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Author Topic: Pantyhose Math  (Read 1725 times)

Offline Kim Mortimore

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Pantyhose Math
« on: January 12, 2011, 03:11:10 PM »

Does anyone know if the percentage reduction in effective venturi size resulting from the use of an air filter made from one layer of nylon stocking material has been calculated (or even can be calculated)?  I'd like to compare the relative effects of layers of this material vs switching to a smaller venturi.  Thanks.
Kim Mortimore
Santa Clara, CA

Offline Russell Shaffer

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Re: Pantyhose Math
« Reply #1 on: January 12, 2011, 06:11:40 PM »
I think it is a "cut and try" thing.  The pantyhose mesh changes size depending on the degree of stretch so it is variable.
Russell Shaffer
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Online Brett Buck

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Re: Pantyhose Math
« Reply #2 on: January 12, 2011, 09:33:01 PM »
Does anyone know if the percentage reduction in effective venturi size resulting from the use of an air filter made from one layer of nylon stocking material has been calculated (or even can be calculated)?  I'd like to compare the relative effects of layers of this material vs switching to a smaller venturi.  Thanks.

    I just to it experimentally. I am primarily using it to generate turbulent flow more than for increasing the fuel draw, and only use one layer.

   Brett

Offline Paul Smith

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Re: Pantyhose Math
« Reply #3 on: January 13, 2011, 07:28:44 PM »
An air filter upstream of the vena concocta will reduce the air velocity and thus the suction.

If you really need filtation, that's one thing.  But if you dispense with the filter and just make the venturi smaller, you will increase suction and get a better run.
Paul Smith

Offline Russell Shaffer

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Re: Pantyhose Math
« Reply #4 on: January 13, 2011, 08:04:38 PM »
Pantyhose does a great job of keeping grass out of the innards. 
Russell Shaffer
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Just North of the California border

Online Brett Buck

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Re: Pantyhose Math
« Reply #5 on: January 13, 2011, 09:45:20 PM »
An air filter upstream of the vena concocta will reduce the air velocity and thus the suction.

If you really need filtation, that's one thing.  But if you dispense with the filter and just make the venturi smaller, you will increase suction and get a better run.

   Perhaps stronger, but not at all necessarily better. I am firmly convinced that you will in most cases do much better with either a flow straightener, a big volume, or some form of diffuser over the intake. On the RO-Jett, with the really stubby intake, the diffuser makes a HUGE difference in the steadiness of the run. Even one layer which is a pretty minimal flow restriction.

    And don't be so sure about the suction, either. It's not just Bernoulli effects. A pretty large fraction of the suction comes from flow restrictions upstream of the intake. If you want to choke the engine, you put your finger over the intake so it draws fuel better - and the velocity of the air passing through the venturi at that point is exactly zero.

     Brett
« Last Edit: January 15, 2011, 02:02:29 PM by Brett Buck »

Offline Paul Smith

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Re: Pantyhose Math
« Reply #6 on: January 14, 2011, 08:49:58 PM »
Of course, when you cut off all the air, it has no choice but to draw fuel. 

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Offline wwwarbird

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Re: Pantyhose Math
« Reply #7 on: January 14, 2011, 09:20:41 PM »
Does anyone know if the percentage reduction in effective venturi size resulting from the use of an air filter made from one layer of nylon stocking material has been calculated (or even can be calculated)?  I'd like to compare the relative effects of layers of this material vs switching to a smaller venturi.  Thanks.

 OK, I would imagine basing anything on any resulting calculation would vary substantially. First, you would have to assume that there is precisely the same amount of "stretch" on each installation of hosiery. Second, I would also be confident that the thread/fiber diameter of various brands of hosiery could also vary. I'm sure it must from Wal-Mart to JC Penney to Macy's etc.

 (...pssst, did someone really ask this question?) ;D
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Online Brett Buck

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Re: Pantyhose Math
« Reply #8 on: January 14, 2011, 09:36:42 PM »
Of course, when you cut off all the air, it has no choice but to draw fuel. 



   The same issue arises with a partial restriction. IF you get everything exactly right, Bernoulli effect may create a situation where you get more fuel draw for the same flow, or more flow for the same fuel draw. That will possibly get you more power - possibly, not certainly, because what happens downstream of the inlet matters, A LOT in some cases. The right conditions appear to be very difficult to achieve and probably rarely if ever occur in real life. A huge fraction of the fuel draw is do to simple flow restriction. I think Frank Williams has some interesting information on the topic.

   But even assuming you do get the conditions just right for more power, they aren't likely to stay that way as you maneuver around nor does more power = better. Power is a non-issue in stunt, we are flying airplanes with piped 75s that we used to fly with ST46s. What is far more important is consistent response in flight. That's where I have found various schemes to ensure consistent flow with different loading and different input air directions to be critical. The diffuser on the inlet had tremendous beneficial effects on the run consistency far out of proportion to any possible restriction effect. And it worked far better with a large inlet and a diffuser than a tiny inlet and no diffuser. My current #5 venturi with a spigot and diffuser (single layer of pantyhose) clearly draws less my initial #12 venturi with a spigot, but it runs much more consistently.

