Building my first multi, a few questions

[CowboyDan]

Hey guys, I'm new here just signed up.. been lurking for a few weeks and reading, finally time to start asking some questions I want to build a quad that is capable of GoPro filming, but that will be modularly upgradeable and will teach me what I need to know to build and fly one capable of carrying 5-10kg of camera down the line.

What I would like in my first quad:

• SAFETY and reliability, these are very important
• Reasonably easy to get in the air
• Decent flight time (10 minutes plus)
• Capable of lifting itself plus a GoPro and gimbal (although I will start out without the gimbal); I will mainly use this quad to learn to fly but I will want to take some footage at some stage
• Maybe capacity to upgrade the configuration for more lift; X8 seems like a good idea
• Easy to add FPV capability
• Foldable into a small package as I travel a lot

I am looking to start with a setup based on this article: http://www.hovership.com/2012/12/beginner-quadcopter-kit-buying-guide/. Right now I've settled on the X666 frame because replacement parts are SUPER cheap, I'm actually planning to buy two (maybe three) X666 frames plus a pack of replacement arms. Although some parts are on backorder, so I might go with the X525. I don't really see a difference in these frames other than the 666 is very slightly bigger.

If I outgrow the X666/X525 I am looking at Randy's UAP2 setup. But I like the way the X666 folds down and it is $20 vs. $200 so I think I will start with that.

TX/RX I plan to go with the OrangeRX T-SIX TX in the article, but with the Orange RX R710 RX + satellite. I don't really want to drop a grand on a radio; if it does the job, it will be fine to start with. I will need a range of around 200-300 metres, it seems that this radio should easily do that.

ESC it seems like the Turnigy Plush are the go, any other recommendations?

FC will be an OP CC3D mounted in one of the pretty 3D printed cases.

Motors and props I am still working on. I've found the Ecalc calculator so I'm busy playing with that to work out what is the best prop/motor combo. But there are only a certain list of motors and props so if anyone has any recommendations for motors and props please feel free to voice them


Now for the questions.......

LiPO batteries are new to me, I raced R/C in the day of NiCad and NiMH were the rich man's batteries... although the 1s/2s/3s thing seems clear to me, that's just one cell two cell three cell... it seems 3s is the way to go for small multis, is that right?

How do props and motor RPM affect stability? I am looking for a quad that is very stable and a bit sluggish, I don't want a stunt chopper to do backflips. I will be doing a LOT of hovering (I'm busy using the Ecalc calculator to find a combination of motors/props that maximises hover time) and slow chasing, I will not be doing anything fancy. Should I be aiming for a large prop/low rpm combo or visa-versa? It seems to me that small props at high RPM will be very twitchy while large props at low RPM will be more docile. Is this right or am I making stuff up?

I will be flying this in temperatures of anywhere from -5degC to -25degC. Does anyone have experience in these sorts of temperatures and is there anything special you need to consider? I have included this in my calculations on Ecalc but I'm more interested in whether the electrics will perform...

Cheers!!
Dan.

 

[PeterLester]

My 2 cents worth.
Temperature specs are typically published for each product..... In the case of the DJI NAZA, the lower temp limit is around -10 but -5 for the WKM. But that does not take into account internal heat during operation. A plastic cap over the electronics will also help to retain PCB internal heat longer and help to eliminate wind chill etc. Also the PMU for the NAZA doesn't put out accurate LED and control signals at low temperatures whereas the WKM's PMU does a much better job.

The Multirotor arm lengths determines the MR stability for the most part.
Also a "+" MR configuration is easier to fly than a "x" configuration...... its about the arm length (angular momentum)
Longer arms are much easier to control but also allow for a much broader range of prop and motor sizes.
The rest of the stability is a matter of setup and tuning the PID loops.

For each motor size there is a limited range of prop diameters/pitches to remain inside the sweet spot for thrust efficiency.

Most common batteries required will be 4S batteries but sometimes there is a need to move to 3S.
You will likely need 4500 to 5000 mAH. (often in parallel)
The DJI F550 will lift a hard mounted GoPro... But if you add landing gear and 2 axis gimbal it will be too heavy until you upgrade the motors etc.

 

[CowboyDan]

Thanks Peter, I planned to enclose the electrics and we use chemical hand warmers (little pouches that get warm when they contact oxygen) a fair bit so when I get more advanced in the build I'll look at whether there's a way to use one of those to warm the brains up a few degrees. But the ESCs might generate a bit of heat, it'll be an interesting experiment I see plenty of snow shots so they must be fairly rugged.

