I spent a very frustrating day yesterday trying to get my OS Max S .35 to run reliably.  She's normally steady, holds a needle setting, runs reliably.  But yesterday I had no success.  She would start readily but jumped around as much as 1,000 rpm, seemed to ignore everything I did with the needle valve.  Would go into violent vibrations for a few seconds at a time that were downright scary.     On couple of the last attempts to run her, raw fuel came out of the uniflow vent hose aimed into clean air from the prop.  These are the things I tried.  The prop was balanced.  I tried another balanced prop.   I changed the NVA (ST) twice, changed the fuel lines, changed back to a previous fuel tank, changed the glow plug, even opened her up to see if the con rod had gotten bent/gone sloppy.

But something I was reading an old post today suggested to me that there's nothing wrong with the engine or fuel tank, etc.   The article I read today was about the Sig Twister (mine is a much abused hand-me-down Francherized Twister) and it was recommended to double the nose to reduce vibration problems.  My model has the nose doubler.  I just repaired her after ANOTHER severe ground contact (crash) and one of the repairs was to re-glue the leading edge to the fuselage.  The crash had produced a visible crack half way back to the spar and I could wiggle the front of the fuselage back and forth one sixteenth of an inch or so.  Aft of the visible crack the wing/fuselage joint appears and feels absolutely solid.   I poured in some thin Super glue and made a fillet of Sig Micro balloons mixed with Sigment over the cracked area.  The wing to fuselage joints feels as solid as can be.  But I've never had the engine vibrate violently like that before.

Is it possible that the broken nose, even though it feels as solid as ever, isn't adequately repaired/solid enough and is the cause of the violent vibration and therefore the cause of the erratic engine run as well?

Thanks,
Joe Ed Pederson
Cuba Missouri

Thanks Dennis and Ty,

Dennis: Thanks for the picture of your uniflow vent set up.   I saw your picture on a previous post and had copied the vent set up a while back and was using it yesterday.

I'm using Sig Champion 10% nitro, 25% all castor and I add a little Klotz to bring the oil up to 26%.  This fuel has worked well for a gallon and a quart.

Ty:  I glue my tanks (metal) to 3/32" plywood.  I'll add a foam pad behind the tank and see if that helps.  It ran fine in the past without a pad, but that was also before the wing to fuselage joint broke.  Thanks for the suggestion.   I always use a fuel filter in the feed line and filter the fuel as it goes into the tank.  I checked the fuel filter yesterday.  There were a few specks in it, but it certainly wasn't clogged.

Joe Ed

Bill,

Yes, this engine has been crashed hard several times.  Always on grass. 

How do you check for a bent crankshaft?

It seems solid in the bushing and the prop tips measure the same from a fixed point near the landing gear.

Thanks,

Joe Ed

     Hi Dennis:

      What is a high compression head for a Fox .35?  I have a Fox S2 high compression head for a Fox .35.  I have not compared to a stock to determine if there is any difference in performance.  What if any, difference have you found?

                                                                                                     Thanks,

                                                                                                     Frank

Wish I was there to see this in person.  I first thought of something in the fuel line/needle valve assembly.  The other thought was fuel line leakage.  I have changed fuel line in planes at times and every thing started working like normal.  I also don't run a lot of oil.  22% is my max with a little castor.   Another way to check for bent crank is to see if the tips of the prop are tracking the same path.   There are so many things that could be causing your problems.  Let us know what you find.

Some engines will vibrate at a certain rpm then smooth out at lower or higher rpm and there is nothing you can do but not run them at that rpm.

   Yes, very possible, but it's just as possible that you made it stiffer enough that the natural frequency of the nose (the frequency at which it wants to deflect) happens to line up with the running frequency of the engine. If I had a guess I would vote from the nose being less stiff, lowering the natural frequency to a bad range for the engine.

  This is typically a significant issue for these ancient super-shaker engines on profiles, it's kind of amazing it doesn't happen more often.

   Brett

 

Thanks Brett,

My plans to go to the Nats and fly beginners got cancelled for this year by circumstances outside my control.  I'm thinking of using the money I would have spent on travel and lodging for a Sig Skyray and a FP .20 or LA .25.

I've cut and pasted nearly all the threads on your Sig Skyray recommendations.  Do you still recommend shortening the tail of the Skyray by 1 1/2" and increasing the tail size to maintain the same tail volume? 

Is the early FP .20 with steel liner as good an option as the later FP .20 ABN? [Never mind.  I found the answer is "no" in a 2006 thread by Dirty Dan titled "The Recipe for 20 FPs."]   

I'm a little reluctant to get an LA .25 because, as far as I have read, it isn't possible to distinguish the less powerful LA .25 from the more powerful LA .25 until after you lay down your money.

Thanks,
Joe Ed

Joe, those .25 sized planes are ok for fun but if you want to move up the ranks go on ebay and get an LA46 and build a Brodak Tanager or something that size. You'll be amazed how much better it flys.

Motorman

Motorman,

Thanks for the suggestion. I'm a rank beginner and I'm still working on putting three consecutive inside loops together that come close to being round much less putting three inverted loops together.  Haven't worked up the courage to even attempt a square loop. 

That being the case, I'm thinking that in a crash (I foresee many more crashes in my future) the Skyray will suffer less damage than the Tanager because the Skyray has less mass to jerk to that sudden stop.   I could be wrong about that though.

I have had my eye on the Tanager for the future.  I like her lines and I wrote in my Brodak catalog your post about the Tanager from  the thread "Beginner Pattern Profile Electric Model???"  You wrote: "It has a real stunt wing airfoil and flies well in the wind."

