Bjorn’s Corner: New engine development. Part 9. The role of the Nacelle

By Bjorn Fehrm

May 24, 2024, ©. Leeham News: We do an article series about engine development. The aim is to understand why engine development now has longer timelines than airframe development and carries larger risks of product maturity problems.

To understand why engine development has become a challenging task, we need to understand engine fundamentals and the technologies used for these fundamentals.

In the last Corner, we looked at why Open-Rotor engines have two fan stages, either both rotating in different directions or one rotating followed by a de-swirling stator stage. Now we study the flow field ahead of and around the fans to understand the role of the nacelle and the open rotor spinner.

Figure 1. A graphic showing the different parts of a Pratt & Whitney PW1100G nacelle. Source: Pratt & Whitney.

The role of the Nacelle

The turbofan engine uses a nacelle to channel the airflow through the powerplant. When doing it, the nacelle has several functions (Figure 2):

Figure 2. A graphic showing the different parts and functions of a turbofan nacelle. Click to enlarge. Source: SAFRAN.

  • Aerodynamic: The nacelle captures the free stream air at about 0.76 to 0.85, depending on the airliner type, and channels it to the fan in an inlet diffusor section, thus lowering the airspeed to around M 0.5. The lower axial speed of the air eases the local Mach number of the fan blades to below Mach 1.3 during high-power operation (takeoff, climb). The diffusor has a perforated wall that absorbs the sound from the fan, including transonic shock waves during high-power operation.
  • Thrust reversing: The rear part of the nacelle houses the thrust reverser doors, which close off the bypass channel and direct the bypass air outwards and forwards during thrust reversing. The nacelle structure transfers the thrust reverser forces to the engine and the pylon (also called strut in the US).
  • Nozzle: The aft part of the nacelle contracts the outer walls and has a core exhaust plug. It ensures the exit speed of the core and bypass channel have the appropriate speeds so the engine has optimal efficiency (the 80% bypass to core airspeed relation).
  • Minimize drag: The external surface of the nacelle creates drag due to air friction. The nacelle keeps this surface to the minimum while ensuring a smooth flow to avoid interference and pressure drag. The internal friction drag is integrated into the engine thrust data.
  • Low weight: With its many functions, a nacelle weighs about one-third of the engine it houses. The heaviest parts are the thrust reverser and the air inlet, which must sustain hail and rain impact at high speeds. Therefore, the nacelle structure and walls are, to a large extent, composite.

An Open Rotor or Fan engine also has a nacelle. For the CFM RISE (Figure 3), it holds the core, and the stator vanes. Its shape controls the Overspeed of the engine fan flow compared with the core exhaust air at the back of the nacelle.

Figure 3. The CFM RISE fans are attached to the spinner and nacelle. Source: CFM.

The fan spinner is more important for an Open Fan engine than for a Turbofan. Unlike a turbofan, an Open Fan engine does not have the luxury of a diffusing inlet. A combination of the spinner and nacelle shape must manage the control of the air speed around the fan and the de-swirling static vanes.

Figure 4. The flow field around the GE36 aft-mounted fans. Source: NASA UDF design report.

Figure 4 shows the flow design around the GE36 fans. The graph is from the GE-NASA GE36 design report. Note how the nacelle diameter is reduced ahead of the aft-mounted fans to lower the airspeed and avoid supersonic flow as the air passes the thick inner section of the fanblades.

The spinner and nacelle of the CFM RISE would have the same role. Here, the fan blades’ inner part is thinner, so the need for a lower Mach should be reduced. But don’t be surprised to see a Coke bottle shape spinner on the flight test RISE.

13 Comments on “Bjorn’s Corner: New engine development. Part 9. The role of the Nacelle

  1. The UDF has an advantage by accelerating some air outside the rotor disc hence increasing the bypass ratio a bit. Still designing a UDF rotor being quiet, variable pitch, anti-ice, efficient up to M0.8 and with a long certified time on wing including its rotor disc is not for the unexperienced nor experienced without massive computing power and advanced software.

    • IMU:
      the front rotor disk driven unshrouded airstream will widen behind the
      rotor. ( you see the reverse effect in wind turbines )
      The secondary rotor or set of guide vanes should cover that larger area !?

      • Its the air viscosity that drags along some air outside the rotor disc. That airstream will not be deswirled by the 2nd stage stationary vanes if they have the same diameter as the first stage, often they have a tad smaller diameter to avoid the tip wortizes for the first stage. In summary you get a thin swirl component at the o.d. of the fanflow.

  2. I wonder if the fan spinner on a UDF will prove to do an adequate and efficient job replacing the turbofan’s nacelle.

  3. I’m still wondering why the open fan will not be another Thunderscreech. What are they doing to control the noise from the sonic booms from the prop tips?

    • * More blades to reduce the individual blade loads
      * Swept blades
      * Rounded blade tips
      * Thinner blades with wider chords
      * Back rotor (or stator) stage having slightly shorter blades/vanes than the front rotor (which avoids chopping through the tip vortexes from the front rotor)

      I’m sure Bjorn will be covering all of this and more in the future.

      • It’ll be fascinating to see how the UDF works in practice. Will it really be as quiet as a current turbofan (as we are told), cruise at virtually the same speed (ditto), and be substantially more efficient than one *all things considered* ?

        That’s setting aside completely the issue of uncontained UDF blade failure- it only takes one or so of those to kibosh the whole project. “Oops!”

        • The public perception of RISE is going to be a disc of death made up of Samurai swords no matter what

          • The Open Fan is a turboprop at higher speed. Props have been around, like…. forever.

          • I didn’t say that this perception was logical

          • More like Dervish scimitar swords…
            They have noise test data from the Safran demo and wind tunnel tests hopefully complying within 1-2% from Computational Fluid Dynamics calculated values.

      • Also, some of those advances (like the thinner, wider blades) couldn’t be implemented until the development/improvement of composites in the 1970s and 1980s, which was decades after the Thunderscreech failed.

  4. The dotted lines are iso mach lines but what are the solid vertical lines around the two stages of udf fans. I was expecting speeds slightly higher than Mach 1 at the tip of the fan blades… Always so educational. Bravo

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