Recently I have been shooting airshows. I have used a camera (Sony HDR CX 500V) equipped with optical image stabilization that works great shooting level but seems to produce very jerky footage when pointed up.This may be due to the CMOS sensor and/or AVCHD format. Yet I have also used a Sony HDR HC1 and got better results (HDV format and electronic image stabilization). Finally even my Canon A1 with CCD sensors and optical image stabilization does not seem to be as good as the HC1. My conclusion is that optical does not work as well as electronic when the camera is pointed up. Am I arriving at the right conclusion?

Hmmm. I don't think it is because it is pointing up, it is because you are shooting a moving object. I shoot air shows a lot with an HPX 170 Panny. But I always follow the rule of "if you use a tripod, turn off the OIS" . And I always shoot airshows with a tripod since I am zoomed to the max so much hand held is horrible. The image stabilization is to counter YOUR movements when operating the camera, not the movements of the image in frame. I would suggest not using the OIS at all if using sticks.

The image in motion will appear jerking because the OIS is locking onto the plane and releasing and locking and releasing giving it that jerkiness you are talking about. The same happens if you do a pan with the OIS on. It will attempt to lock onto images over and over and give that jerky look as well.

You'll find OIS works well for shooting fairly stable objects and YOU are not so stable.

Just my opinion. I don't claim to know it all!

Jim