Airplanes have revolutionized long-distance transportation allowing people to cross continents and oceans quickly, safely, and relatively cheaply. They have changed the meaning of warfare and were an important stepping stone into space. However, conventional aircraft require long runways for both take-off and landing are classified as CTOL ( Conventional Take-off or Landing).
Sometimes long runways aren’t available to use and there is a need for an aircraft to use short or no runways at all. Vertical take-off and landing VTOL technology means aircraft can theoretically take off and land almost anywhere, making them far more flexible. They’re also able to perform various manoeuvres not possible with a conventional plane; a significant advantage for aircraft in combat situations. What’s more, VTOL aircraft, such as drones, that use electric motors are more energy efficient than those using jet engines.
They are highly useful for operations in congested places. Uber is working with manufacturers to meet its goal of starting a flying ride-hail service in Dallas and Los Angeles by the end of 2023. To remove the need for runways, these cars will rely on vertical take-off and landing technology, known as VTOL. Munich-based aviation startup Lilium aims to offer an on-demand flying taxi service that it claims will be five times faster than travelling in a car. Plane builder Airbus is tackling technical and legislative details with Vahana, its flying car project.
Rolls-Royce has confirmed it’s working on a flying taxi. The Back To The Future-style vehicle will be capable of both vertical take-off and landing, bypassing the need for a runway. The hybrid flying taxi will transport five passengers at speeds of 250mph (402 km/h) for up to 500 miles (800 km), the company said. Rolls-Royce believes its upcoming vehicle will fly paying customers over congested roads from early next decade.
NASA has developed the battery-powered GL-10, which take off and land vertically but flies efficiently like a conventional plane. The current prototype is a two-seater aircraft shaped like a conventional plane that uses a VTOL system. VTOL technology is increasingly making its way into more aircraft, from small recreational drones up to the traditional systems in helicopters and military jets.
One of the best knows V/STOL capable aircraft are: Harrier Jump Jet, Bell-Boeing V-22 Osprey, AW609, F-35B. Bell V-280 Valor, Yak 141, XFY-1 Pogo and Fairey Rotodyne. The F-35B Standard Take-off and Vertical Landing (STOVL) is a single-engine, fifth generation fighter aircraft designed and developed by Lockheed Martin. It is the first aircraft to combine stealth technology with STOVL capabilities and supersonic speeds. The tricycle type retractable undercarriage design allows the aircraft to take-off from and land on the deck of small naval ships, unimproved airstrips, rough airfields and roads.
VTOL/STOL aircraft technology
Vertical takeoff and landing (VTOL) aircraft include fixed-wing aircraft that can hover, take off and land vertically, as well as helicopters and other aircraft with powered rotors, such as tiltrotors. The helicopter’s spinning rotors create thrust like a large propeller that is directed vertically, enabling it to lift off. While in flight, a slight tilt in the desired direction pushes some of the aircraft’s thrust and sends the craft forward.
The gyrodyne is also known as a compound helicopter because it has the powered rotor of a helicopter, but a separate forward thrust system. On the cyclogyro, the rotary wing’s axis and surfaces remain sideways across the airflow, similar to a conventional wing.
A tiltrotor is an aircraft that uses a pair tiltrotors mounted on rotating engines at the end of a fixed wing to generate vertical and horizontal thrust. It combines the vertical capability of a helicopter with the speed and range of a fixed-wing aircraft. For vertical flight, the rotors are angled so the plane of rotation is horizontal, like a helicopter. As the aircraft gains speed, the rotors are tilted forward, with the plane of rotation eventually becoming vertical. The wing then provides lift, and the rotor provides thrust like a propeller.
Powered-lift aircraft takeoff and land vertically but behave differently than rotorcrafts while in flight. They often have a fixed-wing design. To take off or land vertically, the powerful exhaust streams from a jet engine can be directed downward as well as backward, and their direction can be changed in mid-flight. This allows fixed-wing aircraft, such as the Harrier or the F-35B, to take off vertically, fly forward, stop in mid-air, back up, and land vertically. They can also take off and land like a normal airplane. A helicopter’s spinning blades create thrust like a large propeller, but the thrust is directed vertically. This allows the vehicle to take off and land vertically and to hover. To move forward, the helicopter tilts slightly to direct some of its thrust forward.
The tailsitter aircraft sits vertically for takeoff and landing, but, once in the air, the whole craft tilts forward to achieve horizontal flight. The convertiplane relies on the rotor for lift when taking-off, but then switches to a fixed-wing lift while in flight.
