I bought a 50 cc USED ARF and need to fix the alignment of the vertical fin and elevators. I can get the elevators to be level with the wings by moving the stab tube in the fuse, but it would take major surgery to get the vertical fin at a perfect 90 degrees. I could make the elevator at 90 degrees with the vertical fin, but then the elevators will not be level with the wings.

Which of these two alignments is most critical and how will it affect this plane?

Thanks for your help

It is as critical as you want it to be... some people cannot build straight, YES, it will affect the flight characteristics, but as long as it is not off terribly, it will fly ok. You should be able to counter act the offset with either a slight adjustment of one of the elevator halves or the trim. Got a picture? If you are a perfectionist, you could run some support wires to pull/twist the tail feathers slightly so that everything is aligned. This could just be from the wood being slightly damp and twisting, it might come back to true if you sat the structure in the sun an hour or 2.
dan

If it's only off a small fraction of an inch, maybe 1/8" or so, I'd leave it. A sport plane will fly ok like that. It may loop a little crooked, and it may turn better one way than another, but it'll still fly fairly nicely. For me, and this is only for me, I build within 1/32" or less of "straight". Your mileage may vary.

Within 2 to 3 degrees of perfect alignment you'll never notice any issues significant to worry about. Perhaps if you were competing at the world class level it would be something to fix. But if you were that good you would not be buying pre-assembled models of this sort of quality. Even then I suspect that any effect would go unnoticed. The key point will be all the know it all pundits on the flight line telling you that your model is crooked.