Some MultiCopter Design Thoughts.


Hi All, Just pulled this image in from SUAS News (Thank you Gary) to illustrate a few really good concepts that it incorporates and talk about better Multicopter design a bit.

This copter has a lot of things right and is really a study in design excellence.

1. Most obvious is a sliding ball full enclosure camera Gimbal, very pro and really something we should be striving for.

2. The motors are on the bottom under the arms: More aerodynamically efficient with no prop wash interference and very little intake interference. Noticeably increased flight times and greater lift capability and much quieter.

3. Large slow high efficiency props and pancake motors: These are way more efficient than faster motors with smaller diameter more conventional props.

4. Carbon fiber aerodynamic frame arms: Again more aerodynamically efficient, light and strong.

5. Fully enclosed framework and a simple spring loaded landing gear that provides as little interference with the prop wash as possible.

Basically this quad incorporates the best design features I have seen so far and definitely provides food for thought.

I thought it might be worth soliciting comments and other thoughts about optimal multicopter design techniques.

Best Regards,


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  • Anything rounded that you add to a flat frontal surface has to be better than square.
  • Guyz, since we are talking about efficiency do you think that square arm thrust obstruction could be reduced to match round tubes with simply ataching round foam sticker on top of it?



    I wonder if it could work with only one foam sticker on top or would it require one on sideway facing incoming propeller as well, as atached to look in direction oposite of propeller direction shown below.


  • Foxtech K130 16kg Fly Time Test

  • And also as somebody mentioned,different propose where silence is needed is another story and than bottom props is solution for sure..

  • Exactly,The Sun where is evidence for above theory?

    Anyway somewhere in this thread about long time flights on RCGroups

    there is data from somebody test where final flight time was 0.1-1 % longer with props at bottom.

    if you interested take a better look,all the things abot efficiency are discused there

    Also there is very big difference if you use 8 inch or 18 inch prop..

    there is so many other ways to improve flight time and this is not important enough.

  • @The Sun
    Everyone here is posting unsubstantiated numbers. What's wrong with another opinion?

    Garry McCray has a post above that seem to make very good sense, are you going to call him on those numbers. Fwiw I think Garry's numbers reflect my experiences to.
  • @Emin Bu

    If you dont have evidence dont post numbers that directly contradict with other conclusions in this thread.
  • Difference u will notice will be under 0.2%(all motors together) I cant find link or remember where I read that

  • Very good points, i definetly agree with you that propeller above boom is a lot less efficient. Gary thank you for the explanation.

  • Hi Dragan,

    Simply for a vehicle that spends it's time primarily hovering, the propeller below the support are is always going to be more efficient than the propeller above the arm, the High speed downward prop wash has considerably more effect against and is more interfered with by the frame arm below the propeller than it is by an equivalent arm above the propeller in static air space.

    Of course, there is some effect of the overhead arm in interfering with air going to the propeller but it is comparatively very small and even that is reduced to an imperceptible amount simply by raising the overhead arm a bit higher relative to the prop.

    Basically since you have flipped the motor over and it is between the frame arm and the prop the intereference of the over head arm can generally be taken as negligeable.

    That is not the case for it below the arm where the high speed thrust of the prop wash is entirely responsible for keeping you aloft.

    The amount of the prop wash that directly impinges on the frasme arm (as flat plate area) is directly deductible from the gross thrust, in addition, turbulence generated is also directly deducted because it is no longer contributing to thrust.

    Square tubing produces a flat plate area equivalent to its size = 1" square tubing = 1" flat plate area times length.

    Round tubing has a flat plate area of approximately 1/2 of its diameter.

    And a wing shaped (aerodynamic) arm can have considerably less flat plate area (and reduced turbulence loss as well).

    Generally the losses with propeller over the frame arm are significant and on the order of 2 to 15 percent (or more for really fat or interfering arms.

    From my understanding those with the propeller under the frame arm are from 1 or 2 percent to being not measureable.

    A Multicopter with round tube Carbon Fiber frame arms appropriately sized for what it is lifting is likely to have no more than 2 or 3 percent loss, but a plastic or molded wide web or unnecessarily large frame arm can easily jump into the 5 to 10 percent net loss range.

    And this is a straight loss, both in lifting capacity and in battery flight time endurance.

    I don't know some engineer might call me on the veracity of the numbers, but not by much and the basic principles are correct.

    Except for very special designs I am not advocating motor/prop below the frame arm, they are just too difficult to put into practice, but optimized size carbon fiber tube arms or better aerodynamic profile arms are really important to keep these losses to a minimum.

    Best Regards,


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