On December 6, 2023, Japan’s "Sankei Shimbun" reported: US President Biden issued a statement on the 5th saying that all 8 people on the CV-22B "Osprey" transport plane that crashed in the waters near Yakushima, Kagoshima Prefecture, Japan on November 29 have died. The US side will continue to collect relevant information and conduct a thorough investigation into the cause of the crash.
The "Osprey" transport aircraft, F-35B stealth carrier-based aircraft, "Zumwalt" class destroyer and littoral combat ship are all products of the "from sea to land" strategic transformation proposed by the US military in the 1990s. When these four equipment first came out, they did give people a refreshing feeling. But as time went by, they have been stumbling and troubled since the finalization test, and they quickly fell from the altar due to poor performance after entering service. Take the Osprey transport aircraft as an example. As of now, as many as 17 of them have suffered "irreparable losses" in accidents. Why is this?
Limitations of traditional helicopter layout
Helicopters, which emerged at the end of World War II, first appeared on the battlefield on the Korean Peninsula in the early 1950s. In the Vietnam War, the large-scale use of helicopters gave the US Army wings to a certain extent, enabling it to evade the ambush circles set up by the Vietnamese army along the highway through air maneuvers and implement the "leapfrog" tactic.
In several local wars that followed, helicopters played an increasingly important role, and even triggered a heated debate in the military on whether to abandon the traditional ground assault force centered on tanks and armored vehicles and rebuild a "flying army". In the end, the debate was largely ended by the shortcomings of the helicopters themselves. Among them, the most prominent is that its flight speed and maximum range are not enough to support revolutionary changes in the army’s combat mode.
Take the US Marine Corps, known as the "army within the army", as an example. Before the MV-22 Osprey entered service, the main model CH-46 "Sea Knight" responsible for personnel transportation had a maximum flight speed of only 267 kilometers per hour. When performing assault landing missions, its combat radius was only 163 kilometers, so the landing forces could only be deployed near the beach. The amphibious assault ship for the CH-46 "Sea Knight" to take off and land cannot be too far from the beach, so it is difficult for the landing forces to use the vertical mobility in the air to seize the key positions in the enemy’s defense depth, and the amphibious landing fleet has to be exposed to the threat of enemy shore fire for a long time.
Why the flight speed and maximum range of the helicopter cannot be increased, which is closely related to its basic flight principle.
The conventional layout of a helicopter is to install a horizontal main suspension wing on the top of the fuselage, and a long tail boom extends from the rear of the fuselage, on which a vertical tail boom is installed. The function of the vertical tail boom is to overcome the reaction deflection torque generated by the horizontal main rotor on the fuselage.
The lift of a conventional layout helicopter is entirely provided by the horizontal main suspension wing. The pulling force that pulls the helicopter forward is also provided by the horizontal main suspension wing. Therefore, when the helicopter is flying in the air, the horizontal main suspension wing is not actually parallel to the sea level, but must be tilted forward at a certain angle. In this way, the component force generated by the main suspension wing in the vertical direction is used to overcome the gravity of the helicopter, so that it can hover in the air without falling; the component force generated by the main suspension wing in the horizontal direction is used to pull the helicopter forward.
Since the horizontal main suspension wing of a conventional layout helicopter can tilt forward at a limited angle, when the helicopter needs to fly at high speed, the fuselage must also tilt forward to allow the main suspension wing to obtain a larger tilt angle. However, the nose sinks and the tail rises, which will greatly increase the windward area of the helicopter and increase the resistance. Not only that, when the main suspension wing of a conventional layout helicopter rotates, the component force generated in the horizontal direction is not all the pulling force that pushes the helicopter forward. When the suspension wing rotates from the rear of the fuselage to the front of the fuselage (called "forward" in jargon), the component force generated in the horizontal direction is completely opposite to the forward direction of the helicopter, which is actually equivalent to resistance. When a conventional layout helicopter is flying forward, the main rotor rotates from the front of the fuselage to the rear of the fuselage (called "reverse movement" in jargon), and the relative speed of the blades to the still air is low, and the efficiency is greatly reduced. In severe cases, it may cause stalling.
