Archive for July, 2010


More about Fankart and the HOLY FUCK!ted Fan

Jul 10, 2010 in Fankart!, Project Build Reports, Stuff

So this is where I expound on the gory details of why there is now a propeller-driven miniature shopping cart buzzing around campus.

The reason: Well, there is no good reason. Is there ever a good reason for anything I do?

Fankart actually has a bit of history behind it. At least, the fan part does. A few months ago when the semester was winding down, I made a post about the proposed ground effect vehicle Chuckranoplan. With me having no aero/astro design skills to speak of, nor meaningful experience operating either airplanes or boats, it was an idea that was bound to end in significant accidental property damage and probably some personal injury too. But the dream lives on.

Researching the vehicles, though, got me interested in aircraft and marine propulsion. For instance, I now have an itching desire to build a Voith-Schneider cycloidal thruster, which is something slower and more Meche-intensive. The other end of the curiosity thread was ducted fan propulsion. Electric ducted fans have been in use for many years on model aircraft, but only recently have they started getting huge because of the power levels the hobby has risen to.

Huge and Electric are two words that appeal to me greatly. Couple that with my history in combat robots which has caused me to lose all sense of self-preservation around high-speed spinning objects, and it meant that I was going to build a vehicle with EDF thrusters sooner or later. Plus, since Chuckranoplan was destined to be electric, it was good practice.

Then came word of MIT Talaris.

In short: Electric ducted fan supported platform reduces the apparent weight of the experimental lander vehicle by 5/6ths in order to test operation in the Moon’s gravity… without actually, you know, going to the Moon. After being shown their test videos, I decided to build myself a ducted fan thruster that was bigger than theirs.

That’s it. That’s the entire reason I’m doing this – so I can have the biggest EDF array on campus. After all, I can’t work without a false sense of competition and engineering machismo.

caution: no aero-astro majors were harmed in the construction of this travesty of engineering. course 16 discretion is advised.

The HOLY FUCK!ted Fan, as it became known as, has been in the works for about a month. By “in the works” I don’t mean real engineering such as fluid dynamics simulations, airfoil selection, motor analysis, manufacturing studies, or any of that. I meant sitting on HobbyKing and window shopping for a few nights.

The overarching goal was getting the most static thrust for the least effort and money. One ducted fan unit (without the motor, even) on Talaris runs a cool 500 dollars. Thrust, as I understood it from Wikipedia and hopefully accurate information from random friends, tends to increase more quickly with diameter than speed. So at the outset, I steered away from having actual ducted fan blades towards what would be better described as a shrouded propeller; that is, using a stock or slightly modified propeller in a tight-fitting circular duct.

The duct would be Schedule 5 or 10 PVC pipe – otherwise known as PVC ducting, used generally in laboratory settings for corrosion resistance where your vapors would eat fucking metal. The wall thickness on duct of this size is 3/16″, which was significantly better than the 7/16″ seen on common Schedule 40 for the size ranges I was considering. The duct alone would weigh more than the Sun.

I decided to begin small – and by small I mean 12 inches across with a power level in the 2 to 3 kilowatt range. After all, if I build it too small, it would overlap with real ducted fans. I decided to try stacking two three-blade propellers to get what amounts to a 6 blade propeller, with caveats such as “it wouldn’t actually be as good as a three bladed propeller of marginally larger size” and “the rear prop would just catch all the turbulence coming off the first”. It just had to look mean. Be glad I didn’t go with THREE stacked props.

What you get: shiny thing, shiny thing, shiny thing, shiny thing, and a tacky sticker sheet

The motor choice was pretty simple. I’ve actually been meaning to write up a post about the newest line of Hobbyking mutant outrunners, because they are actually legit hardware. These motors feature a full “distributed LRK” (i.e. 12 slots, 14 magnet poles) winding with almost outrageous slot-filling percentage, achieved by paralleling many fine strands of magnet wire in a quasi-Litz fashion. The massive external rotor is double-supported, on one end by a conventional shaft bearing and on the other by a ring bearing. This means that they no longer have magnet-ditching issues, which was a sour point with the previous generation of ICBMs (Inexpensive Chinese Brushless Motors)

In other words, it’s like Deathrunner but cheaper, better, and mass-produced – HKrunners. I’m pretty much going to use them on everything now.

