Fox 35 Combat Engine

[Steve Dwyer]

I flew my Tudor 50 today with a used Fox 35 Combat I bought on line a while back.  My question is why couldn't I get a 4-2 break. My other standard Fox 35's all give a nice break with or without a uniflow tank. The tank on the Tudor is standard vented and it did as expected lean out toward the end of the run. I also noted a Fox burp when in an overhead 8 but I could not richen the mixture to a 4 stroke without it shutting down. Is this typical of the combat engine? I had a 10x6 APC on it.

The temperature today was a warm 45F and the burnt castor was dark and like syrup, winter's coming.

Steve

[kenneth cook]

            A combat engine is not designed for a 4-2 break. They're designed to run higher rpm's. In addition, the combat versions are not going to be happy on 10" propellers . Not saying that it can't be done but knowing what version you have is most important. You might have too much venturi area as these engines run better with case or bladder pressure. They can be run on suction but are not typically very happy doing so without restricting it which somewhat defeats the purpose. In addition, the combat versions typically ran higher nitro so the plug temp might also be incorrect for your stunt fuel

[Paul Smith]

There are many many engines called Fox 35 Combat.

Having a 4-2-4 break is not all that important.  Many of us can't even do it with a real stunt engine.

I've flown stunt with combat engines a lot and enjoyed it, break or no break.  I would suggest a 9-6 pr 10-5 prop.

There is no guarantee that a old combat engine with unknown history will do anything.

[Dennis Leonhardi]

Adding a head gasket or two might help, but the timing on the combat engine just isn't designed for a 4-2-4 break.  That said, if you can get the airspeed down it might be OK.  As mentioned, a large venturi likewise doesn't make for slow speeds.

Dennis

[Dan McEntee]

  As others have said, you will not get a Fox .35 stunt type run out of the old Fox .35 combat engines because the cylinder liners are different. If yu went to a lower pitch prop and turned the RPM up to get the speed you need, you will get the engine in a happy place where it will run pretty consistent. The first place I read about the flat pitch/high RPM method of running an engine was in an article by Bob Baron where he used a ST.35 combat engine I believe and a 4 inch pitch prop.  As Brett Buck points out quite often,  that is most people's problems with modern engines, trying to run them in the 4-2 mode when they are not designed for it.  Spend some time trying some flatter pitch props and make other adjustments as necessary and you will learn to like the newer engines. The same for this engine you have. Learn to run it like it was designed for but harness the power with the appropriate prop and line length.
   Type at you later,
    Dan McEntee

[Steve Dwyer]

I'm running 5%N/28%C, what's your best recommendation for NV setting? I ran it up lean to near shut off and backed it off 1.5 turns, I'd estimate there is probably a 3 to 5 turn area with no change in RPM before it becomes too rich and starts to die down. Your thoughts?
I will try a smaller lower pitch prop. and this I'd suspect should be done before adjusting the NV?

Steve

[Steve Dwyer]

Another question, the NV is the straight pointed style, I've seen the spade type and the pointed with the flat spot any thoughts if the valve style would have an effect on metering fuel and changing the sensitivity? Anyone know what needle this engine originally had?
Steve

[Dennis Toth]

Steve,
With the combat motor the low pitch/high rpm set up is the way to go (10x4 prop should be a good starting place), you will need to get to around 10.5 - 11K rpm for normal lap time (4.9 ish on 60'). Seems that many of the combat engines when using this approach will do fine. One reason some engines made it in stunt and other didn't was weight. There were/are lots of engines that make more power then the old Fox 35 but few are lighter (maybe the Foster's). The K&B 35 which became king of combat was an oz heavier as was the McCoy and Johnsons. But when we realized that the low pitch/high rpm worked really good that opened the door to explore other engine options, like the combat motors (ala Bob Baron) and the newer higher timed RC engines.

The NVA will not make it 4-2, it is mostly a cylinder timing thing that gets a reliable 4-2. But if you can't get it to run in a four cycle at all you have a fuel blockage someplace in the system. Clean out the fuel lines, filter, NVA. Put in a hot glow plug (Sig or K&B not a Fox). One thing I would recommend even with the 2-2-2 run is to use a uniflow tank with the uniflow vent on the inboard side of the fuse pointing into the free air stream. This gives a very consistent run.

