Hello,
This is continuation of work that began last year.

I know that adding shims "softens" the engines but how much shimming is too much?
Is there any general rule of "shimming"?

My Evo60NX R/C (Evolution PN EVOE0600) will only run well in 2 cycle because of the porting geometry and timing.
It runs stable 2 cycle at 10,300 launch RPM on 13x4 MAS two blade propeller, using 10% nitro Omega with some castor added.
Unfortunately this gives the lap times 4.8-4.9 seconds.

I have tried to slow this engine down and it can go into unstable 2-4-2 (fluctuating between 8,300-9,800 RPM )when the fuel mixture is rich.

Then, I have tried to add shims hoping to deal with this instability but it only made everything worse.

Any suggestions?
Thanks,
Matt

 

Different prop and/or longer lines, if you're not maxed yet on line length. MAS are crappy props, as were Zingers. APC makes a 13-4W and 14-4W wide blade, one of which might work much better. Also look at XOAR, Vess, and some of the other new brands of wood props. I think most are made by XOAR in China.  Steve

Hello,
This is continuation of work that began last year.

I know that adding shims "softens" the engines but how much shimming is too much?
Is there any general rule of "shimming"?

My Evo60NX R/C (Evolution PN EVOE0600) will only run well in 2 cycle because of the porting geometry and timing.
It runs stable 2 cycle at 10,300 launch RPM on 13x4 MAS two blade propeller, using 10% nitro Omega with some castor added.
Unfortunately this gives the lap times 4.8-4.9 seconds.

I have tried to slow this engine down and it can go into unstable 2-4-2 (fluctuating between 8,300-9,800 RPM )when the fuel mixture is rich.

Then, I have tried to add shims hoping to deal with this instability but it only made everything worse.
 

   I am not at all surprised at all that it made it more unstable to lower the compression. A lot of "stunt lore" (i.e. bullsh*t) has grown up around adding head shims. It's rarely necessary, and usually only small changes are required. Adding head shims reduces the power difference between 4 and 2, and reduces the power overall, and causes the 4-2 switch to take longer. The only engine I ever found to need, very occasionally, a bit mroe clearance (like .003-.005) just to work was the ST46. In that case, it was because the stock head gasket compressed over time and reduced it from what it started.

     Unless it is backfiring, quitting, etc, the compression is not too high. The fact that lowering it made it worse is pretty definitive.

    Patient, probably Misty Rowe: "Doctor, doctor, it hurts when I do this"
    Doctor Archie Campbell from Hee-Haw: "Then don't do that!"

 
     I am also extremely skeptical about the "timing and port geometry" causing it to only 2-stroke. But putting that aside, it sounds to me to be a combination of too much venturi and incorrect prop. You have to get off the Zinger/Master Airscrew train, at least to get it to work at all. APC 12.25-3.75 would be a good starting point.


 Put the head clearance back to what it came with and leave it alone. Reduce the venturi diameter by maybe .010" at a time, trying it after each change, until you get about the right speed AND are running right around the 4-2 break point in level flight. Then, adjust up and down from there to get the right combination.

     If you have no idea where to start, try a venturi with a choke area of about 0.018 square inches. That's about .152" for a flush outlet venturi (like the stock PA or Tee Dee 049), or .271 for a "cross the inlet" venturi with a ST spraybar. This is probably on the small side, so if you start there, run it up, not down.

    Brett

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As suggested by Brett, I will shim the venturi choke area to get 0.271" and run the tests.

Additional problem: ST60 NVA needle keep turning due to vibrations. Tightening the NVA brass nut works for a short time and the connection is getting loose again.
How to fix this annoying thing? There is no point to test in the air if I do not know if the needle holds the position. By the way: the vibrations are "normal vibrations to be expected" not the excessive vibrations caused by the propeller and spinner imbalance. MAS 13x4 and the spinner were balanced before tests.

