EDF (electric fan propulsion)?

[SA_SA_SA]

The gadget show recently attached 4 electric ducted fans to a bike in one of their "quests": they seemed rather effective propelling bike to 4o+mph:


so has anyone tried just the one, to stay within the 15mph electric bike legal max:

as an electric drive such a fan would have the advantage of being completely separate from bike transmission, required no wheel building, and could be easily removable.


They are essentially small electric jet engines (you can get similar sized model real jet engines, but you would then have a moped, thus requiring a licence insurance etc).

[simon1]

Presumably less efficient tha chain drive, so what you gain in ease of attachment you lose in battery life / range.

[SA_SA_SA]

presume nothing

[hubgearfreak]

They are essentially small electric jet engines

i've an understanding of how jet engines work - i fail to see how you could have one that's electrically powered

[[XAP]Bob]

They are essentially small electric jet engines

i've an understanding of how jet engines work - i fail to see how you could have one that's electrically powered

IIRC most modern jet engines are actually ducted fans powered by a small gas turbine engine.

I know this is a toy:
but the tube in the middle is the gas tubine (classic jet engine) and the big surrounding space (and big fan at the front) are a ducted fan, providing much of the thrust of the engine without actually being part of the "jet" section.

So you replace the classic "jet" bit (the gas turbine) with an electric motor. You'll still get a jet of air...


Ducted fans are remarkable efficient - The Gadget show bike looks kind of mean from the back, but with no gears, it just provides a virtual tailwind and ALOT of noise

[hubgearfreak]

so it's an electric fan. i really can't see that being an efficient way to propel anything

[SA_SA_SA]

I am interested in real world data not "it will never work" style posts.

Turbo fans work OK for planes, they propelled the gadget show person to a right old speed, I am interested in the results of someone whose actually tried it.

[hubgearfreak]

Turbo fans work OK for planes

yes they do. with the addition of huge amounts of jetfuel; which contains 42mJ/kG

they propelled the gadget show person to a right old speed

i've no doubt. but how much battery did it use???

[rualexander]

I imagine that the battery capacity would be the main problem with powering a bike for any length of time.
There are loads of videos on youtube of EDF powered model aircraft etc. such as this. Pretty cool.
As an aside I once saw a Japanese cycle tourist in New Zealand with a large petrol driven propeller (similar to a powered paraglider engine) strapped to his back, but he wasn't using it when I saw him so I don't know how effective it was.

[snibgo]

... cycle tourist in New Zealand with a large petrol driven propeller (similar to a powered paraglider engine) strapped to his back ...

I love it, especially as maybe it doesn't then count as a motorised vehicle. Wow. It's just a bike, but does any speed I like (and can afford the fuel for).

But they'd probably get me for "furious cycling" or something.

[hubgearfreak]

I am interested in real world data

OK. look at newtonian physics. see the work that needs to be done, the efficiency of electric fans, weight of batteries, etc.

[[XAP]Bob]

Ducted fans are what they use for personal helicopters (with 2 stroke engines normally)

http://en.m.wikipedia.org/wiki/Ducted_fan

[snailracer]

The main decider of efficiency for a turbofan or propeller is the mass of air it pushes out the back and how fast it pushes it out.

A fan with a large diameter pushes a large mass of air slowly, and this turns out to be more efficient than a small diameter fan that (for the same thrust) pushes a small mass of air quickly.

Thrust: F = ma, where acceleration "a" is proportional to the speed of the blown air.

However, the energy in the blown air is given by E = 0.5 mv^2, i.e. proportional to speed squared, so the faster moving but smaller mass of air requires more energy to generate the same thrust as the slower moving but larger mass of air.

The intuitive explanation is that pushing a small mass of air quickly is less efficient because that mass of fast-moving air wastes more energy as turbulence behind the aircraft.

A turbofan or propeller is only more efficient in the context of aircraft, which don't have anything more efficient to compare against. Neither is efficient in comparison to driving through wheels on the ground.

[[XAP]Bob]

But they can be significantly more powerful (see thrust ssc)

Of course that's not really a consideration on a pedal cycle, but I'd like to know what the legal eagles would try to work out - what is the power output? meaningless.
What is the max speed? meaningless