Why? (Fox needle valve)

[BillLee]

Asking for a friend! 

Over the years I have seen many references to the old Fox needle valve that has a tapered flat side instead of needle-shaped point. Most references were of the type "Get rid of it.".

Question: why?

Since the purpose of the NV is to change the cross-sectional area of the fuel passage as the needle is screwed  in our out, it seems that either form factor would work equally well. Yet the "conventional wisdom"/"rule of thumb"/"old wives tale"/"stunt physics" says otherwise. Why?

[bill bischoff]

Perhaps it is the flat needle in conjunction with the two hole spray bar. When I used to fly Fox-Goldberg racing (long ago), more than once I ran into the situation where once I found the sweet spot on the needle, a couple of clicks either way made a drastic change, but full turns either way made small changes. I presumed this to be about the position of the flat relative to the feed holes in the spray bar. Or not.....


And as long as you're asking for a friend, I'll be your friend. 

[frank williams]

Fox was no dummy, its shape is really not that bad and does have some real advantages.

The semi-circular cross section shape at the very tip transforms into a full circular shape as you go up the "needle"to the threads.  In either case, when inserted into the circular hole of the orifice, it is fully supported and won't wiggle or vibrate.  The beloved sharp needle is a cantilevered beam that will vibrate if the collet is not tightened down properly.  You can sometimes see the wear on the needle slightly back from the point where it vibrates against the orifice hole.  This will effect a stunt run. 

The problem with the Fox configuration is not necessarily the needle but the spraybar.  The location of the "dump hole" in the spraybar is such that an on off effect might be seen by the flow.  added ... The proximity of the orifice to the dump hole and the flat needle can cause a nonlinear effect,

Another advantage of the Fox needle is that its wedge shape gives a finer tuning.  In other words as the needle is turned in or out the amount of fuel restriction is half that of a full pointy needle.  One full turn of needle on a Fox shape results in only half the fuel restriction of a pointy needle.  This is a fun math problem to work out and recommend verifying my assertion.

[Dan McEntee]

Asking for a friend! 

Over the years I have seen many references to the old Fox needle valve that has a tapered flat side instead of needle-shaped point. Most references were of the type "Get rid of it.".

Question: why?

Since the purpose of the NV is to change the cross-sectional area of the fuel passage as the needle is screwed  in our out, it seems that either form factor would work equally well. Yet the "conventional wisdom"/"rule of thumb"/"old wives tale"/"stunt physics" says otherwise. Why?

    The old original needle valve has it's issues, the biggest I think is that there wasn't very good quality control in their production and they were very inconsistent in how they worked. I have had some that worked just fine. The biggest issue is air leaking through the threads causing inconsistent runs. The cure there is the old short piece of fuel line over the threads trick. Next came inconsistency in the depth of the seat in the spray bar. Most times this isn't drilled deep enough and you have to turn the needle out until it almost falls out before the setting is rich enough to start and run. I had always thought that the flat on the needle was the main issue here but in another discussion about this, Brett Buck pointed out the depth of the seat. That made sense. I have successfully filed the flat spot to make it longer and in a sense "shorter" to let more fuel flow and that worked also. Just a touch with the correct number drill against the seat to remove a few thousandths works also.  One of the first cures for the flat spot on the needle was to chuck the needle up in your power drill and use a file to change the profile to a tapered point. This worked also, and in later years Fox changed to a long, tapered point and eliminated the tapered flat spot. The last thing was the two hole spray bar that confused people. In a drawing on some old Fox instructions I have seen it was drawn that the holes should be about 120 degrees apart. That would put both holes on the same side of the spray bar and works effectively, but that was when they had skilled machinists making parts. Most examples just have a hole drill straight through the spray bar and they are more or less 180 degrees apart. It still works for the most part, but people with the thing in their mind that they should not be able to see the holes when looking down the throat of the venturi are bothered by it/. That is pretty much my take on the Great Fox Needle Valve Assembly Mystery!

