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Getting Better Flight Times - Multirotor

#1
I've been flying quadrotors for about a year now and recently tried to design one and build it up. I'm having a hell of a time with my estimated flight times. No matter what combination of motors and props, I can't get my average flight time over 7 minutes. My plan is to make a multirotor that can carry FPV gear and a GoPro (maybe Gimbal) for around 10-15 minutes. Does anyone have special tips on maximizing flight times? Is this sort of flying time even achievable with this setup?
 

joshuabardwell

Senior Member
Mentor
#3
Go spend two dollars for a 30 day subscription to eCalc and play around with different combinations. There are two ways to get long flight times:

The first way is huge batteries. This is a bad strategy, because as you add more mAh of flight time, you also add battery weight. So there is a law of diminishing returns. You might triple your battery weight but only add 30% more flight time (that number pulled out of thin air, but you get the point).

The second way is big props. The lift produced by a wing increases as the square of the area, so double the size, four times the lift. Larger props are more efficient than smaller props. It is more efficient to swing a larger prop slowly than it is to swing a smaller prop quickly. Take a low-kv motor swinging a large prop, and a high-kv motor swinging a small prop, and set them up so they pull the exact same wattage. The low-kv motor will produce more thrust, and will have longer flight time on the same batteries.

I designed a quad in eCalc with an AUW around 1.2 kg, that was estimated to get about a 20-25 minute flight time. It swung 14.5x5" props on something like a 400-600 kv motor. To get the required thrust, I had to go up to a 4S battery, though. There was just no way that a 3S could push those big props enough to generate thrust at a reasonable throttle setting.

I guess that would be the other thing I would say about long flight times. Yes, you can find 3S copters that have 20-30 minute flight times, but they are usually carrying huge batteries--10,000 mAh or thereabouts. People think of 4S copters as agile, fast machines, but if you are running big props with low kv motors, you can get a reasonable TWR with much longer flight times instead.
 

Dumpster Jedi

The One Who Speaks
#5
The only advice I got for this task is: low KV, big props and keep the weight down.
I second this. Just swapped 1200kv for the 935kv I had on my FPV quad... cut my flight times by almost 3 minutes, loaded exactly the same as before. Next set will be in the 7-800kv range for sure.
 

joshuabardwell

Senior Member
Mentor
#6
BTW, with multirotors, there is a down side to big props, and that is that they have more inertia, and so it is harder for them to speed up and slow down quickly, which of course is something that multirotor props do all the time. I don't have personal experience here, but my understanding is that larger props can result in less stability and less "locked in" feel, because the motors can't respond as quickly. But there are a lot of people who fly multirotors with 10"-15" props, so it clearly can be done.
 

joshuabardwell

Senior Member
Mentor
#8
How heavy is the typical quad? With my FPV gear and no additional payload, my quad is just under 2 kg. tips on reducing weight would be great.
2 kg seems heavy, but it depends on the size of your copter. A Blackout Mini H comes in somewhere around 600 g, if memory serves. A DJI Phantom 2 is just over 1 kg. Maybe if you posted your parts list, people could help you figure out where to shave weight.
 

mmeyer

Senior Member
#9
Weight is also very dependant on the frame. You want to be using light material with minimal nuts and bolts and trying to shave off weight everywhere as long as it does not impact on durability.
 
#10
Frame - HobbyKing Super H600 = 580 g
Receiver - Spektrum a800(I think) = 31 g
FC - APM2.6 = 35 g
GPS - uBlox = 28 g
Motors - RCTimer HP2814- 710kv = 127 g (I feel like these were a terrible choice, but you gotta experiment to learn)
Prop - 12x4.5 SF = 12 g
ESC - Turnigy Plush 40A = 30 g
Battery - I have a couple different sizes of 3S batteries from 2200-5000 mAh (192-380 g)
FPV - Fatshark Predator V2 kit = 85 g

I'm open to all suggestions. My previous Hobby was 3D printing so I had a go at printing my own frame, but it was incredibly fragile since I only use PLA in my prints. I chose the Hobbyking frame because it gave me room for expansion (gimbal with GoPro/Mobius, more batteries, misc electronics...)

I'd like to have a machine that I can fly around first person and not have to instantly bring it back because of a dead battery.
 

joshuabardwell

Senior Member
Mentor
#13
Motors - RCTimer HP2814- 710kv = 127 g (I feel like these were a terrible choice, but you gotta experiment to learn)
The motors are the main thing that stands out to me as abnormally heavy. For example, the DT750 that David uses in his tricopters only weighs about 80 grams. I haven't crunched the numbers, but the DT750s obviously make enough thrust to fly a tri or quad for a little while. The HP2184 may be over-specced for your use; there may be a lighter motor that still produces enough thrust.

That motor's specs show that, on a 12x3.8 prop with a 3S battery, it pulled only 15.1 amps at full throttle, so your ESCs seem way bigger than they need to be. You could get them down by about half if you went with a 20-30 amp ESC. With a 12x3.8 prop, the motor generated 1121 grams of thrust. So you're getting about 4 kg of thrust from your motors, which is the minimum amount for a 2 kg copter.

Since neither your motors nor your ESCs are maxxed out, you could consider going to bigger props--except I don't think that frame will take them.

You are pulling about 60 amps at full throttle. A 2200 mAh battery would need to be at least 30C to support this. a 5000 mAh battery would need to be around 15C.
 
