Forces of Flight
The flight of any airborne machine is the result of several forces. The four main forces, commonly referred to as the four forces of flight, are thrust, lift, weight, and drag. The combined effect of these forces enables an aircraft to fly in the air. It is much easier to describe and understand the four forces of flight using the example of an airplane. We will first use an airplane to describe these forces before returning to quadcopters to explore their similarities and differences. To comprehend how a flying machine operates, it is essential to understand these forces and how they are controlled in various types of flying machines.
1. Thrust
Thrust is the forward-pushing force generated by the airplane's engines. In the case of jet engines, it is generated through the expulsion of air or exhaust gases at high velocities. And, in propeller-driven aircraft, propeller blades are responsible for generating thrust. This expulsion of gases, for example, in jet engines, creates a reactionary force, pushing the airplane forward. It is necessary for overcoming drag and achieving or maintaining forward motion. Thrust can be explained by Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction. In the context of an aircraft's jet engine, the action is the expulsion of gases at high speeds, and the reaction is the generation of thrust, pushing the aircraft in the forward direction. It is important to understand that thrust does not directly generate lift, but it does affect airflow patterns around the aircraft, indirectly assisting the wings in generating lift. Generally, thrust is directly proportional to aircraft speed, which in turn increases aerodynamic forces, i.e., lift and drag. Thrust is opposed by drag, which we will be discussing in a moment.
2. Lift
Lift is the force that allows flying machines to overcome gravity and stay aloft. Most of the lift is generated by the wings of an aircraft, but it is not true to say that lift is only generated by the wings. The entire aircraft generates lift to some extent, but the main and essential components that generate lift and keep an aircraft aloft are the wings. When an aircraft's engines produce thrust, and the aircraft begins to move forward, the airflow over the wings increases as a result. As the aircraft moves through the air, the wings create an area of lower pressure above and higher pressure below. This happens due to the purpose-built shape of the wings. The air passes faster over the top of the wings compared to the lower side of the wings, resulting in lower pressure above. This lower-pressure area above the wings effectively "sucks" the aircraft upward. This suction effect is a crucial component of lift generation. The faster the aircraft moves through the air, facilitated by thrust, the more pronounced this effect becomes.
The cross-sectional area at any part of the wing is called an airfoil. The chord line is a straight line from the leading to trailing edge of an airfoil, and the angle between the wing's chord line and the oncoming airflow is commonly known as the angle of attack. Generally, a higher angle of attack creates more lift. Airplanes must generate enough lift to equal or exceed their weight to take off and maintain level flight. Lift is opposed by the weight of the flying machine, which tries to pull it towards the ground.
3. Drag
Drag is the aerodynamic resistance that opposes a flying machine's forward motion and is thus a force that directly opposes thrust. There are many sub-types of drag, which are categorized depending upon the shape, construction, surface smoothness of the aircraft structure, and the disruption of airflow around the aircraft, etc. Reducing drag is critical to improving fuel efficiency and overall aircraft performance.
4. Weight
Weight is the force resulting from gravity acting on the mass of the flying machine. It acts vertically downward, toward the center of the Earth. This means that this force directly opposes lift. To achieve and sustain level flight, the lift must be equal to the weight of the flying machine. Changes in pitch, altitude, and speed necessitate adjustments to the balance between lift and weight.
Now, we will be discussing the sources and effects of basic forces discussed previously in the case of quadcopters and other multirotor aircraft. The propellers generate both thrust and lift. Each propeller generates thrust in a direction perpendicular to its plane of rotation. Keep in mind that the total thrust produced by a quadcopter is the sum of the thrust generated by all of its propellers. For example, a quadcopter with four propellers, each generating 10 newtons of thrust, would have a total thrust of 40 newtons. The combined thrust of four rotors should be equal to the weight of the quadcopter to maintain altitude and should be greater to become airborne and increase altitude. The thrust generated by propellers can vary widely depending on the type of propellers used, the specific design, intended purpose, and the type of brushless DC motors used. Most professional multirotor drones use brushless DC motors. Brushless DC motors have been discussed in detail in another lesson. Assuming that the vehicle is using brushless DC motors, the physical size of the motor and its Kv rating are important factors. Larger motors with higher Kv ratings can generally produce more thrust. Kv rating determines how fast the motor will spin for a given voltage, and higher Kv motors tend to spin at higher RPM, which can generate more thrust when paired with appropriate propellers. Quadcopters vary rotor speeds to control lift, enabling them to ascend, descend, or hover. Unlike airplanes, where lift is commonly decoupled from thrust, in quadcopters, lift and thrust are interdependent.
The rotor blades create lift through the same aerodynamic principles as traditional aircraft wings; however, the mechanism is slightly different. As the rotors spin, they produce a downward flow of air. As a result, a pressure difference between the upper and lower surfaces of the propellers is generated, resulting in an upward lift force.
In the context of a quadcopter, an airfoil typically refers to the shape and design of the rotor blades or propellers used by the quadcopter to generate lift and control its motion. Airfoils are crucial components as they influence the aerodynamic performance and efficiency of the quadcopter's rotors. Different airfoil profiles may be used depending on the specific requirements of the quadcopter, such as efficiency, speed, or payload capacity.
5. Torque
Torque will be discussed in detail in another lesson. However, in very simple words, torque is a force that causes an object to rotate. There are two very distinct torques that play an important role in the case of quadcopters. One is the rotational force produced by the spinning of the quadcopter's rotors. As each rotor rotates, it generates torque in the opposite direction, known as the "yawing moment." If left unchecked, this torque would cause the quadcopter to continuously rotate uncontrollably. To counteract this, quadcopters employ counter-rotating pairs of propellers and flight controllers that adjust rotor speeds to balance the torque.
Conclusion
Understanding the forces of flight is fundamental to aviation, whether we're talking about conventional airplanes or modern quadcopter drones. Lift, weight, thrust, and drag remain the key forces that shape the flight of airplanes, while quadcopters introduce additional considerations, including torque, and
a unique method of generating lift and thrust through the rotation of their rotors. The ability to manipulate these forces through engineering and control systems has revolutionized the way we explore the skies and perform tasks from aerial photography to package delivery."
Image Credits:
Side view of aircraft
File:Boeing 747-400 3view.Svg. In Wikipedia. https://commons.wikimedia.org/wiki/File:Boeing_747-400_3view.svg
Airfoil
By Olivier Cleynen - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=16100403