Replies to This Discussion

Permalink Reply by Ellison Chan on May 4, 2012 at 2:21pm

From an efficiency perspective, a lower speed higher torque motor is recommended running with higher voltage.  Look at the MD4-1000 for instance,  It's all a matter of how  much money, and what engineering decisions you want to make.  A quad is usually more efficient than hexa or octo, since you're carrying less dead motor weight, and the requisite larger cap batteries.  But with his hexa configuration and those 880kv motors, there are limitations.

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Permalink Reply by James Dinh on May 5, 2012 at 3:20am

Wow!  I check my thread and in just one day I have all these knowledgeable people giving me some great advice.  Thanks to everyone who has taken the time to comment.  It will take me some time to learn everything stated above and I will try my best.  So far, besides the fact that I fail behind the sticks (of course that will improve in time), my hexa has maintained a rock stable hover at about 5 feet with less than 40% throttle, using 9x 4.5 cheapy props.  I see literally no vibrations on the frame, and it responds fairly well to my inputs (a little on the slow side, but I think that's because I have Rate P tuned down to only .085, Rate I .022, Rate D .006, and Stab P 3.7.  PID tuning is a topic that I want to ask about in a later discussion.  I think after reading all of your suggestions,  I will be ordering APC props 10x4.7 slow flyers.  I'm confident that these will work out for me since the 9" ones seem to do just fine.  I'm hoping that 10" props will give me extended flight times as well as stability.  When I finally get some videos of my hexa in action I will post them as a tribute.  Thanks again, and I will be eagerly awaiting for more great advice from the pros.

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Permalink Reply by Ellison Chan on May 5, 2012 at 5:47am

Hi Jame, just some final suggestions:

• Don't cheap out on the props.  Get some of those carbon reinforced one.  You'll lower you chance of a crash due to prop breakage, and increase efficiency with less flex on the blades.
• You will probably notice that you need to throttle higher to lift off with the longer props.  Using a 4s will improve that.
• You'll also notice that the craft will be less responsive with the 10" props. It's a difference between driving a pick up and a sporty coupe.

Good luck.

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Permalink Reply by Geeteshkumar Shakyawar on June 13, 2012 at 7:13am

even dont use those black and composite props....... for me both break in mid air........... 

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Permalink Reply by Brad Hughey on May 5, 2012 at 10:44am

@James: A couple things pop into my head.

I ranted a bit about aerodynamics in another thread, and if you really want to calculate what's going on, understand the fundamental physics involved, and also check out some real-world data, please read this:

http://www.diydrones.com/forum/topics/the-case-for-large-scale-elec...

I dare say it's better than relying on "rules of thumb", although there's no argument with something that works.  Beware of turning the props too slowly at this scale, however, as the UofI wind-tunnel data proves that sometimes achieving a higher Reynolds number is more important than the drag coefficient theory.  The efficiency of props they tested was actually greater at higher RPMs for the most part.  It might be counter-intuitive, but much below 100K Re, the drag coefficient goes down as the speed goes up, so the usual energy cubed law doesn't apply in this region.

Also, 6061 is NOT aircraft grade aluminum; it is actually the most common alloy around.  While it is better than the highly corrosion resistant, and otherwise uselessly soft 6063 "architectural" grade for our purposes, it is hardly the best choice.  2024 (or 7075) is the good stuff, but it's 3 times the price and hard to find in a variety of extruded shapes.  I am an avid customer of this site for materials:

http://www.onlinemetals.com/aluminumguide.cfm

My only relationship with them is one of highly satisfied customer.  And I have purchased quite a bit of aluminum.

I am not a mechanical engineer, but the consensus seems to be that torsional rigidity is highly important in multicopter thrust unit arms, suggesting that round tubes are the best shape for strength-to-weight.  Others who are more expert in this area than I are encouraged to weigh in (pun intended).

One of these days, I'm going to post pictures of a wire cutter and an aluminum-bodied turnbuckle that came into contact with each other for a few milliseconds while I was working on my full-sized ship, which has a fault-current of at least 4,000 amps (10S X 36P 3700 mAh 20C).  Managing my best Crocodile Dundee imitation, "That's not a plasma, THIS is a plasma!"  The wirecutter was scarred, but the turnbuckle barely still exists in a recognizable form (this was a full-sized quarter inch open-frame job too).  I escaped with nary a scratch, but have a newly rediscovered respect for high currents. 

