Hello everyone! I’m glad to have found this forum as I am an engineering student at University of South Florida.

I had a question about batteries and motors, specifically what rechargeable batteries to buy for my specific need here. I am building a skateboard powered by 2 electric ducted fans (EDF’s), and I would like to get at least 30 min of use out of them. The EDF I have my eye on is entitled “90mm EDF Ducted Fan CCW System with 1900KV Motor 2-6S LIPO Full Alloy 12 Blades”.

Here’s the link: https://www.sanelky.com/90mm-edf-du...-for-rc-jet-turbo-plane-diy-gliding-parachute

So how do I know what batteries to get for this application? Also, should I use Lithium Polymer, Lithium Ion, etc? Remember, I’ll be powering 2 fans total.

Thanks!

 

The ducted fans are made for a little toy jet airplane, not for pushing a skateboard with you on it. The ducted fan has absolutely NO SPEC's so it is useless.

 

If we look at a well known ducted fan from a reputable supplier on a 6S LiPo pack it gives 3.8kg thrust. So 2 would give 7.6kg, or 76N. (use g=10).

Assume you & board, etc. weigh 100kg and coefficient of friction of wheels etc is .05 then drag is 100 * 10 *.05 = 50N, so 26N available for acceleration.

F = ma, a =F/m = 26/100 = 0.26m/s^2

Walking pace is approx 1.4m/s

V = at, so t = V/a = 1.4/.26 = 5.4s

And that's being generous... With a known good fan. And we've not looked at battery capacity or size yet (>100Ah for 30min)

Don't waste your money...

I would have expected my engineering students to have run the math first...

Unless your numbers are better?

 

Every DF model at out field uses LiPo's. There are a few much older fixed wing propeller planes that use NiCd. I have one of those oldies. I would not recommend NiCd's for your project.

 

Nicads wouldn't cut it! Those fans need 105A @ 22v each... Only a BIG 6S LiPo will come close and that's going to weigh ~18kg for a 30min run (105Ah)

 

I would never suggest NiCd's. No need to repeat.

Taking a different view of skate boarding, I assume it is more efficient than walking, just as bicycling is. Using data for walking, continuous output is about 3 W/kg = 300 W for 100 kg person.

That 300 W with a 6s pack (about 22.8 V @ 3.8 V/cell) would require 13 A. 30 minutes would require 6.5 Ah (6500 mAh), which is well within the range of a 6s2p pack. Obviously, such high-power fans are not needed, but the project is feasible.

As it is a skate board, I assume the motor will not be running continuously and DF's are quite efficient. So, I don't see it as unreasonable, particularly with a 70 kg or less person, such as an average height male college student .

Sources:
Health "guru:"
http://bodytransform.co/Blog/Power+output+during+exercise.html

How much can we generate?
For sustained periods – let’s say about an hour – here’s the athletic power output for different types of people:

The average fit person can produce about 3 watts/kg
top amateurs produce 5 watts/kg and
elite athletes achieve 6 watts/kg

University study for walking:
Original: https://blog.smu.edu/research/2010/...uation-is-first-to-calculate-cost-of-walking/
Table: https://jeb.biologists.org/content/jexbio/suppl/2010/11/15/213.23.3972.DC1/JEB048199TableS1.pdf

Children and adults 3.66 W/kg total (basal 1.13 W/kg)

 

DF aren't efficient... 2200W input for 3.8kg thrust?

 

Open propellers are efficient:
https://www.sciencedirect.com/topic...inRegister-,Propeller Efficiency,0.7lbs/hp-hr.

Propeller Efficiency. Propeller efficiencies are in the range of 82–92% while specific fuel consumption is in the range of 0.5–0.7lbs/hp-hr.

Ducted fans apparently have a narrower range of efficiency:
https://aviation.stackexchange.com/questions/36726/why-dont-drones-use-ducted-fans



Ducted fans are considered safer and more robust in collisions, which is why they are so popular in hobby drones, but not military large drones. For a motorized skate board, it may not be the best choice for propulsion. But if the options are an open propeller or DF, then I think it is.

@Irving
Some of your "facts" are not widely known. Please add citations to support them.

 

Which fact are you referring to?

 

The "No Spec's" ducted fan to be used might burn up if powered from 6s (25.2V when fully charged).
The battery might also burn up if the current from two fans is 210A.

A teen on my street has an electric skateboard but its motors are geared to the wheels. Another guy rides an electric skateboard that has only ONE WHEEL!

 

So if DF’s aren’t a very efficient option, what would be?

 

I would consider an "outrunner" DC motor with a coaster drive to one wheel. Choice of front or back would be up to you. In other words, something like a scooter without handle bars for steering.

 

An electronic speed controller is also needed. What is used as a brake?

 

It's a skate board. That's why they all wear helmets.

I thought about adding regenerative braking/recharging, but figured if it was so equipped, one would want to be able to disable it for coasting.

 

A friend of mine has a homebrew electric skateboard, though really its a handleless scooter. It has a 2-wheel trunnion at the front for lean-steering and a single wide 4" hub-motor wheel at the back. There's at hand-held thumb control which gives power-off-brake - I think thats home-brewed too. There's also, from memory, a heel-operated mechanical brake that acts on the rear tyre.

Or use these

 

Come on. Please provide a direct link. That seems perfect.