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Author Topic: center of pressure vs center of gravity  (Read 11029 times)

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Offline 631

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center of pressure vs center of gravity
« on: June 08, 2014, 02:20:59 PM »
A nice article on CP CG by Eva Hakansson and Bill Dube who campaign their electric side car streamlined motorcycle.  They are also engineers and well qualified to comment on the subject.  The article is filed in the tech section.  Thanks to them for writing it and Jon / Bob for putting on the site.

Offline John Burk

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Re: center of pressure vs center of gravity
« Reply #1 on: June 08, 2014, 08:17:01 PM »
The results from a test I did blowing air at a 3 dimensional wooden scale model of my streamliner found different results than the center of side area mentioned here . I hung it from various points by a .010" wire with a string above as a swivel and with a weighted wire below the nose so it hung level . The aerodynamic neutral point was 42% from the nose . The average area point calculates to 55% from the nose . I agree going sideways the average area point is correct but it's the pointing forward AC we care about . The front 12 ft of my body is 24" tall and rear 12 ft is 36" tall and has no tail and my 18" model is scaled to this . Adding a typical streamliner tail moved the AC back to 47% . Supported at 45% it stayed nose forward but hunted continuously I guess because a flat tail needs yaw to work .

Offline John Burk

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Re: center of pressure vs center of gravity
« Reply #2 on: June 08, 2014, 08:33:33 PM »
I should have said the center of side area is at 58%.

Offline wobblywalrus

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Re: center of pressure vs center of gravity
« Reply #3 on: June 20, 2014, 08:41:45 PM »
Bikes can be problematic.  They tend to have a rearward weight bias.  Things I do are:

1)  Position myself as far forward as possible in the streamlining shell.  The back of my azz is just ahead of the upper rear shock mount and I am tight up against the back of the tank.  The back of my feet are just under the upper rear shock mount.  It is not very comfortable, although it keeps my weight toward the front.

2)  Run with a full tank.  This puts some weight toward the front.

Offline maj

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Re: center of pressure vs center of gravity
« Reply #4 on: June 20, 2014, 10:16:44 PM »
I may be out in the left field here , but always considered there is another aspect were not giving enough attention on the bikes, and i suppose cars will have a similar effect
and thats the amount of drive your putting down , mostly i suppose from the torque reaction and how it interacts with the c of g taking weight from the front and effectively tranferring it higher or to the rear

but also how it changes the vehicle as it presents itself to the wind
front rising, rear squatting moving the cop forward

Am i overthinking this ??



Offline Old Scrambler

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Re: center of pressure vs center of gravity
« Reply #5 on: June 21, 2014, 02:36:37 PM »
Maj and others...........No engineer here.................I agree.........at least to the THINKING part.  I see and hear of high-speed runs with 'light' steering and "the bike wants to wheelie".  I keep wanting to see a wind-tunnel test with the smoke already mixed into the blown air at a larger circumference then the bike or car.  Sort of a BIG PICTURE effect.  Also..........does the percentage of vehicle length change with air-speed? 
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Offline wobblywalrus

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Re: center of pressure vs center of gravity
« Reply #6 on: June 22, 2014, 01:01:45 AM »
Vehicles like jets and rockets with the center of thrust at the same height, or higher, above the ground than the center of pressure will not lift their front ends at speed.  The center of thrust with wheeled vehicles is on the surface.  It is lower than the center of pressure so there is a tendency for the front end to lift.   

Offline panic

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Re: center of pressure vs center of gravity
« Reply #7 on: June 23, 2014, 04:17:07 PM »
For a simple program that calculates CP from your silhouette (as a .bmp drawing), Google "Martin Hepperle" and select "A.C. Calculator".

Offline SPARKY

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Re: center of pressure vs center of gravity
« Reply #8 on: June 23, 2014, 04:31:39 PM »
This statement may be more fitting here  :?


On my car I start out with most of my cooling water 20" behind the rear axle---during the run I actually move about  80# from behind the rear axle to 108" in front which really helps on trying to get CG more in front of the CP.


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Offline John Burk

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Re: center of pressure vs center of gravity
« Reply #9 on: June 23, 2014, 10:55:36 PM »
This says the CP changes with angle of attack .

http://exploration.grc.nasa.gov/education/rocket/Images/cp.gif


Offline Sumner

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Re: center of pressure vs center of gravity
« Reply #10 on: June 24, 2014, 12:46:56 PM »
This says the CP changes with angle of attack .

http://exploration.grc.nasa.gov/education/rocket/Images/cp.gif

That seems to make sense as the area ahead and behind the CP only really see the air when the angle of attack increases.  Hopefully the initial change in angle of attack is enough to straighten the car back out but in our case with the tires constantly spinning the car came quite a ways around before the vertical stabilizers saw enough air to stop the car from coming around more.  Of note was the fact that the constantly spinning tires presented a worst case scenario.  All of the other times when the tires spun and the car started sideways as soon as the throttle was reduced the car immediately straightened out.

One other item I observed was once the verticals caught the car it now took off in the direction the car was pointed until it then rotated the opposite way.  During these events if you look at the in car camera you can see that I was steering into the direction the car was spinning, first one way and then the other.  Just prior to the event I was running down the right side of the track just off the flags.  At the end of the event I was running off course on the left side of the track finally parallel to the course again.  I never steered over there the car went there after the verticals caught the car and sent it in that direction.  You can see the flags and where I started on the course and were I ended up off of the course in the down-track camera video.

If the tires would of quit spinning after the first direction change I think that would of been the new direction when the chute came out, away from the track at an angle,

Sum
« Last Edit: June 24, 2014, 12:55:38 PM by Sumner »

Offline GH

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Re: center of pressure vs center of gravity
« Reply #11 on: June 24, 2014, 01:07:16 PM »
Sumner, maybe you need to push a little easier on the loud pedal and keep the tires hooked.

Offline Sumner

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Re: center of pressure vs center of gravity
« Reply #12 on: June 24, 2014, 01:15:31 PM »
Sumner, maybe you need to push a little easier on the loud pedal and keep the tires hooked.

Throttle stuck WOT. believe me it wasn't intentional  8-).  How is the Atlas 6 running in the pickup?

Sum

Offline John Burk

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Re: center of pressure vs center of gravity
« Reply #13 on: June 24, 2014, 02:46:35 PM »
We've talked before about how tires have a given amount of traction to share between acceleration and resisting yaw . Use it all for accelerating or braking there's none left for side to side traction .

Offline panic

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Re: center of pressure vs center of gravity
« Reply #14 on: June 25, 2014, 11:27:20 AM »
I think it was Mark Donahue who referred to that as the "circle of life" 50 years ago: it's a vector in which you can get more traction shared between accel and yaw, but it's less than the numerical sum.