This description of Helicopter Aerodynamics is a general explanation of how a helicopter flies:
Before we begin, here are some basic aerodynamic terms and their definitions:
Lift - The force that a surface (wing, rotor blade, propeller...) creates from wind passing over it. Usually upward; away from the center of the Earth.
Weight - The force that the Earth's Gravity exerts on an object. Usually downward; towards the center of the Earth.
Thrust - The force that is generated from an airfoil (rotor blade, propeller...) passing through the air. Usually forward but can also be upward as in a hover.
Drag - The force that resists movement of the airfoil (wing, rotor blade, propeller...) through the air. Drag can also apply to anything that moves through the air. Usually opposite to lift or thrust.
These 4 basic forces are responsible for which direction a helicopter moves.
In a stabilized hover: The force of lift equals weight and the force of thrust equals drag. In addition, all the forces are acting vertically (straight up and down). If lift is greater than weight the helicopter will climb in altitude (move upward). If weight is greater than lift than the helicopter will move downward.
In stabilized horizontal flight: The force of lift equals weight and the force of thrust equals drag. The difference between hovering and horizontal flight is the thrust force and drag force are acting horizontally while the lift and weight act vertically. If thrust exceeds drag then the helicopter increases its horizontal speed. If drag is greater than thrust than the helicopter slows down its horizontal speed. If lift is greater than weight then the helicopter climbs. If weight is greater than lift than the helicopter descends (moves down). Because of the unique abilities of helicopters, horizontal flight is not limited to only forward. This lift, weight, thrust, and drag relationship applies to any direction that the helicopter is moving (forwards, sideways, or backwards).
The Helicopter's Flight Controls:
Cyclic - This is the 'stick' that comes up from the floor of most helicopters. The cyclic controls the direction of the tilt of the main rotor (the big spinning thing on top of the helicopter). While the helicopter rotors are spinning, if you move the stick forward the main rotor will tilt forwards. If you move the stick backwards the main rotor will tilt backwards, move the cyclic to the side and it will tilt the main rotor disc to the side. This control changes the lift and thrust force.
Collective - This is the control that is usually to the left of the pilot and it controls the angle of the main rotor blades. If you pull the collective up, the angle of the main rotor blades increase (the front part of the rotor blade (leading edge) will move higher than the rear part of the rotor blade (trailing edge)). This control changes the lift and thrust force also.
Tail Rotor or Anti-Torque Pedals - These are the pedals located at the pilot's feet. They control the pitch of the rotor blades of the tail rotor (there is a small rotor blade at the rear of the helicopter. This is to control the left or right movement of the helicopter). The tail rotor was added to the design of the helicopter because the Torque from the main rotor (Torque is created from the engine turning the main rotor. For every action there is an equal and opposite reaction. The turning of the main rotor blades by the engine results in the cabin or body of the helicopter turning in the opposite direction. To keep the helicopter from spinning around the center axis uncontrollably the tail rotor was added to create thrust opposite to the direction of Torque).
To fly the helicopter, the pilot uses all of these controls in combination to get the helicopter to go where the pilot wants it to. By changing the lift and thrust of the main rotor by moving the cyclic and collective, the pilot can move the helicopter in any direction. Moving the cyclic forward moves the helicopter forward. Moving the cyclic to the left moves the helicopter to the left. Moving the cyclic to the left AND forward moves the helicopter forward and to the left. Lifting the collective up causes the helicopter to climb. Lowering the collective causes the helicopter to descend. Moving the cyclic forward and lifting the collective causes the helicopter to increase its forward speed and may result in a climb depending on how much collective is raised. Using the pilot's feet, the tail rotor pedals control whether the nose (front of the helicopter) moves to the left or the right. Left pedal moves the nose to the left. Right pedal moves the nose to the right.
If you increase the collective and the engine increases power (some helicopters have automatic engine controls that do this for you) to keep the same revolutions per minute (RPM) to the main rotor then your Torque will also increase. This requires more left pedal (or pedal opposite to the direction that Torque is trying to spin the helicopter) to keep the nose of the helicopter in the same place. So, an increase in collective needs to have an equal increase in the tail rotor pedals.
As you can see, reading about how this all works can get confusing. It is actually easier to just get in one and try out what each control and combination of controls will do. It requires a lot of practice to have the control of a helicopter become second nature but in theory all you are doing is changing lift and thrust forces through the cycle and collective.
The Autorotation (or how a helicopter gets to the ground after the engine quits without crashing):
An Autorotation is the mode of flight when the helicopter is "gliding" without engine power. Yes, helicopters can "glide" if the engine quits. The basic idea of an autorotation is changing the flow of air through the main rotor from downward to upward and propelling the main rotor blades using this upward flowing air.
If you have ever seen a maple seed fall from a maple tree you have seen an autorotation. The air passing over the "wing" of the seed propels the wing and keeps the seed spinning down to soft landing. The helicopter's main rotor blade follows a similar idea. There must be pilot input, though, to make the helicopter transition from powered flight to power off flight.