[Brett Buck]

The problem with the Fox configuration is not necessarily the needle but the spraybar.  The location of the "dump hole" in the spraybar is such that an on off effect might be seen by the flow.  added ... The proximity of the orifice to the dump hole and the flat needle can cause a nonlinear effect,

   Bingo!  I had had the same difficulties as everyone else , and had gotten as far as realizing that the way the flat was clocked WRT the holes when it was otherwise set to give the right amount of fuel made a difference - more fuel coming out one of the two holes than the other made it run radically different, and change dramatically with even one click.

  40ish years later, someone posted here that the "seat" of the spraybar was right next to the holes, and all you needed to do to fix it was drill the seat a little bit deeper- presumably to let fuel flow normally for a bit before it got to the holes. I grabbed the Skyray, put the Fox back on it, went out and flew it at the park, and darned if it didn't needle perfectly well and very smoothly.

    The later, tapered Fox needle tried to address the "flat" concern, not by the simple fix of drilling it slightly deeper, but by tapering it - removing all the support from the needle. If you use that needle in the old brass spraybar, it will pound it to bits in a few flights. Apparently Fox realized this and started making steel spraybars with an extended section of thread on the "hex" end to brute force hold it.

     Unfortunately, all of them seem to be tapered, but lopsided (the center for the taper grinding is not the center for the thread cutting and other turning), creating a smaller version of the original problem!  And no two are similarly lopsided, so while I haven't run mine enough to tell, I would be dollars to donuts that it ALSO now matters which needle you have and which way the lopsided taper falls when it is set right. 

    Brett



p.s. Congratulations to Glen Peterson
https://stunthanger.com/smf/engine-set-up-tips/needle-for-fox-35-engine/msg552302/#msg552302

[Dennis Leonhardi]

About a hundred years ago, we filed the tapered needles down a bit so there wasn't as much needle exposed to the elements (terra firma).

Randy's needle assemblies are the real deal, but it's amazing how many awesome flights were put up and how much hardware was won with those "junk" Fox 35 Stunt engines.  We just didn't know any better ... "Ya dance with the girl what brung ya."

 

[Brett Buck]

About a hundred years ago, we filed the tapered needles down a bit so there wasn't as much needle exposed to the elements (terra firma).

Randy's needle assemblies are the real deal, but it's amazing how many awesome flights were put up and how much hardware was won with those "junk" Fox 35 Stunt engines.  We just didn't know any better ... "Ya dance with the girl what brung ya."

   If you somehow get it working, it performed much better than most everything else at the time, for about 10 years. The problem was that you frequently never knew what you did to get it to work, and if you changed something - anything - it might not work at all. Since you never knew what you did in the first place, you couldn't reliably replicate it. Leave it alone, and it will keep working like it is, change one thing even slightly, and all bets are off.

    Other engines, like the 20FP, runs right out of the box very reliably, you an take it apart and put it back together, it will run the same as it did, if you make a change it will have predictable results, and you can make the same change on another engine, and that, too, will have a predictable effect. I have never once had a case where if you slightly tightened a head screw, it never ran right again, and stuff like that happened all the time with Fox 35s.

    Brett

[Sean McEntee]

It's for all of these reasons that folks swap out the Fox NVA for other, more reliable rigs, such as an OS or ST-style NVA.  That was one of several changes that Larry Foster made to his reworked Fox's, back when he was mod'ing motors.  I've used such ST-clone NVAs on all of my Fox's with great success.  However, like the the stock configuration, airtightness cannot be overlooked when diagnosing erratic runs.  Most common leaks occur between the spraybar and venturi.  A couple nylon washers are a good fix.

[Ken Culbertson]

The problem was that you frequently never knew what you did to get it to work....

I know exactly what I did.  I attached an OS35s to the OS needle valve from the Fox.  Worked great.

ken

[Dennis Leonhardi]

   If you somehow get it working, it performed much better than most everything else at the time, for about 10 years. The problem was that you frequently never knew what you did to get it to work, and if you changed something - anything - it might not work at all. Since you never knew what you did in the first place, you couldn't reliably replicate it. Leave it alone, and it will keep working like it is, change one thing even slightly, and all bets are off.

    Brett

That may have been your experience Brett, and I'm not particularly defending Fox or the 35 Stunt engine.  You were fortunate to start when there were better choices and may not remember cutting printed parts from crappy wood, for example, but some damn good builders did it and turned out masterpieces.

