The increased range and speed characteristics inherent to the tilt-rotor concept significantly improve the enroute survivability of the MV-22. With effective intelligence and mission planning, the MV-22 is able to use its range advantage to circumvent enroute threats. Combined with the use of very low-level tactics that take advantage of terrain masking, the increased flight speed of the MV-22 provides far less opportunity (as compared to slower flying helicopters) for a surface-to-air threat system to acquire and fire.
To maximize the Osprey's survivability, design teams integrated all three aspects of survivability into the V-22's design. They reduced the chance of being detected and engaged by an enemy (susceptibility). They reduced the damage to or loss of the aircraft and crew when engaged by an enemy (vulnerability), and they improved the crashworthiness of the aircraft. The overriding goal was to preserve a limited and important national asset.
Maximizing the V-22's survivability begins with making it less susceptible to detection and engagement. The tiltrotor has several inherent advantages over helicopters and the Osprey's state-of-the-art avionics provide the capability to degrade or defeat an enemy's ability to engage. Some of these are:
The ability to fly at turboprop speeds makes the V-22 much more difficult to engage than a conventional helicopter. The ability to transit a threat zone at a high rate of speed decreases the chance that the Osprey will be detected. With five times the range of a CH-46, the Osprey can fly around threats rather than through them. Combined with its speed advantage, the V-22 can fly a safer route and still insert more troops, faster.
Compared to both helicopters and conventional turboprops, the Osprey has a lower acoustic signature due to the tiltrotor's reduced rotor rotational speed. It also uses very low thrust for cruise propulsion. The V-22 flying in aircraft mode produces a distinctive sound, described by observers as a "throaty and muted hum - more like a vehicle than a helicopter." The observers noted that, in combined operations, the steady buzz of the MV-22 was frequently masked until the last minute by the "whop-whop noise" of AH-1 Cobras and UH-1 Hueys that were supporting nearby. Overall, as compared to the CH-46, the MV-22 is less noisy while in the aircraft mode, and provides comparable acoustic acquisition cues while operating in the helicopter mode.
At the rear of each engine nacelle is an IR suppressor. These units mix the hot exhaust gases with cool air and deflects it away from the aircraft. The two engines are widely spread which also reduces the infrared signature.
The V-22 has the option of flying at either high or low altitudes depending on the threat encountered. To avoid small arms fire, the Osprey can fly at altitudes unusable by helicopters. Unlike fixed wing aircraft, it can also fly low speed low level using terrain masking techniques. The CV-22 variant uses its multi-mode radar for high speed low altitude Terrain Following/Terrain Avoidance flying. This allows the CV-22 to fulfill the Special Operations mission in any weather, at night, into denied territory.
The Osprey's cockpit is fully Night Vision Goggle (NVG) compatible allowing it to operate under the cover of darkness. Two retractable search and landing lights are located on the bottom of the fuselage. The lights are selectable white or infrared. The Osprey is designed to operate in weather that would ground current generation helicopters. Ice Detection, Anti-Icing on the engine inlets, windshield, pitot-static/angle-of-attack probes and parts of the proprotors and De-icing of the proprotor blades, spinners, wing and tails are part of the Osprey's design.
The radar warning receiver (RWR) provides passive radar detection, identification, crew alerts, and interface with other electronic warfare (EW) systems. The MV-22 uses the AN/APR-39A(V)2 and the CV-22 uses the ALQ-211 SIRFC. The AN/AAR-47 provides passive electro-optical detection and reaction to/warning of incoming missiles. The AN/AVR-2A Laser Detection System passively detects, identifies, and provides warning of laser threats by comparison of received information with internal threat files.
The Countermeasures Dispensing System (CDS) set provides chaff, decoy, and/or flare countermeasures for cued threats from the MWS/RWR and manual crew-initiated dispense programs. This function is provided by the ALE-47. The CV-22 electronic countermeasures suite incorporates the Suite of Integrated Radio Frequency Countermeasures (ALQ-211 SIRFC) for radar warning, RF jamming, electonic warfare management, and increased situational awareness.
While overall the MV-22 provides reduced susceptibility compared to the CH-46 and CH-53, there are areas of concern. The MV-22 incorporates an existing radar-warning receiver (RWR), the AN/APR-39 in common with other operational rotary-wing aircraft. Other testing has shown that this GFE common RWR produces uncertainties regarding the angle of arrival of some threat signal information. As a result, this was an area waived by the CNO for OPEVAL. In addition, operational testing has shown that the amount of chaff and flares carried by the MV-22 is inadequate to counter radar and/or infrared-guided threats in a typical threat scenario. During numerous missions conducted on both an open-air threat range as well as in simulated threat environments at the Air Combat Environment Test and Evaluation Facility (ACETEF), an anechoic chamber with sophisticated threat simulation capabilities, the MV-22 dispensed all of the chaff and flares ingressing through threat areas, leaving the aircraft with none for the egress. Since the MV-22 can be expected to often fly through areas with an extensive MANPADS threat, additional flares are needed. If exposure to more sophisticated radio-frequency (i.e., radar-guided) threats is anticipated, additional chaff capacity would also be required.
