Andrew Doyle/LONDON
ADVANCED, INHERENTLY unstable delta-canard combat aircraft could be easier to control and have a longer structural fatigue life than conventional designs through the use of active-structural-mode-control (AcSMC) technology, according to a study carried out by Lancaster University in the UK.
The British Aerospace Military Aircraft division, which along with the UK Engineering and Physical Sciences Research Council sponsored the work, hopes that AcSMC can be used to prevent airframe structural vibrations influencing the feedback signals sent to the aircraft's digital flight-control system (FCS), a phenomenon known as cross-coupling, which can result in unwanted aircraft oscillations.
AcSMC technology could also provide increased structural damping, resulting in reduced wing bending and a consequent improvement in fatigue life, according to Dr Stuart Burge of Lancaster University's Engineering Department. Burge says that the study found that "...by controlling the first two symmetric structural modes [of vibration], greatly reduced cross-coupling, together with increased structural damping, can be achieved".
There may, however, be a drag penalty if AcSMC is included on existing designs, such as an eventual upgrade of the Eurofighter 2000, because larger control-surface deflections are required to manoeuvre the aircraft.
Existing unstable aircraft such as the Eurofighter and JAS39 Gripen feature passive structural mode control (PaSMC), in which "digital structural notch filters" are employed to try and cancel out from the feedback signals those vibrations occurring at frequencies known to be specific to the airframe itself. Phase lags, however, can occur which result in structural oscillations being fed back to the FCS, which in turn further drives the oscillations through control-surface actuator demands.
AcSMC differs from PaSMC in that actual structural vibrations are measured by separate sensors and the data is fed to the FCS. "In this way, the AcSMC can be regarded as a logical extension to the normal FCS," says Burge.
AcSMC can also be used on commercial airliners to reduce wing bending and provide gust load alleviation. AcSMC was used by Airbus Industrie on the fly-by-wire A320, but subsequently dropped for the A319/A321 developments and the widebody A330/340. Airbus says that the system works on the A320, but it was not needed on the other types because of "changes in regulations" governing gust loadings.
There is, however, a potential drawback. "The FCS is not as fast as on a military aircraft - the flying responses are much, much slower," Burge adds.
Source: Flight International
