One of the “great debates” in the miniquad world is what the best motor kV is for 5″ racing quadcopters. Generally people take one of two sides: those who champion 2300kV-2400kV motors and those who prefer 2600kV+ motors. Over the last year, I’ve settled into the latter camp, and I’m going to explain why in this article.

Excess Thrust

jet engine thrust

2600kV motors are like strapping a jet engine on your quadcopter. Image courtesy of NASA.gov

The fundamental reason I like higher-kV motors is because they are capable of providing more thrust at higher airspeeds. Having extra thrust at high speed is specifically important when flying acro – my personal favorite form of flying. Excess thrust is the difference between rushing at a tree at a 40 degree tilt and being able to hop over it into an epic gravity maneuver and having to pull up to a lower tilt to perform the same move – probably losing site of the landscape in the process.

Why do I need to go so fast in the first place? Because the basis for quadcopter acrobatic maneuvers is the mastery of the interplay between potential and kinetic energy. More speed means more kinetic energy, which in turn means you can “fling” your quadcopter further. You can go really fast in a line, cut the power and coast through a series of maneuvers as you fly through the air like a sophisticated FPV baseball. Even after hundreds of hours of flying, the thought of it still brings a smile to my face.

Calculating quadcopter thrust at speed

Reserve thrust doesn’t necessarily come at the cost of frying your LiPo batteries, either. The reason is that the faster you fly your quadcopter, the less thrust your props are able to impart to the airstream. This is covered in delicious detail in this gem of an article, and I recommend you check it out if you want to understand the math behind thrust number.

To understand why a motor that consumes 60 Amps on the bench only pulls a fraction of that at full speed, let’s do some math to get some ballpark ideas of how much actual work our motors are doing at top speed. The top speed of any aircraft is the speed where the horizontal drag force equals the horizontal thrust from the props.

If we can calculate the drag on a miniquad, we will also be able to calculate it’s horizontal thrust. If we also know the tilt angle, we can calculate the total thrust from all four motors.

Drag can be calculated by a fairly simple equation:
equation
Where:

Variable Description
D Drag in Newtons
Cd Coefficient of drag. For this rough estimate we'll assume our quadcopters are simple "plates" with a coefficient of ~1.2
A Plate area in meters squared.
r Fluid density of air. Air at sea level and 15C has a density of 1.2 so we'll use that even though the test was done at a higher altitude.
V Fluid velocity. which is equivalent to the airspeed of the quadcopter. I'm going to use the top speed of my Bolt 210 recorded with KingKong 5040 tri-blade props as measured in my RaceKraft 5051 review article. Since the tests were done on a relatively calm day we'll use the ground speed of 107.7KM/hr or 29.9m/s for the fluid velocity.
Computing the plate area of a quadcopter in flight

We have all the numbers needed for the equation but the plate area. This one is a little complicated. I started by taking a top down photo of my Bolt and using GIMP to calculate the amount of pixels in a 10.5″x13″ area of an image that are taken up by my quadcopter:
frame area

This calculation netted me the raw plate area of my Bolt 210: roughly 22 inches squared.

Compensating plate area for tilt

I don’t have any raw numbers, but on the day that I performed the speed tests, my camera was tilted at 40 degrees. With that tilt, a majority of the flight footage still contained the ground – this means the quadcopter was tilted at an angle greater than 40 degrees. I am going to make a rough assumption that my tilt angle was 50 degrees during the speed run. For the final result, this assumption does not matter, because effects of the tilt angle cancels out when calculating the total thrust. I’ll call this out later.
bolt plate area
This is a rough calculation – the area of a quadcopter is not L x W and a shrinkage in L will affect the plate area more than a shrinkage of W. The calculation will come pretty close though, as aside from the small body area my Bolt 210 is pretty symmetric in both axes. So here is the shrunken area, compensated for tilt:
As = 22 * .77 = 16.9 sq inches = .011 sq meters

Calculating thrust required at max speed

Alright, so flying at a 50 degree tilt, my quadcopter is exposing a cross-sectional area of .011 square meters to the airstream at a speed of 29.9 meters per second. How much drag is that?

equation
D = .5 * 1.2 * .011 * 1.2 * 29.9 ^ 2
D = 7.08N = 722 grams force

(De?) compensating for tilt (again)

Alright, we’re getting close. What I’ve shown is that at max speed, where horizontal drag is equal to horizontal force (Th), the drag on the quadcopter is 722 grams. So how much thrust are my 2300kV motors producing? This is requires some more trigonometry:
math writeup
So the total thrust being generated by the quadcopter is:

T = 722 / .77 = 943 grams.

Notice how the effect of the tilt got cancelled out, so not knowing the exact value doesn’t really matter.

70MPH on 950g of thrust

But wait! The motor/prop combination on this quadcopter produces 1143 grams of thrust apeice on the bench. The total thrust from all four 2300kV motors at top speed doesn’t even match the thrust of a single motor on the bench. All that thrust is lost because the props simply cannot do as much work on the airstream at high forward speeds. To fix this, you have two options:

  1. Upgrade your props to have more bite. The best 5″ props on the market that I know of are the RaceKraft 5051’s. These props enjoy a 20% increase in performance over less aggressive props. There is diminishing returns here though – I highly doubt a new prop will be able to be designed that beats the RaceKrafts as handily as they beat my poor KingKong tri-blades.
  2. Increase the maximum RPM’s your motor is capable of – aka increase the kV.

Flying with restraint

This is the reason why I like higher-kV motors. When I’m flying my quadcopter with very high camera tilt angles, I always feel like I have a little bit more “juice”.

Of course, with great power comes great responsibility. With the latest miniquad motors using N52SH magnets, power consumption levels can easily start getting ridiculous. If you’re a fan of jamming the throttle stick to 100% for epic climbouts, your batteries are going to puff, and quickly. Flying with hot 2600kV motors means flying with finnesse. The upper 30% or so of throttle is reserved only for flying at full speed, and even then only for short bursts. You must have an OSD and watch your battery voltage or current meter. With these tools and a little smarts, though, I think high kV is the way to go.

What about torque?

The most common argument for 2300kV motors I hear is that they have more torque. This isn’t necessarily true:

Even if it was, though, I would ask what 2300kV supporters think torque is good for in a miniquad? The only thing I can think of is control responsiveness: motor torque is the primary mechanism the flight controller uses to achieve yaw, for example. Thing is, no pilot I know maxes out their yaw rates. So what’s the point?

At one point last year, I had two quadcopters flying with the same brand of motors. One had RCX 2300kV motors, the other had RCX 2633kV motors. There was absolutely no discernible difference in responsiveness between the two. There was a huge difference in how the two quadcopters handled at high speed, though. Want to guess which one I flew more often?

 

This is our first Propwashed opinion article. We are expressing our opinions and the reasons behind them and make no claims at being right. You should take this article at face value. We’d love to hear your thoughts in the comments below.

 

 

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