MD 81 Unducted Fan test aircraft.

[SkyBaby]

I got curious what this thing sounded like, so I looked it up.

http://www.youtube.com/watch?v=1BMNaXc1rL8&playnext=1&list=PL9D3B4312915CFC58

Here's another vid explaining a little more about it. 

http://www.youtube.com/watch?v=4lip8lPWFLo&feature=related

[NikonGuy]

That's pretty cool, SkyBaby. Thanks for sharing! 

[phantomphan1974]

I remember that, that is OG.

[SkyBaby]

Here's a vid with good footage of the 727 testbed and more footage of the MD 81. 

http://www.youtube.com/watch?v=zxVAaIsfPIY&feature=related

[WARBIRD KEITH]

It sounded like a Piaggio Avanti but a little louder

[SkyBaby]

It sounded like a Piaggio Avanti but a little louder

I noticed that.  Not quite a turbo prop.  Not mellow enough. lol  Got a real nice sound I think. 

[Realbigtaco]

You know that sound is one of the main reason why the Unducted Fan (UDF) has not caught on in commercial aviation.  When McDonnell Douglas was doing those tests they claimed that the UDF would reduce fuel burn by up to 60% of traditional bypass turbofans.  The timing of the program also occurred at a time when oil prices where fairly low and therefore the fuel burn reduction was not as appealing as it is today.

With that said I know either Rolls Royce or GE is looking into UDF's a lot more today.  However the two big areas they need to concentrate on are noise and fan blade out conditions.  In a traditional turbofan if you lose a blade you aim to have your fan cowl contain your blades.  In a UDF you loose a blade and it will possibly destroy the plane.

[NikonGuy]

In a traditional turbofan if you lose a blade you aim to have your fan cowl contain your blades.  In a UDF you loose a blade and it will possibly destroy the plane.

Wow that's pretty scary, how about regular turbo props? Is it not the same?

[nmcbride]

Wow that's pretty scary, how about regular turbo props? Is it not the same?

Apparently not.   Sounds cool, but what would the noise pollution people have to say?

"An unducted fan or propfan is a modified turbofan engine, with the fan placed outside the engine nacelle on the same axis as the compressor blades. Propfans are also known as ultra-high bypass (UHB) engines and, most recently, open rotor jet engines. The design is intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop."

[Realbigtaco]

In regards to the noise I'm sure people would hate it.  I work for the biggest engine nacelle company in the world and you would be surprised at how much money is invested into making those engines quieter.  So the unique situation with the propfan is that it is a pretty substantial step back in the noise abatement rules that the FAA set out.  So much so that I think at the current levels of dB's that are put out by propfans, if airlines used them today they could be subject extreme restrictions in use due to the FAA Stage IV noise requirements.  In fact I'm not even sure you can certify a plane with the FAA unless it meets those Stage IV guidelines.

This is one of the big hurdles that this engine has to make.

But then it is a give and take.  This engine has better fuel efficiency and hence also has less pollution (CO2 and NOX).