Introduction

This guide is designed to help a student helicopter pilot prepare for the oral portion of their check ride. The guide is set up on the following manner for each section.

  • Question
  • Brief statement
  • Expanded explanation
  • Reference(s)

This format was chosen so the student can provide brief initial answers to questions. Then, if additional detail is needed, the student can further expand on the concept. It is recommended that students keep their initial answers short and then expand on the topic as requested by the examiner.

Many people find it more difficult to answer questions verbally than when answering questions on written tests. Do not forget to practice answering questions verbally.

Everything described in this guide relates to counter-clockwise rotating rotor systems, as these are the predominate types of rotor systems used in the United States. Many of the principles described would be the opposite if the pilot was flying a helicopter with a clockwise rotating main rotor system. In addition, all descriptions presented assume a helicopter has a tail rotor for anti-torque control, as this is the most common anti-torque method used on training helicopters. There are designs, such as the NOTOR® system, that use different methods for anti-torque.

Other relevant concepts in this guide are explained in relation to the northern hemisphere. Some concepts would be reversed in the southern hemisphere.

What are the four aerodynamic forces?

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.

Aerodynamic fources with helicopter flight
Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2.2

Other Helicopter Aerodynamic Principles

How is lift created?

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.

how lift is created for helicopters

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.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-3
Principles of Helicopter Flight, 2nd Edition, pg. 17

Other Helicopter Aerodynamic Principles

What is the lift formula or equation?

The lift formula or equation is CL ½ p V2 S.

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.

Reference(s):

Principles of Helicopter Flight, 2nd Edition, pg. 18

Other Helicopter Aerodynamic Principles

What is induced flow?

Induced flow is the downward vertical movement of air through the rotor system due to the production of lift, often referred to as downwash.Helicopter Induced FlowAt 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.

Reference(s):

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

Other Helicopter Aerodynamic Principles

What is relative wind?

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.

relative windWith 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.

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. 1-8

Other Helicopter Aerodynamic Principles

What is rotational relative wind?

Rotational relative wind is the relative wind produced by the rotation of the rotor blades.

Rotational relative wind is parallel to the physical flight path of the rotor blades.  The rotational relative wind is at its maximum velocity at the rotor tips.

rotational relative windReference(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. 1-9

Other Helicopter Aerodynamic Principles

What is resultant relative wind?

Resultant relative wind is angle airflow at the rotor blades considering rotational relative wind and induced flow.

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.resultant relative windReference(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

Other Helicopter Aerodynamic Principles

What is dissymmetry of lift?

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.

dissymmetry of lift

Example: Calculate lift at 100 knots indicated airspeed for the advancing and retreating blade using the lift formula CL*½p*V2*S.

Reference(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

Other Helicopter Aerodynamic Principles

What is flapping?

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

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

Flapping Advancing Blade

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 downRetreating 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.

Reference(s):

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

Other Helicopter Aerodynamic Principles