Aeronavics / Droidworx Skyjib Ti 8 or x4
- Thread starter jjon667
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rotorvista
Member
Build it yourself so then you'll know the ins and outs of it when something goes wrong. You need to know your craft 100% or you'll be spending a lot of time at your builders and if that's Aeronavics or any other international company you won't make any money as it'll all be going on import duty and postage and you'll have too much down time.
MombasaFlash
Heli's & Tele's bloke
It is invariably fairly entertaining reading posts from Aye-Yup Dave coz his speling is so atroxius, but his advice in this case is spot on. What's more it is not idle theory. It is based upon practical experience. Similarly, and based upon my own experience, I would say err towards compact and manageable, rather than all-out grunt. As far as I know there have been no direct X8 vs. Flat 8 comparisons, as a realistic comparison would involve two otherwise entirely identical airframes, and so the statements about the slightly reduced lifting power of the co-axial frame are largely based upon theory - albeit valid theory.
If you are not going to be called upon to carry a 6kg camera on a regular basis, why bother suffering with the large, unwieldy machine for the majority of the time?
Big machines are just a damn pain to transport. Flat 8's are totally the safest but they are also very annoying because there is always a prop poking your eyes or snagging your clothes as you try to work on the centre hub.
I also agree that you should bite the bullet and build it yourself.
If you are not going to be called upon to carry a 6kg camera on a regular basis, why bother suffering with the large, unwieldy machine for the majority of the time?
Big machines are just a damn pain to transport. Flat 8's are totally the safest but they are also very annoying because there is always a prop poking your eyes or snagging your clothes as you try to work on the centre hub.
I also agree that you should bite the bullet and build it yourself.
Rentakill
Member
Planet Oz Sydney Dave, I've had some corro with Aeronavics. They sent me some raw footage of an X4 in flight.
The big question would be using the buy once theory, RTF packages $$$$$ or custom built $$$.
Its for commercial use with a payload of zenmuse Z15 & 5D.
I wonder why people keep on saying that a coaxial propeller is less efficient?
Since as long as the coaxial propellers are contra rotating they are more efficient than single props:
http://en.wikipedia.org/wiki/Contra-rotating_propellers
Contra rotating propellers and fans have been applied in aircrafts, wind tunnels, submarines, outboard motors, gas turbines, wind turbines etc.
The only reason why this concept is not always applied is because contra rotating propellers are more complicated than single props and can be noisier.
But those issues wouldn't apply to a X4 as opposed to an Octocopter.
Since as long as the coaxial propellers are contra rotating they are more efficient than single props:
http://en.wikipedia.org/wiki/Contra-rotating_propellers
Contra rotating propellers and fans have been applied in aircrafts, wind tunnels, submarines, outboard motors, gas turbines, wind turbines etc.
The only reason why this concept is not always applied is because contra rotating propellers are more complicated than single props and can be noisier.
But those issues wouldn't apply to a X4 as opposed to an Octocopter.
SoCal Blur
Member
This is from wikipedia so you have to understand that anyone can add/edit information. The only reference they quote is a password protected .pdf that you can't read.
But let's assume that the statement is correct. The statement is that TWO propellers in a contra-rotating setup is 6% - 16% more efficient than a SINGLE propeller. So that means that in an X8/X4 configuration, 8 props in Contra-Rotating configuration can be up to 16% more efficient than FOUR SINGLE props. In a Flat 8 Octocopter, you have 8 props that are separate so the total arrangement is still more efficient than if they were in a coaxial set up. So based on that statement, for a contra-rotating setup to be more efficient than a flat 8, you would need 16 propellers.
Droider
Drone Enthusiast
I fly Coax on anything more that 4 props!
Why coz they work for me, they are easier to transport and just love flying in british weather. I dont care that much about efficiencies, I know what I can get and build my work round that. I am looking at other setups but what I have works end of.
Dave
Why coz they work for me, they are easier to transport and just love flying in british weather. I dont care that much about efficiencies, I know what I can get and build my work round that. I am looking at other setups but what I have works end of.
Dave
redridinghood
Member
About a year ago I converted my SJ8 to X4. It is better all around and the size is more manageble. Almost the same flying time. What I find makes the biggest difference is the esc you use. Fast esc work much better with DJI fc's. I use Castle Ice, Ultra esc and Herkuless III on my main builds. My SJ X4 with Plettenberg and Herkules III has more than 100 hour flying time problem free. Oh and I never go higher than 5.12 firmware on Wkm.
MombasaFlash
Heli's & Tele's bloke
It sounds good on the face of it but I am not exactly sure whether the whole argument holds up, because it seems to suggest that 4 pairs of contra-rotating props on four booms is anything up to 94% less efficient than a flat 8. But it would be interesting to know the real physics of it all, just to put this question to rest. We need a scientist here!
Jess? Where are you?
MombasaFlash
Heli's & Tele's bloke
Well, not really, although I should have written 74%. It was just to make the point that the way it was worded suggested a co-ax arrangement is really inefficient.
If a flat 4 has 50% of the power of a flat 8 and a coax 8 is 6% more efficient than a flat 4: 4 x 6% = 24; 50 + 24 = 74%.
Of course this is an entirely non-scientific argument because if you take the 16% value and apply the same formula the co-ax ends up being more efficient than the flat 8: 4 x 16% =64; 50 + 64 = 114% !!
As you say, as long as it does the job, who cares what the loss in efficiency of the more manageable co-ax airframe is, but it would be interesting to know what the power loss is.