Best,   DennisT

[kenneth cook]

            I see advice being offered and no one still knows what version is being used. The 1957 Rocket is the closest run your going to achieve to the .35 stunt. It looks somewhat similar but a 4 bolt backplate and a bell mouthed venturi stack. I personally would use 10% nitro myself minimum. While I see the recommendation of uniflow and I'm not opposed to it, if the engine is vibrating, uniflow is not going to work well due to fuel foaming. The Tudor and I'm assuming this is a old first version Top Flite Tudor used 3/8" balsa fuse with 1/16" doublers and was quite long. This combined with a shaker engine is not a good scenario. You now perfected the tuning fork. It sounds to me as if the engine is already doing what I mentioned and it's causing the engine to go overlean due to the foaming in the tank. You can't richen it enough due to this. Sometimes you can set it off a bit lean and it settles down once in the air but there's so many unknowns here that all of this is a guess.

[Wolfgang Nieuwkamp]

Lowering the cylinder liner helps to make the timing similar to the Fox 35 Stunt. I did that many years ago, when I flew I/C.

Regards,
Wolfgang

[Howard Rush]

Show us a picture, please.  Depending on what model you have, someone might trade you a good stunt engine for it. 

[Dennis Toth]

Steve,
I had a Tutor and there are a couple things you can do if it is vibrating. First put a couple drops of Armor-All in the fuel to reduce the foaming. Next make sure the tank vent is on the inboard side of the fuse about 3/8" off the side pointing into the free air stream. This is the one thing that on my Tutor made the engine run consistent. Prior to relocating the uniflow vent to the inboard side I had it just coming out of the top of the tank. The engine run was all over the place with both a Fox 35 and an Enya 35. Once moved the engine runs were solid. Last thing you can do for the vibration is take a prop that has a heavy blade, mount it so that with the piston at bottom of the stroke the heavy side is placed pointing straight up, this adds a little weight on the counter weight side and reduces some of the vibration.

Best,    DennisT

[Steve Dwyer]

Here's a couple of shots of the Combat 35. It does have a 4 bolt back plate a larger more flared venturi and the head is thicker than the standard Fox 35, it's definitely a heavier engine. The Tutor is 1.2 times larger than the RSM 35 model bringing it to a 50" span. I asked Eric to enlarge the plans and cut a kit. It is still smaller than the bigger, longer ARF Tutor by Topflite I have seen.

I was struggling with what appeared to be foam in the fuel line and erratic up and down running but it ran ok once in the air. I will say I did not notice any more than usual vibration causing the resulting foaming but it still could be running at a resonant frequency causing it. Perhaps loading on the airframe once airborne tends to dampen the problem.

I'm anxious to increase the Rpm with a lighter prop to see what happens, I have some 10% nitro I can juice up to 28% castor to try as well.

The Tutor performs well, it was a bit nose heavy requiring .5 oz at the tail to balance at 25% of the chord. The Tutor does have a long nose from the leading edge and with the heavier engine it needed some offset. I guess I'll stay with the standard vented tank for now. I'll try to get back out again this week, and get back to you.

Thanks,

Steve

[Steve Dwyer]

Dennis,

I'll try the prop adjustment as well. Interesting how the uniflow improved your run, this will be next.

thanks,

Steve

[Steve Dwyer]

Here's a shot of the front of the engine.

[Dan Berry]

You're asking that engine to do something that it doesn't want to do.

[Howard Rush]

I'll give you a Fox .35 stunt for it.

[bob whitney]

that is a Fox Combat round venturi Black head,.probably one of his better engines

take the restrictor out of it, put a pressure fitting in the back plate put it in a Wooten Sneaker with a Rev Up 8.5x6.5 and have some fun,RAD

[Steve Dwyer]

Thanks all for the education, think i'll try to see how it's supposed to run before considering trading it.

Steve

[Dan McEntee]

  Well, we've been telling you how it is supposed to be run, and how it will run if you use a proper prop. You just have to decide if you want to keep it for what it is and find an engine more suited for stunt/sport flying, or trade/sell it off for a an engine. Once you get some more experience under your belt and want to try a vintage combat model like the Sneaker that was mentioned or a Voodoo or Combat Cat then you are all set. Even if you ran it on suction with a hard tank it will give you a thrill 11 And then like Bob mentions, put a pressure tap in the back p-late, take the restrictor out and then kick it up a notch!!  It will give you an idea of how the combat guys got to be the way they are!!!
   Type at you later,
   Dan McEntee

[Dan Berry]

Thanks all for the education, think i'll try to see how it's supposed to run before considering trading it.