In order to reduce the resistance when moving forward, the main suspension wing of a conventional layout helicopter needs to reduce the pitch of the blades in the forward stage. In the reverse stage, the pitch needs to be increased. The so-called pitch is actually the angle of attack of the blades. The larger the angle of attack, the greater the force generated by the blades at the same speed. In this way, the left and right forces of the helicopter are unbalanced, and the helicopter can only be balanced by the horizontal tail deflection control surface on the tail beam to create additional resistance. This resistance is also the aerodynamic resistance of the helicopter. part. The continuous adjustment of the pitch causes the aerodynamic load of the blades to change continuously, so a flexible structure must be used to delay the occurrence of metal fatigue as much as possible.
Helicopters with conventional layout can use the lift imbalance on the left and right sides to achieve the left and right swing of the fuselage. The front and rear pitch of the fuselage is mainly achieved by the front and rear lift imbalance.
In summary, the inherent weaknesses of the conventional layout have generated a lot of additional resistance, and at the same time, the structure and control difficulty of the main rotor system have become more and more complicated. The basic size of the helicopter body limits the size of the power system and the main rotor. Even if there is a work-to-weight ratio A higher engine, when the main rotor diameter is limited, will increase the rotor speed, which may cause the wingtip of the main rotor to exceed the speed of sound, thereby generating a detached shock wave, which may cause the wingtip to stall, thereby sharply reducing the aerodynamic efficiency of the rotor. The strong downwash airflow generated by the main rotor forms a strong air curtain around the helicopter, which also greatly increases the resistance to forward flight.
In addition, because there are two rotor systems, the main rotor and the tail rotor, the aerodynamic shape of the conventional layout helicopter cannot be designed as smooth as that of a fixed-wing aircraft, resulting in increased flight resistance. Its engine power is distributed to the two rotor systems, the main rotor and the tail rotor, resulting in power dissipation and reduced efficiency. All of these are the causes of conventional layout helicopters. The reason why it is difficult to break through the speed of the aircraft.
Innovative new models
The Osprey transport aircraft is a hybrid of a traditional helicopter and a traditional transport aircraft. When it is in take-off and landing mode, the propellers at both ends of the wing are vertically deflected to provide lift. When it is in normal flight, it is a fixed-wing aircraft that uses turboprop power to provide thrust.
In the traditional sense, most helicopters install the engine and reducer above the fuselage. Even if 2 or 3 engines are used, they share 1 reducer, so the power system is quite compact. However, like the M The rotors of a tilt-rotor transport aircraft such as the V-22 Osprey are installed on the left and right wingtips of the fixed wing. If the engine and reducer are installed on the top of the fuselage like a helicopter, and the power is transmitted to the two rotors through two transmission shafts, then the transmission shaft must be always under high load. Moreover, if the transmission shaft that is more than 6 meters long wants to successfully transmit the output power of the AE1107C turboshaft engine up to 4590 kilowatts for a long time, it must be made extremely strong, and the vibration and noise problems caused by it must also be solved.
In this way, the weight of the transmission shaft will be high. Since the tilt-rotor is subjected to extremely complex forces during state conversion, this design will lead to low reliability of the power system. Once a problem occurs in any engine or transmission shaft, it will directly cause the wing tip of one end of the aircraft to lose power and lift, and the aircraft will overturn immediately. Not to mention that under this technical configuration, the aircraft power system has no emergency backup disposal means at all. Even if there is, it will be too late to react.
Therefore, after weighing the pros and cons, the Osprey transport aircraft adopts a technical configuration that installs the engine and reducer at the wingtips of both wings to directly drive the rotor. However, there is still a transmission shaft connected between the power systems of the two wings. This shaft is usually idle. Only when an engine fails, this shaft will be "commissioned in times of crisis" to divide a part of the output power of the other engine to replace the failed engine to drive the rotor, so that the aircraft can still maintain flight for a short time and will not overturn immediately, thereby buying precious minutes of emergency response time for the pilot to make an emergency landing.
