Dutch Roll Damping and Washout Filters | Lateral Flight Control Fundamentals | Section 1.1

[@MuhammadAmir-xd8pm]

Excellent explanation of Dutch roll control. Looking forward to the lesson on coordinated turns. It would be wonderful if you can explain how to achieve turn coordination where angle of sideslip(sideslip sensor) is not available. Thank you and I really appreciate your efforts on creating clear and thorough material on the subject

[@LearnGandC]

Thank you Muhammad! I'll keep it in mind.

[@djredrover]

Hey Ben, awesome as always but found a little mistake at 8:08, where you state “signals to the right of the cut-on are attenuated and to the left, preserved” which should be the opposite here as that describes a low pass filter.

[@LearnGandC]

Ah, there I go again, mixing up my left and right. Thanks for letting me know. So begins the errata list! Check the description.

[@foxlies0106]

Thank you for the detailed and informative video. I did have questions though likely they just show my ignorance. 1) The bottom graphs that show the different responses to the tuning and the 1deg rudder step input show yaw rate, r[deg/s]. But the values seem to trend toward monotonic increase toward -40, -60, -80 deg/sec, which are higher and higher yaw rates that seem out of proportion to a 1 degree step input. I'd almost like to ask if the spiral mode term, with its low pole, is causing that. Would one expect a faster and faster yaw rates like those from a 1 degree step rudder input? The other questions I had are 2) I am surprised the roll is so suppressed for a dutch roll oscillation; and 3) whether one should be concerned and if so assess the stability(Margins?) of the feedback and tuning solution(s) that were found, eg via Nyquist criterion? for instance for low frequency pilot commands, washout filter is like a differentiator and adds 90 degrees of phase into FB loop. That sounds helpful but does it need stability check? Thank you for the fine work and teaching, and any consideration.

[@LearnGandC]

Thanks for watching and the questions. They are actually good questions and I think I have some answers. 1. You are correct to suspect the linearly increasing yaw rate magnitude. We have to remember this is a linear simulation and valid for the full nonlinear aircraft dynamics when state changes about the trim condition are limited. The actual aircraft would likely reach a steady state turn with constant yaw rate, essentially flying in a circle, but the linear simulation does not have this fidelity, It's just based on the SISO transfer function. 1b. That said, yaw rate is linearly decreasing because \dot{r} = A*x + b*\delta_r, where \delta_r = 1. Direct integration for steady state control input gives r = b*\delta_r*t, consistent with what we see in the plot. I recall b had -10 in it with a bunch of zeros, consistent with the 10 deg/s/s slope (I think) in the plots. 1b. The spiral mode eigenvalue is 0.004 so e^(0.004*25) = 1.1, so our window is too short for the spiral mode to play a significant effect. Recall its time to double is >170 seconds! 2. For this transfer function which corresponded to a transport aircraft lateral dynamics (trim condition unknonwn), the roll subsidance mode was cancelled by a zero so it doesn't appear. However, roll usually is present in the dutch roll oscillation, even if its a small amount like shown in the X-15 example at the beginning. 3. Yes!! Absolutely one should assess stability margins. I had the Nyquist diagram animation prepared, but chose not to include it in the video because its analysis for this particular case is complicated and I just didn't have time to include it with clarity. I explored this system for very large gains (order of 100 and greater) and it remained stable. The closed loop poles apparently asymptotically approach a complex pair of open loop zeros (not shown in the root locus diagram) and do not actually migrate into the right half plane. Well, this was a longer answer than I expected. I hope it's helpful.

[@foxlies0106]

@@LearnGandC Thank you very much for the thoughtful and helpful reply,, which I will study and learn from, as I am just learning; thx for your hard work and great presentation.

[@pooyankhosravi5337]

I'm probably missing something obvious about how these algorithms were developed and used but I find a bunch of your reasoning, especially considering human caused problems in commercial aircraft accidents, unreasonable. Wouldn't decoupling human input from control loop increase robustness of these kind of systems? For example: pilot controls an artificial point and control tries to achieve that point. Another system gives tactile feedback to pilot based on what control system actually achieved. Am I on the right track or mistaken?

[@LearnGandC]

Let's first clarify the difference between guidance and flight control. Guidance is how the aircraft travels from point A to point B. Flight control (the subject of this video), acts behind the scenes to make the aircraft maneuver in a safe and desirable manner (think passenger comfort) so that it achieves the periodic course corrections from the guidance updates. The methods described in this video allow the piloted aircraft to be safer because due to the roll damping, the aircraft handles in a more desirable manner. As a result, the pilot can focus their attention on the other tasks associated with their flight. In addition, the pilot can remove themselves from the guidance loop by setting a waypoint and letting the aircraft follow the trajectory. In this case, a fully automatic system comprised of a guidance outer-loop and autopilot inner loop takes over. Note that recent history is full of software and hardware faults that underlie accidents, in addition to pilot error. Technology is reaching a state where aircraft can be designed to be fully autonomous (a bigger topic than the scope of this video). There are cases where these capabilities are utilized and cases where it is not. The latter typically involves souls on board, while the former typically does not.

[@pooyankhosravi5337]

@@LearnGandC Thank you for your time! That helped a lot. If I can take more of your time, can you point me to a resource so I can understand why removing pilot from direct control is unsafe? By direct control I mean what you described, as in pilot input is used for physical control with some additions rather than using it to inform and guide flight control.