Principles of Flight

Objective

Upon completion of this lesson, the CFI candidate will demonstrate the ability to effectively instruct on helicopter principles of flight by accurately explaining fundamental aerodynamic concepts, identifying associated risk management considerations, and delivering clear instruction on at least three specific aerodynamic phenomena affecting helicopter flight operations in accordance with HI.III.B.

Content

Four Forces of Flight (HI.III.B.K1)

Lift - The upward force generated by the rotor system that opposes weight. In helicopters, lift is created primarily by the main rotor blades acting as rotating airfoils. The amount of lift varies with collective pitch changes, rotor RPM, air density, and forward airspeed.

Weight - The downward force due to gravity acting on the helicopter’s mass. Weight remains relatively constant during flight but affects performance calculations and power requirements.

Thrust - In helicopters, thrust is the horizontal component of the main rotor’s total lift vector. Thrust is controlled by cyclic inputs that tilt the rotor disc, redirecting lift to provide forward, aft, or sideward movement.

Drag - The rearward force opposing forward motion through the air. Helicopters experience profile drag (form drag), induced drag, and parasite drag. Understanding drag is crucial for power management and performance planning.

Airfoils (HI.III.B.K2)

Airfoil Terminology:

• Chord line: Imaginary straight line from leading edge to trailing edge
• Angle of attack: Angle between chord line and relative wind
• Camber: Curvature of the airfoil
• Leading edge: Front portion of the airfoil
• Trailing edge: Rear portion of the airfoil

Airfoil Types:

• Symmetrical: Equal camber above and below chord line, used in many helicopter main rotor blades
• Semi-symmetrical: Slight camber, common in modern helicopter designs
• Cambered: Pronounced curve, less common in helicopter rotors due to operational requirements

Helicopter-Specific Aerodynamic Phenomena

Torque Effect and Translating Tendency (HI.III.B.K3) Torque effect results from Newton’s third law - the main rotor’s rotation creates an equal and opposite reaction attempting to rotate the fuselage. This manifests as translating tendency, causing the helicopter to drift laterally (typically left in American helicopters with counter-clockwise rotor rotation). Compensated by tail rotor thrust and left cyclic input.

Gyroscopic Precession (HI.III.B.K4) The spinning main rotor acts as a gyroscope. When a force is applied to a spinning gyroscope, the reaction occurs 90 degrees later in the direction of rotation. In helicopters, this affects control response and requires pilots to understand the relationship between cyclic input and rotor disc tilt.

Blade Flapping and Coning (HI.III.B.K5)

• Flapping: Vertical movement of rotor blades to equalize lift across the rotor disc, particularly important during forward flight
• Coning: Upward bending of rotor blades due to centrifugal force and lift, creating a cone shape when viewed from above

Coriolis Effect (HI.III.B.K6) Changes in the radius of mass rotation affect rotational velocity. As rotor blades flap up and down, their center of mass moves closer to or farther from the hub, affecting blade angular velocity and requiring compensation through blade design and control systems.

Pendular Action (HI.III.B.K7) The helicopter’s tendency to oscillate like a pendulum due to the center of gravity being below the rotor hub. This characteristic affects stability and control, particularly during hovering flight and requires active pilot input for control.

Dissymmetry of Lift (HI.III.B.K8) In forward flight, the advancing blade (moving in the same direction as helicopter movement) experiences higher relative wind velocity than the retreating blade, creating unequal lift. Compensated by blade flapping and cyclic feathering.

Retreating Blade Stall (HI.III.B.K9) At high forward airspeeds or aggressive maneuvering, the retreating blade may reach critical angle of attack and stall due to its lower relative wind velocity. This represents a significant flight limitation and safety consideration.

Translational Lift and ETL (HI.III.B.K10)

• Translational Lift: Additional lift gained when helicopter moves horizontally, reducing induced drag and improving efficiency
• Effective Translational Lift (ETL): Occurs around 16-24 knots when helicopter completely escapes its downwash, providing noticeable performance improvement

Transverse Flow Effect (HI.III.B.K11) Air flowing across the rotor disc during forward flight creates different flow conditions at front and rear of the disc, causing vibration and requiring pilot compensation during transition from hover to forward flight.

