I want to build a ~2.5 kg quad with 14 inch props. I’m currently thinking to use 16mm diameter (1mm wall thinkness) aluminum round tubes. Is there anyone who could give me some recommendations/advice whether the size is big enough (I only find motor boat props when I try to google that and I would be very happy to get some reports about first hand experience)?
I would also interested if a BLHeli32 ESC works togtheter with an 700kV motor (I found a lot of different opinions but they are all some years old.)?
T-Motor CF 14" 4.8 props
T-Motor F55A pro BLHeli32 4in1 ESC
12.000 mAh 3S lipo
mRo X2.1 autopilot
Thank you ver much!
I could report about the build results if someone is interested…
Tobias, it might be worth modeling your build in something like eCalc.ch. You are using heavy motors. You are unlikely to find a 3S 12 amp battery. Your motors and battery alone will weight around 1600g. Aluminum tubing is heavy too. Depends on how long you want it to fly and if you intend to have any payload but I would check these things before you buy or build anything.
HI Mike, thank you for your answer, I did model it and I think the results are ok. I also think that the weight I aim for is possible. But I’m not sure whether the construction will be stiff enough. I wand to start with aluminum tubings and get some experience and then I want to switch to carbon tubes… . But I do not like to work with carbon fibre… .
Not all the figures are correct in eCalc. The T-Motor U3 with teh cables are 128g according to T-Motor. Again have to tried to look for a 12amp 3s battery?
I don’t like working with carbon fiber either, and I am very careful, and cut it under running water but it is light.
Most carbon arms for your build would be 16mm. It is also easy to find motor mounts for 16mm.
Yes, the motors are 97g. I calculated with 120 g per motor because I will only use 40 cm of the 60 cm cable.
I have still 16 Multistar 3S 4000 mAh batteries. When they are no longer working I plan to switch for 4S 9000 mAh batteries.
I plan to go for carbon fibre if the build proves to be worth it. If I could use the 16mm aluminum tubes it would be very easy to switch for 16 mm carbon tubes (I also found great source in Germany). I also will order cnc cutted carbonfibre/balsa sandwitch if I switch.
But for the moment I’m willing to accept a 250 g weight penalty (I think that is a conservative estimation, the 16 mm aluminum tube is 130 g/m and a cabon fiber tube probably 70g/m) if I do not have to cut carbon.
The important question for me is whether the 16mm tubes will work or not. @mike do you think it will work? Or do you think the whole concept is wrongly designed? I’m very happy that you provide me with expertise. Could you give more details about the doubts you are having?
Most any frame capable of 14" props will weight 600g or more depending on the landing gear, made out of carbon. Your weight is light at around 480g.
Every 50grams of extra weight represents 1 minute of lost flight time. Long flight time comes from large slow moving props, if flight time is important.
The aircraft performance would be very different with 4s battery vs a 3s battery. Remember the 3s is rotating at 12x700=8400rpm while 4s would be 16x700=11200rpm.
Every 50grams of extra weight represents 1 minute of lost flight time.
Clearly 16mm outside diameter tubes will work but it depends on the wall thickness you choose and or howmuch you plan to crash
Yes thank you, I also followed your great thread. I know that carbon fibre is much better in many aspects.
But for now I’m only interested in the fact whether my tubes will be stiff enough. your fantastic calculater (thank you very much!) tells me that the aluminum tubes are 40% heavier and will bend 2.6 times more (6 mm with 0.9 kg force) than carbon fibre.
As others have stated, the aluminum tubing wall thickness of even the 1mm will be the least of your problems, tube strength won’t be one of them. There isn’t much bending force on the middle of the length of the arm until you crash. Simple test, lift the craft by 2 diagonal motors, does it sag in the center? Also a perfect way to check your center of gravity and balance when fitting the battery and electronics.
I have some 4K 3S1P lipo’s and they are over 300g each, another error in ecalc. Which brand 3S1P 12K lipo weighs 260g?
