Helicopter Flight Terms and Definitions:
This page contains a brief rundown on common terms about RC Heli Flight. Included on this page are:
- Airfoil
- Heli Pilot Directions and Control (yaw, rudder, pitch, elevator, roll, and aileron)
- Torque
- Lift
- Ground Effect
- Translational Lift
- Dissymmetry of Lift
- Autorotation
Airfoil:
A structure with specially shaped curved surfaces designed to give the most favorable ratio of lift to drag in flight.
Airfoils are important, because a flat piece of wood will never create enough lift to get anything off the ground. From a practical standpoint, even a flat piece of plastic costs too much energy for the lift produced; airfoils are great at creating maximum lift on minimum power. Airfoils on planes are known as wings, and on helicopters as rotor blades or simply heli blades. I prefer rotor blades, to avoid confusion.
There are four main types of rotor blades: flat, delta airfoils, semi symmetrical airfoils, and symmetrical airfoils.
Airfoils are important, because a flat piece of wood will never create enough lift to get anything off the ground. From a practical standpoint, even a flat piece of plastic costs too much energy for the lift produced; airfoils are great at creating maximum lift on minimum power. Airfoils on planes are known as wings, and on helicopters as rotor blades or simply heli blades. I prefer rotor blades, to avoid confusion.
There are four main types of rotor blades: flat, delta airfoils, semi symmetrical airfoils, and symmetrical airfoils.
- Flat Rotor Blades are least efficient and least used. However, they can spin in either direction and still create lift, which makes them useful for vertical tail rotors such as those found on 3-channel coaxials.
- Delta Airfoils are most efficient at creating lift but cannot fly inverted; this makes them ideal for smaller helicopters without collective pitch. They are distinguished by their curved look.The 9053 has delta airfoils.
- Semi-Symmetrical Airfoils are a toned-down version of symmetrical airfoils. They create more lift than symmetrical airfoils and can sustain brief inverted flight. They have difficultly with longer inverted flight.
- Symmetrical Airfoils are just that: symmetrical on top and on bottom. Thus, they fly just as well upside down as right side up. Most wings are symmetrical airfoils. Collective pitch RC Helicopters generally use symmetrical airfoils to enable them to fly inverted. They are the least efficient airfoil type, however.
Delta Airfoil
Notice how a delta airfoil is curved on one end, with a shape similar to the top line of a symmetrical airfoil.
Symmetrical airfoils create equal positive and negative lift when level. To create greater positive lift they are slanted up, while for negative lift they are slanted down. |
Symmetrical Airfoil
|
Because of their symmetrical shape, they are less efficient: with a positive increase in pitch, positive lift increases significantly but even when creating significant positive lift, they still create negative lift, although with higher pitch levels they create less opposite lift. Other types of airfoils create less or almost no negative lift.
Directions and Controls for a Heli Pilot:
These definitions are about directional controls for a 3D grid based on the heli. 0 in all 3 dimensions is at the center of gravity of the helicopter, right in the main rotor shaft.
Yaw: A horizontal rotation about the central vertical axis. A heli can yaw to the right or left.
Rudder: The command on an RC Aircraft Radio to initiate a left or right turn (yaw left or right, respectively).
Pitch: A rotation about the short horizontal axis. A heli can pitch up or down, causing backwards or forwards movement, respectively.
Elevator: The command on an RC Aircraft Radio to initiate forward or backwards movement on an RC Heli (pitch down or pitch up, respectively).
Roll: A rotation about the long horizontal axis, running through the canopy. A heli can roll to the right or left, causing right or left movement. This is only available on 4+ channel helis.
Aileron: The command on an RC Aircraft Radio to initiate left or right movement on an RC Heli (roll left or right, respectively).
Yaw: A horizontal rotation about the central vertical axis. A heli can yaw to the right or left.
Rudder: The command on an RC Aircraft Radio to initiate a left or right turn (yaw left or right, respectively).
Pitch: A rotation about the short horizontal axis. A heli can pitch up or down, causing backwards or forwards movement, respectively.
Elevator: The command on an RC Aircraft Radio to initiate forward or backwards movement on an RC Heli (pitch down or pitch up, respectively).
Roll: A rotation about the long horizontal axis, running through the canopy. A heli can roll to the right or left, causing right or left movement. This is only available on 4+ channel helis.
