The rise of the tilt-wing: a game-changer for VTOL aircraft

Sikorsky and Lockheed Martin’s decision to build a hybrid tilt-wing technology demonstrator is a significant step forward for eVTOL aircraft. We are in a unique position to say this, as Dufour Aerospace has been designing, building, and testing tilt-wing aircraft for many years.

We will present some of the reasons that we believe Sikorsky has made the right choice in developing a tilt-wing, including hover, transition, cruise efficiency, smooth transition, robustness, scalability, and of course safety.

What is a tilt-wing aircraft?
Unlike tilt-rotor or tilt-propeller aircraft, a tilt-wing tilts the whole wing instead of just the propeller nacelles. While this may seem a small detail, it is actually a fundamental difference.

Tilt-wing aircraft are not new – they were designed, built, and intensively flown in the 60s and 70s of the last century, at a time when distributed propulsion and advanced electronic control systems were not yet available. Prominent representatives of this first tilt-wing wave are the American XC-142 and the Canadian CL-84 aircraft.

Hover and transition efficiency
Tilt-rotor and tilt-propeller aircraft lose some of their thrust during vertical take-off and landing because the fixed horizontal wing blocks the airflow. This loss of thrust is called “download”. The presence of the wing is a disadvantage for the aircraft until it has accelerated beyond a critical speed.

Tilt-wing aircraft, in contrast, keep the wing aligned with the propeller thrust. This minimizes download and drag, and the airflow energizes the wing, producing additional lift as soon as the aircraft starts to move forward.

This characteristic is a significant advantage for the overall power requirements and configuration of the aircraft.

Smooth transition with large safety margins
Transitioning from hover to forward flight and vice versa in many VTOL configurations is aerodynamically complex and energy-intensive. The tilt-wing design simplifies this transition. By rotating the whole wing forward, the tilt-wing seamlessly converts from a vertical to a horizontal configuration, without drastic changes in airflow. There is no handover between a hover propulsion system and a cruise propulsion system as is the case with lift-and-push configurations. This translates to smoother, seamless transitions.

As the wing provides lift at all times, the power requirements for transition are dramatically lower than with any other convertible concept, resulting in high efficiency. There are also large power margins remaining for control and a wide corridor in the flight envelope, which translates directly to large safety margins at all times.

Cruise efficiency
As an eVTOL aircraft flies 90% of the time in cruise configuration, it is essential that the aircraft is as efficient as possible in cruise. Efficiency comes with what is called a “clean” configuration – as little drag as possible. Some VTOL configurations, such as lift-and-push, carry the burden of non-lifting components (such as propellers that are only used for hover) during cruise flight. These components create drag, reducing the overall efficiency. Everyone who ever flew a propeller aircraft with speed brakes knows the impact of drag. The tilt-wing design is very clean in cruise, so does not suffer from this drawback. So the tilt-wing aircraft is a fuel-efficient long-range cruiser, which is also capable of VTOL operations.

Mechanical simplicity and robustness
Hinges on VTOL aircraft are critical components but also represent potential weak points. They need to be robust and redundant. Losing a hinge in flight and potential cascading failures are a risk that needs to be eliminated as much as possible.

Critical hinges are designed to be multi-loadpath and damage tolerant so that there is no single point of failure. This is a well-known design principle in aviation, and is used for example on the horizontal stabilizer trim mechanisms of all commercial airliners. (These trim mechanisms are designed and certified much the same way as our wing tilt system.)

Building hinges in this rock-solid way adds weight and inspection requirements, of course. So it is an advantage to have a single hinge, as the tilt-wing does, rather than multiple hinges as in a tilt-propeller design.

Handling qualities
When talking about safety, most representatives of the eVTOL scene are almost exclusively talking about component certification. There is very little discussion of what matters equally as much: the flight behavior and control margins of the aircraft.

Technical failure of a component is rarely the reason for an accident. 80% of the accidents are caused by human error – a pilot mistake. So it is critical that pilots (and automated flight control systems) have enough control margin to react if something unforeseen happens.

A direct learning of Dufour Aerospace’s CEO, Thomas Pfammatter, with over 30 years of experience in helicopter operations, is that having large control authority margins is a key element for safety.

That is where tilt-wing aircraft excel. The propellers and fully energized wing with aerodynamic control surfaces provide powerful and direct control authority in all three axes. Additionally, the ability to adjust the wing to any given wind situation allows for redundant control and smooth transitions while keeping the cabin level.

This is not just theory: it has been proven and tested by hundreds of test and army pilots with the CL-84 and XC-142 aircraft, and by Dufour Aerospace in test flights almost every day.

Scalability
‍The air behaves differently at different scales. What works for a multicopter drone, a “flying brick”, does not work so well for larger aircraft. While multicopters, lift-and-push, and tilt-propeller aircraft struggle to scale up, tilt-wing aircraft can comfortably be much larger. This has been proven by the XC-142 at over 20 tons, and is due to the efficiency of the blown wing.

That makes the tilt-wing concept unique in its ability to cover the biggest range of transitional aircraft at a reasonable cost.

Conclusion
At Dufour Aerospace, we are well aware of the advantages of the tilt-wing configuration, as we have been building and flying tilt-wing aircraft for many years, so we understand the reasons behind Sikorsky’s decision.

They have also decided to pursue a hybrid power approach, rather than using only batteries for energy storage. Electric propulsion has many advantages, including noise reduction and redundancy. However, today’s battery technology does not come close to the specific energy available in fuel. The hybrid power configuration will allow Sikorsky to bring together the best of both of these technologies, and achieve much longer range missions, and greater operational versatility, just as we do at Dufour Aerospace.

From our team at Dufour Aerospace to yours, congratulations, Sikorsky!