<p>To fly above ground effect, you need thrust equal to the weight of the aircraft (or slightly more, as you want to climb).</p>

<p>Ground effect doesn't hold aircraft down, it just makes it easier to fly when they are close to the ground. The exact amount varies quite widely depending on exactly how close the wing/rotor is to the ground. For a plane or helicopter it may only be 10% extra lift, but a vehicle designed to maximise ground effect (like a hovercraft) can fly with vastly less power than would be required to fly without ground effect.</p>

<p>Ground effect is particularly noticeable if you hover a model helicopter inverted over a smooth surface. As it descends towards the floor, it looks as if it lands in a big soft cushion - decelerating and bobbing around at about half to a third of rotor diameter. You can hover without the constant adjustments in collective pitch that are normally required, and need to drastically reduce thrust to go lower. When you're half an inch or less off the floor, it's probably only using 10-20% of the pitch required for normal hovering, and there's a self-levelling effect so it's much easier than it looks!
For a typical FPV quadcopter with 5" props on top of the motors, you almost can't feel ground effect if you put the battery under the frame. With the battery on top you can feel a noticeable cushion and you can find a throttle setting that holds the quad off the ground but won't climb out of ground effect. I'd estimate that it's flying at about 80-90% of normal power. </p>

<p>While the power may be reduced, it can be harder to hold a multi-rotor or helicopter in place when in ground effect. The bubble of higher pressure tries to escape out from underneath the aircraft, pushing it in the opposite direction.  </p>

<p>A general rule of thumb is that the reduction in power due to ground effect starts to be noticeable when the altitude is less than around half or a third of the wing span or rotor diameter. The 'air bubble' effect on position holding while hovering is noticeable one or two diameters above the ground. Both tail off gradually so there's no hard limit.</p>

<p>See also:<br>
* <a href="https://aviation.stackexchange.com/questions/311/what-is-ground-effect">What is ground effect?</a><br>
* <a href="https://aviation.stackexchange.com/questions/43026/how-high-does-the-ground-effect-last">How high does the ground effect last?</a><br>
* <a href="https://aviation.stackexchange.com/questions/70731/does-ground-effect-scale-11-with-wingpsan">Does ground effect scale 1:1 with wingspan?</a><br>
and lots more, over at the Aviation stack-exchange.</p>

<p>Also this excellent academic paper:
<a href="https://www.hindawi.com/journals/ijae/2017/1823056/" rel="nofollow noreferrer">Characterization of the Aerodynamic Ground Effect and Its Influence in Multirotor Control</a></p>

CC BY-SA 4.0
2020-05-19T18:38:22.377Z
Given your description that it takes less power to fly in ground effect, how much less energy (thrust) is needed to hover in ground effect than it takes to hover in free air?
HansenJC001
CC BY-SA 4.0
2020-05-19T20:30:28.927Z
@HansenJC001, that depends on how close the propellers get to the ground.
Mark
CC BY-SA 4.0
2020-05-20T13:21:19.807Z
@Robin, since this is a question which is more aligned with SE.Aviation, perhaps your answer could reference their answers as well?
Kenn Sebesta
CC BY-SA 4.0
2020-05-20T13:22:13.537Z
@HansenJC001 It's specifically to do with wing diameter. A spinning propeller is just a rotating wing in a different frame of reference. So the same rules apply. And IIRC, ground effect is generally defined as the boundary layer within 1/3 of the wingspan.
Kenn Sebesta
CC BY-SA 4.0
2020-05-21T08:04:12.937Z
@KennSebesta - good idea, I've added a few that seem relevant. There are a lot!
Robin Bennett
CC BY-SA 4.0
2020-05-24T03:15:36.113Z
I appreciate your answer. However, the post related to ground effect scaling 1 to 1 with wingpspan brings me to ask: "Is the effect valid in exactly the same way for multirotors and wings on an airplane? Multirotors produce lift by directing thrust from propellers in a chosen direction. That thrust is quite turbulent and may be very different than lift of a slow moving wing.
HansenJC001
CC BY-SA 4.0
2020-05-24T03:18:23.623Z
So, I fine-tuned my question to this: "Has anyone validated the vertical distance for the ground effect of multirotors?" and I was not expecting Google to produce such a clean answer, there it is: Characterization of the Aerodynamic Ground Effect and Its Influence in Multirotor Control. 
My initial curiosity was about power differences between ground effect and hovering outside of ground effect. This article identifies a complex environment of ground effect for multirotors that is larger in scale than for fixed wings and also larger than it is for a single rotor such as a helicopter.
HansenJC001
CC BY-SA 4.0
2020-05-26T09:03:37.940Z
@HansenJC001 - thanks, I've added a link to the paper, and modififed the answer slightly. I tried embedding one of the graphs, but SE doesn't like SVGZ files.
Robin Bennett
CC BY-SA 4.0
2020-05-19T10:58:16.710Z
2020-05-26T09:02:57.873Z
2020-05-26T09:02:57.873Z
Robin Bennett
<p>Assuming either a quadcopter or a hexacopter, is there any published data on how much more energy is expended to escape the ground effect? </p>

<p>Is it a function of the thrust being produced to lift the multirotor? And is it different for a quadcopter than for a hexacopter? Is the requirement lower when the props are higher off the ground?</p>

<multirotor><aerodynamics><thrust><stability>
How do thrust requirements change as a multirotor leaves ground effect?
HansenJC001
# How do thrust requirements change as a multirotor leaves ground effect?

