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Tuesday, March 6, 2012

Z8RC UMX Gee Bee Scorches Past 50 mph (introducing the "Piggy Bee")

Update:  I tested a single battery solution by tapping 2S receiver power from the 3S balance port.  While not as elegant as a discrete receiver battery, the current draw is low enough I don't foresee a problem doing it.  This would work equally well for 4S, and effective-4S from running two 2Ss in series.

Doppler Wave Analysis Calculates 61.8 MPH

The stock E-Flite UMX Gee Bee has good speed at 30 mph and it looks fast in the air.  Nice try E-Flite, but you should know that's never going to be good enough for this blog. :)  My exclusive Z8RC "Piggy Bee" tears by your stock Gee Bee racer like it's standing still.

 Z8RC Piggy Bee Highlights:

- No modification to the stock airplane, motor, or electronics
- No main board
higher voltage issues or overload protection problems
- AS3X compatible 
- 8g plus choice of battery
- 20 minute installation requires beginner level soldering (not to UMX)
- Requires DX6i or better
- Works with UMX Gee Bee, Beast, Beast 3D, Sbach, Stryker 180
- Total cost, about $10 (can be a one-time cost to speed all your planes) 

And if you upgrade the motor:

- Add "any-S" brushless power (4S, 5S, and so on...)
- Works to convert most 1S ultra micros to "any-S" brushless prower

About 90% battery - max RPM is 14,800 with the stock prop.
UMX Gee Bee Variant
Thrust (oz)
Speed (mph)
E-Flite UMX Gee Bee R-2
OCP Limited
Stock + Z8RC OCP Fix
$0 Upgrade
Z8RC UMX Gee Bee
$10 Upgrade
    Stock E-Flite UMX Gee Bee for reference:

    Step by step Piggy Bee instructions:

    What a fun way to pick up breakfast. I pressed my steering wheel-mounted volume-control and channel-change rockers down at the same time.  The Speedo and Tach both dropped to 0.   Channel-up two clicks, the Tach snaps to 1000, then 2000, increasing turbo boost from 8.5 (stock) to 11.0 to 14.5 psi.  Volume-down one click to save-out, and my otherwise helpless Bimmer instantly jumps from 300hp to 380hp, dropping 0-60 from high 4's to low 4's.

    That's a $500 piggyback ECU.   I love it.  Instead of modifying the car, you "piggyback" another Engine Control Unit on top of the original one.   Takes about 20 minutes to install.  The only thing it does faster than melt my rear tires is disappear for service.  It takes about 2 seconds to reverse the procedure above, returning the car to stock.

    Chomping on chewy bacon in the car, I had an evil thought.  That anemic UMX Gee Bee sure could use something like this.   An hour later it was running, and the Z8RC Piggy Bee was borne.

    Ok, I admit it upfront.  This concept is so simple I don't know why you didn't already think of it.  Oh well, the best ideas are always simple.   Stir a previous simple concept into this mix, going from single to dual aileron channels in Z8RC UMX Extra 300 flaperon/spoilerons, and soon you're riding piggyback to pylon racer glory.

    The UMX Extra 300's board is similar to the Gee Bee's in that it has extra servo channel or two sitting empty.  The UMX Gee Bee actually kludges 2 of 3 integral servo sockets for it's ailerons.  I remember thinking a Y harness would have been cheaper, but oh, maybe AS3X needs two channels.  I wondered if they stole the flap servo channel for the other aileron, which would make the last open channel for landing gear.  So I played with the flap controls and sure enough, even though only one Gee Bee aileron is plugged into the flap servo channel, a flap channel input moves both ailerons in typically illogical Spectrum fashion.

    On the UMX Extra 300, the only open socket was wired as a Spektrum landing gear channel, and man was it a bear to figure out how to mix into one aileron. A little poking around and yep, the Gee Bee's open channel is wired as retractable landing gear.  Boy, I can't wait to see that on future R-2s.

