Open debate on open rotors

Anyone think open rotors are for real this time round? By open rotor I mean propfans, unducted fans – engines with the fans on the outside. Engine manufacturers are again talking about open rotors, but as part of a long-term technology plan that begins with today’s turbofans, advances through ultra-high bypass engines (those with the fans on inside, like Pratt & Whitney’s Geared Turbofan) and finally gets to open rotors around the end of next decade.

But airlines like easyJet, with its “eco-friendly” aircraft design, are pushing for open-rotor engines by 2015. And NASA, which has begun developing technology for a 2015-timeframe next-generation narrowbody, says it has had requests to revisit open rotors.

Propfans came close to a launch in the late 1980s, but fuel prices were too low. This time round the driver is emissions and the environment.
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Because of the work done in the 1980s, manufacturers could probably launch development of open-rotor engines today, with today’s technoplogy, and deliver substantial reductions in fuel burn – maybe as much as 30% – and therefore in carbon emissions. The problem is noise. Back in the late 1980s the best NASA thought was possible for a propfan was Stage 3, but new aircraft now have to meet Stage 4 – another 10dB reduction in allowable noise.

It is clear open rotors will not meet the noise limits without some help – aircraft configurations that provide noise shielding, or active noise-cancellation techniques – that would take time to develop and would probably push propfan-powered airliners beyond the 2015 horizon. That would likely mean waiting until the next next-generation narrowbody, which airlines like easyJet might not be too happy about.

But the current clamour over aviation’s contribution to global warming could abate, and ultra-high bypass engines (with the fans inside) could deliver substantial enough fuel and emissions savings, combined with lower noise, and mollify the airlines. It may be premature to fire up the propfan, but best to dust off the old reports and take another look.

7 Responses to Open debate on open rotors

  1. Frank J.Valvo 3 July, 2007 at 5:28 pm #

    A big question with “open rotors” is will the flying public accept commercial airliners with big propellers flailing around? It’s a fact that most people prefer to fly in jet powered aircraft rather then a turboprop powered one. Isn’t the “open rotor” concept going to be a glorified propjet from an esthetic point of view, that passengers may not care for – I believe so.

  2. Woracle 3 July, 2007 at 10:13 pm #

    Thanks, guys. I think the resurgence of turboprops shows airlines are more than capable of putting economics ahead of passenger preferences. I wonder if they are also capable of putting their fear of becoming scapegoats for global warming ahead of passenger preferences and industry assurances that open rotors won’t be ready by 2015.

  3. Donald MacDONALD 4 July, 2007 at 1:59 pm #

    The P & W prop fan with front geared fan is very interesting. With the exposed fans at rear and their multi-scimitar blades – is there a prospect of their being variable-pitch – with mechanical reliability? Very good reverse force = thrust for short landings that way if possible.

    NOISE+ PASSENGER APPEAL. Given a choice between jet and turboprop passengers will fill the jet before the turboprop gains seats. Illogical but there it is. Could such problems be solved by a short duct around part of the engine and all of the counter-rotating fans? It could make for greater efficiency as well as greatly reduced noise. People would get used to the appearance: cf first of the 737′s vs current models – the first engines now look like cigars.

  4. Ralf Klocke 6 July, 2007 at 2:22 pm #

    The big issue is safety. Imagine a released blade penetrating the hull during take-off, or even worse at cruise altitude. Noise and vibrations could be decreased, compared to present turboprops, but these engines would definetly require completely redesigned airframes. A flying wing design with engines mounted on top of fuselage might be a solution for the next generation of passenger aircraft. We will see…

  5. Arthur C. Hughes 6 July, 2007 at 9:45 pm #

    They have been called Unducted Fans, Propfans, and now Open Rotors. Regardless of the appelation given them, they are propellers. As propellers, they have the same physics that have been known since their first use. Ideal efficiency remains as described by Glauert. The thrust will diminish rapidly as forward speed increases, which is why we quit using them for high speed (M>0.5) airplanes.

