<p><strong>It depends on what kind of stability you're after.</strong> Nicolas Petit <a href="http://cas.ensmp.fr/~petit/papers/ecc09/pjb.pdf" rel="nofollow noreferrer">wrote a paper</a> on this several years ago, the graphics below are taken from Pages 4 and 5.</p>

<p>The summary is that <strong>a higher CG is better for x-y position stability, a lower CG is better for speed stability</strong>.</p>

<h1>Background</h1>

<p>Before we begin, let's assume we're only talking about a drone flying purely forward and so all dynamics are related to pitch.[*]</p>

<p>Second, let's define the CG, center of gravity, relative to the aerodynamic center (similarly to in <a href="https://aviation.stackexchange.com/questions/19388/what-is-the-difference-between-centre-of-pressure-aerodynamic-centre-and-neutra">https://aviation.stackexchange.com/questions/19388/what-is-the-difference-between-centre-of-pressure-aerodynamic-centre-and-neutra</a>). The aerodynamic center is the idealized point where all the drag forces are acting around it evenly. In other words, it can be thought of as the spot where half the drag rotates the quadcopter into a pitch up orientation, and half rotates it into a pitch down orientation.</p>

<p>Here's the relevant image. It shows forward flight, and that's the same as position-holding flight with a backwards wind.[**] The red arrow is for when the CG is above the aerodynamic center, and the blue arrow is for when it's below the aerodynamic center.</p>

<p><a href="https://i.sstatic.net/tSJni.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/tSJni.png" alt="enter image description here"></a></p>

<p><a href="https://i.sstatic.net/n6g8f.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/n6g8f.png" alt="enter image description here"></a></p>

<h2>Top-heavy CG: increases position stability, decreases velocity stability</h2>

<p>When the CG is above the the aerodynamic center, the drone tends to tilt <em>forward</em> when there's a gust of wind. This ensures positional stability, because as the drone is pushed backwards it naturally leans forward which reduces the speed at which it accelerates backwards. </p>

<p>However, since the CG is above the aerodynamic center, when flying forward the drone wants to trip over its AC and pitch even more forward. This means that forward velocity is somewhat underdamped, results in a less positive-- or even negative-- feedback cycle.</p>

<h2>Bottom-heavy CG: decreases position stability, increases velocity stability</h2>

<p>In this case, the CG is below the aerodynamic center. When there's a sudden gust which destabilizes the drone, the drone tilts away from its holding position, which combined with the gust's force, accentuates the positional error.</p>

<p>But on the other when flying steadily forward the drone tends to want to pitch back toward the vertical, instead of tripping over its own feet. This means that forward velocity is well damped without a positive feedback cycle.</p>

<hr>

<p>[*] Roll functions exactly the same way, but it's distracting to write "pitch/roll" each time we're talking about a rotation.
[**] This is a basic tenet of aerodynamics and fluid flow, which is that it's indistinguishable if the object is moving through the fluid or the fluid is moving around the object.</p>

CC BY-SA 4.0
2020-05-14T08:17:39.427Z
While this is true, I think it's overestimating a minor effect because most drones are fairly dense - there's no large, light surfaces to catch gusts of wind. If you switch between battery-on-top and battery-underneath, you move both the CG and CoP. The question asks about CG vs motor height, which is irrelevant.
Robin Bennett
CC BY-SA 4.0
2020-05-14T09:53:39.307Z
*".. a basic **tenet** of aerodynamics .."*.
Nij
CC BY-SA 4.0
2020-05-14T20:51:40.140Z
>since the CG is above the aerodynamic center, gravity now tends to pull the drone increasingly out of vertical alignment.
uhmmmm.. Isn't that _literally_ the pendulum rocket fallacy?

