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Help me choose a motor for my home built plane

#1
I am designing an airplane that I am going to build. I have built some plane before but not this big. This plane has a wingspan of 1,5 meters. It is a pusher configuration. The the wing and tail surfaces should be hot-wire-cut out of blue foam (styrofoam). I estimate that the weight will be around or slightly less than 2 kg. I want around 1:1 trust weight ratio and as high efficiency as possible when cruising around at below half throttle. I have designed the airplane for a 12 inch propeller but I can go up maybe one inch is it is necessary. Speed is not a priority here.

I have a few 3 cell 8000 mAh batteries that I want to use in this airplane. When I do some research I found that people usually use 4 or 5 cell batteries for this size of airplane. But I am sure this is possible with a 3 cell as well. I also think it is important that the motor should not bee pushed to the limit. I do not want it to get hot after cruising around for 20 minutes.

In conclusion: I want an efficient motor that can spin a 12 inch prop can produce around 2 kg of trust at full speed using a 3 cell battery.

Maybe there are some people with similar sized airplanes flying of 3 cell batteries? If so, please share your motor-propeller combination and how mush static trust you get.

Here are some CAD images of the plane:

plane1.jpg

plane2.jpg
 
#3
Hey, Axbri!

It looks like a 900kv NTM 3548 swinging a GWS 12x8 HD prop will be your best bet here.
Here is a screen shot of the performance estimates, courtesy of Drivecalc.

http://www.hobbyking.com/hobbyking/store/__35377__NTM_Prop_Drive_35_48_Series_900KV_815W_EU_warehouse_.html

Don't forget to order the accessory pack.
Yes, that motor seems suitable! Thanks for the help. I have never tried any software for this before. I will play around more with Drivecalc.

I am still interested if anyone has some real-life experience with motors/airplanes in this size and can recommend something they have tried.
 

TEAJR66

Flite is good
Mentor
#4
I built a 60" span twin boom pusher. It flew on 2 x 3S 2200mah in parallel for 4400mah total. Had great power and cruised at just under half throttle. The DT750 with an 11x4.7 SF prop. I don't know the all up weight, but it had those batteries, and my FPV gear in it. You could also look at the DT850 and go with a 12" prop (I own a DT850 but have not used it yet). 3S is the sweet spot in batteries for those motors. I have not used any software to compare data, just used what I had on hand and it worked well.
DSC01659.jpg
 
#5
I built a 60" span twin boom pusher. It flew on 2 x 3S 2200mah in parallel for 4400mah total. Had great power and cruised at just under half throttle. The DT750 with an 11x4.7 SF prop. I don't know the all up weight, but it had those batteries, and my FPV gear in it. You could also look at the DT850 and go with a 12" prop (I own a DT850 but have not used it yet). 3S is the sweet spot in batteries for those motors. I have not used any software to compare data, just used what I had on hand and it worked well.
Nice airplane! I like those more or less improvised home built planes. However a think a DT750 will be a bit weak in my case. Your plane seems to be lighter than I imagine that my plane will be. I have built a quadcopter with DT750 motors and 10x4.7 props, the it could lift almost 1 kg per motor at full power. I think I will need to push a DT750 very hard and almost overload it to generate 2kg of thrust, and that is not what I am looking for here. The DT850 is just that same motor but with a higher KV rating, meaning a higher RPM. I do not think it can tolerate more power.
 

TEAJR66

Flite is good
Mentor
#6
Axbri,

No worries. I hope you get it sorted.

Those DT Motors work backwards (it seams to me) in that as the kv ratings go up so does the strength. They all run peak on 3S. When you look at the motors side by side, the bells get longer as the kv rating goes up. That's why the 850kv swings a 12" prop while the 750kv works better with 11" props. But you are correct in that you wont get 2kg of thrust out of either. Even the DT100 with an 11X9 three blade only gives 1.2kg. The 850 with 12X6 is 1.6kg.

I am curious to see what you come up with that gives 2kg of thrust on 3S. Let us know.
 

joshuabardwell

Senior Member
Mentor
#7

TEAJR66

Flite is good
Mentor
#8
The best I could do, after some fiddling around on HobbyKing's motor search wizard, was this:

http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8484

Produces 2400 grams of thrust on 3S using a 12x8 prop.

I say, "best I could do," but in fact, this is the only 3S motor I could find that even claimed to break 2 kg of thrust.
That looks good. Not a fan of the shaft through the firewall type mounting that will be required. But that motor seems to meet the specs.
 

SnowRocker88

Amateur pilot and builder
#9
#10
Thanks for all the advice. I think I will go with the 900kv NTM 3548 and a 12x8 prop. It seams to be what I want looking at the numbers, and I know many people use and like the NTM Prop Drive motors in general.

The sugested Turnigy 3648 600KV may also work, but I do not like that it must be mounted inside a firewall, it makes the motor mount that I have to build a lot more complicated.

I have not started building the plane yet, but I will probably make some sort of build-thread here in the forum when I have something to show. Watch out for it if you are interested in my project.