Vertical Take Off & Landing Vertical Take Off & Landing (VTOL)(VTOL)AircraftAircraft

----A ComparisonA Comparison

Lift / Propulsion / Control ApproachesLift / Propulsion / Control ApproachesFor VTOL AircraftFor VTOL Aircraft

(General)

For Shaft-Driven VTOL Aircraft, Need At Least Two Thrusters:

Fixed Thrusters

> One Main, One Anti-Torque

> Two Equal Size (Opposite Rotation)

Vectored Thrusters

> Thruster Tilting

> Exhaust Deflection

Methods of Transition for Various V/STOL Concepts

VTOL ConceptsVTOL Concepts

PURE HELICOPTERPURE HELICOPTER

ADVANTAGES

•Most Efficient Hover/Loiter•Low Downwash•Good Low Speed Maneuverability•Symmetrical Yaw Control•Low Empty Weight

DISADVANTAGES

•Low Max Speed•Lowest Cruise Efficiency (Range)•Limited High Speed Maneuverability•Attitude Depends on Speed/Acceleration•Rotating Component RCS•Highest Vibration Environment•Complexity (2 Fixed Thrusters)

Add a Third Fixed Thruster (Propulsive Thruster)or

Use Vectored Thrust

SOLUTION:

Need to Get Propulsion from Someplace Other than Main Rotor

THERE ARE MAJOR PROBLEMS WITH ADDING A WING

Main Rotor Needs Aircraft to be Nose Down to Get Propulsion

Velocity

Thrust

Wing "Lift"(Download)

Wing Needs Aircraft to be Nose Up to Get Lift

Velocity

Thrust

Wing Lift

COMPOUND HELICOPTERCOMPOUND HELICOPTER(Fixed Thruster)

ADVANTAGES•Good Hover/Loiter Efficiency•Low Downwash•Faster Than Pure Helicopter•Good Maneuverability -- All Speeds•Attitude Independent of

Speed/Acceleration•Symmetrical Yaw Control•Reverse Prop Thrust -- All Speeds

DISADVANTAGES•Low Cruise Efficiency (Range)•Rotating Component RCS•High Vibration Environment•Increased Empty Weight•Complexity (3 Fixed Thrusters)

COMPOUND HELICOPTERCOMPOUND HELICOPTER(Advancing Blade Concept)

ADVANTAGES•Good Hover/Loiter Efficiency•Low Downwash•Faster Than Pure Helicopter•Good Maneuverability -- All Speeds•Attitude Independent of

Speed/Acceleration•Symmetrical Yaw Control•No Anti-Torque Rotor Required•Reverse Prop Thrust -- All Speeds

DISADVANTAGES•Low Cruise Efficiency (Range)•Rotating Component RCS•High Vibration Environment•Increased Empty Weight•Complexity (3 Fixed Thrusters)

COMPOUND HELICOPTERCOMPOUND HELICOPTER(Vectored Thruster - Open Prop)

ADVANTAGES•Good Hover/Loiter Efficiency•Low Downwash•Faster Than Pure Helicopter•Good Maneuverability

(Except Conversion)•Reverse Prop Thrust at High Speed

DISADVANTAGES•Low Cruise Efficiency (Range)•Conversion (Limited Agility)•Attitude Depends on Acceleration at

Low Speed•Unprotected Vectored Thruster•Rotating Component RCS•High Vibration Environment•Increased Empty Weight•Complexity

(1 Fixed + 1 Vectored Thruster)

COMPOUND HELICOPTERCOMPOUND HELICOPTER(Vectored Thruster - Ducted Prop)

ADVANTAGES•Good Hover/Loiter Efficiency•Low Downwash•Faster Than Pure Helicopter•Good Maneuverability

(Except Conversion)•Reverse Prop Thrust at High Speed•Ground Safety/Damage

(Ducted Prop)

DISADVANTAGES•Low Cruise Efficiency (Range)•Conversion (Limited Agility)•Limited / Unsymmetrical Yaw Control•Attitude Depends on Acceleration at

Low Speed•Rotating Component RCS•High Vibration Environment•Complexity

(1 Fixed + 1 Vectored Thruster)

CANARD ROTOR WINGCANARD ROTOR WING

ADVANTAGES•Good Hover/Loiter Efficiency•Low Downwash•Potential for High Subsonic Cruise•Good Maneuverability

(Except in Conversion)•No Anti-Torque Rotor•Reduced RCS in High Speed Mode•Low Vibration Environment in

High Speed Mode

DISADVANTAGES•Limited Maneuverability in Conversion•Power for Yaw Control Near Hover•Moderate Vibration Environment in

Low Speed & Conversion Modes•Rotating Component RCS in Low

Speed Mode•Complexity (Rotor Stopping &

Convertible Engine)

TILT ROTORTILT ROTOR

ADVANTAGES•Good Hover/Loiter Efficiency•Moderate Downwash•Good Max Speed•Good Cruise Efficiency (Range)•Good Maneuverability -- All Speeds•Attitude Independent of

Speed/Acceleration•Ground Safety/Damage

(No Tail Rotor)

DISADVANTAGES•Greater Operating Width•Conversion (Benign)•Rotating Component RCS•Moderate Vibration Environment•Increased Empty Weight•Complexity (2 Vectored Thrusters)

TILT WINGTILT WING

ADVANTAGES•Fair Hover/Loiter Efficiency•Faster Than Tilt Rotor•Good Cruise Efficiency (Range)•Good High Speed Maneuverability•Attitude Independent of

