Aerodynamic Instability: The Holy Grail of Efficiency? Part 1 

Kind of like Concorde and the trim fuel tank in the tail? Boeing has an airplane with fuel tanks in the horizontal stabilizer too....

@@231pilot yes similar to similar to that. it would allow to fly high in the air and testing of the unstable configuration away from the ground.

nice idea to shift weight, esp to ease the exploration for finding what's the good-enough spot for CG

@@231pilot Also the F22 uses air bladders to push fuel forward to counteract the CG change when dropping heavy bombs.

Clever

Exactly. "Crawl before you walk."

He is building Porco Rosso plane! Hard to get from the ground but excellent in the sky.

I don't think this is the case. It's a system, and the servo responses are a part of the dynamics of this system. Many real world rockets such as those flown by SpaceX are not stable, and also use slow actuators. These systems are very well controlled due to the system dynamics being very well understood. I would hazard that faster actuators, if not properly understood, will simply lead to higher frequency oscillations and problems. As other comments have put far better than I will, properly modeling craft dynamics is the solution.

Oscillations can also occur in a perfect "instant" system. Looks more like bad pid tune than rate limiting.

@@KnowledgePerformance7 That's a fair point. Joe Barnard at BPS.Space has done a lot of interesting things in this arena, like vertical landing a model rocket like Falcon 9 does, and his channel is worth checking out. I think, however, that "slow" actuators like SpaceX uses are far more acceptable for large heavy systems. The smaller the vehicle, the more "twitchy" it is, and the faster the control systems have to operate. Also, the less stable, the faster a response is required. Watching this video makes it clear that the vehicle is responding VERY quickly, and you cannot hope to control it unless your actuator can go stop-to-stop in about the time that the vehicle executes a single oscillation. Those actuators in this video are WAY too slow for THAT. Dial down the aerodynamic instability (move the CG closer to the aero center), and it MIGHT work with the current actuators. So in summary, I think you're right to a point, but faster actuators will be required to handle this level of instability, even if the system dynamics are perfectly modeled.

Agree that the smaller the model, the faster the response loop has to be. Would stepper motors give a faster response?? Or perhaps linear actuators?

No. Oscillations are often the result of closed loop feedback system which is unstable. This can be for example caused by delay in the closed loop resulting in the feedback signal being phase shifted to the point where it becomes additive at the input to the control system. One such cause of a delay is the interaction between the human operating the demand controls and the control system, with the human introducing a delay in the feedback loop. It can just be down to adjustment of the parameters in the control system. This control theory subject is not actually taught to students until they get to an undergraduate degree course at university in Engineering. So it it not surprising that many people don't understand it.

I can only imagine that you climaxed when he increased the quantity to 100. I wish I had $4000 in foam money.

With my vast experience with betaflight and high preformance quads, and trying AP on these high preformance quads AP cannot handle or is setup to this task. iNav is a fork of BF and they race with iNav on planes, I know of no high preformance racing done with AP (the pixracer was a total failure in the racing scene). AP is not a low latency pid and filters loop, they are made for larger size, heavier and slower craft and autonomous, I seriously think you will fight the limits of AP slow loop for a high preformance plane, try iNav.

@@Ozzy3333333 well, I won't be using ardupilot but fully self written code on a custom board. However, ardupilot is more than fine for this application as the loop time of the PID is multiple orders of magnitude above the response of the servo actuators, which are the limiting factor here. An ATmega328 would probably still be plenty fast enough for this task, and that is way slower than the ARM uCs in the pixhawk and other ardupilot FCs.

@@tyskStefan Well good to know your writing your own code. AP is NOT successful in any racing task, so imo is is not good enough for this application in this video and maybe not your application (not seeing your application, I dont know). Of course this is not racing, but needs quick response, unlike anything I have seen AP do. AP was a pain just to get a old school mini quad flying w/o oscillating, and it never flew "good". Good luck.

@@tyskStefan Curious why a custom board, because F4 FC are less than $20 nowadays? What do you need different?

@@Ozzy3333333 multiple reasons: first of all because I need dedicated sensor inputs for angle of attack vanes, pitot tube and the likes as well as a beefy onboard BEC to supply the servos with. Additionally I don't need the OSD chip and some of the other features provided by multirotor FCs, I hardly use those on my multirotors... But most importantly it saves me the headache of searching for a new board and writing a new config every time a board gets discontinued.

I'd love to hear about what textbooks you used to learn this stuff in college!

I'd like to hear which textbooks you learned this in as well.

@@marthinwurer When I took Flight modelling and automatic control in college last semester, we used "Flight Stability and Automatic Control" by Robert C. Nelson. A bit dated and somewhat of a tough read but it's great for making mathematical models of aircraft dynamics. Then, making a control system of your own based on that model can be done, Anderson and Moore's "Linear Quadratic Methods" is great but reads like ancient greek and the subject matter is still half black magic to me.

@@Mrissecool Man I felt some relief when you said "subject matter is still half black magic to me" because as an aeronautic engineer I felt kind of bad for not completely grasping the subject, is good to know I´m not the only one. BTW the first book you mentioned, I love it!