Sure, contrary to a rocket, there _are_ aerodynamic forces, but unless the drone is flying bottom-first, they shouldn't be misaligning it more with a higher-up CG vs. a lower CG. It's probably more proper to say that the CG determines the drone's _preferred orientation_ relative to its direction of flight (or vice-versa, the preferred direction of travel relative to its own frame of reference).
FlashCactus
CC BY-SA 4.0
2020-05-15T00:25:27.683Z
+1 for catching a fallacy. I wasn't paying attention, the answer has now been fixed. The issue is not rotational stability, it's velocity stability.
Kenn Sebesta
CC BY-SA 4.0
2020-05-13T13:44:09.910Z
2020-05-16T15:34:31.620Z
2020-05-16T15:34:31.620Z
Kenn Sebesta
<p>As far as I know, it doesn’t make too much of a difference in regards to flight stability because of something called the <a href="https://www.dronependulumfallacy.com" rel="noreferrer">Drone Pendulum Fallacy</a>, which describes how the drone rotates about the centre of mass, not from the point at which the thrust is created as illustrated below in the photo from the Drone Pendulum Fallacy website:
<a href="https://i.sstatic.net/lESX4.png" rel="noreferrer"><img src="https://i.sstatic.net/lESX4.png" alt="Diagram off centre of rotation"></a></p>

<p>In my experience, it may affect handling - many racing pilots prefer bottom mounted whilst many freestyle pilots prefer top mounted, however this is down to the way the quad handles and so is really personal preference.</p>

<p>There are other factors you may want to consider - for example a top mounted battery brings the centre of mass closer to the thrust line, and it can also balance out the impact of having a GoPro on your quad. A top mounted battery also means you don’t have to land on your battery so takes away that risk.</p>

<p>However, a top mounted battery requires more space than a bottom mounted one (assuming you also want to use a GoPro).</p>

<p>Oscar Liang talks about this on their <a href="https://oscarliang.com/mount-battery-mini-quad-bottom-top/" rel="noreferrer">website</a>, so I recommend taking a look there to help make up your decision.</p>

CC BY-SA 4.0
2020-05-13T22:32:36.003Z
The drone pendulum fallacy only applies to internal stability -- in an environment without external forces, the location of the center of gravity relative to the center of thrust doesn't matter, so long as the thrust axis passes through the CG.  Once you start introducing external forces (eg. wind gusts), it indirectly does.  True, the center of thrust itself doesn't matter, but since the propellers and their support structure is the main source of drag for most copters, it's a reasonable approximation to the actual aerodynamics involved.
Mark
CC BY-SA 4.0
2020-05-14T13:53:57.470Z
I always see people talk about bringing the center of mass closer to the thrust line. I don't see why that's important in terms of physics. If the moment of inertia is the same there should be no difference in angular acceleration whether the props are 10ft below the CG or directly in plane (assuming the props are at the same radius from the CG). It seems like what's happening is that the moment of inertia is being reduced, but the performance gain is attributed to CG near prop line.
Luca Scheuer
CC BY-SA 4.0
2020-05-14T21:03:59.663Z
@LucaScheuer Maybe they're talking about _horizontal_ distribution of mass? The actual thrust line (as in, the line on which both the cumulative thrust vector and the point of its application lie) is always perpendicular to the propellers (if we're talking about a regular "flat" multicopter), so the only way to move the CG closer to it is horizontally. This isn't _too_ important, since the line itself is constantly moved around by the props spinning up and down - that's how the quad rotates itself - but having the CoG dead center means the props will spin evenly to maintain level flight.
FlashCactus
CC BY-SA 4.0
2020-05-15T00:27:16.060Z
@LucaScheuer moment of inertia goes up with the cube of distance. So bringing mass the CG closer to the plane reduces the inertia of the motors-- some of the heaviest parts of the vehicle-- cubically.
Kenn Sebesta
CC BY-SA 4.0
2020-05-15T01:20:43.773Z
@KennSebesta Looking at the lists of common moments of inertia's on wikipedia, it goes up with the square, not the cube I think:
https://en.wikipedia.org/wiki/List_of_moments_of_inertia
Bringing all of the mass closer together would reduce the moment of inertia, but there's nothing saying that the center of mass or the moment of inertia is lined up with the prop plane. I agree that the motors are heavy and are a big part of the copter's moment of inertia, but that doesn't mean that with a given set of components the lowest moment of inertia is achieved by putting the CG in the prop plane.
Luca Scheuer
CC BY-SA 4.0
2020-05-15T05:21:09.833Z
Yes, you're right! I was thinking of a area moment of inertia, not mass moment of inertia. You're also right that bringing the CG to the prop plane is not *de facto* the lowest moment of inertia. But it's pretty damned close, and if you're taking about having a low CG and moving it lower, then dollars to donuts gets you that you're increasing the (mass) moment of inertia.
Kenn Sebesta
CC BY-SA 4.0
2020-05-13T13:11:49.363Z
2020-05-13T13:11:49.363Z
Drones and Whatnot
<p>Does it matter where the battery is mounted? Or is it best to have the centre of gravity aligned with the propeller plane where the thrust is generated?</p>

