<p>Ducts and shrouds reduce tip losses; the reason we don't see them more often is that a well designed propeller should not have large tip losses.</p>

<p>Tip losses are greatest when there's a large pressure differential between the upper and lower side of the blade. In other words, the blade is highly loaded.</p>

<p>This question explains why large, lightly loaded blades are more efficient than smaller, faster spinning blades:</p>

<p><a href="https://drones.stackexchange.com/questions/961/why-are-larger-propellers-generally-more-efficient-than-smaller-ones">Why are larger propellers generally more efficient than smaller ones?</a></p>

<p>Notice that none of the answers mention tip losses - they're not the reason the small, fast prop is inefficient, they're just an extra problem. Fixing that extra problem with a shroud still leave you with fundamentally inefficient system.</p>

<blockquote>
  <p>assuming that the shroud can be manufactured with a mass that is less that the incremental improvement in thrust, </p>
</blockquote>

<p>For a well designed prop, a shroud adds more weight that it saves - and it's usually better to use that weight to increase the diameter of the prop instead of adding a shroud.</p>

<blockquote>
  <p>significant benefit of having some built-in protection from horizontal impact with obstacles.</p>
</blockquote>

<p>To work well, a shroud needs to have very little clearance from the blades. That means that if you hit anything, the shroud will hit the blades. You might cause less damage to the thing you hit, but you're still going to crash.</p>

CC BY-SA 4.0
2020-05-04T21:45:51.743Z
I am sure we agree on most of these issue of design optimization and ultimately design compromises.
HansenJC001
CC BY-SA 4.0
2020-05-04T21:54:19.343Z
1) Any propeller, whether well designed or not, is never going to protect itself without a guard or shroud.
HansenJC001
CC BY-SA 4.0
2020-05-04T21:56:33.210Z
2) I need help understanding more about prop loading in a hovering configuration. How does this imact the comparison of an open prop vs a shrouded prop.
HansenJC001
CC BY-SA 4.0
2020-05-04T22:01:10.800Z
3) When you said: "This question explains why large, lightly loaded blades are more efficient...." Did you mean 'the original question' or were you referencing the facts that surround lightly loading a prop. Is there any way to load any prop less when expecting a given lifting capacity in hovering mode while not enlarging the area?
HansenJC001
CC BY-SA 4.0
2020-05-04T22:07:34.087Z
4) This question is not about absolute efficiency in spite of my belief that ultimately a shroud will produce greater net efficiency for the same prop diameter while factoring in the weight of the shroud. The original question is about the need to understand what impact the shroud has on torque.
HansenJC001
CC BY-SA 4.0
2020-05-04T22:14:55.990Z
5) For the available props that are 'well designed" is there published manufacturer's data that supports your statement that they are more efficient than any other props? I do not find claims of efficiency for applications involving hovering. I have looked and I am still looking. I will be looking for the most efficient prop for hovering that can be compared to the shrouded props that I will test. I am comitted to this research.
HansenJC001
CC BY-SA 4.0
2020-05-04T22:19:07.883Z
6) "For a well designed prop, a shroud adds more weight than it saves...." I am working on evaluating this question. But, it will be difficult or even impossible because the prop that is designed for a shroud is not the same prop. There may never be a well designed prop that can be properly shrouded because the tips are different for each application..
HansenJC001
CC BY-SA 4.0
2020-05-04T22:23:10.043Z
7) The premise that a shroud will serve its intended purpose fails when it is not strong enough to survive a collision without deforming and hitting the prop. A fragile shroud is not a case worth testing. No published shrouded test have ever been published that evaluated the Survivability of a shrouded prop. This must be tested.
HansenJC001
CC BY-SA 4.0
2020-05-04T22:27:32.393Z
8) And I am still interested in the question of the torque produced by a shrouded prop.
HansenJC001
CC BY-SA 4.0
2020-05-05T08:02:45.237Z
@HansenJC001 - there are lots of good questions there that would be worth posting separately. Torque is just prop drag when hovering, so a shroud may well reduce it.
Robin Bennett
CC BY-SA 4.0
2020-05-04T08:55:07.263Z
2020-05-04T08:55:07.263Z
Robin Bennett
<p>There are two possibilities, depending on how ducts increase efficiency, but the end result should be the same. It could reduce torque required to spin the motor at a given RPM and thrust, or it could increase the thrust per RPM of the propeller with a constant torque. In either case the torque required to spin the propeller for hover thrust goes down.</p>