Aileron: The command on an RC Aircraft Radio to initiate left or right movement on an RC Heli (roll left or right, respectively).
Torque
In relation to helicopters, torque is simply a spinning effect in the opposite direction of a rotor's spin. Remembering back to Jr High physics, Newton's Third Law states that for every action there is an equal and opposite reaction. For helicopters, this means that for each spin of a rotor, there is an equal and opposite spin of the the helicopter. However, because the mass of the rotor blades is much less than that of the helicopter, the spin of the helicopter will be less than the spin of the blades, at least initially.
If uncorrected by another rotor, torque will cause the helicopter to spin in the opposite direction of the rotor's spin. All helicopters must have some way to correct for torque, or the helicopter would be impossible to control:
If uncorrected by another rotor, torque will cause the helicopter to spin in the opposite direction of the rotor's spin. All helicopters must have some way to correct for torque, or the helicopter would be impossible to control:
- Coaxial helicopters correct for the torque of the main rotor by having two main rotors, each of which corrects for the torque of the other by spinning in the opposite direction. Notice that coaxials with a vertical tail rotor for forward flight induce a torque from the vertical tail rotor spinning (see Tail Rotor Rules).
- Single rotor (or more correctly single main rotor) helicopters correct for the reactive torque of the main rotor by using a horizontal tail rotor, which is used to precisely counter the reactive torque of the main rotor by creating sideways lift.
- NOTAR (No tail Rotor) Helicopters correct for the reactive torque of the main rotor by piping air blown by a fan in their tail boom out of their tail boom in the opposite direction of the rotor's spin.
- Quadcopters are like coaxial helicopters in that each rotor has a companion rotor that spins in the opposite direction, thereby canceling each other's torque. This is why quadcopters always have an even number of rotors (4, 6, etc.)
Lift
Lift is created by different airspeed above and below an airfoil.
To illustrate this, take a standard square or rectangle piece of paper with your hands holding two corners, adjacent to each other. Preferably, take the long end of the paper. For this demonstration, the part of the paper you are holding should be level, with the opposite end curving downward. Your paper, viewed from the side, is a delta airfoil (see above).
Now, blow downward and across on the paper. Almost magically, the end of the paper lifts. Why is this? Basically, the higher airspeed on top of your paper "airfoil" creates less pressure above the paper, causing the higher pressure below the paper to raise it up slightly.
As the picture above shows, the dots (representing air particles) start out even spaced, but then travel over the airfoil faster on top than on bottom. In this picture, air particles travel almost twice as fast over the top compared to the bottom.
To illustrate this, take a standard square or rectangle piece of paper with your hands holding two corners, adjacent to each other. Preferably, take the long end of the paper. For this demonstration, the part of the paper you are holding should be level, with the opposite end curving downward. Your paper, viewed from the side, is a delta airfoil (see above).
Now, blow downward and across on the paper. Almost magically, the end of the paper lifts. Why is this? Basically, the higher airspeed on top of your paper "airfoil" creates less pressure above the paper, causing the higher pressure below the paper to raise it up slightly.
As the picture above shows, the dots (representing air particles) start out even spaced, but then travel over the airfoil faster on top than on bottom. In this picture, air particles travel almost twice as fast over the top compared to the bottom.
Obviously, gravity limits the maximum lift. As shown in the picture below, gravity and lift are opposite forces; to get more relative lift, you can either decrease weight (at some point impractical) or increase lift. However, lift doesn't just happen because of the shape of the wing. There has to be an airflow, as was stated above.
If you want a more detailed explanation, check out Wikipedia's Lift Article. |
There are two ways to increase lift: Increase the collective (or angle of attack) of the blades (which increases airspeed above and decreases it below the airfoil) or increase the motor rpm (which increases airspeed above the airfoil faster than below it). For FP Helis, the only option is to increase the throttle. With CP helis, the preferred option is to increase the collective.
Ground Effect
My 9053 in ground effect.
This is a complicated topic, but well worth understanding. It is usually noticeable when flying less than a foot or two above the ground. More specifically, it is pronounced up until about a rotor diameter below the landing skids. It has two main side effects:
Increased sensitivity: A helicopter flying in ground effect will be more sensitive to control inputs and center of gravity imbalances. This is more pronounced on smaller helicopters.
Increased Lift: A helicopter in ground effect will get off the ground with a lower rpm / collective than it should. In other words, the helicopter will fly when the speed or collective of the rotors normally wouldn't enable it to.
