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Here's a crazy question, which I'm sure some of you just love: I used to have a 1988 Ford Festiva and I figured it would be neat to use an old box fan (20" diameter) and mount it on the back bumper so it could spin while I was driving. I spray-painted it silver and it got lots of laughs. It would spin wicked fast once I got up to about 30 mph (and it was also neat to see it spinning when my car was parked). I've always wondered if the drag against the fan negated any benefit from the whole fan spinning and propelling the car a tiny bit. I'm guessing the former is the case or else we'd see those all over, beyond the little ones on hitches, but it seems like there should be some way to harness the wind power without suffering an aerodynamic loss. And similarly, how efficient are those giant motor-driven fans on those swamp boats?

I think the answer is going to be the same as a perputual motion machine...you never recover all the energy you put into a car.

As great as hybrids are, regenitive braking recovers maybe 30% of the kinetic energy.

Hi Laughing Dog, welcome to the forums.

The fan will create more drag, it's never spinning fast enough to actually propell you even a tiny bit (unless somehow you decelerated extremely fast from a very high speed so it was still spinning quickly). It's spinning passively in response to the wind rushing past it (which creates drag). Delta Flyer is right, it's similar principles as the perpetual motion machine - you can't harness the wind power to create more windpower than harnessed.

Kirk

Here's a crazy question, which I'm sure some of you just love: I used to have a 1988 Ford Festiva and I figured it would be neat to use an old box fan (20" diameter) and mount it on the back bumper so it could spin while I was driving. I spray-painted it silver and it got lots of laughs.

That would be fun to see :) A swamp boat was exactly what came to mind.

Do a google image search of "Chaparral 2J"

-mr. bill

The fan blades make drag, big drag. When an airplane engine quits, the first thing the pilot needs to do is to get the prop to quit windmilling so he can glide farther. When a helicoptor engine dies, the pilot gives the rotor a small amout of negative collective and it glides to a smooth landing with the drag of a windmilling main rotor acting like a parachute. When iceboats and high performance catamaran sailboats go downwind, they zig-zag back and forth instead of just going dead downwind, this increases the speed of these boats so much that they can actually go downwind faster than the air itself is going downwind.

http://img165.imageshack.us/img165/525/sailboatmd0.gif

I remember of this story in highschool. Some guy took an electric leafblower, and rigged it to blow into his cars intake. We all don't know if it actually increased his hp or not, but if it did, I guess you could say it was like a hybrid, since the blower ran off of the battery.

I'm curious laughingdog51, how fast could you go on just the power of the fan? Any time you are going faster than what the fan can propel you will cause a drag that cost you efficiency. And how were you powering the fan, if you weren't it was always nothing but a drag, and if you were it was a drag on your alternator/engine, especially while it tried to overcome the opposite force of your vehicle speed.

You are kidding right.:eek:

I'm curious laughingdog51, how fast could you go on just the power of the fan? Any time you are going faster than what the fan can propel you will cause a drag that cost you efficiency. And how were you powering the fan, if you weren't it was always nothing but a drag, and if you were it was a drag on your alternator/engine, especially while it tried to overcome the opposite force of your vehicle speed.

You are kidding right.:eek:

I think the way to figure this out would be to figure out how many CFM (cubic feet per minute) of air the fan moves. They got CFM ratings for computer fans, but I doubt they have them for cheapo box fans. Then you can figure out how much air would pass through the area of the fan at a certain speed. And then finally once more air would move through the area then the fan can move, resistance is created.

Hook the fan up to a generator, and use it to recharge the batteries in your hybrid retrofit kit, then you'll be making use of that energy.

Hook the fan up to a generator, and use it to recharge the batteries in your hybrid retrofit kit, then you'll be making use of that energy.

However conversion losses would negate any benefit there. The aero losses would be more than the electricity produced.

Kirk

. I've always wondered if the drag against the fan negated any benefit from the whole fan spinning and propelling the car a tiny bit.

Simple answer is YES. Since the fan has no propulsion method in an of itself it will cause a net loss in energy. The fan also adds weight resulting in more loss. The only reason the fan spins is because of drag. The air passing over the fan blades pushes the blades and causes them to spin. The spinning fan can not possibly help the car move forward in any net gain fashion since it is the energy of the car and the drag on the blades making the blades spin.

http://videos.streetfire.net/video/Leaf-Blower-Bolt-on-Dyno_7514.htm

hahahhaha!!!!!!!!!!

thats awesome .. hahhahaha !!

http://img165.imageshack.us/img165/525/sailboatmd0.gif

I remember of this story in highschool. Some guy took an electric leafblower, and rigged it to blow into his cars intake. We all don't know if it actually increased his hp or not, but if it did, I guess you could say it was like a hybrid, since the blower ran off of the battery.

You mean like this. :rolleyes:

http://www.electricsupercharger.com/