July 2002
Volume 10, number 4
NASA HL-20 Lifting Body Airframe Modeled with Simulink and the Aerospace Blockset (continued)
Taking a closer look at the airframe components
Examining the 6DoF (Euler angles) subsystem, you will find that it contains six-degrees-of-freedom equations of motion for the airframe. This subsystem is also one of the equations of motion blocks from the Aerospace Blockset. In this example, the body attitude is propagated in time using an Euler angle representation. A quaternion representation is also available in the Aerospace Blockset.
Next, you can click on the subsystem Environment Models to inspect its construction. The environmental models used in the HL-20 airframe are:
- World Geodetic System of 1984 (WGS 84) for gravity
- Committee on Extension to the Standard Atmosphere (COESA) model for atmosphere
- Wind shear, Dryden turbulence, and discrete gust for wind models
Within this Environmental Models subsystem, the wind models can be found in the subsystem Wind Models. All of these standard environmental models are blocks contained in the Aerospace Blockset. These models are implementations of mathematical representations within standard references, such as U.S. Standard Atmosphere, 1976.
 Figure 4: Environmental Models in HL-20 airframe model. Click to enlarge image. |
 Figure 5: Wind Models in HL-20 airframe model. Click to enlarge image. |
Opening the Alpha, Beta, Mach subsystem, you will find additional calculated parameters that are needed for the aerodynamic coefficient computation and lookup. In addition to calculating Mach number and incidence angles, total velocity and dynamic pressure are computed. Also, wind velocity and wind angular acceleration are added to the body velocity and body rates, respectively.
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Figure 6: Additional calculated parameters for HL-20 airframe model
(Alpha, Beta, Mach Subsystem). |
Following the majority of the calculated parameters' signal flow brings you to the Aerodynamic Coefficients subsystem. Within this subsystem, aerodynamic data and equations for calculating the six aerodynamic coefficients are implemented as in NASA TM4302, however the ground effects and the landing gear effects are not used in this aerodynamic model.
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Figure 7: Aerodynamic Coefficients in HL-20 airframe model.
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