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DummyCatz

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  1. Hi, do you possibly know if the longitudinal system of the F-16CM FLCS should be g-commanded until 15.0 or 15.8 deg AOA? 

  2. Hi, has this bug report been recognized and acknowledged? THIS is the evidence and the video was just used to illustrate the differences.
  3. 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.
  4. 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.
  5. Time history of NASA FLCS control system A performing 1g stall, for example: Ref Figure 18, NASA TP-1538, https://ntrs.nasa.gov/api/citations/19800005879/downloads/19800005879.pdf
  6. By referencing a publicly available FLCS flashcard (https://quizlet.com/309832047/flcs-flash-cards/), the yaw rate limiter is activated when AOA > 35° until AOA is reduced below 32°. A similar description can be found in the dash-1 manual (which is not quoted here), but I'm not seeing this behavior in DCS F-16. In the two tracks below, the activation AOA is 29° as indicated by the suddenly reducing aileron deflection upon crossing 29° AOA. This is an incorrect value by the F-16 version we had. Viper yaw rate limiter activation AOA - track2.trk Viper yaw rate limiter activation AOA.trk
  7. Here's a bug in the CAT I limiter: By just performing a max effort pull to the AOA limit and reducing the throttle to IDLE, the aircraft developed an aggravating pitch oscillation by itself and eventually overshot the AOA limit and resulted in a deep stall. This dangerous behavior is not listed in the dash-1 manual. Viper AOA oscillation.trk
  8. I'm not sure about the Su-33, but the lag filter in Su-27 is to reduce sensitivity in pitch control especially at higher speeds, as it poses a danger in pitch control over-sensitivity. The same can be said for the F-16 as there's a 'pitch prefilter' applied after the pitch 'stick command limiter', and there's also a known issue of pitch control over-sensitivity at transonic speeds in the F-16. According to Figure 3.1 in the paper "F-16 Simulator for Man-in-the-Loop Testing of Aircraft Control Systems", https://apps.dtic.mil/sti/citations/ADA189675, the pitch prefilter is a first-order low-pass filter (a.k.a lag filter so yes it creates lag), with a time constant of 1/N14. The gain N14 is variable, scheduled with dynamic pressure. With a dynamic pressure below 250 psf (~= 266KCAS), the time constant is 1/8.3 sec, which means the shaped g-command will be reaching 63.2% of the commanded g value in 1/8.3 seconds, given a step input. With a dynamic pressure greater than 400 psf (~= 585KCAS), the time constant is 1/6 sec, lowering the control sensitivity at higher speeds. This essentially shapes the desired first-order g-response of the aircraft, with no overshoots nor oscillations. This may explain both the lag and the g-onset. An illustration of first-order response given a step input: The location of pitch prefilter as shown in Figure 3.1, https://apps.dtic.mil/sti/citations/ADA189675
  9. Update flight model for ground effect, takeoff pitch effects, auto-pilot based on FPM, touch and go handling, and other remaining flight model issues [In Progress] Hi, I've already noticed that the Flight Model and FCS has been WIP for more than 5 years. I may ask, what are the other remaining issues and has this particular bug been reported? I'm not seeing the reasoning behind the 'as stated in the road map' cuz I'm doing a bug report (regarding the rudder FCS) of an implemented feature (sideslip feedback), not a wishlist item that is not implemented (like missing logic in FCS). It's basically including all subsequent FM related bug reports into this category automatically, and it's just like saying that all the remaining issues that could possibly exist are WIP. (of course) The fix should be simple and it’s not related to the aerodynamics. Just pure control logic: Remove the sideslip feedback to the rudder, and add it to the aileron and diff-stab instead. See, it’s not a missing logic, but a bug.
