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Author Topic: Is my .281" venturi too large for stunt?  (Read 3823 times)

Offline frank mccune

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Is my .281" venturi too large for stunt?
« on: August 04, 2015, 06:23:45 PM »
    Hi:

     I am planning to power a Mustunt II and Green Box Nobler with an older O.S. .40 Stunt engine circa late 60's-mid 70's, with a venturi that has a bore of .281".  The nva is the standard O.S. unit for this engine.  Will this be o.k. or do I need to have a venturi with a smaller bore ?  I also have ST nva for this engine.

     Thoughts?

                                                                                                                              Tia,

                                                                                                                              Frank

Offline Air Ministry .

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Re: Is my .281" venturi too large for stunt?
« Reply #1 on: August 04, 2015, 07:57:57 PM »
I will check the bore of my 4oS , the 4o FSRS comes with two similar venturies of differant hole sizes .

More Nitro keeps the fires lit , if its a bit craky on that one , or a bigger prop will load it up more ,
I think I used a 10 x 4 three blade on my 40S in my 50 Oz Yak , and tried 10 x 6 & 11 x 5 .

If the Intakes the STOCK one it should be o.k .



If the Noblers under 45 Oz , the 35 or 30'd do the trick , and are lighter .

This Sucker used a Max 40 S ,



though if theres wind and youve got strong lines , go ahead . But theres NO POINT in adding tailweight , a shorter nose might be needed ,
and 43 Oz is a good weight for a NOBLER . a OS Max 30 S will fly one fine on 60s on a 10 x 6 or 10 x 5 .

The 40 S is here , http://sceptreflight.net/Model%20Engine%20Tests/Index.html



says 7.5 mm , 3.9 mm spraybar , 15.8 mm area . used 15.2 on my 25FSR . ! to fly 47 Oz Nobler Size .
Fitted a big max .20 one to choke down the Max 30 in a Gieske Nobler , 43 Oz , back in 76 .
« Last Edit: August 04, 2015, 08:24:32 PM by Matt Spencer »

Offline RandySmith

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Re: Is my .281" venturi too large for stunt?
« Reply #2 on: August 04, 2015, 07:59:48 PM »
If it is an older OS NVA  with a diameter of .157 that is  OK, if it is a newer one that is a 20 size and around 130  diameter  it is way too large  and you should drop down to a .255

Randy

Offline Brian Hampton

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Re: Is my .281" venturi too large for stunt?
« Reply #3 on: August 04, 2015, 08:20:36 PM »
The H40S had a 7.5mm (.295") venturi and 3.9mm (.153") spray bar.

Offline Steve Helmick

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Re: Is my .281" venturi too large for stunt?
« Reply #4 on: August 04, 2015, 11:41:48 PM »
Don't forget that you can put a couple layers of pantyhose material over the venturi (hold it on with a neoprene O-ring, small wire-tie, etc). Keeping dirt, bugs and rocks from going down the venturi is a huge bonus, but it also reduces the effective venturi size and prevents fuel splattering out of the venturi as the engine runs.  y1 Steve
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Offline frank mccune

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Re: Is my .281" venturi too large for stunt?
« Reply #5 on: August 05, 2015, 05:40:10 AM »
         Thanks for the replies Guys!

         This engine appears to be a good running engine that is very easy to start and runs very well. Another "find" in my junk drawer. Lol

                                                                                         "Keep em Flying"

                                                                                          Frank

Offline Brett Buck

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Re: Is my .281" venturi too large for stunt?
« Reply #6 on: August 05, 2015, 09:53:42 AM »
The H40S had a 7.5mm (.295") venturi and 3.9mm (.153") spray bar.

   *Probably* too big, but maybe OK with sufficiently low pitch or reduced diameter. i would certainly have something around a .270-.275 ready to install.

     Brett

Online Dennis Toth

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Re: Is my .281" venturi too large for stunt?
« Reply #7 on: August 17, 2015, 12:20:20 PM »
Frank,
One trick you can use to get just a tad bit better fuel draw is to add an intake filter with a green element. This adds a little choke action and can do wonders if the needle is erratic. Some place I remember a guy making plastic discs the same diameter as the filter element with different size holes and putting them in under the filter element to add more choke without changing the venture out. Some guys have put two elements in the holder to do the same thing.

I used a single green filter on the intake of my OS 25 (Older baffle style) to smooth out the run as it was very weather sensitive. It worked very well and the engine now holds the setting no matter where I set it and has lots of power.

Best,     DennisT

Offline Tim Wescott

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Re: Is my .281" venturi too large for stunt?
« Reply #8 on: August 17, 2015, 12:24:29 PM »
Don't forget that you can put a couple layers of pantyhose material over the venturi (hold it on with a neoprene O-ring, small wire-tie, etc). Keeping dirt, bugs and rocks from going down the venturi is a huge bonus, but it also reduces the effective venturi size and prevents fuel splattering out of the venturi as the engine runs.  y1 Steve

This works very well.  Steve and Mike Haverly (another Seattle pilot) kept at me until I started doing this on my engines, and I'm glad that they did.  Not only will it tone an engine down nicely, but it's very easy to adjust the run.
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Scott B. Riese

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Re: Is my .281" venturi too large for stunt?
« Reply #9 on: September 01, 2015, 11:37:33 AM »
This works very well.  Steve and Mike Haverly (another Seattle pilot) kept at me until I started doing this on my engines, and I'm glad that they did.  Not only will it tone an engine down nicely, but it's very easy to adjust the run.

Gee TIM..we here in PORTLAND do the same thing  >:D LL~
Scott Riese
Portland, Oregon
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Offline Tim Wescott

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Re: Is my .281" venturi too large for stunt?
« Reply #10 on: September 01, 2015, 12:08:49 PM »
Gee TIM..we here in PORTLAND do the same thing  >:D LL~

Well, yes, but apparently you're too polite to say so, unlike those pushy folks up north.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Brett Buck

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Re: Is my .281" venturi too large for stunt?
« Reply #11 on: September 01, 2015, 12:36:26 PM »
This works very well.  Steve and Mike Haverly (another Seattle pilot) kept at me until I started doing this on my engines, and I'm glad that they did.  Not only will it tone an engine down nicely, but it's very easy to adjust the run.

   Richard Oliver clued me in on that one, and I am possibly/probably passed it on. Note that it works even if you make the venturi larger enough to have the same effective flow as without it, even a single layer. I am pretty well convinced that this is a function of acting as a diffuser and breaking up anything like steady streamlines of air going in, making it much less sensitive to maneuvering. Stacking them probably adds no benefit from the diffusing action, but does have the effect you expect a restriction to have. i.e. more fuel draw and less overall flow.

    Brett

Offline Tim Wescott

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Re: Is my .281" venturi too large for stunt?
« Reply #12 on: September 01, 2015, 12:50:07 PM »
I am pretty well convinced that this is a function of acting as a diffuser and breaking up anything like steady streamlines of air going in, making it much less sensitive to maneuvering.

I should try this.  I haven't noticed a difference on my one-layer stacks, but I haven't been looking.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Scott B. Riese

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Re: Is my .281" venturi too large for stunt?
« Reply #13 on: September 03, 2015, 01:37:04 PM »
I should try this.  I haven't noticed a difference on my one-layer stacks, but I haven't been looking.

I 've tried ..."nude" "natural" and "black"....the white itched to much.......... 010! **)
Scott Riese
Portland, Oregon
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Offline Ken Burdick

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Re: Is my .281" venturi too large for stunt?
« Reply #14 on: September 03, 2015, 02:22:36 PM »
you been hanging around Leo too much

Kennyb and the Bod Busters LL~


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