50's chevy 80's ford rear aerodynamics

[mar5ka]

I was just reading road and track classic from 2006 were they take a 1959 4 door chevy and put it in a wind tunnel, they proclaim that the pointed rear fins or gull wings create air seperation and decrease drag for the rear of the car......Is this right? I thought that any sharpe edge on a moving body creates vortex's that increase drag. So then explain to me about the ford bi wing which reduces drag by 5% when the first wing creates air seperation or vortex's?

[Frank Lee]

link?

[Xringer]

Did they yank off the fins to do a before-and-after?
And what was the shape of the empty area where the fins used to be?
Smooth and rounded off or square edges?

[Shawn D.]

First, the plural of "vortex" is "vortices."

Second, it depends on where the vortices are and how they are interacting with the rest of the vehicle. Trailing vortices do indeed represent lost energy, but vortex generators can reduce drag if properly placed.

That having been said, your description of what they said doesn't seem to make sense. I'd like to see the article itself if you can provide it.

[aerohead]

Originally Posted by mar5ka

I was just reading road and track classic from 2006 were they take a 1959 4 door chevy and put it in a wind tunnel, they proclaim that the pointed rear fins or gull wings create air seperation and decrease drag for the rear of the car......Is this right? I thought that any sharpe edge on a moving body creates vortex's that increase drag. So then explain to me about the ford bi wing which reduces drag by 5% when the first wing creates air seperation or vortex's?

I'm with Frank Lee,more data if the article has it.Vorticity forms as a function of two air masses,either with a differential pressure,or a differential velocity commingling together.It happens a lot at wing tips where pressure differences above and below the wing cause the high to flow into the low,spiraling into a vortex.Its also common around A-pillars,and its famous for pseudo-Kammback,fastback car C-Pillars,where air over the roof moving at a relatively higher velocity,collides with slower-moving air along the sides,spiraling into the vortex.Once the vortex is formed,it robs an enormous amount of engine power to feed this circulation. As to the patented bi-wing rear spoiler,the spoiler forces the air to take a "longer" path over the back-lite and boot of the car.A locked-vortex is created by the spoiler,which robs some engine power,however drag reduction gained by the free-stream flowing over the new "virtual" aft body out weighs the loss for a net gain in efficiency.I think the fins on the Chevy were just for styling.That car did not suffer from cross-wind instability,the reason for which fins are usually employed.

[mar5ka]

I will have to scan the mag but I found this, and yes I quoted wrong date and title 1959 Chevy Impala | Concept Photos, Reviews, & More at Motortrend.com

[Xringer]

In the video, they are talking about lift or no lift from the fins. Not much about the CD..

The shape of the body just about 5" under the top of the fins, kinda looks like a boat-tail.

After seeing the smoke falling off the back of the fin into a ball of turbulence,
I'm wondering if that CD might have been a lot better without those crazy fins.?.

[Frank Lee]

Thanks for the link. I'd like to see what the article says. I have opinions on what I saw in the video, but it could be my interpretation is not correct.

[trikkonceptz]

I can serve as a true testament that the 1959 Impala was designed as a 1 year only model because the tail on the back end created lift.

The car would begin to hop @ 75mph, I know because I experienced it first hand in my own car. Now aside from design, poor suspension and 40yr old parts had a lot to to with helping it, but none the less the first it happened to me it scared the crap out of me .. LOL

On the other hand, the tri-power 348cu in engine with a three speed box did a great job at propelling that land yacht to incredible speeds ..

[jamesqf]

Originally Posted by trikkonceptz

I can serve as a true testament that the 1959 Impala was designed as a 1 year only model...

But the 1960 was nearly the same, the major difference being that the 59's curved fins were replaced with an angular design.

I never experienced rear-end hopping in the one I had as a kid, but the 6-cylinder wouldn't go that fast :-)