Author Topic: Fox and fuel tank problem???????????? (Read 2157 times)

frank mccune · « **on:** September 21, 2018, 07:57:37 AM »

Hi All:

I have a Goldgerg Shoestring with a Fox .35 Stunt for power that is giving me a major problem. The problem goes like this. The engine is adjusted to run in a 4 cycle and break into a 2 cycle while being rotated on the ground. The plane is launched in a steady 4 cycle but goes into a strong 2 cycle in about a lap. It will continue into this mode during the first half of the tank. It then goes very rich for the remainder of the tank. No amount of manouvering will cause the engine to go lean again.

The setup is a 10-6 wooden prop.
A three ounce RSM Uniflow tank.
The tank is mounted on the cl of the engine
It does the same whether the tank overflow is plugged or unplugged.
The engine is very easy to start and adjust.
I am running an OS .35 nva. in this Fox.
The plane balances and flies great with this engine but...
There are no problens with vibration and fuel flow.

I could swap the engine for another Fox or swap the fuel tank to help determine the problem but I thought tht I would ask here before I start making major changes. How can a Fox .35 Stunt and a Goldberg Shoestring NOT be a trouble free combination? Lol

Any ideas as to what may be causing this problem?

Tia,

Frank McCune

Robert Zambelli · « **Reply #1 on:** September 21, 2018, 09:35:28 AM »

Frank - did you turn down the middle of the spray bar to .125?

frank mccune · « **Reply #2 on:** September 21, 2018, 09:41:38 AM »

Hi Robert:

No I did not. This engine and nva worked very well for Foxberg for many flights. I will check to see if I have a standard Fox .35 NVA. I can't imagine that the OS nva. would cause these symptoms. But then again what do I know? Lol

Cheers,

Frank McCune

Brett Buck · « **Reply #3 on:** September 21, 2018, 09:55:35 AM »

Quote from: frank mccune on September 21, 2018, 09:41:38 AM

Hi Robert:

No I did not. This engine and nva worked very well for Foxberg for many flights. I will check to see if I have a standard Fox .35 NVA. I can't imagine that the OS nva. would cause these symptoms.

Probably not, but you can generally assume that any of these sort of weird issues on a Fox-powered airplane, particularly a profile, is caused by various parts going in and out of resonance with vibration. This is a typical example - the natural frequency of the tank slosh or tank mount changes as the fuel level changes. 4-strokes also shake a lot, typically, and we have altered this sort of behavior by putting small weights on the tank. We could shift where/when it happened during a tank of fuel by changing the resonant frequency. Note I said altered, I can't recall any cases where it was solved completely. It's remotely possible that the NVA itself is the source, but the OS is generally much less prone to vibration issues than most others, particularly the SuperTigre.

If it was me, I would probably first try putting a stick-on weight on the tank (like an ounce, maybe) and see if it changes the behavior. If so, that's the issue, but what to do about it to actually solve the problem is unclear.

Brett

frank mccune · « **Reply #4 on:** September 21, 2018, 11:02:48 AM »

Hi Brett:

Thanks for the prompt reply.

I think that I should make a Winter Project out of attaching a ST G20-.23 or a Brodak .25 to the Shoestring and stop fighting The Fox Curse. Of course there will be a need for more nose weight but both of those engines should pull a Shoestring quite well.

Be well,

Frank McCune

RandySmith · « **Reply #5 on:** September 21, 2018, 01:33:11 PM »

Frank

The typical setup for a FOX 35 is to NOT put the tank on centerline, but to raise the tank 3/16 to 1/4 inch. Keep the tank as close to the motor as possible, Do Not use a OS NVA in it unless it is the OS 15 or 20, The OS 35S is much thicker and will kill off power on the FOX
I would switch fuels, and switch tanks, one after another, to see what affect it has, the prop, if balanced, should pose to problem.
And make sure your using 25% oil in the FOX or better, and I would recommend part of that synthetic

Let us know

Randy

frank mccune · « **Reply #6 on:** September 21, 2018, 05:40:31 PM »

Hello Randy:

Thanks for the reply!

Here is some information re.my setup:

Tank on CL until I start flying inverted. It is made to be adjusted at the field. First I must get the engine to make a consistent run in level flight.

The tank is very close to the engine

The fuel is 22% castor,5% synthetic, 10% nitro and a dollop of prop.

The prop and spinner are both balanced.

