DATCOM Forces and Moments Block Control Surface Deflection Issues

I am a bit uncertain abou the proper use of the control surface delta block for the DATCOM forces and moments block in Simulink.

I noticed that when using import DATCOM with both the GRNDHT namelist for ground effect and the DELTA namelist for control surface deflections, the static stability derivatives are missing the values at various deflections at free-air ground height. To address this, I copied and filled in all the deflection values (6th index) coping the first value in the 6th index to fill in the free-air paraemters across all deflections. My assumption was that the DATCOM forces and moments block would use these to compute the forces and moments with no deflection, then add the control surface contributions seperately using dcm_sym, dcl_sym etc. However I only see a response to control surface delta inputs when the ground height signal is within the ground effect range, indicating that perhaps the DATCOM forces and moments block is not adding the dcm_sym, dcl_sym [...] contributions and only finding coefficients from the cl, cl [...] tables.

My question is, does the DATCOM forces and moments block read the incremental lift coefficients dcm_sym, dcl_sym [...], or does it only apply the cl, cm [...] coefficients for the current flight conditions, and are you supposed to manually apply aerdynamic coefficients using the aerodynamic forces and moments to account for forces and moments induced by the control surfaces?

Lastly, the units for the delta signal in Simulink state rad/s, I presume this is just a typo?

If anyone has any experience adding control surface contributions to a DATCOM simulink model, I would greatly appredicate your assistance!

Answer 1

[Aditya]

When working with the DATCOM Forces and Moments block in Simulink, especially when dealing with control surface deflections and ground effect, it's crucial to understand how the block interprets and uses the imported DATCOM data. Here are some key points and clarifications to help address your concerns:

1. Handling Control Surface Deflections and Ground Effect

Static Stability Derivatives and Deflections

• The DATCOM Forces and Moments block typically uses the aerodynamic coefficients (e.g., Cl, Cm) provided in the DATCOM data tables for the current flight conditions.
• The incremental coefficients (e.g., dcm_sym, dcl_sym) should be used to account for control surface deflections. These incremental coefficients represent the change in aerodynamic coefficients due to control surface deflections.

2. Combining CoefficientsHow the Block Uses Coefficients

• The block should combine the base aerodynamic coefficients (Cl, Cm, etc.) with the incremental coefficients (dcm_sym, dcl_sym, etc.) to determine the total aerodynamic forces and moments.
• If you notice that the block only responds to control surface inputs within the ground effect range, this might indicate that the block is not correctly combining the incremental coefficients with the base coefficients.

3. Manual Application of Incremental CoefficientsApplying Incremental Coefficients Manually

• If the block does not automatically combine the incremental coefficients with the base coefficients, you may need to manually apply the incremental coefficients using additional blocks in Simulink.
• You can create a custom subsystem that calculates the total aerodynamic forces and moments by adding the contributions from the incremental coefficients to the base coefficients.

4. Units for the Delta SignalClarification on Units

• The units for the delta signal should indeed be in radians (rad) for angular deflections, not rad/s. This does appear to be a typo. Ensure that your control surface deflection inputs are in radians.

Example subsystem for Incremental coefficients:

function [dCl, dCm] = calculateIncrementalCoefficients(delta, dcl_sym, dcm_sym)
    % delta: Control surface deflection in radians
    % dcl_sym: Incremental lift coefficient due to control surface deflection
    % dcm_sym: Incremental moment coefficient due to control surface deflection
    
    % Calculate the incremental lift and moment coefficients
    dCl = dcl_sym * delta;
    dCm = dcm_sym * delta;
end

%Combining forces and moments
function [Cl_total, Cm_total] = combineForcesMoments(Cl_base, Cm_base, dCl, dCm)
    % Cl_base: Base lift coefficient
    % Cm_base: Base moment coefficient
    % dCl: Incremental lift coefficient
    % dCm: Incremental moment coefficient
    
    % Combine the base and incremental coefficients
    Cl_total = Cl_base + dCl;
    Cm_total = Cm_base + dCm;
end

[r47_r47]

Thank you very much for taking the time to get back to me! This helps clarify what I was wondering a lot. I appreciate it!!!