I started flying control line nearly 70 years ago and remember many modelers who never seemed to struggle with so many seemingly minor issues.  Meaning no offense, I've seen relatively few flyers over the years who could consistently get 1-flip starts and launch with very-near-optimum needle settings.  I can't draw a straight line or sing anything in tune, so that's not a personal criticism, just fact.

Isn't the OS LA Series a group of engines "detuned" for neophytes?

I ran Fox Stunt engines for years and years - heck, even won combat contests with them! - so some of us apparently had it figured out and obtained consistency.  And, believe me, I was just a klutz compared to the really good fliers!

Some of the best (and most consistent) stunt flights I ever watched were flown by Bob Gieseke many years ago - was he running an OS .25FP?  (Just asking for a friend ...)

If my mentor in the '50s needed more than 1 flip to start his Fox 35 or launched with less than a nearly-perfect setting, we knew he was having a very, very bad day.  Then again, we were out flying for hours and hours whenever possible and spent no time typing letters on a screen ...

Maybe sometimes the best "engineering" is a lot of trial and error.

Dennis

PS: I'll never forget the day my mentor asked, "You're really serious about combat, aren't you?  OK, bring your (Sterling Mustang with a Fox 35) to school Saturday morning, and at least a gallon of fuel."

[Brett Buck]

Isn't the OS LA Series a group of engines "detuned" for neophytes?

   It is a very inexpensive RC sport engine. It is relatively low output - but what are you going to do with 2 HP at 18,000 RPM in a stunt plane? Without a pipe to selectively "Detune" it, you couldn't us a 46VF, either,

      Brett

[John Park]

I don't know anything about Fox engines except what I've read - and there's one thing I've read about Fox NVAs that doesn't seem to have been mentioned in this thread.  It's on page 164 of Ron Moulton's Control Line Manual, and refers to the problem of 'only getting the desired peak rev. setting at one point in each 360 degree turn of the needle' as a result of a bent needle.  There's a picture of a tapered (not chamfered) Fox needle with a thin T-shaped projection at the tip, as wide as the ID of the spraybar.  This was intended to locate the needle centrally in sll circumstances.  Dies anybody remember this, and did it work?

[dale gleason]

The first "spade" Fox needle I ever saw was on the first run Combat Foxes. about 1956. Worked well.

dg

[Dave_Trible]

Learn something new every day.  I never understood the 'why' of the spade needle.  Now I do.  It wasn't just to clean fingernails.  Weren't they made only for the combat engines?  My 36Xs have them but not the Stunt 35s.

Dave

[Dan McEntee]

I don't know anything about Fox engines except what I've read - and there's one thing I've read about Fox NVAs that doesn't seem to have been mentioned in this thread.  It's on page 164 of Ron Moulton's Control Line Manual, and refers to the problem of 'only getting the desired peak rev. setting at one point in each 360 degree turn of the needle' as a result of a bent needle.  There's a picture of a tapered (not chamfered) Fox needle with a thin T-shaped projection at the tip, as wide as the ID of the spraybar.  This was intended to locate the needle centrally in sll circumstances.  Dies anybody remember this, and did it work?

    As Dave and Dale have mentioned, that needle wasn't for the Fox .35 Stunt. I have had several different vintages of new in box Fox.35 Stunt engines that didn't have these and many more that I have no doubt that the needles they came with were original and they where the typical Fox.35 needle. I don't think the spade ended needle is even the same size and will not fit into a standard spray bar, and have to wonder if a complete NVA for one of these would even fit or work in a .35 Stunt?

    Type at you later,
     Dan McEntee

[Ken Culbertson]

    As Dave and Dale have mentioned, that needle wasn't for the Fox .35 Stunt. I have had several different vintages of new in box Fox.35 Stunt engines that didn't have these and many more that I have no doubt that the needles they came with were original and they where the typical Fox.35 needle. I don't think the spade ended needle is even the same size and will not fit into a standard spray bar, and have to wonder if a complete NVA for one of these would even fit or work in a .35 Stunt?

