So you want to build your first quadcopter and you go out and look at all the parts that you can use to make one. You pick up a frame(say F450)…. but now you’re stuck with a problem. You can’t choose the right propeller, motor, E.S.C. and battery until you know the weight of the frame and you can’t know the weight of the frame until you know which motor, E.S.C., battery and propeller you’ll be using.
So how do we solve this problem?
ASSUMPTION MAN TO THE RESCUE!!! Basically, The weight of a motor, whether it is a 1000kv motor or an 1800kv motor, will be almost the same as long as both of them are of the size 2212(the length of the magnets used is 12mm and the diameter of the motor can is 22mm. Now you know what that 2212 number means!). The weight of the E.S.C. will change by 10-20 grams as you go from a 20A E.S.C. to a 40 A E.S.C. The weight of the battery will change a bit more significantly, but you can start with a 2200mAh battery or a 3000mAh battery for a 450 size frame and your end results will come out more or less the same. Therefore, you can assume that you already have the hardware before you even have the hardware.
Okay, So let’s see the steps:
Step 1: Assume the weight of the multi-rotor (look up the weight of the possible individual components and add it up).
Step 2: Divide that weight (All Up Weight or A.U.W.) by 4 . this is the Thrust that each propeller must produce.
Step 3: Find the largest possible diameter of the propeller that you can possibly fit (or would prefer to fit) on your multi-rotor. The 450 frame can work with diameters up to 11 inches. However, 11-inch diameter propellers would put a little more load on the motor and will require a lower kv motor. Low kv motors are a bit more expensive. Medium range kv motors have cheap equivalents available in the market. therefore a 10-inch or 9-inch prop would be good. However, 9-inch props would require more current for the same thrust(Why? A hint towards the proof is there in the “derivations” section at the end, look at the formula for thrust and power).
Step 4: For an efficient and smooth flight, the pitch of the propeller is ~ 1/3rd of the diameter. For a more aggressive flight, the pitch can be as high as 1/2 the diameter of the propeller. For example, for a 10-inch diameter propeller, the ideal pitch would be ~3.3 inches. (Why 1/3rd? A hint towards the reason is given in the “derivations” section at the end, look at the angle of attack’s relation to pitch).
Step 5: Find the propeller in the market that most closely resembles the propeller that you want (say if you want a 10×3.3 but the market has 10×4.5 and 10×3.8, go for 10×3.8).
Step 6: Find the RPM at which the propeller would have to rotate for generating AUW/4 (calculated in step 2) using this online calculator: https://rcplanes.online/calc_thrust.htm
(Derivation for the thrust formula and the formula for power is there in the “derivations section at the end. It is the first derivation).
Step 7: Multiply that RPM by a factor of 1.5(this is a number I have come up with through experience) and divide it by 50% of the average battery voltage. For example, if the average battery voltage is 12V (3s lipo) and the required RPM is 4000, then the Kv of the required motor is = (4000×1.5)/(12×0.5) = 1000Kv. (Why did I multiply by 1.5? The reason for that “correction factor” is that the kv rating depicts the rpm per volt (1000kv motor will spin at 1000 rpm when driven by a 1V amplitude source) under no load. When the motor is turning a propeller, it is under load, therefore the actual kv rating drops to ~66%(again, this is from personal experience) for medium kv motors (anything above 980kv is medium kv) ).
But wait, how do I know whether I should use a 4s setup with a 770kv motor or a 3s setup with a 1000kv motor (since both of them produce virtually the same RPM at 50% throttle)? The answer lies in whether the propellers you’re using are light and have a low angle of attack and low drag OR whether the propellers you’re using are heavy and have a higher angle of attack and drag. In general, for moving heavier loads, one chooses a lower kv motor. If a fast response is what you’re looking for (a lighter frame), go for a high kv low cell count setup, otherwise, go for a low kv high cell count setup. ( Racing drones use a high kv high cell count setup because they need to spin very small loads to very high RPMs. The cell count and high kv help with achieving those high RPMs.)
Step 8: From the online thrust calculator, find the amount of power spent at double the RPM at which hover thrust was generated (in our example, that RPM would be 4000×2 =8000 RPM). Divide the power by the average battery voltage. For example, if the required power is 275 Watts, then the current (Amps) requirement is 275/12 (12=average battery voltage) = 23 Amps. To account for heat and all that, go for a 30A E.S.C. (30% margin of error). BTW, I have a personal preference for SimonK E.S.C.’s because they have very fast and consistent response.
Step 9: You’re almost done. This step finds the optimistic flight time you would get. The optimistic flight time is (Avg Voltage x Ah rating x 60)/(power consumed at hover thrust x 4). So in our example, that would be (12 x 2.2 x 60)/(17.4 x 4) = 23 minutes (this is wayy more than what you would actually get).
DERIVATIONS (AND EXPLANATIONS):
Okay, so for those of you who watched the video I uploaded
(link – https://youtu.be/D5vC0Eu9LyE ), here are the derivations and extra explanations I promised :
Yes, I am aware that I said double the RPM’s. I chose 10000 RPMS because that is the absolute maximum that I happen to know(from experience) that the motors can handle under this load.