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
Relative wind is the angle of airflow as it impacts an airfoil.
Movement of an airfoil through the air creates relative wind. The direction of airfoil in relation to the air changes the angle of the airflow, or relative wind. Relative wind is parallel but in the opposite direction of the airfoil’s direction.
With fixed-wing aircraft, this is a simple concept. With rotary wing aircraft, there are several other factors impacting the relative wind, mainly rotation of the rotor blades, and the induced flow from lift production. With helicopters, when someone refers to relative wind, they are usually referring to resultant relative wind.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-8 Principles of Helicopter Flight, 2nd Edition, pg. 47 FM 3-04.203-2007 Fundamentals of Flight pg. 1-8
When at a hover in a calm, no-wind condition, resultant relative wind is the combination of rotational relative wind and induced flow. However, the movement of the helicopter and wind velocity also affect the angle of the airflow at the rotor blades. Resultant relative wind is a factor used in determining or describing many aerodynamic factors. Often, relative wind and resultant relative wind are used simultaneously.Reference(s):
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-8 Principles of Helicopter Flight, 2nd Edition, pg. 47 FM 3-04.203-2007 Fundamentals of Flight pg. 1-10
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
Flapping is the vertical movement of a blade up or down to increase or decrease lift in order to compensate for dissymmetry of lift.
To equalize lift across the rotor disc, the advancing blade flaps up and the retreating blade flaps down. Flapping modifies the resultant relative wind by moving with or against the induced flow, which changes a blade’s angle of attack. The effect of the advancing blade flapping up is the same as increasing the induced flow. The increased induced flow will decrease the blade’s angle of attack as the resultant relative wind is influenced more by the induced flow and less by the (rotational) relative wind. With a lower angle of attack and the same rotational speed, the advancing blade produces less lift than without flapping. The opposite is true for the retreating blade. When the retreating blade flaps down, it moves with the induced flow. This movement reduces the induced flow and increases the retreating blade’s angle of attack as the resultant relative wind is influenced less by the induced flow and more (rotational) relative wind.
As a blade’s angle of attack changes, so does the blade’s inflow angle. The inflow angle is the angle between the rotational relative wind and the resultant relative wind. Other factors removed, there is an inverse relationship between the inflow angle and the blade’s angle of attack. If the inflow angle increases, the angle of attack decreases, producing less lift. If the inflow angle decreases, the angle of attack increases, producing more lift.
No Wind Hover: No Flapping
Induced Flow (IF): The downwash. Rotational Relative Wind (RW): from rotation of the blade Resultant Relative Wind (RRW): combination of induced flow and (rotational) relative wind Inflow Angle (IA): RW – RRW Blade Angle (BA): Physical angle of the blade Angle of Attack (AOA): BA – RRW
Advancing Blade: Flaps Up Decreases AOA, less lift
Moving the blade up is the same as increasing the induced flow, like walking into the wind verses with the wind. The AOA decreases as the RRW is influenced more the induced flow and less by the (rotational) relative wind.
Retreating Blade: Flaps Down Increases AOA, more lift
Moving the blade down is the same as decreasing the induced flow, like walking with the wind verses into the wind. The AOA increases as the RRW is influenced more by the (rotational) relative wind and less by the induced flow.
There are several methods of flapping. In a fully articulated rotor system, like the 300CB, each blade flaps individually. In a semi-ridged rotor system, like the Robinson R22/R44, the blades flap as a unit, when one flaps up, the other flaps down.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-19 Principles of Helicopter Flight, 2nd Edition, pg. 92 FM 3-04.203-2007 Fundamentals of Flight pg. 1-13, 1-40
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.