    Type at you later,
     Dan McEntee

I think Dan and Dale are right.  I ran the Stunt 35 and the 36x before switching to the OS. The flat was on the 36.  Lots of Fox bashing but keep in mind how much better they were than most everything else "back in the day".  Some of us were fortunate enough to get one of Duke's "private stock" or have one "blueprinted".  Engines just weren't very reliable like they are now.  The special Foxes were.  I don't think I ever lost the engine starting points with it, probably why they eliminated them!

Ken

[Brett Buck]

Learn something new every day.  I never understood the 'why' of the spade needle.  Now I do.  It wasn't just to clean fingernails.  Weren't they made only for the combat engines?  My 36Xs have them but not the Stunt 35s.

    Frank's point is that the "flat taper" needle accomplishes the same thing for the 35, and was much easier to manufacture. Unfortunately, they never correctly diagnosed the problem it caused, so as delivered it didn't work very well.

      Brett

[Steve Lotz]

The stunt 35's run very well with the "spade" needle valve assembly. The thread is different and you can't use that needle in a standard stunt spray bar. Back in the day, a lot of people used the K&B needle assembly in a stunt 35 with good results.

[Dick Pacini]

And nobody ever mentions the needle in the Fox 15...

[kevin king]

I was surprised that the needle setting for my fox 35 was 11 turns out from the closed position. My guess is I might have 2 different fox 35 parts in the NVA.

[Brett Buck]

And nobody ever mentions the needle in the Fox 15...

    I beg to differ, one of the appendices of the Rules of The Hurl has the results of the CFD model and wind tunnel testing of the Fox 15 needle valve/spring, for purposes of defining a average and standard deviation of the Cd. This is the basis of the Hurl Technical Committee certification process for Engines To Be Hurled as defined in Rule 6.

   Thanks for your continued interest in the international sporting phenomenon known as The Hurl!

       Brett

[Dan McEntee]

I was surprised that the needle setting for my fox 35 was 11 turns out from the closed position. My guess is I might have 2 different fox 35 parts in the NVA.

     That is one of the things I pointed out in my post and what Brett posted the drawing of. The position and placement of the seat in relation to the hole is at issue there. I had never thought to drill the seat in further and had always just filed the flat tapered spot on the needle back, in effect making it shorter and allowing more fuel to pass. Some where on here is a video of me flying Mike Gretz's Fierce Arrow that is powered by a older Fox.35 that is bone stock. When I first tried to run the engine I could not get the engine to richen up to a decent setting before the needle fell out. I used the file on my pocket tool I always carry tp dress the needle back like I described and added the ubiquitous piece of fuel line to the threads on the needle and it runs a sweet 4-2 break now. You can hear it really well on the video. If yours takes 11 turns out to run you are about a turn and a half from falling out. Drill out the seat in the spray bar as per Brett's post and you should be good, or if you can't do that try filing the flat taper back.
 
      Type at you later,
       Dan McEntee

[kevin king]

Thanks Dan.

[Ara Dedekian]

 
, .... and all you needed to do to fix it was drill the seat a little bit deeper....


     I use the Fox remote needle valve on my bladder combat planes. Under pressure there was limited adjustment, in that you had to screw the needle in until it nearly touched the valve body for a somewhat proper setting. The fix was to remove the needle from the holder and drill the seat ~1/16" deeper which moved the needle further in (and I thought of that all by myself!). This mod resulted in a full range of adjustment, even with the coarse thread. I think MECOA still sells them. Never thought to apply that fix to the Stunt 35. Twelve turns out to start and and a bunch of turns in to run was always so bewildering. Now I know; Thanks!, to all the folks above.

    Ara

[Christopher Root]

That MECOA "remote" NVA is my absolute favorite!  Although I think it is out of production,

C R

[Brett Buck]

That MECOA "remote" NVA is my absolute favorite!  Although I think it is out of production,

C R

  You can make something similar out of many RC carbs. The core of the OS carburetor is an assembly with a metering block like the MECOA unit, and OS used to sell it separately as part of a "remote needle kit" before they started putting rear needles as standard. Just unscrew it from the throttle barrel, mount it on a bracket somewhere, use an old spraybar as the "nozzle", and there you go, remote needle.

    And in fact I have used it on a Fox 35 during the early testing of the Skyray (and before Frank discovered the fix to the burp). It worked fine, unfortunately, and (in hindsight) obviously, it didn't fix the burp.

       Brett