Duct fan propulsion?

[swbluto]

Hey, I was wondering, has anybody tried ducted fan propulsion? Like something like this: http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=4235

It appears that these have been tested at 20 pounds force but it looks like 10-15 pounds force would be more reliable - I think that much force would be good for somewhere between 40-50 mph? I'd have to check the calcs. While this would be more inefficient than a chain/belt drive and so you'd get less range, this looks to have the advantage of being fairly easy to implement and the ducted part makes it "look safer" (than a regular fan). Also, the price of this particular unit is $25 which sure seems to beat paying $100+ for all the components involved in setting up a chain drive. Plus, the motor free-wheels, so you can pedal independently from the drive system.

I'm thinking I like this because, maybe, I could strap this to a "propulsion suit" with the fans mounted ontop of my shoulder or on the belt or something, and that way I could provide propulsion to whatever I wanted. Like, a skateboard, scooter, bike, roller blades, whatever. I can definitely imagine having some of those "roller shoes" and then attaching maybe a 200W - 1 kW system to my belt or something. Walk when you need to, roll when you don't wanna.

 

[Doctorbass]

Hmmm... that think with a 130KV 6500W motor ... UNDER WATER would be awsome!!!

Doc

 

[liveforphysics]

I bought a:

5"
4.5"
4"
3.5"
3"

The plan was to stack them in series from largest to smallest, out fit each one with the most powerful brushless outrunner they can hold, and try to come up with something that could provide a useful amount of boost as an electric supercharger. Turned out, I was never able to get more than a 0.4psi differential pressure across any of them, which I half expected. Since I'm looking to get at least 10psi, I dropped the idea.

Keep in mind, you still gotta buy a $50 motor and $60-100 controller to get one of these things working, and they sound like a screaming leaf-blower on crack.

 

[swbluto]

I bought a:

5"
4.5"
4"
3.5"
3"

Turned out, I was never able to get more than a 0.4psi differential pressure across any of them, which I half expected.

Hmmm... is this math correct? pi*5inch^2 = 78.5398163 square inches * .4 pounds / square inch = 31 pounds? So you were getting like 31 pounds of force from the 5" one?

 

[Lock]

http://endless-sphere.com/forums/viewtopic.php?f=3&t=17175#p251251

 

[Canis Lupus]

I enjoyed reading the idea, if for no other reason to know that I am not the only one who comes up with impractical ideas, mine being so stupid it would be pointless to even post them. At least this one makes some sense.

 

[liveforphysics]

I bought a:

5"
4.5"
4"
3.5"
3"

Turned out, I was never able to get more than a 0.4psi differential pressure across any of them, which I half expected.

Hmmm... is this math correct? pi*5inch^2 = 78.5398163 square inches * .4 pounds / square inch = 31 pounds? So you were getting like 31 pounds of force from the 5" one?

That was with the discharge end nearly sealed off, but yes, 0.4psi. The moment you let the discharge end up off the desk a smidge, the pressure drops right off to less than half of the obstructed pressure. This is because obstructed, it's not doing useful work, it's just applying a force, and the useful efficiency is 0%, and it would be useless for moving a scooter. Stacking a bunch of them in series would also be useless for moving a scooter. You would want them as unobstructed as possible, hopefully with nicely radiused inlets, and put units in parallel to gain useful thrust.

The engine I wish to supercharge is only drawing ~400 hundred CFM peak, and around 300CFM average, which is why it acts like an largely obstructed discharge nozzle for a device that would otherwise be moving >5,000CFM unobstructed.

 

[liveforphysics]

A large diameter prop is so much better for providing some thrust, and doesn't sound like a screaming banshee. But... it's also a serious safety hazard to have something like an 18" sharp spinning blade of death following behind or in front of you through a crowded area. lol

 

[Tiberius]

I've done a lot of work on props and ducted fans for hovercraft.

Size is important. It turns out that the thrust that can be generated for a given input power is related to the diameter (more specifically the area, if you ever made a non-circular one). This is why aircraft try to have the largest props possible. A ducted fan can be made equivalent to a bare prop that is about 20% larger.

In the case of a bicycle, its probably not worth considering a prop smaller than about five foot diameter, and then the cost, complexity and weight is greater than arranging a drive to a wheel. Luke has already mentioned one of the other problems .

Nick

 

[Miles]

[youtube]Klh5AyCOvmI[/youtube]

et al.... LOL!

 

[liveforphysics]

[youtube]Rj4moeKDGY4[/youtube]

Mega-fail.


[youtube]WIAE8Mpp4nc[/youtube]

This one would likely be able to provide perhaps 15-20lbs of continous thrust, which would make it useful on flat ground. This guy badly needs to get a servo tester rather than dragging around his trans on his handle bars. lol

 

[Tiberius]

Here's a collection of videos showing why you want a ducted fan rather than an open prop
http://homepage.ntlworld.com/paulzvideo/video_gallery_new.html
Oh, and a helmet and body armour too.

And yes, I am in one of them.

