Replies to This Discussion

DeveloperPermalink Reply by R_Lefebvre on July 25, 2012 at 1:27pm

Yes, but the 20-40% loss goes away in the all-important engine failure, right?

The 2836 is rated at 260W.  If only it didn't have those stupid prop adaptors.

I like the AC2830HE-508 from Jdrones, but I don't think it has enough power?  I think it's limited to 10" props.

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DeveloperPermalink Reply by John Arne Birkeland on July 26, 2012 at 3:26am

A hexa/octo heavy lifter with a gimbal and good quality camera and optics, usually ends up with a AUW in the 6-7kg range.

My latest machine (octo) will be exactly 6kg if everything pans out (waiting on gimbal to arrive), but had no problems lifting 10kg+ AUW in tests. The headroom usually translates to longer flight time since you don't have to push the system so hard. And as you mentioned, enough spare power to still keep the camera up in the air if a motor fails. Actually if we forget about stability for a moment, my system should be able to hover 6kg using only 5 motors.

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DeveloperPermalink Reply by R_Lefebvre on July 26, 2012 at 10:04am

John, but what if I want to build something small, to lift a camera <500g.  My build on ecalc is coming up to 4kg AUW, assuming 800g for the frame and FC, and 1000g total for camera and gimbal.  2836 motors, 20A speed controls, 2x 4S 5000 batts, flies for 13 minutes, on 12x4.5 props and 47% to hover.

Is this reasonable or am I totally out to lunch?

Part of my problem is I am having trouble justifying to myself (and more importantly my wife) dropping $2000+ on a big Octo setup when I still haven't taken a single picture from the air.

The small X8 I'm considering I can do for <$500, and at least start learning about operations, etc.

This is what gets me.  Here's two shots.  One taken with the DSLR, one with a 140g P&S.  Which is which?

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DeveloperPermalink Reply by R_Lefebvre on July 26, 2012 at 10:53am

Meh, the Turnigy Plush 40A is in stock.  I should probably just not chance it, get 8 of those, and the NTM 3548-900kv motors.  I see now that they have "proper" prop adaptors that bolt to the motor face instead of the silly collet style.

If I want to build a real small X8, I'll just convert the 3DR quad.

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Permalink Reply by JB on July 29, 2012 at 4:14am

Everything I have read on coaxial props says that they increase efficiency.  Could you please explain in some detail why this is not the case?

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Permalink Reply by Brad Hughey on July 29, 2012 at 6:13am

Everything you've read?  Please share the sources you are referring to, as this concept is patently false.

As an explanation, all studies of multiple propellers (rotors) compare the efficiencies of two isolated systems versus where the disk areas overlap.  Of course, in the case of coaxial rotors, the overlap is 100%.  We go back to classical mechanics, specifically the two primary equations F = M*A and E = 1/2 M * V^2.  In order to produce an aerodynamic propulsive force F, we must take a mass of air M and give it an increase in velocity A.  However, every increase in velocity takes an exponentially increasing cost on the energy required.

In the case of isolated rotors, they are accelerating separate columns of air to the same velocity.  According to empirical laboratory tests, when the columns of air start to merge at a disk overlap of about 30% (due to flow contraction effects), some of the air flow gets "reaccellerated" and has more velocity, reducing the efficiency of the system.  While this explanation is based on the somewhat simplistic disk actuator model (there are a myriad of vortex and other complex aerodynamic interactions taking place), it is the dominant phenomenon of why overlap of more than 30% reduces efficiency. 

If you're interested in something a bit more detailed, this paper by Gordon Leishman explains the entire situation very well:

Griffiths-Leishman Dual Rotor Paper

P.S.  There are many reason to use coaxial rotors, such as torque cancellation, etc.  But designers know up front that there is an efficiency price to be paid for it.