The "Why": In ground effect, the airflow shooting down off of the main rotors bounces off the ground and hits the helicopter. The helicopter is more sensitive in this case because the air bouncing off the ground interacts with the airflow down from the rotors. Any defect in the rotors or change in the air off of the rotors (like that occasioned by a cyclic input or an unbalanced helicopter) results in a small pitch or roll that tends to destabilize the helicopter.
As ground effect is caused by air from the rotors bouncing off of the ground, it also creates more lift. This is because the air has no place to go after being forced down by the rotors, and thus stays under the helicopter. This creates higher pressure under the helicopter -- which means denser air -- and thus not only increases the air holding the heli up, but also makes more lift per collective / rpm.
An important note is that wind can negate ground effect by "blowing" the high pressure air under the heli away.
Increased sensitivity: A helicopter flying in ground effect will be more sensitive to control inputs and center of gravity imbalances. This is more pronounced on smaller helicopters.
Increased Lift: A helicopter in ground effect will get off the ground with a lower rpm / collective than it should. In other words, the helicopter will fly when the speed or collective of the rotors normally wouldn't enable it to.
The "Why": In ground effect, the airflow shooting down off of the main rotors bounces off the ground and hits the helicopter. The helicopter is more sensitive in this case because the air bouncing off the ground interacts with the airflow down from the rotors. Any defect in the rotors or change in the air off of the rotors (like that occasioned by a cyclic input or an unbalanced helicopter) results in a small pitch or roll that tends to destabilize the helicopter.
As ground effect is caused by air from the rotors bouncing off of the ground, it also creates more lift. This is because the air has no place to go after being forced down by the rotors, and thus stays under the helicopter. This creates higher pressure under the helicopter -- which means denser air -- and thus not only increases the air holding the heli up, but also makes more lift per collective / rpm.
An important note is that wind can negate ground effect by "blowing" the high pressure air under the heli away.
Translational Lift
Translational lift means extra lift generated when a helicopter is in motion, in any direction. It is caused because the speed of the rotors is added to the speed of the helicopter, thus creating more lift per collective / rpm (throttle).
This means that the helicopter flies higher with a lower collective / rpm than it would hovering.
An interesting side effect is dissymmetry of lift.
This means that the helicopter flies higher with a lower collective / rpm than it would hovering.
An interesting side effect is dissymmetry of lift.
Dissymmetry of Lift
This term means that depending on its place in a rotation, a blade produces more or less lift. This is true only on single rotor helis: coaxials have two counter-rotating rotors that cancel each other's dissymmetry of lift. Dissymmetry of lift can either be controlled or uncontrolled.
In controlled dissymmetry of lift, the collective of the helicopter is manipulated by the swashplate to create a maximum high and low collective 180 degrees apart in the rotational cycle. These high and lows are fixed in reference to the helicopter, not the rotors, and are based on the cyclic control commanded on the radio. This creates more lift on one side of the helicopter than another and makes the helicopter roll or pitch, depending on where the dissymmetry is. For a roll, the lift (in other words, collective) is greater on one side than the other. For a forward pitch, the lift is greater in the back than the front; for a backwards pitch, the lift is greater in the front than the back.
To understand uncontrolled dissymmetry of lift, it is important to understand two terms:
Leading edge: The side of the helicopter in which the rotor spins towards the direction of motion. For the purpose of this explanation, this is a positive speed. For a counter-clockwise spinning rotor, this would be the right side if the helicopter were moving forward.
Trailing edge: The side of the helicopter in which the rotor spins towards the direction the helicopter came from. For the purpose of this explanation, this is a negative speed. On a counter-clockwise spinning rotor this would be the tail side if moving to the left; if moving forward it would be the left side.
When moving, the air flowing over the blades is greater than at rest because the speed of the air flowing past the helicopter is added to the air flowing over the blades. In effect, a positive speed is added to the positive speed of the leading rotor and the negative speed of the trailing rotor. This creates more lift on the leading edge and less lift on the trailing edge. This causes the sides of the helicopter to have unequal lifts, and the helicopter rolls in forward or backwards flight and pitches in side to side flight.
This is more pronounced at higher speeds (such as that of larger helicopters), when the air flowing around the helicopter becomes more significant compared to the speed of the rotors. This is also why helicopters have a maximum speed: if the dissymmetry of lift is too great, then in forward movement no roll can compensate for the enforced roll of the unequal lift.
