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 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 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.psd
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.
Separate slides for pictures.
Airfoil Design
Reynolds Number and Mach Number
Airfoil 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 effects
The 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-02
1980s 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.
