i am just curious, since a spoiler makes the tail less sharp, will that increase the Cd (instead of lowering it)?

(since a wing is above the car, so it won't be involved in the sharpness of the tail. but a spoiler is part of the tail so that got something to do with the sharpness )

 

All spoilers increase drag, you are trading that drag (force) for a percentage of it in downforce.

Nick

 

Yup...any time you use the airflow to apply a force (ie a wing) you create drag. You get drag from simply creating that force and also from friction between the air and the surface. The minimum drag a car can have would be zero lift and zero downforce (fyi...most cars actually lift at one end or the other...)

A spoiler does exactly that...it spoils the airflow. Usually, this creates downforce at the expense of added drag. A wing on the other had creates downforce as a separate entity, then transfers it to the car through it's struts. It's basically an airplane wing mounted upside down.

Note that some pieces that look like spoilers may not technically be spoilers...they could smooth the airflow, which would, in fact, lower drag. The airflow around a car is very complicated, mostly because of the underside and the wheels. Convertibles are, of course, more complicated than coupes too. Just because something looks like it may serve an aerodynamic purpose, doesn't mean that it does, and it may actually have the opposite effect.

 

Hmmm...I was expecting the opposite. Assuming that the "Wagon" has a "blunt" end (i.e. like that of rear end of a bullet with some exaggeration or think of that little aero "slap" at the top of the Civic hatchbacks), I would expect the flow seperation is LESS likely there. On the other hand, since a Sedan's rear tends to slop down much steeply (and hence nothing for the flow/votices to attach themselves), I would expect the flow would tend to curl towards that region and perhaps leads to more drag?

BTW, it just occured to me that the "ridges" on top of the tops will actually REDUCE drag, IF our cars can go really really fast, as there is a certain region where the drag becomes less with increasing speed AND slight introduction of turbulece (think ribblets on your golf balls) !

 

Bullets aren't particularly aerodynamic...the flat rear is so the propellant has something to push against. The most aerodymanic shape is a tear-drop... That's the form that rain drops - which can take any shape they want - naturally form into. And since everything naturally goes to it's lowest energy state, that would mean (with more than one jump here, granted) it's the most aerodynamic (well...bellow the speed of sound, at least) The shape is relatively spherical up front, grows until about 1/4 of it's length, then tapers to a sharp point at the tail. This is "ideal", note that wings look something like this, in 2D. Also, look at nature... Note the shape that fish bodies, bidr bodies, whale's, etc. are. Nature is really good at figuring out what the best way to do something is.

You're correct about the the golf balls... Those dimples are there to create a turbulent boundary layer instead of a laminar one. A turbulent boundary layer can stay attached to a surface over more drastic changes than a laminar one can, and since separation is a major cause of drag, turbulence can actually help in some cases. This probably isn't mush of any issue with our roofs though, since there's a lot to trip the boundary layer before them. Oh, and as a side...this is one of the reasons why a steeply raked windsheild is more aerodynamic....the air transitioning from the windsheild to the roof is less likely to separate. Again though...the back is where most of the drag is created.

 

Can anyone explain why Nissan claims to lower the Cd of the G35 with a wing mounted? I've always wondered about that because that goes directly against what a spoiler does.

 

But I think that shape of the rain drop simply represents conditions that are IMPOSED on the surface of the droplet like the pressure distribution and this does not necessarily mean that IT is the most aerodynamic shape.

For all practical purposes, I believe that we want to keep the flow attached as long as possible and keep it laminar if possible (since we are not going to tap into the turbulent drag bucket region)at the backend. And one of the simplest/cheapest thing that can be done is to simply elongate the physical surface on which the flow can attach itself so that the flow can deccelerate smoothly towards the back end. This is well demonstrated in hollow fairings on top of Mack trucks that extends to (and being flushed at) the leading edge of the very unaerodynamic trailer at the back. Granted this will increase the SKIN drag but that's nothing comprared to the penalty/gas bills for not managing the flow and reduing the pressure drag.

 

They added a lot of underbody air ducting which, in combination with the lip, created a car with zero lift front and rear. Remember that whenever force (lift or downforce) is created from airflow, drag is also created...so zero lift is needed for minimum drag. It's possible that the airflow actually was moving up the rear of the car, so the lip merges the air from the under the car to the air over the car smoothly. Or maybe not, that's just one possibility. Like I said, aerodynamics can be very complicated when it comes to these things, so sometimes what appears to be one thing can have another function entirely.

Technically, it's not a spoiler if it doesn't spoil the airflow...ie create a region of "dirty" airflow behind it. It's just easier to call it that, since that's the name people will use anyways. Just like we tend to call shock tower bars "strut" tower bars when the S2000 doesn't have struts...

 

Thanks, that's what I thought. It was quite confusing that I was trying to explain to a Nissan salesman that the wing does NOT decrease Cd, it's done by something underneathe the car! He wouldn't listen, instead he told me to sell my s2k and get a Z!

 

Exactly! This is precisely what the teardrop shape does. It's the smoothest way to decelerate the flow back to the "free stream" velocity, minimizing the possibility of separation. The laminar vs. turbulent thing doesn't have anything to do with this, only that on a non-ideal shape, the tubulent boundary layer will stay attached longer.

The Mack truck example is a good one...those fairings smooth the flow onto the trailer portion. This reduces the drag considerably. However, the major contributor to drag on those trucks is the square back end. Drive behind one with the top down...you'll get buffeted a huge amount... The airflow back there is horrendous. If they could, they'd fair it into a much smoother trailing edgem but that's not practical for other reasons.