    At least the contra-rotating will excel in converting fuel to noise.

  6. Adam 27 July, 2007 at 10:05 pm #

    I think that there are many misguided comments about the Open Rotor/Unducted Fan concept. Let’s look at some of the proposed complaints about the UDF concept.

    First let’s address the idea that it is just a prop. Using that rationale, then all jet engines are also props inside a tube, or for that matter we could just say that they are a series of rotating wings that work in coordination.

    We have to remember that there are a handful of aircraft that are operating regular unswept props at high efficiencies above M0.5 namely the P-3 Orion and the Tu-95/142, with the latter operating in the high-60% efficiency range at high speed cruise.

    The technology that is applied to the design of the UDF fan blades is a three-dimensional sweep which has sweep in both the radial direction as well as longitudinal. This, coupled with advanced airfoils, allows them to operate in the transonic regime.

    According to NASA/GE reports, the efficiency achieved at M0.75 was 84.9% as their target. This target was based upon optimization of a propfan design done by Hamilton Standard and tested at full scale. This design achieved 81.2% efficiency at M0.8 (NASA Report CR180867, Dec 1987, pg 41).

    This efficiency was achieved almost 20 years ago without the ability to use computer modeling as extensively as we have today. The lowest reported extrapolated value for efficiency at higher Mach numbers of about 0.85 show a prop efficiency above 75%. Though of interest, it must be considered that this will be operating as a system, and due to higher bypass ratios, the propulsive efficiency will be increased over that of ducted bypass engines as well as a drag reduction by eliminating the outer nacelle.

    The next issue is the idea of a FBO (fan blade out) event. Currently GE is certifying their engines without having to test for this due to the fact that their composite fan blade technology has been deemed impervious to this type of event and it only has to do it by analysis. Indeed the last time that I heard about a prop losing a blade was due to the fact that it had not complied with an AD notice, and that one was a C-130 operated by the Saudi forces. I can imagine that this prop had seen frequent abuse.

    We have seen though that a similar event is possible, though with dated technology, when a DC-10 lost its fan and it sliced through hydraulic controls. Nonetheless, I am sure that this will be handled during the certification of such engine as was the case of the GE36, which performed such a test. Also remember that most turboprop aircraft have the propellers located near the passenger section where a FBO type event would have a much higher probability of causing a fatality due to the blade passing through the cabin section of the aircraft.

    Noise is the next myth that needs to be tackled. In reading the NASA/GE design report, the design was done to meet Stage 3 requirements and not beyond. I cannot find the particular report at the moment, but it indicated that neither interior noise nor external noise would have been an issue for certification.

    The GE36 was intended for certification, this needs to be remembered. The effort expended to ensure Stage 3 requirements was extensive due to concerns that it might be difficult to achieve, but I have not read any reports which seemed to indicate that this requirement was troublesome. Hence, one might assume that there is still some easy growth in this area and meeting the next stage requirements may not be as difficult as one might imagine.

    The last myth that needs to be addressed is that of preference to jets vs turboprops, which in reality are really jets too. Anyone that tells you otherwise is lying. A turboprop is just an ultra high bypass, geared, open rotor jet. The main reason people fly jets over turboprop aircraft is availability. How many routes are really flown with a turboprop? The ratio is really high in favor of jet routes. Jets have this because they operate faster (0.75<M<0.85) and until now have been the only thing that has been able to compete efficiently for this type of route. Could anyone imagine a flight from LA to New York that was 14 hours long? Or going from LA to Salt Lake City in 4 hours? No not in this day and age.

    Also turboprop airplanes have always been characterized with smaller cabins and bumpier rides. Looking at recent development in technology we see that the A400 military transport is anticipated to cruise between 0.68<M<0.72 with a max speed of 421kts. This plane incorporates swept blade technology and has swept wings. This is not bad for a military transport which is known for being draggy.

