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Thread: The flying Prius

  1. #1

    The flying Prius

    This is a project that has been in the works since August. It has been very challenging and is still in the early stages. So I am starting a blog to present my work and provide future updates on progress.

    In short, this is a fuel - electric hybrid (series hybrid) UAV/FPV plane.

    IC engine ---> Electric motor ---> rectifier/filter ---> ESC ---> Electric flight motor



    - Endurance of an IC setup
    - battery back-up, adding some redundancy (reliability)
    - Turning off generator and flying on batteries allows vibration-free HD filming
    - ability to use an IC engine that has just enough power to sustain flight/ mild climb. (endurance bonus)
    - ability to take-off under both generator output and battery output (power bonus on demand)


    - Weight: This design uses two electric motors (one for generator, one for prop), an IC engine, batteries/esc, bunch of power electronics (large filtering capacitors, rectifier, arduino uno to manage the generator speed (voltage output)).

    - complexity: The IC engine is directly coupled to the generator motor (alignment is critical), starting the engine requires an electric starter (custom design required because of space confinements), power electronics have to be up to spec for the projected current draw, cooling has to be adequate, have to program the arduino to manage throttle (tuning a PID), etc, etc, etc... A lot to go wrong here...

    Setup: (to be updated)

    IC engine: DLE 20

    So taking that into consideration, I first had to figure out what scale to make this on. My first attempt actually used an OS 15 fp. belt driving a small BL motor. It was too heavy and anemic to even think about putting in the air.

    I had a DLE 20 laying around the house so I decided to go big or go home lol. So the IC engine was/is the DLE-20

    Airframe: Slipstream RV-8.

    It just so happened that the DLE 20 had spent some time in the front of a slipstream RV-8 (30 cc). It was also the case that the engine fit upside-down and backwards in a manner that made cooling feasable. It turned out that there was also enough space for the generator and they would fit in the right proximity to make direct shaft coupling possible. It also has an 80 inch wingspan and a 14 inch chord - so lots of wing area.

    Generator: Turnigy SK3 6354

    I needed a low KV motor to get the correct voltage (shooting for 8s) at the operating RPM of the DLE-20 (8-10k rpm). This one was supposed to be 260 Kv and it was cheap. Not knowing if this project would work - I bought two of them. Both for the generator and prop.

    Drive Motor: Turnigy SK3 6354 swinging 17X8 - 18X10 prop

    Rectifier: 50 amp, 3-phase rectifier from digikey with heat sink

    Ripple filter: 14 - series connected, actively balanced, 50-farad supercaps. Max voltage of each cap is 2.7-2.8 vots.

    IC engine controller: Arduino uno using a PID library. It monitors the generator output through a rectified/filtered DC signal going to an analog input. It also looks at a 3 pos switch signal from my Rx. On position 1 the arduino sets throttle to idle. Position 2 sets throttle to high idle; if the generator is not running, arduino turns the starter and choke on, after one second - turns the choke off, and after two seconds turns the starter off. It waits another one second and repeats if necessary. Position 3 activates the PID and attempts to hold the generator output at a given setpoint.

    Battery Charge system: I currently do not have a way to charge flight batteries. In fact - The first time I fly it with the generator on, I will run the generator in parallel with the batteries. Later I am planning on installing a power-diode in the battery circuit to prevent the generator from directly charging it. Then I would like to have some way of charging through the balance leads. I am not sure how to do this - I have read a lot about simple lipo chargers and will likely make my own PCB with 8 switching mode regulators but I am certainly open to suggestions. I would like to charge each individual cell at 4 amps and keep the total weight under say... 250 grams... Supply voltage for the charger will be 27-32 volts.

    FPV/RC stuff:

    Camera: RMRC 480 (non-OSD, with audio which is not hooked up... that will change soon)
    VTx: 800 mW 1.3 GHz with CP antennas

    OSD: ET, I don't like it - but I need to have all that data during test phase. I am currently getting familiar with APM so down the road...

