SIFT Part 1

Aerodynamic Fundamentals and the Atmosphere

Aerodynamics involves the study of the motion of air and the relative forces acting on objects moving through it. For helicopter pilots, understanding the International Standard Atmosphere (ISA) is vital, as air density ($\rho$) directly impacts rotor efficiency. At sea level standard conditions ($15^{\circ}C$ and $29.92$ inHg), the static pressure is approximately $2,116$ pounds per square foot ($lb/ft^{2}$). As altitude or temperature increases, air density decreases, requiring more power to produce the same amount of lift.

Gravity, Weight, and Center of Gravity (CG)

Gravity exerts a constant downward pull on the mass of the helicopter. Weight is the force that pilots must counteract to achieve flight. For instance, a rotor blade weighing $100$ pounds with a surface area of $20$ square feet ($20$ feet long by $1$ foot wide) must have an opposing force to stay aloft.

Crucially, weight acts through the Center of Gravity (CG). In helicopters, the allowable CG range is often very narrow. If the CG is too far forward or aft, the cyclic control may not have enough travel to maintain level flight, specifically during hover or steep maneuvers. Pilots must calculate the weight and balance for every flight to ensure the CG remains within the manufacturer’s specified limits.

Pressure and the Generation of Lift

Lift is the result of pressure differentials created by moving air. According to Bernoulli’s Principle, as the velocity of a moving fluid (like air) increases, its internal static pressure decreases.

1. Venturi Effect and Airfoils

   • An airfoil is shaped to create a "constriction" for the air passing over the top. This increases the air's velocity, causing a decrease in static pressure relative to the bottom surface.

   • A differential of only pounds per square foot across a rotor system can be sufficient to support a thousand-pound aircraft.

2. The Lift Equation

   • Lift ($L$) is mathematically defined as:

     $L = \frac{1}{2} \rho v^{2} S C_{L}$

     • $\rho$ represents air density.

     • $v$ is the velocity of the airflow.

     • $S$ is the surface area of the blade.

     • $C_{L}$ is the coefficient of lift, which is primarily influenced by the Angle of Attack (AoA).

The Four Primary Forces in Flight

Once airborne, the helicopter is a system in equilibrium or transition between four primary forces:

• Lift: Produced by the rotor system to oppose weight. It acts perpendicular to the resultant relative wind.

• Weight: The total load including the airframe, fuel, passengers, and cargo. It acts vertically downward through the CG.

• Thrust: The forward force produced by the rotor system by tilting the Total Aerodynamic Force (TAF) vector. It opposes drag.

• Drag: The retarding force acting opposite to the direction of flight. TAF is the resultant of Lift and Drag acting on the airfoil.

Airfoil Design and Geometry

Helicopter blades use specific airfoil profiles to optimize performance:

• Chord Line: A straight line from the leading edge to the trailing edge.

• Mean Camber Line: A line equidistant from the upper and lower surfaces.

• Symmetrical Airfoils: These have identical upper and lower surfaces. Their Center of Pressure (CP) remains relatively constant as the AoA changes, which provides essential stability for rotor blades.

• Non-Symmetrical (Cambered) Airfoils: These have different upper and lower curves. They produce more lift at lower AoAs but result in shifting CP, which can cause blade twisting or undesirable pitching moments.

Drag Taxonomy

Total drag is the sum of three distinct components:

1. Profile Drag: Created by the frictional resistance of the blades passing through the air. It includes:

   • Skin Friction: Air molecules sticking to the blade surface.

   • Form Drag: Resistance related to the physical shape of the airfoil.

2. Induced Drag: A byproduct of lift. As the AoA increases to produce more lift, the downward deflection of air (induced flow) tilts the lift vector rearward, increasing resistance. This drag decreases as forward airspeed increases.

3. Parasite Drag: Resistance from non-lifting components such as the fuselage, skids, and antennas. It increases with the square of the airspeed ($v^{2}$).

Load Factor and Bank Angles

In a level turn, the rotor must produce enough lift to support both the weight of the helicopter and the centrifugal force generated by the turn. This combined load is often referred to as "Apparent Weight."

• At a $30^{\circ}$ bank angle, the load factor is $1.15$ Gs (a $15\%$ increase).

• At a $60^{\circ}$ bank angle, the load factor is $2.0$ Gs (weight effectively doubles to $3,200$ pounds for a $1,600$-pound aircraft).

• High load factors increase the stall speed of the rotor blades and can lead to structural damage or accelerated fatigue if limits are exceeded.

Relative Wind and Blade Dynamics

• Rotational Relative Wind: Airflow produced by the rotation of the blades. It is highest at the tips and zero at the hub.

• Induced Flow (Downwash): The downward flow of air through the rotor disc.

• Resultant Relative Wind: The combination of rotational wind and induced flow. The Angle of Attack (AoA) is measured between the chord line and this resultant wind.

• Angle of Incidence (AoI): Also called the pitch angle, it is the mechanical angle between the chord line and the rotor hub, controlled by the pilot via the collective and cyclic.

Advanced Aerodynamic Concepts

1. Ground Effect: Occurs when the helicopter is within approximately $1$ rotor diameter of the surface. The ground interferes with the formation of tip vortices and reduces induced flow, allowing the helicopter to hover with significantly less power than in free air.

2. Dissymmetry of Lift: In forward flight, the advancing blade experiences higher relative wind speed than the retreating blade. To prevent the helicopter from rolling, blades use "flapping" (moving up and down) to equalize lift across the rotor disc.

3. Translational Lift: This is the improved rotor efficiency resulting from forward airspeed. As the helicopter moves forward, it outruns its own recirculating vortices. Effective Translational Lift (ETL) typically occurs between $16$ and $24$ knots.