CP vs CG

[tortoise]

Thanks to all for tolerating a nonracer's presence. I love this stuff.

[jl222]

.....You might be better off adding the weight you're talking about in one lump rather than two, at what would be the CG of the two lumps. This gives you the same CG relocation but a lower polar moment of inertia.

I agree with that and probably 80% of the weight we have added to the car is in weight boxes ahead of the rear tires and back of the driver with the CG about where the steering wheel is.  Running out of room there we also have weight under the transmission area and a small amount just ahead of the engine.  Cars are still compromises and we have had to make some.

I once built a 'pump trailer' that was used to do water testing and pump-down tests on wells for an anticipated strip mine.  You could pump or bail wells up to 400 feet deep with it.  To keep the cost down I used the drive-train from a 4 wheel drive Bronco to supply power for the winches and generator and such.  The drive train was pretty much intact except for swapping the front axle and rear axle locations and was mounted on a trailer.  So a lot of weight at both ends of the trailer.  The trailer was a real pain to tow since if it started to wag its tail it only got worst.  The next one I made had a different design and all the weight central to the trailer.  Lesson learned.  That is why I really try and tell people to not put the weight they need for traction back behind the rear axle.  We have nothing back there except for 1 normal size battery and another for data-logging only.

Edit:  Added what is below this at a later time.
=======================================



I added the photo above to show the 'approximate' locations of the CG and CP on the Stude.  Actually the CP would be a little further back as Tom B. pointed out to me that I should use the side view area of the vertical stablizers twice in predicting the 'approximate' location of the CP since both of them see the air.

I'll grant that it is going to be very hard to get a 'precise' location for the CP, but John Burk has pointed to one method to get close and I've used a different method that comes from people with far more knowledge than I have to figure ours.  I outline that method here for anyone interested....

http://1fatgmc.com/car/14-Hooley/14%20-%20hooley-construction-2014-2.html

I'm not saying you are going to be within inches of the CP with John's method or what I presented but it sure beats doing nothing about something that you can do to build a safer car,

Sumner

  Sum...how is traction with the CG so forward? Seems like it would suck.  Whats Hooleys best 2/14 and 1st mile time?

                  JL222

   Ps...its well known that trailers should have at least 10% of their weight on the tongue.
                  

[Sumner]

....  Sum...how is traction with the CG so forward? Seems like it would suck.  Whats Hooleys best 2/14 and 1st mile time?..    

We don't have a good run with the new motor as I was doing license runs and we felt the motor might of gotten hurt so quit after the last run where I got my A license.  On the one B lic. run where I got into probably 75% throttle and then 100% for a second until the throttle stuck the car was running 185 at the 1 1/2 mile marker before it started to spin and I'm sure it would of in the old configuration (pre-vertical stablizers/wing) but didn't.

With the old B motor that maybe made a little over 800 HP Hooley would run about a 217 at the 2 1/4 and set  the record at 249+ with a 253 exit speed.  With a 2.47 rear and a 1.91 first he could run about 50% throttle max in 1st gear (this is data logged data not driver impressions).  Would get to 100% throttle in the 1 to 1 3rd and in the overdrive 4th (.93).

It isn't just about the CG as you can have all different rear axle weights with the same CG.  In our case right now the car with the big block and the 2 foot extended wheel base is at 6420 lbs. minus a driver and has 3320 on the rear....



... other figures are in the pic above.

Now with the wing we will be able to add downforce without having to add weight hopefully (won't help a lot at lower speeds but at some point you will meet the aero wall where the car is going to spin the tires on the top end too and then we will have the benefit of the wing).  Even with the wing working we will still have the CG ahead of the CP as shown above since the wing hasn't moved the CG.

I think the car has a lot of potential.  On my A lic. run I didn't push it at all getting up to speed and ran the last couple miles constant throttle between 212 and 218 with the throttle in the mid 40% range.  We had changed gearing in the transmission and thought we had different drive gears than we really had and went less overdrive instead of more.  According to the tack I thought I was running 240 but that was based on having the ratio in the transmission we thought we had.  Since the timing slip and my calcs didn't agree back home I counted teeth in the transmission and then saw we had different drive gears than what we thought.  Plugging those in the calcs and timing slip now matched.  We were going back to World Finals with the right gears in the transmission but it rained out and then the same happened this year so we have yet to find out what the new combo is going to do but I think it will run well over 217 at the 2 1/4  ,

Not sure what the comment about tongue weight was about but if it was about that pump trailer I built it had well over 10% tongue weight (about 400 lbs. if I remember right).  The problem it had was the weight at the two ends of the trailer which still put the CG close to the axles or ahead but the weight, especially at the far back was not in a good place once inertia became a factor.

