Author Topic: Handling Question  (Read 2762 times)

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Offline Fheckro

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Handling Question
« on: April 17, 2010, 08:13:39 AM »
Hi All,

I have an 80 Monza (see avitar).
I'm considering it for use at Maxton.

I read a comment in one post about Monzas wanting to swap ends at high speed.

That brings up the questions ... what exactly is "Center of pressure"?    How is it measured?
                                           Is it better to have the engine moved forward or rearward for land speed racing?
                                           What is the best relationship between center of pressure and center of gravity?
 
  Fred
9479 Monza C/CGC
Current record Holder @ 180.000

Online bbarn

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Re: Handling Question
« Reply #1 on: April 17, 2010, 08:41:43 AM »
Here is a simple analogy to understand the center of gravity and it's relationship with the center of pressure. Think of an archers arrow, when it is shot from the bow, what makes it keep the pointy end forward? In short, it is the relationship of CG to CP.

Remove the fletching and examine the arrow in your mind, assume that there is no heavy weighted tip and the entire arrow weighs the same from tip to tip. The center of gravity would be the middle of the arrow since the entire structure has the same mass. If you were to shoot that arrow, it would not keep the pointy end forward because the arrow would want to rotate around the center of gravity. The aero forces running around the shaft would make it wiggle and wobble, but it would not fly straight.

Now, take the arrow and put a pointy heavy tip on it. Now, shoot it and imagine what it does. The added mass to the front of the arrow moves the center of gravity forward. This will make the arrow TEND to want to stay pointy end first, but it will not fly true.

Add the fletching back on and viola, it wants to fly true. The fletching provides stability by moving the center of pressure to the rear of the center of gravity. By holding the relationship of CG forward of CP, you gain stability. The reason some cars want to swap ends is that the CP is too far forward, or in front of the CG. If a car that had tendancies to swap ends had some modifications to move the CP aft, it would prevent them from spinning, it would also move you out of several classes and increase the drag on the vehicle.

Wait for the roadster guys to chime in, they have classic examples of swapping, spinning and some tricks to minimize its occurrence.
I almost never wake up cranky, I usually just let her sleep in.

Online bbarn

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Re: Handling Question
« Reply #2 on: April 17, 2010, 10:41:09 AM »
btw, get a copy of this book, its an easy read and has lots of good info on this as well as other aero concepts.
http://www.amazon.com/Race-Car-Aerodynamics-Engineering-Performance/dp/0837601428
I almost never wake up cranky, I usually just let her sleep in.

Offline jl222

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Re: Handling Question
« Reply #3 on: April 17, 2010, 12:34:09 PM »
  After many years of racing and several years ago it was discovered why race cars were unstable at high speed by Chapperal racing and Chevrolet and the rear spoiler was discovered.
  Fred Dannenfelzer has gone 386 mph with his [rear engine] lakester + a wing for even more weight on rear end.
 In some cases center of pressure might help [streamliners] but in others achieving it will result in wheel spin and rear end swapping.
  It was discovered that after 165mph the rear suspensions were going into droop [ lifting up of bodies ] and loosing traction, especialy
in coupes.
  I would suggest the biggest spoiler allowed and some weight over axle.

                  JL222

  bbarns.... were is your center of pressure?
                

 
« Last Edit: April 17, 2010, 02:53:19 PM by jl222 »

Online bbarn

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Re: Handling Question
« Reply #4 on: April 17, 2010, 12:55:36 PM »
On the lakester it was at the rear wing and supports, much like fast Freddie's. On the liner we are building, it is about 3" behind the leading edge of the wheel pants. The center of gravity should be right around a foot forward of the engine.

http://s889.photobucket.com/albums/ac92/bbarnhart_photos/Streamliner/Version4/
« Last Edit: April 18, 2010, 10:46:41 AM by bbarn »
I almost never wake up cranky, I usually just let her sleep in.

Offline racer x

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Re: Handling Question
« Reply #5 on: April 17, 2010, 01:52:21 PM »
I had a 1975 Monza 2+2 in high school. I put a souped up 283 in it. With no rear spoiler the wing shape of the body would lift and feel very  unstable at high speed.
Thank you to all the volunteers

Offline Graham in Aus

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Re: Handling Question
« Reply #6 on: April 17, 2010, 05:17:23 PM »
I guess there is no real way measure Cp, other than ratio of area's front to rear?

What's Ack up to here?





Centre of gravity??  :?

'Chute mount integrity testing?  :-P
« Last Edit: April 17, 2010, 05:19:44 PM by Graham in Aus »

Online bbarn

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Re: Handling Question
« Reply #7 on: April 17, 2010, 05:28:27 PM »
'Simple method' for calculating the CP - http://exploration.grc.nasa.gov/education/rocket/rktcp.html

The important part is here:
For a model rocket, there is a simple mechanical way to determine the center of pressure for each component or for the entire rocket. Make a two dimensional tracing of the shape of the component, or rocket, on a piece of cardboard and cut out the shape. Hang the cut out shape by a string, and determine the point at which it balances. This is just like balancing a pencil with a string! The point at which the component, or rocket, is balanced is the center of pressure. You obviously could not use this procedure for a very large rocket like the Space Shuttle. But it works quite well for a model.
I almost never wake up cranky, I usually just let her sleep in.

Offline John Burk

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Re: Handling Question
« Reply #8 on: April 17, 2010, 07:27:18 PM »
The center of pressure of wings of all types is 24% to 28% from the nose . I did a cp test with a wooden model of my streamliner by hanging it from various points with my air hose aimed at it . At 36% it barely pointed into the air stream and at 38% it didn't know which way to point , the cp . The center of side area is at about 65% . For a landspeed car I think stability is a ballance between cp vs cg and lift vs rear tire grip .

Offline 1212FBGS

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Re: Handling Question
« Reply #9 on: April 17, 2010, 08:19:22 PM »
John
you absolutely did not find your aerodynamic "CP" .... you found the center of "YAW" if you put your pis in the sides and rehang it you will find your center of "Pitch" to help determine lift or down force... if you had a tail and put your pins in the front and pack you could find your "roll" center.... all of which don't tell you where your "CP" is located.... but when you really think about it your "YAW" is what keeps yer car goin straight....
Graham... ya really wanna know what Ack was up to in that shot? Its a really good example of a smarty pants who was able to talk a retired old racer (Jimmy Odem) into doin somethin he really shouldn't do....  in this case you can see witness standing just outside the impact zone (protecting the beer) and Jimmy in the riders seat.... fortunately no one was rushed to the hospital and Ack was able to determine his chute attachment location....
Kent

Offline Milwaukee Midget

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Re: Handling Question
« Reply #10 on: April 17, 2010, 09:18:11 PM »
Are you sure he's testing the bike?  He might be testing the crane . . . :roll:
"Problems are almost always a sign of progress."  Harold Bettes
Well, I guess we're making a LOT of progress . . .  :roll:

We are NOT rebuilding . . . We are reloading.

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Offline Fheckro

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Re: Handling Question
« Reply #11 on: April 18, 2010, 06:51:31 AM »
Ok thanks... just ordered the book. Sounds like the monza might me a good candidate.

 It looks like the CG of the  engine (as manufactured)  is slightly ahead of the front axle, which should give the vehicle a more forward CG

 As for body lift , this is a "cabriolet" top and not a hatch back. I wonder how that affects lift. seems like it should help kill lift.

Fred
9479 Monza C/CGC
Current record Holder @ 180.000