Drone Pendulum Fallacy
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- เผยแพร่เมื่อ 2 มิ.ย. 2024
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There are many questions about adding a hinge between the drone and the pendulum (with a short length of string), so I thought I should address my view here. I'm not 100% sure what would happen as it depends how far the string is mounted from the drones centre of mass, however I think it will be more of a issue than an assistance. With any change of speed/direction, the pendulum will swing and will drag the drone with it. Also, if the string is attached at the drones' centre of mass, it will be unable to induce a torque on the drone and rotate it to level. If the mounting point is away from the drones' centre of mass, I think it'll still crash as the pull from gravity is uniform, so the drone is pulled down as much as the pendulum. Thanks for getting me thinking about this, I should have tested it in the video! Maybe one for the future!
Are you refering to the center of Mass as the center of mass or the point right between all propellers? - It might make a difference even if those two points are very close to eachother.
I'm thinking about towing objects on the water, if you attach them to far back on your tug it will reduce your ability to turn away from the mass, water doest allow for swinging though so maybe attach fins to your weight to dampen swinging motion
what if you had a stick going down halfway from the pendulum and a string going the rest of the way, would the swinging weight pull it back to level or just get caught in the props?
@@MrGrimies ah, but that just dampens rotations. It provides no correcting force.
wouldn't the mass of the weight after the hinge just be a down vector at the hinge?so effectively you have just moved the weight and the CG a bit and you have the same set up?
I keep watching this every year to remember the principal! Thank you!
I read this in your accent
dude you blow up caps, what does this have to do with your profession
This is the most user-friendly explanation of this effect I've ever seen. Nice Work!
This video and the one Scott Manley did do a perfect job of explaining the pendulum fallacy.
I second that!
This one is better than Scott's. I don't say that often.
Can you link Scotts video? Many thanks.
@@MrProfama36 th-cam.com/video/Tx4cjP-GRAY/w-d-xo.html
Scott's is more in reference to building rockets in the spaceflight simulator KSP. But it's the same principle.
I've seen that one too. Especially interesting was the bit at the end where he added flexibility to the pylon mounts, after which it did exhibit a vertical-returning tendency (I think the sideways aerodynamic drag on the main body caused an offset between the CG and the thrustline that established a torque away from the relative wind). Would be really neat to see an attempt at that built physically here.
Tom, I'm the CTO of the Japanese drone manufacturer ACSL. I've had to attempt to explain this so many times, but I doubt I've ever been able to do it as clearly as you have here. I will be sure to point people to this video in the future. BTW, let me know if you have any interest in working in Japan :P
Interesting that you do have pendulum stability when you build an airship rather than a drone - presumably because the lift vector is consistently vertical, unlike a tilting drone, so it is a pivot point, ^oo^
Yes! Though this is only the case if the center of mass is below the center of bouyancy. If the cm is above the cb, then you have pendulum instability, and the airship would be stable only when its upside down.
Yes! I'm thinking it has to do with speed. The slow balloon has time to correct while the fast drone doesn't?
@@ZacLowing A balloon is using buoyancy, so the force upon it is always opposite gravity, unlike a propeller which will generate thrust in any direction you point it.
Yeah but you should not imagine it as a pendulum. It still rotates around the center of mass. Just the ballon part is offset and therefor produces a torque if the airship is tilted to one side.
@WCTA Chicago underground sound No.
Like Pliotgeek said a balloon uses bouyancy which always "pushes" upwards. If you rotate a balloon it will still oppose gravity. You could achieve this on a drone when you rotate the propellers to always be level with the ground.
This is infact how helicopters work, they tilt the whole propeller
Hackaday journalists be like...
Finally he made a new video,time to write another article!
are* like
@@JorgetePanete I think @Alex already knows this ;) X)
@@JorgetePanete it's a slang short for "(must) be like"
@@JorgetePanete Sometimes it's fun to use incorrect English.
@@JorgetePanete r/woooooosh
Great video!!! Always wondered about this when mounting my drone batteries. Falcon 9 example was a great example. Thanks for teaching this. The gas thruster drone is awesome
To the left, to the left
This drone loves to go to the left.
