Vertical Take Off & Landing (VTOL) Aircraft -- A Comparison

Lift / Propulsion / Control Approaches For VTOL Aircraft (General) For Shaft-Driven VTOL Aircraft, Need At Least Two Thrusters: l Fixed Thrusters > One Main, One Anti-Torque > Two Equal Size (Opposite Rotation) l Vectored Thrusters > Thruster Tilting > Exhaust Deflection

Methods of Transition for Various V/STOL Concepts

VTOL Concepts

PURE HELICOPTER ADVANTAGES DISADVANTAGES 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)

THERE ARE MAJOR PROBLEMS WITH ADDING A WING Wing Needs Aircraft to be Nose Up to Get Lift Velocity Thrust Wing Lift Main Rotor Needs Aircraft to be Nose Down to Get Propulsion Velocity Thrust Wing "Lift" (Download) Need to Get Propulsion from Someplace Other than Main Rotor Add a Third Fixed Thruster (Propulsive Thruster) or Use Vectored Thrust SOLUTION:

COMPOUND HELICOPTER (Fixed Thruster) ADVANTAGES DISADVANTAGES 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)

(Advancing Blade Concept) COMPOUND 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)

(Vectored Thruster - Open Prop) COMPOUND 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)

(Vectored Thruster - Ducted Prop) COMPOUND 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 WING ADVANTAGES DISADVANTAGES 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 ROTOR ADVANTAGES DISADVANTAGES 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 WING ADVANTAGES DISADVANTAGES 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 FANS ADVANTAGES DISADVANTAGES 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-WING ADVANTAGES DISADVANTAGES 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 LIFT ADVANTAGES DISADVANTAGES 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, . . . ?

Comparison of Different Types of V/STOL Platforms