<multirotor><batteries><design><drone-frame>
Is it better to have a bottom heavy quadcopter or a top heavy quadcopter when it comes to a quadcopter stability?
Diamondx
# Is it better to have a bottom heavy quadcopter or a top heavy quadcopter when it comes to a quadcopter stability?

(Asked by: username_0 on 2020-05-13)

Does it matter where the battery is mounted? Or is it best to have the centre of gravity aligned with the propeller plane where the thrust is generated?

**Tags:** multirotor, batteries, design, drone-frame


---

## 2 answers

### Answer by username_1 on 2020-05-13. Score: 10

**It depends on what kind of stability you're after.** Nicolas Petit [wrote a paper](http://cas.ensmp.fr/~petit/papers/ecc09/pjb.pdf) on this several years ago, the graphics below are taken from Pages 4 and 5.

The summary is that **a higher CG is better for x-y position stability, a lower CG is better for speed stability**.

# Background

Before we begin, let's assume we're only talking about a drone flying purely forward and so all dynamics are related to pitch.[\*]

Second, let's define the CG, center of gravity, relative to the aerodynamic center (similarly to in <https://aviation.stackexchange.com/questions/19388/what-is-the-difference-between-centre-of-pressure-aerodynamic-centre-and-neutra>). The aerodynamic center is the idealized point where all the drag forces are acting around it evenly. In other words, it can be thought of as the spot where half the drag rotates the quadcopter into a pitch up orientation, and half rotates it into a pitch down orientation.

Here's the relevant image. It shows forward flight, and that's the same as position-holding flight with a backwards wind.[\*\*] The red arrow is for when the CG is above the aerodynamic center, and the blue arrow is for when it's below the aerodynamic center.

[![enter image description here](https://i.sstatic.net/tSJni.png)](https://i.sstatic.net/tSJni.png)

[![enter image description here](https://i.sstatic.net/n6g8f.png)](https://i.sstatic.net/n6g8f.png)

## Top-heavy CG: increases position stability, decreases velocity stability

When the CG is above the the aerodynamic center, the drone tends to tilt *forward* when there's a gust of wind. This ensures positional stability, because as the drone is pushed backwards it naturally leans forward which reduces the speed at which it accelerates backwards.

However, since the CG is above the aerodynamic center, when flying forward the drone wants to trip over its AC and pitch even more forward. This means that forward velocity is somewhat underdamped, results in a less positive-- or even negative-- feedback cycle.

## Bottom-heavy CG: decreases position stability, increases velocity stability

In this case, the CG is below the aerodynamic center. When there's a sudden gust which destabilizes the drone, the drone tilts away from its holding position, which combined with the gust's force, accentuates the positional error.

But on the other when flying steadily forward the drone tends to want to pitch back toward the vertical, instead of tripping over its own feet. This means that forward velocity is well damped without a positive feedback cycle.

---

[\*] Roll functions exactly the same way, but it's distracting to write "pitch/roll" each time we're talking about a rotation.
[\*\*] This is a basic tenet of aerodynamics and fluid flow, which is that it's indistinguishable if the object is moving through the fluid or the fluid is moving around the object.

