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.
the “ducted” part of the fan
T-nuts? In my allegedly aerodynamically sound surface? More likely than you think.
I didn’t put much thought into how the motor would actually be mounted within the fan until I had the propeller assembly built. That became the next task at hand. The above design is what I settled on after about 15 minutes of thought – the cruciform piece replaces the aluminum mounting flange on the motors, and the fins pointing vertically act as lateral support and increase stiffness.
The plan was to epoxy the whole assembly into the tube, so the fins also helped in alignment.
I managed to set a new personal best design-to-fabrication turnaround time of about 1.5 hours from “New Part” to toweling off the aluminum plate after fetching it from the waterjet.
After a bit of sanding work, the… death-shredder-spike-club-stabby-point thing is done.
I countersunk the motor mounting holes to use the same M4 flathead screws that the motor came with.
Step 22: slather in 5-minute epoxy because it’s the closest thing chilling next to you.
I didn’t expect these joints to be very supple or durable because of the lack of surface preparation (you know, in addition to the total lack of engineering discretion).
A few slices of pizza later, I returned to a hardened joint. So, on goes all the flight hardware…
Notice that the center hole in this mount is big enough to seat the hex head of the bolt within – saving me about 3/8″ or thereabouts.
Prop-trimming was a relatively simple exercise involving setting up the indexing head a few inches away from something pointy in the mill spindle.
I rotated the head in the conventional milling direction so the first cut didn’t just rip the prop blade right off (which it would probably have done if I climb milled).
AND IT’S IN!
I even added the cute little spinner cap to complete the experience.
To test the HOLY FUCK!ted Fan, I threw it right back onto Fankart.
This time, the packaging allowed me to just tape it into the baby seat, facing forward.
It means I had to ditch the spinner cone since it protruded forward of the tube, but hey.
Everything else remained the same – same 10S, 10Ah lithium pack, controller, radio, and shady servo-on-a-string-tugging-the-wheels steering setup (which, mind you, worked GREAT.)
Here’s a better shot of the HFF from the back, showing the mounting fins.
…and cruising the hallway!
The test video of FANKART!!! 2 is also on TechTV:
It was beautiful.
I was also totally correct about the gluing job. When the final rollover occured, the shock just busted all the epoxy joints clean off the PVC surface.
Well, that’s what I get for rushing things. These fins will probably get end-drilled and tapped for some real screws to hold them in place later on.
It works? I have yet to retrieve actual thrust numbers or anything, but the whole cart + batteries + fan rig weighed around 25 pounds. Factor in some rolling friction and you get… I’m not sure, at least a few ounces of thrust?
The next steps for the HOLY FUCK!ted Fan is to make it more in line with its namesake. Besides repairing it, I want to get heavier-pitched, larger props and trim them down to the same diameter, ensuring that the blade angle is steep throughout roughly the entire span. Right now, the 13 inch props are very shallow pitched at the tips, which sort of makes the duct not worthwhile. I figure starting with a 16 x 10 or thereabouts and cutting it back down to 13″ will be the next starting point.