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Flywheel Physics Explained


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#1 rhino-aus

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Posted 15 January 2015 - 06:23 PM

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Greeting non-Facebook people!
I am an aeronautical engineer and over the last few days there has been a lot of uninformed things said about how flywheel blasters actually work on the various Facebook groups I normally inhabit. Seeing as how dual wielding 13 dart per second RapidPistols is now my calling card; I have a bit of a vested interest in educating people on this wonderful technology.

I decided to write this document to try and help people understand flywheel blasters, what motors to choose, and more importantly why from a physics point of view. The document will be improved and expanded over time; this link will always have the most current version.

Dropbox Link to PDF

Have a read and let me know what you think. If I get a good response, I might make this into a video or analyze the physics behind other blaster types.

Hope this is the sorta thing that belongs here and is done right; this is my first post to a Nerf forum thing.
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#2 RedFear

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Posted 15 January 2015 - 09:23 PM

just want to point out that this guide only covers one stage of acceleration of a flywheel blaster.
http://nerfhaven.com...showtopic=24891
when you use an afterburner mod or multiple stages, you get more acceleration distance available to you. this rayven topped out at about 134 fps with quite conservative motors (26krpm 2nd stage motors with high torque magnets) so your calculations are pretty much spot on. if the forward motor armatures are swapped for higher-rpm armatures, say 40k or 60k rpm armatures, current draw increases a ton, but at the same time, you open up a whole new range of top-end fps speeds because your starting speed is 80-120fps depending on your first stage. (my first stage was stock stryfe motors, so i'd put that at about 80fps or so) i'm contemplating tacking on another afterburner to make it a 3-stage, but that's likely not going to be space efficient.

so for instance, if i take those shark 40k rpm motors and make a 3rd afterburner stage, my starting velocity is 120-130fps out of the barrel. that means that the 3rd afterburner is given the chance to accelerate the dart another 25 or so fps since the per-stage increase dropoff is exponential. that nets a theoretical aprroximate limit of around 150-160ish fps for 3 stages.

Edited by RedFear, 15 January 2015 - 09:28 PM.

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#3 rhino-aus

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Posted 15 January 2015 - 09:28 PM

As an FNG I have a limited number of replies so I will probably end up just editting this one.

RedFear. You're right. You can use the simple eqn v = √(u^2 + 2as) to find the increase of each flywheel stage, but do remember that the dart will never go faster than the flywheel surface velocity. A Shark 40K could probably get darts to about 210fps with a 2 stage afterburner (3 wheel sets total) but no more. I really, REALLY wanna try that...

Edited by rhino-aus, 16 January 2015 - 12:18 AM.

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#4 RedFear

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Posted 16 January 2015 - 05:01 AM

well, if i decide to get another h-bridge module and some stryfe flywheel housings for project stingray, i might go ahead and do a quad-stage (3 afterburner stages) rayven and tell you how it chronos. i have the motors for it. just not the flywheel housings. The blaster is going to have a long barrel anyways, so might as well make it useful.

Edited by RedFear, 16 January 2015 - 05:03 AM.

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#5 rhino-aus

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Posted 16 January 2015 - 05:49 AM

Just make sure the motors are at a high enough RPM!
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#6 RedFear

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Posted 16 January 2015 - 12:13 PM

Just make sure the motors are at a high enough RPM!

Don't worry! I come from a background of custom built motors, and crazy motors from super high torque to 100k rpm dyno-queen rc car builds. I can manage that. Have you read the guide to motors I wrote?
http://nerfhaven.com...showtopic=24887

Edited by RedFear, 16 January 2015 - 12:15 PM.

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#7 Imperial

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Posted 16 January 2015 - 07:32 PM

good solid information, thankyou for taking the time to write such a detailed overview of the physics related to flywheels. you took what I had in my head but could never put into words, and made something truly enlightening.

I am now seriously considering a twin afterburner setup running of a belt used in rc cars to connect the front and rear drive, and 1 380 sized brushless motor. I have a 380 in my 1/16 rc car that is 4800kv and rated to 12.2 volts that would be somewhere around the 50k rpm mark with huge amount of torque and inertia from the belt and flywheels spinning as one huge rotating assembly. hmm the brain is ticking now, engineering this would take some time.

edit: yes I understand resistance would slow down the 50k rpm figure I gave. these motors can be up to 7100kv from what I have seen, and my 1.3kg rc car will spin all 4 wheels when cruising at 60km/h (40ish mph)and then punching the throttle so torque wouldn't be a problem.

Edited by Imperial, 16 January 2015 - 07:36 PM.