    Since the Gee Bee has an open channel, it is possible to piggyback a second ESC on top of the first ESC.  After all, ESC's are designed to look like simple servos to the receiver, a throwback to servo controlled carbureted throttles.

    This mod is like adding a servo from the Gee Bee's perspective.

    I was hopeful that Spektrum's lack of programming logic could be overcome.  And as it turns out, all you have to do is first logically unscrew Spectrum's illogical landing gear servo behavior, then map the throttle to that now-normal servo.  Once you have that done, this mod is about as simple as it gets:

    Step 1:  Pre-mix the radio.  

    If you don't mix your radio properly first, you'll have a 0 to 15,000 RPM landing gear switch.  Hopefully the plane will bind with the gear switch in the 0 RPM position (mine didn't :).

    You need a radio with two mixes available.  Mix 1 unscrews Spektrum's awkward "gear channel" to behave like a normal servo.  Mix 2 maps the throttle stick to control the landing gear servo.

    Mix 1:
    RATE D-100% U-100%

    Mix 2:
    RATE D+100% U+100%

    Note: When I was in "binary throttle" mode the motor seemed even stronger.  So mixing THRO->GEAR to a value above U+100% might yield ultimate power.  Extending throttle or gear travel might help too.

    Be careful not to reverse the plus and minus signs, or idle throttle could be 15,000 RPM halfway up your neighbor's tree.

    Step 2:  Fashion a suitable piggyback ESC to look like a servo (read entirely before starting).

    I pulled an 8 gram, 7 Amp Techone ESC out of another airplane to get this going, but any number of brushless ESCs could work.  Here are a few ideas.

    You don't need to do anything special to the ESC except put a micro servo plug on the signal and ground leads.  I snipped a micro plug from my drawer full of stripped or dead UM servos, but I'll try to find and post the part number.

    Although receivers can generally accept an external battery pack into any open channel, I elected to snip the ESC's center (positive) lead so (BEC) power does not flow back into the Gee Bee's main board.   I did it this way to protect the main board from an odd-ball ESC power supply, and I'm not sure if the main board runs on 5V or 7.4V.  You only have to connect the negative lead and the signal lead to the two outside pins on the micro servo connector, the center pin/wire stays unused.

    The problem, or good thing, about the previous paragraph is that means traditional battery power is still required to flow into the main board.   So you still need a 2S battery, but it can be very small, as it only supplies power to the receiver and servos, not to the motor.

    The piggyback ESC will need a second, motor battery.  The stock Gee Bee motor is similar to a 2S-3S E-Flite Park 180, but with a higher kV rating.  Here are the Park 180 specs for reference.  I decided to use a 350 mAh 3S battery, but you can certainly use a 2S if you want to do this mod only as an OCP fix.  The beauty of 2S is you can Y-off power to the piggyback ESC and use only one battery.

    The ESC plugs into the open gear channel on the main board.   Oddly, the socket has 4 pins, not three.  That is by design and not a problem.  The correct way to plug-in another servo, or in this case the piggyback ESC, is to place the micro servo plug onto the 3 pins closest to the board's motor and power wires, i.e the front of the airplane.  The rear, 4th pin stays unused.  The plug will fit on the front 3 pins.
    Crayon Diagram
    Step 3:  Switch the motor wires to the piggyback ESC.

    The clean way to plug the Gee Bee's motor into the piggyback ESC is to buy another matching plastic connector.  Whatever.  I tinned the new ESC's wires with solder, ground them into flat pins with a Dremel, then inserted them one by one into the motor's connector.  I secured them with an small piece of electrical tape.

    The exact orientation of attaching the 3 piggyback ESC wires to the 3 motor wires doesn't matter, but if you get it wrong the motor will spin opposite direction.  In that case, disconnect and reverse any 2 wires.

    The motor wires from the original Gee Bee ESC stay unconnected.  If you scavenge the plug for the piggyback ESC, make sure the old motor wires cannot touch anything as they will remain energized.

