Flightloads > Aerodynamics > Unsteady Aerodynamics
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Unsteady Aerodynamics
In order to perform dynamic aeroelastic stability (Flutter) and response analysis, first define the aerodynamics at specific Mach numbers, M (ratio of speed to the speed of sound) and Reduced Frequencies, k (nondimensional frequency). In MSC FLD, these defined data are used as the basis for interpolation to user-defined analysis points. The interpolation is performed on the union of all Mach-Frequency Pair Sets that are used in any given run. Under user control, these data may be interpolated within a Mach number (e.g., frequencies only) or across all Mach-k pairs in the union of sets. Typically only 6 to 10 frequencies per Mach number are required for an adequate interpolation. Too many frequencies can actually degrade the quality of the interpolation!
In this form you are able to define named sets of Mach Frequency pairs. You will then select these sets on a subcase by subcase basis in Flutter Subcase Creation.
The basic approach is to define ranges of values and select Uniform, Biased or Tabular methods to fill interstitial values. The Mach numbers are typically a single value per set, so it is possible to merely specify a Minimum value if only one is desired. The Reduced Frequency may be defined nondimensionally (that is, by k value) or dimesionally by combinations of cyclic frequency (in Hertz) and dimensional speed. The k value is related to these dimensional parameters by the equation:
where F is the cyclic frequency, b is the reference chord length from the Global Data and V is the dimensional Velocity.
You may choose dimensional input as a convenience if you understand the frequency range of your analysis (typically coming from the normal mode frequency range of your vehicle) and the speed range (typically coming from the flight condition and flight envelope).
In either case, the data will be converted into reduced frequency values for computational purposes.
Overview
When the user selects Unsteady Aerodynamics, the first form that displays is Create/MK Pair Set. Shown adjacent to this form are all the different actions that can appear as part of the Unsteady Aerodyanmic option:
Most of the Unsteady Aerodynamic forms are shown and annotated in the following pages, grouped by Action as follows:
• Create
• Delete
• Modify
Unsteady Aerodynamics
The following is a list of the forms included in the Unsteady Aerodynamics section:
• Create/MK Pair Set
• Delete/MK Pair Set
• Modify/MK Pair Set
• Mach-Frequency Pairs Subforms
• Mach-Frequency Pairs/Uniform/NonDimensional
• Mach-Frequency Pairs/Uniform/Dimensional
• Mach-Frequency Pairs/One Way Bias/NonDimensional
• Mach-Frequency Pairs/One Way Bias/Dimensional
• Mach-Frequency Pairs/Tabular
Create/MK Pair Set
MK Pairs are defined by creating a set name and then defining a series of Mach-Frequency Pairs.
 
Note:  
1. The user is able to create duplicates of already existing Mach - Frequency Pair Sets. This can be done by selecting an existing name from the “Existing MK Pair Sets” listbox. This name is entered into the “MK Pair Set Name” databox and the data for this selection is displayed on the “Mach - Frequency Pairs” subform. The name needs to be changed before the “Apply” button is selected otherwise an error message is generated.
2. Selecting “Apply” associates the “Mach - Frequency Pairs” with the “MK Pair Set Name” and stores the data in the database in association with the current Supergroup.
Delete/MK Pair Set
This form allows the user to delete existing MK Pairs from the database.
 
Note:  
1. The user can select any number of Mach - Frequency Pair Sets to be deleted.
2. Selecting “Apply” deletes the “Mach - Frequency Pairs” data and the “MK Pair Set Name” from the database and disassociates it from the current Supergroup.
Modify/MK Pair Set
This form allows the user to modify existing MK Pairs by allowing the set name and the actual Mach-Frequency pairs to be redefined.
 
Note:  
1. The user selects an existing name from the “Existing MK Pair Sets” listbox. This name is entered into the “New MK Pair Set Name” databox and the data for this selection is displayed on the “Mach - Frequency Pairs” subform.
2. This form can also be used to change the name of the “MK Pair Set”. If the name in the “New MK Pair Set Name” databox is different than the selected name in the “Existing MK Pair Sets” listbox, then the new name is associated with the new “Mach - Frequency Pairs” data in the database and the old name/data are deleted.
3. Selecting “Apply” modifies the “Mach - Frequency Pairs” data that is associated with the selected “MK Pair Set Name” and restores the data in the database in association with the current Supergroup.
Mach-Frequency Pairs/Subforms
Mach-Frequency Pairs/Uniform/NonDimensional
This form is used to define the Mach-Frequency Pairs using uniform nondimensional data.
 
