N

npowell

Well Known Member
I have noticed fluctuation in my rpms now and then when flying level at altitude. I have an RV-9A, carburated 0-320, and FP prop. I might be flying at 10,500 ft, 2450 rpm. I will hear the engine change pitch, and notice that the rpm is suddenly at 2510, and actually sounding better (smoother) than before. A more worrisome scenario is it might lower from 2450 to 2400 without me touching anything.

In each case, changing tanks, turning on the aux pump, or turning on carb heat has not seemed to make a difference. When this happens, I usually end up changing the throttle/mixture settings, and so lose the reference of where things were.

I had a mechanic check my carb venturi, and look things over recently, but he thought everything looked fine. I have a Van's gascolator and thought I might go to the hanger today and take that apart a check the screen, etc.
By the way, 90 hours on airframe and SMOH on engine.

I just remembered another symptom. When the rpm has dropped, I have noticed that one cylinder (or maybe two) has higher egt than usual. I am wondering if one or two intermittent spark plugs could be causing this.

Thanks in advance for any thoughts or suggestions.

Neal Powell
RV-9A
 
Last edited:
Are you flying with an Auto-Pilot. If so turn off the altitude hold. I suspect that you are riding a mountain wave. Just like riding a boat in water with swells. You chug up one side and then race down the other.
 
Flying high all leaned out, sometimes the OAT changes which changes the fuel air ratio.

I thought I had an engine failure up high once with a 0235 and all it was was colder air, pushed the mixture forward, no more engine failure. :)
 
My experience with spark plug problems is that that they show an indication of transient lower EGT. There's still some action and some EGT but it's lower than normal and variable.

And a bad spark plug lead shows a solid failure to fire.

They are easy to differentiate when the other side of the ignition is off, at least on my O-470.
 
Interesting. My prop is also a catto, three bladed. Also, each time this has happened it has been over mountains, either Cascades or Rockies.

Neal
 
Normal for me

I guess if someone flew a fixed pitch prop airplane in flat country under stable meteorological conditions all the time they might not get RPM changes.

I get large fluctuations on almost every flight during straight and level flight. Rising and sinking air is very common. If you hold altitude in rising air, airspeed and RPM will increase and vice versa. I commonly see 200+ RPM differences but don't do anything unless the RPM is going up above redline.

I live on the edge of the mountains. In the mountains I get orographic lift and sink and thermal lift and sink. In the flatlands it is more commonly just thermal.
 
Go to an RV supply store (not the airplane, the rec vehicle) and get one of the cards that contains two level bubbles. Fasten the larger of the two on the inside side-wall of the cockpit or canopy and zero it out with some reference; I used a water level from the tip of the LE to the tip of the TE about a third of the way out on the span and jacked the plane 'til this was level. The next time this happens, see if the bubble changes position. I'll bet when the rpm goes up the nose goes down and versa-visa. I, too, used to think my engine was doing something strange until the bubble showed that I was either climbing into descending air, or descending into rising air. And yes, this usually happens over or near mountains. Real mountains, as in the west; not those puny Alleghenys!
 
Thanks for the helpful comments. I guess after four years off from flying, I forgot that the rpm fluctuates. I feel better and appreciate your help.

Neal
 
You must log in or register to reply here.