Hi,
Almost 1 year ago I started a topic on the diydrons forum in wich I asked on how much efficiency loss I will have on converting my quad to an X8. Almost no reaction to my question. Nobody could give some figures wich I could use for my calculations. The only helpfull answer I got was from somebody who said that the efficiency loss depends on prop size and distance between the props.
Time has past, other projects are closed so I'm starting with the build of a Y6 multirotor. And the first thing before building the frame is a thrust test of 2 motors in sequence. Result is a 10% efficiency loss. See full graph and info on my blog at http://wipo-y6copter.blogspot.be/
This figurs can by handy for everyone who is building Y6/X8 copters. Currently I'm building the frame, but I will try to post more efficiency loss tests for other prop sizes asap.
Comments
So is it a general rule to gage up the washed prop 10% per rev up? So that to compensate?
So if that is the case should you gain the prop that is "washed" with either a curve of increased power based on the top prop rpm or just compensate for the overall spin at a thrust per rev of 10% greater?
Hi wim.
thanks, thats great.
Cheers
DAVE
@Dave: you measure thrust like this http://www.rcgroups.com/forums/attachment.php?attachmentid=3122734
Hi. Great work, I was wondering if you could share your method with me / us.
I guess that you used a multi-meeter or amp meter but how did you get the thrust measurements ??
I would like to run some tests on motor spacing, prop sizes, ratios & twin blade VS 3 blade props etc
I have also heard that you could reduce this loss by using different pitch on the top & bottom eg: 5x4.5 top & 5x3 bottom ?? I would like to test this out.
I also would like to know about shrouds, does the ifficency increase counter wind resistance of the shroud
Any info would be much appreciated
Thanks & regards
DAVE
From image it is difficult to see exact differences but they seems to vary just like you found even less than 10% to 16%. Perhaps one reason for these differences between your and my results is distance between propels. You have 9 inch and I have less than six and another test just above four inches. Also size of the propel might make difference. According my tests bigger was significantly worse in efficiency difference compared to smaller eventhough overall bigger propel efficiency was better (1455x2 - 6.5grams/W and 1655x2 - 7grams/W).
Perhaps I'll try 10 inch propels also when I got pair in my hands.
Btw, do you have photo of your setup and meters?
Has anyone tried enclosing both of the props in a shroud? I think it would alleviate a lot of the problems that normally come from coaxial propellers, perhaps especially if you put stators in the shroud to make the airflow more linear. I plan to test this myself but I'd like to know if it has already been done.
Also, for the record here are some tips I've heard (but not personally tested) about using coaxial props that I didn't already see here:
-Use two different number of blades (e.g. 3 top/4 bottom). The reason for this is to reduce resonance which might otherwise occur with the same number of blades or (possibly) with two even or two odd numbered props.
-Don't put the blades too close to each other. I'm not sure what "too close" is, but the theory is that if the blades are too close then when they pass over each other two pockets of high pressure will collide, reducing efficiency and probably putting extra wear on the blades by flexing them.
-Contrary to what I have heard on this thread, I have heard people say to use a smaller prop on the bottom. Something to do with the top blade losing some air to the side and the "disk" of actually downward moving air therefore being smaller. I am not sure how this would be affected by a shroud.
Lastly, for an example of a great Y6 design check out the Bolt http://diydrones.com/profiles/blogs/bolt-very-small-fast-agile-y6. It makes full usage of the advantages of coaxial props to make a very small package that can fit in narrow spaces other multirotors can't even think of going.
@ henri: Interesting point of view and I really appreciate the effort you make to explain it in a way thats is understandable for me (and others).
So what I did is I took a pair of motors placed them side by side and measured the thrust at X watt. Then I took a pair of motors I placed them coaxial and measured the thrust at the same X watt. At the same X watt I then got 10% lest thrust.
In your point of view I should take 2 motors, place them side by side and measure watt at X gram of thrust. Then place the 2 motors coaxial and again measure the watt at the same X gram of thrust.
So if I take the figures on my graph. Lets say at 500 gram thrust I got 65 watt for the side by side pair and I got 75 watt for the coaxial pair. So that is a difference of ~16%
I try to explain it: Lets say that some single prop quad setup produces thrust of 1kg per motor. According the graph two motors should produce about 0.9kg with same power (W), only 10% less. I believe your tests were correct in this.
But
When you add motors you should expect more thrust which in this case would be double, 1kg+1kg=2kg. So you should test so that you try to achieve double thrust and measure how much more than double wattage is. Difference would be according my tests at least 16% and perhaps even more than 30%.
In your tests propels swings only approx half speed compared to single and they might swing in more optimal speed for that specific propel model.
If I remember correct I achieved 2kg using lower wattage with two propels in sequence than one alone. So efficiency looked in the beginning to be better with co-axial setup until I realized the difference.
Hopefully this was understandable explanation.
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