Active Aeroelastic Wing
Sarah Chandrasekar
Department of Engineering, Calvin College
Engineering 315 Final Paper
Professor Ribeiro
Abstract : Aero elastic Wing will soon take the place of today's wing control. These changes ought to generate savings in weight and increases in reliability, employing technologies that have been in development and testing for years. The research and development required for developing MAVs and related systems is technically challenging and requires a number of technological advances that may benefit a broad range of aerospace applications. The development of a vehicle with aero elastic wing could also promote development of component technologies and help to support an emerging growth market for micro aerial vehicles.
1.0 Intr oduction
The Air Force calls it “back to the future”—taking an element of the Wright brothers’ original aircraft, a feature deliberately engineered out of modern planes, and putting it back, literally. In this case, it means restoring preproduction wings on a Navy F/A-18—wings that had been replaced because they twisted in flight. The active aeroelastic wing (AAW) aircraft is a unique joint research effort involving the Navy, the Air Force Research Lab at Wright-Patterson AFB, NASA-Dryden, and the Boeing Phantom Works. Funding for the Navy plane comes from AFRL’s Air Vehicles Directorate and NASA’s Office of Aerospace Transportation Technology, with Boeing performing the F/A-18 modifications under contract to the Air Vehicles Directorate. For his historic first flight on December 17, 1903, Orville Wright used the movements of his hips in the airplane’s “saddle”—in which he lay prone—to twist or warp either the left or right wingtip. This provided flight control without the use of ailerons or flaps. While such aeroelastic warping is inherent in the wings of modern high-speed aircraft, engineers have done everything possible to counteract it, from physically stiffening the wings to incorporating other control surfaces.
The degree of twisting involved is actually rather small—less than 4°. AAW technology you can provide a weight-competitive wing, reduce drag, improve range, and reduce fuel consumption, because you have a more aerodynamically efficient wing with an increased aspect ratio1.
Identify 3 axes of rotation
Aircraft fly in three dimensions, and they move in directions other than straight and level. The axis that extends lengthwise (nose through tail) is called the longitudinal axis, and rotation about this axis is called roll. The axis that extends crosswise (wingtip through wingtip) is called the lateral axis, and rotation about this axis is called pitch. The axis that passes vertically through the center of gravity (when the aircraft is in level night) is called the vertical axis, and rotation about this axis is called yaw.
Figure 1
Role of Aircraft wings in different Maneuvers
Basic flight maneuvers include climbs, descents, turns, and combinations of these. Generally, the basic flight maneuvers are started from what is called straight and level flight. Straight and level flight (also called controlled flight) is a flight condition where the wings are kept level and the altitude and heading constant. Power setting is maintained at 55 percent to 75 percent of available power. If speed is desired a higher setting is required, however if fuel needs to be saved then a lower setting is required. Straight and level flight is a series of slight adjustments or corrections in pitch, yaw, and roll to keep the wings level and heading and altitude constant. Climbs are a combination of power and "up elevator." The amount of power used determines whether the climb is steep or shallow. In order to use all available power, the climb angle must be as steep as possible. This is called the best angle of climb, but it is a short-term climb. A sustained climb at this angle can overheat the engine. The third basic maneuver is the turn. Turns are gentle, medium, or steep; and they may be made when climbing, descending, or while not gaining or losing altitude. Causing the airplane to turn requires smooth coordination of aileron, rudder, and elevator controls; in other words, pressure on the control wheel and the rudder pedal should be applied simultaneously. The moment a wing begins to rise in a banked turn, it experiences more drag because of the lowered aileron and its higher angle of attack. A simultaneous application of rudder compensates for this additional drag by making the airplane also rotate about its vertical axis.