    With the introduction of UDFs to a MD-80 type airfraime, I do not see the general public determining that they are going to fly less. Especially with the big push for going green, many people are going to say: I can fly just as fast and it is better for the environment, or reduces our dependability on oil as it consumes much less fuel than current technology; OK, I like it.

    With the PR machines of Boeing and Airbus, this will be an easy sell to the public. Look at what Boeing was able to accomplish with the 787. They had 15,000 people there at the roll out and more filling a stadium down the road, with thousands watching on TV and the internet. This was just for changing the materials that were used, which also I may note were deemed not suitable before for major structural components by the public.

    I see Open Rotor/UDF technology as the next leap. I think that it will be the thing that keeps the airline industry out of the crosshairs of environmentalists and government regulations. It is exciting to see, and one hopes that it might trickle down to business jets and down to the smallest VLJs.

    Adam

  7. Nigel Eric Rose 18 August, 2008 at 9:11 pm #

    Currently, the biggest single draw back to Open Rotor systems appears to be concern about noise levels produced by the blades themselves. This may not be such a big issue, ultimately, the price of crude is likely to overcome all other dictates. For an airline, it all comes down to how many seats you can sell. And this, me dears, is going to become ever more dependent on the cost of fuel.
    Lets look at this another way, and how about doing this as a clean sheet approach. Lets consider the Advanced Hydraulic Engine diesel aviation power plant and Open Rotor system and change over to a large flying boat configuration. For simplicities sake, we’ll use the Saunders Roe SR.45 “Princess” as a baseline. Whoops, sorry, we accidentally eliminated one of the major future problems of aviation, the need for ever larger and more costly airfields; oh well, I remember Mayor Bloomberg wondering why the industry didn’t go back to large flying boats, for much the same reasons. Incidental to this, as most areas suitable for large flying boats would allow departure over water, this would eliminate climb out at high power settings over built up areas, there goes another major problem, potentially high noise levels of Open Rotor aircraft over built up areas.
    Now lets look at the concept of AHEda&OR powering of the beast. Predictions are the AHEda power plant will have a fuel burn about 30% of a contemporary diesel of equivalent horsepower and be competitive with an equivalent turbo-shaft power plant on a power/weight ratio basis and be far less costly to produce than that turbo-shaft unit.
    Hmmm! Lets see what we’ve got so far:
    1. Lighter aircraft due to reduced fuel load resulting from lower fuel burn and elimination of weight of landing gear.
    2. Simpler aircraft due to elimination of landing gear and the need to address high landing load shocks, normally transferred into aircrafts structure.
    3. Less costly aircraft due to cheaper power plant manufactured cost and elimination of
    landing gear and need to address high associated loads
    4. More comfortable aircraft due to ability to assign more aircraft weight to passenger accommodation, services and baggage
    5. In most cases the elimination of cross wind approaches and departures due to ability to approach and depart thrugh 360 degrees, rather than having to align with a fixed runway
    6. Aircraft that will enjoy the level of reliability inherent to the most advanced forms of diesel engines and the ability to service major components, such as cylinder units, by simply swapping them out in a routine that could, conservatively, see a complete engine rebuild done in 24 hours

    Oh and while I think of it, being a hydraulic drive line, the AHEda&OR system does not have the usual problems associated with contra-rotating propellers or Open Rotor systems, in that there is no requirement for a gearbox, quite simply, the hydraulic motors run in opposite directions driving each blade set through concentric shafts.

    By comparison, it looks like the AHEda&OR system powered SR.45 will return a people kilometre per litre of fuel burn, better than three times that of a Boeing 747-400, now that is what I call a pretty significant commercial advantage and if a high proportion of that saving is passed on to the traveling public, folks, we can all afford to fly again.

    Well that should be enough t’ put the cat amongst the pidgeons and give the industry something to think about.

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