In addition, since this connecting shaft can only be used in emergency response and does not need to work under load for a long time, this transmission shaft called the "intermediate coordination shaft" can be made lighter, thereby greatly saving the investment in reducing the weight of the transmission shaft, increasing its service life, and reducing shock and noise, clearing the way for the real practical application of the tilt-rotor aircraft.
However, the "Osprey" achieves lift-thrust conversion through the overall rotation of the power system located at the two wingtips. Because the rotor needs to provide forward thrust during taxiing takeoff, the power system must be tilted forward. To prevent the rotor from hitting the ground, the rotor diameter must be limited. Because the rotor diameter is small, to achieve the same lift, the rotor’s torsion angle can only be increased to increase the airflow speed, resulting in a higher rotor disk load.
The total propeller area of the MV-22 Osprey is 210.6 square meters, and the vertical takeoff weight is 21.55 tons. At this time, the propeller load is 0.1 tons/square meters, and the power-to-weight ratio is 0.186 megawatts/ton. The CH-47D with a similar takeoff weight has a total propeller area of 526 square meters, a takeoff weight of 24.5 tons, a propeller load of 0.047 tons/square meters, and a takeoff power-to-weight ratio of 0.144 megawatts/ton. In other words, the propeller load of the MV-22 Osprey is 2.13 times that of the CH-47, and the takeoff power-to-weight ratio is 2.19 times that of the latter.
If you only look at this set of comparative data, it will give people the illusion that the vertical takeoff and landing weight of tilt-rotor aircraft has great potential to be tapped. However, calculations show that when an aircraft using this technical configuration is in a hovering state, the aerodynamic efficiency of the large twist angle rotor is significantly reduced, which strictly limits the vertical takeoff and landing weight.
Not only that, the tilt-rotor aircraft adopts a transverse double-rotor aerodynamic layout, and its lift center and its own center of gravity are difficult to coincide, resulting in poor longitudinal stability of the aircraft, and a tendency to rise or fall. Moreover, because the main wing forms a certain degree of shielding for the rotor, when the helicopter’s forward flight speed is very low and the descent speed is high, it will fall into its own downwash airflow, which is very likely to cause the occurrence of a vortex ring state.
In the vortex ring state, the air will flow in a ring around the tip of the rotor blade, forming a vortex similar to a doughnut. The air pressure inside the vortex drops, which causes the rotor to lose some lift. If the pilot tries to compensate for the part of the lift lost due to the vortex by increasing the throttle and increasing the blade’s working angle of attack, the vortex ring movement will accelerate, causing the rotor to lose more lift, and the situation will become worse. If one side of the rotor enters the vortex ring state during flight while the other side works normally, the lift on the left and right sides will be unbalanced, and the aircraft will roll towards the rotor side affected by the vortex ring.
Of course, the above difficulties are not unsolvable. The most core solution is to use a high-performance flight control system and rely on the high level of flight control software to make up for the inherent deficiencies of the tilt-rotor configuration aircraft. The reason why the US military knows that the tilt-rotor configuration aircraft has these technical shortcomings, but is willing to take risks and insist on purchasing it, the most fundamental reason is that once the aircraft using this technical configuration enters the level flight state, its flight resistance is small, so both the maximum flight speed and the key indicator of load range are very attractive.
MV-22 Osprey transport aircraft has a maximum level flight speed of 509 kilometers per hour, and a mission combat radius of 722 kilometers. In other words, after replacing the CH46 Sea Knight with the MV-22 Osprey, the US Marine Corps’ aviation delivery speed is 1.91 times the original, and the delivery radius is 4.43 times the original. The amphibious assault ships equipped by this service can be relatively safely deployed at sea hundreds of kilometers away from the enemy beachhead, and calmly launch over-the-horizon attacks, thereby greatly improving the concealment and suddenness of the "from sea to land" operation.
