Helicopters have the unique ability to hover, take off and land vertically, and fly in any direction. Their main distinguishing feature is the ability to hover for extended periods using a rotating wing. This allows helicopters to perform a wide range of missions. Helicopters face challenges in high-speed forward flight due to unequal lift distribution across the rotor blades and vibrations. Various configurations have been developed to counter the reactive torque created by the main rotor, including tail rotors, coaxial rotors, and tiltrotors. Airfoil design for helicopter rotors must account for the changing airspeed and angles of attack experienced throughout each revolution. Advancements in computer modeling have improved airfoil designs and reduced the need for wind tunnel testing

1. A Brief Introduction to
Helicopters
Robert L. Roedts II
The Pennsylvania State University
Rotorcraft Center of Excellence
PENNSTATE
1 8 5 5

2. What is a Helicopter?
• Unique Features
– Rotating-wing vehicles
– Ability to hover
– Land and take-off vertically
– Fly forward, backward and sideways
• Helicopters are closely related to
autogyros and tiltrotors.

3. Motivation
• The overall unique aspect of a helicopter is
it’s ability to hover for extended periods of
time.
• The ability to hover it a very useful attribute.
– An good example is that of a hummingbird.

4. • With this ability to hover, helicopters can
perform a wide range of missions.
Helicopters at Work

5. Comparison of Fixed-Wing
Aircraft and Helicopters
• Fixed Wing Aircraft

6. Comparison of Fixed-Wing
Aircraft and Helicopters
• Helicopter

7. Configurations of Rotorcraft
• Many different ways to counter Reactive Torque
Other possibilities: Tip jets, tip mounted enginesOther possibilities: Tip jets, tip mounted engines
Question: Why do each of these methods work?Question: Why do each of these methods work?
What are the likely advantages and disadvantages of each?What are the likely advantages and disadvantages of each?

8. Configurations of Rotorcraft
Main Rotor - Tail Rotor Config.

9. Configurations of Rotorcraft
Tandem Rotors (Chinook)

10. Configurations of Rotorcraft
Coaxial Rotors (Kamov KA-52)

11. Configurations of Rotorcraft
NOTAR Helicopter

12. Configurations of Rotorcraft
Tilt Rotor (BA 609)

13. Unequal Lift Distribution
tipV R= Ω
tipV R V∞= Ω +
tipV R V∞= Ω −
Lift ~ V2

14. High-Speed Forward Flight
Limitations
• As the forward speed increases, advancing side
experiences shock effects, retreating side stalls. This
limits thrust available.
• Vibrations go up, because of the increased dynamic
pressure, and increased harmonic content.
• Shock noise goes up.
• Fuselage drag increases, and parasite power
consumption goes up as V3
.
• We need to understand and accurately predict the air
loads in high speed forward flight.

15. Interactional Aerodynamics
There are many ways to deal with these problems. One
example would be the airfoil selection.

16. Rotor Descent States

17. Ground Interactions
At low forward speeds, less power is required.

18. Airfoil Design
• Rotorcraft present an interesting problem for
airfoil design.
• Fixed Wing Aircraft can be designed for certain
conditions.
• The Rotorcraft environment changes rapidly
as the blade travels around the rotor disk.

19. Airfoil Design
• Reynolds Number and Mach Number

20. • Four Rules of Rotorcraft Airfoil Design
– High CLmax
– High MDD
– Good L/D over a wide range of Mach
Numbers
– Low Cm
• Design constraints are much narrower
for rotorcraft. (I.e.: Cm ≤ 0.02)
Airfoil Design

21. • Initially, symmetric airfoils were used
– Low Pitching Moment, Cm
– Cyclic Pitch
• Juan de la Cierva
– Autogyros
– First to use a cambered airfoil
• Resulted in a crash in 1939
• Crash and low torsional stiffness
resulted in universal use of symmetric
airfoil until the 1960s.
Early Helicopter Airfoil Design

22. The 60’s & 70’s Revolution
• Vast Improvements in Modern Computers
allowed engineers to utilized them.
• Panel Methods
– Inviscid Solutions but still insightful
• Conformal Mapping introduced into computer
codes
• Reintroduction of Cambered Airfoils
– Computer design
– Improved Structures
• More concentration on transonic effects

23. • Example: YAH-64 Apache, 1976
– Heavy use of computer during design process.
• Reduced amount of wind tunnel testing necessary
• Design costs and time decrease
– Started with a NACA 63A-410 and ended with the
HH-02
The 60’s & 70’s Revolution

24. • Trailing Edge Tabs
– Offset the pitching moment of a cambered airfoil
– By simple application of thin-airfoil theory, one
may see the effect.
– Research showed that these tabs led to a small
increase in drag and little effect on lift.
1980’s Airfoil Development

25. 1990’s Development
• Change blade geometries
• British Experimental Rotor Program
(BERP)
– Developed to deal with tip effects on blade.
• Transonic Effects (Advancing Side)
• High Alpha Stall (Retreating Side)

26. 2000 and Beyond
• Morphing Technologies
– Gurney Flaps
• Keeps flow attached in high alpha conditions.
• Unsteady Aerodynamics
– Current design methods assume static CLmax & Cm as
in a steady flow condition.
– With blade wake interactions, this is not the case.
– Current research is concentrated in this area
where a N-S solver will be used along side the
Eppler code to design airfoils.

27. References
• Gessow, A. and Myers, G.C., Aerodynamics of the Heilcopter,
3rd
Edition, College Park Press, College Park, MD, 1999.
• J. Seddon, Basic Helicopter Aerodynamics, 2nd
Edition, AIAA,
Washington, DC, 2001.
• Leishman, J. G., Principles of Helicopter Aerodynamics, 2nd
Edition, Cambridge University Press, New York, NY, 2005.
• McCormick, B. W., Aerodynamics of V/STOL Flight, Academic
Press, Inc., New York, NY, 1967.
• Johnson, W. Helicopter Theory, Princeton University Press,
Princeton, NJ, 1980.

28. Final Thought
“Helicopters don’t fly. They beat the air
into submission.”
~ Dr. Ed Smith