#14
I see. I used an electronics calculator that I found in some forum somewhere (similar but less awesome than eCalc) and it led me to those motors. I'll definitely use eCalc this next go around.

Lets say I was to redesign the power system... In what order would I choose components?
(props>motor>esc>battery)

I'm open to a complete redesign if I will get the flight times that I want.
 

stay-fun

Helicopter addict
#15
That's a pretty heavy frame dude. Combining that with low kV motors and (relative to the frame) small props, I have a feeling that a hover is above midstick, is that right?
 

joshuabardwell

Senior Member
Mentor
#17
I see. I used an electronics calculator that I found in some forum somewhere (similar but less awesome than eCalc) and it led me to those motors. I'll definitely use eCalc this next go around.
None of the calculators are perfect, but my impression is that eCalc is the best. For a few dollars (30 day subscription), I think it's worth it. (Note: I'm not associated with eCalc. Just a happy user.)

Lets say I was to redesign the power system... In what order would I choose components?
(props>motor>esc>battery)
I think the first thing to do is to sort of define your copter's mission. An acrobatic mini quad is going to have a different mission than a hobby flyer. A quad that is designed to carry a big DSLR on a gimbal system is going to have a different mission still. The main parameters might be things like flight time, stability vs. acro, cargo capacity, and all-up-weight (AUW). In your case, I think you would be in the low-mid range for cargo and mid range for AUW. It seems like you would prioritize flight time over speed/agility.

If flight time is your priority, then you need to start focusing on low kv and large props. I added up your gear, minus your power system, and came up with 850 g, if memory serves. So put into eCalc a weight of 850 g, "not including power system", and then play with different power systems to try to come up with longer flight times. Make sure that your hover throttle is around 50%. eCalc will not complain unless your copter would require more than 80% throttle on low battery, but I think most of us would find that to be unacceptable.

When I designed my quad (haven't built it yet, mind you), I wanted a flight time of around 20-25 minutes. In the images below, you can see what I came up with in eCalc, and the resulting flight times, etc... Notice a couple things. First, I am using 14" props with 390 kv motors. Second, I had to step up to a 4S battery to push the motors hard enough to keep the hover at around 50% throttle. Third, my airframe, minus power system, is only around 450 g. This is for a home-built H-quad frame, made out of wood. I haven't built it, but I based it off of other people's builds. My AUW comes in around 1.5 kg, with a hover time of 24 minutes, and a mixed flight time of 13 minutes.

I'm open to a complete redesign if I will get the flight times that I want.
Take this with a grain of salt, as I am just getting started with quad design, but I think your frame is holding you back here. That frame is heavy for its size--you should be able to come in under 400 g, I think. But the main way that frame hurts you, IMO, is that it limits you to 12" props. I think you are going to be challenged to get the kind of flight times you want using 12" props. If you were to cut a lot of weight and go to 4S battery, maybe. But given that switching frames may be one of the main ways you can cut weight, you may as well consider whether you want to go with larger props as long as you're at it. Or maybe you can tweak things out in eCalc to get something that works with 12" props, by using different motors and ESCs. It kind of all depends on what exactly you consider to be a long enough flight time. 7-9 minutes is plenty for a lot of people, and they just land and stick another battery in. If you are intending to do long FPV flights, like 15-20 minutes, I think you are going to have to really go back to the drawing board. That is an exceptional flight time for a multirotor, and they really have to be purpose build to achieve that.
 

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#18
Take this with a grain of salt, as I am just getting started with quad design, but I think your frame is holding you back here. That frame is heavy for its size--you should be able to come in under 400 g, I think. But the main way that frame hurts you, IMO, is that it limits you to 12" props. I think you are going to be challenged to get the kind of flight times you want using 12" props. If you were to cut a lot of weight and go to 4S battery, maybe. But given that switching frames may be one of the main ways you can cut weight, you may as well consider whether you want to go with larger props as long as you're at it. Or maybe you can tweak things out in eCalc to get something that works with 12" props, by using different motors and ESCs. It kind of all depends on what exactly you consider to be a long enough flight time. 7-9 minutes is plenty for a lot of people, and they just land and stick another battery in. If you are intending to do long FPV flights, like 15-20 minutes, I think you are going to have to really go back to the drawing board. That is an exceptional flight time for a multirotor, and they really have to be purpose build to achieve that.



This is exactly what I needed to hear. I really appreciate you taking the time to write that up.

I've been playing with eCalc for an hour now and can't find a combo that will work for me with 12" props. The weight of the frame does seem to be the main culprit here. Looks like I will be going back to the drawing board...

Thanks to everyone who chimed in. It's been a real help.
 

Tritium

Amateur Extra Class K5TWM
#19
I am building my big boy with RCT 5010-14 360KV and 17X5.5" CF Props. Planning on 3S battery. Motors and ESC's good for 6S though.

Thurmond
 
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joshuabardwell

Senior Member
Mentor
#20
The more I learn about copter design, the more impressed I am that DJI manages to get around 20 minutes of flight time out of a 5 Ah 3S on the Phantom 2. Its AUW is 1030 g and it uses 9" props, so there's your benchmark. Maybe OP should just look at what motor DJI uses and build a phantom clone! Whenever I read people on the Internet talking about building long-flight-time quads, they are always either using big props (12"+) or they are carrying massive batteries. I dunno...