need evidence Control authority and flight model RUDDER response weak?

[GumidekCZ]

My question for all Hornet users and players:
DO YOU THINK, DCS HORNET HAVE ENOUGH RUDDER RESPONSE IN ITS CURRENT FLIGHT MODEL???

Can you do same things as these guys -- without asymetric thrust -- :
Just if you fly straight and apply full rudder, the nose will slide at the start about 4 deg to side and seconds after only 3 degress off the flight path. Over all the plane will still be flying almost straight ahead without rolling or yawing because the flight computer will be against you, even if FCS GAIN OVERRIDE switch engaged of paddle pressed.

 

 

[DummyCatz]

Haven't had the time to check it thoroughly but I would recommend looking into the IRL control logic for any rudder authority limitations. A good place to start is the paper 'Simulation Model of a Twin-Tail, High Performance Airplane' by NASA (https://ntrs.nasa.gov/api/citations/19920024293/downloads/19920024293.pdf).

Page 113 contains detailed description of the Directional Auto Flap Up CAS with a set of control block diagrams of FCC OFP v10.1 directly from the MDC report.

Quote

The rudder pedal force transducer signal, after it is shaped by a dead band followed by a parabolic gradient (Function 14), is air data scheduled (Function 10) to prevent a command which would exceed the vertical tail load limits.

Air data and angle-of-attack gain schedules (Function 17 and 114) eliminate aircraft departures for full pedal inputs. Full surface authority is available at high angles of attack and low Mach numbers through the rudder pedal-to-rolling surface and rolling surface-to-rudder interconnects described below.

Page 115 and beyond detailing the block diagram and gain schedules:

(Ri = dynamic pressure Qc / static pressure Ps)

So your pedal command is effectively shaped by the PEDAL GRADIENT, that uses Function 10 to reduce the gradient at higher dynamic pressure (or airspeed), which prevents exceeding the vertical tail load limits. Then the command is multiplied by (0.5 - Function 17 * Function 114), and is limited to -/+30°. Function 17 increases the gain at higher alpha, while Function 114 increases the gain at higher mach number. The more gain they produces, the more reduction to the command ratio. The idea is to prevent medium to high alpha yaw departures caused by misplacing the pedal. This concludes the command path and may explain why the rudder command is reduced.

The remaining paths are feedback paths, like yaw rate feedback and lateral acceleration feedback, that are used to enhance directional static stability (provides turn coordination) and yaw rate dampening. A higher feedback gain could increase stability and dampening effect, but could also lower the yaw response and destabilize the system, causing the rudder to oscillate.

Sideslip and sideslip rate feedback is only added in OFP v10.7, and is not provided to the rudder, but to the aileron and diff-stab. But I found no indications that any v10.7 logic is implemented in DCS.

Cheers.

Edited November 20, 2023 by DummyCatz
extra info

[rob10]

Those videos (which ED won't use as evidence anyway since it doesn't show hard numbers or what the specific plane/year/etc is) appear to be mostly airshow demonstrations.  From posts here over the years, a lot of the time the planes used for demo's like that often have a tweaked FCS that gives them different handling characteristics than a stock front line version.  I have no idea if that's the case with these ones, but that's also why ED won't use them (they won't know either).

[DummyCatz]

This might be the root cause: our current FCS is using sideslip feedback to control the rudder, which it shouldn't IRL in the first place. To make things worse, there seems to be a massive sideslip feedback gain being put on the rudder, causing a very weak control response.

Reported here:

 

Edited November 20, 2023 by DummyCatz

[DummyCatz]

I'd also like to point out the yaw rate and sideslip angle achieved in this knife edge maneuver, starting at 1:47:

And a brief sideslip excursion at rolling off, starting at 4:13:

With the current sideslip feedback logic of the rudder that is unique to DCS, it would be impossible that such sideslip excursions are being allowed with the excessive sideslip dampening.

[DummyCatz]

On 11/15/2023 at 12:50 AM, rob10 said:

Those videos (which ED won't use as evidence anyway since it doesn't show hard numbers or what the specific plane/year/etc is) appear to be mostly airshow demonstrations.  From posts here over the years, a lot of the time the planes used for demo's like that often have a tweaked FCS that gives them different handling characteristics than a stock front line version.  I have no idea if that's the case with these ones, but that's also why ED won't use them (they won't know either).

I'd like to differ. Any 'tweaked' FCS would subject to well-recorded, extensive flight testing, and thus the allocation of time and money.  A perfect example would be from https://apps.dtic.mil/dtic/tr/fulltext/u2/a307768.pdf, back in the times when a particular version of FCC software, the OFP v10.5.1, was being flight tested. What was believed to be a very simple change resulted in a serious amount of problems. Handling qualities were dangerously degraded and a lot of time and money was spent trying to fix it. It is not feasible to develop and thoroughly flight test a 'tweaked' version of FCC software just for airshow demonstrations.

[BIGNEWY]

We can not use the videos as evidence as the air show configurations for the FCS can be tweaked ones. 

thanks

[DummyCatz]

30 minutes ago, BIGNEWY said:

We can not use the videos as evidence as the air show configurations for the FCS can be tweaked ones. 

thanks

Hi, has this bug report been recognized and acknowledged? THIS is the evidence and the video was just used to illustrate the differences.

 

[BIGNEWY]

The thread is marked W.I.P as stated in the road map. 

Again we can not use video as evidence for flight models.

thank you