In the field of non-traditional security, the speed and range advantages of tilt-rotor configuration aircraft are also difficult for helicopters to match. At 14:11 on April 25, 2015, an earthquake with a magnitude of 8.1 occurred in Nepal (28.2 degrees north latitude, 84.7 degrees east longitude). The U.S. Marine Corps’ MV-22 took off from Futenma Air Base in Okinawa with relief supplies, stopped at Clark Air Base in the Philippines and U-Tapao Air Base in Thailand, and then flew 2,200 kilometers directly from U-Tapao Air Base to Nepal. Traditional helicopters cannot perform such long-distance maneuvers anyway.
There are many problems
Since its launch, the public has been concerned about the flight safety and quality of the tilt-rotor transport aircraft "Osprey". And this aircraft with a novel configuration does not seem to be very competitive, and accident reports will make headlines every now and then.
In 1991, the fifth prototype of the "Osprey" transport aircraft crashed during its first flight.
1 On June 11, 1991, a prototype of the Osprey transport aircraft lost balance while hovering, and the left cabin hit the ground, causing minor injuries to two people.
In 1992, the fourth prototype of the Osprey transport aircraft crashed due to an engine fire during a flight demonstration for US congressmen, and all seven US Marines on board were killed.
On April 8, 2000, the fourth MV-22B "Osprey" delivered to the US Marine Corps crashed during a test flight due to pilot error, killing all 19 people on board.
On December 11, 2000, an MV-22 Osprey transport aircraft crashed due to a hydraulic system failure, killing four crew members on board.
In April 2012, an MV-22 Osprey transport plane crashed in Morocco, killing two US Marines
On October 2, 2014, an Osprey transport plane crashed into the sea shortly after taking off in the Persian Gulf, killing a US Marine.
In March 2022, an MV-22 Osprey transport plane crashed in Norway, killing four US soldiers.
In June 2022, an MV-22 Osprey transport plane crashed in the United States, killing five US soldiers.
In August 2023, an MV-22 Osprey transport plane crashed in northern Australia, killing three U.S. soldiers and injuring 20.
On November 29, 2023, a CV-22B Osprey transport plane of the U.S. military stationed in Japan crashed in the waters near Yakushima Island, Kagoshima Prefecture, Japan, and all eight U.S. Marines on board died.
According to big data statistics, of the 400 or so Osprey transport planes produced and delivered by Bell, 17 have suffered "irreparable losses" in various accidents so far, with a loss rate of about 5.67%. The total production of the UH-60 Black Hawk series helicopters is about 4,500, and the number of crashes so far is about 350, with a crash rate of about 7.78%. Of course, some of them were destroyed by enemy artillery fire on the battlefield, or other factors.
That is to say, in terms of the flight accident rate caused by aircraft failure, the Osprey and UH-60 Black Hawk are actually almost the same, or even lower. This is also the reason why the US military not only equips a large number of this aircraft, but also refits it as the US presidential plane. The poor safety record of the Osprey transport aircraft in the public’s mind is largely because this novel configuration aircraft will easily become the focus of media attention whenever there is a problem.
Not only that, tilt-rotor configuration aircraft have some unique advantages in terms of safety. On October 1, 2014, when an MV-22B took off from the amphibious assault ship "Makin Island", because the aircraft was set in "maintenance" mode, the flight control software failed to remind the crew in time, and the aircraft took off forcibly when the engine could only generate 80% of the power. Due to insufficient lift, it fell into the sea shortly after takeoff. Fortunately, the MV-22B cabin volume was large enough, and the reserve buoyancy was sufficient. The crew members reduced the weight by emergency fuel release, and finally the aircraft miraculously flew off the sea and returned to the "Makin Island". Traditional helicopters have never had such a "resurrection" case.
It is precisely because of this that the US military purchased a large number of "Osprey" transport aircraft under the pressure of public opinion, and developed various modifications to meet the different needs of various military services.