  10. After several failed attempts at the above described MSRM falling leaf entry procedure, I realised that the aircraft is just too stable for the aileron to generate enough sideslip angle and start oscillating. This is about pure aerodynamics now. I'm using the coefficients and derivatives from the NASA F18 HARV wind tunnel and flight tested data, which shouldn't differ too much with our production F18C, especially the trend of stability reduction at high AOA. There're three main areas of under-representation in our current flight model: 1. Directional static stability (Cn-beta) is just too high above 30° AOA. According to the wind tunnel data, Cn-beta drops below zero (= statically unstable) above 30° AOA, which means any sideslip built up has a tendency to increase by itself. I'm not seeing such instability in DCS. Any sideslip created by the aileron or rudder would just be reduced immediately at all AOA ranges, and this is with MSRM engaged so no FCS augmentation. 2. Insufficient adverse yaw created by the aileron. Please refer to the Cn-dA curve below, which is the coefficient of yawing moment created by the aileron. There should be a huge amount of adverse yaw created above 40° AOA, but I'm not seeing much and it's not generating enough sideslip. 3. Too much roll control power by the aileron. Refer to the Cl-dA curve, rolling moment coefficient is greatly reduced above 40° AOA, but I'm not seeing such trend either and it's just like rolling at 25° AOA. Additional observations by @Maverick Su-35S, thanks for the testing: References: Figure 19, 22, 23 from "AERODYNAMIC PARAMETERS OF HIGH-ANGLE-OF-ATTACK RESEARCH VEHICLE (HARV) ESTIMATED FROM FLIGHT DATA", NASA TM 202692, https://ntrs.nasa.gov/api/citations/19900019262/downloads/19900019262.pdf
  11. Our current Auto Flap Up FCS implementation is using sideslip feedback to control the rudder, which I consider as a bug, because the feedback is erroneously applied to the rudder rather than to the aileron and differential stabilators. The sideslip and sideslip rate feedback should be fed to the aileron and diff-stab above 20 deg AOA, as in FCC OFP v10.7 IRL. For now, I'm seeing rudders move with sideslip changes even if AOA < 20°. Test procedure: 1. Set an extremely turbulent weather. 2. Flaps to auto. 3. Press F4 to get a closer look at the rudder 4. Check if the rudder is moving with turbulence/sideslip changes. Use active pause so that there's no lateral acceleration and yaw rate interference to the rudder. Test with AOA below or above 20 degrees. References: 1. 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 2. 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 3. Simulation Model of a Twin-Tail, High Performance Airplane, NASA TM 107601, including a set of FCC OFP v10.1 block diagrams (https://ntrs.nasa.gov/api/citations/19920024293/downloads/19920024293.pdf) 4. NATOPS manual which is not quoted here but contains relevant info. According to the Directional Auto Flap Up CAS block diagram from reference 3, there's no sideslip or sideslip rate feedback in v10.1 as there was no sideslip measurements available to the aircraft. The feedbacks were only included in v10.6.1 (a test version of 10.7) and v10.7, together with the sideslip estimator. From reference 2 describing the sideslip estimator: From reference 1 describing the sideslip and sideslip rate feedback: Also from reference 1: Hronet rudder moves with sideslip.trk
  12. 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:
  13. A bug related to g-response and pitch instability is fixed in 2.9, which could also affect g-onset if more pitch dampening is applied: Using the example of going from 9g to 1g by releasing the stick, in DCS 2.7 and later 2.8 (with partial fix) this looked like going from 9g to 0.5g rather quickly, undershooting below 1g and then slowly creeps back to 1g, especially at speeds between 400 to 500 kts, displaying a lack of dynamic stability. Now it looks like 9g -> 2g -> 1g as a 1st-order g-response, which is a correct response type as IRL is. But the bug fix introduces pitch creep, which is to say, upon releasing the stick the nose tends to continue creep a bit more degrees until it stops.
  14. The phugoid is gone in 2.9. Many thanks. Now the opposite happen and there's a tendency to overshoot the pitch attitude upon releasing the stick, but the original bug is fixed.
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