<p>For the given hover thrust the yaw torque caused by propeller drag as described above would then go down. The yaw torque produced by the inertia of the propeller and motor changing speed would remain constant (assuming the moment of inertia of that system is the same).</p>

<p>I'm not sure what the ratio of yaw torque generated by propeller drag is to the yaw torque generated by accelerating the propeller, but the inertia based one would eventually saturate like a reaction wheel on a space craft does.</p>

<p>This means that your maximum yaw steady state angular acceleration would go down if weight of the aircraft was exactly equal. I'm not sure how often multirotors get to this point. For smaller movements my intuition says that it would come down to flight controller tuning to get similar yaw performance.</p>

CC BY-SA 4.0
2020-05-07T14:45:38.740Z
2020-05-07T15:30:48.687Z
2020-05-07T15:30:48.687Z
Luca Scheuer
<p>Based on the research presented here: "<a href="https://apps.dtic.mil/dtic/tr/fulltext/u2/a595716.pdf" rel="noreferrer">Hover and wind-tunnel testing of shrouded rotors for improved micro air vehicle design</a>" and other reading that translates similarly well-designed shroud and rotor combinations into more general layman's terms, I have found that some authors indicate that there is a performance increase of 40% to 60% in static thrust for such a configuration. One source has indicated a theoretical increase in static thrust as much as 80%.</p>

<p>If we focus our attention on a multirotor design that is intended as a craft whose principal purpose is for inspections of stationary objects such as buildings and stationary infrastructure, and it is not expected to fly fast nor fly far, then the primary benefit from the shrouded rotor is the improvement of static thrust. Flight in translation will be rare and incidental to maneuvering around the object being inspected.</p>

<p>And assuming that the shroud can be manufactured with a mass that is less that the incremental improvement in thrust, this design will serve its intended purpose with the significant benefit of having some built-in protection from horizontal impact with obstacles.</p>

<p>The need for maneuverability will be equal to most other multirotor applications intended for inspections and as such yaw will still need to be produced by controlling the speed of the propellers by the flight controller.</p>

<p>What impact will the shroud have on the production of yaw by the torque produced by the propellers (rotors)?</p>

CC BY-SA 4.0
2020-05-03T15:38:31.453Z
By shroud, do you mean something like a duct?
Drones and Whatnot
CC BY-SA 4.0
2020-05-03T15:48:20.547Z
Not a duct. A shroud is a very specific shape that extends below the prop and flairs out with a curved lip at the intake. Take a look at the link in the question. I see no drone builders working to achieving these rigorous details of the shroud, and I would like to attempt it and verify the theoretical improvement in efficiency.
HansenJC001
CC BY-SA 4.0
2020-05-03T16:09:20.237Z
[link] https://gyazo.com/7a00e1f5573ffcdcbad5a897794e6130
HansenJC001
CC BY-SA 4.0
2020-05-03T16:12:18.600Z
good to know, thanks.
Drones and Whatnot
<quadcopters><design><thrust>
What impact does a shroud have on the torque produced by the prop(s) of a multirotor that incorporates a properly designed shouded propeller?
HansenJC001
# What impact does a shroud have on the torque produced by the prop(s) of a multirotor that incorporates a properly designed shouded propeller?

(Asked by: username_0 on 2020-05-03)

Based on the research presented here: "[Hover and wind-tunnel testing of shrouded rotors for improved micro air vehicle design](https://apps.dtic.mil/dtic/tr/fulltext/u2/a595716.pdf)" and other reading that translates similarly well-designed shroud and rotor combinations into more general layman's terms, I have found that some authors indicate that there is a performance increase of 40% to 60% in static thrust for such a configuration. One source has indicated a theoretical increase in static thrust as much as 80%.

If we focus our attention on a multirotor design that is intended as a craft whose principal purpose is for inspections of stationary objects such as buildings and stationary infrastructure, and it is not expected to fly fast nor fly far, then the primary benefit from the shrouded rotor is the improvement of static thrust. Flight in translation will be rare and incidental to maneuvering around the object being inspected.

And assuming that the shroud can be manufactured with a mass that is less that the incremental improvement in thrust, this design will serve its intended purpose with the significant benefit of having some built-in protection from horizontal impact with obstacles.

The need for maneuverability will be equal to most other multirotor applications intended for inspections and as such yaw will still need to be produced by controlling the speed of the propellers by the flight controller.

What impact will the shroud have on the production of yaw by the torque produced by the propellers (rotors)?

