High Speed VTOL

Designs for aircraft that can fly at fighter speeds and hover efficiently have been around for over 60 years. However, due to high technical complexity, they’ve not made it off the drawing board. This may change in the next 15 years: appetite for these vehicles is growing alongside enabling technologies.

In this article, we discuss a recent program by the US DOD to develop these so-called high speed VTOL (HSVTOL) aircraft—the HSVTOL challenge. We'll briefly discuss the program and then some of the high-level designs being investigated under it.

HSVTOL challenge

In the spring of 2021, AFWERX launched the HSVTOL challenge for special operations (SOCOM) and the air force (USAF). They received 218 proposals and selected 11 for investment in Phase 1 of the program. We don't have detailed requirements or specifications, but AFWERX says target aircraft need to be capable of

• jet-like speeds,
• vertical takeoff and landing (VTOL),
• rapid air refueling,
• collision avoidance and detection systems, and
• mission-specific payload accommodation.

Example missions for the aircraft include

• infiltration and exfiltration of specials ops forces and equipment,
• personnel recovery,
• aeromedical evacuation, and
• tactical mobility.

High-ranking USAF personnel have stated that their plan to island / location hop to avoid enemy missiles in the Pacific will require “getting away from the fixed runway, so that we can do logistics in a way that is somewhat disruptive, and, frankly, a lot harder to target.”

There’s a wide variety of designs in the 11 selections including a variable diameter rotor, a tilt wing, a tilting ducted rotor, a stop fold rotor and fan-in-wing solutions. We’ll take a quick look at some of these designs below.

Variable diameter tiltrotor

Continuum Dynamics and AVX partnered to propose a variable diameter tiltrotor (VDTR) design. VDTRs have been studied for decades by organizations including Sikorsky. The blades telescope outward to increase rotor diameter for efficient hover and better autorotation performance. At higher speeds, in airplane mode, the blades retract for a more propeller-like diameter.

At high speeds, smaller rotor diameters reduce the potential for proprotor-whirl flutter, and lessen the wing stiffness and weight requirements. Smaller rotors also reduce sensitivity to head-on longitudinal gusts and roll/yaw coupling. By reducing diameter, the rotor can also maintain a more similar rotation speed in hover and airplane mode, typically improving engine efficiency.

More information can be found on Continuum Dynamics website.

Tandem tiltwing

American Aerospace Engineering proposed a tandem tiltwing design, such as the one above. Rotors mounted on tiltable wings have aerodynamic advantages over tiltable rotors mounted on fixed wings (like the V-22). Tiltrotors waste a substantial amount of rotor thrust in hover because rotor downwash impinges the upper surface of the wing (10% for a V-22). The downwash from a tiltwing rotor flows over the leading edge of the wing, which greatly reduces this waste.

A key disadvantage of tiltwings is their susceptibility to wind gusts when the wings are vertical, as they are in VTOL operation. This can pose substantial danger trying to land or takeoff, particularly on a ship or aircraft carrier.

Tiltwings have been pondered for about a century, including the Weserflug P.1003 in 1938. Many were constructed in the 1950s, including the Hiller X-18, but they never received much customer interest.

Stop fold rotors

Bell Flight proposed a stop fold rotor. These aircraft behave like a traditional tiltrotor (e.g. a V-22) until reaching a sufficiently high speed, where a turbofan (jet) begins providing forward thrust in place of the rotors. At this point, the rotors are stopped and folded in flight so that the aircraft resembles a traditional jet at high speeds.

Stop fold rotor designs have been contemplated for over 60 years, but none have made it off the drawing board. The complexity of folding a rotor in mid-flight may be the primary reason for this. Also, these designs have substantial inefficiencies like carrying unused rotors and turboshaft engines in high speed flight, while carrying an unused turbojet engine in hover. Of course, this could be improved with a dual-use engine capable of powering both the rotors and the jet. However, such an engine may be inefficient or otherwise impractical.

Tilting ducted rotors

Piasecki is working on a design with tilting ducted rotors on each wing, in addition to 2 aft-mounted turbofan engines with vectored exhaust. All of these can provide vertical thrust for VTOL operations and forward thrust for high speed flight. While ducts improve the efficiency of the rotor/fan they enclose, they can hurt overall aircraft performance due to the increased weight and drag they introduce.

Ducted tiltrotors (or tiltfans) have been studied for decades, and a prototype named the X-22 was flown by Bell in the 1960s. These tiltfan designs were abandoned until the F-35B employed one for vertical takeoff.

Fan-in-wing

Astro Aerospace proposed a variant of its Horizon Cavorite X5, pictured above. The X5 is powered by a gas engine, which generates power for its many electric motors.

In VTOL mode, the wings open to expose ducted fans that provide vertical thrust. At higher speeds, the wings close over the fans and an aft-mounted pusher propeller is used for forward thrust. The X5 has 12 ducted fans on the rear wings, four on the front canard wings, and one pusher propeller behind the aft wing. It’s unknown how much of this was changed in their HSVTOL proposal.