The baddest of the bad is the 80/100 size, which (besides actually having a 68mm x 54mm stator) weighs 4 pounds and can briefly suck down half a dozen kilowatts. While I already had access to a few of these in the Media Lab because we are converting them into drive motors for the world’s most awesome Powerwheels car EVER, I decided that using a mere 13″ prop on something this huge is just a waste of the motor. Those motors swing 2 foot propellers on model planes 8 feet across in wingspan. When you build a model plane that big, can’t you just get in and fly it yourself?

I settled on the polyp stage of the 80/100 motor, the 63/64. They’re a miniaturized copy of the big motor, and use the same stator size that I used in the skatemotors. In fact, I briefly considered just hacking up a few of them for use in RazErblades, but decided to press forward with the custom motors anyway.

The 280 RPM/V rating ought to net me a bit over 10,000 RPMs at 10S lithium batteries.

I got a pile of Master Airscrew 13 x 8, three-bladed props to start with. When I received the shipment, it was just too tempting to resist pitching a prop on the motor and spinning it up. You can’t grab onto an outrunner when it’s spinning, since the case moves. So I used the included face mounting flange and… well, bolted it to a table.

And stood behind a 1/4″ polycarbonate blast shield.

Watching a spinning propeller isn’t really funny or insightful, so I’ll spare the test video.

Here’s the 12 inch PVC duct from McMaster-Carr. For reference, it’s part number 2051K75, which actually seems to be not a duct in its own right, but a connector for two ducts.

The ID was larger than I anticipated – meaning that it was not a 12 inch pipe, but to go around a 12 inch pipe. The props still need trimming to fit inside, which meant it wasn’t a total loss.

Now I needed to make a custom prop adapter that could hold two stacked propellers. The stock prop mount that comes with the motor suffers from a few issues, such as

  • being made totally of aluminum. Soft aluminum.
  • bolting to the face of the motor without an aligning boss… well, there was one, but it was rather ill-fitting
  • not being long enough to fit two propellors.

All the reason to make one, of course. Pictured above is a bag of M12 bolts, 12mm shaft collars, and some conical washers. I’m going to revert again to the tactic of the split-tube-collar shaft adapter, as seen now on my Die Holder of Convenience and Überclocker 1′s arm drive.

The concept is simple: drill a hole in a larger shaft that is the diameter of the smaller shaft. Slit the larger shaft, then drop a collar over the slit and crank it down. Simple, adjustable, full-contact, and compact.

I decided to splurge and buy some special tools just for this purpose. Along with the McMaster order came a 12mm drill bit for boring out the props to fit the bolt, a 10mm drill bit to hollow out the bolt, and a 0.040″ thick slitting saw. I had to fight my sense of never-buy-specialized-tools-you’ll-only-use-like-once because there was really no other way I could realistically put a 10mm and 12mm hole in something as quickly as with bits that were already that size.

I mean, I couldn’t even waterjet this, and doing it the professional way without a dedicated bit, such as with a generic boring tool, would take longer than 1.2 seconds.

Here’s the first double prop adapter undergoing the slitting operation after being drilled. I got to break out my haute usinage indexer, mostly as a horizontal protruding mount for the bolt.

A completed prop-bolt…

… slipped over the motor shaft and clamped solidly. Gee, this is so convenient I might start doing it for everything that might need attaching to a motor.

The flower of death blooms.

Or something like that. The two props slip on the 12mm bolt and are retained by the stock aluminum prop washer, one spring washer for preload, and the stock M12 nut.

I ordered enough materials to make two propeller assemblies (but not two ducts!). For the next motor, I decided to try rearranging it inface mount mode. Motors like this can be mounted inside the plane (“firewall mount”) or outside (“face mount”). In the former mode, the propeller is on the opposite of the mounting surface as the motor, and in the latter, the motor and prop all hang off the mounting flange.

Straight-up mechanics would dictate that the latter mode is almost always less stiff, but it was worth a shot. To move the shaft on the motor involved undoing the shaft set screws, removing the small retaining ring on the underside of the stationary base , then using a 2 ton arbor press to barely move the shaft through an inch to the other retaining ring groove. That center shaft is well installed.

