Design and Development of Morphing Wing for Swept Wing Tailless UAV
Principal Investigator: A.Smirnov, Assistant Professor andrei@smirnov.mae.wvu.edu
Co-PI: W.Huebsch, Assistant Professor (wade.huebsch@mail.wvu.edu)
Co-PI: G.Campa, Research Assistant Professor (campa@cemr.wvu.edu)
Department of Mechanical & Aerospace Engineering, West Virginia University
Throughout the history of flight, we have strived to improve the efficiency, control, and
adaptability for different flight regimes of the aircraft we design. The swept-wing tailless
aircraft have long been thought of as a chance at significant improvement in flight
performance. These aircraft exhibit a highly efficient configuration with low parasitic
drag and up to 30\% less total drag. The power plant, fuel, and payload are all built into
the wing structure, which allows the design of a very lightweight structure. With one of
the primary design criterion for many of the current and future Unmanned Aerial
Vehicles (UAV) being the range and/or endurance of the aircraft, the swept-wing tailless
configuration seems an ideal choice in many situations. In addition, the smooth lines and
composite materials allow the design of an aircraft with a minimum electromagnetic
signature, which has become an important metric in today's military aircraft.
Unfortunately, there are a number of problems and design compromises inherent in
tailless aircraft, which currently detract from many of the potential advantages. Some of
the problems and compromises include:
1.Tailless aircraft have a narrow c.g. range. Variations from the design c.g. must
currently be trimmed using elevons (or similar control surface).
2.Many designs experience adverse yaw problems during maneuvering. Current designs
mitigate the adverse problems using a relatively large built-in wash out at the wing
tips, which results in a drag increase during all flight regimes, but is only needed
during the maneuvering
3.Extreme elevon throws can initiate separated flow at the wing tips and cause a tip stall.
This problem is compounded by the increased trim needed for the c.g.
compensation. It has been concluded by members in the tailless aircraft community that
the efficiency could be increased and the poor handling characteristics remedied if there
was a way to adjust the washout in flight. With recent advances in materials and actuator
technology, the authors suggest that wing morphing can now aid in the design of tailless
UAV aircraft.
Some preliminary work has been done over the last few years in this area by
groups such as the military and NASA using various complexity levels of wing
morphing. Currently much work is being directed at potential candidates for the material
to be implemented in a morphing structure ranging from flexible composite material to
smart materials (e.g. nitinol). Overall, it is clear that the different components comprising
wing morphing technology have matured to an adequate level to find application on the
next generation UAV.
Wing morphing would be able to immediately address the compromise of
fixed wing washout. Active morphing of the wing through adaptive washout would be
used for control of the aircraft and used only when needed, which eliminates the
corresponding drag for all non-maneuvering flight regimes. In addition, the wing
morphing approach for variable washout would reduce or eliminate the amount of flow
separation inherent in the extreme elevon throws.
The adaptive washout would remedy some of the inherent problems in
tailless aircraft and improve the overall efficiency. However this morphing mechanism
alone would not have significant impact on the mission adaptability of the aircraft as a
multi-role UAV; for example the ability to range from long endurance to attack roles.
The adaptive washout opens the door to a morphing system for variable sweep. The
adaptive washout would be used to, in part, to compensate for changes in pitching
moment generated by the changes in sweep. The implementation of these two morphing
mechanisms would mark a significant improvement for the swept-wing tailless UAV.
The overall objective of the proposed effort is the investigation and
development of a total wing morphing system for swept-wing tailless UAV that will
optimize the aerodynamic characteristics and improve the efficiency, control and
adaptability. The focus of the study will involve a detailed computational fluid dynamics
investigation on application of wing morphing for UAV and subsequent control system
development that will work in parallel with current experimental wind tunnel efforts at
WVU on UAV wing morphing sponsored by NASA Dryden. The proposed work will
have three main components:
1.Use of CFD to augment the experimental wind tunnel testing and help explore
parameter space to determine optimum aerodynamic configurations. This will include
dynamic models for both adaptive washout as well as variable sweep.
