helicopter airspeed - u3asites · berp british experimental rotor programme . 1986 - g-lynx 216kt ....

Helicopter Airspeed

Ray White

November 2015

Helleborre

Heli-bore

• Portadown College & Glasgow

University

• 1974 - Westland Helicopters

• 1986 - Messerschmitt-Bölkow-Blohm

• 1991 - CAA

• 2006 - EASA

• 2014 - Freedom

History Lesson

Sikorsky VS300 29 November 1939

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)

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 SH3/Westland Sea King

First Flight 1962

Maximum speed 145kt/Cruise speed 120kt

Sikorsky S70 Black Hawk

First Flight 1974


Sikorsky S76

First Flight 1977


Sikorsky S92

First Flight 1988


Agusta Westland AW139

First Flight 2001


Eurocopter EC175

First Flight 2009


Agusta Westland AW189

First Flight 2011


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

Forward

Flight

Airspeed

Advancing blade tip speed

= 𝜛R + Airspeed

Rotor dia = 48.56ft (14.8m)

Rotor rpm = 298.5

Hover Tip speed = 759fps

Retreating Blade Stall

Eurocopter EC175

Advancing side Retreating side

Forward

Flight

Airspeed

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)

helicopter airspeed - u3asites · berp british experimental rotor programme . 1986 - g-lynx 216kt ....

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