FPV Drone Motors: How to Choose the Right Size, Shape, and KV

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FPV Drone Motors: How to Choose the Right Size, Shape, and KV

In this blog post, we are going deep on understanding motors for mini quads. I’ll be sharing my insights on how to select the optimal motor for your build and flying style. 

Let’s dive in.

How Motors Generate Torque

First, let’s understand how brushless motors work to generate mechanical torque.

The torque is produced by the magnetic interaction between the rotor’s permanent magnets and the stator’s electromagnets. This interaction is influenced by:

  • The intensity of the magnetic field (magnetic flux density)
  • The contact area between the rotor and stator.

The greater the magnetic flux density and the larger the area of interaction, the higher the force.

How motors generate torque
FPV Drone Motors: How to Choose the Right Size, Shape, and KV 4

This force creates a turning effect (torque) on the motor shaft. The torque is proportional to the force and the radius of the motor.

So, the key factors determining torque are:

  • Magnetic flux density
  • Stator volume (which determines the area of interaction)
  • Motor radius

Stator Volume is King for Torque

Now, let’s dig deeper into stator volume.

The stator area that interacts with the rotor is approximately the cylinder surface area around the sides. It does not include the end caps.

So the interacting area is:

Area = 2πrh

Where r is the motor radius, and h is the motor height.

Substituting this into the torque equation gives:

Torque ∝ (Magnetic Flux Density)2 x (Stator Volume) / (Gap Permeability)

For a given magnetic flux density, the torque depends only on the stator volume!

The stator volume depends on the radius and height of the motor. A taller motor with the same radius will have greater volume and torque.

So, maximizing stator volume is key to maximizing torque for a brushless motor.

How Motor Shape Affects Torque and Responsiveness

What about the shape of the motor? Does a wider pancake motor produce more torque than a narrow one?

The short answer is no, not if their volumes are equal.

Torque depends only on stator volume, not width vs. height.

In fact, a narrower motor often has better torque and responsiveness due to lower moment of inertia.

Let’s unpack this further.

For equal volumes, a wider motor does NOT make more torque than a narrow one. A wider motor only helps if it increases total volume.

Additionally, a wider motor has higher rotational inertia than a narrow one of equal volume. This is due to the r2 term in the inertia calculation:

Inertia ∝ r2 * Volume

So, a short, wide motor bell has much higher inertia than a narrow, tall one, even if their volumes are equal.

High inertia hurts motor responsiveness, which is critical for agile flight. Responsiveness depends on the motor’s torque reserve (excess torque available) divided by its rotational inertia.

Therefore, a narrower motor optimized for volume will be more responsive, especially for freestyle builds.

However, cooling is also a concern with taller motors. A width-to-height ratio under 3:1 can lead to overheating issues in freestyle setups.

My rule of thumb:

  • Freestyle: Optimal width-to-height ratio around 3:1
  • Race/Cinewhoops: Up to 4:1 for better cooling under load
  • Avoid going over 6:1, as responsiveness suffers greatly.

Prioritize torque through stator volume, not width. But don’t go too narrow to compromise cooling.

How to Choose the Right Size Motor for Your Build

Next, let’s discuss how to choose the right size motor for your prop and build.

As a starting point, larger prop diameters need higher stator volumes for optimal efficiency and control.

But it depends on other factors too:

  • Prop pitch – Higher pitch props need larger motors
  • Prop weight – Heavier props need larger motors
  • Desired performance – More power-hungry builds need larger motors

Here’s a handy chart estimating motor volumes for various prop sizes:

Motor volume vs prop diameter
FPV Drone Motors: How to Choose the Right Size, Shape, and KV 5

You’ll want to choose a motor towards the top end of the range for high-pitch/high-load props. And towards the lower end for lightweight, low-pitch props.

Don’t just go by diameter – consider the whole prop design.

Understanding Motor KV and How it Affects Performance

KV is a measure of a motor’s RPM per volt applied.

Higher KV generally equals higher RPMs, all else being equal.

But KV also affects:

  • Torque vs RPM relationship – High KV maintains torque better at higher RPMs
  • Current draw and efficiency – High KV requires more current for a given torque, hurting efficiency

Let’s look at both of these in more detail:

Torque vs. RPM Relationship

Higher KV motors maintain their torque output better as RPM increases. As seen in this chart:

Torque vs RPM for a given throttle position
FPV Drone Motors: How to Choose the Right Size, Shape, and KV 6

So, high KV motors have an advantage in peak power and speed. But…

Current Draw and Efficiency

High KV motors require significantly more current to produce the same torque as low KV ones.

The increased current causes more electrical losses due to:

  • Battery internal resistance
  • ESC resistance
  • Motor winding resistance

Therefore, high KV motors have lower efficiency (thrust per watt) than low KV ones.

This also limits the max torque – at very high current draws, battery voltage sag reduces power to the motors.

So, KV affects efficiency and maximum torque due to current draw issues.

Choosing KV – Two Methods

How do we choose the optimal KV for a build? Here are two approaches:

For freestyle/technical tracks:

  • Look at static thrust data for your prop
  • Find current draw at max throttle
  • Choose KV where your battery can supply the needed current

For top speed/open tracks:

  • Test sustained full throttle current draw with different KVs
  • Increase KV until diminishing returns on current draw due to voltage sag
  • Make sure your battery can supply the needed current

Also consider:

  • Lower KV is better for larger props and heavy quads
  • Higher KV is better for lighter, high-speed quads

Match KV to your build and props for the power system to work optimally together.

Achieving good throttle feel and resolution

The final piece of the motor puzzle is tuning for a good throttle feel and resolution.

Symptoms of poor resolution:

  • Difficult to maintain a steady hover
  • Hard to make precise small throttle adjustments

If you’ve optimized components (right props, KV, etc.), your system may simply have more power than you’re used to.

In that case, use throttle curves in Betaflight to constrain the range and improve resolution:

  • Throttle limits – Reduce max throttle percent
  • Expo – Increase resolution around the center stick

Think of it like PID tuning vs. rates. PIDs make the quad match your input, and rates determine the input curve.

For throttle response:

  • Motor, prop, and KV choices affect the power
  • Throttle curves affect input sensitivity

So get the right hardware, then tune the curves for your preferred feel!

Final Words

I hope this guide gives you the insights to pick the right motors and tune them for peak performance and flight feel! Let me know in the comments if you have any other motor questions.

Happy flying!

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Written By Kristen Ward

My name is Kristen R. Ward. I’m an adventure Filmmaker and I run a production company based out of New York. FPV drones are integral to my business. I'll be teaching you everything I've learned over the years creating videos for clients.

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