The + or X config I will test out, I can just build the thing and change the config in the OP software so I'll try both styles and see which I prefer.



Why do you say 4S is most common; I seem to see mostly 3S setups. What would the difference be? I understand 4S is a higher voltage but wouldn't you then adjust your motors kv rating to achieve the optimum RPM? e.g. say you want your props doing 10,000rpm (arbitrary number) 3S + 900kv or 4S + 675kv would give you the same results, right? I'm guessing there is more to it than that, right?

I have played on ecalc with 3S and 4S, I can only seem to get good flight time results with 3S, 4S setups I can't seem to get hover time over 10 or 12 minutes.

EDIT: after writing the bit below I went back over my post and I can see a bit better how the 3S/4S affects things, I can use bigger props and carry much more weight on a 4S setup... is that the basic advantage?



On ecalc for the 3S setups I am using 1000-1100kv motors and props 8 or 9" x 4 to 5", with a 4S I am using 750-ish kv motors and props 10 or 11" x 4 to 6". Both setups using 5000mah batteries. I can get efficiencies 80-85% and above with 3S but 4S I can't seem to get them over 75%. Hover throttle is around 45-55% in both cases, is this about right? I seem to remember reading 50% hover is about right.

I've got a combo here 4S 5000mah + 30A ESC + Turnigy NTM2830 750kv + 11x4.5" props; hover time is 16min, mixed flight 8.7min, hover throttle 47%, additional payload 1094g. All efficiencies around 75% though, I feel like they should be higher.

But then if I add the extra payload (set the total weight to 2700g) it says it can carry I wind up with the same flight times but hover time drops to 7.8min and hover throttle jumps to 80%. I was working on 650g model weight + drive (total comes out to about 1650g), remember I will mostly be carrying a gopro but I would maybe like the option of carrying the SLR as well if that's possible, so I have to consider that.



I feel like I am sort of heading in the right direction with these numbers and playing with ecalc is starting to give me a good idea on how all these numbers affect each other but it's still a bit greek to me. Any tips on the prop/motor combos?

Thanks

 

[ChrisViperM]

This might also be of some help for you:


http://www.rcdiscuss.com/showthread.php?27325-Chart-for-Motor-Size-ESC-Battery-and-Prop-Size

http://ajmas.dyndns.org/?loc=projects/thrusthp

https://docs.google.com/a/timskoch....Vu_8jK&sig=AHIEtbRwO1HZ5U4KbTnUhmZEQfv-JyIKOA

If you are flying in the cold, make always sure your LiPo's are ~15°C - 20°C. The efficency of LiPo's drops drastrically with low temperatures....also be aware that plastic props get very brittle in low temperatures and could crack.


Chris

 

[CowboyDan]

OK so I have been having a think and I am abandoning the idea of carrying the SLR. A bit of a grounding has made me realise that I want to use this first quad to learn to fly and once I've done that, to carry my GoPro (and *maybe* a gimbal). If I want to carry the SLR I'll look at building a new quad to cope with that.

So with the X666 frame + OpenPilot + wiring etc estimated at 750g (which I think is maybe a little heavy?) then allowing ecalc to work out the drive weight, using Turnigy 2830-1020 motors, APC slowfly 10x4", and a 3S 3300mah battery I wind up with 4.5 minutes 100% throttle and 10.8min hover.. add a second 3300 battery and I get 8.4 full throttle and 16.3 minutes hover... and I have 250g payload to play with. Flight times are sounding better and it's more in line with what I wanted to do with this quad in the first place.

I guess I will keep playing with the motor/prop combo until I get the best I can. It helps that I now understand what motor numbers mean.... all those numbers were blowing my mind but I found a good tutorial that taught me how to read them so I feel a bit better now

 

[CowboyDan]

The efficency of LiPo's drops drastrically with low temperatures....also be aware that plastic props get very brittle in low temperatures and could crack.

Thanks for the links, will check them out. I had not thought of the battery efficiency and props getting brittle, thanks for the heads up.

Any ideas how wooden props will perform in the cold? They might handle the temp ok but they might not like the moisture....