2.Development of preliminary control schemes based on wind tunnel testing and CFD
results for adaptive washout and variable sweep.
3.Design and construct an instrumented 8' swept-wing tailless UAV demonstrator for
flight testing.
The initial push of the proposed work will be to develop a virtual wind
tunnel with CFD to investigate different wing morphing configurations for swept-wing
tailless UAV. Experimental work, sponsored by NASA Dryden, is currently underway to
design, test, and implement wing morphing technology on a UAV-class aircraft with
variable washout on the outer 1/3 of the wing span. This work primarily focuses on
extensive wind tunnel testing of aerodynamic forces and moments for wing
configurations with morphing surfaces and standard control surfaces. Based on the
experimental results, an 8-foot UAV flight demonstrator will be designed and built to
include the variable washout. Future wind tunnel experiments will include variable
morphing sweep as well.
The CFD model will be setup to simulate the flow past the wind tunnel
model with the experimental results being used to validate the CFD simulations. If
needed, the experimental data could be used to calibrate the numerical models to improve
the fidelity of the computations. Once the simulations have been validated, the CFD
model can then be used to efficiently investigate parameter space, which will alleviate
some of the burden of model construction and wind tunnel testing.
CFD simulations will be performed with the commercial code Fluent (either
in RANS and/or LES mode), which is well suited for aerodynamic flows of this nature.
The advanced grid options available with this software will aid in accurately simulating
the flow past a morphed or clean surface. The computer simulations will provide a
virtual test bench that will speed-up the design process and advance the search for the
optimal wing shape and morphing characteristics.
The two focus areas of wing morphing for a tailless UAV will be: 1)
variable washout on the outer 1/3 of the wing span and 2) variable sweep morphology for
the outer 2/3 of each half span. For brevity, the details on the implementation of these
morphing mechanisms is not included, but would be in a full proposal.
Part of the tasks for both the adaptive washout and variable sweep will be
for flight control of the tailless UAV. Obviously, the optimum scenario will be for these
mechanisms to work in concert to affect pitch, roll, and yaw. However, the use of both
variable washout and sweep will also augment the variety of control combinations to
achieve fault tolerance for the UAV. For example, a failed variable washout mechanism
on one side could be countered by the use of variable sweep.
Development of the right control system will be an important part of the
project, since a morphing wing presents substantial challenges from a control system's
point of view, such as: (1) finding the optimal number of actuators and sensors to place
on the wing, in order to obtain the desired capabilities, and %Of course cost, physical
limitations, and technolgical issues must each play %a considerable part in deciding how
many actuators and sensors are needed. (2) identifying the optimal location for sensors
and actuators on the wing.
From a modeling perspective a morphing wing has infinite degrees of
freedom, so it is an infinite dimensional system, which must be described by a PDE.
Control of such high order multi input multi output (MIMO) systems presents some
peculiar characteristics. It is usually impossible, to directly sense all the system states,
therefore, controllers for those systems often include kalman-filter based observers to
internally reconstruct all the states of the system. Techniques like H-Infinity and MuSynthesis can guarantee a certain degree of control robustness, while often placing harder
computational requirements on the control system. A careful mixing of the above
techniques with feedback linearization, gain scheduling and LMI synthesis for piecewise
linear system must then be evaluated to solve this kind of problem.
There will be a total of two UAV flight demonstrators built for testing of
wing morphing technologies. The first will be under the NASA Dryden wing morphing
project and will be based on the wind tunnel testing currently underway. This first 8-foot
demonstrator will make use of the variable washout as a flight control mechanism and to
improve some of the deficiencies in tailless aircraft. The second flight demonstrator will
make use of both variable washout as well as variable sweep wing morphing. The design
will be based on results from the wind tunnel testing, CFD simulations, and control
strategies developed during the proposed project.
This project will blend current experimental efforts with CFD, control
scheme development, and UAV flight demonstrator testing to provide an overall strategy
for investigation of wing morphing on UAV platforms. The participants of the project
have many years of experience in respective areas, and are known for their contributions
to unsteady wing aerodynamics, CFD, control theory, and the design and development of
UAV-type aircraft.