RPM decrease during takeoff roll

[Ronald Cecchi]

Hello all,
As a relatively new Comanche owner, and likewise new to complex, I have a question I would like help with.

Recently I was beginning my takeoff roll and instead of getting close to 2700 rpm I only got about 2500 then as I briefly continue the t/o roll it dropped a little further to about 2350. My MP appeared to increase about 2" during this process until I aborted. The second attempt had similar results.

I shut the plane down and called my a/p and discussed it with him. He really didn't have an answer. I restarted and again did a full runup and cycled the prop etc and all appeared normal again. This time everything appeared fairly normal. RPM was at 2650, perhaps 50 below normal, and MP was where it should be, and I have not been able to reproduce the problem again.

Looking for ideas on how to proceed. I assume their could be tach issues, but it did appear that MP was a little high making me wonder if that was it. And since I can't get it to duplicate I am stumped.

Thanks for any thoughts!

[Clarence Beintema]

Is it possible that something is catching and pulling on the prop control cable in the engine compartment? The fact that MAP went up suggest that the RPM drop is related to prop blade angle and not throttle position.

Clarence

[Kristin Winter]

If inspecting the prop control cable doesn't yield anything, i have to wonder when the governor was last overhauled.

[Michael Bryant]

I recently had something similar. My aircraft is carbureted so this may not apply to you.

At the beginning of the takeoff roll, I was making full power. A second or so later both MP and RPM decreased so I aborted the takeoff. Performed another run-up, same results.

Turns out the coupling boot that connects the air filter/cowling to the carburetor had a couple of fasteners come loose. The boot was folding in on itself as the air in the cowling became pressurized from ram air and was reducing the amount of air the carburetor could take in.

You have your mechanic give it a look if you are carbureted.