Steve

It’s supposed to run wide open. Fast. It’s a good engine for Nostalgia Gas Free Flight- where we run’em wide open, under pressure with 25-35% nitro in the fuel. It’s not docile.

[Dennis Leonhardi]

You're asking that engine to do something that it doesn't want to do.

I spent several years in chemical research working on products for industry.  We reminded ourselves - frequently - "You can't make a silk purse out of a sow's ear".  It's fitting here ...

[Steve Helmick]

It’s supposed to run wide open. Fast. It’s a good engine for Nostalgia Gas Free Flight- where we run’em wide open, under pressure with 25-35% nitro in the fuel. It’s not docile.

Ideal use for it! Maybe a Ramrod 750 (or 600 if you're brave!). Those vibrated with much vigor....even more than the Stunt .35.  Steve

[Dan Berry]

Ideal use for it! Maybe a Ramrod 750 (or 600 if you're brave!). Those vibrated with much vigor....even more than the Stunt .35.  Steve

My buddy Jackie had one on a Lucky Lindy at 700-750 squares. Strong engine.

[frank mccune]

     Hi Steve:

     I had an engine that looked like yours and I called it a 1958 Fox Combat Special .35.  It was a great engine!

     Good luck,

     Frank

[Steve Dwyer]

Tomorrow's is forecast is dry and light winds, I should have a report in the evening as to how this Fox 35 Combat ran with all your recommendations.

Steve

[Howard Rush]

"You can't make a silk purse out of a sow's ear".  It's fitting here ...

I don't think that's the right analogy.  I complained on this forum when people desecrated a Supertigre G.21 .35 by putting it on a stunt plane that they had tried to make a pickup truck out of a vintage Ferrari.  A blackhead Fox is not a vintage Ferrari, but it's not a sow's ear.  It's a '55 Chevy.  It belongs on the nose of a Jerkline Special.  If Steve won't take a Fox stunt as a trade, I'm willing to negotiate. 

[C.T. Schaefer]

Fun thread!  I have been flying these early CS motors in, mostly, appropriate vintage combat planes, for a few years now.  Everything from the Scarinzi Killer and Consolidated  Bandit to  80's vintage 'commie' combat wings.  They are definitely 'hot' motors and run well on pressure or bladder with the insert removed. 9x6 - 9x7 props work well. Depending on the combo speeds are roughly 85 -95.  Because I had the parts I made a 58 with the Gardner ABC that we had made for the 36x. It took a bit of 'fitting' but runs quite well!  It is on a  1960 Vigani  UP  n  At'm.  Sadly we dont get to fly them too much because of noise and  lack of a suitable grass field.   Fly em while ya can boys!   TS

[Andre Ming]

I don't think that's the right analogy.  I complained on this forum when people desecrated a Supertigre G.21 .35 by putting it on a stunt plane that they had tried to make a pickup truck out of a vintage Ferrari.  A blackhead Fox is not a vintage Ferrari, but it's not a sow's ear.  It's a '55 Chevy.  It belongs on the nose of a Jerkline Special.

A very good analogy. Not a Ferrari for sure, but when the era is considered, it was a pretty stout motor for its time. 

If Steve won't take a Fox stunt as a trade, I'm willing to negotiate.

Bless you, my child.

I tip the hat to any "seasoned" (much better sounding that the word "old") combat guy that respects the history of the sport.

Though I wasn't a Supertigre fan, I DO have a Tigre C35 that's in "like new" condition. I purchased it new back the mid-late 60s. It was originally an R/C version that I put into a Roberts "Mauler" for some carrier sport flying. Flew it once as I recall, then it was back to combat planes. The engine languished for decades until here a few years ago Randy Daily offered to rebuild it. (Yes I jumped on the chance. Randy knows his Tigres!)

Somewhere over the dateless past the R/C carb on it was lost, so Randy asked if I'd like it to be refitted with a venturi for C/L use, thus making it a stock C/L version of the C35 that predates the G21.35. I jumped on the chance and now I have a pristine C35 that really needs to be on an appropriate vintage combat plane, but may never make that meager goal while I'm still alive.

