multiple geared motor unit- "single engine"?

Discussion in 'Electric Propulsion' started by 13brv3, May 14, 2019.

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  1. May 14, 2019 #1

    13brv3

    13brv3

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    Greetings,

    One day I'd like to build an electric aircraft. Right now, the ultralight rules don't seem favorable for electric power. LSA has a specific statement that requires a single reciprocating engine, so no electric power there. That leave Experimental Amateur Built as the first viable option.

    I've seen some motor solutions that were actually multiple motors assembled in a power unit, where the motors are geared together to turn one prop. Would this allow a single engine rated pilot to fly it, or would it get classified as a multi-engine? I sort of suspect this would depend on who you ask, and it might be safer NOT to ask. You could argue it either way I suppose.

    I'm wondering if a multi-motor power unit would make sense. Smaller motors are easier to get, and it would add some redundancy if a motor controller failed. It might allow easier scaling of power by simple adding as many motors as needed. Just kinda thinking right now...

    Thanks,
    Rusty
     
  2. May 14, 2019 #2

    akwrencher

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    It's been done, but doesn't seem to be common. I remember seeing an engine in kitplanes 20 plus years ago that was two Mazda roteries driving one prop, but have never heard of it since. Done right it could work but the r&d, weight, cost, and complexities are likely not worth it. Depends on your goals though.
     
  3. May 14, 2019 #3

    poormansairforce

    poormansairforce

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    Do a search. I have a thread about it as do others. Probably better ways to power a plane .
     
  4. May 14, 2019 #4

    13brv3

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    My primary question is whether a multiple motor "power unit" would qualify as "single engine" or would you need a multi-engine rating. I haven't seen that addressed, though I will admit to not reading every word of the forum.

    I do seem to recall situations where gasoline engines were combined, but I don't remember if it was consider single or twin engine.

    Thanks,
    Rusty
     
  5. May 14, 2019 #5

    emir_82

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    Hi,

    I think that the LSA rules now are changingto allow the electric motor.
    The common sense says that two motor on the same axis will act like one, so you don't need to care about asymmetric push if one of the motors fails. Also you can argue that if the double motor is controlled with one lever is like a I.c.e. with more cylinders.
    You can find emrax motors coupled from factory. Also you can put two controllers per motor. So you can have a lot of redundancy.

    Regards
     
  6. May 14, 2019 #6

    13brv3

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    I do hope the LSA rules get changed. It seems pretty short sighted that they excluded electric motors. The ultralight rules will be stuck like they are forever, because everyone is afraid of asking for a change, then getting something they don't want in the process.

    I wish I could find the DIY motor unit I was originally referring to. It was pretty clever using a single frame, with a large gear and thrust bearing turning a single prop. You could then mount some number of smaller motors around the larger gear, so it acted as a gear reduction, and coupling system for all the motors. I haven't researched whether that would be better or worse than a single large motor.

    Rusty
     
  7. May 15, 2019 #7

    Dana

    Dana

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    If there is one set of controls (i.e. one throttle lever, one mixture control, one ignition switch, etc.) for the composite engine, it's just a novel form of multi cylinder engine, so I think you'd have a good argument that it's a single engine from a regulatory standpoint.

    Re the small engines and large gear, seems to me that approach has been used to make a pseudo radial engine. The trick is balancing the power output of the individual engines so they're not fighting each other.
     
    bmcj likes this.
  8. May 15, 2019 #8

    BBerson

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    You don't need a multi rating for any EA-B if single seat.
    Assuming the operating limitations don't mention any such need for a multi rating.
    Can go ultralight also.

    I doubt that coupled hybrids on the future market will need a multi rating.
     
  9. May 15, 2019 #9

    13brv3

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    Thanks for the comments. I agree that it should be considered single engine if built as a package, with a single control.

    I wasn't aware of the exception for single seat experimentals, but I found several references to it FAR 61.31(l). As you said, you also have to get the FAA/DAR to not require a ME rating in the operating limitations. The Cri-Cri is a good example of this. It's really good to know of this potential option, so thanks.

    All in all, it sounds like I could find a way to do this for a single seat aircraft one way or another.

    Thanks again,
    Rusty
     
  10. May 15, 2019 #10

    TFF

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    Usually the operating instructions make a blanket statement like “must have appropriate license”. Really only room to ask forgiveness if caught. In the 80s and 90s maybe, but I doubt modern OI will anything but what is already published as the template.
     
  11. May 15, 2019 #11

    13brv3

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    Yeah, that would definitely be open to interpretation. I'll be using a DAR for another project in the next couple months, and I'll make a note to ask him about this.
     
  12. May 15, 2019 #12

    BJC

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    Does a multi-engine, multi-seat E-AB, flown solo, require a ME rating?

