<p>If you are asking this question here, like this, then I humbly suggest that you probably shouldn't be attempting this.</p>
<p>This is life or death stuff, and without years of professional aviation knowledge and experience the chances of something going horribly wrong are very real.</p>

CC BY-SA 4.0
2023-04-17T22:53:30.683Z
2023-04-17T22:53:30.683Z
strepto
<p>A helicopter needs the highest power in hover. As a first approximation we can use the <em><a href="https://en.m.wikipedia.org/wiki/Momentum_theory" rel="nofollow noreferrer">momentum theory</a></em> which estimates the thrust as:</p>
<p><span class="math-container">$ T = 0.5\sqrt[3]{2 \rho A P^2} $</span></p>
<p>Where:</p>
<ul>
<li><span class="math-container">$A$</span> is the rotor area;</li>
<li><span class="math-container">$\rho$</span> is the density;</li>
<li><span class="math-container">$P$</span> is the power needed to generate that thrust;</li>
<li>and the <span class="math-container">$0.5$</span> is just to take into account the simplifications introduced by the momentum theory.</li>
</ul>
<p>Obviously in hover thrust equals weight and therefore:</p>
<p><span class="math-container">$ A = \frac{(2T)³}{2 \rho P^2}$</span></p>
<p>If you are going to use a contra-rotating configuration then you have obviously to split the thrust between the two rotors but taking into account that the lower rotor will need some 15% more power the thrust being equal due to the fact that it works in the wake of the upper rotor.</p>

CC BY-SA 4.0
2023-04-17T08:03:34.463Z
If the pilot and machine weigh 100kg, that's 1000N of thrust required, which gives an area of 320 square meters, or a blade length of 10meters. 5kW is pretty low power for a helicopter. For comparison, the Mosquito single-person helicopter has 60-95hp engine options.
Robin Bennett
CC BY-SA 4.0
2023-04-14T13:12:36.507Z
2023-04-17T07:53:51.653Z
2023-04-17T07:53:51.653Z
sophit
<p>I am building a personal electric helicopter, it consists of a lightweight steel frame that attaches to the axle of the hub motor. If the motor outputs 155nm of torque @ 5kw, and a max rpm of 700. How much area will the rotors need to have to reach at least 200 pounds of thrust, I weigh 150 pounds plus 50 for the motor frame and battery.
Something like this but with a single 4 blade rotor attached to the hub motor.</p>
<p><a href="https://i.sstatic.net/3ycxS.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/3ycxS.jpg" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2023-04-10T19:37:03.003Z
I'd start looking at some similar design, like the Hiller ROE Rotorcycle, Gyrodyne RON Rotorcycle, Ka-56 and kit-helicopters
sophit
CC BY-SA 4.0
2023-04-13T08:42:56.630Z
Wait, a single 4-blade rotor? How are you going to compensate the torque?
Zeus
CC BY-SA 4.0
2023-04-14T03:51:35.173Z
I was thinking with a rather large tail rudder, it might be a bit twisty at the beginning but hopefully once it gets up to speed there will be enough side thrust to compensate for the torque
Troy E
CC BY-SA 4.0
2023-04-14T07:58:25.793Z
"Hopefully"? With such attitude I wouldn't advise to test it out yourself ;) On this device, once you're off the ground, it will be much easier to spin _you_ than the rotor.
Zeus
CC BY-SA 4.0
2023-04-14T08:34:21.747Z
It can be as large as you want, still in hoover it won't produce any force...
sophit
CC BY-SA 4.0
2023-04-14T15:06:52.610Z
Adjustments can be made to increase side thrust, a safety rope can be attached to the top end of the axle and of course I would be wearing a motorbike helmet and jacket.
Troy E
CC BY-SA 4.0
2023-04-14T15:11:40.497Z
I made some prototype props and they seem like they could work, each one is 36 inches long by 5.5 inches wide using an asymmetrical airfoil. The motor case is 8 inches wide so the overall diameter would be 80 inches, I am planning to have them fixed at about a 15 degree angle of attack.
Troy E
CC BY-SA 4.0
2023-04-14T23:06:52.170Z
"*I am planning to have them fixed*": how are you going to control it?
sophit
<thrust><helicopters>
Building a personal helicopter, I have a 5kw electric hub motor, how big will the rotors need to be?
Troy E
# Building a personal helicopter, I have a 5kw electric hub motor, how big will the rotors need to be?

(Asked by: username_0 on 2023-04-10)

I am building a personal electric helicopter, it consists of a lightweight steel frame that attaches to the axle of the hub motor. If the motor outputs 155nm of torque @ 5kw, and a max rpm of 700. How much area will the rotors need to have to reach at least 200 pounds of thrust, I weigh 150 pounds plus 50 for the motor frame and battery.
Something like this but with a single 4 blade rotor attached to the hub motor.

[![enter image description here](https://i.sstatic.net/3ycxS.jpg)](https://i.sstatic.net/3ycxS.jpg)

**Tags:** thrust, helicopters
> **Comments:**
> **username_1**: I'd start looking at some similar design, like the Hiller ROE Rotorcycle, Gyrodyne RON Rotorcycle, Ka-56 and kit-helicopters  
> **username_2**: Wait, a single 4-blade rotor? How are you going to compensate the torque?  
> **username_0**: I was thinking with a rather large tail rudder, it might be a bit twisty at the beginning but hopefully once it gets up to speed there will be enough side thrust to compensate for the torque  
> **username_2**: "Hopefully"? With such attitude I wouldn't advise to test it out yourself ;) On this device, once you're off the ground, it will be much easier to spin _you_ than the rotor.  
> **username_1**: It can be as large as you want, still in hoover it won't produce any force...  
> **username_0**: Adjustments can be made to increase side thrust, a safety rope can be attached to the top end of the axle and of course I would be wearing a motorbike helmet and jacket.  
> **username_0**: I made some prototype props and they seem like they could work, each one is 36 inches long by 5.5 inches wide using an asymmetrical airfoil. The motor case is 8 inches wide so the overall diameter would be 80 inches, I am planning to have them fixed at about a 15 degree angle of attack.  
> **username_1**: "*I am planning to have them fixed*": how are you going to control it?  

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## 2 answers

### Answer by username_3 on 2023-04-17. Score: 3

If you are asking this question here, like this, then I humbly suggest that you probably shouldn't be attempting this.

This is life or death stuff, and without years of professional aviation knowledge and experience the chances of something going horribly wrong are very real.


---

### Answer by username_1 on 2023-04-14. Score: 1

A helicopter needs the highest power in hover. As a first approximation we can use the *[momentum theory](https://en.m.wikipedia.org/wiki/Momentum_theory)* which estimates the thrust as:

$ T = 0.5\sqrt[3]{2 \rho A P^2} $

Where:

* $A$ is the rotor area;
* $\rho$ is the density;
* $P$ is the power needed to generate that thrust;
* and the $0.5$ is just to take into account the simplifications introduced by the momentum theory.

Obviously in hover thrust equals weight and therefore:

$ A = \frac{(2T)³}{2 \rho P^2}$

If you are going to use a contra-rotating configuration then you have obviously to split the thrust between the two rotors but taking into account that the lower rotor will need some 15% more power the thrust being equal due to the fact that it works in the wake of the upper rotor.

> **Comments:**
> **username_4**: If the pilot and machine weigh 100kg, that's 1000N of thrust required, which gives an area of 320 square meters, or a blade length of 10meters. 5kW is pretty low power for a helicopter. For comparison, the Mosquito single-person helicopter has 60-95hp engine options.  

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