Updated 7/6/2017 with grammatical corrections.

Mobility, the ability to transport troops and provisions from one place to another, is critical to any military operation. While firepower may be more prominently featured in the popular imagination, many campaigns have been won by sheer mobility and maneuvering prowess. Germany’s blitzkrieg operations against France, for example, demonstrated how highly mobile forces could outflank and rout a similarly-armed but less-mobile adversary.

Helicopters are a critical component of 21st-century military mobility because no other vehicle has similar terrain-agnosticism. Planes, while are faster and more efficient, are heavily limited by the need to land on a long and flat strip. Wheeled vehicles, while more cost-efficient than helicopters, are much slower, especially when offroading. Helicopters allow troops to be quickly inserted or extracted from virtually any location with more precision than a paradrop and faster speeds than ground transportation.

Nevertheless, current helicopter designs have some inherent limitations. Helicopters lose performance at high altitudes and high ambient temperatures because these conditions decrease air density and thus lessen the lift generated by the rotor. Further, the maximum speed achievable with a single-rotor helicopter is limited by retreating blade stall. Basically, the blade moving in the opposite direction of the helicopter itself (the retreating blade) experiences relatively low airspeed, decreasing lift. If high enough speeds are reached, the retreating rotor blade will begin to stall, which is a hazardous occurrence. The design of a conventional helicopter is not particularly energy-efficient either, since some of the engine’s power is spent powering an anti-torque rotor to counteract the force produced by the main rotor.

As a result, helicopters, while versatile, have restricted ranges, payloads, and top speeds. Since these shortcomings are inherent to the conventional helicopter design, upgrading existing helicopters can only achieve so much. As a result, designing a truly next-generation helicopter requires a fundamental departure from traditional configurations.

The Future Vertical Lift Program

All military helicopters in the US’s inventory will reach the end of their “useful life” before 2040. The OH-58 Kiowa has already reached this stage and was retired. Future Vertical Lift (FVL) is the US military’s next-generation helicopter program, which is intended to replace all the retiring airframes. The scope of FVL is hard to overemphasize; not only will FVL replace the US’s ~3000 medium lift helicopters, it will also replace hundreds of light and heavy rotorcraft as well. Variants of the FVL will also be adapted to the reconnaissance and attack roles. In other words, nearly all rotorcraft in the US military will be FVL-derived. Note that those figures do not even include the large-scale export sales FVL will inevitably achieve.

Because the US military is such an influential customer, it is safe to assume that FVL may change the face of aviation forever when allies and adversaries alike adapt the design to their own purposes and build licensed copies.

FVL has a myriad of requirements, but two are especially important. First, FVL requires a cruising speed between 240 and 300 knots, which is well above the speed limit imposed on a conventional design by retreating blade stall. Second, FVL must possess a combat radius of at least 229 nautical miles — conventional medium lift helicopters have a radius of less than 100 nautical miles.

FVL must also be scalable. While the first FVL airframes will be medium-lift, the FVL design will eventually be scaled up to produce heavy and ultra lift aircraft and down to produce light-duty aircraft. Similarly to the F-35, FVL will include features critical to each branch of the military; thus, FVL will need to be compactible and corrosion-resistant for stowage on Navy ships. There are a myriad of other features noted in the request for proposals, including automated landing with terrain avoidance, sensor fusion, aerial refueling capabilities, etc. The full list of desired technologies can be found here.

The program originally included four vendors, but two — Sikorsky and Bell — have been chosen to continue development and produce full-scale prototypes for evaluation. For the program, Sikorsky has partnered with Boeing and Bell has partnered with Lockheed Martin.

The Bell V-280 Valor

Bell’s entry into the FVL program is a tiltrotor. Unlike a conventional helicopter, where the main lift rotor is fixed, a tiltrotor has the ability to pivot its engines. During takeoff and landing, the rotors are pointed upwards like a conventional helicopter. During level flight, the rotors are pointed forwards like a propellor aircraft. The idea is that a tiltrotor combines the best of both worlds: with rotors pointed upwards, a tiltrotor can hover and land vertically like a conventional helicopter. After that, the rotors are pointed forwards to achieve plane-like performance. Tiltrotors have a unique rotor design with disk loading (loaded weight per unit are of blade) somewhere in between that of a conventional helicopter and a turboprop aircraft.

With rotors pointed forwards, the tiltrotor completely avoids retreating blade stall and obtains efficiency similar to a fixed-wing aircraft. Tiltrotors also boast increased efficiency because they use a fixed wing in addition to their high-loading rotors. When in level flight, the fixed wing produces all of the necessary lift, allowing the propellers to direct all their thrust rearwards. Because of their airplane-like characteristics, tiltrotors can operate at speeds much higher than a regular helicopter; in fact, tiltrotors have reached over 300 knots in flight, an increase of more than 100kn relative to even the fastest conventional helicopters.

Because the rotors need to pivot while in flight, tiltrotors are mechanically complex machines. Ensuring not only that the system is sound mechanically but also that the transition from vertical to horizontal flight is smooth presents a major challenge. Bell, which produced the V-22 Osprey in a partnership with Boeing, knows this from experience. The V-22, which was the world’s first fully operational tiltrotor, experienced many teething issues, including cost overruns and a spotty initial safety record.

