Aerodynamic Instability: The Holy Grail of Efficiency? Part 1

[christiangunther]

Have you thought about moving the center of gravity inflight, with a servo? This would allow to start your plane in a stable configuration.

[simonl]

when designing good planes is just not enough. this project is pretty crazy. IMO you pushed the cg back very far for a first iteration. I would have worked my way back a bit slower but if anyone can pull this off it's you. Excellent video as always. Cheers!

[WindCatcherRC]

@2:22, thanks for displaying our website briefly in your video, much appreciated. It also reminds me that I really need to add pictures for all of the different sheets of foam.
Cool project! I'm looking forward to the next video.

[Brandon_SoMD]

The pitch oscillations evident in the video indicate that a significant issue here is rate-limited servos. You can have the perfect control loop, but if your servos cannot keep up with the commands, you'll always end up with oscillatory response.

[Jeremy-fysz]

Cool idea. You might want to increase the strength and speed of your servos. They're doing much more work in the unstable configuration than you would usually have. Also if there is any play in your hinge, you may need to tighten things up as any sort of play will have a magnified effect.

[mikegofton]

Hey Kavin, the most compelling thing about your channel for me is the identification and exploration of novel concepts. Can't wait for the next episode !.

[tyskStefan]

Nice to see you pick this idea up! I've been working on something similar for a while. I plan on fully modelling the flight mechanics & actuator dynamics before hand and using that model to set up a custom flight controller. It should be perfectly doable without all that, it'll just take a lot of time to tune the controller parameters right. Most importantly try not to make it too unstable since that'll require greater servo speed & response time. Another big problem is that you won't need a lot of elevator travel while the aileron travel is largely unaffected. So it might be a good idea to split the elevons into separate ailerons & elevator.
Looking forward to the progress & discoveries you'll make along the way!

[markkalsbeek]

This is in essence a really cool control system problem. I'm not sure if you have a background in this, but in case you don't I have some tips.
Key to designing a good controller for your aircraft is having a good model of the dynamics of your aircraft, because that way you can tune the controller in software first behind your desk without needing to go driving for every test.

There's two main ways of getting a good software model. The first is by explicitly modeling the dynamics of the plane using standard equations. If you want I can share some resources on this from my bachelor courses on this. However modeling this fully is hard, there are always some dynamics that are going to be hard to capture.

So that leads to the second approach, which is called system identification. Basically what you do is log the inputs you give the actuators and the response in terms of pitch, yaw and roll and feed that to a program that will infer the plane dynamics from that. Then you can tune your controller based on that.

If you have any questions please let me know, always happy to give my 2 cents.

[jacobfaseler]

Looks like your controller is underdamped.

If possible in your SW, tune your PID with less proportional gain (to reduce overcorrection), and more derivative gain (to anticipate overcorrection and dampen it).

I also agree with those saying to start with a less drastic CG shift.

Very interested to see how this develops - you definitely earned my sub.

[VyarkX]

I think you could try increasing the moment of inertia in the pitch axis, so instability will onset slower and give the servos more time to react. Full sized relaxed stability aircraft or slightly unstable aircraft are flyable (albeit with difficulty) by humans, since it takes a while for deviations in the pitch axis to put the plane out of control.

[justusmetzler]

What about Prandl wings? Efficiency is not speed. If you fly slower and have no tip vortexes and proverse yaw and thus no vertical stabilizers the drag is reduced alot. Look into Albion Bowers Research with Nasa and the Flying like birds series he has on youtube. Birds are flying wings without vertical tails. Dont see many birds with spin departure problems right?

[mtacoustic]

Not quite the same as your study; but I won a fair number of soaring contests using a Hobie Hawk with a very rearward CG. In clear air any thermal would upset the sailplane outward; and by just adjusting the elevator & rudder trim to a preset turn against the upset, the Hawk would catch the thermal and start taking its ride up on the thermal elevator. When the Hawk got too far downwind, the trims were put back to neutral and the Hawk was again sent straight upwind hunting for another upset and thermal.

[bozi]

That guy asking if its a Zagi was awesome. Brought back so may memories of my zagi's, hahah messing with brushed 400 motors and nicads. I miss those days. Great video love it!

[DavidKaden]

Hey, that’s very cool. I’ve done this already with my tailsitter that I built some years ago. Never got it fly stable on high speeds though. Had used all symmetrical airfoil. Maybe you just put the motors on the LE and activate tailsitter mode. I can help you tuning the vertical hover perfect without flying. Then you have a backup if the horizontal flight isn’t stable. BR David

[dasemifake]

I believe moving the CG to the neutral point instead of rearwards would have a better effect for what you are trying to do

[s_cycle]

So I think you will find that you will get the controller to keep things pseduo-stable when the angle of attack is low; but when it deviates too far, the controller won't be able to catch up in time and you'll get a deepening stall or dive. Rather like what you can see. What you could do to counteract this, is an aircraft where there is a lot of mass well in front, and well behind the centre of gravity. That would increase the rotational inertia around pitch axis, and slow down those pitch oscillations. you can still move the CG wherever you want but the inertia would be higher. much higher aircraft inertia around pitch axis and much lower inertia for elevons. v interesting project good luck!

[agsystems]

I think this is going to be extremely hard without active AoA sensing. The flight controller can do it's best to damp oscillations, but this sort of instability is considerably worse than just oscillations.
Lets say you want it stable at 5' AoA. There is an upset, and the aircraft tilts up to 6'. The gyro can detect that, and strongarm the controls to return the aircraft to the original orientation, but the direction of flight will change in that time, so the original orientation might have an AoA of 4'. If the controls then return to the 'neutral' setting (metastable at 5'), the aircraft will still find itself with a pitch down movement. If held perfectly at the same orientation the flight path will start to oscillate up and down. We want the aircraft to be stable (after computer control) with regards to AoA. In order for the computer to do a good job of this, it needs some good way of understanding where it is wanting to stabilise towards.
Even if you manage to get the computer to infer the AoA from inertial measurements, a gust of wind will throw all that off.
One design that I have been looking into is weather-vaning canards. The idea is that the canards are placed behind their axis, and are controlled either by a motor that outputs a certain torque for given control input, rather than setting the position, or a smaller control surface on the back of the canards. They effectively get an AoA term affecting their position automatically.

[turbo]

I built a 23 degree forward swept wing many years ago, motivated by that Nasa plane also. They have they same beneficial drag reduction as a conventional rearward swept wing, but none of the downsides. Having the wing swept forward does do some interesting beneficial things to drag also- it gives positive feedback both because of more of the leading edge is ahead of the balance point and because of slight flexing of the wing. The upshot of the unstable "positive feedback"effects is that it takes MUCH less elevator deflection to change angle of attack. On my plane, it was so unstable and so quick to react it could literally do a zero radius 180, and was more or less unflyable until I calmed down the balance point.

[eckern]

I bet it would be easy to stabilize that with up/down thrust vectoring the two motors, especially at low speed when the elevons don't have much authority.

[Darkfranchise]

How do you find the aerodynamic neutral point?