         I can only imagine that the brass in the NVA is still creeping a bit. You need to tighten the collet until you can barely turn it, and after a few cycles, it should settle in and work OK. It needs to be that tight to obviate the effects of the needle valve ringing like a tuning fork. All bets are off if it is a "clone" version, some of those just don't work at all, and the collet bottoms out on the spraybar without really compressing it.

        Brett

I always defeat the spring NVAs and on PA versions I shorten them as much as practical without beating my knuckles to death to adjust

Field report: April 14, 2017.

Two flights still on MAS 13x4.
Launch RPM stable 8,500 in wet 2 cycle mode with venturi's choke area diameter decreased to 0.265".
EVO60 had still 0.51 mm. head shims in it.
Normal and inverted flight RPM were the same. When flying up and down the RPM are roughly the same.

Lap times: 5.6-5.8 sec. and this is too much on 61' long lines (eye-to eye)

Could not finish some maneuvers because lines were going slack.

Next step: launch RPM 9,000-9,500 in full 2 cycle mode to provide proper lines tension and the lap times around 5.2 sec.

Additional problem: when the model is flying normal level flight the outside wing's tip is down about 1.25". When it is flying inverted the outside wing's tip is up 1.25".
The wings are straight according to the measurements taken today. The flaps are straight. When I hanged the model using the leadouts, the wings follow the line perpendicular to the ground.

I will add tabs to the wings ends to compensate the aerodynamic moment but, having the above in mind, I have no idea where this moment is coming from.

Any suggestions?
Thanks,
M

the wings level problem , tweak the flaps

Hi Chris,
No castor added. The fuel is stock Omega with ~10% nitro.

Hi Mark,
I know that wing warp is there but I cannot see and measure it. The same with flaps.
To counteract the aerodynamic roll moment I have added small "aileron" type tabs set now at + - 3 deg. but they do not work. Perhaps they are too small or they should be deflected + - 15 deg.?

Hi Brett,
On April 11, I have removed the added head shims and tried to run the engine still with MAS 13x4 but it run consistently backwards.
The compression ratio was such that it was difficult to even flip the prop without the kick back from the engine. Also, the engine was flooded and was spewing the raw fuel from the muffler.

After trying unsuccessfully for two hours, I went home and added all the shims as the engine run with them on April 10. I also measured the venturi choke area diameter again after removing the entire venturi assembly from the engine for the ease of access.

This diameter was 0.265 -0.260" depending where I measured. The Al tube I used for the venturi choke area insert is soft and was probably slightly distorted when I was pushing it into venturi.

I took APC 12.25x3.75 with me and went to the circle.

Unfortunately the engine, still with MAS 13x4, was running backwards but was of course much softer.

"What is the root reason that this thing runs backwards without and with shims?" I thought but had no answer.
"Perhaps 0.265-0.260" venturi is too small and the engine is choked to death?"

I was ready to go home knowing less than before but, as the last resort, decided to check the spray bar fuel delivery hole position.   

With larger venturi choke area diameter (0.285"), the fuel delivery hole was positioned sideways and I left it this way when I choked the venturi area diameter to 0.265-260".

Comment: "sideways" means that the fuel delivery hole axis was perpendicular to the air flow direction in the choke area.

The fuel delivery hole was indeed still sideways.

"What will happen if I turn the spray bar 90 degrees for the fuel delivery hole pointing directly towards the shaft?" I asked myself.

I did it and the engine (still with MAS 13x4) started with the second flip.

In the meantime I kept incrementally tightening the NVA brass cup nut and the needle stays now in the set position despite the vibrations.

Comment: I also kept tightening the muffler mounting screws and the muffler vibration, though still present, is now much smaller.

Conclusion: I will try APC 12.25x3.75 propeller with this setup and without the head shims and then increase the venturi choke area diameter to 0.271" but first I have to fix the wings and flaps warp problem that I cannot see and cannot measure.

Comment: I do not have reamers and changes in the venturi choke area insert inside diameter can be measured only by caliper. It may be almost impossible to enlarge the insert inside diameter in 0.001"-0.002" increments using the sand paper rolled on a stick.

Best Regards and thank you all for comments and suggestions,
M