Greetings,

I'm building an FPV for my grad. project this year, and managed to build my Flight Controller using MultiiWii, Arduino nano and R.Pi for some computer vision processing. I'm now in an issue of choosing the optimal propellers to lift this thing up for about 5 mins if possible. I'm in Egypt, so I'm a bit narrowed on options but here is what I've found available in the market.

Drone total weight: 2~2.5kg (incl. batteries mentioned below)
Motors: 2200kv (1800kv & 2000kv are available)
Propellers: yet to decide
ESC: 30A
Battery: mostly flexible, but aiming at a 8Ah or 10Ah with 20C or 25C

I've made my calculations for the battery lifetime during 1 flight and did some research on the forum before posting, but can't join the points together to make sure that I'm doing right calculations as I can't find a trustful datasheet for the 2200kv motors. So It's highly appreciated to get some guidance.

Max ESC amp draw is 30*4=120 Amps
If I went with the 8 Ah 20C I've about 160 max Amps for 3 mins & 200Amps 2.4 mins for 25C which both can be enough to operate the whole system in addition to the processors. not for the whole 5 mins but for a possible 3 mins I believe

now I can't relate this to the propellers, how do they contribute to the pull power with respect to grams, or how to I can translate it's diameter with the feed amperage to the motor to get the thrust. I'm kinda reaching a dead end here with 4 days of researching, soo am I calculating smth wrong, or missing smth?

I've seen a post written by Iyler that I can estimate a static thrust of 5gms per watt.. on my calculations, my system can throw an 8kg on max draw! xd

Thanks in advance!

@xzanatol

Hi Abdel,
Welcome to the forum.

I’ll try to help you with your problem.
But for that I need the following information:

1. Weight of the airframe WITHOUT motors, props and battery
2. Specifications of the motor, Ie, brand, model number or size (Like 2212, 2826 etc), max amps, max watts, kv and weight. (Kv alone is not enough)

If I have a similar motor, I can test thrust and current draw with different props.

Let me know.

Regards
Iyer

Hello Iyer,

thanks for your response, and your willing to help first of all! 

Ok, for the airframe it's 1kg. (953gm to be exact)

Motor is A2212/6T:

• Max Amps: 12A/60s
• Max Power: 239W
• kv: 2200 RPM/Volt
• Weight: 49gm
• Internal Resistance: 90 m ohm

Let’s estimate the potential total weight:

950 gm Airframe
200 gm motors (50x4)
  40 gm 4x prop+adaptor
140 gm 4x Esc
250 gm 2x 1500mah 3s lipos
——
1580 gm total AUW

So you need 400 gms thrust per motor at hover / 50% throttle.

Okay so far?

Yea I'm following along. just a small doubt, why the 1500mah 3s lipos?

Just an approximation to start with. We will change it as we proceed with our calculations...

ah ok great

These A2212 are generic motors sold under many brands. Very low price - USD 5 in India.
The problem is that Prop and performance data varies depending on the source.

The motors you have are probably the ones shown in the pic below:

(Back shortly. Dog wants to go out!)

Yep, That's the one. Same with the other KVs.

(sure, have fun! )

Wife is taking the dog out, so I’m back.

Notice that the data says 239 watts and 732 gms thrust with a 6x3 prop.
BUT AT 21.5 AMPS
And the motor is rated for 12 amps max for 30 seconds at most.
Such inconsistencies are there in every data source.

But let us assume, for a moment, that they can produce 400 gms thrust on a 6x3 prop at 10 amps draw.

(This is a reasonable assumption based on say 3.5 gms/w and 120 watts)

With 25c-30c lipos, I normally avoid drawing over 10c, and don’t use more tha 60% of the capacity.

Using up 1800mah (60% of a 3000mah lipo) @ 40amps will give us a duration of 2.6 minutes
(1800x60/40,000)
Remember that THIS IS AT HOVER!

Why a 6” dia prop?
Because anything larger will typically overload/ overheat/ burn such motors on 3s.
The prop speed will be about 12x2200x75%, say 20,000 rpm!

Conclusions so far:
1. For 5 minute endurance, you’ll need a 3s lipo of 6000mah or more.
2. So the weight and power required go up by 15% or more, reducing the endurance.
3. Small props like 6” dia at such rpm will produce only 3-4 gms thrust per watt.
4. Better to use an 8” prop if the frame can accommodate them
5. Use lower kv motors corresponding to the largest prop dia feasible.
6. Larger prop/ low pitch/ low kv may give about 5gms/w. So for 2kg AUW, say 100w per motor for hover
7. So motors of perhaps 150-200watts

Hope all this stuff if of some use for your project.

@Free Flight

I see that you are watching from the US!
Comments from you are welcome.

Regards
Iyer

Indeed the calculations now make much more sense. ofc, I have to think of way to decrease the weight by around a 500gm (maybe change the camera specs) to be able to move it around. but overall is it safe to say the following?

• 8" prop
• 8000mAh 25-30C

 that it can give around 5mins

8” prop will overload your 2200kv motor.
Need the lower kv version.

oh, forgot to add it to the list. Ok great, I'll test it out and update you soon.

Thanks again for the explanations!