However, it's cool to look at it and hold it in my hand. Almost as much as my very first Fox 36X (purchased new mid-late 60s) that I also still have. I suspect that I will never part with these two engines as long as I'm still drawing breath. They are relic survivors from my past that have too many good memories from that era attached to them.

Sorry for the ramble... but I tend to get verbose at times. Deal with it.

Andre

[Steve Dwyer]

As promised I flew the Tutor with the Black Head 35 on it Thursday. I had changed out the tank and venting, went to a 10x4 prop, and burned 10/28 castor. It hauled the Tutor at a good clip for several laps until it quit in the first loop. I suspect it may have been too lean. I didn't get a second as it started to sprinkle. As expected it was foaming on the ground, I did not attempt the defoamer trick.

A friend suggested I strap on a Fox pressure vented tongue muffler since it cleared up the same issues I'm experiencing with his. I'm digging around for a pressure fitting to tap one of my mufflers, we'll see if this is the cure to a decent flight.

Howard, sure I'm negotiable but I have enough Fox Stunts already, what I'd prefer is an OS FP 35 or an OS LA 40 instead. I also have a nice Torpedo Green Head 35.

Steve

[dale gleason]

 I still have the first edition Combat Fox that came out in 1956. It doesn't have a black head, just silver the way it came.  Is there any difference in the original Combat Fox and the one with the black painted head? There is one in the AMA museum display like mine.

My combat Fox has the 4 bolt backplate with a pressure fitting location, and the steel venturi insert.

Would be pretty on a Nemesis....

dg

Edit: Failed to mention the spade needle valve. It reduced vibration of the needle inside the spraybar and it may have closed completely to facilitate the use of a pen bladder tank.

[Steve Dwyer]

Dale,

The following explanation came from an old Fox enthusiast, he said:

The original Fox Combat Special came out in 1957; the blackhead replaced it in 1958. There are subtle internal differences, probably having to do with port timing, but for all practical purposes they're the same; all parts are interchangeable.

Steve

[kenneth cook]

             One thing I've come across on these engines is crankshaft differences. Some had a full web crank while the others had a counterweighted crank. My Blackhead has the counterweighted shaft. I also have two redhead Rockets which are from the same case and one has the counterweighted shaft while the other has the full web. It's my understanding though that the Redhead Rockets were a collaboration of parts just to sell them.

[C.T. Schaefer]

Steve, If the engine still has the steel venturi insert it is quite large and is difficult to get it to work for casual stunt work. If there is no insert I am amazed that you got it running at all!  So,  sleeve the venturi hole to  .270 and it should work nice.  Or, drill n tap back plate for a pressure fitting .  Trying to fit a muffler to these is against the laws of nature.   TS
PS   10x4 will be pretty revvy. A 10x6 will work ok once you get a good fuel feed.
PPS   Original CS  has a different piston, cylinder and head than later models.  For 58 he went to a 36x style.  Yes, all parts interchange so, basically, ya never know for sure what is 'supposed' to be in there.   TS

[EddyR]

 I remember these motors well when they came out as I was working for Kerns hobbies and was flying a lot of local  combat. Most people were disappointed in them. A Johnson or K&B with cleaned up intake was still better motor. Johnson was better made than K&B but the K&B was lighter and better in smaller planes like the Quicker.
  EddyR

[Dan McEntee]

[quote author=Steve Dwyer 0

Howard, sure I'm negotiable but I have enough Fox Stunts already, what I'd prefer is an OS FP 35 or an OS LA 40 instead. I also have a nice Torpedo Green Head 35.

Steve
[/quote]

   If you decide this engine isn't what you like, I would trade a FP-35 for it, or an LA.40. Just keep me in mind.
   Type at you later,
   Dan McEntee

[Steve Dwyer]

Getting back on the Fox 35 Combat progress. I installed a uniflow tank and a tapped tongue muffler to pressurize the fuel. I also drilled out the tongue muffler with matching holes on the opposite side to reduce restriction. I did not add Armour All even though I brought it along.