SoCal Blur
Member
Well that would be something but unfortunately, the math is wrong. If the flat 4 has 50% power of the flat 8 and you increase the power of the flat 4 by 16% then it's 50 X 0.16 = 8. 50 + 8= 58% power of the flat 8. Think about it. If, for the sake of argument, the flat 4 has 50% of the power of a flat 8, then in order for the flat 4 to be at the same power as the flat 8, you would have to increase the power by 100%. Here you are only increasing the power by 16%.
But, as you say, this is all hypothetical. It would be nice to see some real-world numbers, if for nothing more than to satisfy curiosity. And I'm in agreement its about what works for you not power bragging rights. :tennis:
MombasaFlash
Heli's & Tele's bloke
Quite so.
BTW, I would never presume to make sense, let alone excel, at anything to do with maths (short for mathematics!).
A flat 8 octocopter is not more efficient than a flat 4 quadrocopter, it can just carry double the load (but it also consumes double the power - so the efficiency remains unchanged).
The reason why contra-rotating propellers are more efficient is because the rear propeller recovers the rotational energy in the wake of the front propeller.
Here's an analysis:
http://cafefoundation.org/v2/pdf_tech/Noise.Technologies/AIAA.1986.Prop.Noise.CRot.pdf
http://www.dynamic-positioning.com/dp2006/thrusters_jukola.pdf
Here are commercial examples:
http://www.scana.no/sites/all/files/Contra Rotating Propulsion_0.pdf
http://www.wartsila.com/file/Wartsila/1278512476073a1267106724867-Wartsila-O-P-IHIMU-CRP.pdf
http://www.rolls-royce.com/Images/gasturbines_tcm92-4977.pdf
http://www.volvopenta.com/volvopent...ps_duoprop/pages/the_benefits_of_duoprop.aspx
http://www.kaman.com/aerospace/aerosystems/air-vehicles-mro/products-services/k-max/
Off-topic:
If one wanted to maximize efficiency, one would use a single large propeller or two large contra rotating propellers and for instance two small propellers to control the attitude. But this arrangement may not be practical for other reasons. (In a hovering application a large slow propeller is more efficient than a small fast propeller, because the small propeller has to bring a smaller mass of air up to a higher velocity in order to generate the same amount of thrust and this requires disproportionally more energy, as thrust is v * m and energy is 1/2 * m * v^2.)
There's a reason why helicopters don't look like this:
http://aerostories.free.fr/dossiers/ADAV/robur.JPG
SoCal Blur
Member
The comparison about efficiency wasn't about Quad versus Octo. It was X8/X4 vs. flat 8. Two coax rotating propellers ARE more efficient than a single propeller. But it is still less efficient than two separate propellers thus the argument that an X8 is less efficient than a flat 8. Again, this is all theoretical, your mileage may vary.
If:
Efficiency is essentially thrust/power. (The more thrust you receive for a given power input, the more efficient your system is).
If two propellers are more efficient than a single prop they would need to generate not just twice the thrust but more than double the thrust (@double the power input).
So, if two contra rotating propellers are more efficient than a single propeller, then they do generate more than double the thrust of a single propeller, which also means that two contra rotating propellers are more efficient than two single propellers.
Then the following statement is contradictory:
Efficiency is essentially thrust/power. (The more thrust you receive for a given power input, the more efficient your system is).
If two propellers are more efficient than a single prop they would need to generate not just twice the thrust but more than double the thrust (@double the power input).
So, if two contra rotating propellers are more efficient than a single propeller, then they do generate more than double the thrust of a single propeller, which also means that two contra rotating propellers are more efficient than two single propellers.
SoCal Blur
Member
Read through the thread, you'll see that someone pointed to a Wikipedia reference that says that contra-rotating propellers are 6% - 16% more efficient than single propellers. Saying that if a contra rotating propeller is more efficient than a single propeller than they must double the thrust is not accurate.
You misunderstood.
Again: If two contra rotating propellers are more efficient than a single propeller then they must generate more than double the thrust, if each propeller (of the two contra-rotating propellers) receives the same amount of power as the single propeller.
Or in other words (if one doesn't like the word double): If two contra rotating propellers are more efficient than a single propeller then they must generate more thrust than a single propeller if each propeller (of the two contra-rotating propellers) just receives 50% of the amount of power of the single propeller.
(Also, the sentences above just explain what efficiency means and don't claim anything.)
SoCal Blur
Member
It has already been stated that you only get an increase of 6% to 16% in a Contra rotating setup vs a single propeller. It is more efficient and generates more thrust than a single propeller (agreed) but does not generate more than double the thrust as you again stated above. No matter how you word it, the statement is incorrect based on the information referenced by another poster.
MombasaFlash
Heli's & Tele's bloke
In the valid interests of obtaining real-life data on this subject I will be happy to accept a donation of around $20,000 to build a no-frills SkyJib 8 and a no-frills SkyJib X4 (X8 damn you Droidworx!) with identical components and fly them both around with an ever increasing load of water bottles until the first one drops or cannot get off the ground.
Whatever physics and fancy equations and suppositions are involved will pale into insignificance against the simple results of this one can lift and sustain normal flight with an extra payload of 8kg and this one can do the same with 6.5kg.
Whatever physics and fancy equations and suppositions are involved will pale into insignificance against the simple results of this one can lift and sustain normal flight with an extra payload of 8kg and this one can do the same with 6.5kg.