The Osprey retains the space and power provisions to integrate a turretted nose gun in the future. This would give the V-22 a significant defensive capability.
The V-22 must be resistant to flight critical damage imposed by hits in vital areas by 12.7 millimeter (mm) Armor Piercing Incendiary (API) (threshold) projectiles and by 14.5 mm API projectiles (objective) at 90 percent of their respective muzzle velocities (USMC KPP). Greater levels of ballistic hardening/tolerance are desired and should be incorporated if achievable without significant degradation of aircraft performance or cost penalties. Protection of crew and electro-optical sensors from low to medium powered lasers is desired (Objective). The 12.7mm and 14.Smm API projectiles are fired from heavy machine guns that are normally vehicle-mounted or emplaced. The 23mm and 30mm projectiles are fired from vehicle-mounted anti-aircraft artillery that can fire a mix of Armor Piercing Incendiary (API) and High Explosive Incendiary (HEI) rounds.
Wherever possible, redundant systems have been used to ensure mission success. The tiltrotor design separates critical componants such as engines and transmission lessening the possibility of dual failures. The cockpit seats are armored for protection against 7.62 mm armor piercing rounds.
An onboard inert gas generating system (OBIGGS) supplies nitrogen-rich air to the wing and sponson tanks as fuel is depleted. The inert gas displaces fuel vapor and reduces the possibility of fire. The lower one-third and appropriate walls of each tank are self-sealing to a 12.7 mm AP threat.The fuel tanks are made of a lightweight synthetic rubber with is highly extensible and has high tensile strength. They are designed to meet a drop test requirement of 65 feet when filled with water. An emergency lubrication system allows thirty minutes of operation at cruise power in the advent of a loss of lubricant.
More than 43 percent of the V-22 airframe structure is fabricated from composite materials. The proprotor blades are also made of composites. These structures are fatigue resistant and damage tolerant, features particularly desirable for ballistic survivability. Spaces occupied by personnel are designed for protection against liquid and vapor intrusion by a hybrid combination of over-pressurization and filtration. Lightning and electromagnetic (E3/EMI/RFI) protection is provided by shielding and hardening the aircraft electrical and electronic systems to prevent failure due to external power interference.
Structural damage tolerance requirements in the V-22 system specification require that the V-22 airframe be ballistically tolerant and that destruction of any single frame member not kill the aircraft.
An interconnecting driveshaft allows the Osprey to continue flying in the advent of an engine failure. Either engine can power both proprotors, although with reduced performance. The drive train subsystem is comprised of two proprotor gearboxes (PRGB), two tilt-axis gearboxes (TAGB), one mid wing gearbox (MWGB), an interconnect drive train, and an emergency lubrication system (ELS). The primary purpose of the drive system is to distribute engine power to the two proprotors, which generate lift and thrust. The drive system enables power distribution to the proprotors during all engines operating (AEO) and one engine inoperative (OEI) conditions.
Under normal operating conditions, each proprotor gearbox is powered by the nearest engine via the engine output shaft. In the event of engine power loss, the proprotor gearbox associated with the failed engine receives power from the opposite engine through the interconnect drive system. A sprag-type overrunning clutch between the engine output shaft and the helical input gears overruns so that the failed engine will not be back driven by the PRGB.
A secondary function of the drive system is to distribute and deliver engine power to the various systems and components required for flight and mission completion. The Interconnect Shafting System ISS is identical physically and functionally on each wing. The ISS transfers power to the TAGBs and the MWGB at all times and to the opposite PRGB during OEI conditions. The pylon-mounted drive shaft mechanically links the PRGB and the TAGB, while the wing-mounted drive shaft connects across the aft cove area of the wing, through the mid-wing gearbox, to the opposite nacelle's TAGB.
The AE 1107C engine and installation in the V-22 incorporate numerous vulnerability reduction features. All engine casings are designed to contain a single blade, failed at maximum continuous speed. The volume of fuel contained in engine components and lines is minimized through the use of the FADECs. The AE 1107C Tube design provides a 5-minute run-dry capability to allow a safe transition to One Engine Inoperative (OEI) operation. All sensors are duplicated and separated to provide the FADECs with continuous monitoring of engine performance. In addition, the engine compartment contains a single-shot fire suppression system to prevent uncontrolled engine fires.
