a venturi (for whatever load the engine is driving) just means the airflow through it will be slow which gives low fuel draw. A slightly smaller venturi will speed up the airflow without really restricting the volume of air so the revs would stay much the same. Go smaller still and although the speed of the airflow may keep rising it'll begin to restrict the volume and then max revs will start to drop.

My G51 has a .282" venturi and a .156" needle valve blocking most of it and I run it at 8000 on the ground in a 4 stroke with a 12x5 prop. It's got a few thousand revs still in hand but I certainly don't want it going there. If I used a smaller venturi to limit the max revs then it'd likely be in a 2 stroke at the 8000 ground revs and that would make a mess of how it runs in the air.

So venturi size really depends on what you want from the engine.
Old 08-04-2005, 10:46 AM
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Default RE: VENTURI SIZE vs ENGINE SPEED

thanks for the reply

I can turn out venturis about every 2 minutes on the lathe, my program will make all of them the same and all I have to do is change the bore size. I have a starting point and will work down from there.

I'm sure I don't want to go larger and I have the G-51 needle valve which is about the .156 you described.
thanks
Old 08-04-2005, 08:34 PM
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Default RE: VENTURI SIZE vs ENGINE SPEED

Leonard Neumann over at SSW gives a ratio as a starting point for a good venturi for a stunt engine. This should be in the range of .045-.052 and is calculated by the effective area of the venturi (taking into account the area of the spray bar) in square inches divided by the volume of the engine in cubic inches. A simple spreadsheet would be an easy way to figure out the venturi bore for these ratios. This only gets you close so you'd need to actually fly with them and see how the engine reacts.
Old 08-07-2005, 12:06 PM
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Default RE: VENTURI SIZE vs ENGINE SPEED

Hello Jim

i made another venturi with a bore size of .273 and put in on my TT .42 GP. The mounting is a little different but it fits and runs a lot better than the larger bore venturi that I had made at first. Im running a 10-5 APC prop on the engine. I tried an 11-4 but it seemed boggy when I started loops and wingovers. I think this little engine likes RPM. Here's a pic of what I have.
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Old 09-01-2005, 11:36 AM
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Default RE: VENTURI SIZE vs ENGINE SPEED

Well guys

I have two venturis that I am going to try this weekend. One is a .273 I.D. with a .155 spraybar and the other is a .255 I.D. with a smaller needle valve assembly, about a .125 diameter.

My effective area should be close to the same on both since the smaller nva has less area and the bore is smaller on this setup.

Weather and work have kept me from flying for a while but this weekend looks great around here, no work and really nice weather.

I'll post results after the weekend.
Old 09-02-2005, 08:43 AM
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Default RE: VENTURI SIZE vs ENGINE SPEED

Hey, RW, long time. You can use the venturi, muffler outlet size, and prop to control the run. Most of these motors have plenty of raw HP. The issue is controlling the power and preventing runaway. If it wants to run away or you have fuel draw problems, you may need a little smaller venturi. Why don't you stick with one needle valve assembly and just dial the venturi size? It's the total intake area that matters most.

You want the engine to run at or near its peak torque RPM. That way it can't wind up (speed up in RPM) through consecutive loops. Some of the these Schneurle engines won't slow back down once they wind up, so you do three loops, it winds up, and you spend the rest of the flight trying to keep up with 4 second laps. You control the load on the engine with prop diameter and the airplane speed and lap time with prop pitch and line length.

So you want to shoot for the following: About 5.0-5.2 seconds per lap. As little windup as possible in the loops. The engine running in a slighty rich 2-stroke needle setting in level flight, breaking up slightly in RPM in maneuvers, and going back to the level flight RPM in between. Line tension overhead will sometimes improve with a larger diameter prop. If you're running a 10" prop, but it gets a little loose overhead, try increasing diameter in 1/4 " increments. Your TT 42 ought to pull an 11-4 fairly well, though, or even an APC 11.5-4. What props have you been using?

Also. a lot of the OS clone engines like the TT and Magnum like muffler pressure. I run pressure on everyting that absolutely won't handle it.

Bottom line is a consistent, well-controlled run. It's not an exact science.

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