The “face mount” mode complete. While the machines were warm and set up, I just went ahead and blitzed both prop adapters.

this is where it all went terribly wrong

“I need something to mount this to so I can spin it up.” -me

“Why don’t you just use the little cart or something?” -MITERer

I mounted the motor to the minikart by sandwiching the wire frame using the other motor’s removable flange. Four 10-32 bolts held everything in place. I wasn’t worried about the motor flying off, just… oh god, everything else. Into the basket went a 100A Turnigy HV controller (my favorite, and RazEr’s heart), my spare BR6000 receiver from Cold Arbor, and either a 7S A123 pack (also from Arbor) or a massive 10S, 10AH lithium ion battery pack.

Yesterday’s post contains that test video. It was funnier than it should have been.

After I finally came to terms with the spinning propeller, I was more comfortable gunning it to full throttle. Funny thing – I have no problem operating and standing around machinery that spins solid hardened steel blades at several thousand RPMs, but am chickenshit over a piece of plastic or wood. All things considered, I think a plastic fragmentation grenade is less painful than a hardened tool steel one.

The best run of the night!

The 10AH lithium pack was big in capacity and all, but it had 18 gauge wire leads. That means it couldn’t flow nearly as many amps as the batteries are capable of. The numbers could be higher, but 2kW is getting close to the “maximum recommended” limit for the mini HKrunner.



Jul 09, 2010 in Fankart!, Project Build Reports, Stuff

…because FANKART!

Deathblades/RazErblades: On-the-ground Testing!

Jul 07, 2010 in Project Build Reports, RazErBlades

i’m still alive

A few days ago, I took the RazErblades into Boston proper in an attempt to gather real “in-city” usage data. Needless to say, as someone experienced in mounting wheels to other things, but a relative amateur to the concept of strapping wheels to yourself, I just barely survived the adventure, but emerged with valuable data and a laundry list of practical improvements to the controls.

Before that, though, I had to actually finish the new frames.

Last time, I showed the frames in their mostly complete state. I was able to make the last of the mounting blocks, so here’s a closeup of the mounting arrangement.

I discovered after trying to crank the mounting bolt with all my might that the thread I thought was metric (M6 x 1) was actually 1/4″-28.

How silly.

So, as the world’s worst and laziest machinist, I just drove a 1/4-28 tap right over the existing M6 threads. Because 28TPI and 25.4TPI (1mm pitch) are almost equal, there was a cool thread harmonic thing going on in the distal parts of the through-hole.

Transferring componentry over to the new frames…

The cavity being 1mm wider this time, the batteries slipped right into place. They seem to be resting on the horizontal T-nuts, so I added a layer of foam rubber between them and the frame for some modest level of shock protection.

A bit of stuffing later, and the ‘blades are ready again. I shouldn’t have problems with the whole thing falling off any more.

ground test

Boston is a city of hills.

And traffic, people, potholes, curbs, and don’t forget the broken-ass narrow sidewalks and whole stretches of ancient unmaintained cobblestone.

If I was going to die anywhere while testing motorized skates, it might as well be in Boston, firmly embedded into the front bumper of a T bus.

When a crew of Putzen decided to go shopping in preparation for Otakon, I tagged along – partially because I also needed material to work with, but mostly because I wanted to get in some IRL off-campus, urban testing of the ‘blades.

The test site was Newbury Street, a famed shopping district of Boston that combines literally all of the aforementioned challenges. It was 90 degrees and the day before Independence Day, so people were everywhere. The start of the street is a long downslope with alleyway curb cuts, seemingly random switches between asphalt and sidewalk concrete, and worst of all, brick pavement. Did I mention there were way too many people?

All my hallway cruising paid off, and there was not a single crash or ditch involved.

Well, technically the test site was along the MIT coastline and across the Harvard Bridge, which presented a lesser challenge to help me tune my own stabilizing loop. I only ended up skating one way – the group had dinner at a little Japanese noodle place.

Bad mistake – afterwards, I figured I could barely keep static balance, nevermind dynamic. So I called it a day then, and walked back.

I wasn’t in the mood to take video or to have video taken, so no video of this test! Sad, I know.


The distance: 1.97 miles

The battery usage, as determined by my charger afterwards: 0.52 amp-hours

I estimate that I was using the electric assist around 25% of the time – mostly because the region was hilly enough such that I didn’t think the motor would actually affect my mobility. Most of the traveled distance was cruising, controlling speed, or just straight out skating like they weren’t motorized or something. New concept, I know.

