Props and max rpm

C

Chop

Active Member
Helloo!

I've recently had a look at some new propellers, especially the Aeronaut CamCarbon light 10x4.5 ones.

The problem is now, that Aeronaut gives a max rpm of 15000rpm.
I have the Tiger Air 2212 920kv motors, which, o wonder of math, would spin at 920x16.8 at 15456rpm.

But ecalc gives a max rpm of ~10000?! Is the motor running that much slower because of the prop or is ecalc wrong? I mean, listening to ecalc, it would be save to use that prop, listening to my "professional" calculations, it would spin too fast.

Any tips/ ideas? Is that prop safe to run?
 
Hi mate to work out RPM is easy the KV of the motor is RPM per 1 volt of power so times kv by volts this is max RPM , dont forget you will get RPM loss due to battery running down , and brushless motors are only 90% efficient
 
As i said above, 920x16.8 (4s) gives 15456rpm.

But ecalc gives ~10000rpm.

So my question is, as said above: will the propeller load slow the motor down to a maximum of ~10000rpm, as ecalc said, or will the motor still spinn at 15456rpm?

(I am questioning the maximum rpm at no load vs maximum rpm at load!)

Anyways, thanks for the quick answer! ;)
 
Chop said:
As i said above, 920x16.8 (4s) gives 15456rpm.

But ecalc gives ~10000rpm.

So my question is, as said above: will the propeller load slow the motor down to a maximum of ~10000rpm, as ecalc said, or will the motor still spinn at 15456rpm?

(I am questioning the maximum rpm at no load vs maximum rpm at load!)

Anyways, thanks for the quick answer! ;)
Click to expand...
Hey Chop. The TigerAir 2212-920kv motors are 83% efficient. Most Chinese motors are 80-81% so you are a little bit above. Ecalc tells you that their computations are +/- 15%. So with 17% on the motors and 15% on Ecalc I wouldn't worry about over revving. With 1045 props you'll be lucky to get 10,000 rpm anyway. Ecalc is meant as a VERY general guide at best. It can be used as a baseline for comparison of drive combinations but shouldn't be considered the last word. You will find, as you get farther into this hobby, that reputable motor manufacturers will give you data on thrust, efficiency and amp draw using various propellers. That kind of data is what you need to really fine tune a power train. BTW. Why did you decide to use 1045 props instead of the 9545 props? They may, or not, work better than the 1045 props for you. They will certainly let the motor run a little cooler. Anyway. Run what ya got and change if it don't work......That's my $.02:D
 
Since i am now adding more stuff to the copter (i know i know kiss principe) some more max thrust is nice to have. And i was going to change my props to stiffer ones anyways, so why not a little bigger. Also Aeronauts are much quieter.

And they are completely made in germany... and i am from germany so maybe thats a point ;D
 
Chop said:
Since i am now adding more stuff to the copter (i know i know kiss principe) some more max thrust is nice to have. And i was going to change my props to stiffer ones anyways, so why not a little bigger. Also Aeronauts are much quieter.

And they are completely made in germany... and i am from germany so maybe thats a point ;D
Click to expand...
Ha! That is, indeed, a point. I have another suggestion to make since you are adding more weight and at 920kv this should help. For more lift, not speed, use a high capacity 3s battery like;
http://www.hobbyking.com/hobbyking/...apacity_3S_5200mAh_Multi_Rotor_Lipo_Pack.html
Use a large diameter, low pitch prop such as a 12 X 3.8 or a 12 X 3.3. Maybe even a 12 X 3.0. This will give you more lifting ability and more efficiency at the expense of speed. The quad will fly more gently and be more stable. Great for video. If you decide you want to play, change to a higher C rating 4s battery and a 9545 prop and tear around the sky. To help you see how this works think of thrust as horsepower, the higher the number the more speed. Think of efficiency rating( lb/hp) as torque, the higher the number the more power. Say on a 4s battery, 9545 prop at 75% throttle your motor has an efficiency rating of 8 and a lot of speed. Now on a 3s battery, 12 X 3.3 prop and 75% throttle your motor has an efficiency rating of 12 and a lot of lift ( remember lb/hp) but less speed. I hope I haven't added to your confusion. Perhaps others on the forum can explain it better. Here is a link that may help clarify things. This manufacturer has the best technical data in the industry in an easy to understand format;

https://cdn.shopify.com/s/files/1/0...ting_-_KDE2315XF-965.pdf?13250022135093590036

Check the differences in efficiency between the prop sizes and 3s and 4s for this particular motor.
 
Yup, those LiPos are indeed nice. Im using the 4s 5200mah ones already. 12 inches would be better, yes. But its a very tight fit an a 450 sized quad, maybe 11 inches on 3s is okay.
And also, when i take similar prop size but lowering voltage, i will of course loose maximum thrust.
So i was looking for the max diameter prop i can have with 4s and then what is the max prop i can fit on the frame.
I guess max prop on 4s would be ~10 inches
and max prop for the frame ~11 inches
That makes me think if its worth having a tad more efficiency on 3s and 11* or rather having more maximum lift on 4s and 10* but also relativly good efficiency.

I will most likely stay with 4s and 10x4.5 but thanks to all for making me feel safe about not over-revving the props and helping out! :)

@tallrider This chart from Tigermotors explains it a little better imo. Just jumped in paint and added some fancy lines ;)
20150202031604541.jpg
 
Good example. I happen to have a 600mm quad with those motors on 4s 8000mah Multistar with 15 X 5.5 props. AUW is 1.75g. I don't fly very aggressively and I consistently get over 30 minutes (barely). Plus I could take a small dog for a ride there is so much lift available. Using a 6s battery I could give 250 racers a scare, but for a very short time. You can see this in the 1400 rpm difference at 100% with the same 15 X 5 prop. Because of the low KV on the 3508's and the higher KV on yours it shows more dramatically the difference in efficiency(torque) which is lift. I have put 12" props on a 450 and it is a close fit, but it does fit. A big part of the fun in this hobby is the experimentation. Thinking about things from a different angle. Remember. Thrust is horsepower(speed) and efficiency, lb/hp is torque (lift). There is a point where they intersect and that is when your quad flies like a dream.
 
Chop said:
As i said above, 920x16.8 (4s) gives 15456rpm.

But ecalc gives ~10000rpm.

So my question is, as said above: will the propeller load slow the motor down to a maximum of ~10000rpm, as ecalc said, or will the motor still spinn at 15456rpm?

(I am questioning the maximum rpm at no load vs maximum rpm at load!)

Anyways, thanks for the quick answer! ;)
Click to expand...
Depends a lot on the pitch of the propeller, and the load on it but the short answer is yes. You an use a tachometer to test this.
 
I have 1400 kv motors, 3 Amp ESCs, 450 frame, 0845 props and a 3300 3S 30C battery. All the motors are functional but with 100 percent throttle also it not taking off. Kindly help in telling what could have gone wrong and how to correct so that this quad takes off
 
I assume you meant 30amp ESC's? Are the motors 2212 1400kv or another size?
Would it be possible for you to send us links for your components or at least tell us exactly what the are?
Oh, and welcome to the forum!
 
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