Bit Char-G - Doing research long after it matters. And making a 1/64 scale proportional car.

I want some r/c cars to drive around my computer room.  The room is about 10x10, and has a bunch of furniture and other stuff in it.  That means even 1/32 scale cars are a bit big, if I want to set up a course and do laps.

Several years ago, I was into these cars called Bit Char-G.  They were made by Tomy, and sometimes were sold on the MicroSizers name by Hobbico in the US.  I got most of mine from a little Japanese mall, and mail order.

Here's a Bit Char-G next to a AAA battery...

Their turning radius is about a foot, so they're great for rolling around on linoleum floors.

So lets talk about the cool stuff on these cars.  First, they have replaceable final drive gears.  They come in three color coded sets.

Now, this seems great, excepting they're not proportional.  They use a magnet and field coils to steer, and have what amounts to a pager motor to make them go.  Admittedly, they have enough power to really go, and be challenging to drive... so that's not the problem.

Now.. how do you give proportional steering to a car that's 55mm between body mounts.

Happily, there are servos that are "about the right size".  The one thing that's missing, is a decent micro speed controller.  "micro" speed controllers seem to start in the "few amp" range, and go up from there.  The smallest brushed one I can find, is a 3amp one for airplanes.

What we need here, is a 1 amp speed controller, with coast, brake, and reverse.



But.. do we really need 1 amp?  I pulled 1 amp out of the air, based on the run time of the car.  Which is "a few minutes" on 50ma.  I think we need to do some testing....

So testing we shall do.  From a fully charged 50ma NiCd cell...

No load: 33ma
Some load: 55ma (finger dragging on the wheels making them slow down)
Full stall: 99ma

I was expecting "amps" not under a tenth.  But.. I'll take that.

The usual answer for "I need a really tiny ESC" is to gut a larger r/c servo, glue the pot  on center, and call it a day.  I want a really good tiny ESC.  If this project pans out... I'll need to do some research to find that sort of thing.


I started digging through my gear to see what I have that might suit this build.  

How are we going to squeeze a battery, ESC, Servo, and RX into that?  Specifically this gear:

It looks like the receiver would fit upside down, without interfereing with most bodies.

And the servo looks like it will fit flat  if I carve out the bottom

But.. in the end, those weren't going to work tonight.  If i want a chassis like that, I'm going to need to build this up as a pan car.  (a completely sane, if time consuming option..)  That, was not in the cards tonight.

Speaking of tonight, I needed an ESC.. now.  Thankfully, a servo was willing to spill it's guts for me.

I'm decent at soldering, but lets say this was a challenge.  Much to my joy, it worked.

A practical airframe. The QFO could fly.

Finding time to work on hobbies is.. a challenge.  Sometimes, you have a cat sleeping on your work surface.  Everyone, meet Al.  

Building inside a QFO frame is an interesting task.  In order to get all of the power wires going the right way, the speed controls all needed to face the same way.  And to have clearance for the battery, and flight controller, the speed controls needed to be tucked into the sides of the airframe.

To get the motor wires going the right way, I soldered them on backwards.

Trying to take a photograph of the inside of the QFO isn't easy.  Here's where I mounted the speed controls.  

I still dislike PDBs.  So I built another one of the Warpquad style wiring harnesses.

And here's the power harness in place.  Somehow, I didn't get a picture of the cable end I built, that includes a JST lead to power the lighting system.  I still plan on using the balance connector to power the video system on this quad.

With everything on board, excepting an antenna, and a camera, it comes to about 400 grams.  

It's all a really very tight fit.  I need to dig up a receiver with CPPM so I can get rid of most of that wiring harness.  Somewhere, I have a couple of those.  

Someday soon, it will fly.  

Next up, is figuring out the camera situation.  I plan on putting an 808 based camera under the airframe.  And.. so long as I don't try to do any high performance flight, I could use the video feed off of the 808.  I'm also considering cutting a hole in the side of the fuselage to stick a tiny camera through.

I should also finish the lighting rig.

Building a QFO - That motor situation.

The QFO is a fiberglass shell, 250 class, quadcopter frame.  The bare shell is about 65 grams. HobbyKing claims 47 grams.. but I think that's without any landing gear, or magnets installed.  "My QFO, ready to install motors" was 65 grams.


http://www.hobbyking.com/hobbyking/store/__58182__FunRC_QFO_250mm_High_visibility_Composite_Quad_Copter_Frame.html

Speaking of motors, that's the subject of today's blog post.  They say the frame is designed for 1506-2208 motors.  If you look closely, you'll see that my motor is an 1806.  It will not fit in that hole. 

 

I mean, that "really" does not fit.  My Turnigy 1704's will fit, but these DYS 1806's are a no go.

 

 

The material that makes the frame is very nearly paper thin.  It's thinner than cereal box cardboard.  So running a dremel around in the hole very quickly takes care of the hole size. 

If you're sharp eyed, you'll see that's not actually a dremel.  It's a $9 "multitool" from Harbor Freight.  It's DC, quiet, and I expect it to last all of say.. one project. 

 

What I really like about this airframe, is that the motors end up recessed.   I think it's a really neat look.  I'm also wondering how you'd get a 22mm motor on there.  As you put in bigger motors, you're going to have less and less fiberglass to mount to.  I think putting 22mm motors on there would probally just lead to them ripping off in the first crash.  I wouldn't try mounting anything bigger than a 18mm motor internally.

I found that mounting the motors to the bottoms of the arms is a valid mounting solution, and while they don't end up recessed, you get around the prop clearance issue, and you retain all of the strength of the arm.  And with the additional prop to frame clearance

 

The QFO doesn't have a whole lot of internal space.  Stuffing this airframe is going to be a challenge, especially since I plan on getting some reasonable endurance out of this, and hanging an 808 based camera underneath it.  

Here I have all the motors installed, a 1500mah 3s battery, a 200mw 5.8ghz transmitter, a LED power supply, an OrangeRX reciever, an ESC, and the Flip32+, are all in the frame. 

 

The only way to get the 1500mah 3s battery in there is across the X.  It will fit longitudinally, or transversely if you rotate it after you install it, but having to rotate it after it's put in the airframe means I can't use the corners to hold the VTX or reciever, or LED power supply.  This does make me wonder how it will fly, as the big battery is going to make the polar moment very different across one axis than the other.  It will induce a diagonal pitch movement every time it tries to tip to go forward, or slide left and right. 

We'll need to see how that works out. 

Getting good flight time from an airframe has a lot of factors.   You need raw watt hours to power the motors.  You need lift to get those watt hours into the air.  You need to not waste energy.  Not wasting energy, really means having an aerodynamic frame, and efficient props.  The QFO is "designed" for 5x3 props, which aren't exactly stellar when it comes to turning motor watts to thrust.  So I decided to try a 6x3. 

 

 

It fits!  But only just barely.  That really is just 3mm of clearance under that blade tip.  But "any" clearance is enough to make this work.  Hopefully it won't induce to much airframe noise.  I plan on mounting the FPV camera through a hole in the side of the frame, without any vibration damping, so I may be forced back to 5x3 props in the end.  

 

 

5x3x3's always have looked good to me.  So I threw that on a motor to see how it looked.  Sadly the extra x3, while it does produce more lift, you also get 50% more tip losses.  I think 5x3x3's will lead to reduced flight time versus 6x3s.

 

 

 So that's it for the first night of building.   I still need to relieve each motor hole just a little.  And then I can get to figuring out how the ESCs will mount.  I think they're going to get foam double stickied tape to the inside top of the airframe.