This approach is used for adaptive controllers. First part entails system identification, then it can program the pids on the fly. However what he is suggesting is to use just the identification for creating a mathematical model, from which you can create a stable PID setting. This can be done in MATLAB once you have the stored the data. You need the transfer function from inputs to sensor readings (outputs). Typically this requires an iterative algorithm which over time converges to the correct model.

I will probably use the autotune feature and slowly walk the CG back. I am not great at tuning PIDs, very curious to see (if it can stabilize it) if it follows your suggestion.

Kavin, PROVERSE YAW! Prandtl wings are one to explore.

But they can finely tune their wing twist according to angle of attack. A Prandtl wing is only good at a very specific angle of attack from what I saw

@@mortache is seem to make sense if u mean most optimal at least? Any reference? Anyways, too much proverse yaw is better than too little (in terms of stall spin). And how about pitch axis stability of prantdl? Is it 'autostabilizing' provided good trim (it likely seems). also do you know if prantdl is autostabilizing the pitch provided slightly bad trim (like say e186 should be, as airspeed raises or lowers the changed pitching moment then fixes the angle of attack)?

@@vidmasze i don't know a LOT about these wings, only that they hqve exceptionally high twist making the wingtips neutral or negative angle of attack. Something like 20-30 degrees

😅😮😅

Cheaper, or analog servos can not. (typically will emit an audio buzz if pushed too many cycles). Digital servos are mare capable.

Extra fast tail-rotor servos will be worth the cost, for development. One less thing that might be the problem.

I was thinking that too. Isnt having the CG either forward or aft of the CP going to force your elevators to generate some lift up or down to counteract the CG/CP moment? Thats just additional drag.

It seems that drag reduction is a primary driver here. When CG is forward of the AC, a small amount of up-trim is needed to maintain level flight. That amount of up-trim results in drag. This is a stable configuration. Since gravity is always pulling the nose down, this up-trimmed configuration seems to work pretty well for landings too. So, in an effort to eliminate that built-in drag, it seems that the designer is moving the CG aft of the AC in order to eliminate the up-trim. There is more to it, I'm sure but I think that's one of the starting points for exploring this dynamic.

Another issue with canard configs is that the flow over the main wing gets heavily modified by the canard ahead of it. If the canard is for high maneuverability at high AoA, like on some modern fighters, then that's a great feature; but if efficiency is your primary goal, it's a bug.

In order for a canard airplane to not deep stall, the canard has to stall before the main wing. That means the main wing can never fully develop it's CLmax, so the wing is oversized to begin with. It also means that high lift devices like flaps are ineffective because you're still limited by your canard. As a result, a canard with a wing sized to meet a given takeoff performance will need a larger wing, and thus be slower and draggier at cruise compared to a conventional plane that can achieve the same takeoff performance with the use of flaps and smaller wings. To get around this you either need flaps on the canard that deploy with the main wing flaps to raise the CLmax, move the canards forward during landing and takeoff to achieve a greater moment to counteract the pitching moment of the flaps and high AOA main wing, or accept that if the plane stalls it will be unable to recover. Using flaps on the canard is problematic because that's where your elevators go and it also makes the canard lift curve steeper, degrading your static stability. Moving the canards forward is viable, and was done via adjustable sweep in the Beechcraft Starship, but it's heavy, complicated, and also steepens the canard lift moment curve (thanks to the longer lever arm) degrading static stability. Finally, accepting the risk of deep stall poses similar control system issues as presented in ThinkFlight's video, but hobbyist grade stall recovery software, stall sensors, and AoA sensors are far less developed than hobbyist control software to deal with aerodynamic instability, thanks to multirotors which are inherently unstable.

If you are correct there is no fix to the issue as it is a non linearity in the control loop.

hm control engineering was a long time ago but out of the top of my head. Even on 50 Hz, letting the control loop run at 400Hz, it could improve still help for noise from input sensors. It could still help as reading of inputs and acting on them is not necessarily close together when on the same frequency (no idea how the code is setup). And if you don't adjust gain values then it could still have a fortunate effect too but that could be easily fixed by putting in the correct gains. But actually the servos are doing 400hz now too 9:30 . ;)

not that I'm qualified to answer but yes and no. In some ways no because as you're thinking, the servos can only move at 50hz..but a 400hz sample rate allows much finer analysis of what's ahppening to teh craft and as this is r&d I'd assume that's valuable in itself. In analagous areas having high quality data that you then downsample to get from in this case 400hz sampling to 50hz action can still be useful as at least you have a better picture of what's happening to base your 50 actiosn per second on if that makes sense. ANyway, I wondered the same thing...hope TF replies to your question ;-)

You'd need servos capable of updating at 400Hz. I think these sorts of servos are available but I don't think they're very common.

I don't know, learning with you :)

Butts, S. L.; Hoover, A. D. (May 1989). "Flying Qualities Evaluation of the X-29A Research Aircraft". U.S. Air Force Flight Test Center. AFFTC-TR-89-08.