Figure 2: Elements of a turn
2. 0 History
When Orville Wright first took to the air on Dec. 17, 1903, he didn't have ailerons or flaps to control his airplane. Instead, the Wright brothers had chosen to twist or "warp" the wingtips of their craft in order to control its rolling or banking motion. Rather than using one of the craft's two control sticks to make the wingtips twist, they had devised a "saddle" in which the pilot lay. Cables connected the saddle to the tips of both wings. By moving his hips from side-to-side, the pilot warped the wingtips either up or down, providing the necessary control for the Wright Flyer to make turns. The test aircraft chosen for the AAW research is a modified F/A-18A obtained from the U.S. Navy in 1999. Begun in 1996, the AAW flight research program has completed detailed design and the wing modifications required for the program have been completed. The test aircraft has been extensively instrumented, and reassembly was completed by early 2001. Over the course of the year, the AAW test aircraft was subjected to extensive structural loads, wing stiffness and vibration tests, installation of the initial control software into the aircraft's research flight control computer, systems checkout and flight simulation activity. The first parameter identification flights in the two-phase flight test program are expected to begin in mid-2002 and continue for about six months. These flights will be used to measure the forces available from each surface to twist the wing and control the aircraft. That will be followed by a yearlong period of data analysis and control software redesign to optimize the performance of the flexible wing. The final phase of flight tests are expected to be flown in 2003, and will evaluate the handling and performance qualities available from the flexible wing concept.2
3 .0 Model for F14 Jet aircraft
The following diagram displays the top level of the model of a flight controller for the longitudinal motion of a Grumman Aerospace F-14 aircraft
Figure 3: Model of F14 Jet aircraft
The model simulates the pilot's stick input with a square wave having a frequency of 0.5 (radians per second) and an amplitude of ± 1. The system outputs are the aircraft angle of attack and the G forces experienced by the pilot. The input and output signals are visually monitored by Scope blocks.
Figure 4: Pilot G-Force Scope
The angle-of attack range extends from -10 to 90 deg, with tunnel data used where it is available and estimates where it is not. Linear interpolation of tabulated data is used to model nonlinearities in aerodynamic coefficients. Control surface effects are linear in their respective deflections, and static lateral-directional effects are linear in sideslip angle. The derivatives for both vary with angle of attack to the limits of data presented in the reference and are constant at higher angle. Angular rate derivatives are constant throughout the angle of attack range.
Figure 5: Angle of Attack
The different components of the model of the F14 are described in the following sections, Controller, Aircraft dynamics model and the Nz pilot calculation
3 .1 Controller
Figure 6: Controller
Stick Input
The stick input is the input from the pilot to control the aircraft. Maneuvers are made possible through the stick input.
Figure 7: Stick Input
Alpha (Rad)
The Alpha is calculated in radians from the vertical velocity w (ft/sec) once it is passed through a gain. The vertical velocity w (ft/sec) is explained in section 3.2
Figure 8: Alpha
q (rad/sec) (also called Pitch Rate)
Pitch rate is explained in detail in section 3.2.
Figure 9: Q (rad/sec)
Output of the controller
The output of the controller is also called the Elevator Command (deg) which one of the inputs to the Aircraft Dynamics Model
Figure 10: Output of the controller
3 .2 Aircraft Dynamics Model
Figure 11: Aircraft Dynamics Model
The aircraft is subject to random gusts of wind. The aircraft responds quite strongly to the gusts of wind, thus making it difficult for the human pilot to maintain control. The gusts of winds can differentiate as Vertical Gust (ft/sec) and Rotary Gust (rad/sec). The inputs of the Vertical Gust referred to as wGust is given in Figure: 12, and the Rotary gust referred to as qGust is given in Figure: 13.
Vertical Gust (ft/sec)
Vertical Gust is the component of gust winds that exhibits vertical motion and is measured in ft/sec.
Figure 12: Vertical Gust(ft/sec)
Ro tary Gust (rad/sec)
Rotary Gust is the gust winds that exhibit rotary motion and is measured in rad/sec
Figure 13: Rotary Gust (rad/sec)
Vertical Velocity, w (ft/sec)
The vertical velocity is the final output after the Vertical Gust is passed through the transfer function coupled with the pitch rate.
Figure 14: Vertical Velocity (ft/sec)
Pitch Rate, q (rad/sec)
The pitch rate is the output of the rotary gust passed through a transfer function, gains and coupled with the value of the velocity rate.
Figure 15: Pitch Rate (rad/sec)
2.3 Nz Pilot Calculation
Figure 16: Control system for Nz Pilot Calculation
Output signal before gain
Figure 17
Pilot g Force (g) (Output signal after gain)
This is one of the final outputs for the F 14 jet aircraft. It is explained in detail in section 3.0
Figure 18: Pilot Force
3. 0 Design requirements For the Pitch Controller
The next step is to set some design criteria for the pitch controller. We want to design a feedback controller so that the output has an overshoot of less than 10%, rise time of less than 2 seconds, settling time of less than 10 seconds, and the steady-state error of less than 2%. For example, if the input is 0.2 rad (11 degrees), then the pitch angle will not exceed 0.22 rad, reaches 0.2 rad within 2 seconds, settles 2% of the steady-state within 10 seconds, and stays within 0.196 to 0.204 rad at the steady-state.
