Open Rotor: Jet Engine with a Mohawk

July 9, 2009 1:18 PM

Ultra-High Bypass (UHB) or "propfan" jet engines received attention in the late eighties as an economical and greener alternative to currrent GTF and ATF jet technologies. Adoption was partially prevented by industry fears that the external propellers would being seen as a step backwards. Evidently, General Electric and NASA are reinvesting in the technology.
posted by ...possums (29 comments total)

Oh man, these are gonna be a huge boon for R-rated action movies. It's been awhile since a solid decapitation-on-the-runway scene.
posted by Greg Nog at 1:26 PM on July 9 [5 favorites]

Wow, that's cool. Though if I saw one in a film like Sky Captain and the World of Tomorrow, I'd chuckle at the absurdity of the "technology" they were trying to pass off as real.

I wonder if the revitalized interest is due to the fact that the V-22 Osprey, with its exposed props, is showing some signs of success.
posted by quin at 1:41 PM on July 9

Arg, I want this.

I love the idea of our technology starting to look more and more alien, less and less like the steps before it in its evolution.

There's something about a turbine covered in sets of contra-rotating spikes whizzing at nearly supersonic speeds that hits that sweet spot.
posted by generichuman at 1:42 PM on July 9

I reckon with the exposed blades they're a little more resistant to bird ingestion/strike than turbofans.
posted by exogenous at 1:46 PM on July 9

Aw yeah, science.
posted by Faint of Butt at 1:49 PM on July 9

Seems from the WP article that the main problem that hindered their adoption wasn't their appearance, but noise concerns. As cool as they are, I'm not exactly dying for any aircraft propulsion developments that substantially raise the in-cabin noise level even further above what it is right now. And airports are already having trouble with noise ordinances and time-of-day operation restrictions due to residential encroachment (or airport encroachment on residential areas).

But there are an awful lot of cargo aircraft around that might be less sensitive to noise issues, at least the in-cabin ones. They might be good initial candidates for these engines, if they were offered.
posted by Kadin2048 at 1:56 PM on July 9

I was reading about this earlier in the week and the reservation seemed to be that it was new technology, needed lots of R&D yet, and then fuel prices tanked, and so no one cared about how much they were going to save... ahhh, economics!
posted by From Bklyn at 2:11 PM on July 9

It's FPPs like this that make me really want to mount a turbofan to my Corolla.
posted by infinitewindow at 2:33 PM on July 9

For every instance when efficiency is sacrificed for maximizing profitability or marketability, a little slice of the future is stolen. Naturally... this is cumulative.

Would increased cabin noise be a concern when the vast majority of travellers could have been transported safely, quickly, and comfortably between high speed rail hubs linking cities large and small? Airframes can and will be redesigned to accommodate for the new characteristics of these engines.

It's rather unfortunate that we can't get the passed years since GE shelved this tech.

The Antonov design bureau have had some success with their An-70. 90% efficiency at cruise altitude at jet aircraft speeds. GE may want to give Ivchenko-Progress a call.
posted by PROD_TPSL at 2:44 PM on July 9

That's "90% efficiency at at cruise altitude at jet aircraft speeds." [citation needed]. That's the most important part of that very lofty claim.
posted by luckypozzo at 2:58 PM on July 9

90% efficiency? There's a M. Carnot who'll be very interested to hear that.
posted by haltingproblemsolved at 3:08 PM on July 9

"90% efficiency at at cruise altitude at jet aircraft speeds." [citation needed].

The GE36 also tested in that range. See the "economical" link in the post:

"Performance - demonstration of high propulsive
efficiency levels in the 90's. as illustrated in Figure 1, at
high subsonic Mach numbers (.75-.85 range). This
would translate into the installed benefits shown
typically for commercial application in Figure 2 and for
military applications would provide enhancement of
payloadirange, time-on-station, and endurance
capability."
posted by ...possums at 3:09 PM on July 9

Thought one: "Dude, seriously, that could shred a goose like a blender does a banana."

Thought two: "Man, those things look alien. And slightly dangerous."

Thought three (after reading articles): "Wow, those are nifty. Want."
posted by strixus at 3:10 PM on July 9 [2 favorites]

Wikipedia on propulsive efficiency.

Serves me right for snarking before reading.
posted by haltingproblemsolved at 3:15 PM on July 9

90% efficiency? There's a M. Carnot who'll be very interested to hear that.

Note, though, that they're talking about propulsive efficiency. That is the percentage of power fed into the propeller that the propeller converts into forward propulsion. It does not represent the percentage of chemical energy in the fuel that is converted to forward propulsion, for which 90% would indeed be impossibly high.

To use a car analogy, 90% propulsive efficiency is equivalent to a car with 10% drivetrain losses. It tells you nothing about the efficiency of the engine itself.
posted by FishBike at 3:19 PM on July 9 [1 favorite]

If I remember my jet engines right (and I certainly don't), the various components of modern turbofans have efficiencies ranging from around 89-96 or so. 90% overall certainly sounds feasible. Also, 100% efficiency would be possible in a frictionless world (this does not violate either conservation of energy, or the second law), but of course our world usually has friction and other losses in it.

