Exhaust Flow and Aero

[jimmy six]

Hey! What would I know; I drive a roadster with 32 shell. Point the exhaust toward the back if you can and step on the throttle......May the best man win 

[Blue]

Please excuse me, I wish to make a correction, on the 2nd to last sentence, I meant to say “The downforce from the underbody is virtually drag free though”

Whoa, whoa, WWWHHHHHOOOOOOOOOOAAAAAAAAAAA!  That's something for nothing.  "The second law of thermodynamics declares the impossibility of machines that generate usable energy from the abundant internal energy of nature by processes called perpetual motion of the second kind."

IOW, there is no free lunch.  Someone a lot smarter than me figured that out.

Drag from the underbody whether it produces downforce or not already exists on many cars.  Subtle changes to the underbody may make significant improvements to downforce without increasing drag beyond the drag that is already there.  At no point is this downforce "drag free".  The fact that this is possible on a high drag design does not in any way mean the downforce is free.  The drag is already there due to poor design.

In fact, changes to the underbody can both reduce separation drag, increase downforce, and with it the induced drag.  Then the car makes the same total drag with more downforce.  This is still not "free".  Drag is still present and a detailed drag analysis can separate the drag due to downforce (induced drag) from the rest of the vehicle drag.

On any vehicle, downforce comes with a drag penalty.  This is called the lift to drag ratio or L/D.  In ground vehicles we would use the term -L/D.  Typical ground racing vehicles that generate significant downforce have -L/D ratios of 0.3 to 3 due to large amounts of separation and low aspect ratios.  Air vehicles have L/D from 5 to 20 to 50 for fighters to airliners to sailplanes. 

Note the progression: higher aspect ratio wings and smaller fuselages have higher L/D.  Cars are the opposite.  While we might lump all kinds of "drag" together for a given vehicle, it is more accurate to break it down to each piece and each type of drag produced.  Then the induced drag specific to the lift (downforce) can be dealt with against all of the rest.

There is no free lunch.

[SPARKY]

  Damm there is that inconvient truth rearing its ugly head again!!!! 

[WOODY@DDLLC]

After becoming an American citizen an old man was asked what he found most interesting about America!

He said, "Free lunch is 99 cents!" 

[Don't make me explain this!!] 

[tortoise]

Please excuse me, I wish to make a correction, on the 2nd to last sentence, I meant to say “The downforce from the underbody is virtually drag free though”

Whoa, whoa, WWWHHHHHOOOOOOOOOOAAAAAAAAAAA!  That's something for nothing.  "The second law of thermodynamics declares the impossibility of machines that generate usable energy from the abundant internal energy of nature by processes called perpetual motion of the second kind."

Since there is no downward motion, downforce isn't perpetual motion. There's no second law violation. No actual work is being done by downforce.  No need to overstate the case to make your point.

[superford317]

Colin Chapman, the father of modern ground effects, was asked during a news conference about his Lotus Model 78 Formula 1 car on its debut and he replied “we found something for nothing” referring to the downforce from the underbody. The downforce from the underbody is not totally drag free, that was why I said it was virtually drag free, but it is as close as you can get, being much more efficient than any other source of aerodynamic downforce.

[manta22]

"Colin Chapman, the father of modern ground effects..."

I think Jim Hall earned that title.

Regards, Neil  Tucson, AZ

[Buickguy3]

   Touche!
Doug 

[John Burk]

F1 ground effects

http://youtu.be/xdqgY9YOaBo

[SPARKY]

Whooops   the propster may take this as a wakeup call and try to make a return

[Jon]

It's "virtually drag free" because there is a heap of shear drag happening underneath the car already, tidying it up and adding some order to the flow offsets the majority of the ground effects drag.

It's like saying someone gives you $10 and you buy something for $11 is "virtually free", only because you didn't bank the $10, still cost you $11.

My opinion anyway.
jon

[Blue]

Colin Chapman, the father of modern ground effects, was asked during a news conference about his Lotus Model 78 Formula 1 car on its debut and he replied “we found something for nothing” referring to the downforce from the underbody. The downforce from the underbody is not totally drag free, that was why I said it was virtually drag free, but it is as close as you can get, being much more efficient than any other source of aerodynamic downforce.

This reflects a 40 year old understanding of race car aero limited by restrictions on wings.  The fact is that high aspect ratio wings, well separated from the rest of the vehicle, are far more efficient at creating downforce (-L/D) than ground effects.  They are not used because every racing body since the early 70's Can Am days restricted wings area, width, and especially height in an effort to control downforce.  The use and science of ground effects are a result of this restriction, they are not the best solution.  The best solution has been outlawed everywhere but LSR.

[Blue]

Please excuse me, I wish to make a correction, on the 2nd to last sentence, I meant to say “The downforce from the underbody is virtually drag free though”

Whoa, whoa, WWWHHHHHOOOOOOOOOOAAAAAAAAAAA!  That's something for nothing.  "The second law of thermodynamics declares the impossibility of machines that generate usable energy from the abundant internal energy of nature by processes called perpetual motion of the second kind."

Since there is no downward motion, downforce isn't perpetual motion. There's no second law violation. No actual work is being done by downforce.  No need to overstate the case to make your point.

If we press up on an immoveable object, there is no work being done.  However the air is not immoveable, it is a fluid.  The first law of motion is that for every action there is an equal and opposite reaction.  When we create downforce by moving through a fluid (air) that force equals moving a mass of air up.  

F = m x a

So for a given downforce (F), we must accelerate (a) a mass of air (m) up.  This is power, and over time, work.

Similarly, drag is the exchange of momentum from the vehicle to the fluid (air) that we are traveling though.  If we do this without any separation or recirculation, the drag is limited to viscous effects.  Most LSR vehicles have significant separation, recirculation, and stagnation, leading to high levels of pressure drag and viscous shear momentum exchange.  It takes power to create downforce just as it takes power to overcome drag.  That power went into all of the wake turbulence behind the car and the salt spray going up and back.  We produced the power, we burned the fuel;  where else did the work go?.  

While this seems esoteric, the fundamental physics are what separate good design decisions from bad.  Look at the ALSR programs: they let their (m) get out of hand, and can't make their design speed with the space and fuel designed even if they won't admit it.

[Blue]

F1 ground effects

http://youtu.be/xdqgY9YOaBo

The video clearly shows that Peter Wright was the "discoverer" of modern ground effects, not Colin.  FYI, I went to high school with Finn O'neil, who's father invented the cylinder cut-out for fuel management in F1 for Lotus working with Colin.  Finn taught me how to race motorcycles.

Peter doesn't mention drag, nor the difference between -L/D ratios from wings vs. downforce.  No one in the film mentions that their wing size/width/height was being restricted every year.  More telling is the later focus on vacuum with fans: clearly a sub-optimum approach dictated by rules, not physics.  LSR is much more open.

Wide span wings work better because it is better to push up a little bit on a lot of air with small edges than to push up a lot on a little bit of air with long edges. 

[SPARKY]

  Thanks to BOTH you guys---our grasshopper AERO minds are  soaking it up---thanks again for so much to ponder as we work toward enlightment!!