ORIGINAL: davidbegg
Yes we thought that the spinner balance would not be to much of an issue as the mass of the spinner is close to the centre of the engine and as you say nothing like an out of balance prop. However two different balanced props and the vibration is still there. The plane was supposed to have a 20 engine and it has a 29 OS engine. Is it possible that the larger engine could itself be causing the problem? Surely if the larger engine is not run flat out and the plane is balanced the larger engine would not be an issue?? Apart from the severe vibration the plane flies really well.
It might be the larger engine versus the very light, smaller airframe. IC engines aren't really in balance at any rpm, but are usually better balanced at their design rpms. Run them slow and they sometimes are worse than expected. They also are shaking a lot more mass of their own than the frame was designed to deal with.
Spinners really aren't usually the cause, if ever. They really are something that can be ignored.
It is worth a try to simply swap out the prop. Very, very few of them actually read different pitch on the blades. Very few, and less nowadays with better mfg methods, but it could happen.