Pendulum Fallacy,
At its best.
Was a great video,
I'll be waiting for the next!
everything you own in the drone to the left?
@@peterjensen6844 Don't you ever, ever get to thinking, drone's aren't replaceable?
Shakespeares "A Force, A Force, My Kingdom for a Force!"
Should have been
"A Torque, A Torque, My Kingdom for a Torque!"
To the left to the right to the middle
My head is spinning around
To the left to the right to the middle
I am going under ground
Its funny when you see some of your favourite youtubers in a comment section
Thank you Tom. I've been curious for years how a space rocket with rockets on the bottom managed to remain so stable during lift off. You explained it perfectly! Great video! Thanks again!
Obviously rocket designers know this very well. Just google 'rocket' and 'stability'. So how did they do it? They put the dense payload on top, and the geometric! (unit density) center as low as possible.
I've known this stuff for years, but sadly it still hasn't crept into my intuition.
Brilliant. the physical demonstration with thrust vectors overlayed really hammered this home for me.
I think there's a semantics issue here. A fixed pendulumn won't be SELF stabilising. However it will add stability in pitch and roll (in your experiements).
Stability is the state of being stable and more stability means you're less inclined to "give way" or "overturn", adding rotational inertia makes that take longer to happen and so makes the system more stable. However you are right in that it won't be self stabilising.
Target archers regularly use long rod stabilisers (their literal name) which is a stick with a (usually dampend) weight on the end, attached to the front of the bow. It provides yaw and pitch stability through inertia (they can also use a V bar to give some roll stability, some balance to the bow and even more pitch and yaw stability).
Even car designs use mass placement to affect the stability of the platform (i.e. front vs rear vs mid engined vehicles, whether brake calipers are on the inside (lower angular moment) or outside (higher angular moment) of the wheels, etc).
When i saw the thumbnail, i was like "sure this wont work, the pendulum action doesnt have an effect on a flying machine." It was then that i realised that i knew this because it was exactly what i learned from tom's chinook build....🤣🤣🤣🤣
This is not entirely true. It may have a stabilizing effect on vehicles that have something that provides an "always up" vector. Most notably, blimps/airships.
And hang gliders that are controlled by shifting the human mass
Worth noting that if can control the pendulum's position (using servos) you can affect the location of the CG relative to the centre of thrust, which *can* be used to aid stability.
...which is basically what your Chinook is doing (in one axis at least).
James Bradwell this needs more likes. Amazing explanation!!
@@mitchellwright6676 It's not an 'amazing explanation'. If you're controlling a pendulum's position, IT'S NOT A PENDULUM.
Great video mate. I've been getting into building drones lately and your channel has definitely enlightened me to a bunch of useful physics to keep in mind when I'm building. But nothing beats a good ole fashioned physical test like these to truly explore quadcopters and drones.
Brilliant! Thank you Tom, every video you post is the promise of a good time watching it.
But what would happen if you replaced the rigid stick with string/rope/hanging by the cable?
It would swing.
You wouldn't even need a cord, just a pivot point in the stick with springs to create torque I guess.
@@fransoto8343 yeah, it would swing. I meant whether it would give similar results as the rigid stick, make it slightly more stable as even before starting the weight would be directly below the vehicle or go full on 'double pendulum'. Cord/rope/string/cable would work better than stick with a pivot point as it would give movement in all directions
@@Durrdalus well, I doubt it stabilises anything... But it would be an interesting follow-up
@Michael Smith
It would be like a heli lifting a cargo package by a cable...
You have t learn how to adjust fr the momentum of the weight hanging under the aircraft. The weight has momentum and there is drag of the package plus the line between the package and the aircraft.
Early attempts would likely crash, but with practice you could fly circuits of the flying site at speed.
FPV would likely be a fail. You nee to see r feel the effect of the swinging weight on the aircraft.
Someone creative could develop a set of load sensors and programming for the autopilot to compensate. Current drone autopilots would have a very hard time.
