The four aerodynamic forces are thrust, drag, lift, and weight.
These aerodynamic forces work together for controlled flight. Thrust is the force needed to move the aircraft forward, overcoming drag. Drag is the force that keeps an aircraft from moving forward. Lift is the force that opposes weight and keeps the aircraft flying. Weight is the mass of the aircraft that is affected by gravity.
Lift is created through the combination of Bernoulli’s principle and Newton’s 3rd law.
Bernoulli’s principle states that as air speeds up its pressure reduces. An airfoil is used to create increased air flow on one side of the airfoil, which creates a lower pressure. In an effort to equalize itself, the airfoil moves to the lower pressure, creating lift.
Newton’s 3rd law states that for every reaction, there is an equal and opposite reaction. With an airfoil at some angle, there will be airflow impacting the bottom side of the airfoil. As such, the airfoil is pushed in the opposite direction, producing lift.
To see these two forces in action, complete the following experiment. While holding a piece of paper horizontally from one end, blow air over the top of the paper from the end being held. The increased airflow should draw the paper upwards. This is an example of Bernoulli’s principle. Now, blow air underneath the paper. This airflow should push the paper upwards. This is an example of Newton’s principle.
This formula is used to quantify the factors or components that influence lift production. The factors are coefficient of lift, air density, velocity, and surface area. Not all factors of the equation are equal.
CL is the coefficient of lift. In general, this is the angle of attack on the rotor blade. Until the stalling angle is reached, an increase in the CL will produce more lift.
½ p V2 This section of the formula is Dynamic Energy or Kinetic Energy. Basically, dynamic/kinetic energy is derived from the movement of air. The p is for pressure or air density.* The greater the density (lower pressure altitude) the more lift produced.
V2 is for velocity or the rotor RPM with regards to helicopter flight. As referenced by the squared component, velocity is a major factor in lift production. A slight change in velocity can have a significant impact on lift. This fact is one reason that low rotor RPM is a significant issue with helicopters.
S stands for surface area. In helicopter flight, the surface area of the rotor blades does not change. Unlike fixed-wing aircraft, rotor systems do not have flaps that can increase or decrease the surface area.**
* The p is m for mass in some equations. With reference to lift, mass is the density of the air. ** There are some experimental systems, but in general these are not available to most pilots. In addition, this discussion does not consider stabilizers or other systems that may change the surface area slightly, as these are not a significant factor in helicopter flight.
Principles of Helicopter Flight, 2nd Edition, pg. 18
Induced flow is the downward vertical movement of air through the rotor system due to the production of lift, often referred to as downwash.At a hover in calm, no-wind conditions, the induced flow is at its greatest because there is no horizontal air flow affecting the rotor disc. Induced flow increases as the angle of attack of the rotor blades increases.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-10 Principles of Helicopter Flight, 2nd Edition, pg. 47 FM 3-04.203-2007 Fundamentals of Flight pg. 1-9
Dissymmetry of lift is the unequal rotor thrust, or lift, produced by the rotor disc due to forward flight or wind.
With forward flight, one blade is advancing into the wind while the other blade is retreating, or going with the wind. Uncorrected, the advancing blade produces more lift than the retreating blade, as the airflow over the advancing blade is greater. If left uncorrected, the helicopter would be difficult to fly and would roll to the left due to the increased lift from the right side of the rotor disc. The lift is equalized across the rotor disc through a process called flapping. With flapping, the rotor blades are able to move vertically to increase or decrease their angle of attack and thus increase or decrease the lift produced by an individual blade.
Example: Calculate lift at 100 knots indicated airspeed for the advancing and retreating blade using the lift formula CL*½p*V2*S.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-18 Principles of Helicopter Flight, 2nd Edition, pg. 91 FM 3-04.203-2007 Fundamentals of Flight pg. 1-39
Coning is an upward sweeping angle of the rotor blades as a result of lift and centrifugal force.
Centrifugal force is caused by blade rotation. This force pulls the rotor blades horizontally and provides rigidity to the blades. The faster the rotation of the blades, the more centrifugal force. In contrast, lift acts perpendicular to airflow or resultant relative wind. The lift generated by a rotor blade increases from the root to the tip. The coning angle increases when more lift is generated as compared to centrifugal force.
Conversely, the coning angle decreases when the centrifugal force increases as compared to the lift generated. When a helicopter transitions from the ground to a hover, the increase in coning angle is easy to see. There are several flight conditions that effect the coning angle. Lower rotor RPM reduces the centrifugal force, which results in an increase in coning angle if the lift requirement remains the same. If the centrifugal force remains the same, the coning angle will increase with an increase in lift. High gross weight and high-G maneuvers require more lift.
With low rotor RPM, a dangerous situation can result when the blades cone due to the inadequate centrifugal force. The blades can cone to a level where it is unable to support the helicopter’s weight.
Effective translational lift (ETL) is when the lift generation from the rotor disc is more efficient due to increased aircraft speed or wind.
When at a hover in calm, no-wind conditions, the induced flow is a significant factor affecting the resultant relative wind. As a result, the blade angles are significant and it takes more power for flight. As the aircraft increases speed, approximately 16-24 knots indicated airspeed, the impact of induced flow is reduced. Because of forward movement or wind, there is undisturbed air* meeting the front of the rotor disc. As speed increases, the portion of the disc receiving undisturbed air increases. As a result, the inflow angle is decreasing as more air is received horizontally versus vertically. As such, a lower angle of attack will produce the same lift, resulting in less power needed for flight.
The tail rotor also becomes more efficient with an increase in forward speed. There are two factors involved. First, like the main rotor, the tail rotor becomes more efficient when it operates in undisturbed air. Second, the forward movement of the aircraft reduces the amount of anti-torque thrust needed as the horizontal stabilizer or similar component, becomes more effective. As the need for anti-torque lessens, there is more power available for the main rotor.
It is a common mistake for pilots to refer to flying through ETL. An aircraft achieves ETL. Once in ETL, the aircraft is receiving the benefit of transitional lift until the relative wind is changed so the ETL is no longer achieved, such as by slowing down, flying downwind or a change in wind velocity. In general, the increase in horizontal speed or wind, the more efficient the rotor system.
* Some people will make reference to clean/dirty air instead of undisturbed/disturbed air. The use of the term clean air and dirty air should be avoided as the level of particulate matter in the air is not a factor, it is the turbulence of the air that is relevant.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-20 Principles of Helicopter Flight, 2nd Edition, pg. 64, 99 FM 3-04.203-2007 Fundamentals of Flight pg. 1-41, 1-43
The rotor disc becomes more efficient within close proximity of the ground, so it takes less power to provide the same amount lift.
The benefits of ground effect are received when the rotor disc is about ½ its diameter from the ground, usually about 3-4ft skid height. The benefits of ground effect are the greatest over hard surfaces.
When over grass or similar surfaces, more power will be required then when over a smooth hard surface. When over a hard surface, such as a taxiway, there is some pressure received which lowers the velocity of the induced flow which reduces the angle of attack. With a lower angle of attack, more power is available for lift. In addition, the tip vortices are reduced, which lowers the induced drag, providing more power available for lift. When in ground effect over grass or similar surfaces, the horizontal movement of the air along the ground is reduced, increasing the induced flow, and requiring more power.
Another factor with grass or non-hard surfaces, pilots tend to slightly increase their altitude. Any increase in altitude, significantly lessens the benefit of ground effect.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-10 Principles of Helicopter Flight, 2nd Edition, pg. 62 FM 3-04.203-2007 Fundamentals of Flight pg. 1-34