Flight Control: CIFER®

System Overview

The identification and analysis procedures have been implemented in a comprehensive set of user-oriented programs supported by extensive databasing and utilities. Such integrated software packages are critical in the application systems identification tools; CIFER is the first such integrated package for the frequency response methodology. A functional layout of CIFER is shown in figure 1 and a detailed description of the engineering methods and results for the BO-105 helicopter are given in ref 1.

The CIFER user interface was designed and implemented in such a way as to relieve the user of bookkeeping concerns. All the information needed to run a particular program is saved in a database which can be reassessed by simply specifying the "case name". The program parameters comprising this case are then presented to the user on a series of screens, through which the user steps (forward or backward), changing default and/or saved values as needed. When all the input choices are made for a particular program, the system submits a batch job to conduct the computations and generate any requested plot files. This allows rapid evaluation of alternative models and flight data.

An example of the user interface is shown in figure 2. The screen presented is for a typical session using DERIVID. This is representative of the complexity and flexibility of the state-space model that can be identified. As shown in figure 2 the section of the F matrix displayed consists of parameters with fixed and variable parts. Each element can be given a derivative name and each derivative can be fixed or freed. Further, any fixed derivative can be constrained to be a linear function of any free derivative. Changes in the model definition are achieved by simply moving the cursor around on the user screens and changing the default or previously saved setups. The user can update the database with the changes that are made

Case information from the various programs as well as all output results are stored in a series of databases that are accessed by CIFER. The user can update the database with the changes that are made at each screen using a single keystroke. Utilities have been developed to allow quick inspection, searching, plotting, or tabulated output of the contents of the database. The extensive and integrated databasing in CIFER is a key requirement for organizing and analyzing the large amounts of data which are generated for flight test identification projects.

CIFER is a uniquely suited tool for comprehensive analysis of aircraft and component dynamics. The CIFER system has been exercised on a wide range of rotary-wing and fixed-wing aircraft flight programs including: XV-15, Bell-214ST, BO-105, AH-64, UH-60, V-22, AV-8 Harrier, and OH-58D. Applications to analysis of simulations/simulators include the ASTOVL, LHX, and VMS. Examples of CIFER state-space models (6dof and 9dof) identification results for a highly coupled helicopter (BO-105) are shown in figure 3, figure 4, and figure 5, Lateral/directional identification results for a fixed-wing aircraft (XV-15 in cruise) are presented in figure 6, figure 7, and figure 8 In both cases, the verification results are for data NOT used in the identification and so demonstrate the excellent predictive capability of the models. As can be seen from the results the off-axis responses are very closely matched, which are often missed by simulation models, especially for helicopters. Simulation model fidelity analysis and improved modeling via CIFER identification is demonstrated for the UH-60 (ref. 2) in figure 9. An application of CIFER to handling-qualities studies is given in ref 3 with typical results shown in figure 10. Identification of AV-8B Harrier wing bending modes from telemetered flight test data at 120Kts is shown in figure 11. A detailed case study using CIFER for aeroelastic analysis is given in ref 4.

The CIFER software is available for Windows and UNIX/Linux platforms. All graphics and computational libraries for CIFER are self-contained and do not depend on access to proprietary packages. The final dependency is in the way time history data is made available to the software. You will only have to modify the time-history data interface routine to access your database before being able to run CIFER.

CIFER is available and distributed via UARC. A comprehensive 25 hour video of engineering lectures and computer demonstration on CIFER has been developed to train new users. CIFER is a proven analysis tool that is operated by over 20 US and international research organizations. More information on CIFER is available by contacting Dr. Mark B. Tischler (650) 604-5563.

REFERENCES

1. Tischler, M.B., Cauffman, M. G., "Frequency-Response Method for Rotorcraft System Identification: Flight Applications to BO-105 Coupled Rotor/Fuselage Dynamics," Journal of the American Helicopter Society, Vol 37, No. 3, pg 3-17, July, 1992.

2. Fletcher, J. W., "Identification of UH-60 Stability Derivative Models in Hover from Flight Test Data," 49th Annual meeting of the American Helicopter Society, May 19-21, St. Louis, Mo.

3. Ham, J. A., Gardner, C. K., Tischler, M. B., "Flight Testing and Frequency Domain Analysis for Rotorcraft Handling Qualities Characteristics," AHS Specialists' Meeting on Piloting Vertical Flight Aircraft, 20-23 Jan, 1993, San Francisco, Ca.

4. Acree, C. W., Jr., Tischler, M. B., "Determining XV-15 Aeroelastic Modes from Flight Data with Frequency-Domain Methods," NASA TP 3330, USAATCOM TR-93-A-004, 1993.