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Principles Of Rotary Flight
Air Moving over a surface decreases air pressure on the surface. As your air speed increases, the surface air pressure decreases accordingly. Basically, as your airfoil* moves through the air, it divides the air molecules. The molecules that move above airfoil must move faster in order to meet up with the air molecules moving under the airfoil. This is due to the fact that the air molecules moving above the airfoil have longer distance to cover. This Creates a low pressure above the airfoil, and high pressure beneath the airfoil. The Higher pressure pushes the airfoil up, creating lift. Figure 1 illustrates this principle *an airfoil is a device used to create lift when passing through air
Figure 1 Fast Moving Air
How does this relate to rotary flight?
As the rotor moves through the air, the shape of the blade produces a high pressure under the blade, and a low pressure above the blade. When the blade moves at a high enough speed, lift overcomes the force of gravity, allowing for positive vertical motion otherwise known as flight.
The Four Acting Forces Lift- the force that overcomes the weight of the aircraft (+ vertical motion) Thrust- the force that moves the aircraft through the air (+ horizontal motion) Weight- the force exerted on the plane by gravity (-vertical motion) Drag- the force of resistance by the air (-horizontal motion)
The Four Forces Diagram
Lift Drag Thrust Weight
The Main Rotor The main rotor produces the lift for the helicopter
Each rotor blade produces an equal share of the lifting force As the speed of the rotor increase, the more lift is produced, allowing for a higher altitude
Torque Reaction As the engine rotates the rotor one direction, it will want to move in the opposite direction. This is due to Newton’s Third law of Motion, for every action, there is an equal and opposite reaction. This is know as torque reaction, and is a serious problem in rotary aircraft. Direction of Rotor Rotation Direction of Rotor Rotation The Helicopter wants to move in the opposite direction as the Rotor
How to compensate for torque reaction
In order to counter balance torque reaction, an additional rotor was added to the tail This is Known as the Tail rotor The tail rotor pulls against the torque reaction in order to straighten out the Helicopter Direction of Rotor Rotation
The Tail Rotors other function
Not only does the tail rotor compensate for torque reaction, but acts as a rudder By slowing down the tail rotor, torque reaction takes over, allowing the helicopter to move in the opposite direction of movement of the main rotor By speeding up the tail rotor, torque compensation becomes too great, an turns the helicopter in the same direction as the rotation of the main rotor Figure 2 illustrates this
Figure 2 Decreasing Tail Rotor speed Increasing Tail Rotor speed
Direction of Rotor Rotation Direction of Rotor Rotation Direction of Helicopter Rotation Direction of Helicopter rotation
Can Helicopters not have tail rotors?
Not all helicopters need tail rotors! The torque reaction of a spinning rotor can be counterbalanced by adding another rotor that spins in the opposite direction This is how helicopters with two main rotors stay straight
How does a Helicopter move forward?
Many People believe that the exhaust from the engine give the helicopter thrust, THIS IS FALSE Helicopters move forward by tilting the main rotor forward as illustrated below The Helicopter moves towards the direction of the tilt The helicopter begins to move forward because of Cycle Pitch Change, and the velocity of each individual blade
Cycle Pitch Change This changes the pitch of each blade per one revolution. The angle of attach for each blade is increased Every time the Blade is in the opposite direction of desired flight, thrust is created
The Speed of the Rotor in Thrust
For Forward Movement The velocity of the rotor is added to the velocity of the forward thrust, on the advancing side of the rotor On the retreating side, these two are subtracted This means that one blade is moving significantly faster than the others This creates different amounts of lift, and gives you thrust Figure 3 Illustrates this Principle
Figure 3 Slower Blade Faster Blade
Left and Right Movement
Once again, the blade can tilt towards the left or the right to create thrust in one direction The rotor Speed on one side will be greater than on the other creating lateral thrust, giving you movement Right and Left Movement is illustrated in Figure 4
Figure 4 Left Movement Right Movement Faster Blade Speed
Unique Helicopters On this helicopter, the rotors produce vertical lift, but then can be rotated 90 degrees to create thrust
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Review Questions What force does lift overcome?
Why does the shape of the rotor create lift? Which one of Newton’s Laws of Motion is the explanation for torque reaction? How do you compensate for torque reaction? What is the other function of the tail rotor? What does increasing the speed of the main rotor do? Do all helicopters need a tail rotor? How does a helicopter generate forward thrust?
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