Sumner

[wobblywalrus]

In hydraulic engineering we have a term called center of buoyancy, center of buoyant pressure, or sometimes informally called the center of pressure.  Lets say you need to hold a car or dead horse shaped styrofoam shape under water with a stick.  The procedure done in the previous post would be used looking at the front, top, and sides of the shape.  Looking at the front would tell how far down the CB is in the shape, looking at the top would tell how far back the CB is from the front of the shape.  Looking at the side will tell how far back the CB is from the front of the shape.  Note there are two distances back from the front.  In practice we average them to get the distance from the front.  The gets us close.

Next, we look at the CG.  It should be at the CP if the thing is to stay horizontal under water.  Picture in your mind a freight submarine carrying a load of lead.  Carrying the lead too far towards the nose will cause the submarine to always want to dive.  Constant correction will be needed on the diving planes and it will need to be moving to stay level.  The plane angle will vary in proportion to the sub's speed.  Similar but opposite happens if the load is too far towards the rear.  Subs have ballast tanks they can partially fill with water to adjust their buoyant trim.

How the heck this method applies to a moving land vehicle is beyond me.  My feeling is it is the wrong method and its value is it is making people do the right things.   

[jl222]

....  Sum...how is traction with the CG so forward? Seems like it would suck.  Whats Hooleys best 2/14 and 1st mile time?..    

We don't have a good run with the new motor as I was doing license runs and we felt the motor might of gotten hurt so quit after the last run where I got my A license.  On the one B lic. run where I got into probably 75% throttle and then 100% for a second until the throttle stuck the car was running 185 at the 1 1/2 mile marker before it started to spin and I'm sure it would of in the old configuration (pre-vertical stablizers/wing) but didn't.

With the old B motor that maybe made a little over 800 HP Hooley would run about a 217 at the 2 1/4 and set  the record at 249+ with a 253 exit speed.  With a 2.47 rear and a 1.91 first he could run about 50% throttle max in 1st gear (this is data logged data not driver impressions).  Would get to 100% throttle in the 1 to 1 3rd and in the overdrive 4th (.93).

It isn't just about the CG as you can have all different rear axle weights with the same CG.  In our case right now the car with the big block and the 2 foot extended wheel base is at 6420 lbs. minus a driver and has 3320 on the rear....



... other figures are in the pic above.

Now with the wing we will be able to add downforce without having to add weight hopefully (won't help a lot at lower speeds but at some point you will meet the aero wall where the car is going to spin the tires on the top end too and then we will have the benefit of the wing).  Even with the wing working we will still have the CG ahead of the CP as shown above since the wing hasn't moved the CG.

I think the car has a lot of potential.  On my A lic. run I didn't push it at all getting up to speed and ran the last couple miles constant throttle between 212 and 218 with the throttle in the mid 40% range.  We had changed gearing in the transmission and thought we had different drive gears than we really had and went less overdrive instead of more.  According to the tack I thought I was running 240 but that was based on having the ratio in the transmission we thought we had.  Since the timing slip and my calcs didn't agree back home I counted teeth in the transmission and then saw we had different drive gears than what we thought.  Plugging those in the calcs and timing slip now matched.  We were going back to World Finals with the right gears in the transmission but it rained out and then the same happened this year so we have yet to find out what the new combo is going to do but I think it will run well over 217 at the 2 1/4  ,

Not sure what the comment about tongue weight was about but if it was about that pump trailer I built it had well over 10% tongue weight (about 400 lbs. if I remember right).  The problem it had was the weight at the two ends of the trailer which still put the CG close to the axles or ahead but the weight, especially at the far back was not in a good place once inertia became a factor.

Sumner

  Good job Sum on building a safe car.

            JL222

[wobblywalrus]

The center of buoyancy does not consider pressure differential over the object's surface.  It is like the center of gravity in this respect.  It depends on the volume of water displaced by the various parts of the shape and it stays pretty constant if the shape in not changed.

This is the question to ask yourself during the design.  Is the CP I am using the center of buoyant pressure or the center of the dynamic pressure.  They are different.  One is static for practical purposes and one is not.

[Interested Observer]

Wobbly:
Actually, the center of buoyancy does depend on the pressure acting at the various points of the object’s surface.  It is the surface integral of the pressure distribution. 
However, for the special case of complete submergence, the sum of all the surface forces works out to be equivalent to the weight of the displaced fluid, as you have stated.

For Pete’s sake, don’t tell JL222 that his car is losing “weight” because it is submerged in air!  (Which would be an advantage of running at Bonneville vs. El Mirage.)  That’s liable to start another endless and pointless discussion.

[Sumner]

.... Good job Sum on building a safe car.

