introduction to helicopter aerodynamics

A Brief Introduction to Helicopters

Robert L. Roedts IIThe Pennsylvania State University

Rotorcraft Center of Excellence

PENNSTATE1 8 5 5

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.

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.

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

Helicopters at Work

Comparison of Fixed-Wing Aircraft and Helicopters

• Fixed Wing Aircraft

Comparison of Fixed-Wing Aircraft and Helicopters

• Helicopter

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?

Configurations of RotorcraftMain Rotor - Tail Rotor Config.

Configurations of RotorcraftTandem Rotors (Chinook)

Configurations of RotorcraftCoaxial Rotors (Kamov KA-52)

Configurations of RotorcraftNOTAR Helicopter

Configurations of RotorcraftTilt Rotor (BA 609)

Unequal Lift Distribution

tipV R=Ω

tipV R V∞=Ω +

tipV R V∞=Ω −

Lift ~ V2

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.

Interactional Aerodynamics

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

Rotor Descent States

Ground Interactions

At low forward speeds, less power is required.

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.

Airfoil Design• Reynolds Number and Mach Number

• 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

• 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

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

• 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

• 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

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)

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.

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.

Final Thought

“Helicopters don’t fly. They beat the air into submission.”

~ Dr. Ed Smith