Was on my second flight today and the engine sputtered and died in the air. Brought it in and notice the prop wouldn't turn through the compression stroke. Even if you forced it. I removed the top of the engine and there were small metal shavings on the top of the piston. The glow plug was toast as well. I cleaned out the cylinder and it fired up again but there was a small knocking sound every now and then and didn't seem to run as smoothly. Any idea what happened? I'm hesitant to take it all apart but if I have to do it I will.