9.2 Aerodynamics & Four Forces of Flight

Aerodynamics & Four Forces of Flight

Quick Summary: Atmospheric flight is governed by four vector forces: Lift, Weight, Thrust, and Drag. In steady, unaccelerated flight, these forces are in equilibrium. Understanding how airfoils generate lift through Bernoulli's principle and Newton's third law, angle of attack stalls, rotary-wing hazards like Vortex Ring State, multirotor translation, and load factor in steep banked turns is critical for passing the Transport Canada Small Basic exam.

Whether flying fixed-wing drones or multirotors, flight is governed by universal aerodynamic principles. While multirotors alter motor speeds rather than control surfaces, their propeller blades act as spinning rotary wings. Under Transport Canada standard TP 15263, pilots must master these aerodynamic concepts under CARs Part IX.

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The Four Forces of Flight in Balance

Every aircraft in flight is acted upon by four opposing forces:

1. Lift: Upward aerodynamic force produced by wings or rotors perpendicular to relative wind. Opposes Weight.
2. Weight (Gravity): Downward gravitational force acting on aircraft mass toward Earth's centre. Opposes Lift.
3. Thrust: Forward propulsive force generated by motors and propellers. Opposes Drag.
4. Drag: Rearward retarding aerodynamic force caused by friction and lift generation. Opposes Thrust.

Equilibrium in Steady Flight

In steady, unaccelerated level flight (or zero-wind hover):

• Lift equals Weight (Lift = Weight)
• Thrust equals Drag (Thrust = Drag)

Forces in balance produce zero net acceleration. If Lift exceeds Weight, the RPA climbs; if Weight exceeds Lift, it descends. If Thrust exceeds Drag, the RPA accelerates; if Drag exceeds Thrust, it decelerates.

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Aerodynamic Drag: Parasite Drag vs. Induced Drag

Total drag is the sum of Parasite Drag and Induced Drag:

Total Drag = Parasite Drag + Induced Drag

The lowest point on the total drag curve is the minimum drag speed (L/D_max), yielding maximum glide range for fixed-wing drones and optimal cruising endurance for multirotors.

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Principles of Lift Generation: Bernoulli and Newton

Propeller blades and wings are airfoils designed to generate lift efficiently:

• Chord Line: Straight imaginary line connecting leading edge to trailing edge.
• Camber: Curvature of airfoil surfaces. Positive camber has greater upper curvature.
• Relative Wind: Airflow direction relative to the airfoil, moving opposite to the flight path.

1. Bernoulli's Principle

As fluid velocity increases, static pressure decreases. Air moves faster over curved upper camber, creating a low static pressure zone above and higher pressure beneath, generating upward lift.

2. Newton's Third Law

Airfoils deflect air downward (downwash). Accelerating air downward creates an equal, opposite reaction pushing the airfoil upward. Both Bernoulli and Newtonian effects produce lift simultaneously.

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Angle of Attack (AoA) and Aerodynamic Stalls

The Angle of Attack (AoA) is the acute angle between the airfoil's chord line and the relative wind.

Lift increases with AoA up to the Critical Angle of Attack (typically 15° to 18°):

• Exceeding this angle causes airflow to detach from the upper surface.
• The boundary layer separates into a turbulent wake, producing a sudden loss of lift and severe drag increase.

Crucial Exam Rule: An aerodynamic stall is caused exclusively by exceeding the critical angle of attack. A stall can occur at any airspeed, flight attitude, or motor throttle setting.

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Rotary-Wing Aerodynamic Hazards

RPAS rotary wings are subject to two specific aerodynamic hazards:

1. Vortex Ring State (VRS / Settling with Power)

Vortex Ring State (VRS) occurs when a multirotor descends vertically or nearly vertically at high sink rates (> 300 to 500 ft/min) with power applied and low forward speed (< 10 km/h):

• The RPA sinks into its own rotor downwash. Tip vortices recirculate in a toroidal ring, destroying lift.
• Pilot Trap: Applying upward throttle accelerates the vortex ring, worsening the plunge!
• Recovery: Immediately pitch or roll horizontally to fly into clean air before applying power.

2. Retreating Blade Stall

In fast forward flight, advancing blades see higher airspeed while retreating blades see lower airspeed. The flight controller increases the retreating blade's AoA to equalize lift. At excessive forward speeds, the retreating blade exceeds critical AoA and stalls, causing roll and pitch instability.

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Multirotor Flight Dynamics: Control Axes and Translation

Multirotors maneuver across three axes intersecting at the Center of Gravity (CG):

In hover, rotor thrust points straight down balancing Weight (T = W). To translate forward, the airframe pitches forward, tilting the thrust vector into a vertical component (T * cos(theta)) balancing Weight and a horizontal component (T * sin(theta)) driving forward motion. Because the vertical vector diminishes, total motor thrust must increase during forward flight to maintain altitude.

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Banked Turns, Centripetal Force & Load Factor

In a turn, banking tilts the lift vector inward:

• The horizontal component of lift acts as centripetal force, pulling the aircraft into the turn.
• The vertical component of lift must continue to balance the aircraft's Weight.

Load Factor (G-Loading)

As bank angle steepens, total lift must increase to maintain level flight:

Load Factor (G) = Total Lift / Weight = 1 / cos(Bank Angle)

At a 60° bank angle, load factor reaches 2.0 G, doubling the effective weight of the aircraft and demanding twice the normal motor thrust, which rapidly drains batteries and increases stall vulnerability.

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Practical Exam Scenarios & Common Traps

• The Steep Bank Altitude Drop: Executing a 60° bank turn demands 2.0 G of lift. If motors are already operating at 70% throttle, they cannot generate 200% thrust, causing the RPA to lose altitude.
• The VRS Descent Plunge: Rapid vertical descents produce violent wobbling and uncommanded sinking. Applying upward throttle worsens the descent; the pilot must pitch horizontally into clean air.
• Airspeed vs. Groundspeed: A multirotor hovering stationary in a 20 km/h headwind has a groundspeed of 0 km/h, but an airspeed of 20 km/h. Parasite drag and lift depend entirely on airspeed, not groundspeed.