Effect of top cowling on static rpm

Has anyone noticed a difference in static rpm with the top cowling off (e.g. for carb balancing) compared to with it on as normal for flight?

Had a bit of a problem recently (long story ? best not to ask!) with the exhaust having become displaced from one of the stub pipes that resulted in blow by which caused a significant reduction max rpm and rough running when the top cowl was on but not when it was off. A consequence of which is that an increase in my awareness of max static rpm has ensued and after the problem was resolved it seems as though with the cowling on the static rpm is about 100 rpm lower than with it off (ambient temp 20C/68F, density altitude approx. sea level).

The difference is possibly not unreasonable, since the air temp under the cowling will be warmer than with the cowling off but not having been aware of any difference in the past I wondered if anyone else had similar experience. If not I have more investigating to do!

Alan

p1lhs said:

it seems as though with the cowling on the static rpm is about 100 rpm lower than with it off
Alan

Click to expand...

Interesting observation! Did you try taking it back of and on about twice to verify if it was static in the data?

Alan,
I have done a lot of different ground run engine tests with the top cowl on and off, and I have never noticed enough of an RPM difference that couldn't easily be explained by a slight variation in wind, etc.

If differing air temps were to have that much effect then we would see the same level of change between a hot and cold day with the same prop pitch, which by my experience we don't. It does change some because of different prop effectiveness and different engine HP in the differing air densitys, but it is never as much 100 RPM.

But you have raised my curiosity.... The next time I do some ground testing I will make a point to look at RPM changes more closely

If your not flying with the top cowl removed who cares?

Thanks for the replies.

I have verified the rpm difference several times with cowl on/off and there is always a drop with the cowl on although it has been a bit variable which was possibly due to a bit of a breezy, changeable wind.

The rpm drop might always have been there (the airplane has been flying for 3 years) and it may be irrelevant since I don't fly without the top cowling. On the other hand it could be symptomatic of an underlying problem which I would rather identify whilst still on terra firma!

Alan

Cowl Off RPM Difference

I had a power loss experience after TO. In the process finding the problem I did static WOT rpm ck with the cowl on and off. I also did static WOT rpm with the carb air filters removed with the cowl on and off. In our Hot Texas weather cowl on and off makes a 100+ rpm difference. Removing the carb air filters did not make a rpm difference. I discussed this with an Arizona Rotax mechanic and he explained that when you do a static WOT with the cowl on in the RV-12 it is like doing a WOT with the carb heat on. Makes sense to me. All my WOT checks were made after letting the oil temp get to the green.Bob Kibby N712BK 255 hrs.

Bob,

Did you determine the cause of the loss of power?

Rich

Power Loss Cause

No, not to the point that one thing caused the power loss. One carb set of floats weighed 6.7 gr but I later weighed them individually. One float weighed 3 gr and the other 3.7 gr. No one liked the 3.7gr float. So, I put in new floats that weighed 3 gr or less. The carbs were reinspected and serviced and the diaphragm Pistons were found to be "stickey". This problem was corrected and carbs were returned and installed. This is when I experienced the rpm difference between cowl on and off WOT rpm. After consulting with several experts I decided the aircraft was airworthy and did a high speed taxi test were the rpm exceeded 5,000 rpm. That was later followed by a test flight where the climb out rpm at 85 kt was5,050. Have flow 5 hrs without a problem. I believe the power loss was a carb issue. The Dynon data showed a large egt difference durning the period of the power loss. After engine warm up I am now seeing the egt's are within 10 deg or so of each other. I now check the egt tracking in addition to yellow and red line. Will provide an update if any power loss indications happen. Bob Kibby N712bK

I remember reading someplace that the minimum RPM during full power climb should be 5200 or more with an airspeed of 75 knots.

Bob, any chance it was vapor lock causing the power loss?

Power Loss

No. See my post on this forum under "Power Loss" Bob Kibby

Thanks, Bob. That's the peril in just reading the most recent posts - us old guys tend to forget the earlier info!