The paper begins with a brief historical overview of pressure adaptive materials and structures. By examining avian
anatomy, it is seen that pressure-adaptive structures have been used successfully in the Natural world to hold structural
positions for extended periods of time and yet allow for dynamic shape changes from one flight state to the next. More
modern pneumatic actuators, including FAA certified autopilot servoactuators are frequently used by aircraft around the
world. Pneumatic artificial muscles (PAM) show good promise as aircraft actuators, but follow the traditional model of
load concentration and distribution commonly found in aircraft. A new system is proposed which leaves distributed
loads distributed and manipulates structures through a distributed actuator. By using Pressure Adaptive Honeycomb
(PAH), it is shown that large structural deformations in excess of 50% strains can be achieved while maintaining full
structural integrity and enabling secondary flight control mechanisms like flaps. The successful implementation of
pressure-adaptive honeycomb in the trailing edge of a wing section sparked the motivation for subsequent research into
the optimal topology of the pressure adaptive honeycomb within the trailing edge of a morphing flap. As an input for the
optimization two known shapes are required: a desired shape in cruise configuration and a desired shape in landing
configuration. In addition, the boundary conditions and load cases (including aerodynamic loads and internal pressure
loads) should be specified for each condition. Finally, a set of six design variables is specified relating to the honeycomb
and upper skin topology of the morphing flap. A finite-element model of the pressure-adaptive honeycomb structure is
developed specifically tailored to generate fast but reliable results for a given combination of external loading, input
variables, and boundary conditions. Based on two bench tests it is shown that this model correlates well to experimental
results. The optimization process finds the skin and honeycomb topology that minimizes the error between the acquired
shape and the desired shape in each configuration.
A new type of adaptive structure is presented that relies on pressurized honeycomb cells that extent a significant
length with respect to the plane of the hexagons. By varying the pressure inside each of the cells, the stiffness can
be altered. A variable stiffness in combination with an externally applied force field results in a fully embedded
pressure adaptive actuator that can yield strains well beyond the state-of-the-art in adaptive materials. The
stiffness change as a function of the pressure is modeled by assigning an equivalent material stiffness to the
honeycomb walls that accounts for both the inherent material stiffness as the pressure-induced stiffness. A finite
element analysis of a beam structure that relies on this model is shown to correlate well to experimental results of
a three-point bend test. To demonstrate the concept of embedded pressure adaptive honeycomb, an wind tunnel
test article with adaptive flap has been constructed and tested in a low speed wind tunnel. It has been proven
that by varying the cell pressure the flap changed its geometry and subsequently altered the lift coefficient.
The dynamic response of a new class of flight control actuators that rely on post-buckled precompressed (PBP)
piezoelectric elements is investigated. While past research has proven that PBP actuators are capable of generating
deflections three times higher than conventional bimorph actuators, this paper quantifies the work output and power
consumption under various axial loads, at various frequencies. An analytical model is presented that supports the
experimental findings regarding the increasing work output and natural frequency shift under increasing axial loads.
Furthermore, increasing axial loads shows an increase in open-loop piezoelectric hysteresis, resulting in an increasing
phase lag in actuator response. Current measurements show an electromechanical coupling that leads to power peaks
around the natural frequency. Increasing axial loads has no effect on the power consumption, while increasing the work
output by a factor of three, which implies a significant increase in work density over the piezoelectric material itself.
This paper describes a new class of flight control actuators using Post-Buckled Precompressed (PBP)
piezoelectric elements mounted within a transonic missile fin. These actuators are designed to produce
significantly higher deflection and force levels than conventional piezoelectric actuator elements. Classical
laminate plate theory (CLPT) models are shown to work very well in capturing the behavior of the free, unloaded
elements. A new high transverse deflection model which employs nonlinear structural relations is shown to
successfully predict the performance of the PBP actuators as they are exposed to higher and higher levels of axial
force, which induces post buckling deflections. A 6" (15.2cm) square rounded diamond transonic fin was made
with integral PBP actuator elements. Quasi-static bench testing showed deflection levels in excess of ±7° at rates
exceeding 21 Hz. The new solid state PBP actuator was shown to reduce the part count with respect to
conventional servoactuators by an order of magnitude. Power consumption dropped from 24W to 1.3W, slop
went from 1.6° to 0.02° and peak current draw went from 5A to 18mA. The PBP actuator was wind tunnel tested
and shown to possess no flutter, divergence or adverse aeroelastic coupling characteristics.
