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Engine basics => Engine set up tips => Topic started by: Steve Scott on July 05, 2007, 08:01:31 PM

Title: Single vs. dual NVA spraybar holes... Why?
Post by: Steve Scott on July 05, 2007, 08:01:31 PM

Just received some replacement NVAs for our Brodak 40.  I had never really inspected the assembly but my son pancaked his ship last year and the needle got bent.  Straightening it out - it just plain wouldn't run.

I noted the Brodak NVA has 2 spray holes like the Fox 35 used to.  Compared to a similar ST type NVA with it's larger, single hole, is there any real advantage to utilizing dual holes?

And yes, I have the holes aligned East and West.

Title: Re: Single vs. dual NVA spraybar holes... Why?
Post by: Dick Fowler on July 06, 2007, 06:02:13 AM

In theory... the smaller holes will produce smaller drops which are more easily broken up in the flow of air. Not sure with the amount of fuel that we use and the volatility of the methanol that there is any distinct advantage.

Title: Re: Single vs. dual NVA spraybar holes... Why?
Post by: L0U CRANE on July 07, 2007, 02:28:51 AM

...and, to add to Dick's comment...

If they are perfectly located, small holes may be in a better location to find the maximum pressure drop through the venturii. The rate of feed through the jets is very slow - for a Fox Stunt, it is something like 2 minutes per fluid ounce - so a small restriction isn't that much problem; we can compensate with the needle setting...

I thought the standard Fox Stunt NVA was still two holes...

OS and ENYA single-jet spraybars also have very small jet holes, and run VERY well.

Gag Q: How do you keep the holes aligned East-West all the way around a lap??

Title: Re: Single vs. dual NVA spraybar holes... Why?
Post by: Jim Thomerson on July 07, 2007, 05:53:12 AM

I wonder, given two holes, if you orient them off a little so that one has a considerable lower pressure (more suction) than the other, if you actually have a one hole spraybar? 

Title: Re: Single vs. dual NVA spraybar holes... Why?
Post by: L0U CRANE on July 07, 2007, 12:15:08 PM

Hmm...

Interesting thought. I doubt it would work as well as a dual-jet properly oriented, or a single-jet several degrees past visible down the choke...

The 'upper' hole - the one exposed to the incoming air hitting it - meets some ram pressure, which - to whatever slight extent - would seem to disrupt the pressure drop and feed at the downstream jet. On the other side of port closing, the 'upper jet' could be aiming to spritz directly away from the shaft port, with the rebound flow.

Anyone with a high speed motion camera want to get some specifics on this?