# How to decide drone motors?

I'm planning to design a quadcopter with a payload capacity of 5 kg. I would need the thrust power from the motors to cater for at-least 7 kg with extra part weights.

I would like to know,

1. How do I decide the motor power? (I can do only one-off investment for that with my budget and no chance for trial and error.)
2. How do I decide the blade sizes? (for this I can spend little extra trying a few different blades.)
3. Deciding the battery. But when I know the motors, I guess I could do the maths to decide the battery, but any comments on that are most welcome.
• If your budget is so limited that you can't even buy new motors after a prototype, you'll have to buy a ready-made drone, in all honesty. Buying a ready-made drone will be cheaper than designing one from scratch – economics of scale. Anyway, questions 1 and 2 are mechanical engineering, not electrical engineering question.
– Marcus Müller
Sep 19 '20 at 13:55
• Find a dedicated drone building forum, not a general purpose engineering forum like this, or even a general purpose mech engineering forum. You need a bunch of hobbyists who build these things, so you can tap into their experience. They might even point you at a half-designed/completed kit of parts that you can customise, that will allow you to fool yourself that you're designing it, while tapping into the economies of scale and the design expertise of 100s of people before you.
– Neil_UK
Sep 19 '20 at 14:18
• yep, otherwise I can promise you that you'll have multiple iterations of your mechanical design, you'll crash your drone repeatedly before you get it into control, you'll buy the wrong things... you simply won't get far if you think "from scratch" and "no money for a second prototype" are compatible.
– Marcus Müller
Sep 19 '20 at 14:19
• Let me amplify what Marcus is saying. When you set out to build something from scratch, you will build the first to throw away. You'll modify it, fix it, learn from it, and by the time you know what you're doing, it will be so mashed and modified you'll want to build a second clean one, even if it's exactly the same design (which it won't be). That will happen, whether you plan for it or not. At least if you plan for it, you'll have the budget and time to cope with it.
– Neil_UK
Sep 19 '20 at 14:22
• Hi @Sency, how can we help? Has the above discussion changed what you're asking about?
– ifconfig
Sep 19 '20 at 20:05

This is an age old problem, there is no solution, only methodology.

First, your budget is a HUGE constraint. To build a drivetrain, without years of experience, is very difficult, I doubt you will nail it.

The approach you use to decide motor is critical, more on that as we progress.

Some things to consider FIRST:

• what are your power train expectations? 10+:1 power:weight is what I use and my drone get's the hell out of town, but I also cook batteries; able to set them on fire punching out if I choose.
• What is your desired type of flight, the spectrum goes from freestyle/race to sit-n-spin (hover and take photos).
• What flight time would you like (short or long?)

As I am sure you know already, pitch and KV are related. High KV leans lower pitch (faster spin with blades that push less air). Low KV leans higher pitch (low KV should have more torque and allow steep pitch to push more air).

High KV build tend to have shorter flights but are more maneuverable. Lower KV builds stay up longer and are amenable to cinematography (generally). There are tuning specific problems across the spectrum.

The characteristics of motors generally in XXYY configuration vary greatly. xx is the rotor diameter and yy is the motor/rotor height. Wider diameter motors have a different inertial footprint than narrow, carrying more weight to help keep the big props swinging. Narrow can stop/accelerate/decelerate faster, again inertia is a big part of the formula. Height of rotor is less impactful from my experience in the context of this discussion, generally a product of the diameter and desired KV (more height to wrap more wire, for example).

A 2000kv motor [example] ranges from 15xx to 25xx sizes, so KV is a hard indicator to use alone. KV is key for calculating the target max rotational speed in conjunction with your voltage. I have used the exact same KV motors in a small and large format and the difference was astonishing. The takeaway is that KV helps determine relative spin rate and is not helpful as a stand alone metric.

Keep in mind that rotational speed of the propellers is huge. Bigger slower moving props do not provide the same gyroscopic byproduct as a fast spinning motor, this may be something to consider. There are all sorts of RPM specific problems along this spectrum.

On to props. Generally props use a AABB format where AA is the diameter and BB is the pitch. Pitch, generally speaking, is how much "screw" the prop will move along the motor shaft axis per rotation. When teaching new pilots I explain pitch as "how far a prop will move in a block of jello spinning 360 degrees". So a steep prop will move far while a shallow prop will not move very far [in this mental exercise]. A good internal understanding of pitch takes a while.

Battery choice is difficult. Decision 1 is cell count (voltage). Decision 2 is mah (amount of energy stored) and generally an indicator of weight for the battery. Last is C rating (abuse factor). Mellow units can fly "well" on 25c, high performance need 75C+, my 5" cinema/chase drones need 95c+ (usually 120c).If you miss the C rating (too low) you will puff battery, decrease life dramatically, and generally be pretty upset with performance.

So to answer your question, there are a few ways to go, but first you will need to have budget to fail and try again, that is the nature of scratch builds, especially for someone lacking extensive power train building experience. Drone building is a constant repair cycle, that is why people who take this seriously travel with mirror+ builds, I usually carry 4+ units for a shoot or session.

The easy route is to buy a pre-built where payload and performance as well as demo videos are available.

The next route is to join a flying club or commercial group and check out other builds and get some local friends to guid the choice. This is my #1 recommendation to new pilots - let a friend help. Otherwise you will be flailing in places like here listening to a lot of armchair pilots pushing their agenda and favorite builds.

The next approach is to copy a build that fits your needs. The largest repository of builds is RotorBuilds (https://rotorbuilds.com/), nice filters, tons of videos, and possibly contact info to ask questions directly.

The last way to go is take a stab at it. That is what I did in 2013, I failed and essentially smoked a whole drone worth of parts before my first flight. I copied Charpu's build (he was #1 at the time) and it was a painful experience without any coaching. Granted times have changed, but the information on the web is mostly garbage.

It feels safe to assume that you wanted XXYY motor and AABB prop on target voltage. I can't deliver than information, I doubt anyone without intimate knowledge of your use case will be much help. Ergo, the approach and methodology to solving the problem becomes key.

DroneTrest has a good reference page (https://www.dronetrest.com/t/brushless-motors-how-they-work-and-what-the-numbers-mean/564) if you have not been here already.

If you want to dig in then seek out motor specification sheets. There are many sites dedicated to this (I have no favorite). Look at motor+volt+prop mah draw (avg/max) and heat if available. This information is great, but unfortunately it will not help a lot without drivetrain building experience.