    All done!

    You can change motor batteries while the board remains powered up by the receiver pack, no problem.  The motor initializes properly, with 3 beeps indicating proper 3S operation unlike the Z8RC exclusive 3S battery mod I posted several months ago for UMXs, that only signals 2 beeps on 3S. 

    CG and Other Observations:

    First a word on CG.  The batteries I happened to have on hand are both are too big.  I used a 9g 2S 120mAh for my receiver pack (the smallest 2S I have) and a 31g 350mAh 3S for the motor battery.  This config adds 26g (.88 oz) of total battery weight above the stock battery, but adds big power and a lot of additional flying time.  The piggyback ESC added another 9g.  Ready-to-fly weight with two big batteries is 134g (4.5 oz).  The stock weight is listed as 102g (3.6 oz).

    There are any number of ways to arrange the two batteries to keep the CG in check.  I elected to velcro my receiver pack to the roof, and place the motor battery in the bottom of the cowling area (see photo below).  The plane balances perfectly, and surprisingly it does not feel heavy in the air.
    My battery capacities are obviously overkill, they are what I had on hand.  But the plane still has unlimited vertical and stays airborne for 12-20 minutes depending on profile ferocity.  I don't think I put a dent in the receiver pack's charge all day long.

    Before I discuss some all-important caveats, it might be worth noting that this system is only limited to 3S by the stock motor.  If you are willing to replace the motor, you can move up to 4S, 5S, and so on.  Even twin or inline twin motors are theoretically possible with a Y-harness.

    Important Caveats:

    1) Be careful propping up too far.  The stock ESC is supposedly rated to 4.9A and it OCPs stone dead on an average battery, so we can assume the stock motor is pulling at least 5A. 

    I've been running UMX E-Flite 180 motors on 3S for a long time without issue, but this is uncharted territory.  The smart thing to do is vary Mix 2 to limit the full throttle U+XXX percentage to a value you are comfortable with.

    Prop selection is important too, I do not recommend using the stock prop on 3S unless you mix your full throttle value down at least some.  After my first post-flight with the stock prop, I noticed the cheap plastic motor mount getting a bit soft from heat.  I think this is a typical case of Horizon Hobby's POS plastic motor mounts failing, rather than a catastrophic motor melt down. The motor seems fine.  I've lost count of how many ridiculously unsafe HH plastic motor mounts have catastrophically failed on me over the years.

    2) Spektrum's auto motor power off feature probably won't work if you lose the link.

    3) The motor battery will still warble then die if you fly til a dead stick, but this is no longer related to the receiver battery's charge level.  While the receiver now has it's own power source and could last for days, when it does fail the plane will stop working without warning and become un-flyable.  No different than flying with a gas motor.

    Similarly, you are no longer at risk of losing radio control if you exhaust the motor battery.


    We know the stock plane draws more than 4.9A since it OCPs the defective ESC, and has a top pitch speed of 30 mph +/-.

    On my set-up, the stock 5.25x3.5 prop develops 75W and draws 6.7A.  On a fresh 3S 350mAh it spins to 14,850 RPM for a 50 mph pitch speed.  Doppler analysis came in at 61.8 mph in the air, but that seems a little high to me.

    The E-Flite 5x2.75 prop draws 60W and 5.4A on 3S.  It spins to 15,900 RPM for a 41 mph pitch speed.  Flight time is 12-20 minutes depending on throttle management.

    The GWS 5x3 does 17,180 RPM with 7.2 oz of thrust at 70W and 6.5A for a pitch speed of 49 mph.  It is obviously much more efficient than the E-Flite 5x2.75 and should be used instead - you can always de-tune the throttle mix to match the 5x2.75's maximum current draw, but with a lot more performance.  Or, you could match the performance with a lot less current.

    Enjoy the Z8RC Piggy Bee!

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