@Michael Smith so long as you didn't overload the craft it would not crash. Helicopters carry loads by string all the time. A quadcopter or drone would be the exact same.
Your videos are always such high quality and rich with information. I'm a big fan! keep it up!
This is the BEST video explaining the pendulum fallacy. The example and reasoning behind different drones and how they work is extremely helpful to visualize. Thankyou.
Thanks for making this video, Tom. It is very through, clear, and excellently visual.
Brilliantly demonstrated!
yeah
Great video and explanation 👌
Really enjoying your channel these days. Thanks for making the effort you deserve to do well from it. :)
Amazing video Tom! Sooo much work must have went into this video! Great production value, great explanations, great footage! Very good job explaining the concept :)
Talking about sticks, rctestflight made a fun flying stick video about 2 months ago. This complements it. Great video!
Had an argument on the pendulum fallacy with someone in the comments of that video haha
What if you use an unfixed pendulum attached to the drone, instead of using the drone as part of the pendulum?
Basically, weight with a couple of strings attached so it can move independently of the drone and the extra strings on it avoid causing turbulence movement
Are you suggesting this for a drone experiment that has no gyro control such as his initial experiment? It won't help anything because there would be nothing compelling an "up" oriented thrust vector
This will correct the orientation of the pendulum, but I'm not sure it would affect the drone's trajectory. You would need a way to selectively couple and decouple the pivot when it's advantageous to the drone. A pretty tricky ask.
I double down on his idea but with a slight change. Taking the example at 4:30 what if you let the middle of the wooden stick be free to rotate? That way, the lower half of the stick would want to be perpendicular to the floor and that would create some torque unless the upper half is perpendicular to the floor as well.
I thought of something similar to this. What if the stick with a weight at the bottom was able to swing in all directions. It would naturally hang down. Maybe there is a way to mechanically tilt the motors based on the angle of the drone to the stick. Since I don’t have a disposable drone for all the failed attempts for this idea, i’ll be looking for a good physics simulator.
That works with a fixed pivot like a regular pendulum, but since the drone accelerates, all it would do is point away from the direction of aceleration. Same reason you can get a bubble level to stay level on a plane by doing a coordinated turn, even though you are banked 45 degrees.
Looking sharp Mr Stanton. (I really like the increase in editing and detail, such as the gif-progress style bar towards the end.)
Thank you so much for this. I really loved your slo-mo footage of the Drone flying with a really low Center of mass.
It is fascinating to watch the Drone (motors) attempt to do the impossible.
this will help me when play KSP, I'll stop building unstable death machines instead of space ships :v
I actually learned this too in KSP, when I made rockets that were top heavy I always thought it was a problem then realized it was actually more ideal! And it kind of makes sense, if you try to balance a stick on your hand, if there is a weight on top it's much easier than if you were to put the weight at the bottom.
No you won't. None Of us will ever stop with the death machines
@@redsquirrelftw that's a great example
Its written in the ksp wiki .. keep your CG high and nice
What are you putting your engines on the top or something?
The drone just loves that hedge what can you say?
(Nothing because its true love)
Some trees eat kites and some hedges eat drones.
@@TDOBrandano Or is the kite trying to eat the tree?
Been following your great videos for a while and this one in particular was very interesting. Thanks for sharing your knowledge with the world.
I see those editing skills have improved quite a lot. Love those graphics in motion all over the drones
If an offset center of mass produces torques that rotate the aircraft, could a drone be controlled only by shifting weights along tracks?
Interesting idea ! I think it should. If you have a propeller providing thrust in the center, a mass moving away from the CG (offset) should tilt the drone and thus redirect the thrust towards to desired direction... would be amazing if he saw it and tried !
@David N
Yeah, but in this case shifting the centre of mass from the centre of thrust would induce a moment, as the thrust is acting at some distance from the pivot.
Great work Tom! Could you do an electro-static thrust vectoring system? E.g. creating air currents via differing high voltage electric charges on different areas of the drone.
