Helicopter Airspeed

Ray White November 2015 Helleborre Heli-bore • Portadown College & Glasgow University • 1974 - Westland • 1986 - Messerschmitt-Bölkow-Blohm • 1991 - CAA • 2006 - EASA • 2014 - Freedom

History Lesson Sikorsky VS300 29 November 1939 Igor Sikorsky If you are in trouble anywhere in the world, an airplane can fly over and drop flowers, but a can land and save your life.

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) The Challenge Sikorsky S51/Westland Dragonfly First Flight 1946 Maximum speed 90kt, Cruise speed 74kt Sikorsky S55/Westland Whirlwind First Flight 1949 Maximum speed 95kt Sikorsky S58/Westland Wessex First Flight 1956 Maximum speed 115kt Sikorsky S58/Westland Wessex First Flight 1956 Maximum speed 115kt Sikorsky SH3/ First Flight 1962 Maximum speed 145kt/Cruise speed 120kt

Sikorsky S70 Black Hawk First Flight 1974 Maximum speed 195kt/Cruise speed 163kt Sikorsky S76 First Flight 1977 Maximum speed 155kt/Cruise speed 150kt Sikorsky S92 First Flight 1988 Maximum speed 165kt/Cruise speed 151kt Agusta Westland AW139 First Flight 2001 Maximum speed 167kt/Cruise speed 165kt Eurocopter EC175 First Flight 2009 Maximum speed 175kt/Cruise speed 155kt Agusta Westland AW189 First Flight 2011 Maximum speed 169kt/Cruise speed 155kt

The Problems What are the obstacles?

• Installed Power • Rotor flight mechanics • Handling qualities • Loads • Vibration Installed power

• Helicopters are “Power hungry” • The following aircraft have approximately the same installed power ATR 42

2 x 1787 shp - 42-52 passengers AW 139

2 x 1679 shp - 16 passengers AW 609

2 x 1940 shp - 9 passengers EH101 Merlin

3 x 1725 shp Up to 800shp to the tail rotor in some flight phases Rotor Mechanics

• Handling qualities • Loads • Vibration

• Advancing blade tip mach number • Retreating blade stall Advancing Blade Tip Mach Number Eurocopter EC175 Focke-Wulf Fw61 First flight 1936 Focke-Achgelis Fa223 First flight 1940 Eurocopter EC175

Advancing side Retreating side

Airspeed

Rotor dia = 48.56ft (14.8m) Rotor rpm = 298.5

Hover Tip speed = 759fps Forward Flight

Advancing blade tip speed = 휛R + Airspeed

Retreating Blade Stall Eurocopter EC175

Advancing side Retreating side

Airspeed

Forward Flight

Rotor dia = 48.56ft (14.8m) Rotor rpm = 298.5

Hover Tip speed = 759fps

The “solutions” 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 1. BERP

British Experimental Rotor Programme 1986 - G-LYNX 216kt 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 2. ABC

Advancing Blade Concept • 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? Sikorsky S-69

(1973)

As a helicopter - 156kt As a compound - 263kt Sikorsky X2

2008 - 2011

• 250kt level • 260 in shallow dive 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 S-97 Raider First Flight May 2015

3. Eurocopter (Airbus) X3 X3 2010 - 2014

• 255kt level flight • 263kt in dive

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 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

Piasecki X49 Deja vu

4. Agusta Westland AW609 AW609 2003 - present day

• 260kt level flight • 293kt in dive 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 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

5. Sikorsky S72 1976 - 1988

X-Wing 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

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)