Adaptive compliant wing
A flexible wing that changes shape in flight for reduced drag.
An adaptive compliant wing is designed to be flexible, allowing its shape to shift during flight. This flexibility offers key advantages over conventional wings, which rely on hinged control surfaces. Those hinges create airflow disruptions, vortices, and sometimes flow separation, all of which increase drag, reduce efficiency, and raise fuel consumption. In contrast, flexible airfoils can adjust aerodynamic forces with far less flow disturbance, leading to lower drag and better fuel economy.
Changing a wing’s shape directly alters its aerodynamic behavior. Depending on the airflow conditions and the wing’s initial form, variations in curvature, angle of attack, or twist produce different forces and moments. Adaptive wings exploit this by using distributed flexibility to achieve smooth, continuous shape changes without gaps. By adjusting these geometric parameters, the wing can be tailored for specific flight conditions—such as reducing drag—or for maneuvers like rolling.
Shape adaptation can be grouped by the type of motion involved. Changes affecting the wing’s overall planform—its shape as seen from above—include altering span (wing length), sweep (angle between wing and fuselage), chord (cross-section length), and dihedral (angle between wing and horizontal plane). Changes to the airfoil itself include adjusting twist, camber, and thickness distribution.
Ongoing research includes several projects. FlexSys Inc. has developed an adaptive compliant wing with a variable-camber trailing edge that can deflect up to ±10°, functioning like a flap but without the gaps and separate segments of a traditional flap system. The wing can twist up to 1° per foot of span and change shape at 30° per second, making it useful for gust load alleviation. The U.S. Air Force Research Laboratory sponsors this work. After wind tunnel tests, a 50-inch (1.3 m) section of the wing was flight-tested on a Scaled Composites White Knight aircraft over seven flights and 20 hours from the Mojave Spaceport. Control methods are being proposed.
At ETH Zurich, adaptive compliant wings are being studied as part of the Smart airfoil project. The EU-funded Flexop program aims to develop higher aspect ratio wings for airliners—reducing induced drag—while making wings lighter and more flexible, along with active flutter suppression.
- Deflection range
- ±10°
- Twist rate
- 1° per foot of span
- Shape change rate
- 30° per second
- Flight tested span
- 50 inches (1.3 m)
- Flight test aircraft
- Scaled Composites White Knight
- Flight test program
- seven-flight, 20-hour program
- Flexop demonstrator span
- 7 m (23 ft)
Lore & Background
Shape adaptation is actively pursued in adaptive wings which, by nature of their distributed compliance, can attain shape changes in a continuous, smooth, gap-free manner. By altering geometrical parameters such as curvature, incidence, twist, camber, and thickness distribution, the forces and moments can be modified to tailor them to specific flight conditions or to perform maneuvers. Shape adaptation can affect the overall planform, including changes in span, sweep, chord length, and dihedral, as well as changes to the airfoil shape itself.
A design by FlexSys Inc. features a variable-camber trailing edge that can be deflected up to ±10°, acting like a flap-equipped wing but without the individual segments and gaps typical in a flap system. The wing can be twisted up to 1° per foot of span and its shape can be changed at a rate of 30° per second, which is ideal for gust load alleviation. The development is sponsored by the U.S. Air Force Research Laboratory. The wing was tested in a wind tunnel, then a 50-inch (1.3 m) section was flight tested on board the Scaled Composites White Knight research aircraft in a seven-flight, 20-hour program operated from the Mojave Spaceport.
The EU-funded Flexop program aims to enable higher wing aspect ratio for less induced drag with lighter, more flexible airliner wings, along with developing active flutter suppression. On 19 November 2019, a 7 m (23 ft) span jet-powered UAV demonstrator with an aeroelastically tailored wing for passive load alleviation was flown in Oberpfaffenhofen, Germany. The flexible wing was 4% lighter than the rigid one. The 54-month, €6.67 million project ended in November 2019, followed by the €3.85 million FLiPASED program from September 2019 until December 2022, using all the movable surfaces.
Reader's Guide
The significance of the adaptive compliant wing lies in its potential to reduce aerodynamic drag and improve fuel economy by avoiding the disruptions to airflow caused by conventional hinged control surfaces. The article notes that flexible aerofoils can manipulate aerodynamic forces with less disruption, resulting in less drag and improved fuel efficiency. The FlexSys design demonstrates a practical implementation, with a variable-camber trailing edge that achieves flap-like deflections without gaps, and a twist capability of 1° per foot of span. Its rapid shape change rate of 30° per second is noted as ideal for gust load alleviation. The Flexop program further highlights the legacy of this technology, aiming for higher aspect ratio wings with passive and active load alleviation. The demonstrator's flexible wing was 4% lighter than a rigid counterpart, and the program targeted a 5% fuel-burn reduction with an aspect ratio of 12.4, with 7% targeted. The FLiPASED program continues this work, focusing on active flutter suppression for flexible wings. The article also places this work in context with earlier demonstrators like the Grumman X-29, noting more refined fiber orientations.
Did You Know?
- The FlexSys adaptive compliant wing can be deflected up to ±10° at the trailing edge.
- A 50-inch (1.3 m) section of the FlexSys wing was flight tested on the Scaled Composites White Knight.
- The Flexop program's flexible wing was 4% lighter than its rigid counterpart.
- The FlexSys wing can change shape at a rate of 30° per second.
More in Aircraft Components, Part 2 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced