helicopter airspeed - u3asites · berp british experimental rotor programme . 1986 - g-lynx 216kt ....
TRANSCRIPT
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Helicopter Airspeed
Ray White
November 2015
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Helleborre
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Heli-bore
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• Portadown College & Glasgow
University
• 1974 - Westland Helicopters
• 1986 - Messerschmitt-Bölkow-Blohm
• 1991 - CAA
• 2006 - EASA
• 2014 - Freedom
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History Lesson
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Sikorsky VS300 29 November 1939
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If you are in trouble anywhere in the world,
an airplane can fly over and drop flowers, but
a helicopter can land and save your life.
Igor Sikorsky
In 1947, queried about helicopter speeds, he
pointed out that, while special designs can and
will be made to go faster, operation efficiency
will hold speeds to around 150 mph (130kt)
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The Challenge
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Sikorsky S51/Westland Dragonfly
First Flight 1946
Maximum speed 90kt, Cruise speed 74kt
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Sikorsky S55/Westland Whirlwind
First Flight 1949
Maximum speed 95kt
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Sikorsky S58/Westland Wessex
First Flight 1956
Maximum speed 115kt
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Sikorsky S58/Westland Wessex
First Flight 1956
Maximum speed 115kt
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Sikorsky SH3/Westland Sea King
First Flight 1962
Maximum speed 145kt/Cruise speed 120kt
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Sikorsky S70 Black Hawk
First Flight 1974
Maximum speed 195kt/Cruise speed 163kt
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Sikorsky S76
First Flight 1977
Maximum speed 155kt/Cruise speed 150kt
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Sikorsky S92
First Flight 1988
Maximum speed 165kt/Cruise speed 151kt
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Agusta Westland AW139
First Flight 2001
Maximum speed 167kt/Cruise speed 165kt
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Eurocopter EC175
First Flight 2009
Maximum speed 175kt/Cruise speed 155kt
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Agusta Westland AW189
First Flight 2011
Maximum speed 169kt/Cruise speed 155kt
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The Problems
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What are the obstacles?
• Installed Power
• Rotor flight mechanics
• Handling qualities
• Loads
• Vibration
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Installed power
• Helicopters are “Power hungry”
• The following aircraft have approximately
the same installed power
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ATR 42
2 x 1787 shp - 42-52 passengers
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AW 139
2 x 1679 shp - 16 passengers
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AW 609
2 x 1940 shp - 9 passengers
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EH101 Merlin
3 x 1725 shp
Up to 800shp to the tail rotor in some flight phases
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Rotor Mechanics
• Handling qualities
• Loads
• Vibration
• Advancing blade tip mach number
• Retreating blade stall
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Advancing Blade Tip Mach Number
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Eurocopter EC175
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Focke-Wulf Fw61 First flight 1936
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Focke-Achgelis Fa223
First flight 1940
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Eurocopter EC175
Advancing side Retreating side
Forward
Flight
Airspeed
Advancing blade tip speed
= 𝜛R + Airspeed
Rotor dia = 48.56ft (14.8m)
Rotor rpm = 298.5
Hover Tip speed = 759fps
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Retreating Blade Stall
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Eurocopter EC175
Advancing side Retreating side
Forward
Flight
Airspeed
Rotor dia = 48.56ft (14.8m)
Rotor rpm = 298.5
Hover Tip speed = 759fps
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The “solutions”
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Why don’t you - - -?
Reduce rotor rpm
• Reduces tip speed
• Reduces retreating blade stall regime
• Loss of lift
• Loss in rotor efficiency - Thrust vs Power
• Loss of Cf stiffening brings torsional instability and disaster
Reduce rotor diameter
• Reduces rotor lift and performance
Reduce rotor diameter and increase number of blades
• Increases rotor thrust
• Increases profile drag
• Increases power requirement to overcome drag
• Increases rotor head and control system complexity
and weight
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1. BERP
British Experimental Rotor Programme
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1986 - G-LYNX 216kt
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Advantages
• Sweep reduces compressibility effects
• Moves CoL aft = instability
• Leading edge extension moves it forward again
• Highly swept leading edge acts like a delta wing at high AoA
• Notch vortex also improves performance of tip
∴Acts like a larger diameter rotor without increased tip speed consequences
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2. ABC
Advancing Blade Concept
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• Retreating blade stall can be reduced (but not eliminated) by reducing blade pitch on that side
• Downside is lift assymetry
• ABC means there is an advancing blade on each side
How?
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Sikorsky S-69
(1973)
As a helicopter - 156kt
As a compound - 263kt
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Sikorsky X2
2008 - 2011
• 250kt level
• 260 in shallow dive
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Principle
• Retreating blade on each rotor is at very flat pitch
• Little or no retreating blade stall
• Co-axial contra-rotating rotors
• Advancing blade on each side
• Symmetrical lift across the disc swept area
• No anti torque rotor necessary
• Tail drive used to drive a propulsor
Disadvantages
• Rotor interference
• Lower rotor in the turbulent wake of the upper
• Huge complexity in the main rotor drive
• Huge complexity in the rotor pitch control systems
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S-97 Raider First Flight May 2015
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3. Eurocopter (Airbus) X3
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X3
2010 - 2014
• 255kt level flight
• 263kt in dive
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Principle
• VP Propellors are driven from main rotor gearbox
• At higher speeds, stub wings provide approx 20% of lift
• Main rotor rpm is reduced by around 10% to reduce
advancing tip speed
• Torsional and bending blade stiffness remains adequate
• Main rotor pitch is reduced by lower thrust requirement
• Resultant lower pitch of retreating blade reduces stalled
region
• No tail rotor
• Hover yaw control by differential propellor thrust
• Forward flight yaw control by conventional rudders
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Disadvantages
• Complexity of rotor and propellor drive system
• Prototype flight controls not ideal (FBW would solve this)
• Cabin noise - high speed propellor tip path noise
• No protection for cabin occupants if a blade is shed
• Difficulty in loading passengers with rotors running
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Piasecki X49
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Deja vu
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4. Agusta Westland AW609
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AW609
2003 - present day
• 260kt level flight
• 293kt in dive
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Principle
• Tilt rotor
• Nacelles vertical for hover and slow speed work
• Nacelles horizontal for high speed operations
• One engine in each nacelle
• Cross shaft with freewheels couples both rotors
• Following an engine failure, remaining engine drives both
rotors
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Disadvantages
• Extremely complex rotor drive system
• Prop-rotor design must be a compromise between 2 modes
of operation
• Engines must be capable of operating for extended periods
when vertical
• Handling challenges - roll inertia with large masses
outboard
• Changes in aerodynamic state during transition from one
mode to the other - rotor downwash
• Change in engine/propellor control laws between helicopter
an aeroplane modes
• Very high energy rotor downwash in hover
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5. Sikorsky S72 1976 - 1988
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X-Wing
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Principle
• Very stiff rotor
• Not variable pitch
• Leading edge bleed air outlets to modify boundary layer
• Creates a virtual aerofoil section
• Boundary layer changes as blade rotates around head,
• and to generate thrust vector variations to manoeuvre the
aircraft.
• As airspeed increases, wings take more of lift loads
• Rotor slows, then stops and becomes a wing with bleed air
augmentation
• When stopped, 2 of the blade trailing edges became wing
leading edges. Bleed air needed there.
Disadvantages
• Beyond technical capabilities of
the day
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Igor Sikorsky
In 1947, queried about helicopter speeds, he
pointed out that, while special designs can and
will be made to go faster, operation efficiency
will hold speeds to around 150 mph (130kt)