In controlled dissymmetry of lift, the collective of the helicopter is manipulated by the swashplate to create a maximum high and low collective 180 degrees apart in the rotational cycle. These high and lows are fixed in reference to the helicopter, not the rotors, and are based on the cyclic control commanded on the radio. This creates more lift on one side of the helicopter than another and makes the helicopter roll or pitch, depending on where the dissymmetry is. For a roll, the lift (in other words, collective) is greater on one side than the other. For a forward pitch, the lift is greater in the back than the front; for a backwards pitch, the lift is greater in the front than the back.
To understand uncontrolled dissymmetry of lift, it is important to understand two terms:
Leading edge: The side of the helicopter in which the rotor spins towards the direction of motion. For the purpose of this explanation, this is a positive speed. For a counter-clockwise spinning rotor, this would be the right side if the helicopter were moving forward.
Trailing edge: The side of the helicopter in which the rotor spins towards the direction the helicopter came from. For the purpose of this explanation, this is a negative speed. On a counter-clockwise spinning rotor this would be the tail side if moving to the left; if moving forward it would be the left side.
When moving, the air flowing over the blades is greater than at rest because the speed of the air flowing past the helicopter is added to the air flowing over the blades. In effect, a positive speed is added to the positive speed of the leading rotor and the negative speed of the trailing rotor. This creates more lift on the leading edge and less lift on the trailing edge. This causes the sides of the helicopter to have unequal lifts, and the helicopter rolls in forward or backwards flight and pitches in side to side flight.
This is more pronounced at higher speeds (such as that of larger helicopters), when the air flowing around the helicopter becomes more significant compared to the speed of the rotors. This is also why helicopters have a maximum speed: if the dissymmetry of lift is too great, then in forward movement no roll can compensate for the enforced roll of the unequal lift.
Auto-Rotation:
*Windflow in an autorotation*
An auto-rotation is a procedure in which the power to the main motor is shut down for an unpowered glide to the earth. In essence, the helicopter becomes a glider and can safely recover and land in an emergency. It is generally initiated in one of three conditions:
An auto rotation requires two things: a way to disconnect the rotors from the motor (so that they can spin freely) and collective pitch. It relies on the kinetic energy of the spinning main rotor to slow down the helicopter.
To initiate an auto-rotation, first the main motor of the helicopter must be shut off. Then, the pitch of the main rotors is decreased quickly to negative, usually about 4 degrees or so. For added airflow, the helicopter usually follows a glide path of about 45 degrees forward to provide translational lift. This causes the main rotor to begin to spin for the same reason a pinwheel or paper helicopter does. This spinning creates friction, which slows the descent and builds kinetic (or moving) energy in the main rotor. Then, when close enough to the ground, the collective pitch is increased to positive, generally around 15 to 20 degrees. The kinetic energy generated earlier now is dissapated by creating lift, which stops the descent for long enough for the helicopter to flare out to stop forward movement and gently land (as it is near the ground when the descent is stopped).
- Loss of tail rotor control (usually due to hitting something in the air or servo failure; for real pilots sometimes due to wind moving in the same direction of the tail rotor spin, causing the tail rotor to stall)
- Engine failure (for any reason)
- Just for fun (a common reason for RC) or learning (common for both RC and real pilot training)
- It is not initiated if the main rotor breaks
An auto rotation requires two things: a way to disconnect the rotors from the motor (so that they can spin freely) and collective pitch. It relies on the kinetic energy of the spinning main rotor to slow down the helicopter.
To initiate an auto-rotation, first the main motor of the helicopter must be shut off. Then, the pitch of the main rotors is decreased quickly to negative, usually about 4 degrees or so. For added airflow, the helicopter usually follows a glide path of about 45 degrees forward to provide translational lift. This causes the main rotor to begin to spin for the same reason a pinwheel or paper helicopter does. This spinning creates friction, which slows the descent and builds kinetic (or moving) energy in the main rotor. Then, when close enough to the ground, the collective pitch is increased to positive, generally around 15 to 20 degrees. The kinetic energy generated earlier now is dissapated by creating lift, which stops the descent for long enough for the helicopter to flare out to stop forward movement and gently land (as it is near the ground when the descent is stopped).