P.S. The UofI site is here, and you'll note that they tested nearly everything mentioned:

http://www.ae.illinois.edu/m-selig/props/propDB.html

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Permalink Reply by Alessio Pennatini on June 13, 2012 at 4:17am

Hi guys, I'm quite new to the world of multirotor drones, but I'm currently developing an heavy-lift for aerial photography, and recently I've read a lot of stuff concerning many topics to reach some basic knowledge and carry on my project (a total of 5Kg of flying drone with included equipment)

I read with interest this discussion, because it deals with many of my current issues and I'd like to add a question here, without open a new thread, just because it's very affine to mine and it's quite recent too;

I tried to use the mentioned calculator eCalc to figure out what my drone can do before to spend money, but actually the shown results don't convince me at all. Instead of using my project, which is still only imaginary, to expose my doubt I'd like you to consider this (probably well-known) heavy-lift hexacopter

http://www.rcgroups.com/forums/showthread.php?t=1372744

It's capable of flying with a total weight of almost 10Kg using APC 12x3.8 SF and some powerful motors not much different from AXI 2826/10 present in eCalc and 3s batteries

According to eCalc this should fly only up to 7Kg, but...

...here is my main doubt: accepting the fact that eCalc probably is not so accurate for such very extreme systems, even with lower loads and other motor/battery configuration it shows results (with apc 12x3.8SF propellers) largely exceeding the 5400 maximum rpm rated by APC manufacturer through the formula 65000/diameter; is it possible to spin a SF propeller up to 8000-9000 rpm without incurring in broken props or drastically lowered fliying efficiency? Or... is the guy of the video spinning them within or not much over the APC suggested limit? (this would means that eCalc totally fails)

Thanks to anyone would like to add a comment!

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Permalink Reply by James on June 13, 2012 at 5:05am

im also fairly new to this, but do have a hexa with 8 hours flying time on it now, and can at least comment on the correlation between ecalc and the real world.

all these configurations with APC 12x3.8SF and 5000mAh 3S.  i generally land with close to 1000mAh left in the battery, so i could stretch these times a bit:

• minimum flying weight is about 1.8Kg - this is no load and gives me a flying time of about 12 minutes before i start getting a battery alarm.  ecalc gives 12.73 minutes at this weight.
• with a heavy aluminium landing gear and camera tray my AUW is around 2.1Kg - this gives me about 10 minutes before hearing the alarm.  ecalc gives 10.44 minutes at this weight.
• finally with a medium weight "superzoom" compact camera my AUW is close to 2.5Kg and i can fly for about 8 minutes.  ecalc gives 8.69 minutes at this weight.

obviously im talking about a much lighter craft - but ecalc seems very close to reality in my case.  i would say that guys running the props way over the rated 5400 rpm, even at the lower payloads.

james

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Permalink Reply by Brad Hughey on June 13, 2012 at 5:58am

Here's a good rule of thumb.  If 4.2 volts times the number of LiPoly cells in series times the KV rating of the motor exceeds the maximum RPM rating of the prop, don't use it.  This means even in the event of something sticking at full throttle, you'll never risk over-revving the props.  This is hazardous in the extreme, not just because one of the propeller blades might become a projectile, but what's left behind on the motor shaft will most certainly be horribly out of balance and could vibrate the rest of your ship to destruction.

I've never used eCalc, because calculating your ideal hover power is easy (see aforementioned thread).  Just take your ideal power and multiply it by 2.2, (assuming .50 FM and 30% for headroom).  This will give you a decent maximum power to shoot for.  Then, look at the motor curves for that maximum power at a KV rating that won't exceed the prop's maximum RPM at full voltage.

That said, I've "run the numbers" for FM on many of the props on the UoI site and the vast majority of them have higher efficiency at higher RPM.  So, the "take away" notion here is that for props under 14" or so, run them as fast as you can without going over the maximum.  I know this is counter-intuitive, as we've all been taught that faster is less efficient.  However, for this Reynolds number range, it is a verifiable fact.

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Permalink Reply by Douglas D. on June 13, 2012 at 4:39am

GP(great planes) or GRAUPNER 1045 for  rcTimmer 2836 or 11x6 you will get the best amp draw and max thrust. 

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Permalink Reply by JeffBetts_KK4MTC on June 13, 2012 at 6:57am

The EPP props are junk. I've had two come apart in mid-flight on my 3DR Quad with 880kv motors. The material is just too brittle. I ordered some Gemfan carbon-core props and they look exactly the same as the EPPs, and make the same "popping" sound when spinning up. I'm looking to move to APC Slo-fly props this week. I can't take another crash! 

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Permalink Reply by Geeteshkumar Shakyawar on June 13, 2012 at 7:07am

same here jeff....... today its second time............. my http://store.jdrones.com/product_p/ac1245comp.htm props just broke in mid air......... the quad crashed........... its painful.......... can you send me the link of those props you just ordered........ i want more reliable props.....

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Permalink Reply by JeffBetts_KK4MTC on June 13, 2012 at 8:49am

http://www.espritmodel.com/apc-propellers-slo-flyer-1.aspx

http://www.espritmodel.com/apc-propellers-slo-flyer.aspx

Here's what I just ordered after doing a bunch of forum reading here and in a couple of other quadcopter forums. This is the type of prop I fly on every plane I own, and have never had one come apart in mid-air. Not everyone sells the "clockwise rotation" version of this 12x3.8.  From what I've read the 3.8 compared to 4.5 pitch should be an improvement in stability, but may decrease top speed, which is of no concern to me.

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