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Why airlines not interested in Ultra-High bypass engines?

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  • Why airlines not interested in Ultra-High bypass engines?

    Ultra-bypass (or so-called unducted fan or propfan) engines were developed in the mid-80s. The initial design of MD90 utilized this type of engine instead of conventional turbofan. Boeing had also come up with a twin engine T-tail model called 7J7 that uses ultra-high bypass fans.

    But some how airlines were not interested. MD90 switched back to conventional turbofan and Boeing killed the 7J7 project.

    Supposing higher the bypass ratio is, more economical and efficient the engines are to operate. Ultra-high bypass engines have a bypass ratio several times higher than the current high-bypass turbofans. Why weren't the airlines interested in such a design? Does anyone know?

    Raytheon's Starship executive jets uses unducted fan but that didn't sell very good either.
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  • #2
    maybe this sounds veeerrryyyyyyy stupid but what is an ultra-bypass engine?
    My New Photo Name on JP.net is DutchAviation!!!!

    why should we fight about wich aircraft manufactor is better..............


    thats so childish....



    because hey we all know boeing is better

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    • #3
      Starship used regular turboprop engines mounted in a pusherprop configuration, just like the P180 Avanti.

      It wasn't the airlines but the aircraft and engine manufacturers that killed the engine.
      It was envisioned as being more efficient and less noisy than jets.
      It was indeed less noisy and more efficient, but didn't give the required performance to compete with similar-sized turbofans.
      While it could have competed with large turboprop engines those were already on the way out being replaced with jets leading to engine-commonality across airline fleets thus reducing operating cost.

      To make a long story short, a promising technology that turned out to be the wrong tech at the wrong time.
      10 years earlier it might have revolutionised the regional airliner market, using those engines instead of jets. But those RJs were already being built and certified with jets by the time the unducted fan engines came on the market and redesign was too expensive for the potential gains.

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      • #4
        All engines have bypass ratio associated with it. Higher the bypass ratio is, higher the efficiency of the engine is. An engine with higher bypass ratio burns less fuel and produces less noise.

        For example, turbojet engines (used to power de Havilland Comet and Boeing 707-120, etc.) has a 1:1 bypass ratio. That's the lowest bypass ratio you can get to.

        Engines like P&W JT8D (powers 727, DC9, etc.) and Rolls Royce Olympus (the one powers Concorde) are low bypass ratio turbofans, while PW4000, GE90 and RR Trent series are high bypass ratio turbofans. High-bypass turbofans have a bypass ratio between 1:5 to 1:15. However, unducted fans are untra-high bypass ratio engines, which has a bypass ratio of 1:25.

        A low bypass ratio turbofan:

        Description:
        For supersonic flight, a low bypass fan is utilized, and an augmentor is added for additional thrust.

        A high bypass ratio turbofan:

        Description:
        A turbofan engine is basically a turbojet to which a fan has been added. Large fans can be placed at either the front or rear of the engine to create high bypass ratios for subsonic flight. In the case of a front fan, the fan is driven by a second turbine, located behind the primary turbine that drives the main compressor. The fan causes more air to flow around (bypass) the engine. This produces greater thrust and reduces specific fuel consumption.

        An ultra-high bypass ratio unducted fan:

        Description:
        During the 1980s, GE developed the Unducted Fan UDF® engine which eliminated the need for a gearbox to drive a large fan. The jet exhaust drives two counter-rotating turbines that are directly coupled to the fan blades. These large span fan blades, made of composite materials, have variable pitch to provide the proper blade angle of attack to meet varying aircraft speed and power requirements. Powerplants such as the UDF® engine are capable of reducing specific fuel consumption another 20-30 percent below current subsonic turbofans.

        The question is: if it is that good, why no one uses it?
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        • #5
          Originally posted by CathayPacific
          The question is: if it is that good, why no one uses it?
          Jwentwing is correct in saying that the performance of UHBP engines was below expectations. However the airlines might still have ordered them based on the cost savings alone anyway but for the passenger perception factor. In the early 80's when these engines were first announced most regionals were flying aircraft like the Beech-99, the Fairchild Metroliner, and the Embraer EMB-110 Bandierante. The Brasillias, CASA 212's, DeHaviland Dash-7's and Dash-8's, BAe Jetstreams, Shorts 330's and Beech 1900's were just hitting the market and the ATR's were in development. The average passenger equated propeller equipped aircraft with small, noisy, and a bad ride.

          This is still true even though 20 years later most of the regional turboprops still flying in passenger service are larger and quieter than the typical regional of the early 80's. But passengers in general still equate pure jets with better service and a better ride. This is why the CRJ's and EMB RJ's are taking over the regional market so quickly today. The airline marketers of 20 years ago couldn't dare take the risk of appearing to have stepped backward by flying planes with props unless they knew for certain that all the competition would be flying the same type of planes. Had majors like UAL or AAL ordered the original MD 90 or the 7J7 and an upstart like Southwest had come along flying only 737's the marketing guys feared that the pure jet operator would grab all the passengers on any competing markets. So they didn't dare order anything that passengers might perceive as not as good. Keep in mind that in the early 80's the airlines were just recovering from some very bad times in the 70's due to recession and increased fuel costs, and deregulation had put them all in serious fare wars with upstart carriers like People's Express, New York Airlines, and Air Florida.

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