I'm not sure you can create an embedded very high voltage generator (roughly 100 000V to ionise air)
@@kehnyalexandre5248 I'm not sure either ... I've seen recent research into small lightweight test aircraft doing it, but it was still quite a bit larger than a normal private drone.
@@PhilosopherRex
Yep, the MIT has made a good prototype recently ^^
The force is extremely low on these I believe. There's a newer take on this that creates plasma/heating with microwaves instead of combustion, but that's probably out of the realm of diy
Absolutely amazing video. Started off with me unsure of what the solution to the problem would be and then concisely and clearly explained the solution. Thank you!
Thanks for this great video! I feel quite educated as your diagrams and models are truly simple and informative!
Put fireworks on it like Colin furze
The world can't possibly contain another Colin Furze. There can be only one.
@@MisterItchy i wouldn't worry, Tom is much calmer and more methodical than Colin
@@JoeBissell you just made me imagine a calm and methodical furze
Mm yes
I find myself wondering if attaching a mass to the bottom via string (either one string at the CoM, or one string attached to each propeller arm) would make it more stable?
I'd guess that a single string probably wouldn't make a difference (or maybe make it less stable), but I think it's worth an experiment.
My idea behind attaching the strings to each arm is that when the drone tilts, some of the strings gain some slack, then the tension in the others would pull that side down. Then when it overcorrects, the tension on the new side would bring that side back down, and so on.
I have no idea if it would work, but I'd love to see a video about it (partially because I don't have money to buy or make a drone to test it myself...).
As long as the string is attached to a point some distance away from the center of gravity of the vehicle. The only problem is it would introduce oscillation.
I still don't think this would work, because ultimately the force of gravity will always be acting at the pivot point by definition, so no torque can be generated.
Excellent Tom ...thanks for sharing your understanding.
Thanks for your videos Tom. They're very well explained, very well recorded and arranged, and I love airplane since 10 and i'm learning a lot with you. Thank you!
The way I see it that a pendulum is a lever where the force is the direction of gravity and the fulcrum is a fixed point. Once the fulcrum is no longer fixed the lever doesn't work, neither does the pendulum.
Yeah, but it's more than that because the angle of the thrust changes, which makes the drone try to accelerate more, but the inertia of the mass will hold it back. (Video explained it well, but just wanted to make sure people knew it's more than just a moving fulcrum)
That's what Tom said in his video...
I think you missed the point by not including a clip showing a weight the same distance above the props.
*Dance the Skies*
exactly! i think the real answer is that on a normal multicopter battery placement does matter so little, its nothing to really worry about. but i guess changing the center of gravity further away in every direction makes it worse exponentially. i also guess lower center of gravity always will be better even just the tiniest bit because its still a difference to go uncontrollable up than down. at the end, if someone talks about lower center of gravity they surely dont mean "add a 1m stick to it".
th-cam.com/video/7F3SFpIBP8g/w-d-xo.html
:-) Batterie in the upper part, props low.
@@stephanbre you really missed the point didn't you... like a mile
Outstanding video Tom, really high production values and easy to understand graphics, Well done sir!
Fascinating as usual. I love how this model of smart TV that you and others like Applied Science are creating seems to be really sustainable. So much so that I might have to jump in!
3:58
That’s really crisp video, have you upgraded your camera?
I think he must have, it did really stand out.
Hi Tom what would happen if you had a pivot at the power source connection point.
If the drone could pivot about the connection point *and was using a flight controller to keep it level*, then you effectively have a regular drone with a weight hanging off it. If it could pivot freely, it wouldn't be any more stable
Same as the chopper. Watch the bit without the body. You will see that's doing exactly what you're asking.
Im pretty sure he meant without a gyro
Phillip Shcherbakov yes without the giro. Would this then become stable. And eventually right it's self ??
PKMartin great answer 😬
Thank you for sharing your knowledge with us Tom!
Nice!
As always, nice video Tom;) and perhaps like one of the coments below, like colin furse...put some fireworks of one of your flying machines🥰
Thanks for the intuitive explanation. I have heard you always want a rockets center of mass to be away from the center of thrust but it next clicked before now
I absolutely LOVE your demonstrations!
string vs wood as pendulum would've made any difference?
