Hello Everyone,

I need advice for a project I want to do, and I will start by apologizing for terminology mistakes and grammar, I will eventually get better.

The project is a spherical drone, 45 cm in diameter, in which the propellers are enclosed inside the sphere as much as possible. I am setting my expectations to the very minimum, I would just need it to hover for a very limited time with an extra load of 5 kg for the sphere and other components (conservative estimates).

I have seen some examples online that resemble a fancy version of what I want to do, like the Fleye, but before I start experimenting by myself I wanted to ask the community.

- I am well aware that any enclosed propeller needs an air intake and an outtake, I would just like to have as much of the sphere closed as possible.

- For the purpose of discussion, let's asume my budget is limitless (ha, I wish). 

1: If I were to enclose a quadcopter inside the sphere's equator, how would I calculate the optimal size for the air inlet and outlet?

2: Would two coaxial propellers enclosed in a vertical shaft inside the sphere give a better result than a multicopter approach?

3: Most importantly, would I be able to fit rotors powerful enough to lift the sphere inside the limited space within it?

I know these are a lot of questions and I can only hope they are not as stupid as I suspect, but any answer that can guide me through the feasibility of this project will be greatly apreciated. 

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Let's think out of the box, since you have a "limitless" budget.

If you want to keep the openings as small as possible then you need to increase the speed of the air.

In that case your best bet is a little jet engine or rocket engine with thrust vectoring. Was thinking to do something similar for a marketing gag, since the flight time is pretty short (fuel consumption).

F = dm/dt x V     dm/dt = A x roh x V

F = A x roh x V^2

That is actually a creative idea, it didn't even cross my mind before. However it comes with a set of limitations, mainly the availability of parts and technical advice.
On the other hand, it got me thinking about electric ducted fans and how their smaller diameter would help me (at least) to fit them inside the sphere.
The first ones I found are shown in the following link:

https://hobbyking.com/en_us/dr-mad-thrust-70mm-11-blade-alloy-edf-1...

According to my calculations, 6 of these should be able to lift 4.5 kg in addition to their own weight at 50% throttle, but how much weight would come from the batteries needed to power them for even a minute I don't know. If anyone could give an estimate it would be greatly appreciated.

Then again I found myself thinking about the fuel engine, and how much more energy dense liquid fuel is compared to LiPo, so I guess a gas powered ducted fan would be the way to go?

Maybe you should start a spread sheet with all the possibilities which come into your mind and all there limitations and dis/-advantages ans so on, if you haven't done it already.

That might help you for your calculations:

www.ecalc.ch

I would roughly estimate a flight time in the range of about 3 min with this configuration.

With six EDFs you built basically a multi rotor with EDFs. Keep in mind that you have to alling them in a way that you can create torque around the vertical axis since the EDFs crate usually very low net torque compared to propellers.

If you want to have small openings than the speed of the air has to go up to get the same thrust which also means lower efficiency. I you want to have at the same time long flight time than you need to look in deed towards high power draw high density storage.

If you decide to go with a gas powered ducted fan than let me know I have one right now in my test stand.

I think the idea of the hexacopter might work. I will start saving to buy the components (buying all the pieces might be in the range of 600 euros) and doing the calculations. I think I will go with electric ducted fans, 3 minutes flying time is enough for the show and as I will probably fly it indoors I won´t have to worry about fumes. I can worry later about how to make the openings as small as possible.

Anyway thank you very much for all the advice! Once I start my build I will post it in the appropriate forum. :)

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