Interesting. My wife was the exact opposite. She doesn't like headsets (or even headphones like the Bose ANR's I have for airline flights). For her it was the (relative) quiet of not having to wear a headphone and being able to chat with a friend in the back compared to a 421 that steered us to the Mits...

As Mike said, everyone's interpretation is different and decibel readings don't mean as much as how it sounds/feels to you...

This is great input guys. My wife dislikes the headsets and my kids HATE the headsets, so this is a larger than customary issue for us.

David, if your airplane is still with Mike in 3 weeks, we may very well go take a look at it as we're in NC for Thanksgiving and this is something that I'll try to get Kris over to check out for herself.

What other factors are involved in 3 versus 4 blade props on the MU2? I assume one couldn't just swap 4 for 3 bladed or vice versa?

The 4 blade props were meant to reduce noise. So they are also longer blades (from memory, 98" vs. 90", but don't quote me...), and they slowed them down by adding one additional reduction gear to the TPE gearbox. The 4 blades turn at 1591 RPM, the 3 blades at 2000 RPM. Because they only added one gear, the 4 blades also turn counterclockwise when viewed from behind, the 3 blades turn the (more typical) clockwise. This, of course, also changes which engine is the critical engine in a twin, and Mits changed the rudder stops slightly so that there is an extra 2° travel on the side relevant to handle the critical engine (i.e. 4 blade has 2° extra travel one way, 3 blade has it the other way).

Net result is no, you can't just swap a 4 blade for a 3 blade. The engines are slightly different as well, and a very minor airframe adjustment also.

Bigger prop, more efficient.

Actually not... the 3 blades are a bit faster in cruise, when all else is equal. The "all else is equal" clause means equal gross weight, equal engines, so it's not quite that simple. But Mike's (3 blade M model -10 upgrade) airplane is a bit faster than mine (4 blade Solitaire) in cruise.

This is huge for family travel. It was a critical criteria in my aircraft selection...untethered comfortable travel experience. No oxygen masks and no headsets.

Agreed on the noise levels and how huge that can be for families if you are doing longer flights. The realities is people just expect headsets in a light twin or single but once you move up to a cabin class airplane, the ability to not wear headsets becomes more of an expectation of pax. For those interested in how noisy a Marquise (long body, four blader) is, you can listen to the audio on some of the youtube videos I have up. All have a blend of two recordings...audio from the audio panel and the cockpit audio picked up by a GoPro camera's microphone (ambient sound). The channel is: https://www.youtube.com/user/drklain/videos.

As pointed out, the less blades the more efficient a prop is (ideal is a one blad prop like some motor gliders have). The reason Mitsubishi went to 4 blades was due to the noise issue. Dropping prop RPM by almost 25% makes the noise significantly quieter.

If you'd like to see the plane, let me know or give Leah a call at Air 1st. She can set it up for you to see the plane. Have a couple of other people who have also expressed an interest but not sure what their timing is.

Maybe. I find it a bit more "throbbing" in the 4 blade machines but I do think the sound meter says they are quieter. They are certainly quieter for the pilot, further from the props.

I know some who shut down one bleed valve which reduces the noise from the air system. So the engine noise must be not too bad if the air blowing noises are significant enough to shut one bleed off.

You can make half the noise go away by shutting down one engine. :-)

If quiet travel is your aim, the MU2 is not going to top that chart. 441 would be better (props more forward, but 2000 RPM), King Air probably better still (props even further forward). PC12, TBM are probably pretty good, too. I'm not sure about Commanders, they have slow props but the props are mid section. 421 is probably one of the best cabin class piston options.

If quiet is what you want, a jet is best.

Mike C.

Actually not... the 3 blades are a bit faster in cruise, when all else is equal. The "all else is equal" clause means equal gross weight, equal engines, so it's not quite that simple. But Mike's (3 blade M model -10 upgrade) airplane is a bit faster than mine (4 blade Solitaire) in cruise.

This has been my experience as well. I owned a C340 before buying a Marquise and felt the cabin noise level was comparable. One other factor is some MU2s are quieter than others, even those of the same model. Best way to decide is to fly in it.

No, not so.

https://www.youtube.com/watch?v=ugvWNAXgswE

What happens is that at high true airspeed (such as an MU2 in cruise), the prop blade angle has to get so high that the lift the blade produces is not as directly forward. As you see in the above video, the blade angle is roughly 45 degrees. So only about 70% of the lift the blade produces moves you forward, the rest contributes to the spiraling of the prop wash (vortex). Putting energy into the vortex is not useful.

If the plane was going 600 knots and the prop turning 500 RPM, The blade angle would be almost feathered. For an MU2 sized prop, I compute the tip blade angle to be 78 degrees for this condition (90 degrees would be feathered).

This is why you can't have supersonic prop airplanes among other reasons, the prop blade angle gets so high that you lose the forward lift.

At slower airspeed, then the slower turning prop with larger disc area wins on efficiency.


Turbofans have much higher RPM plus they have can have stators to convert the spiraled air into more trust.

Helicopters don't run with high airspeed through the disc. In hover, they have some and it takes more power.

This is one reason why tilt rotors are in a bad place when it comes to rotor optimization. They need zero airspeed high lift, which favors slow turning large diameter, and high forward speed, which favors faster turning smaller diameter. They compromise which leads to high disc loading in hover and limited forward speed for the power exerted.

Mike C.

You can have supersonic props. A fanjet is a prop.

But, changing the twist of the blade is only one way to compensate for the increasing thrust as the diameter widens. But you could just as easily design a blade where the angle of attack towards the airstream is at the optimum angle across the whole blade, and then you change the chord width instead towards the tips. This would result in a blade that has very high chord at the root, but is very thin at the tip.

More like this:

You're not getting the high true airspeed part of this.

Those boat props don't operate at 300 knots.

A 98 inch diameter prop, 500 RPM, at 600 knots has a minimum 78 degree blade angle at the tip (which is just zero AOA, so the angle would even be larger to generate lift). When you get to the root, it is almost 90 degrees. Almost none of the blade lift is providing forward thrust, it is all "sideways".

Mike C.