Alper ÇALIK / a.calik@milscint.com
Significant developments are being experienced in the field of rotary wing aircraft technologies, especially in the US market, and many innovative designs are competing within the scope of different programmes. Of these, the programmes being conducted under the Future Vertical Lift (FVL) initiative aim to identify which vertical take-off and landing aircraft (VTOL) will be a part of the US Armed Forces inventory in the future. These programmes, which specify in their requirements aircraft with VTOL capability rather than conventional helicopters, allow the companies participating in the related tenders to develop atypical aircraft such as compound helicopters that incorporate new technologies. In this first part of our two-part article series, which discusses the FVL programme in detail, our focus is on the FARA programme under FVL.
There are various ongoing studies carried out by the private sector on the development of new generation VTOL technologies. However, these studies do not turn into state-sponsored R&D or procurement projects, probably because end users do not have a request in this regard. For this reason, such studies generally do not push the boundaries of the basic design of helicopters which is based on providing lift power from a single main rotor, and remain limited to relatively simple efforts such as introducing different geometries for the blade tips or optimising the aerodynamic form of helicopters.
In this respect, the United States’ FVL programme stands out as the most important state-supported technology development programme in the world. There are two programmes being actively executed under the umbrella of FVL. The programmes and the tasks to be undertaken by the vehicles to be developed under each of them are as follows:
- Future Attack Reconnaissance Aircraft (FARA): Armed Reconnaissance
- Future Long-Range Assault Aircraft (FLRAA): General Purpose / Assault
The latest development about both programmes was that the companies that qualified for the second phase of the programmes were announced in March. While the companies that made it to the finals in the FARA programme were Bell and Sikorsky, Bell and the Sikorsky-Boeing partnership was entitled to continue in the FLRAA programme. In this phase, the prototypes produced by the companies will compete. The helicopters to be manufactured under both programmes are expected to start entering the inventory in the period of 2028–2030.
New Reconnaissance Helicopter to Take on the Role of Veteran Kiowa
FARA programme is about the procurement of new generation armed reconnaissance and light attack helicopters for the US Army. US Army officials state that FARA is the highest priority programme among all other FVL programmes. The reason is that as OH-58 Kiowa helicopters are retired, there is currently no dedicated platform with armed reconnaissance capability in the force inventory. Now, this task is performed by AH-64 Apache helicopters, operating in coordination with MQ-1C Gray Eagle UAVs. Therefore, the lack of this capability, which is intended to be eliminated as soon as possible, takes the priority of the programme to the top, causing the programme schedule to be very tight. With the completion of the FARA programme, it is aimed to retire the AH-64s used for reconnaissance missions, which corresponds to about half of the AH-64s in the inventory.
The winner of the FARA programme is expected to serve in complex airspaces with advanced and integrated air defence systems. FARA helicopters are also required to be small enough to be able to hide behind tree-like obstacles (radar clutters) which absorb radar waves, and between tall buildings in mega-cities. Other requirements include a minimum cruising speed of 180 knots, and a main rotor diameter of less than 40 feet to enable flight between buildings in urban areas. To make a comparison, the main rotor diameter of OH-58 is 35 feet, while that of AH-64 is 48 feet. The engine, 20 mm automatic cannon and weapon systems that the companies will use when developing their prototypes will be provided as Government Furnished Equipment (GFE).
The first phase of the three-phase FARA programme saw the competition of a total of five companies including the AVX Aircraft and L3Harris partnership, Bell, Boeing, Karem Aircraft and Sikorsky. While the first phase ended after the preliminary design reviews and risk assessments made by the procuring authority, Phase 2 will be completed after the flight tests to be conducted in the autumn of 2023. In the third and final phase of the programme, the procurement authority will make its assessments before serial production and determine the winner accordingly.
Having managed to proceed to Phase 2, Bell and Sikorsky offer designs involving various innovations. So far, no prototypes have been produced by any of the two companies.
