Helicopters are one of the wonders of the modern industrial world but making them work took some serious brain pain from the early helicopter pioneers! Something as simple as turning seems simple but to accomplish it takes some brilliant engineering.
To turn a helicopter while in a hover the pilot uses the pedals to control the tail rotor by adjusting the amount of sideways thrust it creates. To turn while in forward flight the pilot tilts the main rotor disk in the direction they wish to turn by using the cyclic control and the fuselage follows.
This statement is about as basic as I can word it to give you a quick answer. To find out the more in-depth answers to how a helicopter turns and banks be sure to read on…
Types of Turn
With a helicopter there are two types of turn:
- Turning while in a Hover – Known as YAW. This is similar to being sat in a spinny office chair and someone rotates you round and round.
This type of turn is accomplished by the Tail Rotor.
- Turning while in Flight – Known as ROLL. This is similar to leaning on a bicycle when entering a fast turn. This type of turn is accomplished by the Main Rotor.
Let’s look at each one of these types of turn individually…
For this article, we will keep it simple and talk about a helicopter with skids and not wheels and built in North America so the main rotor rotates anti-clockwise when viewed from the above. (For reference: helicopters built in Europe tend to have the main rotor turn clockwise when viewed from above).
For European machines like the Eurocopter/Airbus helicopters, the explanations will follow the same fundamentals but in the opposite direction.
Turning While In A Hover
Turning a helicopter while in the hover (for helicopters without wheels) or for turning while taxiing (for helicopters with wheels) is done by the Tail Rotor.
Some quick, simple Fundamentals of Flight theory to set the scene:
In a hover, the forces acting on a helicopter are all equal, thus it should not move.
When the engine turns the main rotor in one direction, the fuselage will want to turn in the opposite direction due to Newton’s Third Law – For every action, there is an equal and opposite reaction.
To keep the helicopter from spinning around, the thrust produced by the tail rotor matches the force of the fuselage wanting to turn (Known as Torque). Therefore when everything is in equilibrium, the helicopter keeps pointing forward.
To Turn Left:
While in the hover, if the pilot pushes on the left foot pedal:
- The pitch of the tail rotor will increase on both/all its blades via a mechanical and/or hydraulic linkage
- As the blade pitch increases, the thrust produced will be greater than the fuselage torque
- This will push the tail of the helicopter to the right
- The cockpit of the helicopter will then rotate to the left around the mast of the main rotor until the pedals are centered
To Turn Right:
While in the hover, if the pilot pushes on the right foot pedal:
- The pitch of the tail rotor will decrease on both/all its blades
- This thrust produced will be less than the fuselage torque
- The torque will pull the tail of the helicopter to the left
- The cockpit of the helicopter will then rotate to the right around the mast of the main rotor until the pedals are centered
Turning While In Forward Flight
This is where it gets a little more complex so I hope I can explain this clearly. I’m not going to go into deep fundamentals like Pendulum Effect, Dissymertry of Lift, Gyroscopic Precession and things like that as the mantra of this site is in the tagline “Everything Aviation – Simply Explained”
When a helicopter is in forward flight at a constant speed and constant height the following forces are acting upon the aircraft:
To begin a turn, the pilot moves the cyclic control either left or right.
What happens next is where all the wizardry comes in! The cyclic is connected to the stationary half of the swashplate that is mounted on the main rotor mast via linkages and hydraulic actuators in most helicopters.
The swashplate is a device that has a stationary half that the cyclic and collective controls are linked to, then the rotating half is connected to each rotor blade by a pitch link.
Most helicopters have 3 Cyclic/Collective linkages connected to the swashplate. The swashplate is used to increase or decrease the pitch of each rotor blade.
If the Collective is raised or lowered (Flight control in the pilot’s left hand), all the linkages/actuators raise or drop the entire swashplate together. This increases/decreases the pitch on all the blades ‘Collectively’ so the helicopter climbs/descends.
If the Cyclic is moved Fwd, Aft, Left, or Right each actuator moves independently to effectively tilt the stationary part of the swashplate in the direction the cyclic was moved. This changes the pitch on each rotor blade individually.
To turn or bank the aircraft, the Cyclic is moved left/right to tilt the stationary part of the swashplate and because the stationary half tilts, the rotating half mirrors the tilt. This then begins to change the pitch of each rotor blade at varying degrees throughout each blades full rotation.
Think of putting your hand flat out of the car window when you were a kid. You tilt your hand up and you can feel the wind pushing it up. This is the same as a rotor blade, but instead of the car moving and giving your hand lift, the spinning rotor blade is creating the lift.
To bank the aircraft RIGHT, the entire rotor disc raises on the left side of the aircraft and drops on the right side of the aircraft. To accomplish this, when each blade is at the 9 o’clock position it is at its maximum pitch causing it (your hand) to rise.
As each blade moves around to the 3 o’clock position it is at its flattest pitch.
At 12 & 6 each blade is either increasing or decreasing its pitch as it rotates.
This will continue until the cyclic is moved back to the center. The opposite happens to turn left, and also forwards and backward. The blade’s pitch is lowest in the direction you want the aircraft to move.
This video shows how a blade changes pitch during each rotation:
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As the rotor disk begins to tilt in the direction of the turn the aerodynamics change and the lift vectors move from being vertical towards horizontal and this begins the turn.
Because some of the lift has moved from being vertical the lift/weight ratio will be reduced and the aircraft will begin to descend. The more the pilot banks the helicopter, the more the lift vector tilts away from vertical, and the more the helicopter will descend.
To counteract this, the pilot must raise the collective to increase engine power, increase the pitch of blades collectively and increase the lift. Once Lift and Weight are matched again, the aircraft will turn without climbing or descending. This is a level turn.
When rolling out from the turn, the pilot must remember to reduce power or the helicopter will have too much lift being produced and the helicopter will begin to climb once flying straight and level in forward flight.
Flying all the controls in balance, at the right time, and by the right amounts is what takes time to learn, hence the reason why students pilots look like they are all over the place in their early stages of helicopter lessons!
Turning in a helicopter seems simple but the aerodynamics and mechanical engineering to complete that task are complex. There is a lot more flight physics involved than what I have shown here in this synopsis that’s what makes learning to fly a helicopter an incredible challenge, not only controlling one but learning the theory behind it!
So next time you watch a helicopter turning in a hover or as it flies over you just have a think of the forces and mechanics involved for just doing something as simple as turning!