Three independent hydraulic subsystems provide control power to the digital fly-by-wire flight control actuators. Two hydraulic subsystems (PC 1 and PC 2) are dedicated to flight control functions. The third subsystem (Utility/PC 3) provides power for the utility functions described above and to two flaperon actuators and one elevator actuator, in addition to serving as a back-up to the other two hydraulic systems for the swashplate and pylon conversion actuators. Each of the three subsystems is designed to provide 5000-psi system pressure.
Hydraulic system lines are separated to preclude single-shot ballistic defeat of all subsystems. Lines are routed in the fuselage along cabin walls, under floor, ceiling and frames. System separation in the wing and nacelle installations is achieved by routing lines along the forward spar in the leading edge area, along the aft spar in the wing cover area, and inside the wing torque box.
Ramp system hydraulic pressure is required for ramp operation. A ballistic perforation of any Utility/PC 3 hydraulic line or other normally pressurized component will result in insufficient ramp system pressure to lower the cargo ramp. In this situation, the ramp cannot be lowered by any means, i.e., there is no emergency back-up. The V-22 Hydraulic system is computer controlled with specific logic steps taken based upon three data inputs. The Flight Control Computers (FCC) constantly monitor system pressure, reservoir fluid levels, and rates of change in reservoir fluid level to detect hydraulic leak or loss of subsystem pressure. If a problem is detected, specific corrective action steps are directed to the Local Switching Isolation Valve, and Remote Switching Valve, based upon these inputs. If a problem is detected, the Local Switching Isolation Valve and Remote Switching Valve limit the amount of fluid lost. The V-22 hydraulic leak detection system response time is about 0.3 seconds. This means that most hits on the hydraulic system should be isolated before a quart of fluid is lost.
Traditionally, autorotation is a required air-worthiness capability for military rotorcraft. High rotor disk loading and low rotor inertia places V-22 well outside the nominal autorotation envelope of existing rotorcraft. Basic rotorcraft engineering analysis indicates that the V-22 will have a difficult time achieving a stable autorotation following a sudden power failure at high power setting, and that the probability of a successful autorotational landing from a stable autorotative descent is very low.
The V-22 Osprey combines the flight regimes of both helicopters and high-speed turboprop fixed wing aircraft. The need to make the Osprey crashworthy within this expansive flight envelope led to many unique features. The basic layout of a tiltrotor puts the large mass items such as engines and transmissions out away from the crew and cabin areas. In the event of a crash landing the occupants are not jeopardized by these items entering the occupied areas.
In the event of a crash landing the wing is designed to fail outboard of the wing/fuselage attachment. This "mass shedding" absorbs kinetic energy from the crash. Otherwise, this energy must be absorbed by the landing gear, structure or the occupants. In the nose of the aircraft is a very strong structure called the antiplow bulkhead. This reduces plowing, or "digging in" of the nose during a crash. The forward fuselage is designed to absorb crash forces of 4 g upward and 6 g rearward. The tricycle landing gear on the V-22 is designed to absorb a high sink rate of 24 feet per second (0.4 ft/min, 7.3 m/s)
The cockpit and cabin structure is designed to be 15 percent stronger than the wing's failure load. The overhead wing design also makes the occupied areas inherently stronger and safer in a roll-over or inverted impact. The occupied cabin area is designed to maintain 85 percent of its volume during a crash. The cockpit and cabin seats stroke vertically to absorb crash energy. Each seat has a restraint harness. The design of the cockpit has minimized the number of head strike hazards.
In the event the V-22 must land with it proprotors in the horizontal or cruise position, the occupants are protected from flying proprotor shards. The blades simply fray into individual strands that pose no harm to the occupants.
The cargo area of the V-22 is designed to keep cargo secure during a crash landing. The system has been designed to withstand loads of 16 g forward and down, 5 g aft and up, and 10 g laterally. Crash induced cargo displacement should not impede access to emergency exits. The V-22's fuel system meets MIL-STD-1290 and is designed to prevent fuel spillage and post-crash fires. The system includes self-sealing breakaway valves, steel overbraid high elongation fuel hoses, and a frangible attached vent system.
There are eight exits points in the V-22. In addition to the crew door and the hydraulically activated ramp there are three pyrotechnically released panels in the cabin. There is also a manually released maintenance access hatch located overhead in the aft fuselage which also serves as an emergency access point. Two pyrotechnically released side canopy windows provide emergency escape points for the cockpit crew. The emergency exits will permit evacuation of the flight crew within 30 seconds and all passengers within 60 seconds using half of the aircraft exits.
The V-22 can withstand a ditching of 6 feet per second sink rate and up to 30 knots forward speed. Emergency landings can be made in sea states up to two (2 foot wave heights). After landing, the aircraft can remain upright for 10 minutes at sea state five (8-12 foot waves, wind speed of 28-40 knots).
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