Some VTOL aircraft can operate in other modes as well, such as CTOL (conventional takeoff and landing), STOL (short takeoff and landing), and/or STOVL (short takeoff and vertical landing). Others, such as some helicopters, can only operate by VTOL, due to the aircraft lacking landing gear that can handle horizontal motion.
Military requirements of S/VTOL Aircrafts
The main problem in contemporary aviation today is that a jet fighter requires a good runway however there are many situations where good runways are no available such as urban canyons, dense forests, high-elevation mountainous terrain, and/or off boats, ships, and offshore platforms with limited open deck area. In addition aircraft hangers are one of the first targets on any adversary attack. The use of VTOL technology would allow aircrafts to operate fron such inaccessible areas as well as enhance their survivability from first-strike attack.
For instance,VTOL aircrafts shall also be necessary for countries like Taiwan against adversary like China. China is developing large number of fighter aircraft, surface-to-surface ballistic missiles, land-attack cruise missiles and bomber aircrafts that could pulverize Taiwan’s air bases within hours of a war, according to RAND report.
None of Taiwan’s fighter aircraft would survive or be deployable on runways turned into a lunar landscape. The Ministry of National Defense said on 16th March 2017 in “Taiwan’s Quadrennial Defense Review” (QDR) that the country plans to acquire stealth fighters and vertical/short takeoff and landing (V/STOL) aircraft to strengthen its defense forces, particularly as China has announced it will increase its military budget 7 percent to a reported US$147 billion this year.
This technology has advantage that you can always operational, can be in place a small landing can be in building or any other ground easily camouflage. On the battlefield survival rate is the highest. Followed by the aircraft carrier can be equipped with a large number of landing and landing aircraft, for sea warfare is a great advantage. The last point is eliminating the preparation time on the runway, operational response time greatly improved.
The VTOL design would enable strike aircraft to be deployed even aboard small aircraft-carrying cruisers, perhaps even foregoing the need to build a new, expensive, conventional aircraft carrier. “With the fielding of the F-35B, the Navy almost doubles its theoretical “first day of war,” fixed wing capable, carrier force. This means that more ships capable of operating high-performance, low-observable, multi-role fighters, can be in more places at a single time,” writes Tyler Rogoway. Customer orders of the F-35B include USMC (340) and Italian Navy (22). The UK ordered two F-35Bs in 2009. The third F-35B was ordered by the UK Government in January 2010. The UK Ministry of Defence plans to procure up to 138 F-35Bs for the Royal Air Force and the Royal Navy.
“Conventional fighters can carry out combat missions in conditions of partially destroyed airfield infrastructure from shortened airstrips of less than 500 m,” the analyst noted. “The military’s plans to build a carrier fleet is something else entirely, however. There, the use of VTOL aircraft would indeed be highly rational.”
VTOL Aircraft are also more suited to future Urban Warfare. Future battlefield is also increasingly urban. 80% of the world’s population will live in urban environments by the year 2030. Urban Warfare poses unique challenges because of Physical constraints like size, weight, and maneuverability, Wind tunneling effects, Line-of-sight obstructions and Public intolerance for collateral damage. While conventional Airplanes can only fly in straight lines, forward only, VTOLs have much more control to move forward, backward, and up and down, spin around, etc which may be able to better counteract turbulence and wind.
Transformable Multirotor/Fixed-Wing sUAS announced in 2020
Auterion Government Solutions Inc (AGS Inc) and Quantum-Systems GmbH have announced that the two companies have partnered to develop a new combination multicopter/fixed-wing small unmanned aerial system (sUAS) for the U.S. Government defense and security markets. The new drones will be based around Quantum-Systems’ Vector and Scorpion drones and the Auterion OS secure open-source operating system. Auterion OS has already been employed on a variety of unmanned aircraft, including small multi-rotors and hybrid VTOL Group 2 aircraft.
The Vector and Scorpion feature a modular construction with a configurable base fuselage. Scorpion is a tri-copter UAS that can be utilized in dynamic urban environments when a combination of maneuverability and hovering capabilities are required. It is ideal for the gathering of ISR (intelligence, surveillance and reconnaissance) and situational awareness data, and can be fitted with an optional tethering system to enable 24/7 operations.By adding fixed wings and a tail section to the base fuselage, Scorpion can be transformed into Vector, an energy-efficient, fixed-wing VTOL UAS for longer range, longer endurance ISR missions.
