Aerospace Blockset™

Digital DATCOM Forces and Moments

Compute aerodynamic forces and moments using Digital DATCOM static and dynamic stability derivatives

Library

Aerodynamics

Description

The Digital DATCOM Forces and Moments block computes the aerodynamic forces and moments about the center of gravity using aerodynamic coefficients from Digital DATCOM.

Algorithms for calculating forces and moments build up the overall aerodynamic forces and moments (F and M) from data contained in the Digital DATCOM structure parameter:

F = F_static + F_dyn

M = M_static + M_dyn

F_static and M_static are the static contribution, and F_dyn and M_dyn the dynamic contribution, to the aerodynamic coefficients. If the dynamic characteristics are not contained in the Digital DATCOM structure parameter, their contribution is set to zero.

Static Stability Characteristics

Static stability characteristics include the following.

Coefficient	Meaning
C_D	Matrix of drag coefficients. These coefficients are defined positive for an aft-acting load.
C_L	Matrix of lift coefficients. These coefficients are defined positive for an up-acting load.
C_m	Matrix of pitching-moment coefficients. These coefficients are defined positive for a nose-up rotation.
C_Yβ	Matrix of derivatives of side-force coefficients with respect to sideslip angle
C_nβ	Matrix of derivatives of yawing-moment coefficients with respect to sideslip angle
C_lβ	Matrix of derivatives of rolling-moment coefficients with respect to sideslip angle

These are the static contributions to the aerodynamic coefficients in stability axes.

C_{D static} = C_D

C_{y static} = C_Yββ

C_{L static} = C_L

C_{l static} = C_lββ

C_{m static} = C_M

C_{n static} = C_nββ

Dynamic Stability Characteristics

Dynamic stability characteristics include the following.

Coefficient	Meaning
C_lq	Matrix of rolling-moment derivatives due to pitch rate
C_mq	Matrix of pitching-moment derivatives due to pitch rate
C_Ldα/dt	Matrix of lift force derivatives due to rate of angle of attack
C_mdα/dt	Matrix of pitching-moment derivatives due to rate of angle of attack
C_lp	Matrix of rolling-moment derivatives due to roll rate
C_Yp	Matrix of lateral force derivatives due to roll rate
C_np	Matrix of yawing-moment derivatives due to roll rate
C_nr	Matrix of yawing-moment derivatives due to yaw rate
C_lr	Matrix of rolling-moment derivatives due to yaw rate

These are the dynamic contributions to the aerodynamic coefficients in stability axes.

C_{D dyn} = 0

C_{y dyn} = C_yp p(b_ref/2V)

C_{L dyn} = C_L (c_bar/2V)

C_{l dyn} = (C_lpp + C_lqq + C_lrr) (b_ref/2V)

C_{m dyn} = (C_mqq + C_m ) (c_bar/2V)

C_{n dyn} = (C_npp + C_nrr) (b_ref/2V)

Dialog Box

Units

Specifies the input and output units:

Units	Force	Moment	Length	Velocity	Pressure
`Metric (MKS)`	Newton	Newton- meter	Meters	Meters per second	Pascal
`English (Velocity in ft/s)`	Pound	Foot-pound	Feet	Feet per second	Pound per square inch
`English (Velocity in kts)`	Pound	Foot-pound	Feet	Knots	Pound per square inch

Digital DATCOM structure

Specifies the MATLAB^® structure containing the digital DATCOM data. This structure is generated by the Aerospace Toolbox function datcomimport.

Force axes

Specifies coordinate system for aerodynamic force: Body or Wind.

Interpolation method

None (flat) or Linear

Extrapolation method

None (clip) or Linear

Process out of range input

Specifies how to handle out-of-range input: Linear Extrapolation or Clip to Range.

Action for out of range input

Specifies if out-of-range input invokes a warning, an error, or no action.

Inputs and Outputs

Input	Dimension Type	Description
First	3-by-3 matrix	Contains the angle of attack.
Second		Contains the sideslip angle, in radians.
Third		Contains the Mach number.
Fourth		Contains the altitude, in selected length units.
Fifth		Contains the dynamic pressure, in selected pressure units.
Sixth		Contains the velocity, selected velocity units and selected force axes.
Seventh (Optional)		Contains the angle of attack rate, in radians per second.
Eight (Optional)		Contains the body angular rates, in radians per second.
Ninth (Optional)		Contains the ground height, in select units of length
Tenth (Optional)		Contains the control surface deflections, radians.

Output	Dimension Type	Description
First		Contains the aerodynamic forces at the center of gravity in selected coordinate system: Body (F_x, F_y, and F_z), or Wind (F_D, F_y, and F_L).
Second		Contains the aerodynamic moments at the center of gravity in body coordinates (M_x, M_y, and M_z).

Assumptions and Limitations

The operational limitations of Digital DATCOM apply to the data contained in the Digital DATCOM structure parameter. For more information on Digital DATCOM limitations, see Section 2.4.5 of reference [1].

The Digital DATCOM structure parameters alpha, mach, alt, grndht, and delta must be strictly monotonically increasing to be used with the Digital DATCOM Forces and Moments block.

The Digital DATCOM structure coefficients must correspond to the dimensions of the breakpoints (alpha, mach, alt, grndht, and delta) to be used with the Digital DATCOM Forces and Moments block.

References

[1] The USAF Stability and Control Digital Datcom, AFFDL-TR-79-3032, 1979.

[2] Etkin, B., and L. D. Reid, Dynamics of Flight Stability and Control, John Wiley & Sons, New York, 1996.

[3] Roskam, J., "Airplane Design Part VI: Preliminary Calculation of Aerodynamic, Thrust and Power Characteristics," Roskam Aviation and Engineering Corporation, Ottawa, Kansas, 1987.

[4] Stevens, B. L., and F. L. Lewis, Aircraft Control and Simulation, John Wiley & Sons, New York, 1992.