Video summary

How an Open Fan Differs from Turboprop & How it Beats Turbofans

Main summary

Key takeaways

Technology

Technological concepts: What an open fan engine is (and why it exists)

An open fan engine (also called an unducted fan or propfan) is described as a hybrid lineage of turbofan + turboprop.

  • Think of it as:
    • A turbofan with the duct removed, plus
    • Redesigned fan blades to work efficiently without the duct.

Goal

The goal is to replace some turbofan roles on airliners by achieving:

  • Higher propulsive efficiency
  • Better fuel economy, especially at cruise speeds

Claimed performance/efficiency comparisons

Open fan designs have demonstrated:

  • ~86% propulsive efficiency at Mach 0.78 (and potentially more)
  • Claims that this matches/exceeds turboprop efficiency levels while operating at ~50% faster speeds
  • Claims of ~20% better fuel efficiency than today’s turbofans

Core physics: Why ducting/unducted changes efficiency

Turbofan thrust principle

Turbofans produce much of their thrust via:

  • A large front fan
  • Bypass air

This improves fuel efficiency and takeoff performance.

Efficiency limit for open propellers/turboprops

Propeller efficiency is limited by blade tip speed:

  • As tip speed approaches the speed of sound, shock waves and drag rise sharply
  • This can crush propulsive efficiency
  • Efficient turboprop cruise has a “hard” limit around Mach ~0.5

Turbofan duct benefits

The duct helps by:

  • Delaying compressibility effects (shock formation), allowing tips to run closer to or beyond local sonic speeds
  • Reducing tip vortex drag
  • Helping with noise suppression
  • Improving airflow/pressure behavior

Why open fans are proposed: Turbofan scaling penalty

To improve turbofan efficiency further, engineers push for higher bypass ratios, which requires bigger fan diameters.

But as diameter increases:

  • Ground/wing clearance becomes problematic
  • Duct/cowling weight increases faster than diameter (a nonlinear weight penalty)

The video argues that the duct that solved tip-speed problems becomes the bottleneck for further gains—so engineers consider removing it.

Open fan engineering solutions (key “how it beats turbofans” features)

The video highlights three main efficiency enablers, plus earlier aerodynamic constraints.

1) Aggressive blade sweep (main fix for compressibility/shock issues)

Open fans need blades that can handle high tip speeds without a duct.

  • Blade sweep reduces the effective component of velocity driving compressibility:
    • Swept blades move through air at an angle, delaying transonic effects

Benefits cited:

  • Delays transonic drag
  • Weakens shock waves
  • Can reduce tip vortex intensity (especially with taper)

Trade-off:

  • Aggressive sweep requires stronger blades due to higher structural loads:
    • centrifugal tension + torsional bending
  • Sweep can reduce low-speed effectiveness due to altered flow characteristics along the blade

2) Anti-swirl / swirl reduction strategies (to reduce efficiency losses)

A major efficiency loss in propulsors is swirl:

  • Only the axial component contributes to useful thrust
  • Swirl wastes power

Two approaches discussed:

Anti-swirl (swirl recovery) vanes

  • A stationary blade row behind the fan redirects the slipstream to be more axial
  • Similar concept to stator vanes in turbofans:
    • also act as diffusers converting velocity to pressure

Contra-rotating rotors

  • Two rotors spin opposite directions to cancel swirl
  • Efficiency comparison is complex:
    • contra-rotating needs ~twice the shaft horsepower
    • but can deliver nearly double thrust with marginal nacelle drag increase

The video claims modern designs often trend toward:

  • Single rotor + anti-swirl vanes
  • Likely for:
    • mechanical simplicity
    • cost
    • placement flexibility

3) Variable pitch blades (matches turboprop-like flexibility)

Variable pitch allows blades to adjust angle to keep optimal RPM/loading across flight phases:

  • Fine pitch for takeoff (high thrust at low airspeed)
  • Coarse pitch for cruise (maintains proper blade angle of attack)

A governor adjusts pitch automatically to maintain efficient RPM.

The video claims this enables peak efficiency in:

  • takeoff
  • climb
  • cruise

…unlike fixed-pitch turbofans.

Design/layout considerations: Duct removal interacts with installation and airframe

The video describes an evolution from older rear “pusher” concepts to modern tractor/underwing layouts.

Mounting concepts

Pusher / Rear mounting

  • Pros: more uniform airflow into the core (inlet not disturbed by rotor wake)
  • Cons: rotor operates in disturbed air from the nacelle

Tractor / Underwing mounting

  • Pros: rotor sees cleaner airflow → better rotor efficiency
  • Cons: wing interference and potential increased drag; also ground clearance issues vs tail mounting

Tail-mounted designs

  • Stress CG and structural weight limits, reducing flexibility

Retrofit risk

The video highlights that even with underwing mounting:

  • open fans may require purpose-built airframes, not easy retrofits

Retrofit difficulty is framed as a major barrier to widespread adoption.

Programs, history, and adoption challenges

Past attempts (oil crisis-era)

In the 1970s oil crisis, efficiency-focused work intensified:

  • NASA’s Advanced Turbrop (propfan) demonstrated about 15% fuel burn reduction by the 1980s

Development was curtailed due to:

  • unresolved structural and noise issues
  • falling oil prices → shift back to incremental turbofan improvements

Renewed interest (2000s onward)

Renewed interest came from:

  • rising fuel prices
  • environmental concerns

The video argues modern open fan success required leaps enabled by:

  • advanced aerodynamic simulation software
  • composite materials

Noise and public perception

Even if certification-level noise may be near turbofan quietness in cruise, the video notes:

  • open fans become significantly louder at:
    • high power
    • high angles of attack during takeoff with high weights

Public perception:

  • Passengers may distrust propeller aircraft due to association with older/smaller planes
  • Blade separation events are said to be rare, but perception and communication remain hurdles

Specific engine references mentioned

  • GE RISE (CFM International / GE + Safran partnership)

    • Presented as the current push
    • Uses single-rotor + anti-swirl vanes (video suggests it may be chosen for reduced noise and mechanical simplicity)
    • GE claims bypass ratio exceeding ~5x advanced turbofans
      • estimates range ~40:1 to 75:1 depending on measurement

  • Soviet Progress D27 on the Antonov AN-70

    • Cruise around ~400 knots
    • Optimized for short/rough operations rather than high-speed cruise
    • Still exceeds the classic Mach ~0.5 efficient propeller barrier

Main speakers/sources (as indicated by the subtitles)

  • Primary narrator / presenter (unnamed): delivers the full technical explanation and references
  • Referenced organizations/companies:
    • GE (General Electric)
    • CFM International
    • Safran
    • Pratt & Whitney
    • NASA
  • Referenced historical engines/figures (examples noted):
    • Junkers Jumo 004
    • Progress D27
    • PW1100G
    • F-35, etc.

Original video