The two drones currently utilise Quantum-Systems’ proprietary flight control stack and qBase command and control software. For this new development they will be integrated with Auterion Enterprise PX4 software, Auterion Ground Station software, and the Auterion Hand-Held Ground Control Station (H-GCS), forming an open ecosystem that is aligned and integrated with the U.S. Department of Defense’s Group 1 UAS Architecture and requirements for a common Group 1 control system. This integration enables these sUAS to be extensible, tailorable and interoperable for customers in both the US Defense and Security markets.
David Sharpin, CEO of AGS Inc, commented: “We are excited to be working with Quantum-Systems to bring forth a new, integrated, rucksack portable sUAS that we feel will transform the way our customers collect, process and disseminate ISR and Situation Awareness information, in all environments. AGS Inc and Quantum-Systems are standing up manufacturing operations in the US and are utilizing assured subsystems and software to provide a secure solution offering for our customers.”
“Everything is a kit that you can put together in the field without any tools, and it can be assembled and ready to launch in under five minutes,” he said. “That’s important for soldiers in the field today and into the future.”
F-35B Lightning II Joint Strike Fighter STOVL Variant, United States of America
However, the latest F-35B STOVL powered by a single Pratt & Whitney F135 afterburner turbofan engine rated at 125kN of dry thrust overcomes some of the deficiencies. The engine can produce 191.3kN of thrust afterburner. The aircraft is fitted with a refuelling probe on the right side of the front fuselage to carry out mid-air refuelling during combat missions. The weapon loads and cockpit layout of the F-35B are similar to those of the F-35 Joint Strike Fighter (JSF).
The F-35B Standard Take-off and Vertical Landing (STOVL) is a single-engine, fifth generation fighter aircraft designed and developed by Lockheed Martin. It is the first aircraft to combine stealth technology with STOVL capabilities and supersonic speeds. The F-35B will supersede the F/A-18 Hornet and AV-8B Harrier II fighter aircraft currently in service with the United States Marine Corps (USMC). It will also replace GR7, GR9 and Sea Harrier aircraft deployed in the fleet of the Royal Navy and Royal Air Force.
The aircraft is fitted with a refuelling probe on the right side of the front fuselage to carry out mid-air refuelling during combat missions. The weapon loads and cockpit layout of the F-35B are similar to those of the F-35 Joint Strike Fighter (JSF).
The F-35B STOVL is powered by a single Pratt & Whitney F135 afterburner turbofan engine rated at 125kN of dry thrust. The engine can produce 191.3kN of thrust afterburner. It is the successor of F119-PW-100 turbofan engine. It is equipped with full authority digital engine control, a gearbox, and health and usage monitoring system.
A shaft-driven LiftFan propulsion system built by Rolls-Royce is incorporated at the aft of F-35B’s cockpit to accomplish the STOVL capabilities. Doors fitted above and below the vertical fan open as the fin spins up for vertical lift of the aircraft. The counter-rotating LiftFan produces more than 20,000lb of thrust with the help of the gas turbine.Three-bearing swivelling exhaust nozzle is appended by two roll control ducts on the inboard section of the wing. The engine combined with the vertical LiftFan renders the requisite STOVL capability. The length and diameter of the engine are 5.5m and 1.3m respectively. The inlet diameter is 1.1m.
The F-35B can fly at a maximum speed of 1,960km/h. The combat radius and maximum range of the aircraft are 833km and 1,667km respectively.
The F-35B is fitted with a 25mm GAU-22A Gatling cannon which has 220 rounds per gun of firing capacity. It has two internal weapon pods and four external underwing hardpoints to expand its mission lethality.
The aircraft can carry 6,803kg of weaponry payload. It is equipped with AIM-120C AMRAAM medium range air to air missiles, air to surface missiles, two GBU-32 JDAM guided bombs, six GBU-38 bombs and munitions dispensers.
Countermeasures and Radars
The F-35B is equipped with AN/APG-81 Active Electronically Scanned Array (AESA) multi-functional radar built by Northrop Grumman. It also houses AN/AAQ-37 Distributed Aperture System (DAS), Barracuda AN/ASQ-239 electronic warfare system, Multifunction Advanced Data Link (MADL) communication system and missile warning system.
Russia developing a new vertical-takeoff aircraft—and new carriers to launch them.