The Engine of the FARA Has Already Been Selected
The engine to be used in the vehicles, which will be serially produced under the FARA programme has already been determined. As part of the Improved Turbine Engine Programme (ITEP) it has put into practice, the US Army also initiated the development process of the turboshaft engine in parallel with the platforms. The winner of this programme has become General Electric Aviation (GE Aviation), with its T901 design that is intended to have 3,000 shaft horsepower. This is an important point; because the user opted for GE’s single-spool design instead of the dual-spool engine of the competing manufacturer. Dual-spool engines have two shafts rotating in each other coaxially. This design increases the efficiency of the engine, but at the same time it increases the complexity as well. Meanwhile, the T700 engine used in the AH-64 Apache and UH-60 Black Hawk helicopters in the inventory of the US Army has a single-spool design. Logistically, it can be said that as the existing maintenance system is more accustomed to what it is already using, it will not be very difficult to maintain the new, single-spool engine. This engine choice of the user also gives signals about its preference for simplicity and sustainability.
Bell Advocates Risk-Free Design with its 360 Invictus
Although it incorporates various innovations, Bell’s single-engine, two-seater 360 Invictus helicopter has a relatively conventional design. Its single main rotor enables the helicopter to both generate lifting power and perform basic manoeuvres, while the short wings mounted at the centre of the fuselage create 50 percent of the total lift at speeds of over 180 knots. Thus, at high speeds, the main rotor is used primarily to provide propulsion. In this way, Invictus prevents, to some extent, the retreating blade stall, in other words from being unable to generate lift (see the box titled “Theoretical Limits of Helicopters”). In addition, the main rotor focuses on creating thrust rather than generating lift power at high speeds. The main rotor is a fully articulated type, which is the type used in most helicopters around the world. The tail section of the helicopter is a canted fenestron. The positioning of this fan-like rotor at an angle to the ground provides a certain amount of additional lift when the helicopter is hovering. The weapons can be carried internally or integrated onto the short wings. Likewise, the retractable landing gear reduces aerodynamic drag.
Another visible asymmetry on the helicopter is that the exhaust outlet is located roughly in the middle of the tail cone, but at the top right. Typically, exhaust outlets in helicopters are located in a symmetrical layout. On the other hand, the design of conventional helicopters is already asymmetrical due to their tail rotors. Also, aircraft that are not naturally balanced often have higher manoeuvrability. Bell’s asymmetry-based approach may also make the helicopter more agile than expected.
Perhaps the most interesting feature of the helicopter is that it will have a Supplemental Power Unit (SPU) in addition to the 3,000 horsepower T901 main engine. According to a news article in Flight Global, this system, which is actually a Pratt & Whitney PW207 turboshaft engine, is significantly different than the conventional auxiliary power units (APUs) that help start the main engine and provide first power to the relevant systems when the helicopter is on the ground. When desired, this nearly 586 horsepower SPU functions as a normal engine and supports the 3,000-horsepower main engine and increases cruise speed. It is also stated that the maximum cruise speed of the helicopter will be over 185 knots.
In the 360 Invictus, which has fly by wire system, the pilots’ seats are arranged in a tandem configuration. At this point, it is also worth mentioning that another remarkable feature of the helicopter is that it is optionally piloted. In this way, the helicopter will be able to fly without any or both of the pilots.
As an additional advantage, Bell offers the 360 Invictus as an aircraft with proven, low-risk technologies. In the brochure prepared by Bell, which includes a list of the top five outstanding features of the helicopter, this feature ranks first as “The Lowest-Risk Path to Affordable FARA Capability”.
RAIDER X Ready to Break Records
The solution offered by Sikorsky for the FARA programme is a single-engine and two-seater compound helicopter called RAIDER X. The lifting power of RAIDER X, which has a rather extraordinary design, is provided by two coaxial rigid rotors rotating in opposite directions. The design of the helicopter removes the need for a tail rotor, which normally serves to stabilise the helicopter on the horizontal axis. In addition, the helicopter also features a rearward propeller at its tail, used for propulsion.
Although RAIDER X has an extraordinary design, this is actually not a new technology. The design of the helicopter is based on the X2 technology for which Sikorsky had initially laid the foundations with its S-69 helicopter that performed its maiden flight in 1973. Other designs of the company that are based on this technology and that have performed various flights with prototypes include the X-2, which takes its name from the same technology, the S-97 RAIDER and the SB> 1 DEFIANT being jointly developed with Boeing. Although there is no prototype of RAIDER X yet, company officials say that the helicopter is a 20 percent enlarged version of the company’s S-97 RAIDER model, whose flight tests have been going on for some time.