     Brett

Online Brett Buck

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Re: Pantyhose Math
« Reply #9 on: January 14, 2011, 09:43:56 PM »
OK, I would imagine basing anything on any resulting calculation would vary substantially. First, you would have to assume that there is precisely the same amount of "stretch" on each installation of hosiery. Second, I would also be confident that the thread/fiber diameter of various brands of hosiery could also vary. I'm sure it must from Wal-Mart to JC Penney to Macy's etc.

 (...pssst, did someone really ask this question?) ;D

     Its a sensible question and people are definitely making fine adjustments in their "venturi size" by stacking on varying layers of pantyhose. So it's relevant, I just don't think anyone knows the answer. I think in my case the change in draw is negligible, but I only use one layer of "nurse grade" i.e heavy-duty pantyhose.  And I also seriously doubt that the basic effect is the same as making the venturi smaller. It had a marked effect on the run consistency but it didn't seem to change the needle or the needle sensitivity, or fuel usage.

     Brett

Online Brett Buck

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Re: Pantyhose Math
« Reply #10 on: January 15, 2011, 01:08:18 PM »
If the area of the crotch is used, you get better breathing as it is less dense  (more open weave) at that point.

   You just wanted to say "crotch", didn't you?

   Brett

   

Offline Rafael Gonzalez

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Re: Pantyhose Math
« Reply #11 on: January 17, 2011, 07:06:30 AM »
I think it would depend whether the modeler uses black pantyhose or stockings. Not much difference on "net" nylons...

(Just a little humor)

:-)

Offline john e. holliday

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Re: Pantyhose Math
« Reply #12 on: January 17, 2011, 11:08:48 AM »
Accordig to what Richard Oliver stated some time ago.  It takes experimentation with the nylon hose you use.  When the wife wonders why there are a hundred holes cut in her panty hose you can explain what it is for.  Best to use panty hose that has been discarded.  The O rings to hold the nylon peices in place is available at your local auto parts store.  Take your engine so you can get the right size.  Remember to keep notes as some days it may require more nylon peices than other days.   H^^
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Offline Chris Wilson

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Re: Pantyhose Math
« Reply #13 on: January 17, 2011, 05:07:45 PM »
This has as much relevance as a lot of replies here so -

Team Race fliers often use a layer of hose for two other reasons, it catches fuel sputter thus returning it back to the engine (and range is paramount with these events) and it can make starting easier as the fuel soaked mesh vaporizes easier.

I see the addition of a flow screen as creating a more chaotic system downstream of it, and chaos is harder to influence with in flight changes.
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Offline Kim Mortimore

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Re: Pantyhose Math
« Reply #14 on: January 18, 2011, 11:13:34 AM »

   The same issue arises with a partial restriction. IF you get everything exactly right, Bernoulli effect may create a situation where you get more fuel draw for the same flow, or more flow for the same fuel draw. That will possibly get you more power - possibly, not certainly, because what happens downstream of the inlet matters, A LOT in some cases. The right conditions appear to be very difficult to achieve and probably rarely if ever occur in real life. A huge fraction of the fuel draw is do to simple flow restriction. I think Frank Williams has some interesting information on the topic.

   But even assuming you do get the conditions just right for more power, they aren't likely to stay that way as you maneuver around nor does more power = better. Power is a non-issue in stunt, we are flying airplanes with piped 75s that we used to fly with ST46s. What is far more important is consistent response in flight. That's where I have found various schemes to ensure consistent flow with different loading and different input air directions to be critical. The diffuser on the inlet had tremendous beneficial effects on the run consistency far out of proportion to any possible restriction effect. And it worked far better with a large inlet and a diffuser than a tiny inlet and no diffuser. My current #5 venturi with a spigot and diffuser (single layer of pantyhose) clearly draws less my initial #12 venturi with a spigot, but it runs much more consistently.

     Brett

This information proved to be extremely helpful to me.  I tried a nylon filter/diffuser on a somewhat skittish Magnum 36 yesterday and the difference was like night and day--much smoother, more consistent run.  I was amazed at such a large result from a seemingly small change.

This was in cold, heavy fog conditions.  It will be interesting to see if warmer, drier conditions make a difference.

Thanks, Brett.
Kim Mortimore
Santa Clara, CA

Online Brett Buck

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Re: Pantyhose Math
« Reply #15 on: January 18, 2011, 11:50:21 AM »
This information proved to be extremely helpful to me.  I tried a nylon filter/diffuser on a somewhat skittish Magnum 36 yesterday and the difference was like night and day--much smoother, more consistent run.  I was amazed at such a large result from a seemingly small change.

This was in cold, heavy fog conditions.  It will be interesting to see if warmer, drier conditions make a difference.

Thanks, Brett.

  You are quite welcome. It won't always be a dramatic change but I haven't seen a case where it hurt anything. And the space required, unlike a conventional air filter, is negligible. To give credit here credit is due, Richard Oliver suggested it to me in early 2003.

     Brett
« Last Edit: January 18, 2011, 06:07:43 PM by Brett Buck »


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