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Principles Of Rotary Flight
Eric Wasacz
Bernoulli’s Principle
• 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
The Four Forces Diagram
The Main Rotor
• The main rotor produces the lift for the helicopter
• Each rotor blade produces an equal share of the lifting
• 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
• 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
Direction of
Rotor Rotation
Direction of
Increasing Tail Rotor
Direction of
Rotor 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
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
• 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
Information taken from
Images Taken From
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?