**Tags:** quadcopters, design, thrust

> **Comments:**
> **username_1**: By shroud, do you mean something like a duct?  
> **username_0**: Not a duct. A shroud is a very specific shape that extends below the prop and flairs out with a curved lip at the intake. Take a look at the link in the question. I see no drone builders working to achieving these rigorous details of the shroud, and I would like to attempt it and verify the theoretical improvement in efficiency.  
> **username_0**: [link] https://gyazo.com/7a00e1f5573ffcdcbad5a897794e6130  
> **username_1**: good to know, thanks.  

---

## 2 answers

### Answer by username_2 on 2020-05-04. Score: 3

Ducts and shrouds reduce tip losses; the reason we don't see them more often is that a well designed propeller should not have large tip losses.

Tip losses are greatest when there's a large pressure differential between the upper and lower side of the blade. In other words, the blade is highly loaded.

This question explains why large, lightly loaded blades are more efficient than smaller, faster spinning blades:

[Why are larger propellers generally more efficient than smaller ones?](https://drones.stackexchange.com/questions/961/why-are-larger-propellers-generally-more-efficient-than-smaller-ones)

Notice that none of the answers mention tip losses - they're not the reason the small, fast prop is inefficient, they're just an extra problem. Fixing that extra problem with a shroud still leave you with fundamentally inefficient system.

> assuming that the shroud can be manufactured with a mass that is less that the incremental improvement in thrust,

For a well designed prop, a shroud adds more weight that it saves - and it's usually better to use that weight to increase the diameter of the prop instead of adding a shroud.

> significant benefit of having some built-in protection from horizontal impact with obstacles.

To work well, a shroud needs to have very little clearance from the blades. That means that if you hit anything, the shroud will hit the blades. You might cause less damage to the thing you hit, but you're still going to crash.

> **Comments:**
> **username_0**: I am sure we agree on most of these issue of design optimization and ultimately design compromises.  
> **username_0**: 1) Any propeller, whether well designed or not, is never going to protect itself without a guard or shroud.  
> **username_0**: 2) I need help understanding more about prop loading in a hovering configuration. How does this imact the comparison of an open prop vs a shrouded prop.  
> **username_0**: 3) When you said: "This question explains why large, lightly loaded blades are more efficient...." Did you mean 'the original question' or were you referencing the facts that surround lightly loading a prop. Is there any way to load any prop less when expecting a given lifting capacity in hovering mode while not enlarging the area?  
> **username_0**: 4) This question is not about absolute efficiency in spite of my belief that ultimately a shroud will produce greater net efficiency for the same prop diameter while factoring in the weight of the shroud. The original question is about the need to understand what impact the shroud has on torque.  
> **username_0**: 5) For the available props that are 'well designed" is there published manufacturer's data that supports your statement that they are more efficient than any other props? I do not find claims of efficiency for applications involving hovering. I have looked and I am still looking. I will be looking for the most efficient prop for hovering that can be compared to the shrouded props that I will test. I am comitted to this research.  
> **username_0**: 6) "For a well designed prop, a shroud adds more weight than it saves...." I am working on evaluating this question. But, it will be difficult or even impossible because the prop that is designed for a shroud is not the same prop. There may never be a well designed prop that can be properly shrouded because the tips are different for each application..  
> **username_0**: 7) The premise that a shroud will serve its intended purpose fails when it is not strong enough to survive a collision without deforming and hitting the prop. A fragile shroud is not a case worth testing. No published shrouded test have ever been published that evaluated the Survivability of a shrouded prop. This must be tested.  
> **username_0**: 8) And I am still interested in the question of the torque produced by a shrouded prop.  
> **username_2**: @username_0 - there are lots of good questions there that would be worth posting separately. Torque is just prop drag when hovering, so a shroud may well reduce it.  

---

### Answer by username_3 on 2020-05-07. Score: 3

There are two possibilities, depending on how ducts increase efficiency, but the end result should be the same. It could reduce torque required to spin the motor at a given RPM and thrust, or it could increase the thrust per RPM of the propeller with a constant torque. In either case the torque required to spin the propeller for hover thrust goes down.

For the given hover thrust the yaw torque caused by propeller drag as described above would then go down. The yaw torque produced by the inertia of the propeller and motor changing speed would remain constant (assuming the moment of inertia of that system is the same).

I'm not sure what the ratio of yaw torque generated by propeller drag is to the yaw torque generated by accelerating the propeller, but the inertia based one would eventually saturate like a reaction wheel on a space craft does.

This means that your maximum yaw steady state angular acceleration would go down if weight of the aircraft was exactly equal. I'm not sure how often multirotors get to this point. For smaller movements my intuition says that it would come down to flight controller tuning to get similar yaw performance.


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