Note:  
1. This form is valid for both the Create action and the Modify action. If an existing MK Pair Set is selected, then the data associated with that Mach - Frequency Pair is displayed in the spreadsheet. If this is a modify action, these data can be modified.
2. This form should not be displayed if the “Reference Chord” and the “Reference Span” widgets on the “Global Data” form have not already been defined. A message to direct the user should be issued.
3. M = Mach, k = Reduced Frequency, F = Cyclic Frequency, and V = Velocity.
4. The “Min Mach”, “Max Mach”, “kmin”, and “kmax” databoxes are all of type Real and blank by default.
5. The two “Number” databoxes are both of type Integer and are blank by default.
6. The spreadsheet has 20 rows to start. When the “Add” button is selected, the data entered above is transferred into the spreadsheet below any rows that are already filled in. If there are not enough rows in the spreadsheet to hold the new data the “Add” button causes new rows to be created.
7. When the “OK” button is selected, all the information that is currently in the spreadsheet is stored in the database. This data is to be stored in the same way that the “Trim Parameter” data for the “Static Aeroelasticity” solution type is currently being stored.
8. The “Mach Set” option menu and the “Frequency Set” option menu work independent of one another so they do not have to be set to the same values.
9. If only 1M or 1k is desired Min or Max Uniform with 1 should be allowed.
10. Must have at least 1 Mk pair at the time the OK button is selected.
11. The data from this form is used with the MKAERO1 card.
12. The user should be able to cycle the Mach set and Freq. Set and press the “Add” button multiple times - thereby accumulating pairs.
TMach-Frequency Pairs/Uniform/Dimensional
This form is similar to the main Mach-Frequency Pairs form defined on the previous page, but this version uses Uniform Dimensional data.
 
Note:  
1. F = Cyclic Frequency and V = Velocity.
2. The “Min Mach”, “Max Mach”, “Fmin”, “Vmax”, “Fmax”, and “Vmin” databoxes are all of type Real and blank by default.
3. The two “Number” databoxes are both of type Integer and are blank by default.
Mach-Frequency Pairs/One Way Bias/NonDimensional
This form is similar to the two previous forms that define the Mach-Frequency pairs but this one uses one way bias nondimensional data.
 
Note:  
1. This form is valid for both the “One Way Bias” and the “Two Way Bias” option. The only difference between the two forms is the icon. For the “Two Way Bias” option the icon looks like this:
 
2. k = Reduced Frequency.
3. The “Min Mach”, “Max Mach”, “kmin”, “kmax”, and the two “L2/L1” databoxes are all of type Real and blank by default.
4. The two “Number” databoxes are both of type Integer and are blank by default.
5. These icons are the same ones that are used on the “Mesh Control” forms.
Mach-Frequency Pairs/One Way Bias/Dimensional
This form is similar to all the previous forms that define the Mach-Frequency pairs but this one uses one way bias dimensional data.
 
Note:  
1. This form is valid for both the “One Way Bias” and the “Two Way Bias” option. The only difference between the two forms is the icon. For the “Two Way Bias” option the icon looks like this:
2. F = Cyclic Frequency and V = Velocity.
3. The “Min Mach”, “Max Mach”, “Fmin”, “Vmax”, “Fmax”, “Vmin”, and the two “L2/L1” databoxes are all of type Real and blank by default.
4. The two “Number” databoxes are both of type Integer and are blank by default.
5. These icons are the same ones that are used on the “Mesh Control” forms.
Mach-Frequency Pairs/Tabular
This form is similar to all the previous forms that define the Mach-Frequency pairs but this one uses tabular data.
 
Note:  
1. There are 25 rows in each spreadsheet.
2. The user enters the data (a real number) in the “Input” databox and presses the Return/Enter key to store the data in the “Mach” or “Reduced Freq.” spreadsheet.
3. The user deletes the data in the “Input” databox and presses the Return/Enter key to remove data from the selected cell.