It may also be limited by structural strength (windscreen resistance to air flow or bird strikes)
I would imagine its more to do with prop tip speeds approaching Mach 1?
I would imagine that resistance to airflow would be an IAS limit and bird strikes would be a TAS limit.
The Vmo is 246 KIAS which transitions to a Mmo of 0.52 at 17,800' under ISA conditions. Mmo is maintained until the max operating pressure altitude of 31,000'. Max TAS will be at 17,800' and decrease up to 31,000'. Since max prop tip speed is based on TAS, the max tip speed (TAS) happens at 17,800'. At 17,800' the helical tip speed is Mach 0.874 - at 31,000' the helical tip speed is Mach 0.907. The calculations take into account the local speed of sound at each altitude. Therefore, it doesn't make sense that the Mmo limit of 0.52 would be directly related to propeller tip speed and it would not be related to a bird strike limit as the TAS decreases with altitude when following a constant Mach number.
Mmo(perating) is just that - an operating limit. In many cases it may be a fairly significant limit like Vne
Just as a technical note, Vmo is related to Vc which is related to Vno. That is probably why you don't normally see a yellow range on the ASI of an airplane limited by Vmo. Vmo shouldn't be exceeded but it's not as serious as exceeding Vne (related to Vd).
That's how I understand the regulations - if someone involved in testing airplanes could correct me on that or confirm it, that would be great.
The inlets for the TPE 331 engines are situated very close to the propeller blade roots. Seems to me that the airflow to the engine inlet might be the first mach imposed limitation reached.
I hadn't really thought too much about the inlet. I understand that the airflow entering can't be above Mach 1.
http://www.nationalflight.com/images...cutout-big.jpg
In that image the inlet looks like it gets smaller which would increase airspeed before it contacts the first compressor stage. However, the image is a bit misleading - it doesn't quite show how the intake is carved out. The air comes in and wraps around the center shaft that connects the engine to the gearbox and contacts the entire surface of the compressor face. Without the exact specs it's hard to say if it's narrowing, staying the same, or expanding. Having looked in there in real life, I don't think it changes much. It would have to almost double the speed of the air to get it to hit Mach 1 - I don't see that happening.
Regarding the interference caused by the lower part of the blade - I'm not sure how that would affect it.