Biologically inspired concepts are rapidly expanding the range of aircraft technology. However, almost all previous work on actively controlled lifting surfaces has concentrated upon distributed actuation across the area and on hinged trailing edge treatments.
Biologically Inspired Flight Vehicle Lifting
The proposed technology involves membrane concepts for biology inspired flight vehicle lifting and control surfaces that do not require complex actuation, guide tracks or support structure. MORFS truly emulates the control exerted by some biological organisms in as much as the location of applied strains around the perimeter of a closed section such as an airfoil can be changed with dramatic results.
The MORFS System
MORFS combines reinforcements and a soft matrix continuum into a unique composite where structural coupling can reasonably be two orders of magnitude larger than in any other current material. In this way a relatively small actuation strain can be amplified 10 - 200 times in the orthogonal directions purely as a material response. No literature has been found which discusses material response of this magnitude and which may not require distributed actuation to be effective.
The key relies on MORFS highly unorthodox material architecture, suitable for managing fluid flows in flight vehicle control, pumping and propulsion applications. Stimulating input strains might be axial tension, torsion, or thermal, to mention a few, but amplified responses trade off dimensions in orthogonal directions. For example, applied torsion on a wing section results in an increased airfoil perimeter and reduced wall thickness. The choice of boundary conditions determines whether the newly cambered airfoil exhibits nose up or nose down attitude. Conceivably the morphing shape might react fast enough to cancel unwanted buffet in addition to major control inputs.
Aeroelastic tailoring used in conventional composite wings can improve lift/drag ratios and stability only over a portion of the flight envelope without deploying high drag devices. Forward swept wing designs might finally become practical using adaptive clean profiles over outboard portions of a wing.
Application in Helicopter Blade Tips
Helicopter blade tips employing adaptive membrane technology might reduce noise and vibration and reduce or eliminate power loss at the swashplate via a novel approach to pitch control.
This strain amplifying material may only need to be sheared or tensioned in a manner consistent with typical loading actions to produce a doubling of lift from a clean wing without leading or trailing edge surfaces. The response should be at the speed of sound in the material and inherent damping in the elastomeric matrix composite may be serendipitous. Avoidance of traditional mechanical wearing parts promises large weight reductions and increased reliability in all fluid management devices of a suitable size.
Manufacturing MORFS Components
MORFS giant response material can take advantage of many of the manufacturing expedients that have been developed for conventional composites including hand lay-up, pultrusion, vacuum infusion and resin transfer molding.
Composites configured using soft matrix materials inevitably must be deployed in structures which avoid significant compression loads – just like natural organisms. While a number of techniques can be used to achieve this, it is also true that such materials are not fatigue limited. That is, the static design limit is also the fatigue endurance limit.
Actuation of MORFS Components
In addition to conventional means of applying small mechanical strains for amplification, high frequency and highly controllable piezo-electric, magneto-strictive, electro-strictive, and similar means can be used.
By packaging this layered, composite into an airfoil the giant muscular-like response can be manifested as a change in lift and hence used for flight control. It may allow flight vehicle control at reduced weight, reduced part count and with increased reliability with no traditional moving parts. In cylindrical or similar configurations paired, co-axial membranes might react against one another using differential tension to create a volume displacing device suitable for pumping or propulsion (see fig. 3).
Figures1, 2, and 3 all show the remarkable behaviour of this strain amplifying material when packaged in a useful geometry. Figure 2 is the end view of the wing section shown in figure 1 and clearly shows the large change in camber with the incidence unaffected. Changing the location of applied strains and reactions around the perimeter can also produce large changes in incidence.
Figure 1. Boundary conditions and applied strains are shown for a NACA006.
Figure 2. Orthogonal response of a strain amplifying membrane.
Figure 3. Cylindrical response of strain amplifying membrane. 0.71” reduction in 2.0” radius results from 0.2” extension of 20.0” long cylinder. Fixed boundary conditions do not inhibit boundary response.
Given the muscular-like, highly oriented nature of the membrane, differential tensile loads are most suitable. This means that the lift loads generated must be beamed to the centerline of the aircraft using either a parallel ‘skeleton’ or a pre-tensioned skin which morphs via selective release of the pre-tension. A large number of permutations of both loads and fixities are achievable commensurate with practical blade or wing root attachments. Alternatively, torsional moments might be introduced between ribs for a smaller but still very large response. Principal difference is in the requisite fiber architecture and the resulting ability to sustain larger fluid pressures.
Figure 3 shows a cylindrical response with limited restraint offered by the root fixity.
The originating company has experience with flexible matrix composites and is well equipped for composite manufacturing, testing and analysis. In particular it has worked toward a viable continuous pultrusion process using these materials.
In December 2002 the company became the only company worldwide to meet Boeing’s pultruded composite strength specifications.
Additional composite innovations include fully integral flexible composite driveshafts which match the performance of current helicopter driveshafts at less than half the weight.