Author Topic: Weight distribution / CG what is ideal  (Read 15653 times)

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Offline Salty Nuts

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Re: Weight distribution / CG what is ideal
« Reply #15 on: October 13, 2008, 12:05:36 AM »
Thanks for the suggestions all!
Panic- how do I calculate AC?

Offline Bville701

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Re: Weight distribution / CG what is ideal
« Reply #16 on: October 13, 2008, 02:14:51 PM »

  I asked Les Leggitt how much weight they had in the 333+mph Lindsley-Leggitt Firebird and he said ''more than you would believe'' this in altered class with roots blown hemi with max engine setback.

                                        JL222

The Kugel & LeFevers Firebird weighed a little over 5K Lbs., and that car had a top speed of 307 MPH.

I think the Lindsley-Leggitt Firebird weighed quite a bit less, but is one heck of a car!!!

Ryan
Ryan LeFevers

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Offline panic

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Re: Weight distribution / CG what is ideal
« Reply #17 on: October 13, 2008, 07:30:25 PM »
Unless you have a wind tunnel wide enough to place the car sideways, it's not easy.
However, you can approximate it using a wide F stop elevation photo (side view, as little perspective as possible) of the car.
Use any photo program, and color in the entire silhouette into a solid (wing sideplates, roof fins, hood scoop) primary color with no outline.
Now convert it to a .bmp, and use a little program I found (e-mail me for a copy).
Using the program, open the .bmp, and select the color, and it maps the color field and locates the AC and CG (but this is useless since weight not related to area).
The location will be at a proportion to the length in pixels, so just crank in your total length in inches blah to get the real AC on the physical car.
Not exact because cannot compensate for radii, effect of open wheelwell, etc. but a good start, and you can save the file, and photo the car again with new details (bigger scoop?), and see how far the AC moved.

Offline manta22

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Re: Weight distribution / CG what is ideal
« Reply #18 on: October 13, 2008, 07:38:27 PM »
Panic;

I'd like to have a copy of that .bmp calculator.

Thanks, Neil  Tucson, AZ    neil@dbelltech.com
Regards, Neil  Tucson, AZ

Offline bvillercr

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Re: Weight distribution / CG what is ideal
« Reply #19 on: October 13, 2008, 07:39:03 PM »

  I asked Les Leggitt how much weight they had in the 333+mph Lindsley-Leggitt Firebird and he said ''more than you would believe'' this in altered class with roots blown hemi with max engine setback.

                                        JL222

The Kugel & LeFevers Firebird weighed a little over 5K Lbs., and that car had a top speed of 307 MPH.

I think the Lindsley-Leggitt Firebird weighed quite a bit less, but is one heck of a car!!!

Ryan

The Lindsley-Leggitt Firebird is the fastest one way car ever.  I think he went 300 in the first mile, and 323 in the middle mile and shut down. :-o

Offline bvillercr

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Re: Weight distribution / CG what is ideal
« Reply #20 on: October 13, 2008, 08:26:41 PM »

  I asked Les Leggitt how much weight they had in the 333+mph Lindsley-Leggitt Firebird and he said ''more than you would believe'' this in altered class with roots blown hemi with max engine setback.

                                        JL222

The Kugel & LeFevers Firebird weighed a little over 5K Lbs., and that car had a top speed of 307 MPH.

I think the Lindsley-Leggitt Firebird weighed quite a bit less, but is one heck of a car!!!

Ryan

The Lindsley-Leggitt Firebird is the fastest one way car ever.  I think he went 300 in the first mile, and 323 in the middle mile and shut down. :-o

Ooops that was 333mph.  :-o

Offline Sumner

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Re: Weight distribution / CG what is ideal
« Reply #21 on: October 13, 2008, 10:06:51 PM »
Thanks for the suggestions all!
Panic- how do I calculate AC?

You can get pretty close by taking a picture like panic suggested.  Print it as large as possible.  Cut the car out.  Lay the cutout on some thicker cardboard and trace the car on the cardboard.  Cut that out.  Now take some thing with a sharp edge on it like a knife.  Put the sharp edge up and put the cardboard cutout on it like a seesaw.  Move the cutout back and forth until it balances.  Where it balances is close to the Center of Pressure since there will be the same area (weight of cardboard) on both sides of the balance point.

