Introduction

Helicopter Rotor Dynamics is a very interesting but complicated area of study. An online project has been developed by us to create a new way of learning. We created our project with a view to helping students understand rotor dynamics better. It is a visual aid to understand the working phenomenon of various complex mechanics and dynamics of helicopters.

Rotor Dynamics is not a simple concept. It is always counter-intuitive to the way helicopters behave.

We created all these in Scratch developed by MIT. All the individual projects have been uploaded to scratch.mit.edu and have been a very helpful tool to learn and create along the way.


Scratch not only enables the user to view the project but also to view the code and the way it actually works.

The process was not so easy. Visualizing what to create and how to create and then actually creating the entire project took days and months. Hopefully, the end result is fruitful and the students of the next generation will be able to implement their practical knowledge and establish it to the next.

The topics we will cover are as follows:

1. Helicopter Hinges

2. Coning Angle

3. Phase lag & Advance Angle

4. Recirculation

5. Ground Effect, IGE, OGE

6. Helicopter Hover Condition with Variable Wind

7. Helicopter Hover Condition with Variable Wind and Variable CG

8. Autorotation

9. Flapping to Equality


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Worth Watching

Helicopter Phase Lag

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In the aerodynamics  of  rotorcraft  like  helicopters ,   phase lag   refers to the angular difference between the point at which a control input to a rotor blade occurs and the point of maximum displacement of the blade in response to that control input , said displacement occurring in the direction of rotor rotation.  ' Phase lag'   differs from  ' advance angle'   in the latter referring to the mechanically fixed angle between the pitch link attachments at the blade and the swashplate.   Phase lag   may vary depending on rotor tilt rate, the ratio of aerodynamic damping to blade inertial forces (Lock number), offset of flapping hinge from the axis of rotation (e/R ratio), and coupling of blade flap, drag, and feather motions, and often results in   cross-coupling   between the aircraft control axes;   advance angle   is fixed and cannot vary. Consequent to phase-lag, rolling a rotorcraft to the left or right would theoretically require a forward or backward cyclic i
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Helicopter Hinges

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The rotor blades in a fully articulate blade system are attached to the rotor hub through a series of hinges so that it can move in three ways and each blade can move independently of the other. The blades can move up and down, back and forth in the horizontal plane, and can change in the pitch angle (the tilt of the blade). The hinges present in the articulate blade system are: Flapping Hinge/Horizontal hinge Lead-lag hinge/Drag hinge/Vertical hinge Feathering Hinge/Pitch change hinge Flapping hinges The flapping hinges are horizontal hinges of a helicopter rotor that allow the blades to move up and down. This up and down motion of blades is called flapping. The flapping motion created by flapping hinges can compensate for the dissymmetry of lift. For this purpose, the flapping hinges may be located at varying distances from the rotor hub. When a blade is on the advancing side, its increased lift causes the blade to flap upwards, which effectively reduces its incidence. The
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Coning Angle

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Coning angle is the angle formed between the plane of rotation of a helicopter rotor blade when it is producing lift and a line perpendicular to the rotor shaft. Also, Coning Angle is formed by the rotor blades with the tip path of the plane. ( Tip Path Plane is the imaginary circular plane outlined by the rotor blade tips in making a cycle of rotation) Coning happens primarily because of the variation of rotor thrust from root to tip. The degree of the coning angle is determined by the relationship between the centrifugal force acting on the blades and the aerodynamic lift produced by the blades. From the figure, as the blade is rotating, there is a centrifugal force, which is throwing the blades outside. The force between upward and the centrifugal force when balanced results in one particular cone angle, which is called coning angle. An increase in helicopter weight results in an increase in coning angle. There are upper limits and lower limits of coning angle. The upper limi
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Flapping to Equality

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During   the   movement   of   a   helicopter   rotating   blade ,  the   speed   of   airflow   over   the   advancing   blade   is more   than   the   retreating   blade ,  resulting   in  the  production   of   an   unequal   amount   of   lift .  This   is   called   a        dissymmetry   of   lift .  To   correct   this   aerodynamic   imbalance ,  the   blades   are   allowed   to   perform   a   series  of   controlled   movements   to   produce   equal   amounts   of   lift .  This   process   of   equalization   is   called   flapping   to   equality .          Nil wind hover, no flapping, AOA is constant, the lift is equal on all sides Advancing Blade, Flapping up, AOA is reduced, the lift is reduced Retreating Blade, Flapping down, AOA is increased, the lift is increased The following project demonstrates the flapping to equality of helicopters. The project is interactive and the user can increase or decrease the velocity of the helicopter. The following link will redirect
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Recirculation