> **Comments:**
> **username_2**: While this is true, I think it's overestimating a minor effect because most username_5 are fairly dense - there's no large, light surfaces to catch gusts of wind. If you switch between battery-on-top and battery-underneath, you move both the CG and CoP. The question asks about CG vs motor height, which is irrelevant.  
> **username_3**: *".. a basic **tenet** of aerodynamics .."*.  
> **username_4**: >since the CG is above the aerodynamic center, gravity now tends to pull the drone increasingly out of vertical alignment. uhmmmm.. Isn't that _literally_ the pendulum rocket fallacy?  Sure, contrary to a rocket, there _are_ aerodynamic forces, but unless the drone is flying bottom-first, they shouldn't be misaligning it more with a higher-up CG vs. a lower CG. It's probably more proper to say that the CG determines the drone's _preferred orientation_ relative to its direction of flight (or vice-versa, the preferred direction of travel relative to its own frame of reference).  
> **username_1**: +1 for catching a fallacy. I wasn't paying attention, the answer has now been fixed. The issue is not rotational stability, it's velocity stability.  

---

### Answer by username_5 on 2020-05-13. Score: 5

As far as I know, it doesn’t make too much of a difference in regards to flight stability because of something called the [Drone Pendulum Fallacy](https://www.dronependulumfallacy.com), which describes how the drone rotates about the centre of mass, not from the point at which the thrust is created as illustrated below in the photo from the Drone Pendulum Fallacy website:
[![Diagram off centre of rotation](https://i.sstatic.net/lESX4.png)](https://i.sstatic.net/lESX4.png)

In my experience, it may affect handling - many racing pilots prefer bottom mounted whilst many freestyle pilots prefer top mounted, however this is down to the way the quad handles and so is really personal preference.

There are other factors you may want to consider - for example a top mounted battery brings the centre of mass closer to the thrust line, and it can also balance out the impact of having a GoPro on your quad. A top mounted battery also means you don’t have to land on your battery so takes away that risk.

However, a top mounted battery requires more space than a bottom mounted one (assuming you also want to use a GoPro).

Oscar Liang talks about this on their [website](https://oscarliang.com/mount-battery-mini-quad-bottom-top/), so I recommend taking a look there to help make up your decision.

> **Comments:**
> **username_6**: The drone pendulum fallacy only applies to internal stability -- in an environment without external forces, the location of the center of gravity relative to the center of thrust doesn't matter, so long as the thrust axis passes through the CG.  Once you start introducing external forces (eg. wind gusts), it indirectly does.  True, the center of thrust itself doesn't matter, but since the propellers and their support structure is the main source of drag for most copters, it's a reasonable approximation to the actual aerodynamics involved.  
> **username_7**: I always see people talk about bringing the center of mass closer to the thrust line. I don't see why that's important in terms of physics. If the moment of inertia is the same there should be no difference in angular acceleration whether the props are 10ft below the CG or directly in plane (assuming the props are at the same radius from the CG). It seems like what's happening is that the moment of inertia is being reduced, but the performance gain is attributed to CG near prop line.  
> **username_4**: @username_7 Maybe they're talking about _horizontal_ distribution of mass? The actual thrust line (as in, the line on which both the cumulative thrust vector and the point of its application lie) is always perpendicular to the propellers (if we're talking about a regular "flat" multicopter), so the only way to move the CG closer to it is horizontally. This isn't _too_ important, since the line itself is constantly moved around by the props spinning up and down - that's how the quad rotates itself - but having the CoG dead center means the props will spin evenly to maintain level flight.  
> **username_1**: @username_7 moment of inertia goes up with the cube of distance. So bringing mass the CG closer to the plane reduces the inertia of the motors-- some of the heaviest parts of the vehicle-- cubically.  
> **username_7**: @username_1 Looking at the lists of common moments of inertia's on wikipedia, it goes up with the square, not the cube I think: https://en.wikipedia.org/wiki/List_of_moments_of_inertia Bringing all of the mass closer together would reduce the moment of inertia, but there's nothing saying that the center of mass or the moment of inertia is lined up with the prop plane. I agree that the motors are heavy and are a big part of the copter's moment of inertia, but that doesn't mean that with a given set of components the lowest moment of inertia is achieved by putting the CG in the prop plane.  
> **username_1**: Yes, you're right! I was thinking of a area moment of inertia, not mass moment of inertia. You're also right that bringing the CG to the prop plane is not *de facto* the lowest moment of inertia. But it's pretty damned close, and if you're taking about having a low CG and moving it lower, then dollars to donuts gets you that you're increasing the (mass) moment of inertia.  

---

