RPM Fluctuations on Take Off

[jlitwin]

An engine/tachometer concern.

 

On takeoff with full power applied MP goes up normally and remains constant. However, RPM increases to about 2500, then fluctuates between 2300 and 2500 for about 3 seconds, then increases to 2700 and remain there. There is no apparent decrease or fluctuation in power output.

 

In flight RPM behavies normally. And, this anomaly does not occur during power increase for a go-around or on a touch & go landing. But, it does reoccur during a taxi back and takeoff.

 

Should we be concerned?

 

 

[Cruiser]

Which model airplane? Mechnical or electric tach?

 

First step is to verify the tach is reading correctly.

[N601RX]

If there is any question about the RPM, the Engine RPM app on the I-phone will confirm the problem

[Cody Stallings]

What prop an Gov?

[BigTex]

If your engine is bouncing between 2300 and 2700, you'll definitely be able to feel it. Since you said that you didn't notice any power changes so I'd suspect a faulty tach.

[N201MKTurbo]

When a mechanical tach needs lubrication of the input bearing, it will stick. When it sticks the cable will wind up until there is enough force to overcome the friction the input shaft will spin causing the needle to jump.

The bottom line is it is time to get an electronic tach. Trying to fix the mechanical tach is a waste of money. For the same money you can get an electronic tach that will work perfect forever.

[captainglen]

^{The observed effect is caused by the governor hunting. All governers opperate on the same principal a cam surface on a set of fly weights pushes on a spring loaded swash plate. The spring tension is set by the input from the propeller control; the oil pressure output represents the difference between handle position and actual RPM. Flat spots can form of the fly weight cam surface causing the prop to prefer a particular pitch angle when the wengine is opperated near the flat spot the prop will hang there until the RPM change is large enough to get the fly weights to move off of the flat spot. It is possible but less likely that the propeller blads are binding so this needs to be checked before replacing the governor especially since the observed fault occurs near or at the minimum pitch mechanical stops in the propeller hub. Flat spots of the fly weights are most common near cruise RPM.}

[N201MKTurbo]

If the governor was doing as you describe it would have been extremely noticeable. The poster said that he perceived no change in RPM.

[DAVIDWH]

First the free baro app, now the free rpm app,

 

who said a glass panel is expensive?