I'll add takeoffs too. A few weeks ago on a breakfast run, I let my buddy fly over, and I flew back. The climbout was very bumpy and I was using both hands on the yoke for while. I was over very rough terrain and was only climbing a couple hundred feet per minute and couldn't understand why. RPM was at 2700, flaps were in, and needless to say, I was concerned. I told the right seater about it and said I don't understand why it isn't climbing better cause I'm at 100% power, and he looked at the JPI and said, No you're not". Sure enough, he had loosened my throttle friction on the flight over and unbeknownst to me, it had crept out and I was at 50% power.