Box fan, free spinning in the bed of a pickup - what would that do for drag ?
 

We have probably all seen it : a guy driving along in his pickup truck with a box fan in the bed spinning full speed.
I wonder what this does for the trucks aerodynamics.

Since the fan spins because of the fan being within the vortex of air spilling off the cab, I'm guessing the fan adds velocity to the air facing towards the front of the truck.

I would think this would make the drag even worse.

But what if that fan were spinning the other direction ?
I know it won't, but just ignore that for the moment.

If a free spinning fan blade could be placed within a duct facing into the wake of a vehicle, would it draw in more air into the wake ?
Would the drag of the fan blade cancel out any benefit from forcing more air into the wake ?

Is that just a really stupid question ? One that begins to sound like perpetual motion nonsense ?

( I should mention that I was just thinking of adding more air into a wake. I didn't mean to imply any sort of free energy )

The spinning of the fan blade is simply a reaction to air that is already present, and will not thereby draw in more air into the fan.

Would that be a correct assumption ?

The simplest way to think of it is, it takes energy input to cause an unpowered fan to spin. That energy is coming from the vehicle's forward motion hence it must increase drag.

Think of it this way: a helicopter with an engine failure falling through sky with the blades autorotating is survivable.

Thanks to you all for the replies, rather than ignoring the question, or saying "Your en idiot !".
I remain the idiot, and don't learn if people keep it to themselves, without explaining what makes the question idiotic.

I understand that the motion produced by the fan blade is a result of forward motion from burning fuel.
I'm just trying to find out if any additional air can be added to the cars wake purely by a free spinning fan.( To reduce the cars wake )
My guess is that it most certainly would not add any additional airflow - ( unless the fan had left over momentum after the airflow slowed.)

But I just don't know.

I have some NACA ducts installed on my cars box cavity / aero extension-do-ma-hitchee
The airflow through them is surprisingly good.
( I tuft tested them )
The idea is to channel air into the dead zone in the cars wake, and hopefully reduce its size.
I think adding small fan blades behind the ducts would be a waste of time, but it did make me curious about what they would do.

Were you able to get a good throttle stop setup going?

That's going to take some time.
I live in the city, and need to find a road that has minimal traffic, as well as a fast speed limit.
It also needs to be close to where I live, so that I can drive home, and remove aeromods to run tests A-B-A.

There is a good road that is nearby, but the speed limit is 50 mph.

I also have to find the "sweet spot" to have the screw in the wooden block that goes under the throttle.

To find it, I think I will use a piece of soft clay under the gas pedal.
I will have to accelerate exrremely slow to get to the desired top speed,
It might be tricky.

Can this be moved to the Unicorn Corall ?

I'm still curious about this though, and asked a math teacher I work with if a windmill draws in any additional airflow once it begins spinning.
He said it probably would
I then called the A&M wind tunnel and got to speak with one of the aerodynamicists.
I asked if the boxfan in the bed of a truck would draw in any additional airflow once it began spinning.

Her answer was " Potentially " - followed by a quick " So is this pertaining to any test you intend to perform here at the tunnel ? " ( Because if not, go away ! Lol )

I certainly do not intend to add fans to my car.
The idea was just a passing curiousity.
I would think that the only extra airflow the fans could provide, would be from a few seconds of momentum as the windspeed changes, and would just be a tiny puff of air.

Useless.

I would imagine that to decrease a cars wake by blowing air into it, you would need a massive amount of wind.

curious
 

I've been thinking about it since you first posted it. Some random thoughts:
* It's very easy to spin an unloaded fan.
* The mechanical advantage of any induced aerodynamic force on the blades, acting away from the shaft center, can easily overcome any initial bushing / bearing resistance to movement.
* Once spinning, it requires very little energy to maintain the rotation.
* After all, they're not harvesting any power from the airstream to produce any useful work.
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* The fan is extremely 'porous'.
* A lot of air bleeds through the unit, whether through the blades or not.
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* The air which makes it through may be deflected in direction a bit, or blasted into turbulence, if it wasn't turbulent to begin with.
* As an additional source of resistance, any chaotic, random turbulent air it's initially embedded within, may end up a little less energetic due to viscous shearing effects, however I don't know that they'd be significant enough to even measure.
* Conservation of energy dictates that whatever kinetic energy existed in the air before it encountered the fan would be preserved, minus a minute amount lost to atmospheric heating due to the viscous shear.
* It couldn't amount to too much. We're adding a smidgeon of additional disorder to a completely turbulent/ chaotic environment.

Depends on how you do it.

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