With this conservative estimate, the “fuel mileage” of the ‘blades is (.52Ah * 22.2v nominal ) / (25% of 1.97 miles) = approximately 23 watt-hours per mile. For reference, RazEr itself seems to hit around 25 Wh/mi.

I suspect that with more electric assist and less me-assist, this number will drop precipitously.

Some observations, lessons, and anticipated changes:

  • Speed control down a hill is a horrible bitch. I could only learn so much from watching Youtube videos, and having to do it myself is no fun… especially when dodging shoppers and tourists.
    • Consequently, I’m going to enable the motor braking on the DEC modules. All this entails is keeping the enable line high such that the controller tries to hold zero speed, fighting my motion.
    • To do this, I’ll have to add another sensor or two to the wristpad controller such that it can distinguish between when I want to accelerate, coast, or brake. I’ll probably make the brake position the old throttle position, which is wrist down, since not only does that trigger when I make the corresponding motion, but as mentioned in the post, also triggers in the palm-open position. You know, like catching an impending faceplant.
  • Electric mode is wonderful for crowd-mingling. I found myself not having enough space to actually keep up the kicking motion to move forward. This was when the electric mode shined – slow, walking pace mingling in close quarters. I definitely received a few weird stares for seemingly moving with no effort while next to someone for 100 or more feet.
  • Electric mode works when skating normally. The sensored motor control means that the controller never has guess where the motor is – it always knows, even if the speed is varying greatly. I haven’t definitively tested if actually using the motors while kicking contributes to speed that much, but it’s something I’d like to get to.
  • Cobblestone and brick pavement is a travesty to anything without pneumatic tires

    • Especially on a downhill section like the Boston end of the bridge.

The motors showing some battle scars from stopping and being accidentally run into a curbside or three!

So what about the new frames? They held up great. I had no issues with loose hardware or flexiness, so the T-nut design proves itself.

the day after

…was July 4th.

Every year, the giant barge of fireworks parks literally right in front of Killian Court, on the river, so MIT gets the best possible view without even trying. The problem comes when everyone tries to crowd the riverfront to watch – and this is hundreds of thousands of people on both sides of the river easily. Roads get closed and blocked off, and the police roll out in full force to make sure nobody drunkenly riots (or if they do, it’s not too outrageous).

Wait, did someone say “roads get closed”?

The three major streets that define the “MITmuda Triangle” were closed down to road traffic – limited to bikes, pedestrians, and…

…small electric vehicles!

We basically rolled out as many small EVs as we could find – the BWD, RazErblades, RazEr itself (before a mysterious controller malfunction grounded it), and the secret MITERS electric pocket bike.

I also unchained LOLrioKart from the ceiling and took it out. As usual, it was a total attention fairy, and I’m sure there are now a few hundreds of pictures of it floating around on Facebook with people making strange faces in them. The rollout was, all things considered, a success, and we even managed to squeeze in a trip to 7-eleven (in full EV regalia) for slurpies, which constitutes the strangest mutation of cruising I have personally witnessed.

We need more small EVs. I get hits from 18.*.*.* all the time, so I KNOW you MIT people are actually reading this site. BUILD MORE SMALL EVs. Swarm and destroy!

Here’s a short video of Shane taking the ‘blades out for a spin near the south end of campus. This is still with 50% motor capacity, by the way.

Once I’m able to get the left side motors magneted up, I’ll gather some people to take real running video. Hopefully, it will be interesting with all 4 motors, and unless something really dumb happens (like I blow all the controllers or… like… die or something), the thinly-veiled-ground-test at Otakon is on schedule.

Deathblades/RazErBlades Structural Redux

Jul 02, 2010 in Project Build Reports, RazErBlades

Alright, so the last test of the RazErblades ended in a mild disappointment as my shoddy metallurgy brittle-failed under load. Personally, I don’t blame it – I might as well have put the metal together with center punch dots the way I speed-brazed the thing together. No matter – a chance to rebuild the frame is also a chance to optimize the frame geometry for better component placement. As I was entering a dead week of waiting for parts, I took a few hours to think about the new frame design.

I decided to go back to using my classic T-nuts and slots method of building, just because everything goes together so quickly using it. For well-constrained structures with lots of fasteners, it’s also fairly strong.

This is the updated rendering of the frame after a few hours of speedCADing, with few structural T-nuts designed in. Most of the changes are to minor aspects of the design that will have a substantial impact on the way the electronics sit inside the frame and the integrity of the motor mounting.