    RC Rx/Tx - TSLRS V6+ with Futaba T8fg (with new firmware so it is a "super"). (actually using 9 channels: Ail, Ele, Thr, Rud, OSD menus, IMU gain, Flaps, Generator modes, Pan.) If you look at my vids in posts 3 and 4, you can see that the cam angle sucks for flight view. I will use my cam switch and have another under the wing - one day... So that would be 10 channels.

    In the following posts I will detail some build points and ground/flight testing as well as technical discussion and pictures/videos.

    Comments, suggestions, criticism and questions are more than welcome.

    I also want to thank the people/organizations for helping make this come together.

    FPVLAB - great info about FPV and other technical stuff

    RCG - Some of the guys in the IC engine forum helped me develop the starter when I was miserably failing.

    Microchip support forum - helped me with programming PIC controllers - really helpful bunch of people.

    Pixturethis - FPVLAB/RCG member/personal friend who has mentored me in FPV and has the ability to fly any cobbeled-together POS that I hand him - and get it back on the ground in one piece!
    Last edited by chemnut220; 7th January 2013 at 09:38 AM.

  2. #2
    Build summary:

    Engine installation in airframe

    I cut the floor out of compartment just behind the original firewall. I then installed a second firewall about 10 cm behind the original. I ordered a standoff mount from seaplane supply so I could use
    standoffs instead of the beam mount which I wanted for the starter.

    Click image for larger version. 

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    The above photo basically sums it up. The shaft coupler is a bellow-type. It tolerates some shaft misalignment and also provides some dampening from the power pulses of the IC engine (wind-up like a coil).

    The generator motor also got its own fire-wall that was glued and braced to a central bulkhead

    Cooling is accomplished by the gaping hole in the front of the plane

    Click image for larger version. 

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    And airflow can either go through the entire air frame or exit through the bottom

    Click image for larger version. 

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    Starter development:

    Version 1: This was my first attempt. The only thing that I did right here was the "flywheel" gear. I had a machine shop bore/hone it so I could press-fit a sprag bearing into the gear. So it grips the crankshaft when the starter is turning over, but when the engine runs - it just free-wheels. The photo and video below demonstrates the design and function of the V1 starter.
    Click image for larger version. 

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    The first time I tried to actually start the DLE-20, it backfired and destroyed the small starter gear.

    V2: I thought that I needed more torque so I built a 2 stage reduction drive. I went from a V1 reduction ration of about 10:1, to a V2 reduction of about 30:1.


    This version was built on the same aluminum chassis as the V1. That aluminum was cheap crap I bought at the hobby shop. It was only 1/16th inch thick and had a lot of flex. Needless to say it failed. The first time I tried to start the motor, the IC engine backfired and ripped the gears up.

    V3: Success! So it turned out that RPM was the key. As the starter is turning the engine over, the ignition fires about 26 degrees or so before the piston hits TDC. That means that the starter/flywheel/piston/etc... needs to have enough inertia to overcome the beginning of the ignition event. Otherwise - you get backfire... So I increased the chassis thickness and used better grade aluminum to 1/8 inch 6061 aluminum which practically eliminated flex. This enabled me to crank up the RPM and start the engine!


    Kind of a crappy video - the fuel tank actually gets mounted in the cockpit so when I was testing without the cockpit attached, there was no good place to set the fuel tank.
    Last edited by chemnut220; 8th January 2013 at 10:12 AM.

  3. #3
    Ground Testing/info log:

    Here is a video where I am testing the generator output.

    RPM is from the IC engine
    CHT is cylinder head temp of the IC engine (deg C)
    ReT is the temp of the rectifier (deg C)
    Amps and volts are what the drive motor is pulling on the generator.

    It is interesting to note that as I pull a big load on the generator, the voltage sags - or KV goes up. that is expected and opposite of what happens when you load a BL motor being used to drive a prop. KV goes down as you load it.

    Also the big filtering caps are doing their job. Castle link allows me to look at a graph of ripple voltage. It never goes above a volt during the above test; which while that is significant - it is right on par with the crappy zippy 8s setup I used on battery only flight.