· Overshoot: Less than 10%
· Rise time: Less than 2 seconds
· Settling time: Less than 10 seconds
· Steady-state error: Less than 2%
3.1 Transfer function
Figure 19: Shows longitudinal, lateral and vertical axis
The longitudinal equations of motion of this aircraft are the following, assuming zero initial conditions. To find the transfer function of the above system, we also need to take the Laplace transform. W hen finding a transfer function, zero initial conditions must be assumed
After doing some calculations we are able to calculate the following transfer function
These values are taken from the data from one of the Boeing's commercial aircraft.
3.3 Matlab representation and open-loop response
Now, we are ready to observe the system characteristics using Matlab. First, let's obtain an open-loop system to a step input and determine which system characteristics need improvement. Let the input (delta e) be 0.2 rad (11 degrees). Create an new m-file and enter the following commands.
de=0.2;
num=[1.151 0.1774];
den=[1 0.739 0.921 0];
step (de*num,den)
Running this m-file in the Matlab command window should give you the plot on the next page.
From the plot, we see that the open-loop response does not satisfy the design criteria at all. In fact the open-loop response is unstable.
3.2 PID Controller
In order to correct the problem with the open loop response we can use a PID controller in a feedback loop.
Using the following equation we can determine the value of the proportional (Kp), integral(Ti) and derivative(Td)
After a lot of calculations we get the following template with the transfer function
We use the following code in m file in order to execute the function in Matlab
de=0.2;
Kp=9;
Td=3;
Ti=4;
numc=[1.151*Kd 1.151*Kp+0.1774*Kd 0.1774*Kp];
denc=[1 0.739+1.151*Kd 0.921+1.151*Kp+0.1774*Kd 0.1774*Kp];
t=0:0.01:10;
step (de*numc,denc,t)
When we use only Kp = 9, we get the following graph
We see that both the overshoot and the settling time need improvement. Thus, we use a derivative gain Td=4. Then we get the following graph
We add the integral in order to get a smoother curve. Ti=4. Then we get the following graph
3. Fundament al s of Active Aeroelastic Wing (AAW)
Figure 1 shows the functionality of the aeroelastic wing.
Figure 20: Difference between conventional aileron and active aeroelastic wing, while performing Right Roll command
Figure 21: Structural testing on AAW
The wings from Dryden's F-18 #840, formerly used in the High-Alpha Research Vehicle (HARV) program, have been modified for the AAW flight research program and installed on the AAW test aircraft. Several of the existing wing skin panels along the rear section of the wing just ahead of the trailing-edge flaps and ailerons have been replaced with thinner, more flexible skin panels and structure, similar to the prototype F-18 wings. Original F-18 wing panels were light and flexible. During early F-18 flight tests, however, the wings were observed to be too flexible at high speeds for the ailerons to provide the specified roll rates. This was because the high aerodynamic forces against a deflected aileron would cause the wing to deflect in the opposite direction. In addition, the F/A-18's leading-edge flap has been divided into separate inboard and outboard segments, and additional actuators have been added to operate the outboard leading-edge flaps separately from the inboard leading-edge surfaces. By using the outboard leading-edge flap and the aileron to twist the wing, the aerodynamic force on the twisted wing will provide the roll forces desired. Now, a flexible wing will have a positive control benefit rather than a negative one. In addition to the wing modifications, a new research flight control computer has been developed for the AAW test aircraft, and extensive research instrumentation, including more than 350 strain gauges, have been installed on each wing.
References
[1]ACTIVE AEROELASTIC WING: A NEW/OLD TWIST ON FLIGHT
a.org/aerospace/Article.cfm?issuetocid=256&ArchiveIss u eID=30
[2]a.gov/centers/dryden/news/FactSheets/FS-061-DFRC . html
[3]in.umich.edu/group/ctm/examples/pitch/digPCSS.html
[4]Modeling Aircraft Wing loads from Flight data using Neural Networks www.dfrc.nasa.gov/DTRS/2003/PDF/H-2546.pdf
[6]labs.arc.nasa.gov/vms/controls.html
[7] MATLAB toolbox