Why Many Helicopters Are Not Built for Speed

To understand our approach, it’s crucial to recognise that many early helicopters simply weren't designed for high-speed transit. Their missions revolved around their unique vertical takeoff and landing capabilities, focusing on utility lifting and accessing tight landing sites. Consequently, aerodynamic streamlining was often a secondary concern.

Consider this: in a conventional helicopter, the fuselage might only account for about 40% of the aircraft's overall parasitic drag. The exposed rotor mast and hub assembly can contribute a staggering 30%, and unfaired skids can easily add another 15%. These large, drag-inducing mechanical elements, not found on fixed-wing aircraft, traditionally pin helicopters down to lower cruise speeds.

What Really Limits Helicopter Velocity

Aspiring pilots are often taught that helicopter speed is intrinsically limited by factors like dissymmetry of lift, retreating blade stall, and advancing tip Mach numbers. While these are very real, their effects don't truly begin to significantly impact performance until around 180 knots.

By 200 knots, the resulting vibration can become prohibitive, with a practical hard cut-off for conventional helicopters around 250 knots (the current record held by a Lynx is about 216 knots).

Our target cruise speed of 140 knots sits well below these challenging technical barriers. Therefore, achieving it becomes a question of aggressive and intelligent drag reduction.

The Power of Aerodynamic Refinement

If you design a helicopter fuselage to be more akin to that of a sleek aeroplane, you naturally tend towards much lower drag levels. Between 1959 and 1963, Bell experimented with a UH-1B "Huey." By simply fairing the aft fuselage, rotor pylon, skid legs, and making other modest airframe modifications (while keeping the same engine and rotor system, minus the teeter bar), they achieved a remarkable 66.5% reduction in drag. This translated to a 30-knot increase in cruise speed for the same aircraft at the same gross weight.

This demonstrates that meticulous aerodynamic design is key to pushing cruise speed envelopes without merely relying on excessive power.

This principle is central to the HX50. Our aircraft, roughly the size of a Gazelle (though slightly smaller and lighter), boasts a vastly superior drag profile. We've achieved this through:

• Streamlined Fuselage & Retractable Undercarriage: Eliminating the drag from exposed landing gear is a significant first step.
• Faired Rotor System: The entire rotor system, including the typically draggy hub and mast, is fully faired. Careful attention has been paid to the blade roots to minimise drag.
• Optimised Airflow: Internal airflows for engine cooling and ventilation systems have been meticulously managed to prevent "leakage" and momentum losses.
• Advanced Rotor Blade Design: Utilizes modern RC3 and RC4 series aerofoils, which are 17-20% more efficient than traditional helicopter rotor aerofoils. Features a parabolic swept tip to minimise the impact of higher speed flight.
• Level Flight Attitude: The mast is tilted 5 degrees forward, and a large horizontal stabiliser ensures the aircraft maintains a horizontal attitude in high-speed flight. This presents the lowest possible projected area to the airflow.
• Composite Rotor Blades: These allow for careful management of the rotor's structural dynamics in faster forward flight, keeping vibration to a minimum.

Ensuring Performance Targets are Met

Our confidence isn't based on theory alone. The HX50's design has undergone extensive validation:

• Computational Fluid Dynamics (CFD): The latest CFD methods were employed to design and optimize the fundamental shape of the fuselage, mast fairing, and rotor head fairing.
• Wind Tunnel Testing: The aircraft fuselage underwent wind tunnel testing in 2019, and the results correlated very closely with our CFD calculations.
• Proven Rotor Technology: The basic rotor topology, planform, and aerofoil distributions are well-understood and are currently in service on existing helicopters delivering similar levels of performance.

Simply put, the strategy is to dramatically reduce drag so less power is needed to fly fast, and then to configure the aircraft for comfortable, level, and efficient forward flight.

Robust Hover Performance

While speed is a key focus, the HX50 has also been designed with robust lifting capability. We are confident in its ability to meet stated performance margins from sea level up to 10,000 feet on an ISA+15 day. This is well-supported by comparator aircraft with similar power, gross weights, rotor diameters, and solidities.

The path to achieving the HX50's forward flight performance is s the result of deliberate, intelligent design focused on aerodynamic efficiency. By minimising drag and optimising the aircraft for its intended cruise speed, we are confident in achieving our performance targets.

Combined with our advanced composite main rotor design, we are poised to deliver an unparalleled flight experience. We are confident in our engineering, our validation, and our ability to deliver on these performance promises.