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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  • @andrew rabbit

    I disagree. I think improvements to efficiency would be much greater than you seem to expect, especially on bigger props with large amounts of inertia. For the most part the Reynolds number reasoning is pretty solid though, I doubt you would see any improvements on something like an 8 or 10 inch prop simply as an inherent result of the nature of the propeller and the likely large proportion the added weight would have to the copter. But on a 15 or 17 inch prop? I haven't done any actual calculations but that doesn't seem like a crazy suggestion. 

  • Euan, I think you can push the fuel injection analogy too far.  You have to understand where the inefficiencies are before electronics have any chance at addressing them.  Much of the inefficiencies in quad/rotor design is one of scale and Reynolds number effects.  Things at the scale we are using are inherently draggy.  Another risk is by solving one problem we introduce another - lots of solutions improve aero efficiency at the cost of weight - ducts are but one example.  In quads, (parasitic) weight is inefficiency in itself.

  • @ Choppy, re: the rig

    Impressive looking thing! I would take a stab that any efficiency gains from the ducting are lost because of the 4 beams directly in the prop wash. And they don't look like big, wide, shallow props, so the mass/shape of moved air will match the width of the beams quite closely, causing the loss to be more significant.


    And yeah, the weight...that's gotta be heavy...

  • The crucial bit I see is the feedback loop. In car injector systems, lambda sensors, air sensors etc all monitor combustion efficiency, and adapt as necessary. We need to be able to do the same. But what data can we use?

    We already have: Amps, rpm, inertial and gyroscopic inputs. But for a truly effective "closed loop" variable pitch model, what else do we need? What data can we use without having to create specific models for inidividual motor and ESC types? what data would allow a FC to decide whether to use RPM or pitch to effect a movement?

  • @ Choppy: Yes, exactly.

    Not sure about the gear drive system, but in principle, if you can tune for performance, you can tune for economy. We have the sensor and signal resolution already in today's hardware - we just need a flight model to run it all.

    PS - have you seen Curtis' gasser version...holy crap...

  • For good examples of how this could work, look at Automotive fuel injection systems. The increase in resolution now means most engines run at much higher efficiency that carbs ever could. I foresee a "variable pitch" future where this advancement occurs in MR's - ESC's, gyro's and pixhawk already have enough resolution to do it. We just need efficient mechanics and a flight model to control it all.

  • Euan Ramsay check out:

    Not sure on endurance tho as its more for performance than ecconomy.

  • Re: Ducted fans - Agree Danny; I hear this a bit, people advocating that gaps at the tip/tunnel interface should be as small as possible - but that's for compressors!

    Re: props. Surprised nobody has mentioned variable pitch yet...another area future FC's and ESC's can possibly improve flight performance/endurance in MR's - by choosing the most efficient method to respond to an input at ESC speeds (eg: Do I increase pitch, or increase rpm?) we should see some efficiency gains assuming the weight & complexity of variable pitch doesn't negate it."Two dimensional" ESC's I like to call them. I'd hate to be the guy who had to work out the flight control model for that though...

  • Well, Denny, you may be right but I have been using this:3692929320?profile=originalThe makers claim the ducts make the unit more efficient. At my altitude of 2000'AMSL, I fly 10 mins using 87% of a 5000mah 4s battery. Kind of ordinary endurance but its not light and the battery could be a little bigger.

    Its made from a 3D printer (Shapeways?) and is hollow but rigid.

    I think the augmentation OUTSIDE the duct is what helps this design out.

    I wonder what it would be like without the ducts? I'll never know as its a company asset....

  • T3

    Simon, I am using RCTimer 5010-14 360KV with 4s.

    I once made a test (because of some yawing issue) and moved the motors from bottom to top. A higher efficiency on top should have solved the problem. However, it did not. So I moved them back to the bottom. Unfortunately, I have no numbers. 

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