Bill_H

Bill_H

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Item A: I notice that my RV12 RPM "jumps around" a bit in increments of 10. Like 5000,5010,5020,5010,5020... Seems to be running very smooth. I figured this was an artifact of the Dynon sensing and display mechanism. But when I was going to get a prop balance, Walt A. was getting the same thing from the reflection-based readings of the prop itself - actually too much rpm variation to do the balance. Tried 4000 rpm (good throttle lock) and even WOT. No avail. Note: the prop balance was showing within limits, improvable, but rpm instability made the balancing process impractical.

Later I checked the plugs - all fine, 25 hrs since replacement. 75 hr TT on engine. "Mag" drop is good and even, "B" seems very slightly 'rougher' than the "A."

Walt thought it might be due to prop blade pitch differences. I was going to slightly repitch the prop anyway. So a couple of days later a check showed that the blades were still within 0.1 degree of each other, (so they were during the test) both before and after the repitch.

Do I have an ignition module problem? What to check? This engine has the soft start built in.

Item B: The prop test is done with the upper cowl off. I noticed about a 100rpm increase in WOT compared to my normal number with the cowl on. Also the idle with throttle full back was higher. Is induction significantly limited to the air filters by the cowl? (The idle phenomenon seems funny because it was originally set with the cowl off!) With the cowl back on, idle and static WOT readings returned to prior values.

Anyone else notice this? Would better induction to the air filters mean more power? Hood scoop anyone?


OH- I downloaded the Dynon data from after the test but haven't put it in Savvyanalysis.com yet.
 
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Could it be just a rounding effect? As the unit is not displayed rpm is rounded at the tens; if you are operating around 2015, sometimes you get 2014 rounded at 2010, sometimes 2016 rounded at 2020. Is it possible that the gentleman who tested by reflection has got the same rounding problem?
 
Bill_H said:
Anyone else notice this? Would better induction to the air filters mean more power? Hood scoop anyone?
Click to expand...

I have done a lot of ground running testing on RV-12's with the top cowl on and off.
I have never noticed it effect full throttle static RPM or idle RPM.
Idle RPM is significantly effected by engine temp. (particularly oil temp).

Because of the rather course pitch (for this HP range of engine anyway) we use, even a small amount of wind will effect the idle and full throttle static RPM.
If the the wind is variable (direction and/or speed), it will cause a slight variation in engine RPM.
 
Good points. He had balanced two Lycomings right before the attempt on mine. I don't know a lot about the instrument but he has also balanced a lot of Rv12s. The engine temp could account for "B" since normally those checks are done soon after reaching 120 degrees but for the test we had been running at higher rpms for awhile.
 
RPM variation

Hi Bill,

As Steve mentions any wind will cause a slight variation and as Wingedfrog mentions rounding takes place within the Dynon program because it only displays in tens. Air flow through a cowl on and off could cause some rpm change and this can also be affected by air filters. (i.e. owners applying too much oil on a K&N for instance) Some air filters can have a slight effect on air intake. I had a Jabiru in once that had high EGT's on all 6 cyl. He was using a paper air filter. I threw it away and put in a K&N. All EGT's went down almost 150F across the board and the owner was floored. No more high EGT issues. So it can make a difference. Even when flying, your rpm may fluctuate 10-40 rpm at times due to several factors and it isn't a problem. Remember air pressure changes on a Bing 64 can cause variation because that is how it works for the piston. Wind gust direction on a prop can cause this very minor fluctuation.
The engine was not changing rpm from 5000-5020 that fast in reality. I balance props and most Rotax engines do this and it still balances. I do however try to balance only on calm days. A 10+ mph wind can cause you headaches.

I think if you go back and try another dynamic prop balance and don't worry about the minor rpm variation you'll be okay.

p.s.
Out of curiosity what plug gap are you using?
 
When I checked them after the attempt, they were all at about .027. I just regapped them to .030 from looking at the manual. Probably the wrong thing to do! I do use the thermal paste per directions. Have K&Ns.

Smaller gap, hotter but smaller spark, better for winter, OK. Why is a bigger gap better for summer?

Thanks, Bill.
 
If your manual is telling you to gap at .030 then you have not updated it. At least a couple of years ago Rotax changed their manuals to recommend .023 - .027.
 
Sandpiper is correct. The new gaps are .023 - .027.
Use the wider gap for the hotter summer months and the narrow gap for the colder winter months. Going wide gap now in cold weather will make your problem worse.
 
Bill_H said:
Item A: I notice that my RV12 RPM "jumps around" a bit in increments of 10. Like 5000,5010,5020,5010,5020... Seems to be running very smooth. I figured this was an artifact of the Dynon sensing and display mechanism. But when I was going to get a prop balance, Walt A. was getting the same thing from the reflection-based readings of the prop itself - actually too much rpm variation to do the balance. Tried 4000 rpm (good throttle lock) and even WOT. No avail. Note: the prop balance was showing within limits, improvable, but rpm instability made the balancing process impractical.
Click to expand...

Bill,
Guess I need to clarify my observations which may or may not have anything to do with the RPM variations you were seeing.

Normally during a balance I have a reasonably steady level of vibration at a specific polar location. What I was seeing on your particular setup was a significant and continuously changing vib level / polar location. This is indicative of an engine or prop problem (or perhaps in the case of the rotax a gearbox issue). I have seen similar things before and it is "usually" from a rough running engine thats why my first suggestion was to check the plugs and carbs. I have also seen propellers cause this when a prop has been bent and straightened (causes a continuously changing aerodynamic imbalance) hence my next suggestion was to carefully check the prop, pitch angles and tracking.

The varying RPM indication "may" be the result of one of these other issues, but the RPM variation itself is not the cause of my not being unable to balance your setup (small variations are averaged out during the balancing process). I've done enough 12's to know that what I was seeing was not normal.
 
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Then my first diagnostic item would be a carb sync check. If this is off then those vibrations will show up because one side of the engine is trying to run at a different rpm than the other. The engine is pretty new to have a gearbox issue.
 
Bill_H said:
When I checked them after the attempt, they were all at about .027. I just regapped them to .030 from looking at the manual. Probably the wrong thing to do! I do use the thermal paste per directions. Have K&Ns.

Smaller gap, hotter but smaller spark, better for winter, OK. Why is a bigger gap better for summer?

Thanks, Bill.
Click to expand...

winter or summer, if the Rotax is running at 200? or maybe 230? because it is liquid cooled, what difference does it make?

Conceivably make starting a little easer? Other than that, I'm open for discussion on the reason why an engine at operating temperature needs different gaps from summer to winter. I don't recall Rotax saying anything about this, but if they have I'd like to see the reference.
 
My Aces system says not to try a dynamic prop balance on windy days. If you were turned with the prop down wind then it gets its own prop wash. Even on a carb sync I always turn into the wind.
 
Did a carb balance today, think that might have been it. They were balanced at 50 hours on the plane, now at 75 hours and I think the cables have settled/stretched. Seems smoother. Also regapped the plugs to 0.025 - the incorrect .028-.032 range was in the original Vans maintenance manual.
 
Bill_H said:
the incorrect .028-.032 range was in the original Vans maintenance manual.
Click to expand...

That's because when the manual was originally written it was the correct value.

If you look, you will notice that the more recent revisions have been removing information related to products supplied by others (Rotax most specifically).
Lesson learned....
 
No criticism implied, I am very familiar with similar issues in the industrial control systems world!
 
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