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Author Topic: Center of pressure calculations  (Read 5884 times)
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Slide
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« on: January 10, 2017, 06:50:59 PM »

I'm sitting here with my 17 month old daughter and we both have a cold and are miserable... so I'm trying my hardest not to be lame and asking to be spoon fed... but please forgive me because I barley have the mental capacity to put my socks on right now.

Because the whole center of gravity vs center of pressure is pretty dang important for a safe run... what am I in for to trying to decipher/calculate/rock paper scissors the center of pressure on my car?

Thanks for the heads up! Avoid my house like the plague...cuz we would spread it... (send tissues)...
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John Burk
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« Reply #1 on: January 11, 2017, 02:13:55 PM »

The only way to find the CP of a car is hang a model from a thread and blow air at it . The center of area method is not valid .
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Slide
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« Reply #2 on: January 11, 2017, 02:27:02 PM »

How accurate does addition of flat floor, and any other aero modifications need to be?
Luckily so far I don't have much...
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WOODY@DDLLC
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« Reply #3 on: January 12, 2017, 10:12:46 AM »

3D CP does not coincide with the 2D CG of a piece of cardboard.  angry


* CP Location.jpg (58.12 KB, 605x838 - viewed 172 times.)
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All models are wrong, but some are useful! G.E. Box (1967) www.designdreams.biz
bbarn
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« Reply #4 on: January 12, 2017, 10:43:40 AM »

Print a 3D model of it and place holes every 1/4" or so. Stick a rod through the holes and take a drive in the passenger seat of a friends car. Move the rod to different holes and present the model to the airflow. Once you get a hole that is stable, press the nose/tail of the car to the side and release it. Observe the effects. Once you have determined which hole gives you the best performance you can calculate the distance from the nose to the location on the full-size model.

Print a couple of the models so that when you figure out where it is roughly you can drill the second model with a different hole spacing close to where your CP is. This way you can hone in on the exact location.

Rob and I did this with the liner on a sub-freezing night at about 45mph down a country road. Not real high-tech but it is accurate.
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I almost never wake up cranky, I usually just let her sleep in.
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« Reply #5 on: January 12, 2017, 11:14:15 AM »

What scale should I be looking at? I can find diecast models of my care in 1:43... anything else and I'll need to find someone with a 3D printer...

But if I do the 3D printer I'll try to incorporate as much of the potential aero ideas that I have.
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tortoise
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« Reply #6 on: January 12, 2017, 01:18:05 PM »

Or, you might just read the rulebook, do everything allowed to move the CP back as far as possible, and call it good enough. What class will the car be?
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Slide
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« Reply #7 on: January 12, 2017, 01:25:07 PM »

Its not a scta event. Its going to be for airstrip attack. so standing mile and mile and a half. Rules are very relaxed. After that, who knows...fun track day car, or maybe some time attack stuff.
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wobblywalrus
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« Reply #8 on: January 13, 2017, 12:11:53 AM »

That virtual modeling that Woody does makes sense.  It can be used to determine CP and a lot of other useful things.
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Sequim Jim
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« Reply #9 on: March 25, 2017, 07:56:07 AM »

Sorry for sounding stupid, but what does this mean? I've been in the aviation business for many years and built drones and full scale aircraft. When you know your CoP, are you trying to balance the weight of the vehicle or motorcycle to match that location?
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Jim
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« Reply #10 on: March 25, 2017, 08:26:54 AM »

Land vehicles need the CoP aft of the CG so that they have inherent straight line stability, otherwise they can spin out at the "drop of  a hat"....

Think of the stability issues of "flying wing" aircraft vs. conventional tail design aircraft ...

Robert
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bbarn
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« Reply #11 on: March 27, 2017, 07:16:22 AM »

Sorry for sounding stupid, but what does this mean? I've been in the aviation business for many years and built drones and full scale aircraft. When you know your CoP, are you trying to balance the weight of the vehicle or motorcycle to match that location?

I use the example of a weather vane as it uses extreme CP/CG ratio to work.

If you think of a traditional weather vane, you have a large weight on the front (arrowhead, chicken head....) and a large tail of some sort on the back end (arrow feathers, chicken tail feathers...). What is happening with the weather vane is that you have the weight of the arrowhead (CG) at the front and the aerodynamic drag of the feathers (CP) at the rear. In the arrangement of a weather vane you want the arrow to point into the wind. The further the CG is ahead of the CP the easier it is for the weather vane to point into the wind.

Now, if you have a calm still day and the wind suddenly picks up on the rear of the vane it will snap around to point into the wind. A smaller example of the same principle is the arrow. Point (CG) in the front and the tail feathers (CP) in the rear. With the ratio that it has it helps to hold the arrow in a straight line. If you drop an arrow tail first from enough height it will turn around and fall point first.

The problem with extreme CP/CG ratios is that on a vehicle (car, rocket, motorcycle...) with extreme ratios it will want to weather vane into the wind. Using toy rockets as an example if you make it nose heavy with big tail finds it will launch and immediately turn into the wind. If you want to get it to track straight or close to straight you want a small ratio between your CP/CG. In model rockets about one caliber of the rocket body (diameter if you will) between the CP and CG is good. This will keep the pointy end forward and reduce the effects of the rocket wanting to weather vane.

Same thing for a car. If you have a wide ratio crosswinds will affect you more severely. If you have a reversed ratio (CP ahead of CG) you will have the car wanting to turn around when the aero becomes strong enough to affect the vehicle.

In your weight and balance sheet on the airplane you are actually calculating the ratio of CG/CP. The main difference is you are also calculating the force of the elevator to be able to maintain leverage over the pitch of the aircraft based on weight. The vertical tail and elevators on the airplane are sized so that at MTOW the CP/CG ratio is in check. In aviation terms it is more important to maintain the weight and balance so that the elevator can maintain control since the CP/CG ratio is already engineered into the calculation. It is likely you can't move the CG rearward enough to effect the ratio without first being outside the balance/elevator control envelope. You would likely just climb vertical at V2min into a stall and end up on your lid if you could.
« Last Edit: March 27, 2017, 07:25:34 AM by bbarn » Logged

I almost never wake up cranky, I usually just let her sleep in.
Sequim Jim
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« Reply #12 on: March 27, 2017, 07:58:22 AM »

If you transfer your calcs to an airplane, you are making your vehicle nose heavy. Depending on how nose heavy, you need more speed for takeoff and lots of elevator movement. Tail heavy makes the elevator very sensitive and the aircraft unstable but works for aerobatics, which is bad on the salt.
On a motorcycle, how nose heavy? Slightly to very?
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Best regards
Jim
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« Reply #13 on: March 27, 2017, 10:16:42 AM »

A 1/10 wooden model can be band sawed in a few minutes . Draw the plan view and side view profiles on a block , saw and round the corners . It's surprising how far foreword the neutral point is with air blowing on it . Tail's don't give directional stability . They reduce the instability .
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