1. How Do Helicopters Fly? An Introduction to Rotor Aeromechanics

2. Rotor Aeromechanics Quiz
A helicopter’s engine must always supply power to keep a rotor turning - True or False?
When flying forward, a helicopter’s rotor blades flap up and down – True or False?

3. Helicopters vs. Airplanes
What’s the difference?
Lift
Propulsion
Lift and Propulsion
Lift and propulsion are separated on an airplane: Lift is done by the wing, propulsion is done by the propeller
Lift and propulsion are both done by the rotor on a helicopter

4. What affects the amount of LIFT on a wing?

5. What Affects the Amount of Lift on a Wing?
Size
(𝑆)
Lift
Lift
Bigger wing = MORE LIFT

6. What Affects the Amount of Lift on a Wing?
Density
(𝜌)
Lift
Density is higher at lower altitudes and lower temperatures
air molecules
Lift
Greater air density = MORE LIFT

7. What Affects the Amount of Lift on a Wing?
CL is called the Lift Coefficient – it is a factor that depends on shape and orientation
Orientation and Shape
(𝐶 𝐿 )
Lift
As the airfoil pitch (angle of attack) is increased, lift increases until the wing stalls
Lift
Higher Angle of Attack = MORE LIFT
Thicker Wing = MORE LIFT

8. What Affects the Amount of Lift on a Wing?
Velocity
(𝑉)
Lift
Velocity
Lift
Velocity
Greater velocity = MORE LIFT

9. Measuring Wing Lift 𝐿= 1 2 𝜌 𝑉 2 𝑆 𝐶 𝐿 Lift Equation: 𝜌 Density of Air
Size of Wing
Velocity of Air Orientation and Shape of Wing 𝑆
Lift effects 𝑉
𝐶 𝐿
𝐿= 1 2 𝜌 𝑉 2 𝑆 𝐶 𝐿
Lift Equation:

10. Rotary Wing
Velocity V
Helicopter rotors produce lift by rotating their wings
The velocity over the rotor wing in a hover is zero at the center and highest at the tip

11. Rotary Wing Lift in Hover
Lift is zero at the center and highest at the tips
𝐿= 1 2 𝜌 𝑉 2 𝑆 𝐶 𝐿

12. Rotor Lift - Where is the lift the greatest on the spinning rotor?
- Where is the lift the least?
- How can you increase the total lift?

13. Maple Seed Flight
The maple seed will rotate in flight without power

14. Helicopter Autorotation
Without any power supplied to the rotor, it is capable of producing thrust approximately equivalent to a parachute of the same diameter.
Similar to a fixed-wing aircraft, a rotary-winged aircraft can glide to a landing in the event of a power failure. Unlike a fixed wing aircraft, a rotorcraft can land with zero forward airspeed.

15. Helicopter Autorotation
Similar to a fixed-wing aircraft, a rotary-winged aircraft can glide to a landing in the event of a power failure. Unlike a fixed wing aircraft, a rotorcraft can land with zero forward airspeed.

16. Forward Flight
Forward velocity
Thrust
Tip Path Plane (TPP)

17. Hover V
Velocity evenly distributed along blades

18. Forward Flight Hover V V Forward flight speed Flow backward over blade
Increased velocity on advancing blade
Decreased velocity on retreating blade
Velocity evenly distributed along blades

19. Forward Flight Changes Lift on Rotor
Forward speed
Less Lift
Rolling moment V
More Lift
Uneven lift makes the helicopter want to flip over

20. Blade Flapping Equalizes Lift
If the rotor blades are allowed to flap up and down, the lift will be balanced side to side
Same amount of lift on either side
Flapping Hinge
Retreating blade flaps down increasing angle of attack
Advancing blade flaps up reducing angle of attack
No rolling moment

22. Rotor Phase Lag Top view
The response of a flapping rotor blade lags the force by 90o
Maximum flapping occurs 90o after maximum angle of attack is applied.
OR maximum flapping occurs 90o after maximum velocity is applied.
Higher velocity here so higher lift here
Top view

23. Rotor Phase Lag Side view
The response of a flapping rotor blade lags the force by 90o
Maximum flapping occurs 90o after maximum angle of attack is applied.
OR maximum flapping occurs 90o after maximum velocity is applied.
Velocity
Higher velocity on the advancing blade = increased lift
Side view

24. Rotor Phase Lag
The response of a flapping rotor blade lags the force by 90o
Maximum flapping occurs 90o after maximum angle of attack is applied.
OR maximum flapping occurs 90o after maximum velocity is applied.
Velocity
Phase lag causes rotor to flap up 90o later
(front flaps up)

25. Which way will this rotor want to tilt next?
front
left
right
back
More lift here due to rear tilt

26. Rotor tilts to the right!
Blade flaps UP here
left
front
rear
right
Rotor tilts to the right!

27. How does this helicopter tilt its rotor?

28. Rotor Activities Rotor Build \ Test Whirly gig flight target practice
Operate rotor in vertical and forward autorotation
Whirly gig flight target practice
Attempt to land whirly-gig on target anticipating flapping response
Parrot UAV flying skills course
Fly the Parrot quad-rotor, determine how quad-rotor is controlled

29. Rotor Build Instructions
cork
bend
wire
Insert wire through the holes in the cork rotor head and bend as shown

30. Rotor Build Instructions
Balsa rotor blade
tape
Attach rotor blades to rotor head with masking tape. Wrap masking tape around the blades several times to ensure blades do not separate in flight!

31. Rotor Build Instructions
Turn rotor upside down and insert skewer into rotor head perpendicular to the surface

32. Adjust rotor blade angle of attack (both blades must have equal pitch)
Determine direction of rotation for positive lift
Spin rotor to test operation
Observe downwash
Observe coning

33. Slide straw over skewer
Secure small cork into skewer
Test over fan and adjust pitch to achieve maximum thrust and maximum forward flight speed. Observe flapping.

34. Questions?

35. Rotor Aeromechanics Quiz – Post Lesson
A helicopter’s engine must always supply power to keep a rotor turning - True or False?
When flying forward, a helicopter’s rotor blades flap up and down – True or False?

36. Video Links Early video of rotor blade motion from blade perspective
Animation of rotor control in operation
Early NASA autorotation studies for spacecraft recovery:
Roton demonstration (rotor powered by tip jets)