Joe Ed

After the crash ... did you dis-assemble the engine?  If so might the sleeve have been put in backwards?

I only took off the venturi and the back plate.  But thanks for the suggestion.

Joe Ed

Joe,
One item to check is if you have some varnish build up causing some tightness in the stroke. To check, remove the plug and slowly rotate it through and feel if it gets tight as you come up on top of the stroke or across the top. To clean out simply run several tanks of all synthetic fuel through it. Check again, sometimes takes several more runs but should get better each tank or so. Could pull the head and do the same check, might have some other material stuck up there.

Best,  DennisT

I got a chance this evening to try again with a third fuel tank and I changed the fuel filter and tried fuel from a different jug.  No improvement.  She starts readily but surges, pays little to no attention to adjustments of the needle valve, and it took 15 turns out (Super tigre NVA) to get her to begin to back off of 2 cycle to an erratic 4 cycle where usually she would start and run a steady 4 at 11 1/2 turns out.

I looked inside the cylinder and top of the piston with an LED flashlight to see if the piston appeared to have a castor build up.  Couldn't detect anything unusual.  With the plug out I turned her over with my fingers on the crankshaft (no prop) and couldn't detect any increase in the drag near top dead center.  She is smooth all the way up and all the way down without any change in the force required to turn the crankshaft through a full rotation.

I checked the crank and with the prop pulled forward I could get the prop tip to deflect 1/64" (just guessing).  Thought I had the problem solved (bent crank).  So I mounted my other Max S .35 that was running fine the last time I used it and it had the exact same prop tip deflection.

Tomorrow I will see if the other motor runs the way it should on this model or does the same thing the first motor has been doing. I'm expecting to find the problem is the fuselage damage and repair is the source of the problem.

Can't think of much else to try.

Thanks to everyone for all the suggestions.

Joe Ed

Your idea to swap out the engine and see if the problem follows the plane or follows the engine is a classic plan.  If you don't have a second plane, a test stand run is also an idea to see if the same needle setting is required, etc.

Peter

In the one brief 10 minutes between rain, I ran the other OS Max S .35 on the repaired Francherized Twister.  Same fuel tank, fuel line, and fuel filter.  Same exact results and same exact problems: excessive vibration, wouldn't hold a needle and didn't respond to needle valve adjustments.

Therefore the problem (at least in the case of this second motor) is either a) in the fuel tank, fuel line, or fuel filter or. b)  in bad harmonics from the fuselage break/repair making the fuel foam.

I made a test stand this evening and will try running the motors on the test stand with the same fuel tank, fuel line, and fuel filter tomorrow.

Bill P.: I took the NVA out of the first motor and it was perfectly clean and unobstructed.  This NVA is also new and never flown (and therefore never bent in a crash).

I don't think this is the problem, but what is the shelf life of fuel stored in heated/air conditioned basement?  I bought my fuel from Sig in the Spring of 2018.  I had to wait for them to mix up a batch so the fuel was fresh when I got it.

The thought crossed my mind this evening to check the thread on changing the Twister to electric.  I have a hard time throwing flyable models away.

Joe Ed

OS needle valves have a habit of fracturing internally. The tapered end can float back and forth.
You can guess what I would try.

Fuel lasts a really, really long time.  Twenty year old fuel is fine.  If anything, the fuel gets a little stronger when some of the methanol evaporates, leaving behind more nitro.  The castor oil never evaporates.  If the castor did evaporate, our "flying clothes" would release the castor oil in the clothes dryer.  Instead, myself and others I know launder separately.

Just to add...about fuel....Tightly sealed OPAQUE containers of 40 year old Dukes fuel and Missile mMist can still be good/////  fairly frequently someone buys a pint of Cox fuel from the 70s.....some minor rust to filter out but the 25% nitro stuff still runs a .049 very well

ALL atmosphere Air has water in it.....Methanol is hydroscopic...it readily absorbs the H2O
Nitro Methane is a relatively stable chemical but is easily deteriorated into sub compounds by UV radiation (sunlight)

Dark amber or steel containers are prefered....That it is sold usually in clear plastic jugs,  is simply for cost and convenience... MOST customers by a Gallon of fuel to USE...NOT store for a decade

My last two batches of Nitro from TORKO came in White OPAQUE plastic jugs

I thought this different from my 2014~2015 buys ( in typical clear plastic jugs...)
 
So I sent an e-mail to TORKO....the answer I got was More and more customers were afraid of no nitro in the future so they were stocking UP.....  read into that a tad.....

Yep , the rubber O ring , on the intake , despite being fearfully modern & built after the war , as often as not has gone hard, cracked , and desintigrates on remooveal .

Therefore a new O ring wont do any harm .

That may be so , but as someone brought up the seals , its not entirely erelephant .
About as important as sealing the N V A .
Not all intakes fit the case ' leak free ' . In fact some double O Ringed ( above & below ) the ST peripheral jet type  .
standard early OS 35S type are hardly a precision fit in the case , You get the odd O ring thats well past it .

If a O ring has hardened up , split , cracked , or has a chunk out , its unlikely to be air tight .
Automotively , a intake leak is about the fastest way to wreck a motor . Some relevance even though were not running V eights .

  Well, bring that sucker up to Buder this Saturday. I'll watch the weather. We'll have you doing the complete beginner pattern before you go home!!!!  Bring the old OS engine with you and let me take a gander at it.
   Type at you later,
    Dan McEntee