Propellers are crucial for aircraft propulsion, generating thrust by accelerating air backward. They work like rotating airfoils, creating lift and drag forces. Thrust depends on propeller diameter, blade pitch, and rotational speed, with larger propellers and higher speeds producing more thrust.

Different propeller types suit various needs. Fixed-pitch propellers are simple and low-cost, while adjustable-pitch and constant-speed propellers offer more flexibility. Blade element theory helps optimize propeller design by analyzing forces on blade sections. Efficiency factors include advance ratio, blade angle, and airfoil shape.

Propeller Fundamentals

Principles of propeller thrust

• Propellers generate thrust by accelerating a mass of air in the opposite direction of the desired aircraft movement following Newton's third law: for every action, there is an equal and opposite reaction
• Propeller blades act as rotating airfoils, creating lift and drag forces
  • Lift force generated by the blades has a forward component, contributing to thrust (Bernoulli's principle)
  • Drag force acts in the opposite direction of rotation, reducing efficiency (form drag and induced drag)
• Thrust depends on factors such as propeller diameter, blade pitch, and rotational speed
  • Larger diameter propellers accelerate a greater mass of air, generating more thrust (increased disk area)
  • Higher blade pitch angles increase the angle of attack, resulting in greater lift and thrust (up to the critical angle of attack)
  • Increased rotational speed leads to higher airflow velocity and greater thrust generation (rpm)

Types of aircraft propellers

• Fixed-pitch propellers have a blade pitch angle that is fixed and cannot be changed during operation, designed for optimal performance at a specific flight condition (cruise), and offer a simple design, lower cost, and easier maintenance compared to other types
• Adjustable-pitch propellers have a blade pitch angle that can be manually adjusted on the ground to optimize performance for different flight conditions but changes cannot be made during flight, providing some flexibility in performance
• Constant-speed propellers automatically adjust the blade pitch angle during flight to maintain a constant rotational speed using a governor system that controls the pitch angle to maintain the desired RPM set by the pilot, offering the best performance across a wide range of flight conditions (takeoff, climb, cruise) but with a more complex design, higher cost, and increased maintenance requirements compared to fixed-pitch propellers

Propeller Design and Performance

Blade element theory in design

• Blade element theory divides the propeller blade into small, independent sections called blade elements, each analyzed as a two-dimensional airfoil with its own local velocity and angle of attack
• The forces acting on each blade element are calculated based on the local flow conditions
  • Lift and drag forces are determined using the airfoil characteristics and the local angle of attack (lift coefficient and drag coefficient)
• The total thrust and torque produced by the propeller are obtained by integrating the forces acting on all blade elements along the blade span (calculus)
• Blade element theory allows for the optimization of propeller design by adjusting:
  • Airfoil shape and thickness distribution along the blade span (NACA airfoils)
  • Chord length and twist distribution (washout)
  • Number of blades and blade planform shape (rectangular, tapered, elliptical)

Factors of propeller efficiency

• Advance ratio ($J$) is the ratio of the forward speed of the propeller to the rotational speed, calculated as $J = \frac{V}{nD}$, where $V$ is the forward speed, $n$ is the rotational speed in revolutions per second, and $D$ is the propeller diameter, with higher advance ratios generally leading to lower propeller efficiency due to increased drag
• Blade angle (pitch angle) is the angle between the blade chord line and the plane of rotation, with the optimal blade angle depending on the specific operating conditions (takeoff, climb, cruise) and affecting the angle of attack and the resulting lift and drag forces
• Airfoil shape is the cross-sectional shape of the propeller blade, influencing the lift-to-drag ratio and the stall characteristics, with high-performance airfoils having high lift-to-drag ratios and delayed stall being desirable for propeller efficiency (laminar flow airfoils)
• Other factors affecting propeller efficiency include:
  • Blade surface finish and roughness (polished vs. rough surfaces)
  • Blade tip losses due to three-dimensional flow effects (tip vortices)
  • Compressibility effects at high rotational speeds (Mach number limitations)