 

[CowboyDan]

Just re-read your post, I thought you were talking about my LiPo's being rated 15-20degC or something........ I just realised what you meant, there is no way on earth that I can keep my LiPo's 20degC in the environment I shoot while flying, it's impossible. It's -15degC almost every day, and only gets above -5 when spring comes. The WARMEST temp I can see myself using this quad for work would be 0degC.

I can keep them warm until they go on the copter but once they are in flight they'll be in the open air.

I'll investigate an enclosure possibly making use of the chemical hand warmers to keep them at a decent temp. Thanks for the idea, I definitely didn't think of that.

 

[PeterLester]

Thats a lot of questions all at once!!! But so many questions just help to demonstrate that this is in fact a task in multi-discipline engineering!!! i.e., Mechanical, Electrical with a bit of aeronautical tossed in for good measure because of the props. ECalc makes an attempt to get past all that engineering in a hurry but can spur a lot more questions in the process!

Electronic sensor accuracy is often adversely affected by rapid temperature changes so for cold weather, always store your MR left out in the cold for about 15 minutes, but keep the batteries in a toasty warm place until you are about to fly. In this way the batteries will retain internal temperature until you are flying. And at that point they will continue to retain most of that internal temperature while discharging thru the motors during flight.

4S batteries may or may not be the most common depending on which MR you started out with. In the case of the F550, one typically starts out with the 4S batteries and smaller diameter props, then switches back to 3S as prop diameter increases since less rpm range is required with larger props to get the same amount of lift.

For the four prop MRs, if you intend to fly in manual mode..... "+" configuration gives you a longer leverage arm for both pitch and roll which makes pitch and roll control slightly more docile than does the "X" configuration.... For the 6 prop MRs the difference is extremely small. If you fly in Attitude and GPS/Attitude mode you will not notice a difference since the controller is doing 98% of the control automatically...... allowing you small override control.

Thanks Peter, I planned to enclose the electrics and we use chemical hand warmers (little pouches that get warm when they contact oxygen) a fair bit so when I get more advanced in the build I'll look at whether there's a way to use one of those to warm the brains up a few degrees. But the ESCs might generate a bit of heat, it'll be an interesting experiment I see plenty of snow shots so they must be fairly rugged.

The + or X config I will test out, I can just build the thing and change the config in the OP software so I'll try both styles and see which I prefer.



Why do you say 4S is most common; I seem to see mostly 3S setups. What would the difference be? I understand 4S is a higher voltage but wouldn't you then adjust your motors kv rating to achieve the optimum RPM? e.g. say you want your props doing 10,000rpm (arbitrary number) 3S + 900kv or 4S + 675kv would give you the same results, right? I'm guessing there is more to it than that, right?

I have played on ecalc with 3S and 4S, I can only seem to get good flight time results with 3S, 4S setups I can't seem to get hover time over 10 or 12 minutes.

EDIT: after writing the bit below I went back over my post and I can see a bit better how the 3S/4S affects things, I can use bigger props and carry much more weight on a 4S setup... is that the basic advantage?



On ecalc for the 3S setups I am using 1000-1100kv motors and props 8 or 9" x 4 to 5", with a 4S I am using 750-ish kv motors and props 10 or 11" x 4 to 6". Both setups using 5000mah batteries. I can get efficiencies 80-85% and above with 3S but 4S I can't seem to get them over 75%. Hover throttle is around 45-55% in both cases, is this about right? I seem to remember reading 50% hover is about right.

I've got a combo here 4S 5000mah + 30A ESC + Turnigy NTM2830 750kv + 11x4.5" props; hover time is 16min, mixed flight 8.7min, hover throttle 47%, additional payload 1094g. All efficiencies around 75% though, I feel like they should be higher.

But then if I add the extra payload (set the total weight to 2700g) it says it can carry I wind up with the same flight times but hover time drops to 7.8min and hover throttle jumps to 80%. I was working on 650g model weight + drive (total comes out to about 1650g), remember I will mostly be carrying a gopro but I would maybe like the option of carrying the SLR as well if that's possible, so I have to consider that.



I feel like I am sort of heading in the right direction with these numbers and playing with ecalc is starting to give me a good idea on how all these numbers affect each other but it's still a bit greek to me. Any tips on the prop/motor combos?

Thanks

 

[CowboyDan]

Thanks Peter, the help here and my daily fiddles with ecalc are starting to make this stuff a lot more clear. I'm getting a bit more confident and starting to get close to ordering my first parts

Cheers!
Dan.