Me 2 that would just increase the mass. It wouldn’t be able to swing
Just make it harder on the motors, as the string would flop down without generating torque on the drone, creating a downwards force
Hey folks, Tom and everybody... I've got a problem:
I've seen the "pendulum effect" on a simple hand launched propeller (yes, the propeller on a stick).
I launch it at an angle but it always pivots around the nose of the propeller during flight, and so during my younger years, I never got the clue of this fallacy.
But then again why does this simple toy defy your explanation? air drag? gyroscopic precesion?
I haven't noticed that myself. What I _can_ say is if you hand-make a paper spinner (I thought they were called whirlygigs, but that seems to be other things, or at least covers other things) they have bad aerodynamics unless a weight is added to the bottom of them.
And I think that with such a "device", adding a weight to the top would not help just as much as adding a weight to bottom. In this case one factor is the rigidity/stability of the paper itself that the weight at the bottom helps with, but I don't think that's the only factor.
That being said, I'd agree that there are factors that probably do have a small degree of self-correction under the right circumstances, but it's possible that your propeller on a stick would still work fine without the stick at all, just like with this drone (but like you said, yours self-corrected at least a bit apparently)
@@MsHojat
The reason those paper spinner's work is kinda like a reverse pendulum effect, except the force isn't gravity, it's drag. Because the spinner has a higher area at the top (further away from the centre of mass near the bottom) it has a greater drag force at the top, so if it's falling through the air at a slight angle, there's an off-centre force that induces a torque around the centre of mass. The reason this works is because the drag force is always roughly perpendicular to the ground (because the spinner is falling) and it doesn't rotate about the point where the drag force acts.
Great video ! Very interesting subject !
Your production quality has really improved, I love the new visuals !
I always learn so much watching your videos, thank you so much.
Add a pivot point, have the stick on a hinge
That wouldn't do anything to stabilize.
If you attach the stick to the drone through a ball joint it moves the pivot point. The props could act as fetching if the battery is heavy enough.
@_ David _ the string I think would have no stability leverage instead just exerting dragon since there is no more lever. Also the string would add Bounce you need the rigidity from a stick to the reduce extra forces when changing direction.
@_ David _ only if there was someone who could try it for us.
things leaving a static frame. Still funny in 2019 haha
Nice explanatory video.
I did some similar tests when I've built my #Rocket #Drone (moving the battery down the fairing to see whether or not the drone would have flown better) and, in the end, I've decided to keep the battery at the top. In this way I can also keep the CoG closer to the nosecone, balancing in a better way the rocket during the launching phase.
Love your videos! Thanks for sharing.
5:40 is that green screen?
So, if you want to make your model/drone easier to control, don't stick a giant pendulum to it- that's the moral of the story here, right?
No! Absolutely not! The moral is that our umbrellas should absolutely hover autonomously over our heads with no need to hold them, even if can't stabilize them via a pendulum effect that isn't there...
Very nice explanation of various scenarios, well done!
Magnificent video! Well done Tom 👍 I thinks it’s worth mentioning the DJI inspire series of drones, they raise the props up for clean air and in the main the get out of the way of the camera! And FPV race drones, lots of people have the battery underneath for racing, it’s not for stability, but in a crash you’re more likely to land props up and therefore maybe able to take off again. Both cases not related to the pendulum and stability topic. Thanks and keep up the good work 👍
When she says she's home alone 1:53
Why didn't you hand launch the drone straight up and see how it would work?
Later brofesser! 😎💨
@TyTy The Great That's with him flying it and there was no launch shown so he could have launched that one at an angle as well. In the experiment, he had no directional control. THAT'S what he was demonstrating there, that it was difficult for him to correct it's flight. But if it had started out level it might have also been more difficult for it to get OUT of balance in the first place! That's why we need a level launch.
Well I watched it again and I must be blind because I still didn't see anyone hand launch it was the best flight @ 4:45 still a bad idea!