  • The bottom of the frame is now flat. Previously, there was a slight (less than 5 degree) slope downwards to the wheels. I’m not really sure why this was ever put in, but it impeded the sanding of the brazed frame surfaces and made machining setups much harder.
  • It also screwed me out of about 1/4″ of “cavity space”. This was 1/4″ that could have been (will now be) used to make sure I wasn’t stepping on the radios.
  • I changed the angle of the motor shaft slot such that it wasn’t directly tangential to the load path the wheel sees. There’s now a bit of downward “hook” to lessen the risk of ditching a whole wheel.
  • The boot mounting plate has become a mounting block. Instead of using the Weird Shady Nut, the center hole of 1/2″ worth of aluminum will be threaded to accept the M6 x 1 screw. This new mounting interface is two 1/4″ aluminum plates screwed to eachother, the bottom having a solid slot interface with the frame’s side plates. In other words, the mounting should no longer be the weak point.
  • The two notches in the middle don’t serve any useful purpose (wire passing, perhaps?) but I thought it made the whole rig look more like its namesake.

Now with more T-nuts! As mentioned above, the internal cavity is much taller. In fact, it’s almost twice as tall as the 6S battery now.

Oh, yes, one more thing. I’ve made the cavity 1 millimeter wider to let the battery slip in without requiring… you know, additional machining. The ends of the wheel shafts will be shimmed slightly to account for this.

There is now certainly the possibility of switching to an even bigger battery – 22.2v and 2.6Ah is already quite substantial to stuff inside, but I’ve been shopping for 3AH and larger packs on Hobbyking. Unfortunately, they all seem to be much wider than the 40-42mm range, which isn’t helpful. Laying the cells sideways would increase my “width capacity” to 46mm or thereabouts, so that’s a possible solution.

design for manufacturing

A day and sheet of 1/8″ and 1/4″ aluminum later.

Not everyone has abrasive waterjets on call at the tap of an ID card. Fortunately, optimizing of designs for waterjet machining (for myself, mostly out of laziness) is complemented by new online manufacturing shops that will take your flat parts drawings and lets you purchase the parts cut in different materials.  It’s one version of the PCB-fab-but-for-MechEs that I want to see more of, and believe will be key in aiding the United States’ manufacturing industry while encouraging the DIY technical arts, the TE part of STEM, and makerdom as a whole.

But enough soapboxing. I keep plugging Simon’s abrasive waterjet (and LASER!) service Big Blue Saw because he and I go way back and are homies. Isn’t that right, Simon?

The way I fasten all this stuff together is with square machine nuts. Überclocker and Cold Arbor are both chock full of 6-32 and 1/4″-20 square nuts.

1/8″ thick material is a difficult one to size up square nuts for. Even a 6-32 nut is far too wide to be useful – it can be done, but it’s excessive because of the amount of overhang at 5/16″ wide each. A 4-40 square nut is generally 1/4″ wide, which is far more palatable. But there was a problem: McMaster, my go-to guys for hardware, don’t stock any kind of 4-40 square nut. I was concerned, because that meant I was potentially spec’ing out hardware which didn’t exist. I mean, if McMaster doesn’t carry it, why should it exist?

A search through the Intergoogles took me to Amazon of all places (seriously, did you know they have a whole industrial components division?), but unfortunately all the 4-40 square nuts came in packs of 5000.

That’s alot of nuts. In fact, it’s 7 and a half pounds of nuts. I decided to press forward and bought a pack, figuring I would never, ever need to buy these things again.

hey mcmaster, you should carry 4-40 square nuts in packs of smaller than 5000.

The 4-40 square nuts are standardized to be 0.25″ wide and0.098″ thick, for reference.

And here are the new plates all loaded with square nuts, ready for fastening. All the screws are standardized to be 4-40 x 3/8″ long socket head cap screws.

And the “version 2″ frames assembled, but without hardware. I still need to modify the top 1/4″ aluminum mounting blocks slightly – they need clearance holes for 4-40 cap screws to go all the way through to the second layer. Both layers also need the center hole tapped for a M6 x 1 thread.

Hopefully, my custom magnets from Super(custom)MagnetGeorge will arrive soon, so I can get to some 4 wheel drive testing. Adding 100% more motor will make a world of difference in terms of handling.