Airframes can and will be redesigned to accommodate for the new characteristics of these engines.

Everything in aircraft design comes down to weight. When you carry more weight, you need larger wings, or larger engines, and guess what, those add yet more weight. Designing an airplane is a series of terrible compromises to find the lesser of many evils. If the engine gives you a 3% increase in efficiency, but the extra damping required to make the cabin suitably quiet adds another 20% of weight, you've gained nothing. This problem will be solved, but it's mistaken to think its trivial.
posted by !Jim at 3:30 PM on July 9

I reckon with the exposed blades they're a little more resistant to bird ingestion/strike than turbofans.

OTOH, there's no cowling to catch the exploding blades if you do hit something, which is bad for the passengers. While you wouldn't have the debris ricocheting around in the cowling like you do now, I think losing a blade or two would still lead to the engine destroying itself due to the imbalanced weight.
posted by smackfu at 3:34 PM on July 9

I remember reading about unducted fans in either the late 80's or early 90's. Once every few years, I try to find out what's happening with them these days. Nice to read that they might be trying again.

If they do manage to make a go of them this time, they're going to have to be enough of an improvement to get designers to put the engines on the tail, or I suppose go to a high-wing design. It doesn't seem that there would be enough ground clearance to put them under the wings otherwise.

There are some drawbacks to doing that. For example, putting engines on the wings reduces the bending load the wings have to support in flight, so not putting the engines there means you need a stronger and heavier wing structure. I'm curious if the improved efficiency of an unducted fan is enough to offset the reduced efficiency of the design compromises they require.
posted by FishBike at 3:36 PM on July 9

Good points, smackfu.
posted by exogenous at 3:38 PM on July 9

Cool beans. Hope something comes of it.
posted by HighTechUnderpants at 4:02 PM on July 9

Meh, GTF or GTFO. These UDFs have been around for a while and never went anywhere..
posted by SirOmega at 4:17 PM on July 9

Eh, FishBike? If the engines are under the wing, the wing outboard of the engines still has a "bending load". There are other advantages to tail-mounted engines, like surface debris avoidance ( and human avoidance! ). Cabin noise may be a problem though.

I wonder if prop-like reverse thrust is possible with the fan? You'd think so, with variable pitch blades. That would be another design advantage.

If these things are really 30% more efficient that turbojets, they're coming. Jet-A or it's replacement isn't going to get any cheaper.
posted by sea at 4:49 PM on July 9

I always thought contra-rotating propellers were less efficient than conventional set ups and were used only to counter asymmetrical torque especially in early helicopter and VTOL aircraft. It's a good thing I didn't bet anyone. Contra-rotating propellers
One question this raises though: Why don't they use contra-rotating fans in turbofan engines?
posted by vapidave at 4:57 PM on July 9

Eh, FishBike? If the engines are under the wing, the wing outboard of the engines still has a "bending load".

The bending moment relief of wing-mounted engines is apparently well-known enough that the Wikipedia article on podded engines talks about it a bit.

Why don't they use contra-rotating fans in turbofan engines?

One reason they don't is because it's unnecessary in a ducted fan. You can just have stator vanes after the fan to straighten out the flow. Another reason is that it's a pain in the ass to do, mechanically. The fan is usually mounted on the same shaft as, and directly driven by, the low-speed turbine. Having a counter-rotating fan means either introducing a gearbox, or an additional shaft with a counter-rotating turbine to drive it.
posted by FishBike at 5:22 PM on July 9

The noise problem could probably be ameliorated by using some version of this noise suppression system.
posted by crunch42 at 6:31 PM on July 9

Wouldn't a pod mounted engine above the wing convey the same wing loading benefit with the exception that the strut would be in tension rather than compression?
posted by Mitheral at 8:44 PM on July 9

Having a counter-rotating fan means either introducing a gearbox, or an additional shaft with a counter-rotating turbine to drive it.

Perfectly doable, though - Rolls-Royce Pegasus (all flavours), Pratt & Whitney F-135 and GE/Rolls-Royce F-136 are all turbofans (the former High Bypass, the latter two both Low Bypass) and all have contra-rotating High Pressure and Low Pressure spools and hence Fan/Compressor stages. The reasoning, however, as alluded to by vapidave, they're designed for VSTOL aircraft.

The reason why nearly all other turbofans don't have contra-rotation is because it isn't needed - there's a (relatively) small increase in complexity and it doesn't give any benefit in situations where you're not relying primarily on jet thrust for control power.
posted by Nice Guy Mike at 9:44 PM on July 9

I reckon with the exposed blades they're a little more resistant to bird ingestion/strike than turbofans. [exogenous]

Can't deep-link PDFs. Gruesome and hilarious findings start halfway down page 7.
http://www.int-birdstrike.org/Helsinki_Papers/IBSC20%20WP40.pdf
posted by ...possums at 6:51 AM on July 10

Gruesome and hilarious findings

Oh, possums, it's like you know the magic words that unlock my heart!
posted by Greg Nog at 9:20 AM on July 10