    Thanks,


    BJC
     
  13. May 15, 2019 #13

    13brv3

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    Good question. It sounds to me like you could fly it solo without a ME rating. Here's an excerpt from 61.31.

    (l) Exceptions. (1) This section does
    not require a category and class rating
    for aircraft not type-certificated as airplanes,
    rotorcraft, gliders, lighterthan-
    air aircraft, powered-lifts, powered
    parachutes, or weight-shift-control
    aircraft.
    (2) The rating limitations of this section
    do not apply to—
    ...
    (B) An experimental certificate, unless
    the operation involves carrying a
    passenger;
     
  14. May 15, 2019 #14

    13brv3

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    I just looked at the operating restrictions for my RV-3B and this wording would make it really straightforward if this is what they're still using-

    The pilot in command of this aircraft must hold a pilot certificate or an authorized instructor's endorsement. The pilot in command also must meet the requirements of FAR Part 61.31 (e), (f), (g), (h), (l), and (j) as appropriate.
     
  15. May 15, 2019 #15

    BBerson

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    That's a gray area.
    My helicopter instructor told me anyone other than Student Certificate does not need endorsements.
    That is one of the privileges of Private Certificate, don't need solo endorsements.
    So he said I could buy and solo a multi seat helicopter without an endorsement. That is TC aircraft and his opinion.
    EA-B is different with specific operating limitations.
     
  16. May 16, 2019 #16

    Dana

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    While the regulations allow you to fly solo in an experimental that you don't have a category or class rating for, the operating limitations template they're using nowadays says:
    So it looks like they've closed that loophole, unless you have an older aircraft with older operating limitations.

    I don't think that's correct, except for experimentals as discussed above... although, oddly, I can't seem to find the specific regulation that actually says a pilot certificate and/or rating is required to operate any aircraft. It must be there... somewhere?
     
  17. May 16, 2019 #17

    BBerson

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    Could be. I think he meant the endorsement is good forever instead of 90 days and doesn't need instructor supervision.
    FAR 61.31 (d) (3) is vague about "required" endorsements for Private Pilots.
    Whereas FAR 61.87 gives specific detailed endorsement requirements for Student Pilot level.
     
  18. May 16, 2019 #18

    wsimpso1

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    Sounds heavy to this retired transmission engineer. Here is why:

    Two stroke singles have one strong firing pulse per rev. Four stroke singles do two turns to get one firing pulse. Both are slowing down between firing pulses. The propellor inertia is much bigger than the inertia of the single engine flywheels or the gear train, and so it wants to spin pretty steadily. When you put a spur gear on three of these singles, align them to a central gear, the gears will go back and forth through their lash twice per operating cycle on each single and a four stroke single will also have a smaller vibe of once per rev. Crash-crash-crash will go the gears.

    The gears will have to be high quality, big and heat treated to stand this duty. That is heavy.

    Other ways to handle it? Put significant flywheels on the singles will allow you to make the gears a little smaller, but will heavier. Put elastomeric springs between the flywheels and the driving gears will to, but that gets complicated to keep the gears aligned well enough, and gets heavy too.

    My thoughts on engine reliability in this range? Go with a modern engine with a good history in flying machines - port fuel injection, oil injection on two strokes, electronic ignition, integral alternators, and good gearboxes. That will buy you a lot of reliability. If you are scared about all the electronics (you should not be, but if you are) you can run two ECU's with a switch and a redundent alternator - there are excellent lightweight permanent magnet alternators out there.

    Billski
     
  19. May 16, 2019 #19

    13brv3

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    All valid points, but my interest is in coupling electric motors, not gas engines.

    Cheers,
    Rusty
     
  20. May 16, 2019 #20

    wsimpso1

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    That did not occur to me. And "engine" was used in the title...

    Why would you do that? Seems like a solution looking for a problem. Are there not motors of the right capacity available, but there are some at 50%? If it is for redundant power, you will play merry hell driving a prop designed for two motors with only one of them making power. In fixed pitch props, torque to turn goes with square of rotational speed and power expressed goes with cube of rotational speed. There is some flight speed effect, but it is not huge and works against you as you slow to the airspeed that reduced power can produce. With half the torque available, the prop will only be able to run 71% as fast, and so can only run to 35% of the power you previously had when one out of two has left the party... With controllable props, your operating range gets wider, but you will still will not get close to 50% power when one of a pair dies - blade area is big to run with two motors, but when you go to one, you will have to drag all of those big blades even, even if you are flattening them out.

    I suspect that the smart way to go is multiple motors, each with a prop, spread along the wing. Then losing one out of six or eight is a way more modest control issue and less power loss... Given the multitude of multirotor and multimotor electrics being flown and proposed, I suspect that they will be legal soon, if not already.

    Billski
     

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