Currently, the Osprey transport aircraft family includes the V-22A basic model, the basic transport model MV-22B used by the US Marine Corps, the ship-borne transport model CM V-22B, the airborne early warning model EV-22, the naval anti-submarine model SV-22, and the naval combat search and rescue model MV-22B. Among them, the MV-22B is still under development. The MV-22B can be equipped with an aerial refueling kit to transform into a tanker. In addition to the US Air Force, Navy and Marine Corps, Japan also bought 17 aircraft to equip the First Helicopter Regiment of the Ground Self-Defense Force. In addition, the Israeli Air Force also bought 6 aircraft, and it is said that Indonesia is also interested in ordering 8 aircraft.
Solutions
Of course, the inherent technical defects of the Osprey series of transport aircraft have not been ignored by the United States. In 2009, the US Army launched the "Future Vertical Take-off and Landing Aircraft of the United States" (FVL) program. The program consists of two parts: the "Future Reconnaissance Attack Helicopter" FRAA project and the "Future Long-Range Assault Aircraft" FLRAA project. Among them, the FLRAA project requires the minimum maximum sustained cruising speed of the winning model to be 250 knots (463 kilometers per hour), and it is expected to reach 280 knots (518.56 kilometers per hour). This speed indicator is more than twice the maximum sustained cruising speed of general-purpose helicopters. In 2016, the US military further refined the requirements for the FLRAA project, requiring the winning aircraft to have a combat radius of 424-795 kilometers in high temperature and plateau environments, a cruising speed of 426-574 kilometers per hour, an internal load of 1587-1814 kilograms, and a hanging load of 2722-3629 kilograms.
More. In 2016, the U.S. military further refined the requirements for the FLRAA project, requiring the winning aircraft to have a combat radius of 424-795 kilometers in a high-temperature and plateau environment, a cruising speed of 426-574 kilometers per hour, an internal load of 1587-1814 kilograms, and a hanging load of 2722-3629 kilograms.
Such technical requirements can only be met by aircraft with a tilt-rotor configuration. So it is not surprising that the V-280 Valor tilt-rotor aircraft developed by Bell won the bid for the project in December 2022.
Of course, the Valor is not a simple copy of the Osprey, but has made many improvements based on the problems exposed by the latter in long-term use. For example, the engine of the Valiant does not have to rotate with the rotor like the Osprey, but the universal joint transmits power to the rotor, which can switch between level flight mode and hovering mode within 12 seconds, and can also fix the rotor at a 45-degree forward tilt angle to enable the aircraft to take off in a short distance. This design greatly reduces the burden on the steering mechanism, makes the engine’s working state more stable, and improves working reliability. Since the engine does not tilt with the rotor, the V-280 does not need to worry about the high-temperature exhaust gas discharged by the engine burning the flight deck during vertical landing.
In terms of aerodynamics, the Valiant replaces the H-shaped vertical tail of the Osprey with a pair of upward-inverted V-shaped tails. Under the condition of achieving the same projected area and the same stability, the upward-inverted V-shaped tail has a smaller wing area, is lighter, and has less aerodynamic resistance, and is conducive to increasing the aircraft’s rolling stability. Its main disadvantages are that the operation is more complicated and the yaw efficiency is relatively low. However, for modern flight control systems, there is enough confidence to make up for the above shortcomings.
In addition, the Hero abandoned the Osprey’s rear-opening springboard tail door design and opened a 1.8-meter-wide side door on each side of the fuselage to facilitate the 14 soldiers on board to quickly leave the aircraft. In terms of speed, load range and other indicators that the US Army values most, the V-280 has a maximum level flight speed of 560 kilometers per hour when unloaded and a maximum level flight speed of 520 kilometers per hour when manned. When performing different tasks, it has a combat radius of 930 to 1,480 kilometers and a maximum range of 3,900 kilometers. When carrying 4.5 tons of supplies, the maximum flight speed can reach 280 kilometers per hour. In these important indicators, the V-280 is a step up from its predecessor, the Osprey.
