Basic Helicopter and Propeller Design - Midterm

Hovering

A state of flight where, in the absence of wind, all rotor blades travel at equal speed relative to the surrounding air.

Dissymmetry of Lift

The difference in lift between the advancing blade side and the retreating blade side of the rotor disc during relative air motion (caused by forward speed or wind).

Advancing Blade

The blade moving into the relative wind, experiencing higher relative airspeed.

Retreating Blade

The blade moving away from the relative wind, experiencing lower relative airspeed.

Dissymmetry of Lift

Because lift increases with airspeed, the advancing side generates more lift than the retreating side. Uncompensated, this causes the helicopter to roll toward the left (retreating side), an effect that intensifies as airspeed increases.

Translational Lift

The additional lift gained as the helicopter transitions from a hover into forward flight.

Effective Translational Lift

Occurs between 16 and 24 knots airspeed, where the rotor encounters more horizontal airflow and reduced induced flow, increasing the angle of attack. This transition produces a noticeable vibration and a distinct increase in lift.

Transverse Flow Effect

An aerodynamic imbalance occurring at slow airspeeds (below 20 knots) where air passing through the rear portion of the rotor disc is accelerated downward longer than air at the front.

Autorotation

A power-off flight condition where the main rotor is driven solely by upward airflow rather than engine torque.

Coriolis Effect

An effect caused by blade flapping or teetering aka blade hunting

Fully Articulated Rotor Systems

Use drag hinges alongside hydraulic or composite dampers to allow limited lead-lag (hunting) movement and absorb acceleration forces.

Semi-Rigid Rotor Systems

Utilize an underslung rotor head arrangement (Delta Hinge design) where the teetering axis sits above the feathering axis. As the rotor teeters, it swings to keep the rotor's center of mass aligned with the driveshaft/mast.

Rear Section

Connects the main body section to the tail structure. Built from three frames linked by beams, it features a stainless steel firewall acting as the engine attachment point, an internal baggage compartment, and mounting points for the tail boom.

Tail Boom

A structural assembly constructed of circular frames, stringers, and outer skin. Stringers and stiffeners provide structural rigidity to support attached components: the tail rotor gearbox (TGB), tail rotor drive shaft (TRDS), and vertical/horizontal stabilizers.

Lower Vertical Fin

Features a symmetrical airfoil profile and is fitted with a tail rotor guard at its base for ground protection.

Top Vertical Fin

Features an asymmetrical airfoil profile designed to generate aerodynamic side force in forward flight, offloading thrust requirements from the tail rotor

Horizontal Stabilizer

Features an asymmetrical airfoil profile mounted through a slot in the tail boom. It generates a downward force to keep the fuselage level during forward flight.

Anti-Vibration Device

Located beneath the pilot's seat, this device creates a vibration node within the cabin to isolate the crew from airframe oscillations.

Propeller

Converts engine rotational power into thrust. Rotational motion produces aerodynamic lift (thrust $F$) while creating aerodynamic drag (requiring engine torque $Q$).

Pitch (Blade Angle)

The angle formed between the blade chord line and the plane of rotation.

Geometric Pitch

The theoretical linear distance a propeller would advance forward in one full revolution without slippage.

Effective Pitch

The actual linear distance the propeller advances forward in one full revolution through the air.

Propeller Slip

The difference between geometric pitch and effective pitch. Higher efficiency corresponds to lower slippage.

Tractor and Pusher

Types of propeller

Fixed-Pitch Propeller

Blade angle is permanently set during manufacturing and cannot be adjusted in flight.

Variable-Pitch / Constant-RPM Propeller

Blade pitch alters during operation to optimize engine speed and power across varying flight regimes.

Feathering Propeller

Designed to rotate blade chord lines parallel to the relative airflow following an engine failure. This aligns blades with the airstream to reduce aerodynamic drag and prevent engine windmilling.

Reverse-Pitch Propeller

Allows pitch to pass through zero into negative angles to generate reverse thrust, serving as an aerodynamic brake during landing rollout.

Shrouded Propeller

Ducted blade assemblies commonly applied in marine propulsion to direct airflow/waterflow and improve thrust efficiency.