I can beleive that the OS nva is too large in diameter for the Fox .35 Stunt. Perhaps this is why this engine gave 62 laps per 2 oz. tank in Foxberg. My Fox .35 Stunt with the standard Fox nva gave 34 laps per tank but was faster. I will see in I can find smaller nva.

In any case, things to keep me out of the bars.

Be well,

Frank McCune

Dan McEntee · « **Reply #7 on:** September 21, 2018, 07:05:03 PM »

" Tank on CL until I start flying inverted. It is made to be adjusted at the field. First I must get the engine to make a consistent run in level flight."

Having the tank on the center line will give you issues in level flight. If the tank you are using is a wide wedge type, that causes needling issues also. With a Fox .35 I usually start out with the tank center line even with the by pass port on the engine and be able to adjust 1/4" or more up or down. I know the Shoestring doesn't have much room for a tank but search out here on the forums for what guys have made or used to get as much fuel capacity as they need. I would shoot for 4 ounces. 3 ounces on a Fox won't take you very far. And just for giggles and grins, drop down to a 10-5 prop while you are sorting things out.
Type at you later,
Dan McEntee '

Balsa Butcher · « **Reply #8 on:** September 21, 2018, 08:39:14 PM »

Add some castor to your fuel. I like 25% castor, 2 % synthetic. I agree with the recommendations. 9x6 or 10x5 work the best for me.

pmackenzie · « **Reply #9 on:** September 21, 2018, 08:55:53 PM »

I would try armor-all, a few drops in the jug till the fuel does not foam when you give it a shake.

Pat MacKenzie.

Dennis Toth · « **Reply #10 on:** September 24, 2018, 06:24:14 PM »

Frank,
Couple things to start, put the stick in the bypass of the Fox 35. Raise the tank as Randy said 3/16" above the engine centerline. If the engine has a muffler drop the prop to a 9x6 (the Brodak BYO prop works well). Last make sure the vent line is on the inside of the fuse pointing into the air flow. I had a problem just as you describe. I had the tank vent up in back of the engine. I remembered old advice about the position of the vent being located to the inside of the fuse. As soon as I moved the vent to the inside of the fuse all the run issues ended. I run 10% N fuel with 27% (50/50) oil. I run the 9x6 prop (I run an RSM 9x6 heat pitched up to 6.5, the BYO is near this pitch out of the bag). Last thing is make sure the engine does not have any binding. Check this by removing the plug and backplate but keep the muffler on and tight, pull the engine through a full revolution. If you find a bind check the muffler, loosen it and check again (if you have only the tabs you need to add support tubes down the exhaust between the tab and muffler). If that isn't it check the head bolts, sometimes they bind on the side of the bolt recess in the head. Easy fix here is to file down the offending bolt (put it in a drill and run it against a file). When you mount the engine in the ship if you have hard aluminum pads put 1/32" plywood "crush" pads under the mounts and tighten down evenly.

Let us know how you do.

Best, DennisT

Air Ministry . · « **Reply #11 on:** September 24, 2018, 06:36:19 PM »

Theyall usedtosay you put the tank up 1/4 inch from center , on a side mounted fox .35 .

kenneth cook · « **Reply #12 on:** September 24, 2018, 08:26:33 PM »

I have found the Shoestring to be very problematic with the Fox.35. I was able to eliminate a lot of problems with reworking the wing construction. I have found the two center ribs to be the problem. The entire front end is like a tuning fork. Some of this was resolved by installing a 1/2" center rib in between them. Unfortunately, the dado joints aren't spaced enough apart to accept a full 1/2" center rib and sanding is needed to insert the solid piece in between the two ribs. I removed the fuse and installed the solid piece and 90% of the problem disappeared. I glassed the cheek cowl to the sheeting and the fuse to the sheeting from the doublers to the trailing edge on both sides using light weight glass. The other 10% was resolved using a spinner which has ball bearings within it.

Brett Buck · « **Reply #13 on:** September 24, 2018, 08:52:57 PM »

Quote from: kenneth cook on September 24, 2018, 08:26:33 PM

The other 10% was resolved using a spinner which has ball bearings within it.

That's amazing, I have never seen anything like that.