Nick

 

[Miles]

Let me know when you have an electric version of this, Nick....

http://news.bbc.co.uk/1/hi/world/asia-pacific/8543202.stm

 

[coln72]

I know that school trialled a prop powered trike a number of years ago. Apparently it worked ok, but the project was shelved after it threw a prop blade. It went through a window and embedded itself into a classroom wall

 

[liveforphysics]

I know that school trialled a prop powered trike a number of years ago. Apparently it worked ok, but the project was shelved after it threw a prop blade. It went through a window and embedded itself into a classroom wall

I would imagine it's this kids bike...

http://www.popularmechanics.com/home_journal/workshop/4218427.html

It shows in the video how the blades attach to the hub. They slide over a piece of dowel, then have a single worm-type hose clamp squeeze a bit of rubber hose to retain them... I can see why he wanted a setup to enable pitch adjustments for testing, but get some T-bar type hose clamps, and some pin/cable/bolt/notch blade retention device! You can nearly strip out the worm-type from tightening them with your fingers...

 

[AussieJester]

[youtube]Klh5AyCOvmI[/youtube]

et al.... LOL!

And it looks so damn safe to Miles im surprised you haven't done it already yourself!

KiM

 

[liveforphysics]

Here is a comercial version...

http://www.gizmag.com/go/6731/

 

[liveforphysics]

[youtube]Klh5AyCOvmI[/youtube]

et al.... LOL!

And it looks so damn safe to Miles im surprised you haven't done it already yourself!

KiM

I've watched it twice now, and it was even better the second time. Notice the on-off switch requires reaching behind you, towards the spinning blade of death, look for the cringe on his face as he flicks on the switch. Notice that they boldly announce they are going to be coasting downhill, and notice for some reason the guy won't put his feet on the pedals, but you can see that beautiful instability wobble he's got going on from the massive dual car battery weights hanging off the sides of the rear wheel. Then, to top it all off, it looks like he accelerates at the rate a non-power assisted bike would roll down that hill. lol That is a gem of a video Miles. Thank you for the chuckles.

 

[erutangis]

Forget that.... you need a couple of these bad boys!

http://www.youtube.com/watch?v=EQ7zbta79vo

 

[Miles]

Then, to top it all off, it looks like he accelerates at the rate a non-power assisted bike would roll down that hill. lol

I wondered if he had the propeller running the wrong way.......

 

[deecanio]

and the winner of most pony and lethal looking bike is.......
:lol:

 

[swbluto]

Well, 20 pounds thrust doesn't sound bad. While nothing like a motorcycle, it'd be like a fairly decent bike although I'd probably prefer a regular bike for the greater range.

I only brought the query up because wikipedia listed these two advantages of ducted fans over regular fans:

-For a similar amount of force, they can be smaller than a regular fan.
-The ducting shields the fan and reduces tip vortices, thus reducing sound.

So, from the experience voiced here, it sounds like it might be practical for 8 mph speeds on something like this:

That honestly would seem pretty cool to me. I think it would only take like a pound or two of force to maintain 8 mph.

How does efficiency for one of these fare? I think a chain drive system can reach 96-98% drive efficiency while an ideal prop could reach 60%-70% efficiency or so. What kind of efficiency would be practical with a "small" duct fan? Like 50%? 10%? I'm really looking for an order of magnitude.

 

[Lock]

So, from the experience voiced here, it sounds like it might be practical for 8 mph speeds on something like this:

Heeley walking stick here:
http://endless-sphere.com/forums/viewtopic.php?f=3&t=1086&p=16621#p16621

tks
Lock

 

[j3tch1u]

i'm wondering if it would be possible to do a micro rc friction drive on those heely's?

it would be fun to race my dogs in the park.

 

[amberwolf]

You can nearly strip out the worm-type from tightening them with your fingers...

That seems to depend on the brand and the material. I've got some by "Ideal" that are pretty good, and some that are very old with an unreadable octagonal logo that are excellent, both stainless steel. The ones by Ideal vary in age from 15-30 years, depending on what car I got them off of, and the others are probably closer to 50+ years old; having sat unused in a drawer in a friend's father's shop for most of that time with the envelopes they came in rotting around them.

Those older ones have a finger-type tightener, but I cannot grip things well enough with just two fingers to get them tight so when I used them to hold some of CrazyBike2 together before I welded things, for test rides, I used pliers to tighten them, and went so far that I could no longer turn them even with the pliers, and did not strip them out.

The Ideal clamps are all flatblade/hexnut style tighteners, of varying sizes, and I haven't stripped one yet despite tightening them with a socket driver as tight as I could turn them. I still use those to hold the seat onto CrazyBike2, and they don't work loose, either (well, the wood blocks used for vibration damping crush a little each ride, so *that* makes them loose, but the clamps don't work loose).

The ones I can typically find at auto parts stores and Harbor Freight, however, I can strip just turning them gently tight. Heck, the HF ones can disintegrate just trying to turn the screw by hand without even putting the strip thru it!