Open Fan

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West

New Member
Joined
Oct 19, 2010
Messages
3
Location
Graz, Austria
Hi everybody,
I hope there is no thread on this topic already, I did a search before but didn't find anything.
I lately just stumbled upon the open fan technology being developed by Boeing and Rolls-Royce.

Aerospace Firms Investigate Open-Fan Propulsion Technology | Defense Technology News at DefenseTalk



The benefits seem to be a huge increase in fuel efficiency but there doesn't seem to be very much information on this and I didn't find an answer to a few questions I have:

1. Does the open fan need a turbine directly attatched to it to work efficiently or would it also be suited to use it with a turboshaft setup or even a rotary engine?

2. I think the open fan would only be useful for high speeds and not so much for small airplanes flying at slower speeds (~200mph)?
Or would there be a way of adapting this to an airplane of this size?

3. I might be totally wrong here but the fan design somewhat reminds me of the P-51D. I'm aware that the double-rotor was used to balance the torque, but are there any other benefits to this?



Thanks in advance!
West
 

orion

Well-Known Member
Joined
Mar 2, 2003
Messages
5,800
Location
Western Washington
There's several subjects herein and they're not really connected. The fan with the turbine you show above has been commonly referred to as an unducted fan, the technology of which dates back to the sixties, but that has gained attention in the eighties or so when fuel efficiency became the hottest topics of airline propulsion. The UDF concept was said to be able to deliver increased efficiency over the turbofans of the day, which was quite true.

But as you guessed, the efficiency discussed was at speed and at altitude - neither of which have significant application to a typical GA airplane. The problem though was that the fans tended to be fairly loud so would have not been popular around airports and also had a tendency for catastrophic failure if hit with substantial ice, which is commonly shed from the fuselage. Then, if placed next to the fuselage, the pressure pulses tended to create substantial noise inside the airplane too, which would have required a lot of heavy insulation. The problems were significant enough that virtually all research was dropped at the time.

Today there seems to be a bit of a resurgence of the technology but so far no-one has stepped forward for realistic application.

Regarding your questions, no the fan does not need a turbine - it can function with other engines as well but as with most things, its function needs to be closely matched to the engine and the speed of the airplane. As such, it's almost a point design. For airlines this works fine since on the ground they are substantially over-powered, so a bit of efficiency loss on take-off and climb is not all too significant. For more marginally powered light planes this would be more problematic. Applications to smaller, slower airplanes would be unlikely, especially when you consider the propulsion benefits are only when compared with the older turbo-fans. A recip with a prop is still much more efficient.

But the P-51 you show (and many similar prop configurations) are totally different in that the counter rotating props are there to essentially remove vorticity of the flow and thus recover some measure of lost efficiency. Yes, some UDF applications look to do the same thing but not all UDF configurations are dual disk. Totally different environment and dsign goals.
 

Robby

Well-Known Member
Joined
Apr 8, 2010
Messages
123
Location
Texas
Google Thunderscreech if you want to see how this has been tried before.
I read an hilarious article about in in an old issue of Smithsonian by someone who worked no the project.
Basically they were trying to improve 'throttle response' in a jet for aircraft carrier landings ( to avoid landing at high power settings ). You can't just spool the engine up in a second or two but you CAN change propeller pitch to increase thrust MUCH faster !!
The biggest problem was noise.
Since the prop had a relatively small diameter ( to avoid
'self-destruction !! ) but the tips still exceeded the speed of sound so what you basically had was a continuous sonic bomm coming from the thing.
The writer stated that you really could not be anywhere near the plane of the prop without getting literally knocked over by the sound. He also stated that he lived 27 MILES from Edwards and could hear the thing when it was being tested at full throttle !!!!
 

orion

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Mar 2, 2003
Messages
5,800
Location
Western Washington
The Thunderscreech program was amusing at best and yes, a fairly complete failure. The biggest problem was that like the P-51 above, it tried to incorporate a fairly conventional prop to the application so efficiencies were poor and yes, the sonic noise was "impressive". Although the UDF technology tries for a similar configuration, the blade design is much different in that it is designed for high subsonic operation, and of course it is being designed and optimized more for cruise than low speed thrust.

But as far as airline application is concerned, the ice impact issue is still a major stumbling block. Most of the programs have been working with graphite blades but their application has proven less than ideal since ice caused damage can be significant. The only positive results I heard of were when the blades were fabricated from Titanium, but that program also faded away - don't know why.
 

autoreply

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Jul 7, 2009
Messages
10,748
Location
Rotterdam, Netherlands
But as far as airline application is concerned, the ice impact issue is still a major stumbling block. Most of the programs have been working with graphite blades but their application has proven less than ideal since ice caused damage can be significant. The only positive results I heard of were when the blades were fabricated from Titanium, but that program also faded away - don't know why.
If I recall correctly, EASA/FAR require the turbine to stay inside the housing with a birdstrike. A prop might disintegrate, but any critical structure should be adequately protected. That's quite hard to do, especially with such big fans/props, and even more so with a pressure vessel and all those control cables going to the tail.

As for carbon blades; rumor goes that if you mention carbon, graphite or cfrp at RR, you're immediately thrown out of the building. I only checked that at Le Bourget, but yes, they were really pissed. :gig:
 
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