Brett

Dave Hull · « **Reply #14 on:** September 24, 2018, 09:44:25 PM »

That type of damper was also used on some Lycomings when installed in experimental aircraft. Obviously, its purpose is to soften the torsional vibration. Problem generally is that it adds nose weight. If I recall correctly, the case was segmented, so that the damping media would never be a significant source of imbalance. I don't remember if it was also silicone-filled (super high viscosity.)

Dave

RandySmith · « **Reply #15 on:** September 25, 2018, 01:36:58 AM »

Quote from: frank mccune on September 21, 2018, 07:57:37 AM

Hi All:

I have a Goldgerg Shoestring with a Fox .35 Stunt for power that is giving me a major problem. The problem goes like this. The engine is adjusted to run in a 4 cycle and break into a 2 cycle while being rotated on the ground. The plane is launched in a steady 4 cycle but goes into a strong 2 cycle in about a lap. It will continue into this mode during the first half of the tank. It then goes very rich for the remainder of the tank. No amount of manouvering will cause the engine to go lean again.

The setup is a 10-6 wooden prop.
A three ounce RSM Uniflow tank.
The tank is mounted on the cl of the engine
It does the same whether the tank overflow is plugged or unplugged.
The engine is very easy to start and adjust.
I am running an OS .35 nva. in this Fox.
The plane balances and flies great with this engine but...
There are no problens with vibration and fuel flow.

I could swap the engine for another Fox or swap the fuel tank to help determine the problem but I thought tht I would ask here before I start making major changes. How can a Fox .35 Stunt and a Goldberg Shoestring NOT be a trouble free combination? Lol

Any ideas as to what may be causing this problem?

Tia,

Frank McCune

Hi Frank
Let us know how the hints here work, I can also tell you that I had several of ALL the Goldberg Planes, Cosmic Wing Shoestring, Buster , and all 3 of them I used FOX 35s in, they all ran fine with no problem, I ran them with the uniflow tank, open to atmosphere, tank raised 3/16 to 1/4 inch above center line, I used 28% oil 1/2 castor Hot RC plug ( back then, today I may use hot non idle bar) and normally a 10 x 6 or 10 x 5 wood prop.
I also had these 3 with OS 35s engines, they also ran well. I did nothing trick to the planes except good solid construction, flat motor mounts and foam under the tanks, I kept the tank very close to the engines, and I had both silkspan painted and Monocoat covered ones. By the way the Shoestring seems to be the be flying one .

Randy

Dennis Toth · « **Reply #16 on:** November 01, 2018, 08:54:52 AM »

Frank,
I have had the same problem and fought it for a long while, recently I remembered some advice that was given at one of the GSCB forums in the 70's about putting the uniflow (or even the conventional vent) into the free airflow stream on the circle inside side of the fuse. I made this one change and all my Fox (and other profile mounted engine) run problems went away. I was able to back to the RMS 10x6 prop with the same muffler I was running on the 9x6. Try it is simple to make up the tab and tube.

Best, DennisT

Phil Krankowski · « **Reply #17 on:** November 02, 2018, 08:40:05 AM »

If you haven't tried this already, kick the back of the tank(where the pickup is located inside) out 1/4 inch, for an outside mounted tank. This will keep the fuel puddled where it is supposed to be, especially if the plane flies more nose out than expected.

Phil

Author Topic: Fox and fuel tank problem???????????? (Read 2157 times)

frank mccune

Fox and fuel tank problem????????????

Robert Zambelli

Re: Fox and fuel tank problem????????????

frank mccune

Re: Fox and fuel tank problem????????????

Brett Buck

Re: Fox and fuel tank problem????????????

frank mccune

Re: Fox and fuel tank problem????????????

RandySmith

Re: Fox and fuel tank problem????????????

frank mccune

Re: Fox and fuel tank problem????????????

Dan McEntee

Re: Fox and fuel tank problem????????????

Balsa Butcher

Re: Fox and fuel tank problem????????????

pmackenzie

Re: Fox and fuel tank problem????????????

Dennis Toth

Re: Fox and fuel tank problem????????????

Air Ministry .

Re: Fox and fuel tank problem????????????

kenneth cook

Re: Fox and fuel tank problem????????????

Brett Buck

Re: Fox and fuel tank problem????????????

Dave Hull

Re: Fox and fuel tank problem????????????

RandySmith

Re: Fox and fuel tank problem????????????

Dennis Toth

Re: Fox and fuel tank problem????????????

Phil Krankowski

Re: Fox and fuel tank problem????????????