    13JAN13: New ground test vid.


    With the new rectifier installed (heat sink on the outside of the fuse for better cooling), I was able to do more ground testing. In this latest video I wanted to see if the engine could stay cool on the ground with the cowling installed (take-off conditions). So I loaded it with about 20 amps and watched the video feed for about a minute. It approached normal operating temp (150-160 C is normal for this motor) after about a minute of running. I shut it down when it got to this point - further testing in this regard will have to be done in flight.

    Also, I set up the battery on a relay which is activated by an RC switch. It seems to work nicely.

    In the video you can hear the motor RPM oscillate around the set point (also see it in the voltage and RPM). I need to do more tuning on the PID to get this worked out, then pick a set point that causes the IC engine to operate at an RPM with lower vibration levels.
    Last edited by chemnut220; 13th January 2013 at 04:00 PM.

  4. #4
    Flight testing:

    Ok so as of 06JAN13, This plane has flown twice. However - both of those flights were battery only with a 17X8 prop. The plane has since gained about 2lbs (auw is now ~ 17-18lbs) with the incorporation of all the power electronics and start system. Flight vid link below.


    Also you will notice that the prop is terribly unbalanced!
    Last edited by chemnut220; 7th January 2013 at 09:11 AM.

  5. #5
    Penguins Do Fly! paintz2007's Avatar
    Join Date
    Feb 2011
    Along for the ride

  6. #6
    WWJ5D? JohnVH's Avatar
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    Nov 2012
    Lynden, WA

  7. #7
    HeliMedic ColoradoFlightMedic's Avatar
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    Nov 2011
    Colorado, USA
    Extremely impressive! Not sure why this thread doesn't have more traffic?

    Brooks, N7BLD

  8. #8
    Crashing Vitamin J's Avatar
    Join Date
    Mar 2011
    Denver, CO
    Who this is crazy, definitely in for the ride!

  9. #9
    Probably Not Flying Legot's Avatar
    Join Date
    Mar 2012
    United States, AZ
    How did I miss this!? That's great!
    Quote Originally Posted by LexusFPV
    I'll trust David22, he seems to know what he's talking about....

  10. #10
    Hey guys and thanks for the interest! This project may move somewhat slowly for a few reasons. For one, its freaking hard! and I am not an engineer or anything like that. Secondly, I am in grad school and writing my dissertation (chemistry - hence the name chemnut). And lastly, I do not have a very good place to safely fly this beast that is close to me (for the time being). I make a 2.5 hour commute for each test flight - but it is worth it!

    I suspect the lower traffic is due to the fact that this thread is a lot to digest for some. I am not the best at presenting information in an easy to digest way (working on it though). I plan on posting a lot of technical stuff about modifying the code for my IC motor controller, tuning the PID, adjusting the power electronics circuitry if necessary, ground testing results, as well as the fun stuff (flying!).

    At this point, if you are uninterested in programming then you may want to skip the rest of the post:

    I posted in another tread regarding arduino, an early version of the code to control the IC motor.

    Here is a link to my google drive containing my old code (I don't know any better to share code and if someone else could help me out I would be happy!)


    Well... I am also not a programmer so the code sucked and really didn't work well. In fact, the PID was not even seeing input from my voltage signal coming from my generator (realized that when the IC motor hit 12k rpm as you can see in the vid in post 3). So after a lot of tweaking around (trial and mostly error) I FINALLY got all the variables and commands in the right place and got the PID working! New code below:


    Sort of... Currently I am waiting on a new rectifier, therefore I could not test it under load, but at least now it will not hit 12k rpm and holds the setpoint where I want it.

    The proportional gain has to be tiny because a gasser responds very very quickly to small inputs in the throttle. At a gain of 0.009 it oscilates moderately. I settled on a value of 0.003 which has very very mild oscilations under no-load conditions. Right now the I and D terms are set to 0. I will play more with them when I get the rectifier back up!

    Again, thanks for the interest in my project and I hope to have more updates.

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