At the instant of an engine failure, the helicopter pilot must change the main rotor blades' angle so that it can take advantage of the up flowing air. If the pilot waits too long, the up flowing air will "stall" the rotor blades and autorotation will not occur (in which case, the pilot and the helicopter become victims of gravity.) To change the angle of the main rotor blades, the pilot immediately lowers the collective when the engine quits. By lowering the angle of the blade, the main rotor blade will be able to use the air flowing over the wing to create lift that will "pull" the main rotor blade and keep the rotor blades spinning.
Remember the discussion about the tail rotor (the rotor blades at the back of the helicopter that control the direction of the nose of the helicopter and help compensate for torque)? Well, the tail rotor is important in an autorotation because you need to have some control over the "yaw axis" (left and right axis). During autorotation the tail rotor still turns because it is connected to the main rotor via a transmission; And as long as the main rotor is turning (and the transmission is functioning properly), the tail rotor will be turning. Torque does not exist during an autorotation but there is a little bit of drag/friction from the main rotor's and tail rotor's transmissions that causes the helicopter to turn in the direction of the main rotor spin. This is controlled by input from the pilot through the tail rotor.
Autorotation Flare - The autorotation is a great way to "glide" the helicopter once the engine quits but as the helicopter gets closer to the ground the helicopter's descent must be slowed down somehow. To slow down the descent, the pilot starts a Flare by slowly pulling back on the cyclic to slow the descent rate, increasing the collective to help cushion the landing, and hoping that the spot the pilot chose is somewhat level. This is a very simplistic description of the Autorotation Flare but this is the basic idea.
Other Aerodynamic Forces Affecting Helicopters:
There are many other aerodynamic forces that influence a helicopter in hover and horizontal flight. The following are the major ones:
Translating Tendency (Drift) - Tendency for the entire helicopter to move in the direction of tail rotor thrust.
Translational Lift - The additional lift the helicopter gets when it flies out from it's own downwash.
Gyroscopic Precession - Applies to any spinning disc: a force applied to a spinning disc has its effect happen 90 degrees later in the direction and plane of rotation.
Coriolis Force - Ice Skater Example: When an ice skater spins in a circle and her arms are out, they have a certain spin speed. If she pulls her arms in, the spin accelerates in speed because the center of mass of the skater's arms are closer to the axis of rotation. Same thing in a helicopter except replace skater's arms with rotor blades.
Transverse Flow Effect - Air flowing over the rear portion of the main rotor disc is accelerated downward by the main rotor which causes the rear portion to have a smaller angle of attack. This results in less lift to the rear portion but because of Gyroscopic Precession, the result is felt 90 degrees later.
Before we begin, here are some basic aerodynamic terms and their definitions:
Lift - The force that a surface (wing, rotor blade, propeller...) creates from wind passing over it. Usually upward; away from the center of the Earth.
Weight - The force that the Earth's Gravity exerts on an object. Usually downward; towards the center of the Earth.
Thrust - The force that is generated from an airfoil (rotor blade, propeller...) passing through the air. Usually forward but can also be upward as in a hover.
Drag - The force that resists movement of the airfoil (wing, rotor blade, propeller...) through the air. Drag can also apply to anything that moves through the air. Usually opposite to lift or thrust.
These 4 basic forces are responsible for which direction a helicopter moves.
In a stabilized hover: The force of lift equals weight and the force of thrust equals drag. In addition, all the forces are acting vertically (straight up and down). If lift is greater than weight the helicopter will climb in altitude (move upward). If weight is greater than lift than the helicopter will move downward.
In stabilized horizontal flight: The force of lift equals weight and the force of thrust equals drag. The difference between hovering and horizontal flight is the thrust force and drag force are acting horizontally while the lift and weight act vertically. If thrust exceeds drag then the helicopter increases its horizontal speed. If drag is greater than thrust than the helicopter slows down its horizontal speed. If lift is greater than weight then the helicopter climbs. If weight is greater than lift than the helicopter descends (moves down). Because of the unique abilities of helicopters, horizontal flight is not limited to only forward. This lift, weight, thrust, and drag relationship applies to any direction that the helicopter is moving (forwards, sideways, or backwards).
The Helicopter's Flight Controls:
Cyclic - This is the 'stick' that comes up from the floor of most helicopters. The cyclic controls the direction of the tilt of the main rotor (the big spinning thing on top of the helicopter). While the helicopter rotors are spinning, if you move the stick forward the main rotor will tilt forwards. If you move the stick backwards the main rotor will tilt backwards, move the cyclic to the side and it will tilt the main rotor disc to the side. This control changes the lift and thrust force.
Collective - This is the control that is usually to the left of the pilot and it controls the angle of the main rotor blades. If you pull the collective up, the angle of the main rotor blades increase (the front part of the rotor blade (leading edge) will move higher than the rear part of the rotor blade (trailing edge)). This control changes the lift and thrust force also.
