I originally built my quadcopter using someone else's plans.  It flew well, but I quickly found that it "needed" additional equipment (camera gimbal, GoPro, longer landing legs, FPV transmitter, etc.).  These add-ons seem to have pushed the aircraft past its optimization point and eCalc estimates my hover throttle at 85% and flight time with a 3S/2200mAh battery to be 3.4 minutes for the aircraft's current weight (the flight time seems to be spot on, I need to test out the hover throttle).  I'm in the process of trying to shed weight from the aircraft and am even considering going with larger motors.  Unfortunately, I'm a bit confused about how to spec motors for this thing.

My question for you guys is, how do YOU go about designing a multicopter?  Do you use a proven setup and slap an APM on it?  Poke and prod at eCalc until it spits out something reasonable?  Have a better understanding of motor specs and the thrust capabilities than I'm familiar with?

I'm particularly concerned about how you approach selecting a specific motor/prop combo for your aircraft.

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start with the load, endurance, pack-ability, and reliability requirements then find a motor prop combination that comes close and design around it.

The weight can vary greatly. For example, I have two heavy quads with basically the same power system. One is an all metal, collapsible frame made in house from whatever alum material we had around. The other is all carbon, fixed config (arms not easy to remove), and uses off-the-shelf components. The carbon frame is MUCH stiffer, but it is also 300gm heavier for the same size. For us, the cheap in house alum frame is better but the other "looks" better.

Think hard and define your requirements, no matter what they are. Then go from there. 

To answer your question, motor/prop comes first and carbon wide blade props always win out over plastic. Same alum quad above, 30+ min on carbon 14x4.7 rctimers. 22 min on APC 14x4.5. go figure. And ecalc does not have good prop data particularly for multirotor optimized carbon props. 

 

Here is quick and cheap one....

motors TURNIGY MULTISTAR 4822 390kv

http://www.hobbyking.com/hobbyking/store/__26957__Turnigy_Multistar...

props 17x5.5carbon,

tarot quad frame,

4s 10000mAh

http://www.ebay.de/itm/14-8V-10C-10000MAH-Nano-Mars-LI-Po-AKKU-BATT...

for best flight times you need frame(with motors,esc,wires) to be the same as payload(batts+camera)

so if u want to fly your quad for 45min+ it has to be 2kg AUW with 4s 10Ah

check simple calculations in attachment

AND THIS THREAD http://www.rcgroups.com/forums/showthread.php?t=1880665

Attachments:

Wow!  I thought that specifying a 15 minute flight time would be really stretching it, but it sounds like I should try to be more optimistic.  So let's say I want to target a flight time of 30 minutes.  I need to add up the weight of my components like GoPro, gimbal, APM system, R/C receiver, FPV transmitter, etc.  That's easy to do.  I'll just bust out the kitchen scale when I get home from traveling for Christmas.  How about estimating the weight of the frame itself, though?  Come up with a design and take some rough measurements of the parts that would be used to build it?

You suggest to "find a motor prop combination that comes close and design around it".  That's the step that I can't seem to wrap my head around.  With so many brushless motors out there in so many different sizes, how do I identify the right one for lifting my estimated X kg of frame and payload?

I'm not sure what you mean by "for best flight times you need frame(with motors,esc,wires) to be the same as payload(batts+camera)".  Can you help clarify that?  I've started reading the RCGroups link that you gave.  Looks like I'll be at that a while.  I'll also review the spreadsheet that you attached once I get back to a computer (only have a tablet on me now and it won't open the file).

Thanks for your help.

I believe the fundamental question you need to have a ballpark answer to as the very first step in design is what is the total weight you want to lift? Frame, wiring, electronics, FPV, motors, batteries, and whatever else. You can't start looking at motor/prop combos until you have an idea of what you need to lift. Once you have an idea of that weight, then multiply it by 2. That's the total lift you'll need to hover around 50% throttle.

Now divide this total lift value by the number of motors. Are you designing a tri, quad, hex or octo? This will give you the amount of lift each motor/prop combo must provide.

Next you spend a lot of time on eCalc plugging in different motor/prop combos and see what you get. If you are looking for long hover times you typically want slow, large KV motors turning big props. A sporty, acrobatic copter will be more likely to use higher KV motors with smaller props. Select typical motors based upon price or quality, whichever is most important to you. You can also identify a battery size to give you the run time you are looking for. Once you have a potential motor/prop/battery combo, go back through the calculations now using a known motor and battery weight.

Repeat the whole process until you converge on an answer. Or until you get so frustrated you just give up and buy something someone else has already put the thought into. Because the penalty for excess weight is very large it's absolutely important to design and construct the most efficient copter possible. And that can be pretty damn hard.

Bo,Tom explained all much better than I would do bcs English is not my native...as you can see in link I gave you,some people build frame of only 100-200gr.(just bare frame) but that is very fragile..each normal frame u can see how heavy it is on web site...that excel sheet is not that important and is only to give you rough idea how things should be...here is ecalc link Tom mentioned http://www.ecalc.ch/xcoptercalc.htm?ecalc&lang=en

and here is some very nice batteries you want for long flight time

http://www.gensace.de/lipo-battery.html?___SID=U&l=1&p=4&am...

they are expensive but believe me it will payback during their lifetime wich will be 2-3-4 times more than cheaper Turnigy nanotech or Zippy

here is another link of very interesting build

http://www.rcgroups.com/forums/showthread.php?t=2046468

do your exploration.....

also here on support forum I gave more detailed explanation what to buy to one begginer

http://ardupilot.com/forum/viewtopic.php?f=37&t=2404

For filming I like using higher Kv's (rpm/Volt). I use 1000Kv with 3s 2200 25c battery, 20 amp esc, 10X4.5 carbon fiber props, and my favorite designs are like a H copter or a TBS Discovery. Those copters let you counter weight the gopro and gimbal with the battery and get the props out of the shot. :)

P.S. carbon fiber props use less power, are quite,increase performance, and decrease the vibration by alot!

Aaron

Thanks for the info.  May I ask what type of flight time you're get with this setup?  Why do you prefer about the higher kV motors when filming?

I would say max 10min...

I went down this path as well. What I figured out is that there is no best design.

Nothing gives you long flight times,  no vibrations, acro performance, room for a gimble and so on. Build the lightest thing you can for acro, and big big slow props for AV and mapping. What you can build is probably dictated by cost. More motors is more money.

Build 3 or 4 machines and you'll probably be able to look at a spec sheet and know if it suits your needs.

Build smaller cheaper machines until you need something more. You may find a mobius cam on an ecks frame is all you'll ever need.

Yea, around 9min I think, Its been REALLY cold so now I get around 7 mins.

Heres a video of me flying a little bit ago.

Aaron

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