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Author Topic: Creating Negative Pressure Underneath Car to Prevent Lift  (Read 11088 times)
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fredvance
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« Reply #30 on: February 17, 2016, 10:59:54 AM »

Dave we like to  Dead Horse Dead Horse around here. cheers
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« Reply #31 on: March 03, 2016, 11:13:10 AM »

Just as Sparky said, there is no free ride when it comes to aerodynamics. Just as lift creates drag, so does negative lift. The drag increases logarithmic-ally the faster you go unless you can somehow control it using adjustable wings. But gravity is free and does not care how fast you go. Using the right tires and adding weight is the best way to get traction.

Don

I don't want to open a can of worms here but two top guys who have good reputations, one being Joe Katz and one other who has been well over 400 disagree with your statement. wink
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Ron Gibson
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« Reply #32 on: March 03, 2016, 02:19:32 PM »

Someone may correct me (memory not what it used to be) but I think Roger Lessmans liner was a ground effects car = negative pressure for downforce. If or when you get out of shape in yaw, you lose downforce = many problems.  cry cry Not saying negative pressure is bad, just don't depend on it entirely.

Ron
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« Reply #33 on: March 03, 2016, 05:05:46 PM »

Just as Sparky said, there is no free ride when it comes to aerodynamics. Just as lift creates drag, so does negative lift. The drag increases logarithmic-ally the faster you go unless you can somehow control it using adjustable wings. But gravity is free and does not care how fast you go. Using the right tires and adding weight is the best way to get traction.

Don

I don't want to open a can of worms here but two top guys who have good reputations, one being Joe Katz and one other who has been well over 400 disagree with your statement. wink
Perhaps we can say that if a given car has lift naturally, and that if it is modified/designed further to generate down force (negative lift) to counteract the positive that is present (ie making it neutral) that there is (may) be no drag penalty?
Gravity is free but it does care as rolling resistance is proportional to velocity. From that stand point, lighter is better. Traction and staying straight may make the penalty worthwhile though. (put the weight in the right place)
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Jack Iliff
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SPARKY
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« Reply #34 on: March 03, 2016, 08:03:04 PM »

the downside to weight---it is still a drag race

 for the 3+ cars especially the 4+ ones and the want-a-b's
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« Reply #35 on: April 18, 2016, 04:43:46 PM »

Perhaps we can say that if a given car has lift naturally, and that if it is modified/designed further to generate down force (negative lift) to counteract the positive that is present (ie making it neutral) that there is (may) be no drag penalty?[/quote]

Jack, as an exercise, I think it quite possible to create neutral lift by generating negative lift to counteract positive lift but would likely try reducing positive lift first. But drag will still be a question how you went about generating that negative lift. If you use a wing you may generate much more drag than using ground effects. Ground effects can be tricky to get right, especially on a long car.

This reminds me of an old Jimmy Stewart movie about Charles Lindbergh's flight across the Atlantic. The story tells of a fly in the cockpit and Lindbergh asking himself quizzically if the fly landed on the airplane, would it add weight or if it flew around in the airplane, would it not add weight. I hadn't thought about this until reminded of a similar experience when at Lake Gairdner. I had a fly in the cockpit with me and wondered if he would get into the 200 MPH club too.     

John
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jacksoni
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« Reply #36 on: April 18, 2016, 05:27:11 PM »

It is my understanding, and sort of what I was trying to say, is that if you have a body that is neutral, any changes that produce lift- positive or negative-will also produce some drag as well. There is no free lunch, it all costs horsepower. Wings may be very efficient at lift production but as you say are more complicated and may have a higher penalty. Seem to work well when horsepower is in excess- and traction is good too. All a trade off.
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Jack Iliff
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« Reply #37 on: April 18, 2016, 05:56:42 PM »

Having two separate forces (positive lift and negative live will cause two sources of drag. Wayno
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« Reply #38 on: August 18, 2016, 08:29:05 PM »

There is at least one ground effects car out on the salt I can think of.

Ground effects is a good way to create downforce (negative lift) but it is subject to some limitations and here are a coupe of things to bear in mind:

  • Increased Pitch Sensitivity
  • Increased Yaw Sensitivity (in relation to downforce created at larger cross wind angles)
  • Limits Suspension Geometry
  • Can impact upper body flow


Unless you have access to a rolling road wind tunnel (I wouldn't trust CFD for absolute lift values - especially not steady state) I would tread very carefully with developing grounds effects. Make speed increments slowly, check live downforce data (easiest way is tracking suspension loading at speed) for signs of the ground effects "stalling" (not really the right term - but it makes the point).

Teams with big budgets have backflipped cars into tree lines when getting ground effects wrong (cough... Mercedes).

You can build a scale model to test the effects.... however, you will need to flow much faster than the full scale car to get in the correct reynolds regime. Once at the right reynolds number you can essentially scale the force values. Then you will need to consider the design of the scale tunnel you are constructing carefully, incorrect boundary layer (i.e. anything above a 1mm displacement thickness) will give false results.

This is a very tricky subject, F1 teams invest billions on this question alone.

Don't want to discourage you, just trying to keep you safe. Please try it, but increment the design of the ground effects slowly, use the great white dyno and read the data carefully.
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