Later brofesser! 😎💨
I've been building Arduino projects for 4 years now, but your channel makes me feel like I have wasted those 4 years on boring projects. Your channel excites my creative side and that it awesome! Keep up the great work!
Nice lighting and motion tracking! well done tom.
Glad you included the bit about it being good with thrust vectoring. You started to make me doubt my own design decisions.
On the topic of your pinned comment addressing the idea of attaching a hinge to the pendulum, I had an idea where you attach a weight to the drone directly below its center of mass, and attached it to the drone via four strings that connect to the drone's extremities, in this case being the blades. The idea is instead of a pendulum, the weight pulls down on whichever extremity is the highest until each extremity is under equal tension from the strings.
I once worked on a coaxial dualcopter layout aircraft with the two rotors mounted on the top on a gimbal mechanism. The top gimbal was set in a stablize mode such that it always pointed itself parallel to gravity. I believe this was closer to how an ideal pendulum worked. The aircraft flew and I had positive control but it is definitely no contest to a properly tuned multirotor or traditional helicopter. In addition it had really bad stability when in free fall.
Brilliant clarification of something you’ve mentioned a lot thanks👍
This has been eye-opening. Thank you, Tom. Much appreciated.
This man is so humble but still with the confidence to make these awesome videos and that's such a fun combination.
Thanks for clearing that up.
I really like the way you explain things. I have some understanding of what's actually going on, now! Thank You.
This is such a good video! Well done Tom!
Tom. Thanks for this video. It explains this issue so well.
Next level production value dude! Hat's off!
Excellent explanation, thanks Tom.
Always love your videos
I recently built a 550 quad I flew it in different configurations first with duel batteries mounted atop. It had slight stability issues. Second, single batt. mounted atop that made the stability worse. Third and the best I could get without messing with the PDI was the batt. mounted below with the large landing gear. It hovered perfectly except for a little trim adjustment. So I believe the "Drone Pendulum Fallacy" is a fallacy. I moved the battery roughly 3" from top to bottom and made all the difference in the world. Other than that Great Channel I dig what you'er doing.
Great video, love your work.
Fantastic videos clear and concise with excellent examples. Thanks 👍
thanks for this beautiful animation !
Great editing and explanation 👍!!
What great information, love your work!
This needed to be done. Well done.
I've enjoyed watching all sorts of flying machines on your channel. From hovercraft, rockets, drones to helicopters. One machine I would love to see attempted would be the tip jet, as in the Fairey Rotordyne.
The best explanation I've ever seen for the pendulum fallacy! It was never quite clear how it worked before, since it's very counterintuitive normally.
It is a great video, and as part of your subscribers I want to point out that you might be able to stabilize the drone by increasing the aerodynamic drag at the top. The COG in front of the COA is what makes an airplane stable in any direction. But essentially this "any direction" is what you don't want. So if you put two sheets of plastic forming a cross (when looking down on the drone) on top of it, this should counteract the vertical velocity, which would probably make it asymptotically stable w.r.t the vertical velocity. Depending on the battery's weight, you might also be able to move this cross down the shaft, such that it sits right below the drone's frame.
On the last drone, lowering the battery puts it closest to the rest of the bodys center of mass, decreasing its moment of inertia, which makes it more suseptible to wind but also allows you to have greater control.
An excellent video. Well explained.
Very great work!
Dude your channel is awesome.
Yet another great video. Thanks very much
Well done, clear and down to the point!
Thank you! As an aero engineer, former mass properties engineer, pilot and participant in an unhealthy amount of enthusiast and pilot fora this, in the form of high wing vs low wing stability (and the associated dihedral fallacy), are high up on my list of regularly appearing pet peeves.
Your video will probably make a few people catch on. Well done.
Love your projects, they actually got me into mechanics :)
Excellent video as always.
When I saw the Space X Falcon 9 model rocket flying, along with your explanation, it reminded me of trying to balance a pole on the palm of your hand. The hand is like the thruster keeping it from hitting the ground and moving your hand to keep it balanced.