Missing important features in FCC OFP v10.7 (lack of high AOA roll and yaw performance as if with v10.5.1)

[DummyCatz]

Hi,

This thread is about the FCC OFP version 10.7 control logic implemented in DCS, and it's missing important features.

 

Several observations (currently as of 2.9.4) show that a core function, which is described as 'Opposite Differential Stabilators for Roll', is not implemented. This function utilizes adverse yaw to generate yaw rate, enhancing roll coordination and performance at high AOA.

A description of opposite diff-stab function, quoted from reference [2]:

Quote

The differential stabilator is deflected opposite the intended roll direction at high angles of attack and low airspeed to allow that surface to provide added yawing moment for enhanced roll coordination and performance. The benefit is that the aileron deflection can be increased to provide a net improvement in the coordinated roll performance at high angles of attack. The aileron-to-stabilator ratio begins to reduce above 30 degrees angle of attack, and reverses above 36 degrees angle of attack.

 

A description of the sluggish roll performance of v10.5.1, quoted from reference [1]:

Quote

Differential Stabilator (Diff-stab) was used as a primary yaw device instead of a rolling device at elevated AOA (above 30°): This is because a significant amount of adverse yaw is generated by differential deflection of stabilators at greater than 20º AOA.
The stabilator that is deflected trailing edge down (TED) creates very high induced drag on that side, producing yaw opposite the commanded roll. The Legacy Hornet’s old v10.5.1 FCC software used considerable rudder (up to full 30º deflection) to coordinate even small lateral stick inputs due to this diff-stab induced adverse yaw. The old software therefore drastically limited roll command gains (diff-stab + aileron) when the rudders were saturated, resulting in sluggish roll performance when greater than 30° AOA.

 

Video evidence of Opposite Differential Stabilators, from 0:42:

 

This would be very useful in generating proverse yawing moment in the direction of intended yaw, increasing the yaw/sideslip maneuverability of the Hornet at elevated AOA, which is currently lacking in DCS.

 

References:

1. Mitchell, Eric John, "F/A-18A-D Flight Control Computer OFP Versions 10.6.1 and 10.7 Developmental Flight Testing: Out-of-Controlled Flight Test Program Yields Reduced Falling Leaf Departure Susceptibility and Enhanced Aircraft Maneuverability. " Master's Thesis, University of Tennessee, 2004. https://trace.tennessee.edu/utk_gradthes/2372

2. Park, David J., "Development of F/A-18 Spin Departure Demonstration Procedure with Departure Resistant Flight Control Computer Version 10.7. " Master's Thesis, University of Tennessee, 2004. https://trace.tennessee.edu/utk_gradthes/2312

3. NATOPs manual which is not quoted here but contains relevant info.

Edited Thursday at 01:23 PM by DummyCatz

[Hulkbust44]

Flight model/FCS is WIP, This is the correct OFP  and if this doesn't change when we get an official update in the FM/system we'll need to push for a bug report.

[DummyCatz]

On 11/7/2023 at 12:35 AM, Hulkbust44 said:

Flight model/FCS is WIP, This is the correct OFP  and if this doesn't change when we get an official update in the FM/system we'll need to push for a bug report.

Since our Hornet has gone through the long awaited FM refactoring, I think it's time for a bug report.

The trackfile is for the opposite diff-stab function. As quoted, the aileron-to-stabilator ratio begins to reduce above 30 degrees angle of attack, and reverses above 36 degrees angle of attack. So I'm expecting the diff-stab to reverse above 36 deg AOA, but it didn't:

F18 opposite stab test.trk

 

EDIT: This thread is still tagged as 'wip please read roadmap'. And now the roadmap shows that:

• Update flight model for ground effect, takeoff pitch effects, auto-pilot based on FPM, touch and go handling, and other remaining flight model issues [completed]

So it suggest that this has not been wip at all and has slipped through the already done FM refactoring.

Edited Friday at 05:31 AM by DummyCatz