The RAIDER X is capable of completely stopping its tail propeller or reversing the thrust with a special clutch system. This system enables the helicopter to perform unconventional manoeuvres, such as accelerating and decelerating while remaining completely horizontal and decelerating during a dive, which are not possible with conventional helicopters. These various capabilities enable the helicopter to perform many additional manoeuvres and demonstrate a wide range of tasks especially on the battlefield. For example, the helicopter, which can slow down during the dive to the target, can thereby extend the engagement time and fire at the target area more intensely in a single pass. Similarly, since the helicopter can speed up and slow down without changing the nose angle, pilots can better focus on the target or terrain in front of them. The helicopter can also hang in the air with its nose down or up, and accordingly, the helicopter does not need to move in order to fire the targets that are at a lower altitude than itself. In this way, when hidden behind a high hill, the helicopter can rise with its nose facing down, and after firing its weapons to the target at a lower altitude, it can descend back to the point where it was hidden. All these are important features that expand the helicopter’s engagement envelope.
When the RAIDER X completely stops its tail propeller, it becomes quieter and reduces its acoustic signature. In this case, the RAIDER X can continue to manoeuvre like a normal helicopter. It is also stated that when the tail propeller is operated as a thruster, the helicopter can reach very high speeds. Since RAIDER X does not have a prototype yet, company officials cannot pronounce an exact figure for maximum speed, but they say that S-97 RAIDER, which embodies the same technology, can reach 207 knots in level flights while X2 can reach 250 knots in the same type of flight. Therefore, it would not be wrong to say that RAIDER X can reach similarly high speeds. When RAIDER X reaches such high speeds, 90 percent of the engine power is used by the rear propeller while 10 percent by the main rotors. At these speeds, the main rotors provide only lift. Company officials state that the main factors determining the speed limit of this helicopter are the engine and gearbox, noting that more powerful engines and gearboxes will increase the maximum speed of the helicopter even more.
In the RAIDER X images published so far by Sikorsky, the location of the exhaust is not clearly seen, however, it is known that the exhaust of S-97 RAIDER, which has similar features, is located at the rear end of the tail and is designed in the form of an upward-facing grille. It can be assumed that the exhaust of RAIDER X, whose fuselage is designed as a 20 percent enlarged version of the fuselage of S-97, will also be located at the same position.
The horizontal elevator and vertical rudders located at the tail of RAIDER X are able to move like in fixed wing aircraft. This enables the helicopter to manoeuvre at high speeds just like an airplane. Such manoeuvres include turns made at 60 degrees angle of bank. The test pilots of Sikorsky say that S-97, which has a similar design, flies like a jet aircraft.
The helicopters with coaxial dual main rotor feature an inherited problem, namely, high vibration. To overcome this vibration, RAIDER X uses active anti-vibration technologies. This technology, which functions like the technology used in noise-cancelling headphones, creates a vibration that is the opposite of the waveform of the vibrations created by the rotors, and these two vibrations absorb each other.
Low Risk or Innovative Approach?
Currently, none of the two helicopters have prototypes and it is too early to comment, however, the helicopters can be compared in terms of the following features, based on the images and announcements shared by the companies:
- Maximum Cruise Speed: One of the most important features for armed reconnaissance helicopters is speed. At this point, it is worth noting that both vehicles will be powered by the same main engine with 3,000 horsepower. Considering this, 360 Invictus is said to be capable of reaching speeds of around 185 knots by also utilising the supplemental power of 586 horsepower. Using the same technology as RAIDER X, S-97 RAIDER proved in the flight tests that it can reach a speed of 207 knots without any requirement for additional power. Meanwhile, it is worth mentioning that the programme’s lower limit requirement for maximum cruise speed is 180 knots. Thus, it seems that both vehicles will meet this condition.
- Thermal Signatures: Hiding from the enemy is also an important capability for reconnaissance helicopters. At this stage, it is not possible to compare the helicopters’ sizes with respect to visual detection and their radar cross-sectional areas in terms of radar detection. However, it is possible to examine the platforms in terms of thermal signatures. From this point of view, it is obvious that 360 Invictus will spend more fuel considering the SPU it will use to reach high speeds, and thus will produce higher volumes of exhaust gas. Meanwhile, the exhaust of 360 Invictus is located on the right side of the helicopter, under the main blades, behind the engine. This exhaust outlet is in a position that can be seen when looked from the right and from below. Assuming that the RAIDER X’s exhaust will be located behind the tail and facing up, we can say that it will almost be impossible to be seen directly when viewed from below. It seems that this feature will better hide the helicopter’s thermal signatures. When all of these are considered together, RAIDER X seems to have a higher survivability especially against heat-seeking missiles launched from the ground.
