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Drone Physics

iahmed.me

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Re: Drone Physics

#11

Post by kurthr »

fragmede wrote: post #10
ivanjermakov wrote: post #8

I studied this subject when I was building FPV sim from scratch. I noticed that yaw action is often misunderstood, falsely believing that it's caused by a change in angular momentum (think reaction wheels). There is a good SE answer about deriving net torques given each props' thrust force in less PhD language than this article: https://drones.stackexchange.com/a/416/11402 If anyone is interested in tackling UAV simu…

Why does four seem optimal? Why not three or five? or eight?

Not the OP, but due to neighboring rotor interactions an even number (or a single helicopter like lift rotor and a tail rotor ignoring airfoil lift possibilities) rotor makes design easier and improves performance significantly. Think of the rotors like gears meshed together and realize that an even number spins easily, while an odd number is locked.

Of course there are 6, 8, and larger numbers of rotors used in actual drones. The advantage of more rotors is that redundancy to failure can be built in, and that rotor tip speed for a given lift can be somewhat reduced at the cost of efficiency.

Re: Drone Physics

#12

Post by chasd00 »

MadnessASAP wrote: post #7
hnuser123456 wrote: post #4

Earlier quoted context omitted.

I always found it interesting that basic quadcopters work on nothing but a gyroscope which measures angular velocity in 3 dimensions, and optionally an accelerometer for auto-levelling, with 3 dimensions of acceleration. Then of course you can add on GPS for absolute position and route planning in 3 dimensions, and a ground height sensor for auto-landing, then you can add distance sensors on the sides for obstacle av…

> The hard part seems to be smooth rapid vertical descent. It's impossible to predict how the prop wash will interact with the wind and push the drone around as it descends into its own turbulence. I was tracking betaflight development for a while and was wondering if we'd ever see some kind of prop-wash calibration. Is there some adjustment of PID gains while descending through prop wash that could improve stability…

Why not just flip over and accelerate? I bet some in the drone racing crowd do this.

Re: Drone Physics

#13

Post by hnuser123456 »

chasd00 wrote: post #12
MadnessASAP wrote: post #7

Earlier quoted context omitted.

> The hard part seems to be smooth rapid vertical descent. It's impossible to predict how the prop wash will interact with the wind and push the drone around as it descends into its own turbulence. I was tracking betaflight development for a while and was wondering if we'd ever see some kind of prop-wash calibration. Is there some adjustment of PID gains while descending through prop wash that could improve stability…

Why not just flip over and accelerate? I bet some in the drone racing crowd do this.

You could, but once you need to turn around and thrust upwards to avoid hitting the ground, you'll need to run the motors HARD, which will make a lot of turbulence right below you, which you then fall into. So you're falling into wind that is already blowing towards the ground, so you need to push it down even faster, and so on.

Ducted fans might help a little, since they can send the turbulence further away faster, but then you have the additional mass of the ducts, and the inefficiency of accelerating the air to higher speeds, since larger slower props are more efficient (but less responsive), to the point that this is possible: https://www.youtube.com/watch?v=emK-qIbuJ-k

Re: Drone Physics

#14

Post by YZF »

hnuser123456 wrote: post #4
the__alchemist wrote: post #3

Earlier quoted context omitted.

Yes and no. The math here is useful, albeit not required. It is not specialized PhD material; it's linear algebra. (And not the abstract quantum mechanics/chem kind!) What I mean by not required is, I've written drone firmware and didn't directly use this; the core can be done with a PID for rate controls (Compare measured rate along each axis with commanded; nudge motor power proportional to the diff), and commandin…

I always found it interesting that basic quadcopters work on nothing but a gyroscope which measures angular velocity in 3 dimensions, and optionally an accelerometer for auto-levelling, with 3 dimensions of acceleration. Then of course you can add on GPS for absolute position and route planning in 3 dimensions, and a ground height sensor for auto-landing, then you can add distance sensors on the sides for obstacle av…

My intuition is that you don't want to touch your gains/tuning for external disturbance. The tuning should be the most aggressive that maintains stability margins. The external input doesn't matter because you're looking at open loop stability margins (See: https://en.wikipedia.org/wiki/Nyquist_stability_criterion ). All this assumes linear systems and such but as as general principle.

Re: Drone Physics

#15

Post by blt »

hnuser123456 wrote: post #4
the__alchemist wrote: post #3

Earlier quoted context omitted.

Yes and no. The math here is useful, albeit not required. It is not specialized PhD material; it's linear algebra. (And not the abstract quantum mechanics/chem kind!) What I mean by not required is, I've written drone firmware and didn't directly use this; the core can be done with a PID for rate controls (Compare measured rate along each axis with commanded; nudge motor power proportional to the diff), and commandin…

I always found it interesting that basic quadcopters work on nothing but a gyroscope which measures angular velocity in 3 dimensions, and optionally an accelerometer for auto-levelling, with 3 dimensions of acceleration. Then of course you can add on GPS for absolute position and route planning in 3 dimensions, and a ground height sensor for auto-landing, then you can add distance sensors on the sides for obstacle av…

One can predict forces involved in landing for specific hardware using machine learning, e.g. https://arxiv.org/abs/1811.08027.

Re: Drone Physics

#18

Post by [deleted] »

fragmede wrote: post #10
ivanjermakov wrote: post #8

I studied this subject when I was building FPV sim from scratch. I noticed that yaw action is often misunderstood, falsely believing that it's caused by a change in angular momentum (think reaction wheels). There is a good SE answer about deriving net torques given each props' thrust force in less PhD language than this article: https://drones.stackexchange.com/a/416/11402 If anyone is interested in tackling UAV simu…

Why does four seem optimal? Why not three or five? or eight?

[deleted]

Re: Drone Physics

#19

Post by hnuser123456 »

fragmede wrote: post #10
ivanjermakov wrote: post #8

I studied this subject when I was building FPV sim from scratch. I noticed that yaw action is often misunderstood, falsely believing that it's caused by a change in angular momentum (think reaction wheels). There is a good SE answer about deriving net torques given each props' thrust force in less PhD language than this article: https://drones.stackexchange.com/a/416/11402 If anyone is interested in tackling UAV simu…

Why does four seem optimal? Why not three or five? or eight?

4, 6, and 8 are all fairly standard, as quadcopter, hexacopter, and octacopter, because you have a pair of motors rotating in each direction, which allows you to slow down all the motors spinning in one direction and speed up the others (as long as each set spinning the same direction is on opposite sides of the center of gravity), to maintain constant lift while yawing left or right.

Tricopters are possible as tilt-rotors, where you have two motors spinning opposite directions on mirrored sides, and the third motor is on the midline in front or back and able to tilt left and right. This allows the vehicle to control yaw despite having an inherent yaw imbalance in hover. I suppose you could do any odd number of rotors this way, and more rotors would mean less inherent yaw imbalance if there's only one extra motor spinning a given direction.

Two is possible if the payload hangs below the props, and the props are able to independently tilt.

One requires collective pitch and still at least a small tail rotor to cancel out the yaw, as a helicopter, but that is far more complex and fragile. But it is the most efficient, as fewer larger props are always more efficient than more smaller ones.

So four just turns out to be minimum to have no moving parts besides the props themselves, and still have full control authority, and the control logic is more straightforward than tilt rotors and such.

Re: Drone Physics

#20

Post by katzgrau »

> This article describes drone physics. Audience should have familiarity with introductory linear algebra, introductory calculus, and introductory classical mechanics.

Having familiarity with all 3, as soon as I read this sentence I knew it was all down hill for me.

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