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Permalink Reply by JB on August 6, 2012 at 5:25pm

I'll admit that I am quite new to this subject, so I might very well be reading this wrong, or it may just be incorrect, but from the Disk Loading article on Wikipedia:

"In reciprocating and propeller engines, disk loading can be defined as the ratio between propeller-induced velocity and freestream velocity. Lower disk loading will increase efficiency, so it is generally desirable to have larger propellers from an efficiency standpoint. Maximum efficiency is reduced as disk loading is increased due to the rotating slipstream; using contra-rotating propellers can alleviate this problem allowing high maximum efficiency even at relatively high disc loadings.[6]"

^ Birdsall, David (1996). Aircraft Performance. Cambridge: Cambridge University Press. pp. 99. ISBN 0-521-56836-6. "contra-rotating propellers this rotational loss can be eliminated and maximum efficiencies approaching 0.9 can be obtained even with high disc loading"

For context, I am working on the design of a quad tail-sitter with a rigid wing and obviously in terms of propeller diameter and disc loading the compromise between high thrust and efficiency during hover versus high speed in forward flight is a major factor in the design - I'm just trying to figure out whether there would be any advantage in using 4 pairs of contra-rotating smaller-diameter propellers (1 pusher and 1 tractor on either side of each arm) over just using 4 single larger-diameter propellers.  My thoughts being that the contra-rotating pairs could improve the efficiency of the high disc loading during hover, but still enable maximum speed during forward flight.

My math isn't good enough to really understand Griffiths-Leishman's paper that you have linked to above, but am I correct in thinking that this research has contradicted and superseded the previous research from Cambridge University Press linked to in the Wikipedia article above?

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Permalink Reply by Brad Hughey on August 6, 2012 at 6:15pm

Ah, now I understand your confusion.  And while the statement you quote actually seems to contradict the WikiPedia author's own explanation of actuator disk theory, there is something quite different going on when you analyze high-speed aircraft propellers.

If you get disk loading high enough, as sometimes happens with very high performance airplanes (we're talking airspeed ranges at significant percentages of mach), standard momentum theory predictions start to fall apart due to fluid compressibility effects.  This is not unlike the "cavitation" phenomenon which happens to submarine props if the ship tries to accelerate too quickly. 

In this rather special case, it is better to have two disks, each adding its share of velocity to the flow, so that these efficiency-robbing disturbances don't occur.  The whole 1/2 M * V^2 rule still applies to the entire system, however.  Classical mechanics is always operative.

If you build an electric multicopter that can go fast enough to worry about this, please DO share the videos.  At our relatively pedestrian velocities of less than 200 MPH (.3 mach or so), coaxial props only cost you power.

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Permalink Reply by Duran - SteadiDrone on July 26, 2012 at 3:34am

Our SteadiDrone octos are also failry over powered and most of our clients shoot tvcs and commercial stuff with SLRs which give us at least 10min flying time and as mentioned before, I've flown our octo without a motor perfectly and when we get a gap would love to test mid air motor failure. You might have also seen us flip/loop our big octo :) Soon I will do this with fully loaded camera gimbal :)

We've just gotten the new sony cx760 and are speechless, the video stab is just something else, here is the vry first flight outside our building with hour smaller SteadiDrone H6X, av200 gimbal and the Sony CX760, this is NOT post stabilized and straigt out of camera. We are doing a bit of a roadtrip this weekend and will have plenty of stuff to post when we get back next week, keep your eyes on our SteadiDrone website and also our Facebook page, https://www.facebook.com/SteadiDrone

Heres that video

First Flight - SteadiDrone H6X with Sony CX760 from Motion Pixel on Vimeo.

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Permalink Reply by Duran - SteadiDrone on July 26, 2012 at 3:38am

The hex weighs in at 4.2kg WITH camera and lipos.

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DeveloperPermalink Reply by R_Lefebvre on July 26, 2012 at 10:15am

Yeah, that CX760 looks pretty awesome to me.  I have an XR500V, and the digital anti-shake on it is pretty good, better than Canon and Panasonic, which is why I bought it.  That, and "low light" performance is what makes or breaks an amateur video, IMO, so it's more important than some of the super high-end features, or miniscule picture quality differences.

Which motors on that, the Tmotor 2814-11 as shown in your RTF kits?

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Permalink Reply by Pablo Zeron on August 1, 2012 at 5:50am

Hi,

This is my heavy custom octo, don't fly yet, but I will do soon.

It have a width of 100cm shaft to shaft and I will use 6s lipos with 400 kv motors.

What do you thing about it? Any suggestions?

Thanks.

P.D. Sorry for my english.

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