- Cockpit Configuration: In 360 Invictus, pilots are seated in tandem configuration while in RAIDER X, they sit side by side. In this regard, Bell claims that reconnaissance helicopters require pilots with high situational awareness, and argues that this can only be achieved by pilots sitting in tandem configuration. On the other hand, Sikorsky claims that pilots sitting side by side can work in a much higher coordination and can alert each other about important things even by touching each other’s arm whenever required in the combat environment where cognitive load is very high.
- Fuselage Design: Also due to its cockpit configuration, 360 Invictus is much thinner than RAIDER X. So when viewed from the front, it presents a very small target. The thinness of 360 Invictus also means that it has much less aerodynamic resistance. It is obvious that the RAIDER X, whose cruising speed is already high compared to 360 Invictus, does not involve any problems in terms of aerodynamic resistance. Also, the old aviation proverb “If it looks good, it flies good” is worth mentioning here.
- Weapon Systems: A 20 mm autocannon, and missile and rocket systems will be provided as GFE for both vehicles. Both vehicles can carry their weapons internally, except for the 20mm gun. However, there are some differences between the vehicles in terms of payloads carried within the fuselage. When the images published by the manufacturers are examined, it is seen that 360 Invictus, due to its thinner structure, can carry internally two AGM-114 Hellfire missiles on each side of the fuselage. When needed, 360 Invictus can also carry additional weapons under its short wings. It is worth noting that such a configuration will of course affect the speed of the helicopter negatively as it increases its aerodynamic drag. On the other hand, the images related to RAIDER X show that in addition to two internally carried AGM-114s on each side of the helicopter, there are also two more internal stations that can be equipped with different weapons. Therefore, in terms of internal payload capacity, it can be said that RAIDER X has more superior features while 360 Invictus features more payload carrying options in return for slower speed.
- Sensors: Perhaps the most important feature for armed reconnaissance helicopters is the platform’s sensors. While 360 Invictus carries the electro-optic sensor system on the nose of the helicopter, the location of the sensor system cannot be seen in the images of RAIDER-X. In addition, as this type of sensor systems are often developed independently from the platform, the final location of the sensor in question may vary according to user requests. For example, the user may choose a sensor integrated into a mast placed on the main rotors. In such a case, sensor integration can be performed without a need to change the fuselage design of RAIDER X.
When all these factors are evaluated together, it is seen that RAIDER X is a little more advantageous than its competitor in terms of armed reconnaissance missions. However, as FARA is currently the top priority programme of the US Army, the user can opt for the lower risk helicopter, namely 360 Invictus, like in the ITEP, which was launched to determine the engines to be used in helicopters.
Theoretical Limits of Helicopters
Due to the conventional design of the helicopter, which has a single main rotor, there is a theoretical limit on its maximum speed. Although it varies according to helicopter type, the speed limit is around 180 knots in level flights even for the fastest helicopters, due to a phenomenon called Retreating Blade Stall, which can be explained as follows:
During forward flight, the air speed of the forward rotating blade attached to the main rotor of the helicopter is higher compared to the backward rotating (retreating) blade. For this reason, the blades are given different angles of attack so that both blades can generate equal amounts of lift. Thus, the angle of attack of the forward rotating blade is lower than that of the retreating blade.
As the speed of the helicopter increases, the speed of the retreating blade decreases further compared to the air around it, and a need arises for further increasing the angle of attack in order to create enough lift. When the helicopter gets closer to its maximum speed, the angle of attack of the retreating blade increases so much that the blade stalls and becomes unable to generate lift, causing the helicopter to lose its balance.
This is how the phenomenon of Retreating Blade Stall limits the speed of conventional helicopters. On the other hand, such a problem does not occur in helicopters with coaxial dual main rotors because in both main rotors rotating in opposite directions, there are blades that move both forward and backward at the same time, and the aerodynamic forces created by these blades balance each other.
In the next part of this article series, we will be covering the FLRAA programme and the platforms competing in this programme namely the Bell’s V-280 Valor tiltrotor and Sikorsky-Boeing’s SB> 1 DEFIANT compound helicopter.