Basic Principles of Flight

A drone is a type of aircraft. To understand how drones fly, you need to understand the physics and basics of how an aircraft flies.

Four Forces of Flight

An aircraft flies because of four forces, and these four forces are lift, weight, thrust, and drag.

Lift – Lift is the force that holds an aircraft in the air. The wings create most of the lift used by aircraft.

Weight – Weight is the force caused by gravity.

Thrust – Thrust is a force that moves an aircraft in the direction of the motion. It is created with a propeller, jet engine, or rocket. Air is pulled in and then pushed out in an opposite direction. One example is a household fan.

Drag – Drag is the force that acts opposite to the direction of motion. It tends to slow an object. Drag is caused by friction and differences in air pressure. An example is putting your hand out of a moving car window and feeling it pull back.

In order for an aircraft to be able to fly, thrust has to equal drag, and lift has to equal weight.

TO FLY >> THRUST = DRAG & LIFT = WEIGHT

If for any reason, the amount of drag becomes larger than the amount of thrust, the plane will slow down. If the thrust is increased so that it is greater than the drag, the plane will speed up. Similarly, if the amount of lift drops below the weight of the aircraft, the plane will descend. By increasing the lift, the pilot can make the aircraft climb.

Each force has an opposite force that works against it. Lift works opposite of weight. Thrust works opposite of drag. When the forces are balanced, a plane flies in a level direction. The plane goes up if the forces of lift and thrust are more than gravity and drag. If gravity and drag are bigger than lift and thrust, the plane goes down. Just as drag holds something back as a response to wind flow, lift pushes something up. The air pressure is higher on the bottom side of a wing, so it is pushed upward.

Newton’s laws of motion

Newton’s three laws of motion are inertia, acceleration, and action/reaction. These laws apply to the flight of any aircraft.

Interaction between the laws of motion and aircraft mechanical actions causes the aircraft to fly.

INERTIA – A body at rest will remain at rest, and a body in motion will remain in motion at the same speed and in the same direction unless acted upon by an external force. Nothing starts or stops without an outside force to bring about or prevent motion. Inertia is a body’s resistance to a change in its state of motion.

ACCELERATION – The force required to produce a change in motion of a body is directly proportional to its mass and rate of change in its velocity. Acceleration refers to an increase or decrease – often called deceleration in velocity. Acceleration is a change in magnitude or direction of the velocity vector with respect to time. Velocity refers to the direction and rate of linear motion of an object.

ACTION/REACTION – For every action, there is an equal and opposite reaction. When an interaction occurs between two bodies, equal forces in opposite directions are imparted to each body.

Bernoulli’s Principle

This principle describes the relationship between internal fluid pressure and fluid velocity. It is a statement of the law of conservation of energy and helps explain why an airfoil develops an aerodynamic force. The concept of conservation of energy states energy cannot be created or destroyed and the amount of energy entering a system must also exit.

Airflow and The Airfoil

Airflow around an airfoil performs similar to airflow through a constriction. As the velocity of the airflow increases, static pressure decreases above and below the airfoil. The air usually has to travel a greater distance over the upper surface; thus, there is a greater velocity increase and static pressure decrease over the upper surface than the lower surface. The static pressure differential on the upper and lower surfaces produces about 75 percent of the aerodynamic force, called lift. The remaining 25 percent of the force is produced as a result of action/reaction from the downward deflection of air as it leaves the trailing edge of the airfoil and by the downward deflection of air impacting the exposed lower surface of the airfoil.

Characteristics of Airfoil

Helicopters and conventional aircraft are able to fly due to aerodynamic forces produced when air passes around the airfoil. An airfoil is a structure or body designed to produce a reaction by its motion through the air.

• Airfoils are most often associated with the production of lift.
• Airfoils are also used for stability (fin), control (elevator), and thrust or propulsion (propeller or rotor).
• Certain airfoils, such as rotor blades, combine some of these functions.
• Airfoils are carefully structured to accommodate a specific set of flight characteristics.

Blade Span – The length of the rotor blade from point of rotation to tip of the blade.

Wing Span – The length of the wing from tip to tip.

Chord Line – A straight line intersecting leading and trailing edges of the airfoil.

Chord – The length of the chord line from leading edge to trailing edge; it is the characteristic longitudinal dimension of the airfoil section.

Mean Camber Line – A line drawn halfway between the upper and lower surfaces. The chord line connects the ends of the mean camber line.

Camber refers to the curvature of the airfoil and may be considered curvature of the mean camber line. The shape of the mean camber is important for determining the aerodynamic characteristics of an airfoil section. Maximum camber (displacement of the mean camber line from the chord line) and its location help to define the shape of the mean camber line. The location of the maximum camber and its displacement from the chord line is expressed as fractions or percentages of the basic chord length. By varying the point of maximum camber, the manufacturer can tailor an airfoil for a specific purpose. The profile thickness and thickness distribution are important properties of an airfoil section.

Leading-Edge Radius – The radius of curvature given the leading edge shape.

Flight-Path Velocity – The speed and direction of the airfoil passing through the air. For Fixed-Wing airfoils, flight-path velocity is equal to true airspeed (TAS). For helicopter rotor blades, flight-path velocity is equal to rotational velocity, plus or minus a component of directional airspeed.

Relative Wind – Air in motion equal to and opposite the flight-path velocity of the airfoil. This is rotational relative wind for rotary-wing aircraft and will be covered in detail later. As an induced airflow may modify flight-path velocity, relative wind experienced by the airfoil may not be exactly opposite its direction of travel.

Induced Flow – The downward flow of air (more distinct in rotary-wing).

Resultant Relative Wind – Relative wind modified by the induced flow.

Angle of Attack (AOA) – The angle measured between the resultant relative wind and chord line.

Angle of Incidence (Fixed-Wing Aircraft) – The angle between the airfoil chord line and longitudinal axis or other selected reference plane of the aircraft.

Angle of Incidence (Rotary-Wing Aircraft) – The angle between the chord line of a main or tail-rotor blade and rotational relative wind (tip-path plane). It is usually referred to as blade pitch angle. For fixed airfoils, such as vertical fins or elevators, angle of incidence is the angle between the chord line of the airfoil and a selected reference plane of the helicopter.

Center of Pressure – The point along the chord line of an airfoil through which all aerodynamic forces are considered to act. Since pressures vary on the surface of an airfoil, an average location of pressure variation is needed. As the AOA changes, these pressures change, and the center of pressure moves along the chord line.

Aerodynamic Center – The point along the chord line where all changes to lift effectively take place. If the center of pressure is located behind the aerodynamic center, the airfoil experiences a nose-down pitching moment. The use of this point by engineers eliminates the problem of center of pressure movement during AOA aerodynamic analysis.

BLADE TWIST (ROTARY-WING AIRCRAFT)

Because of the lift differential along the blade, it should be designed with a twist to alleviate internal blade stress and distribute the lifting force more evenly along the blade. Blade twist provides higher pitch angles at the root where velocity is low and lower pitch angles nearer the tip where velocity is higher. This increases the induced air velocity and blade loading near the inboard section of the blade.