This paper describes a new class of flight control actuators using Post-Buckled Precompressed (PBP)
piezoelectric elements to provide much improved actuator performance. These PBP actuator elements are modeled
using basic large deflection Euler-beam estimations accounting for laminated plate effects. The deflection
estimations are then coupled to a high rotation kinematic model which translates PBP beam bending to stabilator
deflections. A test article using PZT-5H piezoceramic sheets built into an active bender element was fitted with an
elastic band which induced much improved deflection levels. Statically the bender element was capable of
producing unloaded end rotations on the order of ±2.6°. With axial compression, the end deflections were shown to
increase nearly 4-fold. The PBP element was then fitted with a graphite-epoxy aeroshell which was designed to
pitch around a tubular stainless steel main spar. Quasi-static bench testing showed excellent correlation between
theory and experiment through ±25° of pitch deflection. Finally, wind tunnel testing was conducted at airspeeds up
to 120kts (62m/s, 202ft/s). Testing showed that deflections up through ±20° could be maintained at even the highest
flight speed. The stabilator showed no flutter or divergence tendencies at all flight speeds. At higher deflection
levels, it was shown that a slight degradation deflection was induced by nose-down pitching moments generated by
separated flow conditions induced by extremely high angles of attack.
This paper presents the use of a new class of flight control actuators employing Post-Buckled Precompressed (PBP) piezoelectric elements in morphing wing Uninhabited Aerial Vehicles (UAVs). The new actuator relies on axial compression to amplify deflections and control forces simultaneously. Two designs employing morphing wing panels based on PBP actuators were conceived. One design employed PBP actuators in a membrane wing panel over the aft 60% of the chord to impose roll control on a 720mm span subscale UAV. This design relied on a change in curvature of the actuators to control the camber of the airfoil. Axial compression of the actuators was ensured by means of rubber bands and increased end rotation levels with almost a factor of two up to ±13.6° peak-to-peak, with excellent correlation between theory and experiment. Wind tunnel tests quantitatively proved that wing morphing induced roll acceleration levels in excess of 1500 <i>deg/s<sup>2</sup></i>. A second design employed PBP actuators in a wing panel with significant thickness, relying on a highly compliant Latex skin to allow for shape deformation and at the same time induce an axial force on the actuators. Bench tests showed that due to the axial compression provided by the skin end rotations were increased with more than a factor of two up to ±15.8° peak-to-peak up to a break frequency of 34Hz. Compared to conventional electromechanical servoactuaters, the PBP actuators showed a net reduction in flight control system weight, slop and power consumption for minimal part count. Both morphing wing concepts showed that PBP piezoelectric actuators have significant benefits over conventional actuators and can be successfully applied to induce aircraft control.
This paper describes a new class of flight control actuators using Post-Buckled Precompressed (PBP) piezoelectric elements. These actuators are designed to produce significantly higher deflection and force levels than conventional piezoelectric actuator elements. Classical laminate plate theory (CLPT) models are shown to work very well in capturing the behavior of the free, unloaded elements. A new high transverse deflection model which employs nonlinear structural relations is shown to successfully predict the performance of the PBP actuators as they are exposed to higher and higher levels of axial force, which induces post buckling deflections. A proof-of-concept empennage assembly and actuator were fabricated using the principles of PBP actuation. A single grid-fin flight control effector was driven by a 3.5" (88.9mm) long piezoceramic bimorph PBP actuator. By using the PBP configuration, deflections were controllably magnified 4.5 times with excellent correlation between theory and experiment. Quasi-static bench testing showed deflection levels in excess of ±6° at rates exceeding 15 Hz. The new solid state PBP actuator was shown to reduce the part count with respect to conventional servoactuators by an order of magnitude. Power consumption dropped from 24W to 100mW, weight was cut from 108g to 14g, slop went from 1.6° to 0.02° and current draw went from 5A to 1.4mA. The result was that the XQ-138 subscale UAV family experienced nearly a 4% reduction in operating empty weight via the switch from conventional to PBP actuators while in every other measure, gross performance was significantly enhanced.