12.1 Principles of Flight & Four Forces

12.1 Principles of Flight & Four Forces

Quick Answer: Flight is a balance of lift, weight, thrust, and drag. Bernoulli (pressure difference on aerofoils) and Newton (action–reaction / momentum change of air) both explain how wings and rotors work. Multirotors fly mainly by rotor thrust—accelerating air downward—and they vector that thrust by tilting the airframe. Know induced vs parasite drag and when forces are in equilibrium.

TP 15263 Section 7 (Theory of Flight) expects Advanced candidates to reason about why an RPAS climbs, stalls, sinks in a hover, or needs more power in a turn—not only which button is Return-to-Home. Flight-review examiners under Standard 921.02 / TP 15395 often probe the same concepts when they ask about performance, wind, and control. This section builds the force model you will reuse for aerofoils, stability, load factor, and multirotor hazards in the next two sections.

Bernoulli’s principle (pressure and speed)

Bernoulli’s principle (for steady, incompressible flow along a streamline) links fluid speed and static pressure: where air moves faster, static pressure is lower, and where air moves slower, static pressure is higher.

On a classic cambered aerofoil (wing section):

1. Airflow splits at the leading edge.
2. Upper-surface flow is accelerated (path length / flow geometry / circulation).
3. Lower static pressure above the wing and relatively higher pressure below produce a net force roughly perpendicular to the relative airflow—lift.
4. The chord-wise pressure distribution also contributes to pitching moment about the aerodynamic centre.

Exam-useful caveats (avoid cartoon physics):

• Bernoulli alone is not a complete story of lift; real wings also deflect air downward (Newtonian momentum change). Both views are valid and complementary.
• “Equal transit time” (air parcels above and below reunite at the trailing edge at the same moment) is a myth—do not use it in explanations.
• Propellers and rotor blades are rotating aerofoils: the same pressure/momentum physics applies to each blade element, with relative airflow set by RPM, pitch, and freestream speed.

Newton’s laws applied to rotors and wings

Rotor / propeller thrust as momentum change

A propeller or multirotor rotor increases the downward momentum of a mass of air per second. Thrust is essentially the rate of momentum change of that slipstream (plus pressure terms in full actuator-disk theory). Practical implications:

• More thrust needs more power: higher RPM, more blade pitch (where variable), or denser air.
• In thin air (high density altitude), the same RPM moves less mass → less thrust → higher power for hover (links to performance planning in flight-ops chapters).
• Ground effect (hover near the surface) can increase efficiency slightly because the downwash is constrained; leaving ground effect suddenly demands more power.

Fixed-wing lift as turning the airflow

A wing in flight imparts downward momentum to the air (downwash). Lift is the reaction to that force. Angle of attack and camber control how strongly the flow is turned—until the flow separates (stall; next section).

Fixed-wing vs multirotor vs helicopter — parts and roles

TP 15263 expects recognition of major components even if your daily aircraft is a quadcopter.

Fixed-wing RPAS (aeroplane configuration)

Fixed-wing craft need airspeed for wing lift. They are efficient for long mapping lines but cannot pure-hover (unless VTOL hybrid).

Multirotor RPAS

Lift in hover is almost entirely vertical rotor thrust. There is no large wing providing cruise lift (some hybrids excepted). Control is by differential RPM (and sometimes blade pitch on advanced designs).

Helicopter (rotary-wing manned/RPA overview)

Helicopters are in the Advanced syllabus mainly for recognition and hazard literacy (torque, vortex ring, dynamic rollover)—covered in Section 12.3—not for full type-rating depth.

The four forces of flight

In conventional fixed-wing teaching, four forces act on the aeroplane:

1. Lift — aerodynamic force primarily perpendicular to the relative airflow (wing/rotor).
2. Weight — gravity acting through the centre of gravity (CG).
3. Thrust — force from propeller, jet, or (for multirotors) the horizontal component of tilted total rotor thrust.
4. Drag — aerodynamic resistance opposite to the flight path / relative airflow.

Multirotor force picture (hover and cruise)

In a steady hover:

• Total rotor thrust is essentially vertical and equals weight (plus small corrections for wind).
• “Lift” and “thrust” language collapses: the rotors provide the upward force; there is no separate wing lift.
• Horizontal forces are near zero unless fighting wind.

In forward flight, the airframe tilts; the thrust vector tilts forward:

• Vertical component ≈ weight
• Horizontal component ≈ drag (steady speed)
• Body drag, induced losses, and profile power all rise with speed—hence a practical maximum efficient cruise.

Equilibrium

Equilibrium means the vector sum of forces is zero (and moments balance about the CG for steady attitude). Examples:

Exam trap: assuming “equilibrium” means the aircraft is not moving. Equilibrium means no net force—constant velocity, including constant zero velocity in a hover.

Drag: induced vs parasite

Induced drag

Induced drag is the drag associated with producing lift. Wingtip vortices and the rearward tilt of the lift vector on a finite wing create a drag component. For a given weight:

• Induced drag is highest at low speed / high angle of attack (more lift coefficient needed).
• It falls as speed increases (for the same lift) because less AoA is required.

Multirotor analogue: “induced” power/losses related to accelerating a large mass of air for hover—very high power just to stay aloft with no forward speed.

Parasite drag

Parasite drag is drag not primarily from lift production:

Parasite drag rises roughly with the square of speed. Clean configurations and streamlined payloads matter more as forward speed increases.

Total drag and practical RPAS effects

Total drag ≈ induced + parasite. Fixed-wing aircraft have a classic U-shaped total-drag curve and a speed for minimum drag / best glide. Multirotors show high hover power, a dip at moderate forward speed (translational lift / better rotor inflow), then rising parasite costs at high speed.

Operational links:

• External payloads and open gimbal cages raise parasite drag → more power, less endurance, more wind susceptibility.
• Slow, high-AoA fixed-wing legs or aggressive multirotor pitch attitudes raise induced costs and stall risk.
• Contaminated or damaged props increase drag and destroy efficiency (see propeller care in 12.3).

How multirotors create lift via rotor thrust vectoring

Creating lift (hover thrust)

1. Flight controller commands motor RPM (via ESCs).
2. Rotors accelerate air downward (Newton) / create pressure difference across the disc.
3. Upward reaction force supports the mass.
4. Opposite-spinning props cancel net torque about the yaw axis (Section 12.3).

Vectoring for control and translation

Multirotors do not have ailerons. They change attitude so the thrust vector points where net acceleration is needed:

Thrust vectoring here means using the airframe as a steerable thruster, not a rocket nozzle. Autopilots stabilize attitude dozens of times per second so the pilot’s stick commands become smooth ground track.

Limits of the thrust model

• Maximum climb is limited by excess thrust and power, not by “unlimited lift.”
• Near hover ceiling or high density altitude, thrust ≈ weight with no margin—gusts cause sink.
• Tilted flight reduces the vertical component available for weight support: aggressive forward flight needs more total thrust to keep altitude—power and heat rise.

Putting Section 12.1 together for the exam

When a stem mentions Bernoulli, choose pressure decrease with increased flow speed and lift from pressure difference—without the equal-transit myth. When it mentions rotors, lead with Newton / momentum and action–reaction. When it asks about four forces, place them in equilibrium for steady flight and identify induced (lift-related, low-speed) vs parasite (speed-squared, form/friction). When it asks how a quadcopter goes sideways, answer attitude change → thrust vector tilt, not ailerons.

Bottom line: Wings and rotors both work by changing air momentum and pressure. Master the four forces, drag split, equilibrium, and multirotor thrust vectoring—and every later Theory of Flight topic becomes application instead of memorization.