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A Brief Introduction to Helicopters
Robert L. Roedts II
The Pennsylvania State University
Rotorcraft Center of Excellence
What is a Helicopter?
Unique FeaturesRotating-wing vehiclesAbility to hoverLand and take-off verticallyFly forward, backward and sidewaysHelicopters are closely related to autogyros and tiltrotors.Motivation
The overall unique aspect of a helicopter is its 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.Helicopters at Work
With this ability to hover, helicopters can perform a wide range of missions.Comparison of Fixed-Wing Aircraft and Helicopters
Fixed Wing AircraftComparison of Fixed-Wing Aircraft and Helicopters
HelicopterConfigurations of Rotorcraft
Many different ways to counter Reactive TorqueOther possibilities: Tip jets, tip mounted engines
Question: Why do each of these methods work?
What are the likely advantages and disadvantages of each?
Configurations of Rotorcraft
Main Rotor - Tail Rotor Config.
Configurations of Rotorcraft
Tandem Rotors (Chinook)
Configurations of Rotorcraft
Coaxial Rotors (Kamov KA-52)
Configurations of Rotorcraft
NOTAR Helicopter
Configurations of Rotorcraft
Tilt Rotor (BA 609)
Unequal Lift Distribution
Lift ~ V2
28.psd30.psdHigh-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.Separate slides for pictures.
Airfoil Design
Reynolds Number and Mach NumberAirfoil Design
Four Rules of Rotorcraft Airfoil DesignHigh CLmaxHigh MDDGood L/D over a wide range of Mach NumbersLow CmDesign constraints are much narrower for rotorcraft. (I.e.: Cm 0.02)Change straightforward
Early Helicopter Airfoil Design
Initially, symmetric airfoils were usedLow Pitching Moment, CmCyclic PitchJuan de la CiervaAutogyrosFirst to use a cambered airfoilResulted in a crash in 1939Crash and low torsional stiffness resulted in universal use of symmetric airfoil until the 1960s.The 60s & 70s Revolution
Vast Improvements in Modern Computers allowed engineers to utilized them.Panel MethodsInviscid Solutions but still insightfulConformal Mapping introduced into computer codesReintroduction of Cambered Airfoils Computer designImproved StructuresMore concentration on transonic effectsThe 60s & 70s Revolution
Example: YAH-64 Apache, 1976Heavy use of computer during design process.Reduced amount of wind tunnel testing necessaryDesign costs and time decreaseStarted with a NACA 63A-410 and ended with the HH-021980s Airfoil Development
Trailing Edge TabsOffset the pitching moment of a cambered airfoilBy 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.1990s Development
Change blade geometriesBritish 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 TechnologiesGurney FlapsKeeps 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 dont fly. They beat the air into submission.
~ Dr. Ed Smith