            JL222

Thanks but Hooley did 90% of the build and John and I and a few others filled in the other 10%  .  Hoping for 1300-1400 HP, but that is only about 1/2 of what you guys make  ,

Sum

[jl222]

Wobbly:
Actually, the center of buoyancy does depend on the pressure acting at the various points of the object’s surface.  It is the surface integral of the pressure distribution. 
However, for the special case of complete submergence, the sum of all the surface forces works out to be equivalent to the weight of the displaced fluid, as you have stated.

For Pete’s sake, don’t tell JL222 that his car is losing “weight” because it is submerged in air!  (Which would be an advantage of running at Bonneville vs. El Mirage.)  That’s liable to start another endless and pointless discussion.

  IO..Some cars actually do loose weight at speed and the suspension can go into droop. Racers couldn't figure out the
ill handling at speed for years then Jim Hall and the Chapperall  put on a spoiler, then a wing.

  Didn't you know that ?

               JL222

[wobblywalrus]

That is something I do not know.  The work I did, and do, on this subject is on objects submerged in liquid fluid.  The center of dynamic pressure moves forward as velocity increases.  Centroids by definition are at the center of what they define.  More pressure on one side of something due to movement in a fluid shifts the dynamic centroid of pressure in that direction.  The problem with using buoyancy CP is that it might work for awhile.  Eventually, as the vehicle goes faster and faster, the dynamic CP moves too far ahead for stability.

This was demonstrated in a flume when I was in skool.  An object was hung by a cable and lowered into the flume where it was submerged.  Water velocity is slow at first and the object hangs from the cord without wiggling.  The water velocity speeds up.  The object starts to wiggle at higher flow velocities.  The dynamic CP is too far forward.  We monkeyed around with different shapes, fins, where we attached the cord, etc.  These had effects on the flow velocities when wiggling started.

I never figured out a method of determining dynamic CP that I am proud enough about to share.  A problem of that nature comes up once every eight to ten years so I never put much effort into refining the method.  My profession is civil and environmental engineering so I rough in a math based design and "field adjust" to get it to work.   The mechanical and structural folks on our staff get paid to think so they get the problem if it needs to be done correctly.     

[Interested Observer]

Didn't you know that ?

               JL222

Spoilers and wings on cars happened long before Paul Lamar, Chevy R&D, and Jim Hall made notable use of them.


Wobbly:
From your water flume description, it sounds like the “wiggling” was more a product of vortex shedding than a stability issue.  Vortices forming alternately on either side of the object travel down the side and then are shed off the tail, inducing an alternating pressure distribution and “wiggling”.  Have you ever heard or seen a powerline “singing” in the wind?
If it was an instability, the object would have turned and remained in another orientation (and possibly wiggled there).

[tortoise]

Vortices forming alternately on either side of the object travel down the side and then are shed off the tail, inducing an alternating pressure distribution and “wiggling”. 

I seem to remember reading on this forum about one of the prominent motorcycle streamliners of our day having a cover enclosing the chute exit. They ran without it because early testing showed just such a problem.  Is there a straightforward fix for this?

[John Burk]

The problem with discussing CP vs CG is knowing where the CP is . The simplest method is a wooden model on a string and an air hose like I've done which showed the CP to be 40% back from the nose.

The closest things I saw on the internet that look like a streamliner and mention CP are aircraft fuselages and bullets .

"The subsonic center of pressure of the fuselage will be located at or forward of the quarter-length point , and since the airplane c.g. is usually considerably aft of this point , the fuselage contribution will be destabilizing" .

http://thearmsguide.com/wp-content/uploads/2013/11/Featured-static-stab-620x350.jpg

My test and these disprove the CP is at the center of body area . Why is it so far forward . From what I've seen it's because pressure on the nose is a bigger part of drag than skin friction . That disagrees with "skin drag is 70% of total drag" . Still hoping to answer that contradiction . Door cars have the same forward CP but still looking for information on that .

[floydjer]

And  one proposal from Harry Miller while building the " Golden Submarine" for Oldfield was to install a moveable wing  atop the car for downforce.  In 1917

[maj]

That bullet stuff is interesting , i could not find further reference from those pages , but was CP dependant on velocity ?

Other info i found after this yrs stability issues on our bike was rocket design where they aim for one body diameter between CG and CP as ideal for stability
http://www.rockets4schools.org/images/Basic.Rocket.Stability.pdf, not sure i can achieve that sort of spread

also i wonder on 2 vehicles with same Cg and CP but different wheel placement (probably more on liners and motorbikes) , one with wheels near either end of the vehicle and other with one or both wheels closer to centre , just how stability would compare , i suspect any instability would be exaggerated by closer wheels