Tail Rotor or Anti-Torque Pedals - These are the pedals located at the pilot's feet. They control the pitch of the rotor blades of the tail rotor (there is a small rotor blade at the rear of the helicopter. This is to control the left or right movement of the helicopter). The tail rotor was added to the design of the helicopter because the Torque from the main rotor (Torque is created from the engine turning the main rotor. For every action there is an equal and opposite reaction. The turning of the main rotor blades by the engine results in the cabin or body of the helicopter turning in the opposite direction. To keep the helicopter from spinning around the center axis uncontrollably the tail rotor was added to create thrust opposite to the direction of Torque).
To fly the helicopter, the pilot uses all of these controls in combination to get the helicopter to go where the pilot wants it to. By changing the lift and thrust of the main rotor by moving the cyclic and collective, the pilot can move the helicopter in any direction. Moving the cyclic forward moves the helicopter forward. Moving the cyclic to the left moves the helicopter to the left. Moving the cyclic to the left AND forward moves the helicopter forward and to the left. Lifting the collective up causes the helicopter to climb. Lowering the collective causes the helicopter to descend. Moving the cyclic forward and lifting the collective causes the helicopter to increase its forward speed and may result in a climb depending on how much collective is raised. Using the pilot's feet, the tail rotor pedals control whether the nose (front of the helicopter) moves to the left or the right. Left pedal moves the nose to the left. Right pedal moves the nose to the right.
If you increase the collective and the engine increases power (some helicopters have automatic engine controls that do this for you) to keep the same revolutions per minute (RPM) to the main rotor then your Torque will also increase. This requires more left pedal (or pedal opposite to the direction that Torque is trying to spin the helicopter) to keep the nose of the helicopter in the same place. So, an increase in collective needs to have an equal increase in the tail rotor pedals.
As you can see, reading about how this all works can get confusing. It is actually easier to just get in one and try out what each control and combination of controls will do. It requires a lot of practice to have the control of a helicopter become second nature but in theory all you are doing is changing lift and thrust forces through the cycle and collective.
The Autorotation (or how a helicopter gets to the ground after the engine quits without crashing):
An Autorotation is the mode of flight when the helicopter is "gliding" without engine power. Yes, helicopters can "glide" if the engine quits. The basic idea of an autorotation is changing the flow of air through the main rotor from downward to upward and propelling the main rotor blades using this upward flowing air.
If you have ever seen a maple seed fall from a maple tree you have seen an autorotation. The air passing over the "wing" of the seed propels the wing and keeps the seed spinning down to soft landing. The helicopter's main rotor blade follows a similar idea. There must be pilot input, though, to make the helicopter transition from powered flight to power off flight.
At the instant of an engine failure, the helicopter pilot must change the main rotor blades' angle so that it can take advantage of the up flowing air. If the pilot waits too long, the up flowing air will "stall" the rotor blades and autorotation will not occur (in which case, the pilot and the helicopter become victims of gravity.) To change the angle of the main rotor blades, the pilot immediately lowers the collective when the engine quits. By lowering the angle of the blade, the main rotor blade will be able to use the air flowing over the wing to create lift that will "pull" the main rotor blade and keep the rotor blades spinning.
Remember the discussion about the tail rotor (the rotor blades at the back of the helicopter that control the direction of the nose of the helicopter and help compensate for torque)? Well, the tail rotor is important in an autorotation because you need to have some control over the "yaw axis" (left and right axis). During autorotation the tail rotor still turns because it is connected to the main rotor via a transmission; And as long as the main rotor is turning (and the transmission is functioning properly), the tail rotor will be turning. Torque does not exist during an autorotation but there is a little bit of drag/friction from the main rotor's and tail rotor's transmissions that causes the helicopter to turn in the direction of the main rotor spin. This is controlled by input from the pilot through the tail rotor.
Autorotation Flare - The autorotation is a great way to "glide" the helicopter once the engine quits but as the helicopter gets closer to the ground the helicopter's descent must be slowed down somehow. To slow down the descent, the pilot starts a Flare by slowly pulling back on the cyclic to slow the descent rate, increasing the collective to help cushion the landing, and hoping that the spot the pilot chose is somewhat level. This is a very simplistic description of the Autorotation Flare but this is the basic idea.
Other Aerodynamic Forces Affecting Helicopters:
There are many other aerodynamic forces that influence a helicopter in hover and horizontal flight. The following are the major ones:
Translating Tendency (Drift) - Tendency for the entire helicopter to move in the direction of tail rotor thrust.
Translational Lift - The additional lift the helicopter gets when it flies out from it's own downwash.
Gyroscopic Precession - Applies to any spinning disc: a force applied to a spinning disc has its effect happen 90 degrees later in the direction and plane of rotation.
Coriolis Force - Ice Skater Example: When an ice skater spins in a circle and her arms are out, they have a certain spin speed. If she pulls her arms in, the spin accelerates in speed because the center of mass of the skater's arms are closer to the axis of rotation. Same thing in a helicopter except replace skater's arms with rotor blades.
Transverse Flow Effect - Air flowing over the rear portion of the main rotor disc is accelerated downward by the main rotor which causes the rear portion to have a smaller angle of attack. This results in less lift to the rear portion but because of Gyroscopic Precession, the result is felt 90 degrees later.
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