Author Topic: It did NOT work for me (Read 1953 times)

frank mccune · « **on:** June 01, 2014, 05:46:11 PM »

Hi All:

Today I attempted to start a Fox .35 Stunt that had the spray hole turned 90 degrees to the air flow. This was an O.S. nva. The engine would start easily but would NOT draw any fuel until the spray hole was turned to be at the rear of the the air flow that passed over the spraybar.

I read somewhere that when the spray hole was turned at 90 degrees to the air flow, this wouuld be the place for maximum vacuum to be formed. I had always placed the hole in the spraybar at the bottom of the spraybar. This has worked for me for a very long time. Most drag on a bullet is at the end opposite of the dircection of travel. Not on the side! Why would a spraybar be any different?

Perhaps I misread the information. Anybody else find this to be disinformation!

Perhaps the article had been placed on 1 April.

Any thoughts or comments re this idea?

Stay well.

Frank

BillLee · « **Reply #1 on:** June 01, 2014, 05:55:58 PM »

Certainly NOT "disinformation"! It has been shown to work by any number of tests, but also that hitting the "sweet spot" by eye is damned near impossible. For you: point the hole down and go fly. Or put a 2-hole Fox needle valve back in the engine.

Dennis Toth · « **Reply #2 on:** June 02, 2014, 05:37:06 PM »

Frank,
This does work, the reason is that you have a venturi and when the spraybar passes through it further reduces the area causing the air velocity to increase at the pinch point with the corresponding decrease in pressure. With your bullet the sides are open and the back being flat causes a vacuum to form at the rear.

I like to put the hole facing rearward to the back of the venturi, but either way works. First mark the position of the hole in the solid nut side of the spraybar. You want to position it just below the line of sight. The trick is to make sure you are looking straight down the venturi, start with the hole (assuming a single hole spraybar) pointing straight up so you can see it clearly (have a good strong light shining helps).

Snug the nut so there is some drag on it. Now slowly rotate the solid nut in the direction you want to position the hole (front or back) and when the hole just slips out of sight note the position of the mark you made on the solid nut and tight the spraybar down. Then recheck that the hole is in the same position. Make sure you have the threads of the NVA sealed (I like to put a piece of silicon fuel tubing over the needle head and onto the threads). Check that with the needle set to the approximate run position it will draw fuel when you choke it and you should be good to go.

Best, DennisT

frank mccune · « **Reply #3 on:** June 03, 2014, 08:54:41 AM »

Hi Dennis:

Thank you for the reply.

The Fox/O.S. nva would only burn off the prime until I rotated the hole in the spray bar to be at bottom of the spraybar. A friend off mine noticed that the hole was on the side of the spraybar and suggested that I turn the spraybar until the hole is at the bottom. After if did this the Fox started and ran on the next flip.

I did use all of the methods that you had suggested regarding installation of a nva. I have been doing it for years and it has always worked very well.

Perhaps the bullet, round ball etc. and a spraybar was not the best anaology to use in this case. It may be comparing aples to oranges. In pointed bullets, peoople have attempetd to lessen the drag by using a boat tail design. The navy tried to use shell that had a hole the full length of the projectile to lesson the drag caused by the base of the bullet. This did not work. I think that boat tail and hollow shells only work at subsonic velocities. Above this, there is just to large of a shock wave in front of the projectile.

After studying race car dynamics, it appears that most of the drag is created by the front and rear of the car. I can not remember reading anything about reducing drag caused by the side of the cars. Again I may be confusing different discilplines of aerodynamics.

In any case, thanks again for taking the time to give me some information.

Stay well my friend,

Frank

Dennis Toth · « **Reply #4 on:** June 04, 2014, 04:31:57 PM »

Frank,
It is interesting that rotating the spraybar and boom the engine starts. Did you try the next experiment which would be to now rotate it back to the side and try it? Sounds like something was blocking the fuel flow and when you move it the obstruction was removed. I know its a pain to reset it once you get it working but still would be interesting if you could. I have used it both ways and the stock Fox spraybar has two holes that you position just across the venturi throat just out of sight (the holes are drilled off center so if one puts it in with the wrong side up you will see both hole). All of them have worked as long as fuel flows free from the tank. I have recently had a bad pluggage problem with crud plugging the pickup line. Took several flushes to clean it out, worth checking the tank. Just put some fuel in the tank tip the ship on end and pull it out with your fuel needle (ahead of the filter). Look at the fuel sucked back into the fueler and see if anything is floating. If there is floaters repeat with 1oz shots until its clear.

Best, DennisT

frank mccune · « **Reply #5 on:** June 05, 2014, 04:58:20 PM »

Hi Again Dennis:

Yes, I cleaned all of the fuel system prior to returning the spray hole to the six o clock posistion. The only thing that I can think of was an inadequate flow or vacuum at the side of the spraybar.

When I can get back into the airplane flying mode, I will attempt to run my engines with the spray hole positioned at 3 o clock. I will keep you apprised of what I find.

Say well,

Frank

Richard Logston · « **Reply #6 on:** June 09, 2014, 10:01:16 PM »

This is very interesting as I have a post relating to a O&R23 FRV that has been giving me fits. It has a two hole spray bar installed, so now I'm thinking it has replaced the original by a previous owner. My engine acts like it wants to run but won't do anything except pop with each flip of the prop. Today I soldered one of the holes but it still does the same thing. I have tried different glow plus and fuel to no avail. You can read more on my post here at O&R 23 FRV won't start Richard

Dennis Toth · « **Reply #7 on:** June 10, 2014, 05:33:04 PM »

Richard,
Check the condition of the gasket on the crankcase. If she's leaking base compression it would put fuel in or blow it into the cylinder which results in "no go". Also check the seal of the cylinder sleeve in the case. Check this by putting a few drops of fuel in the exhaust port and rotate the piston through the compression and down. As you come down through the down stroke watch through the exhaust flange at the cylinder and see if there are a lot of bubbles. Some bubbles are normal but a lot means you have no base compression. Heavy castor can sometimes seal it up.

Best, DennisT

Author Topic: It did NOT work for me (Read 1953 times)

frank mccune

It did NOT work for me

BillLee

Re: It did NOT work for me

Dennis Toth

Re: It did NOT work for me

frank mccune

Re: It did NOT work for me

Dennis Toth

Re: It did NOT work for me

frank mccune

Re: It did NOT work for me

Richard Logston

Re: It did NOT work for me

Dennis Toth

Re: It did NOT work for me