Aircraft propellers come in an astonishing variety—high blade, low blade, high pitch, low pitch, ducted, non-ducted, coaxial, contra-rotating, co-rotating, Q-tip, toroidal, and even electric ducted fans (EDFs). But which ones are truly the best? What technological breakthroughs are shaping their future? And how can choosing the right propeller make your aircraft more efficient?
Whether you’re designing the next-generation eVTOL or trying to understand the engineering behind them, this comprehensive guide will cover everything you need to know about modern propeller technologies.
Understanding the Fundamentals of Propeller Design
Before we explore specific types, let’s establish four fundamental truths that apply to all propeller types:
1. Tip Speed Limitations
The tip of a propeller moves much faster than the root. This increasing speed toward the tip must stay subsonic—below Mach 0.8—to prevent shockwave formation, drag spikes, and increased noise. Most modern designs keep tip speeds around Mach 0.6 (approx. 480 mph or 770 km/h).
Notable exception: the Tupolev Tu-95 features propellers with supersonic tips, achieving aircraft speeds up to 575 mph (925 km/h). The blade’s shape and pitch must match the aircraft’s cruise speed to ensure maximum efficiency.
2. Blade Count vs. Efficiency
More blades mean more thrust at the same RPM, but this comes at a cost—higher drag and greater power consumption. Importantly, thrust gain from additional blades diminishes with each added blade. While longer blades could be more efficient, they’re constrained by tip speed and landing gear height.
3. Variable Pitch and Blade Twist
There’s no one-size-fits-all propeller. At low speeds, a low pitch offers better acceleration; at high speeds, high pitch is more efficient. Modern propellers are twisted to vary the angle of attack from root to tip—just like a gearbox changes torque and speed in a car.
In eVTOLs, pitch management is crucial. Since these aircraft operate in vertical takeoff, transition, and cruise regimes, they either use different sets of propellers for lift and cruise or opt for variable-pitch systems that adapt to each phase.
4. Tip Vortices and Efficiency Loss
As a propeller spins, low pressure forms on one side and high pressure on the other. These pressure differences seek equilibrium—causing air to spiral around the tips, forming vortices. This not only reduces thrust but also increases noise. Managing these vortices is key to efficient, quiet designs.
Types of Propellers in eVTOL Applications
With the basics out of the way, let’s examine the various types of propellers used in modern eVTOL aircraft and assess their pros and cons.
Open Propellers
The most common and straightforward type, open propellers are:
Pros:
- Lightweight, simple, and low-cost
- Low drag
- High maneuverability
Cons:
- Noisy
- Vulnerable to debris
- Hazardous in exposed environments
- Less efficient at low speeds
Aircraft like Joby S4, Archer Midnight, and Vertical Aerospace VX4 use 5-bladed variable-pitch open propellers. Notably, they feature Q-tip blades—curved tips that act like winglets, reducing vortices and noise.
Emerging Tech: Toroidal Propellers
Gaining attention for their quiet operation and enhanced control, toroidal props (popular in marine tech) are being tested for aircraft. They’re complex to manufacture but promising in performance.
Coaxial Propellers
Found on aircraft like Jetson ONE, these involve two propellers mounted on the same axis.
Pros:
- More thrust in a compact footprint
- Greater maneuverability
- Can cancel out torque in contra-rotating setups
Cons:
- Less efficient (up to 22% more power needed for same thrust)
- Mechanical complexity
Contra-rotating coaxials are superior to co-rotating ones due to:
- Zero net torque
- Better stability in gusty conditions
Ducted Propellers
Once shelved due to complexity, ducted fans are back thanks to small, powerful electric motors that eliminate the need for complicated transmissions.
Performance Gains:
- Up to 94% more thrust or 62% less power compared to open rotors
- Tip vortex reduction (≈15% gain)
- Venturi effect (entrains more air for more thrust)
Challenges:
- Added weight
- Susceptibility to crosswinds during takeoff
- Increased drag in forward flight
Solutions include:
- Lightweight composite ducts
- Adaptive ducted fans (ADF) that change geometry
- Gimbal mounts to counter crosswind effects
Notable Design:
Lilium Jet uses advanced EDFs with:
- Multiple blades
- Stator vanes (to reduce swirl losses)
- Adaptive nozzles (boosting thrust by 40%)
- Acoustic liners (reducing noise)
Electric Ducted Fans (EDFs)
Originally used in hobbyist RC models, EDFs have evolved into serious aviation components.
Key Features:
- High blade count (12+)
- Compact, high-thrust configuration
- Often integrated with stator vanes and adaptive nozzles
These fans provide thrust in tight spaces, crucial for streamlined eVTOL designs.
Coaxial Ducted Fans
These are being explored in aircraft like the Alauda Airspeeder Mk4 and VRCO XP4.
Advantages:
- High thrust from compact power units
- Improved control with dual motors and blades
- Potential for smaller, more aerodynamic aircraft
Design Concepts:
- Two-stage EDFs with different blade pitches
- Enhanced airflow velocity in each stage
- Increased efficiency from smaller units
The Road Ahead for Propeller Technology
While open propellers remain widespread due to their simplicity, the push toward quieter, more efficient urban air mobility solutions is making advanced propeller types like ducted fans, toroidal blades, and adaptive systems increasingly viable.
Key Design Takeaways:
- Tailor pitch to flight phase (or use variable-pitch props)
- Manage vortices for noise and efficiency
- Use coaxial or ducted systems when compact power delivery is required
Whether you’re building your own drone or leading the next eVTOL startup, understanding propeller dynamics is essential. And now, you’re equipped with a strong foundation to evaluate, choose, or innovate the next big leap in electric aviation.