Saturday, June 02, 2007

So You Want To Put An Outrunner On A Mugi Evo But Don't Know How To Mount It . . . .

Well never fear, because I'm about to make that task a whole lot easier. The design of the Mugi Evo naturally lends itself to a can type motor, but I've devised a simple method of attaching a brushless outrunner using inexpensive and readily available parts. With only a few simple modifications to a GWS 400 gearbox frame, you too can take advantage of the increased efficiency of brushless power, especially now that there are several inexpensive brushless motors available, many rivaling even brushed motors in the price department. I've been using the Tower Pro 2408-21 myself, and have found the performance to be almost identical to my brushed Speed 400 when running the same props, with half of the current draw of the brushed motor I used to run. This power system, including the mount that we're about to make, also happens to weigh about 1/4 of an ounce less than a brushed Speed 400 that it does a great job of replacing. The TP 2408-21 is readily available, especially on Ebay, for $10 - $15, which is about the same price as a typical brushed 400 motor. As an added bonus, many of the TP motors on Ebay include a grey plastic gearbox mount that will work equally as well as the GWS gearbox frames for an outrunner mount conversion. When one considers the benefits in efficiency and power, then factors price and weight into the mix, there really isn't any reason to settle for the short lifespan and inefficiency of brushed motors.

Ready to go brushless? Here we go.

First thing's first, assemble all of the parts and tools necessary. You'll need a GWS 400 gearbox frame, not the entire gearbox, just the plastic frame. I ordered a box of them from All E RC, they cost $1.25 each. Like I said, this is an inexpensive project. If you happen to have an old GWS gearbox laying around, you can use that too. Just remove the spur gear and shaft, and push the bearings out of the frame. You'll also need a Dremel tool with a cutoff wheel, a short (2 or 3 inches) scrap of .188" carbon fiber tube which just so happens to be the same tubing I use for the wing spars in my Evos, a drill bit the same size as your carbon tube, 3 nylon ties to attach your finished mount to your plane, some thin CA, and a hobby knife or two with different shaped blades.

First, we need to ream out the gear shaft hole to fit the carbon rod that we're going to use to reinforce the mount and create a third tie down point to make the motor more secure. I don't recommend using a drill to turn the bit since the plastic will melt if you're not extremely careful. I just twist the bit through with a pair of pliers.

When you've finished enlarging the hole, test fit your carbon tube. The fit should be reasonably tight, but if it can wiggle around a bit don't worry. We'll secure the tube in the hole with a dab of CA later.

Next we'll cut off three sides of the stick mount portion of the gearbox frame. Try not to cut into the top portion of the stick mount, leave a nice flat portion on the bottom of the mount as this will be the platform the entire mount sits on when we zip tie it to the Mugi Evo.

This is what it should look like when you're done, no more stick mount, just a nice flat platform. Don't worry if it's a little rough, the sharp edges will just help to keep the mount from sliding around.

Next we need to make a place for the center nylon tie to sit. If you hold one of your nylon ties around the mount, centered on the large section that's designed to hold the motor, you can easily make a little mark on either side of it with the blade of a hobby knife. When properly modified, these little bumps make a keen retainer for our nylon ties.

Then carefully grind out the material between the marks with the Dremel so that the nylon tie will sit flush like this. Try not to cut too deeply, or you may weaken the mount.

Mount the motor to the plastic mount facing rearward, like this. Think about which direction you want your motor wires to face before installing the screws, and don't overtighten the screws or you'll crack the plastic mounting ears.

Now place the carbon tube into the hole that we drilled earlier. Position the carbon tube so that it hangs just a bit onto the rear portion of the motor can. Make sure that it can't rub on the rotor however, or you'll likely let the magic smoke out of your powerplant in very short order.

Cut the carbon tube off flush with the extended front portion of the mount.

This is what your mount should look like at this point with the motor attached and the carbon rod properly positioned.

Drop a little thin CA onto both ends of the carbon tube, allowing it to wick in between the carbon tube and the plastic mount. Use extreme caution not to allow any CA to get into the motor.

If you use a motor like this one with a long, threaded shaft, you'll likely want to cut the shaft off and use a set screw or collet type prop adapter instead of mounting the prop directly. A long shaft like this would likely get bent on the first landing with a Mugi Evo, so cut it off as short as possible while still leaving enough material to mount your prop adapter securely.

Well, here it is, the completed mount. Now let's take a moment or two to blow/wipe off the dust and shavings, then we'll attach our brushless outrunner to our Mugi Evo.

Start by making a small mark on the centerline of the top doubler, approximately where you want the front of your motor mount to be. I prefer to use a pencil since the marks will virtually disappear if you're not looking for them.

Draw a faint line from the center of the rear of the fuselage, to the mark that you just made. This way we'll have a definite centerline with which to align our motor mount.

Using the tip of an Exacto knife, poke a small hole next to the carbon rod at each end of the mount, and next to the large portion of the mount in line with the grooves that we ground earlier. When you're certain that all six holes are where they need to be, poke the holes all the way through the airframe and enlarge them with a small screwdriver just enough so that you can easily pass your nylon ties through.

This is what the pattern of holes should look like when you're done.

Next we'll secure the motor and mount to the plane. I start by threading the rearmost zip tie around the carbon rod of the mount, then through the holes in the coroplast. Don't pull this tie tight yet however, since we want to tighten the front tie first to get the proper thrust angle. If you use a pair of pliers and a screwdriver like this, you can ratchet your zip ties down extremely tight, just be careful that you don't suck them right through the coroplast!

And here's the finished installation. If you look carefully, you can see the third nylon tie peeking out from under the motor. By installing the carbon tube, and using it to secure the mount to the plane, we can now switch motors simply by removing the screws. No zip tie cutting involved. All that's left is to install the 3mm prop adapter and prop and hook up the wiring.

Here's a view from the underside. When properly tightened, the nylon ties shouldn't allow anything to move around, but they shouldn't be threatening to pull through the plastic either.

Here's the latest Mugi Evo to come from the Big J Aviation production facility. Now I'm off to bed as I'm planning on test flying this bad boy tomorrow. If all goes as planned, the next tutorial will be on how to light this sucker up for some kick ass night flying action. Stay tuned.


Moogs said...

Now that looks positively evil!
Once you've gotten that bad boy up and illuminated, how about a glow in the dark Tea Racer? You could have a different colour in front of and behind the wing spar :-)
Ok, maybe just try it 'as is' first.
Liking the combination of translucent and black. Ebony and ivory...

Justin said...

Unfortunately, it looks a little TOO evil. Actually, it looks a little too invisible. I flew it today and it flies excellently, just a bit hard to see. I put some black stripes on the bottom for orientation, but it isn't enough. I'm thinking maybe some black and clear checkers or something, but it needs a bit more visibility. Perhaps once I get the lights in it it'll be fine in the dark, but in the daytime it's a bit hard to tell which way is up. It does fly good however, even inverted. ;)

Oh trust me my friend, a night fly Tea Racer is definitely in my plans, but not for the first one. I'm thinking black with red scallops, like a Gee Bee Model Z or a Travelair Mystery Ship. The only appropriate scheme for a race plane methinks. Or maybe black and yellow . . . . . ;)

Table Mountains said...

: )

Table Mountains said...

looks like that one didn't quite fit. try again, ://

Table Mountains said...

not having a good day at all.

combine the three lines with no breaks.

Justin said...

That's pretty cool Wayne, thanks for taking the time to make it. I'll have to post that one. ;)