I think there might be more to what initially meets the eye. Currently, the shaking occurs at high altitudes when running less than full throttle and more than idle, but disappears when going to idle or full throttle.

I did some experimenting, based on someone else's observations with the ingame Hurricane and something i had read in a manual for an FSX add-on. In that manual it was advised to set propeller pitch to full fine for takeoff, unless the runway altitude was significantly high whereby it was advised to set it a bit coarser than normal.

That led me to think if the low-density air has some kind of effect on the propeller's "pull", making it harder to generate the same amount of thrust at higher altitudes for a given pitch and manifold pressure setting. Since running coarse pitch gets a bigger "bite" of air, i decided to test if i could get any improvement by running lower RPM.

So, i started climbing with the 109 at 1.2 Ata and sure enough, i got the engine shakes at some point around 4km. I then came back on the prop pitch and reduced RPM to a lower setting that i usually run only for accelerating in dives and level flight cruise at lower altitudes, the pitch indicator at 10:10 and RPM at 2100 or so.

Not only did the shaking disappear almost completely (i was still getting it but on a much smaller intensity and only as i was getting close to 6km), but the aircraft started climbing much better as well. When i trimmed out the climb it was accelerating much better and getting higher speeds.

I'm not sure how much tuning the CEM needs (the auto-mixture mechanics definitely seem in need of adjustment) but maybe there's more to it. Maybe the exact effects are over-pronounced but not entirely unrealistic, for example it might a realistic effect that's simply overdone and needs to be fine-tuned and toned down a bit.

Also, apart from the effects of low-density air at altitude, there might be another bit we are missing. What we do know is that there are limits to how low an RPM can be run at high manifold pressure settings (if you substantially reduce RPM the MP climbs and you need to throttle back to keep it within limits). What i suspect is that there might be not only a correlation between optimal RPM at different altitudes due to thin air, but also a limit as to how high an RPM we can run with reduced MP settings.

If you are diving and don't want to accelerate too much for example, you can go to idle in order to keep the RPM in check and then use full fine pitch. The prop wants to turn at full RPM but the engine doesn't provide the torque at idle, so it's windmilling as the prop blades are facing the airflow almost head-on, acting like an airbrake (that's how i prevent the 110 from going above 500km/h in dives, where the flutter and "snaking" currently seems to occur).

Since available MP drops as we climb maybe a similar effect occurs, namely a lack of engine power for the commanded pitch/RPM setting to work as it should and the propeller losing efficiency. This would be like climbing at full available power but with a bit of an airbrake effect thrown in, which might be possible to create shakes and buffeting.

Of course, the engine running rough due to the auto-mixture routines currently used complicates things and makes it harder to identify what really happens. It's true that the engine does run rough, it can be easily confirmed by the in-cockpit sound because the cut-outs are easily perceptible. I'm just wondering if there's also a second, completely aerodynamic effect which spoils things up high and depends on the ambient air pressure, available engine power and prop pitch mechanics.

I'm sure people like Viper2000 could shed some light on this, namely whether there's an upper RPM limit for a given (lower than full power) MP setting. Any ideas?

Generally the combination of rpm and mp are indicated in the plane's handbook and low rpm with high manifold presssure should be avoided because of the detonation problem (I've made some posts in this thread before about CEM where i speak about MP, RPM and prop pitch systems). For a lower MP than full power you should normally use a lower rpm than max rpm (again as the handbook indicate).

The airbrake effect you are speaking about is right, but normally as long as you keep MP and RPM in the right range/settings there shouldn't be any problem because the thrust to drag ratio is optimised when using the rights settings.

The shakings are not constant for me but there is a kind of stutter every 10 or 15s, I don't think this is related to graphic problems but only to a (maybe flawed) behaviour of the plane, because when I'm in external view or in cockpit view but without CEM or headshake activated the problem dosen't appear. For the vibration I still can't tell if it's a normal reaction of the plane due to a resonance effect induced by a certain frequency or an overmodelling/flaw.

Just coming back to this. I've been flying the Hurricane (Rotol) for quite a while now - almost exclusively in fact - and I have to admit that with the mixture lever fully back it seems ok. I don't know if they changed anything without saying but I know when I started using CoD that I had problems starting and running the engine unless the lever was forward. Could have been me of course but I wasn't the only one in our Squad that had the problem. Now I get vibration if I lean the mixture (lever forward) although I can partly lean it off under some circumstances. I haven't tested it in any meaningful way - too busy having fun - but I must try some climbs to high altitude etc and see if I get the same mixture/vibration problems as some other guys. Of course they may not be 'problems', it may be how it is supposed to be. :)