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Imagine the airflow which was directed to the ground to create the air cushion in a  ground effect is now rebounding off the ground going back up into the top of the rotor system. When it passes back through the rotor again it gets accelerated. This process may continue with the air velocity increasing each time as it passes through the rotor. Eventually, the velocity is so great that the air going into the rotor from above causes a loss of lift and the helicopter will sink toward the ground unless the pilot increases power. This means that i f recirculation is occurring, the helicopter will need mone power to hold a constant height. Recirculation will not always happen but will be aggravated by the type of ground or nearby obstacles causing the air which is trying to escape out to the sides of the helicopter to be directed back up toward the rotor system.  The result is 'recirculation' of downwash air. The following Scratch Project explains the whole process of recirculation i
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Ground Effect

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Ground effect is the positive influence on the lifting characteristics of the horizontal surfaces of an aircraft wing when it is close to the ground. This effect is a consequence of the distortion of the airflow below such surfaces attributable to the proximity of the ground. IGE (In-Ground Effect) IGE conditions are usually found within heights about 0'5 to 1 times the diameter of the main rotor. So if a helicopter has a rotor diameter of 48 ft, the IGE region will be about 24-48ft above the ground. The height will vary depending on the type of helicopter, the slope and nature of the ground, and any prevailing winds. The air impacting the ground creates a small build-up of air pressure in the region below the rotor disc. It seems as if the helicopter is floating upon a cushion of air. The stagnated air will form a high-pressure zone just underneath the fuselage body of helicopters. There is basically no more motion inside this. Which will cause the air to not enter into it. Hence,
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Autorotation

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Autorotation is a condition of helicopter flight during which the lifting rotor of a helicopter is being turned by the force of the relative wind with no power from the engine. It is a maneuver where the engine is disengaged from the main rotor system and the rotor blades are driven solely by the upward flow of air through the rotor. The most common reason to perform an autorotation is engine failure. But autorotation is also performed during complete tail rotor failure as it produces virtually no torque, and a safe landing is possible. At the instant of engine failure or tail rotor failure, the main rotor blades are producing lift and thrust from their AOA and velocity. By immediately lowering the collective pitch, the lift and drag of the helicopter are lowered and the helicopter begins to descend producing an upward flow of air through the rotor system immediately. This upward flow provides sufficient thrust to maintain rotor rpm throughout the descent. Airspeed and rotor rpm
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Hover Condition Variable CG

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During hovering flight, a helicopter maintains a constant position over a selected point, usually a few feet above the ground. For a helicopter to hover, the lift and thrust produced by the rotor system act straight up and must equal the weight and drag, which act straight down. If there is a wind in a specific direction, the rotor blades must be tilted to the opposite direction to counter the wind movement. Cyclic to be adjusted accordingly. The phenomenon is as simple as this. But if the CG is shifted to say forward, the cyclic must be adjusted to tilt the aircraft back and vice versa. The following project demonstrates the hover condition of helicopters with variable CG. The project is interactive and the user can change the position of CG as well as change the direction and speed of the wind. The following link will redirect you to the project page. (For the mobile version, please go to the bottom of the page) Hover Condition Variable CG on Scratch Press the Left or Right arrow key
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Hover Condition

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Hover Condition with Wind During hovering flight, a helicopter maintains a constant position over a selected point, usually a few feet above the ground. For a helicopter to hover, the lift and thrust produced by the rotor system act straight up and must equal the weight and drag, which act straight down. If there is a wind in a specific direction, the rotor blades must be tilted to the opposite direction to counter the wind movement. Cyclic to be adjusted accordingly. The phenomenon is as simple as this. The following project demonstrates the hover condition of helicopters with variable wind speeds. The project is interactive and the user can change the direction and speed of the wind. The following link will redirect you to the project page. (For the mobile version, please go to the bottom of the page) Hover Condition on Scratch Press the Left or Right arrow keys to change the wind speed. Minus means the opposite direction.  This project demonstrates how the lift and weight react to c
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