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#8 RedFear

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Posted 16 January 2015 - 09:30 PM

If cost is no issue, the fastest and highest torque type of motor is brushless. The problem is that a 4-motor brushless setup (if you go without a belt) is around 50-60 us dollars, and the control scheme of each one requires digital control through PWM on brushless-specialized speed controls. though driving that cost down with belts will still pull up costs of axles, bearings, and pulleys. additionally, belt slack while spinning at high rpm is something to worry about since that can impede the dart badly if it misfeeds.
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#9 rhino-aus

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Posted 17 January 2015 - 03:58 AM

If cost is no issue, the fastest and highest torque type of motor is brushless. The problem is that a 4-motor brushless setup (if you go without a belt) is around 50-60 us dollars, and the control scheme of each one requires digital control through PWM on brushless-specialized speed controls. though driving that cost down with belts will still pull up costs of axles, bearings, and pulleys. additionally, belt slack while spinning at high rpm is something to worry about since that can impede the dart badly if it misfeeds.


Brushless is a great idea; the problem with them is the shafts. Very few brushless motors have 2mm shafts; the ones I could find were $30 each. Also; brushless motors are controlled by PPM not PWM.
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#10 RedFear

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Posted 17 January 2015 - 11:37 PM

Brushless is a great idea; the problem with them is the shafts. Very few brushless motors have 2mm shafts; the ones I could find were $30 each. Also; brushless motors are controlled by PPM not PWM.

it really doesn't matter. both can be done with the pwm channel off an arduino. insignificant detail to gathering a parts list. and yes brushless motors are pricey little buggers. though i've found brushless motors that could work for around 15 each motor. there's things you can do to change how it moutns into the blaster and how the flywheels are attached.

Edited by RedFear, 18 January 2015 - 01:56 AM.

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#11 rhino-aus

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Posted 18 January 2015 - 08:12 PM

though i've found brushless motors that could work for around 15 each motor. there's things you can do to change how it moutns into the blaster and how the flywheels are attached.


I have tried to swap the shafts on some of the micro brushless 1220, 1230 sized motors from hobbyking. Ended up breaking 3 of the 4 magnets I had trying to get the shafts out. If you do get those, the shafts at 1.5mm at the attachment, but have been lathed down from 2mm so a shaft from a stock motor will fit nicely inside. I will retry when I can make a jig to press out the shafts; hammering them out is a terrible idea...
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#12 RedFear

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Posted 21 January 2015 - 01:05 PM

I have tried to swap the shafts on some of the micro brushless 1220, 1230 sized motors from hobbyking. Ended up breaking 3 of the 4 magnets I had trying to get the shafts out. If you do get those, the shafts at 1.5mm at the attachment, but have been lathed down from 2mm so a shaft from a stock motor will fit nicely inside. I will retry when I can make a jig to press out the shafts; hammering them out is a terrible idea...

don't machine the expensive brushless motor, get some spare 2mm steel shafts to put the flywheels on and then put in a gear system to sync both top and bottom. you don't need to cut the shaft of the motor. heck you're not supposed to be doing that. if anything, machine the cheap plastic to fit the BL motor shaft, not the other way around. brushless motor magnets are a part of the rotor, not the sator, so you should not separate the rotor shaft like that. that's just asking for the thing to break. impact breaks magnets. simple basic rule of thumb.

Edited by RedFear, 21 January 2015 - 01:06 PM.

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#13 J-Andy

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Posted 21 January 2015 - 06:49 PM

I have considered the brushless route myself. My plan was to create a simple rectangular plastic motor cage, use readily available aluminum slot car wheels (with the set screws) and trued rubber tires as flywheels (which you can find in almost the exact same size as standard RS flywheels). If the diameter of the wheel bore is too small there should be sufficient room to drill them out / if it is too large I have also found sleeve adapters. The only thing that turned me away was that I have no Arduino experience and I can think of no other way to control a mob of ESC's other than buying a mob of servo adjuster and parallel wiring them to throw the 1ms signal when the circuit is open and the 2ms signal when the circuit is closed. My plan was not for an "afterburner" setup but for a double barrel full out RS which would have had a total of 5 brushless motors. I wasn't concerned with expense but my lack of knowledge makes the cost irrelevant.
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#14 Ultrasonic2

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Posted 27 January 2015 - 02:16 PM

Hi i agree with this Doc that ultimately states the limiting factor is not RPM but friction.

I have dome some testing and have found it's pretty much possible to reach the RPM limit with the stock motors at high voltage (not that they would last to long) So excluding spoolup time and life expectancy there seems little point in changing the motors when you can simply crank up the voltage at a far lower cost.