The ground run was smooth and even, once in the air I noticed a couple of rpm blurps but it ran at a consistent speed for the duration of the 3.5 oz tank for about 4.5 min. I had adjusted the NV to about 1/2 half-turn rich from lean hesitation. Because the engine shows little variation in mixture adjustment I wonder if I should run it just off too lean or can this damage the engine. It does seem to be a fuel guzzler at 1/2 turn off, I'm now wondering if I can improve the duration? It has a flatter 10 - 4.5 prop on it vs the 10 -6 I had on originally. The fuel was 10 - 28 castor.

BTW this is not your ordinary Fox 35, the Tutor 50 was really moving.

Steve

[Dan McEntee]

Getting back on the Fox 35 Combat progress. I installed a uniflow tank and a tapped tongue muffler to pressurize the fuel. I also drilled out the tongue muffler with matching holes on the opposite side to reduce restriction. I did not add Armour All even though I brought it along.

The ground run was smooth and even, once in the air I noticed a couple of rpm blurps but it ran at a consistent speed for the duration of the 3.5 oz tank for about 4.5 min. I had adjusted the NV to about 1/2 half-turn rich from lean hesitation. Because the engine shows little variation in mixture adjustment I wonder if I should run it just off too lean or can this damage the engine. It does seem to be a fuel guzzler at 1/2 turn off, I'm now wondering if I can improve the duration? It has a flatter 10 - 4.5 prop on it vs the 10 -6 I had on originally. The fuel was 10 - 28 castor.

BTW this is not your ordinary Fox 35, the Tutor 50 was really moving.

Steve

    Well, the flatter pitch prop is doing it's thing, the extra holes in the muffler are doing their thing, and the muffler pressure is doing it's thing if you got what you think is a satisfactory run for that engine. To get increased duration you need to do one of the following;

   1( Put in a bigger tank
   2) Spike the fuel with a bit of lantern fuel. I'm sure you can do a search in the engine section to find how much to add to a gallon.
   3) Restrict the venturi more. This can be layers of panty hose to the old fashioned stick stuffed in against the spray bar and the side of the venturi.
   4) Go to 5% nitro. This may be the easiest for you and get you enough to do the pattern.

    Adjusting the needle to just off lean should be in the ball park. If all is tight and not leaking air in the fuel system, it should go a bit richer once you pick up some speed and centrifugal force. that is the way this engine is more or less designed to run for combat. Fly it some more with the 5% nitro fuel and see what you think of it.
  Type at you later,
  Dan McEntee

[Brett Buck]

Getting back on the Fox 35 Combat progress. I installed a uniflow tank and a tapped tongue muffler to pressurize the fuel. I also drilled out the tongue muffler with matching holes on the opposite side to reduce restriction. I did not add Armour All even though I brought it along.

The ground run was smooth and even, once in the air I noticed a couple of rpm blurps but it ran at a consistent speed for the duration of the 3.5 oz tank for about 4.5 min. I had adjusted the NV to about 1/2 half-turn rich from lean hesitation. Because the engine shows little variation in mixture adjustment I wonder if I should run it just off too lean or can this damage the engine. It does seem to be a fuel guzzler at 1/2 turn off, I'm now wondering if I can improve the duration? It has a flatter 10 - 4.5 prop on it vs the 10 -6 I had on originally. The fuel was 10 - 28 castor.

   I would suggest just making the venturi smaller until it needles the way you expect. Sounds like you have more than enough power. 28% castor is probably more than necessary, that is not helping you. It's like trying to suck jello through a straw.

    Brett

     

[Howard Rush]

Oh, no, not a muffler.  This is torture.

[Steve Dwyer]

All interesting points, but how do I pressurize this engine without a pressure tapped muffler to the tank? I know in the old days you'd tap the backplate and pressurize the crankcase from a tank, I think?? Was the tank simply vented with angled tubing facing forward and because the crankcase was slightly pressurized would this improve performance?

Stay safe and wear a mask.

Steve

[Brett Buck]

All interesting points, but how do I pressurize this engine without a pressure tapped muffler to the tank? I know in the old days you'd tap the backplate and pressurize the crankcase from a tank, I think?? Was the tank simply vented with angled tubing facing forward and because the crankcase was slightly pressurized would this improve performance?

  Bladder tank.

     Brett

[George Fruhling]

Combat Special with Missal Mist fuel running an  8-8 prop on a Voo Doo combat ship. And a pen bladder.   A Stunt 35 on planes like the Sig Banshee.  The Combat Special loves to rev.  Should be able to arrange an engine trade.