However, Bell eventually worked through the V-22’s issues and appears eager to apply the lessons to the V-280 Valor. One main difference between the two aircraft is size: the medium-lift V-280 will have a payload capacity significantly less than that of the V-22 (11-14 combat kitted troops versus 24). Engine configuration differs slightly between the two as well; the whole engine unit of the V-22 rotates, while only the rotor of the V-280 pivots. The V-280 also cruises at 280 knots, which is somewhat faster than the V-22’s tactical cruise of 240 knots. The V-280 has a lower disk loading because it weighs significantly less than the V-22 but has similarly-sized rotors. This means the V-280’s rotors can operate at lower and more efficient RPMs than the V-22, a welcome change considering the massive amount of downwash and noise generated by the V-22’s propellers/rotors. The V-280 also features a different wing and tail configuration than the V-22; these changes were made in part because of requirement discrepancies between the two programs but also based on lessons learned while constructing and operating the V-22.

The Sikorsky SB-1 Defiant

The SB-1 Defiant approaches the challenge presented by FVL in a different matter. The SB-1 is largely similar to a conventional helicopter, with two important exceptions.

First, the SB-1 has coaxial rotors. In a coaxial system, instead of one main rotor, the aircraft has two, with one on top of (coaxial to) the other. The rotors spin in opposite directions, meaning no anti-torque system is necessary. Also, the effects of lift dissymmetry (a phenomenon resulting from blade stall) are reduced; when one rotor is retreating, the other is advancing, so equal lift is generated on both sides regardless of airspeed. Coaxial rotors are relatively complex compared to a traditional layout but have a number of benefits including moderate efficiency and power increases, higher maximum speed, smaller aircraft footprint, etc.

Sikorsky’s rigid coaxial rotor system is improved relative to current coaxial systems such as those used by the Russian KA-52 Alligator attack helicopter. Sikorsky’s rigid composite rotors have a low drag-to-lift ratio and enable high cruising speeds. The rotor hub is also covered by a low-drag fairing. Because the rotors are in close proximity, the wake generated by one rotor serves to lessen the resistance experienced by the other, increasing propulsive efficiency.

The SB-1’s second innovation is the use of a pusher propeller mounted at the rear of the helicopter’s tail. The principle behind the pusher propellor is simple: it contributes forward thrust in order to reach higher speeds. Helicopters of this type are referred to as compound helicopters or gyrodynes. While simple in concept, the compound helicopter layout proved difficult to execute; Sikorsky had been making prototypes for decades before it finally created the X-2, which was a success and validated the concept. Sikorsky’s compound helicopter roadmap is as follows: first, a small scout helicopter called the S-97 Raider was produced and began testing in 2015. Next, Sikorsky will test a slightly downscaled technology demonstrator in 2017. Tests with the full-scale FVL airframe will begin in the 2020s.

Weighing the proposals

With a cruise speed of around 250 knots, the SB-1 will be somewhat slower than the V-280 and possibly less efficient in high-speed cruise due to its lack of a fixed wing. However, Sikorsky’s design has its advantages. For one, the compound design will likely be simpler to produce than Bell’s tiltrotor. The SB-1’s footprint is also smaller than the V-280, which has a large wing. This means that the V-280’s entire wing assembly will need to fold in order to fit aboard ships, adding complexity and extra cost. The SB-1 will probably only need a folding tail, similar to existing helicopters.

Handling will undoubtedly differ between the two aircraft. The V-22 handles like a large helicopter when nacelles are oriented upwards and like a turboprop plane when nacelles are rotated forwards. The SB-1, on the other hand, generates lift with its rotors throughout the flight envelope. According to Sikorsky, the SB-1 is nimble at low speeds.

While the SB-1 may be somewhat slower, it will likely be more receptive to large throttle inputs than the V-280, which has to transition between modes before accelerating past certain thresholds. Indeed, the V-22 has a stall speed in airplane mode, and transitioning to helicopter mode from high speed requires substantial deceleration. Of course, these are my assumptions based on the available information; only extensive flight testing will truly reveal how each aircraft handles.

Fielding is not planned until the 2030s, but both competitors claim they could finalize their designs earlier if needed. As the program progresses, much more information regarding payloads, ranges, avionics, and powertrains will become available, so more complete analysis will be possible in the future. FVL has the potential to produce a new vertical lift aircraft with vastly expanded capabilities compared to current helicopters; the implications of a rotorcraft with a 250+ knot cruise speed and a 250nmi+ mission radius would be game-changing.

However, there are risks as well: while FVL is so critical that its cancellation is unlikely, the F-35 Joint Strike Fighter program illustrated the kinds of issues that can plague large and ambitious multi-service programs. Complicating matters further, Lockheed Martin has a stake in both competitors, being partnered with Bell but also owning Sikorsky. While both aircraft are based on proven concepts and designed by competent contractors, FVL must nevertheless have proper oversight and follow good procurement practices or else the future of US rotary wing aviation may be as costly as it is revolutionary.