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Author Topic: Creating Negative Pressure Underneath Car to Prevent Lift  (Read 10416 times)
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Jim Phelps
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« Reply #15 on: January 26, 2016, 08:42:38 PM »

I has some questions regarding drag, lift and top speed, etc. with doing something like this:

1.) Will creating a vacuum (negative pressure) underneath the car cause a "drag" on the car when it's moving?

Additional negative pressure will cause at a minimum an increase in rolling resistance by the increase in downforce.  How the downforce is applied can have an external influence that could increase drag.  It depends on this is accomplished. 

2.) How would I figure out the "correct amount of vacuum" to create underneath the car to keep it from "lifting" at very high speeds? 

At very high speeds the force caused by the vacuum has to overcome lift.  Lift or downforce, depending on the shape of the vehicle, increases with the speed squared.

3.) Would too much "Negative Pressure" slow the car's acceleration & reduce the potential top speed?

Same as for 1.)

4.) What is more important in regard to "negative pressure" sucking the car down...a large surface area or a large volume?

The surface of the underside facing downward.  The negative pressure times the area equals force.  The pressure may not be constant along the length or width of underbody.  Depends on how this done.

5.) Would creating a "negative pressure" be any different than just any weight to the car?

Same as 1) and 3.

6.) Would you have to increase the vacuum under the car as the car increases speed and lower the vacuum at lower speeds?

Lift and drag will increase with the square of speed.  So suction on the underbody may also have to increase to overcome that lift increase.  How you do this will influence the external shape and contribute to lift.

Any formulas that are related to weight & down-force, acceleration, speed, lift and drag as related to "negative-pressure" would be great!!!

My main goal is I'm trying to increase the down-force without increasing drag (like using a wing).

Any direction of how to figure some of this stuff out before I build a "scale model" of it would be very helpful.

Thanks a ton!
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DaveB
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« Reply #16 on: January 29, 2016, 10:55:14 AM »

Down Force = V^2 x A x RH / CD  +  HP x RPM / AC^3
                                               ULC

RH relative humidity
CD coefficient of drag
A area
HP horsepower
AC acceleration
V velocity
RPM rev/min
ULC universal L Constant

The ULC is an experimentally determined constant. Sometimes 42 is applied but this is not suggested. Please read Douglas Adams' research on 42.

The fictitious equation above should be applied under adiabatic conditions.
« Last Edit: January 29, 2016, 11:02:15 AM by DaveB » Logged

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Rex Schimmer
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« Reply #17 on: January 29, 2016, 06:23:13 PM »

DaveB,
Interesting formula, any chance you can point me to were I can see where and how it was derived?

Rex
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Dynoroom
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« Reply #18 on: January 29, 2016, 06:43:00 PM »


Quite funny really.... If I can get the adiabatic part working I could do anything.
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DaveB
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« Reply #19 on: January 29, 2016, 08:59:59 PM »

The proper background of the fictitious and adiabatic equation can be quickly determined by checking the Douglas Adams derivation of 42 on Google.

You may also notice that I did not specify units for any of the variables. Using any units you prefer yields the same quality of answer.
« Last Edit: January 29, 2016, 09:10:41 PM by DaveB » Logged

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jdincau
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« Reply #20 on: January 29, 2016, 09:10:18 PM »

But you would need the Infinite Improbability Drive to apply it
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DaveB
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« Reply #21 on: January 29, 2016, 09:12:03 PM »

Absolutely!  grin
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Polyhead
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« Reply #22 on: January 29, 2016, 10:07:07 PM »

Well the real key to all of this is now do they make SFI rated towels.
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Rex Schimmer
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« Reply #23 on: January 30, 2016, 09:34:53 PM »

DaveB said "You may also notice that I did not specify units for any of the variables. Using any units you prefer yields the same quality of answer."  I did really miss the units part and you are so correct regardless of the units used the answer is BS! 42 is obviously the universal fudge constant, it is all so clear now!

Perfect!

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DaveB
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« Reply #24 on: January 31, 2016, 10:06:47 PM »

 cheers                cool
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« Reply #25 on: January 31, 2016, 10:13:34 PM »

No one pointed out that properly solving this equation also requires a firm understanding of bistromathics.  cheers
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Ah, this is obviously some strange usage of the word 'safe' that I wasn't previously aware of.  Douglas Adams
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« Reply #26 on: February 01, 2016, 09:44:13 PM »

You guys are wacked out on goofballs. LOL
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« Reply #27 on: February 02, 2016, 09:16:48 AM »

Thank you   grin
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« Reply #28 on: February 09, 2016, 02:36:03 AM »

Down Force = V^2 x A x RH / CD  +  HP x RPM / AC^3
                                               ULC

RH relative humidity
CD coefficient of drag
A area
HP horsepower
AC acceleration
V velocity
RPM rev/min
ULC universal L Constant

The ULC is an experimentally determined constant. Sometimes 42 is applied but this is not suggested. Please read Douglas Adams' research on 42.

The fictitious equation above should be applied under adiabatic conditions.
ROTFLMAO!!!

Seriously, the equations expressed before this post are so far off of reality they defy description.
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« Reply #29 on: February 17, 2016, 10:55:08 AM »

I hope that everybody by now realized this equation was a joke. I did call it fictitious and others also joined in on the silliness in their  posts.  smiley It seemed like an appropriate way to answer the same repeated questions. My post #14 did show that any equation to determine down force would have to be derived for the specific application. Because of this, any equation that did it in terms of generalities would be a joke. Come on, engine RPM directly relating directly to down force?  rolleyes   cool
« Last Edit: February 17, 2016, 10:56:59 AM by DaveB » Logged

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