(Asked by: username_0 on 2020-05-18)

Assuming either a quadcopter or a hexacopter, is there any published data on how much more energy is expended to escape the ground effect?

Is it a function of the thrust being produced to lift the multirotor? And is it different for a quadcopter than for a hexacopter? Is the requirement lower when the props are higher off the ground?

**Tags:** multirotor, aerodynamics, thrust, stability

---

## 1 answers

### Answer by username_1 on 2020-05-19. Score: 3 | Accepted answer

To fly above ground effect, you need thrust equal to the weight of the aircraft (or slightly more, as you want to climb).

Ground effect doesn't hold aircraft down, it just makes it easier to fly when they are close to the ground. The exact amount varies quite widely depending on exactly how close the wing/rotor is to the ground. For a plane or helicopter it may only be 10% extra lift, but a vehicle designed to maximise ground effect (like a hovercraft) can fly with vastly less power than would be required to fly without ground effect.

Ground effect is particularly noticeable if you hover a model helicopter inverted over a smooth surface. As it descends towards the floor, it looks as if it lands in a big soft cushion - decelerating and bobbing around at about half to a third of rotor diameter. You can hover without the constant adjustments in collective pitch that are normally required, and need to drastically reduce thrust to go lower. When you're half an inch or less off the floor, it's probably only using 10-20% of the pitch required for normal hovering, and there's a self-levelling effect so it's much easier than it looks!
For a typical FPV quadcopter with 5" props on top of the motors, you almost can't feel ground effect if you put the battery under the frame. With the battery on top you can feel a noticeable cushion and you can find a throttle setting that holds the quad off the ground but won't climb out of ground effect. I'd estimate that it's flying at about 80-90% of normal power.

While the power may be reduced, it can be harder to hold a multi-rotor or helicopter in place when in ground effect. The bubble of higher pressure tries to escape out from underneath the aircraft, pushing it in the opposite direction.

A general rule of thumb is that the reduction in power due to ground effect starts to be noticeable when the altitude is less than around half or a third of the wing span or rotor diameter. The 'air bubble' effect on position holding while hovering is noticeable one or two diameters above the ground. Both tail off gradually so there's no hard limit.

See also:  
\* [What is ground effect?](https://aviation.stackexchange.com/questions/311/what-is-ground-effect)  
\* [How high does the ground effect last?](https://aviation.stackexchange.com/questions/43026/how-high-does-the-ground-effect-last)  
\* [Does ground effect scale 1:1 with wingspan?](https://aviation.stackexchange.com/questions/70731/does-ground-effect-scale-11-with-wingpsan)  
and lots more, over at the Aviation stack-exchange.

Also this excellent academic paper:
[Characterization of the Aerodynamic Ground Effect and Its Influence in Multirotor Control](https://www.hindawi.com/journals/ijae/2017/1823056/)

> **Comments:**
> **username_0**: Given your description that it takes less power to fly in ground effect, how much less energy (thrust) is needed to hover in ground effect than it takes to hover in free air?  
> **username_2**: @HansenJC001, that depends on how close the propellers get to the ground.  
> **username_3**: @Robin, since this is a question which is more aligned with SE.Aviation, perhaps your answer could reference their answers as well?  
> **username_3**: @HansenJC001 It's specifically to do with wing diameter. A spinning propeller is just a rotating wing in a different frame of reference. So the same rules apply. And IIRC, ground effect is generally defined as the boundary layer within 1/3 of the wingspan.  
> **username_1**: @KennSebesta - good idea, I've added a few that seem relevant. There are a lot!  
> **username_0**: I appreciate your answer. However, the post related to ground effect scaling 1 to 1 with wingpspan brings me to ask: "Is the effect valid in exactly the same way for multirotors and wings on an airplane? Multirotors produce lift by directing thrust from propellers in a chosen direction. That thrust is quite turbulent and may be very different than lift of a slow moving wing.  
> **username_0**: So, I fine-tuned my question to this: "Has anyone validated the vertical distance for the ground effect of multirotors?" and I was not expecting Google to produce such a clean answer, there it is: Characterization of the Aerodynamic Ground Effect and Its Influence in Multirotor Control.  My initial curiosity was about power differences between ground effect and hovering outside of ground effect. This article identifies a complex environment of ground effect for multirotors that is larger in scale than for fixed wings and also larger than it is for a single rotor such as a helicopter.  
> **username_1**: @HansenJC001 - thanks, I've added a link to the paper, and modififed the answer slightly. I tried embedding one of the graphs, but SE doesn't like SVGZ files.  

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