I have some thoughts on CG/CP here...........

http://purplesagetradingpost.com/sumner/bvillecar/bville%20-%20LSR%20Thoughts-4.html

.......... but they are the same as what panic is saying.

c ya,

Sum

Offline panic

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Re: Weight distribution / CG what is ideal
« Reply #22 on: October 13, 2008, 10:11:00 PM »
Yup - and if you feel like living dangerously, you could "adjust" the cardboard by trimming off an extra (example only: .010" for every 1" of radius) of the outer edge surface, windshield, roof etc. to tilt the shape away from accepting the silhouette literally as if it were a billboard.

Offline hotrod

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Re: Weight distribution / CG what is ideal
« Reply #23 on: October 14, 2008, 03:43:44 AM »
I have to respectfully disagree with the centroid method of estimating the center of pressure.

The side on center of pressure (even if the centroid method worked which is highly debatable), only matters if the car is already spinning.

You want to know the center of pressure when the car is in its normal attitude and pointing within a few degrees of the local air flow. The bad news is that the center of pressure moves forward as the car goes faster so the center of pressure at 150 mph will be different than the center of pressure at 200 etc. That is why some cars have a "magic speed" which you can put money on them spinning if there is the slightest upset to the car. That is the speed where the center of pressure gets very close to the center of mass. Once the center of pressure is no longer behind the cars center of mass, there is no aerodynamic stability (tail feathers on an arrow) provided by the air flow, and the car is just as happy going backwards as it is frontwards.

Once the center of pressure gets ahead of the center of mass (if you can keep it straight that long) you have absolute aerodynamic instability where the aerodynamic forces are trying very hard to turn the car around so the center of pressure a behind the center of mass.

The only way I know of to get a good guess on the location of the aerodynamic center of pressure at low angles of attack (car pointing more or less in the direction it is supposed to be) is to use an accurate model and suspend it so it is free to swing about the support point and blow air over it or flow water over it. When you get the point of suspension at the center of pressure the model will become very unstable and try to spin in the flow.

Either that or buy wind tunnel time or find an aeronautical engineering student who can use some of the newer numerical aerodynamic modeling software to get you a center of pressure.

The centroid method only gives you good results when the car is way out of shape ie nearly sideways, and if and only if the side on drag of the front half of the car body is similar to the rear half. If you have a shape that is smooth and sleek from the side in the front so it has low drag like many tubular streamliners but has a flat fin like body in the rear your area centroid and side on center of pressure will be totally wrong, even for side on air flow.

See the last post in this thread --
http://www.landracing.com/forum/index.php/topic,2991.0.html

and this thread --
http://www.landracing.com/forum/index.php/topic,1594.0.html


Larry
« Last Edit: October 14, 2008, 03:49:38 AM by hotrod »

Offline panic

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Re: Weight distribution / CG what is ideal
« Reply #24 on: October 14, 2008, 10:15:39 AM »
And I have to respectfully disagree with your comment.

Nothing said even suggests that any prediction of a tendency is precise, or includes all relevant factors, or should be maintained against empirical evidence.

Your comment, by making the case that since a prediction based on the CP in a static profile is not accurate at higher speeds it has no value and must be disregarded completely, suggests that no attempt at all at prediction (i.e., a random guess) is a better basis on which to begin construction and testing.

You're also offering a truism (CP ahead of CG is unstable) as if it favors only your method, and if not corrected precludes construction.
Inherently, all RWD cars are unstable under power - their stability relies on geometry external to the problem. A 400 mph car with CP < CG can still be driven - it just takes more skill, more precise controls, and fewer design errors. Many, many cost-no-object military aircraft are unstable immediately outside their design environment, that's why we spend millions to train pilots.

Offline hotrod

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Re: Weight distribution / CG what is ideal
« Reply #25 on: October 14, 2008, 06:03:34 PM »
Quote
Your comment, by making the case that since a prediction based on the CP in a static profile is not accurate at higher speeds it has no value and must be disregarded completely, suggests that no attempt at all at prediction (i.e., a random guess) is a better basis on which to begin construction and testing.


No not at all,I am also saying it could be in accurate at low speeds too. It is highly dependent on the body shape. On one car it might be close and on another car be very far off the mark.

I am saying bad data is worse than no data, especially if you do not realize that the data might be bad. You need to know what the limitations are of the testing process you are using. Yes the projected side on silhouette centroid will give you a scientific wild A** guess of where the center of pressure is when the car is only yawing 5 degrees to the air flow but it could just as easily be off by 10% -50% of the true location of the center of pressure at a 5 degree yaw. In short, unless your car fits a very small family of shapes that have very similar aerodynamic drag both front and rear and very little "fin area" in the rear, it might give you totally useless information, and even very bad information.

It is sort of like trying to do corner weighting on an unleveled floor with bathroom scales. It can be done but the value of the effort is doubtful.

Larry