Say again?

Airplanes make music

I agree with Larry and Jim, the majority of the noise is coming from the compressor inlet at any rpm below 96% airflow is spilling out the inlet creating excess noise. Years ago Marquise N64MD - (the one with the cargo door mods that was converted back to a corp configuration as one of the owners required wheelchair access to the aircraft), bent two blades on the left engine compressor due UFOD and was ferried to I-jet to have the engine repaired.

One could hear that airplane from 5 miles on final howling. Luckily the ferry crew kept the engine at 100 percent and shut it down via the emergency stop as it was murder on the ears.

Have also taxied them around on the start lock just using exhaust thrust. -- Don't do this for long - Mark James and I both saw the damage done to an Mu-2 when the CASI 135 Mu-2 maintenance crew tossed a set of T wheels after running on the start locks at high power; they taxied it about 3 miles on just jet thrust and wondered why the engine came apart.

Mike C - some of it is the exhaust noise but the majority is the compressor. IF you wish go up to 96% with the blades on the locks..... As we used to do during some leak checks .... Noise goes down with the prop not producing any thrust.

Regards,
Nigel

Then every TPE331 airplane should be as noisy as the MU2.

They don't seem to be. Have a 441, Merlin, B100, Commander taxi, doesn't seem as noisy.

Something is different about the MU2. Best I can figure it is the engine under the wing resonating somehow, or the short exhaust causing noise.

Mike C.

Most pilots I know of go to STOP, the engine briefly spools up a bit from the EPA kit firing, then flames out, and then as the RPMs decline through about 50%, they go to reverse. The idea behind waiting to reverse until 50% is to not blast reverse airflow on the line people at higher RPM, or risk having the prop pick up something like the welcome mats. 50% is where there is sufficient oil pressure to put the prop blades behind the locks, but not enough RPM to cause a lot of airflow.


In most cases, at least for TPE331, probably not. The fuel and ignition are introduced at 10% RPM and I don't think you can get to 10% with feathered blades, the air drag would just be too high. So the engine would get to 7% RPM or so and stagnate with no fuel or fire.

The only case I could imagine you might be able to reach 10% with feathered props is a series (48V) start. Even then, I am doubtful. Due to the wear and tear this would cause on the starter, I'm not going to test it on my plane.

Mike C.

I think that the MU-2 sound is unique among TPE-331 installations. The signature "vacuum cleaner getting sucked through a coffee grinder™" sound must be a combination of compressor noise, exhaust noise, and prop noise. It's an amazing and unmistakable sound, clearly a combination of sources.

For you MU-2 Drivers, you can wave at me, I'll give you a nod in return

As to the mighty CJ-610 causing hearing loss... You must have been on the wrong side of the door, up front it's nice and quiet

Most pilots I know of go to STOP, the engine briefly spools up a bit from the EPA kit firing, then flames out, and then as the RPMs decline through about 50%, they go to reverse. The idea behind waiting to reverse until 50% is to not blast reverse airflow on the line people at higher RPM, or risk having the prop pick up something like the welcome mats. 50% is where there is sufficient oil pressure to put the prop blades behind the locks, but not enough RPM to cause a lot of airflow.


In most cases, at least for TPE331, probably not. The fuel and ignition are introduced at 10% RPM and I don't think you can get to 10% with feathered blades, the air drag would just be too high. So the engine would get to 7% RPM or so and stagnate with no fuel or fire.

The only case I could imagine you might be able to reach 10% with feathered props is a series (48V) start. Even then, I am doubtful. Due to the wear and tear this would cause on the starter, I'm not going to test it on my plane.

Mike C.

Clear, flat and capped would've solved this issue. I can't imagine one of these starting in the feather position without serious damage in the form of a hotstart.

It was a 1000 with the SRL's. "Hung" start is probably the key word here.................the crew was normally flying Lear 31's at the time. They got put on the Commander due to dispatch................it wasn't their normal ride. I always preach "it's pretty hard to hurt a 331 once it's on-speed". Starting, however, is quite another story.

Are you 100% sure that they were trying to start the engine with the propeller feathered? If using the unfeathering pump to put the propeller back on the start locks, about one to two unsuccessful attempts will drain the oil reservoir. By motoring the engine, the scavenge oil pump(s) (can't remember how many at the time) will collect the oil from the engine and return it to the oil tank. I'm only assuming that the crew felt the oil reservoir was empty and there was not enough oil to operate the unfeathering pump. We would spin it by hand to protect the starter motor, but not everyone cares to protect the motor.

The slower engines are 1591RPM. They are easier to identify when sitting on the ground because they turn the opposite direction of the 2000RPM engines due to the additional gear in the gearbox. The 2000RPM engines turn clockwise (when viewed from behind), and the 1591's are counter clockwise.

Jason