Now i have been considering making new wheels to increase the friction. However This also seems pointless because with the stock motors at high RPM it's already destroying the ends of the darts. I would think increasing friction would increase acceleration leading to increased wear on the darts making this option out of the question.

So i would think there is a practical FPS limit in a blaster like this

Edited by Ultrasonic2, 27 January 2015 - 02:17 PM.

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#15 meishel

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Posted 27 January 2015 - 02:42 PM

Now i have been considering making new wheels to increase the friction. However This also seems pointless because with the stock motors at high RPM it's already destroying the ends of the darts. I would think increasing friction would increase acceleration leading to increased wear on the darts making this option out of the quesion.


So we just need better darts at that point!! Part of the reason we're shredding darts at that speed is due to lack of grip. All that sliding is melting the darts. I could be wrong though. I'm working with a friend with industrial manufacturing contacts to try and find some sort of rubber or silicone 34-35mm OD ring to incorporate into custom flywheels. I'll make a post here when/if that comes to fruition.
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#16 Ultrasonic2

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Posted 28 January 2015 - 02:09 PM

Well i tried to chrono my Stryfe with my new motors Atomic ASF Spec motors. While being much faster this did not translate to an increase of FPS,But it did turn my gun into a wood chipper or maybe i should say a dart chipper.

I will relook at making new wheels but it's clear i need to make a new housing too as the motors are not parallel and therefore the wheels aren't too. Machining new housing would fix this and reduce the deflection when the dart gets squeezed. Also it would allow me to make make large diameter wheels and make them close or further apart. The only problem being is that machining new housings that are correct would be difficult. Oh yeah other benefits is that machined wheels should be more round.

Doing all this is the only way to increase FPS so maybe i can look into this.

Edited by Ultrasonic2, 28 January 2015 - 03:08 PM.

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#17 meishel

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Posted 29 January 2015 - 01:08 PM

Well i tried to chrono my Stryfe with my new motors Atomic ASF Spec motors. While being much faster this did not translate to an increase of FPS,But it did turn my gun into a wood chipper or maybe i should say a dart chipper.

I will relook at making new wheels but it's clear i need to make a new housing too as the motors are not parallel and therefore the wheels aren't too. Machining new housing would fix this and reduce the deflection when the dart gets squeezed. Also it would allow me to make make large diameter wheels and make them close or further apart. The only problem being is that machining new housings that are correct would be difficult. Oh yeah other benefits is that machined wheels should be more round.

Doing all this is the only way to increase FPS so maybe i can look into this.


What FPS are you getting out of it? Do you have a layer of dart foam built up on the flywheels yet (this aids grip)? If you're not hitting 110-120 FPS, something is wrong. The stock flywheels top out around 120FPS with the right motors.

I've been looking in to making custom flywheels as well. I'm working with a friend who works at a local machine shop right now to get some larger flywheels made from aluminum with rubber rings inserted into the outer surface to see if that will increase grip. He's going to make me an adjustable flywheel cage so I can move the flywheels closer or farther apart. I should have it in a couple weeks or less hopefully. He's about an hour away and I need to bring them a set of flywheels, motors, the rubber rings I have, and a flywheel cage. I'll be sure to post something about my findings. I'm also looking in to getting some smaller rubber rings that could fit the stock flywheels made.

There is another way to increase FPS. You can have multiple sets of flywheels (called an Afterburner). There's a video on youtube of Jodocast's Banshee hitting 158 FPS max.
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#18 Technician Gimmick

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Posted 02 February 2015 - 04:32 PM

Greeting non-Facebook people!
I am an aeronautical engineer and over the last few days there has been a lot of uninformed things said about how flywheel blasters actually work on the various Facebook groups I normally inhabit. Seeing as how dual wielding 13 dart per second RapidPistols is now my calling card; I have a bit of a vested interest in educating people on this wonderful technology.

I decided to write this document to try and help people understand flywheel blasters, what motors to choose, and more importantly why from a physics point of view. The document will be improved and expanded over time; this link will always have the most current version.

Dropbox Link to PDF

Have a read and let me know what you think. If I get a good response, I might make this into a video or analyze the physics behind other blaster types.

Hope this is the sorta thing that belongs here and is done right; this is my first post to a Nerf forum thing.



Very informative. Thank you very much. It's good to hear someone who speaks with authority on a subject. Some of it whet way over my head, but I enjoyed it none the less.

I have a question if you don't mind. I'm working on developing a PCB for a micro-controller for my rapidstrike. Up to this point, I've been far more interested in increased rate of fire, than in higher muzzle velocity, and I've designed specifically for that.

