As aeronautics engineers develop innovative distributed electric propulsion systems, they face new challenges in ensuring that these innovative aircraft are safe as well as fuel efficient. In particular, these systems involve a large number of electrically driven fan motors mounted across a wing that induce vibrations that negatively affect the aircraft’s stability. These vibrations cause problems regardless of whether the motors are bottom-mounted, top-mounted, or wing-embedded.

In considering how to address this vibration problem for electric propulsion systems, researchers discovered a new control system concept based on the principles of forced aeroelasticity. The concept uses harmonics to mitigate — or even cancel out — unwanted wing vibrations brought on by the electric propulsion system, as well as those that naturally occur in the air stream. Specifically, it combines the aircraft’s lift- and drag-induced flutter with forced oscillations from multiple wing-mounted ducted engines/fans that can rotate at independent frequencies (i.e., revolutions per minute or RPMs).

Applying constructive interference to actively control flutter first requires measuring the shape and vibration modes of the wing. This measurement can be achieved via a fiber optic Bragg reflection device. Then the system commands the specific spinning frequency for each engine/fan accordingly. These engine/fan oscillations drive their own vibrations into the wing structure, thereby twisting the wing. This induced vibration actively mitigates the aeroelastic flutter caused by drag through the freestream. As the wing approaches a freestream air velocity that is near the wing structure’s aeroelastic flutter node, the fiber optic detector system and the control algorithm will send signals to command the fan motors to adjust their collective RPMs. Because the fan motors have a spatial, lengthwise placement across the wing, a harmonic combination of motor RPMs can create a standing wave that counteracts the drag-induced flutter. Thus, the wing can pass through the aeroelastic flutter vibration node with minimized structural stress.

This vibrational interference concept not only would prevent catastrophic failure but also would increase the structural lifetime of the wing by avoiding repetitive bending fatigue. As a result, this innovation positively contributes to the full development of distributed electric propulsion systems.

This work was done by Kurt Kloesel of Armstrong Flight Research Center. For more information, contact the NASA Armstrong Technology Transfer Office at This email address is being protected from spambots. You need JavaScript enabled to view it.. Refer to DRC-013-015.

NASA Tech Briefs Magazine

This article first appeared in the February, 2015 issue of NASA Tech Briefs Magazine.

Read more articles from this issue here.

Read more articles from the archives here.