Russia is developing a prototype of a completely new vertical take-off plane on the instruction of Russian President Vladimir Putin, Deputy Prime Minister Yuri Borisov said in August 2018. “This work has, indeed, been included in the state armament program and is being conducted on the instruction of the supreme commander-in-chief. Now work is underway to develop conceptual models and prototypes,” the vice-premier said. Then Yuri Barisov, deputy prime minister for defense, said Russia had been developing a VTOL aircraft since 2017, may lead to a flight of the first experimental prototype in 2022-2023 and the launch of the machine in series production in the late 2020s.”
This work is focused on developing a completely new aircraft rather than creating a plane based on some existing model, Borisov stressed. “No doubt, this is the future for all aircraft-carrying ships and a new fleet of airplanes will be needed and precisely for this purpose various technologies are being used to provide for shortened take-off and landing or simply a vertical take-off,” the vice-premier said.
“Conceptually, such work has been carried out at the Defense Ministry since last year,” the deputy prime minister said. “The timeframe is defined by the technological cycle of the [plane’s] creation. Normally, this takes 7-10 years, if this goes into serial production,” the vice-premier added.
Denis Manturov, the head of the Russian Ministry for Trade and Industry, said Russia would not build helicopter carriers. “I should only note that the shipbuilding program envisages construction of universal landing craft whose landing facilities (helicopter wing, landing craft, and floating armor) are supposed to be placed inside the ship hull or on the ship deck,” he said.
Russia built two Moskva-class helicopter carriers in the 1960s, and then Kiev-class carriers flying vertical short takeoff and landing (VSTOL) aircraft. The Admiral Kuznetsov has been the only Russian carrier able to launch and recover regular fixed-wing aircraft as the U.S. Navy does.
Chinese forums reported on 25 April 2011 that Beijing had probably began testing new J-18 Red Eagle short takeoff and vertical landing fighter aircraft. China has successfully tested its vertical takeoff and landing fighter during field test was reported by Japan in 2016.
But China might still need some time before a STOVL fighter jet is built, Chinese military observers said in June 2019. If China was to develop such aircraft, it needs to make reliable engines that are capable of not only providing thrust, but also lift, together with lift-providing devices in the middle of the aircraft, like lift fans, Wei Dongxu, a Beijing-based military analyst, told the Global Times 16 June 2019, noting that flight control systems could also be very complicated.
Once developed, the STOVL fighter jet could make an amphibious assault ship a light aircraft carrier and play unique roles in protecting islands and safeguarding China’s territorial integrity. Secessionist forces in Taiwan are still taking the chance that the mainland’s amphibious combat capabilities could be insufficient to reunify Taiwan by force. But once the mainland has amphibious assault ships with STOVL warplanes, it may become a strong deterrent against Taiwan secessionists and help solve questions involved with Taiwan, analysts said.
Challenges of VTOL/STOL aircrafts
They also have disadvantage some deficiencies: first is the fuel consumption is too large, take-off requires a third of the amount of oil consumed, so on the voyage is greatly reduced. Second is the bomb load small. Take for instance the British harriers, the warplanes of the bomb load only 2271 kg is one-third of the conventional aircraft, but also unable to truly vertical take-off and landing, it is better to develop conventional aircraft. The last point is that the operation is too difficult, the requirements of the pilot is very high, bad training, the accident rate is very high, very much time and money. Oleg Panteleev, editor-in-chief of Russian aviation news agency Aviaport.ru, said that VTOL fighters’ heavy consumption of fuel on takeoff, combined with the flexibility of traditional aircraft designs, makes fielding a large fleet of air force VTOL fighters impractical.
The advantages of vertical lift, however, can be offset by some key challenges, such as poor efficiency, payload limitations and low endurance. VTOL systems use rotors to generate lift and thrust, requiring more power compared to fixed-wing aircraft, which only require a propulsion system, as lift is generated by its wings.
“The most efficient vertical landing platform is a single rotor helicopter, but that’s also the most fragile system because one defect in that rotor can become a catastrophic failure,” David Arterburn, a UAS expert and director of the Rotorcraft System Engineering and Simulation Center at the University of Alabama, Huntsville noted. “That said, we’re seeing designs in the future vertical lift community to make VTOL more capable than it’s ever been.”
As part of its “what’s next” approach, Lieutenant Colonel Ellis Smith leads the modernization effort of future unmanned aircraft systems for Army Futures Command. Vertical lift is a key priority. “VTOL capability is going to be a really big advantage for the soldier in the future so that they’re not relying on systems that are tied to an airfield,” Smith said. “Operationally, runway independence really changes the predictability of where we’re going to deploy these systems from. When you bring in VTOL, you no longer have to worry about some of the constraints that put our soldiers in harm’s way.”
U.S. Army looks to use silent technology for next-generation aircraft
The U.S. Army has announced that its researchers in cooperation with Uber research lab are working on silent and efficient VTOL, or vertical takeoff and landing operation, for the next generation fleet of Army air vehicles. Currently, stealthily moving of troops and supplies is Army modernization priorities for future vertical lift aircraft. The U.S. Army Combat Capabilities Development Command (DEVCOM), Army Research Laboratory, researchers collaborated with Uber and the University of Texas at Austin to investigate the acoustic properties of electric vertical takeoff and landing aircraft, which use distributed electric propulsion to power flight.
According to a recent team’s paper published in the Vertical Flight Society 76th Annual Forum Proceedings, these eVTOL vehicles may aid the Army with important tasks such as aerial surveillance and cargo transport; however, they feature smaller rotors than traditional helicopters. As a result, eVTOL rotors may emit a different sound signature that researchers will have to take into consideration. Initial experimentation of this concept has revealed the potential for stacked co-rotating rotors be significantly quieter than traditional paired rotor approaches and improve performance for a flying craft. To date, stacked co-rotating rotors have not been deployed in existing flying craft.
“The noise you hear from these smaller rotors is generated through fundamentally different physical mechanisms,” said Dr. George Jacobellis, Army research engineer at the laboratory’s Vehicle Technology Directorate. “Traditional modeling techniques need to be improved to account for all of the noise generated so that vehicle designers can be aware of what will actually be heard.” Standard helicopter noise simulations focus primarily on predicting thickness noise and loading noise, because they constitute the dominant noise sources for large helicopters. Thickness noise stems from the displacement of the air by the rotor blades, while loading noise occurs when lift and drag forces act on the air that flows around the rotary wings. Together, they make up what experts refer to as tonal noise.
In contrast, Army researchers suspected that eVTOL rotors generate more broadband noise, which refers to sounds caused by turbulence, than tonal noise. “We didn’t know whether broadband noise was important or not, but we knew that the tonal and broadband noise scaled differently,” Jacobellis said. “We thought that as rotors became smaller there would be some point at which broadband noise would be the dominant source.” The team confirmed their hypothesis in their research study, which not only measured the acoustic characteristics of various eVTOL rotor configurations but also assessed the modeling capabilities of helicopter noise simulations for eVTOL rotors.
During the field experiment, the researchers set up a test stand with two electrically-powered rotors and recorded the noise generated above and below the rotor plane with nine microphones placed in a circular array surrounding the rotor hub. For the simulations, the team utilized the Rotorcraft Comprehensive Analysis System coupled with a separate program called PSU-WOPWOP, a routine noise prediction code named after an onomatopoeia for the sound that helicopter blades make.
“RCAS computes the aerodynamic loads, or forces, on the blades as well as the bending and twisting of the blades,” Jacobellis said. “This information is necessary to use as inputs to PSU-WOPWOP, which computes the noise generated by the rotor. Connecting the two programs required a significant amount of work, which was done by our group as well.” The researchers modeled the broadband noise in PSU-WOPWOP with Pegg’s method, one of the two settings present in the simulation program with the other called Brooks’ method. While Pegg’s method makes broadband noise predictions based off of experimental data of the entire rotor, Brooks’ method accounts for the unique distribution of lift along the blade.
“[These technologies] are promising in their ability to predict the noise of eVTOL rotors, but we need to do more work to obtain acceptable accuracy,” Jacobellis said. “The next steps are to implement the Brooks method for higher accuracy acoustic predictions and to compare the unsteady loads between the simulation and experiment to check the accuracy of the simulations in predicting the unsteady loads. The Brooks method should be better at capturing unique configurations and loading distributions.”
Researchers also found that co-axial co-rotating rotors, or stacked rotors, may potentially offer better performance and lower noise than a conventional rotor. Unlike conventional rotors which employ blades arranged in a single plane, stacked rotors place blades in multiple planes. According to the results of their study, stacked rotors with equally spaced rotor blades produced the lowest noise level, around the same level as that of a traditional rotor. By investigating different values of axial spacing, Army researchers believe that they may uncover a stacked rotor configuration that produces lower noise than the conventional rotor. “I do think that a stacked rotor can be beneficial for eVTOL applications,” Jacobellis said. “The added degree of freedom for the design will allow for gains in efficiency and control over the acoustic signature, which has been shown in the results. More investigation is needed, however, to quantify the noise reduction with axial spacing.”