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.
Total drag is the sum of parasitic drag, profile drag, and induced drag.
Parasitic drag is caused by non-lifting portions of the aircraft, such as the rotor mast, landing gear, etc. Parasitic drag is present anytime the aircraft is moving. Parasitic drag increases significantly with airspeed.
Profile drag develops from frictional resistance as the rotor blades passing through the air. Profile drag is comprised of form drag and skin friction. Overall, profile drag increases moderately with increases in speed, but does not change significantly with changes in the airfoil’s angle of attack. Form drag is the result of turbulent wake caused by separation of airflow from the surface of a structure. A flat plat creates more form drag than a symmetrical airfoil (teardrop).
Skin friction is caused by surface texture. The smoother something is the less skin friction. Dirt, ice, and other items that impact the surface texture have a significant effect on friction drag.
Induced drag is a result of the production of lift. Lift production generates downward velocities and vortices that increase induced drag. As the aircraft increases forward speed, induced drag decreases.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-5 Principles of Helicopter Flight, 2nd Edition, pg. 28 FM 3-04.203-2007 Fundamentals of Flight pg. 1-28
When flying for endurance, operate the helicopter at the speed using the least amount of power.
To stay in the air for the longest amount of time, power use must be at its minimum in order to conserve fuel. When looking at the horsepower required curve, the best speed for endurance is the speed that corresponds with the bottom of the curve. This speed does not correlate with the lowest drag. The speed for endurance will be slightly less than that needed for range.
Principles of Helicopter Flight, 2nd Edition, pg. 111
The three primary forces that affect wind are pressure gradient force, Coriolis force, and friction.
The atmosphere has different areas of pressure. Wind is created as the pressure systems try to equalize. The airflow moves from high pressure to low pressure.
The Coriolis force causes the wind to deflect to the right in the Northern hemisphere. he Coriolis force is directly proportional to the speed of the wind. The greater the wind speed, the greater the Coriolis force at a given latitude. The Coriolis force is zero at the equator, and more pronounced at middle and higher latitudes. The Coriolis force affects all moving objects. This is the same Coriolis force that affects the rotor system, such as when coning.
Wind is slowed down near the surface due to friction with the earth. The rougher the terrain, the greater the frictional effect. Also, the stronger the wind speed, the greater the friction. The frictional drag of the ground normally decreases with height and becomes insignificant above a few thousand feet.
Local winds are small-scale wind systems driven by heating or cooling of the ground. Air temperature differences develop over adjacent surfaces. Air in contact with the ground heats during the day and cools at night. Local winds are a significant factor for helicopters because their flight is generally local and often close to the ground.