You may lift off but you will probably have to max out your throttle. Flight time will be horrible on 3S, 4S will help. 700kV motors aren’t optimal for this, too high a kV, in my experience. The 14" props are too small, go with at least 15" carbon fibre props. The 15C/25C lipo will also drain quickly, have you tested this to see what the actual total amp draw is on the battery pack?
Anyway, just some things to think about. Good luck, keep us informed.
For your copter, I would go for a 4S with 13*4.4. You would get 2.900 Kg thrust at 50% throttle. That would be perfect for your drone. In my experience, final design is always heavier than initially calculated.
Then, I prefer to make simple maths for material strenght, but I wanted to test the last version of Freecad 0.18 FEM, so I got…
Max. VM Stress: 48 MPa. Aluminium would stand up to 300 MPa, so it enough.
Max displacement: 4,21 mm. It would be more, as the deflection would come from the arm mount more than from the arm itself (depends on the design, but you should consider it).
Carbon fiber would stand up to 1200 MPa, and displacement should be less than 1 mm.
I’ve been down the same road, ecalc is a starting point. If you have specific values for ecalc like the true battery weight, by all means plug it in. The only true test is with your real hardware in it’s real configuration. See if ecalc matches on your current craft. I’m not knocking ecalc, it is a valuable tool to begin with.
I use the amps at hover (mid throttle) to tune my multirotors. First I trim as much weight as possible, then I play with the props. I’ve a Tarot 650 quad with 770kV motors that I settled on 15" CF props after starting with 10" APC MR props, then worked up to 12", 14" and so on. Got the amps to be reasonable with over 25 minutes of flight time. The next step with that one is to change from lipo to 18650 li-ion.
No matter what you do, all up weight is the killer.
I think that the 3S 14" 4.8 configuration is more attractive since it offers more thrust per watt and still has more than enough thrust to handle the 2,500 kg. If I manage the meet the 2,500 kg goal, of course.
I see that 15" will give more efficiency. But I’m also interested in responsiveness to cope with gusts. We have almost all the time gusts with 10m/s and it would be great if gusts with 15m/s could also be handled while flying in wind with 10m/s. I think that there is a trade off there when getting bigger props. Don’t you think so?
I agree, but my experience is that ecalc is conservative. All my builds perfom better than ecalc’s calculations. So I wonder what is wrong with this build.
I’m also terribly affraid of wobbling arms and I have the impression that slow turning motors and large props a prone to that and do not allow too much design shortcomings. I’m not so much interested in getting bigger props.
ARE THOSE DEFAULT NUMBERS REALISTIC?
Yes. Those numbers are taken from a previous drone I built as a group project. It used 380kv motors with 15" props, and the current draw at hover was between 9.5 and 10.5 amps. I got fairly consistent 40 minute flight times with an 8000mAh 6S battery. It had a GoPro HERO4, separate FPV camera, remote switcher, and PIP board.
If I look at those numbers from the tube calculator website I wonder if the difference between those different motor speeds and props sizes are that big. My (very badly constructed as you can read on another thread…) hexa did also draw 10.5 A 6S equivalent at 3S and with 6x9.5" props. That would also result in a flight time next to 40 min if I had traded in my 360 g camera & RTK GPS for a GoPro and added another 250g battery.
Here is a graph of some hours of flight time with this copter:
I also thought about going for a MN3508 380 kV 15" and 4S solution, but therefore I would need a 200g frame. Otherwise there is not much difference according to ecalc. At similar weight this build would hover at 71 watt per motor vs 74 watt with the other build.
@rob215x watched youre frame build videos - sorry for my ignorance! I just do not have the resources for such a build. If you ever plan to sell such a frame I would be very interested! Meanwhile I go with my primitive builds and hope they fly… .
I built a similar sized quad using 3508-380kv motors, 1547 props on 6S with a 8000mah battery. Carrying no payload I got ~ 45minutes of flight time. 6S was definitely the way to go with this craft. eCalc attached.