Autorotative Flight Aerodynamics (HI.III.B.K12) During autorotation, upward airflow through the rotor disc drives the blades, maintaining rotor RPM without engine power. The rotor disc is divided into driven region (outer portion), driving region (middle), and stall region (inner portion). Understanding these regions is critical for safe autorotative flight.

Rotor System Characteristics (HI.III.B.K13)

• Fully articulated: Three hinges per blade (flapping, lead-lag, feathering)
• Semi-rigid: Two-blade system with teetering hinge
• Rigid: No flapping or lead-lag hinges, flexibility built into blade root

Risk Management Considerations (HI.III.B.R1)

Understanding aerodynamic principles is fundamental to safe helicopter operation. Pilots must recognize how aerodynamic limitations affect flight envelope, particularly:

• Operating within retreating blade stall limitations
• Managing power requirements during different flight phases
• Understanding performance degradation in adverse conditions
• Recognizing aerodynamic factors affecting autorotation performance

Schedule

Equipment

Required References:

• FAA-H-8083-21A Helicopter Flying Handbook
• FAA-H-8083-4 Helicopter Instructor’s Handbook
• FAA-S-ACS-29 Helicopter Airman Certification Standards

Visual Aids:

• Helicopter rotor system model or diagram
• Airfoil cross-section models
• Four forces diagram
• Rotor disc illustration showing different regions
• Dissymmetry of lift diagram
• Autorotation airflow diagram

Materials:

• Whiteboard and markers
• Helicopter flight manual for performance examples

Instructor Actions

The CFI candidate will demonstrate instructional ability by:

1. Presenting clear explanations of the four forces using analogies (lift like invisible hands pushing up, weight like gravity’s constant pull)

2. Using progressive disclosure starting with basic four forces, then building complexity with helicopter-specific phenomena

3. Demonstrating teaching techniques including:

   • Visual aids to illustrate abstract concepts
   • Analogies to relate complex aerodynamics to familiar concepts
   • Questioning techniques to assess understanding

4. Explaining practical applications of each aerodynamic principle to actual flight operations

5. Delivering instruction on minimum three elements from K1-K13 as required by HI.III.B.S1

6. Connecting aerodynamic theory to risk management by explaining how understanding principles prevents accidents

7. Using appropriate instructional techniques such as building block method and demonstration-performance method

8. Managing instructional time effectively while covering all required elements

9. Encouraging student questions and providing clear, accurate answers

Student Actions

During this lesson, the student (DPE) will:

1. Listen actively to instruction on aerodynamic principles
2. Ask relevant questions about helicopter aerodynamics
3. Demonstrate understanding by explaining concepts back to instructor
4. Participate in discussions about practical applications
5. Take notes on key aerodynamic principles and their effects
6. Identify relationships between different aerodynamic phenomena
7. Apply knowledge to hypothetical flight scenarios presented by instructor

Completion Standards

The CFI candidate successfully completes HI.III.B when they demonstrate the ability to:

1. Accurately explain fundamental aerodynamic concepts including all knowledge elements K1-K13 with correct terminology and clear understanding

2. Deliver effective instruction on minimum three aerodynamic elements from the knowledge requirements, demonstrating teaching ability rather than just knowledge

3. Use appropriate instructional techniques including visual aids, analogies, and progressive disclosure to enhance student understanding

4. Connect aerodynamic principles to practical flight operations and risk management considerations

5. Demonstrate mastery of subject matter by answering questions accurately and providing additional relevant information

6. Maintain student engagement through effective communication and appropriate instructional methods

7. Complete instruction within reasonable time limits while covering all required elements thoroughly

8. Show evidence of lesson planning and organization with logical flow and clear objectives

Performance meets ACS standards when the CFI candidate demonstrates competency in instructional knowledge, risk management awareness, and teaching skills specific to helicopter aerodynamic principles as outlined in HI.III.B.