Mitsubishi MU 2 sound
 

Does anybody know where this loud whining noise originates from when especially the MU2 Garret TPE331 engines are running on idle power? I've heard other planes fitted with the same engine but there was not such a striking sound.

Thank you! :-)

All the 331s howl like a banshee don't they! :eek:

The MU-2 does so most distinctively indeed. The inlet on the rice rocket has the shortest distance from the inlet lip to the compressor of any of the 331 installations. This may explain the difference in sound from Commanders, Metros and Jetstreams. Those funny harmonics vanish the moment the speed levers are pushed forward into the takeoff range though. And the noise output diminishes significantly as the torque is applied for takeoff. To my ear, the 331s make less noise at takeoff power than the PT6. Too bad about about the ear-splitting noise at ground idle!

Thank you very much! :ok: Unless its a 5-bladed King Air you can hear a pin drop ;-)

Having flown the CASA 212-200 and the RAF Tucano T1, both TPE-331 engined, I would have said they are all noisy brutes. :cool:

It's all to do with the RPM. The TPE 331 unlike most turbo props is a constant speed fixed shaft engine.

I suspect this could be compressor noise. At normal power settings with a ~20 'blade' centrifugal compressor at ~40,000 RPM the rotor-stator interactions are occuring at about 13KHz and propagating against a relatively high Mach no of intake flow. That frequency is sufficiently high that most people will not be sensitive to it. Higher frequencies also dissipate more rapidly as they propogate away from the aircraft.

If idle is 60-65% (guess), then the frequency will drop to around 8KHz, to which the ear is much more sensitive.

As to why it's more noticeable on the MU2, I don't know, but high frequency noises can be more directional in nature and sensitive to intake geometry and acoustic treatment (if fitted) more so than lower frequencies.