However I am considering adding a RPM sensor to the flywheels. It's purpose would be to maintain a constant RPM using stock motors. At first glance it would seem pointless, however the idea is that upon each shot being fired, the loss of RPMs would trigger a higher than stock voltage to be applied in order to maintain the speed. Special consideration would have to be made to prevent burning out the motor, but assuming this could be accomplished, do you think this would be worth the effort on stock motors?

My thoughts:
From what I can tell, this would allow me to push a stock motor far more than normal, while still limiting overheating/burnout from the higher voltages. However if I were to use upgraded motors that are already rated for 12V, I can't see any benefit to the RPM sensor since I wouldn't be able to boost any amount anyway.

Again, thanks for the great post!

- Gimmick
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#19 rhino-aus

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Posted 02 February 2015 - 06:05 PM

I'd definitely suggest upgrading the motors instead of building the suggested control system. Stock motor performance is just too low to justify that.
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#20 ravetrooper

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Posted 03 February 2015 - 04:53 PM

Thanks for the article! I downloaded it and will use it as a reference for my latest project.

I have one question and I hope it's not too off-topic. I'm considering making my own motor housing that would fit inside the front of a longstrike without having to cut the shell (I've tested stryfe and barricade motor housings and they fit but don't line up with the muzzle). I had the idea to make my own out of polycarb where the motors are closer together and the flywheels are smaller- thus the question: Will smaller flywheels help or hinder performance in any way as long as the rpm/friction is still good? Will the size decrease actually increase rpm?
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Gut the electronics, drill a hole in the shell, and attach a crank to the gear. Bam, crank-action stampede that doesn't require batteries, or even a trigger.

...(also judging by your past posts, I would consider you pretty dang wise elder like in the modding community :lol: )


#21 rhino-aus

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Posted 03 February 2015 - 05:09 PM

I had the idea to make my own out of polycarb where the motors are closer together and the flywheels are smaller. Will smaller flywheels help or hinder performance in any way?


So a few things happen with smaller; closer together flywheels:

- The flywheels have a smaller radius so the edge velocity is lower. This may be problematic if they are so small to the point where the edge velocity is lower than the potential maximum velocity: the dart will never travel faster than the flywheels.
- The smaller flywheels will likely have reduced inertia which will cause more slowing during the shot; however this may be offset by the higher density. For reference the moment of inertia of a stock flywheel is 805g.mm^2.
- If inertia is reduced your spin up times will be reduced, otherwise they will be increased.
- Flywheel RPM will be mostly unaffected; the max RPM is based off the no load speed. Assuming nothing is rubbing on the flywheels, they should always eventaully reach this max RPM.
- If the flywheel gap is reduced you are likely to get more normal force applied to the dart which may improve performance assuming the other previously mentioned factors dont limit maximum velocity
- If the surface roughness is higher than the stock flywheels you are also likely to get better performance due to a higher friction coefficient; again as long as your surface velocity remains high enough.

All in all I feel that great care will need to be taken to avoid hindering performance; smaller, lower inertia flywheels will most likely perform worse with the same motors.
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#22 ravetrooper

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Posted 03 February 2015 - 05:39 PM

So a few things happen with smaller; closer together flywheels:

- The flywheels have a smaller radius so the edge velocity is lower. This may be problematic if they are so small to the point where the edge velocity is lower than the potential maximum velocity: the dart will never travel faster than the flywheels.
- The smaller flywheels will likely have reduced inertia which will cause more slowing during the shot; however this may be offset by the higher density. For reference the moment of inertia of a stock flywheel is 805g.mm^2.
- If inertia is reduced your spin up times will be reduced, otherwise they will be increased.
- Flywheel RPM will be mostly unaffected; the max RPM is based off the no load speed. Assuming nothing is rubbing on the flywheels, they should always eventaully reach this max RPM.
- If the flywheel gap is reduced you are likely to get more normal force applied to the dart which may improve performance assuming the other previously mentioned factors dont limit maximum velocity
- If the surface roughness is higher than the stock flywheels you are also likely to get better performance due to a higher friction coefficient; again as long as your surface velocity remains high enough.

All in all I feel that great care will need to be taken to avoid hindering performance; smaller, lower inertia flywheels will most likely perform worse with the same motors.



Thanks for the info. I'll be sure and post the results
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Gut the electronics, drill a hole in the shell, and attach a crank to the gear. Bam, crank-action stampede that doesn't require batteries, or even a trigger.

...(also judging by your past posts, I would consider you pretty dang wise elder like in the modding community :lol: )



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