Speed/Acceleration•Symmetrical Yaw Control

DISADVANTAGES•Marginal Downwash•Conversion (Limited Corridor)•Rotating Component RCS•Increased Empty Weight•Complexity

(1 Fixed + 2 Vectored Thrusters)

TILTING DUCTED FANSTILTING DUCTED FANS

ADVANTAGES•Enclosed Thrusters (Safety)•Symmetrical Yaw Control

DISADVANTAGES•Low Hover/Loiter Efficiency•Limited Low Speed Maneuverability•Conversion (Limited Corridor)•High Empty Weight•Complexity

(1 Fixed + 2 Vectored Thrusters)

FAN-IN-WINGFAN-IN-WING

ADVANTAGES•High Max Speed•Good Cruise Efficiency (Range)•Attitude Independent of

Speed/Acceleration•Good RCS (High Speed Mode)•Low Vibration Environment

DISADVANTAGES•Low Hover/Loiter Efficiency•High Downwash / Temperature•Limited Low Speed Maneuverability•Conversion (Limited Corridor)•High Empty Weight•Complexity (3 Vectored Thrusters)

VECTORED JET LIFTVECTORED JET LIFT

ADVANTAGES•Highest Max Speed•Highest Cruise Efficiency (Range)•Excellent High Speed Maneuverability•Attitude Independent of

Speed/Acceleration•Symmetrical Yaw Control•Low Vibration Environment•Moderate Empty Weight

DISADVANTAGES•Poor Hover/Loiter Efficiency•Extreme Downwash / Temperature•Limited Low Speed Maneuverability•Conversion (Benign)•Rotating Component RCS (Forward)•Jet Exhaust IR•Complexity (1 Thruster + 8 Nozzles)

Summary

• VTOL Aircraft Are Inherently More Complex Than Conventional Take-Off and Landing (CTOL) Aircraft– Mechanization Required to Change Direction of Thrust

With Respect to Aircraft– Additional Controllers (e.g., Collective Stick, Conversion)

• “Best VTOL Concept” Only Has Meaning in the Context of the Mission to be Performed– What Do You Need Most? Hover Time, Fast Cruise,

Long Range, . . . ?– What Do You Have Available? Runway, . . . ?

VTOL Concept Helicopter Compound Compound Compound Compound Tilt Rotor Tilt WingCanard Rotor

WingFan-in-Wing Jet Lift

Features (Tail Rotor +

Aux Prop)(Advancing

Blade Concept)(Vectrd Thrst -

Open Prop)(Vectrd Thrst - Ducted Prop)

ExamplesAH-64, RAH-66

AH-56 Sikorsky XH-59Sikorsky AAFSS

Piasecki VTDPXV-3, XV-15,

V-22CL-84, TW-68,

XC-142Boeing CRW

XV-5, Grum-man ACAS

AV-8B

Hover / Loiter / Ground Ops

Efficiency (Endurance) Best Good Good Good Good Good Fair Low Low Poor

Downwash / Temperature Low Low Low Low Low Moderate Marginal Marginal High Extreme

Ground Ops Protection Optional Open Prop No Anti-Torque Open Prop Ducted Prop --- Optional No Anti-Torque --- Jet Exhaust

Cruise / Dash

Max Speed (Dash) Low > Helicopter > Helicopter > Helicopter > Helicopter Good > Tilt Rotor High High Highest

Cruise Efficiency (Range) Lowest Low Low Low Low Good Good Good Good Highest

Maneuverability / Agility

Low Speed Good Good Good Good Good Good Fair Limited Limited Limited

High Speed Limited Good Good Good Good Good Good Good Good Excellent

Conversion --- --- --- Limited Agility Limited Agility Benign Lmtd Corridor Lmtd Corridor Lmtd Corridor Benign

Attitude vs Accel (Low Speed) Coupled Independent Coupled Coupled Coupled Independent Independent Independent Independent Independent

Reverse Thrust Capability --- All Speeds --- High Speed High Speed --- High Speed --- --- ---

Yaw Control Symmetrical Symmetrical Symmetrical SymmetricalLimited /

UnsymmetricalSymmetrical Symmetrical

Limited in Low Speed

Symmetrical Symmetrical

Survivability

RCS (Low Speed Mode) Rotating Blade Rotating Blade Rotating Blade Rotating Blade Rotating Blade Rotating Blade Rotating Blade Rotating Blade Doors / Louvers Short Inlet

RCS (High Speed Mode) Rotating Blade Rotating Blade Rotating Blade Rotating Blade Rotating Blade Rotating Blade Rotating Blade Good Good Short Inlet

IR (Engine Exhaust) Lowest Power Low Power Low Power Low Power Low Power Low Power Medium Power High Power High Power Short Exhaust

Cost / Supportability

Vibration Environment Highest High Very High High High Moderate --- Moderate Low Low

Empty Weight Low Increased Increased Increased Increased Increased Increased Increased High Moderate

Complexity (Min # of Thrusters)

2 Fixed 3 Fixed 2 Fixed1 Fixed +

1 Vectored1 Fixed +

1 Vectored2 Vectored

1 Fixed + 2 Vectored

Convertib le Engine

3 Vectored1 Thruster + 8 Nozzles

Key to Table: Advantage Neutral Disadvantage

Comparison of Different Types of V/STOL Platforms