center of pressure ?

[hitz]

Started to build a G/GL a couple of months ago and need to know something about estimating the center of pressure. Is there a method to estimate it? I've got the cage done and the shape drawn up but haven't got a clue if I'm headed in the right direction.The drawing came out looking alot like The Grumpy Old Men lakester.Landracing.com has answered a lot of questions for me. Thanks for your hard work Jon.
 THIS IS FUN!

[JackD]

Take a cutout to scale picture that is the exact shape from a side view of your vehicle. The entire picture must be of the same material.
 Hang it from a thread at various points along the top until you find the balance point.
 The side preassure is from a side wind and the center of preassure is in the area seen by the wind.
 If you want to further prove the concept, build a complete rear engined roadster and throw it off a cliff to see which end gets there first. This will be a lesson in CP and CG.
 ENJOY
 
  <small>[ December 31, 2004, 04:23 AM: Message edited by: JackD ]</small>

[hitz]

Jack, Thanks for the information.One more question: Is the best location for the CG on a vertical line with the CP? Thanks again.

[JackD]

Panic is exactly correct.
 I worry about his name though.
 Two things, I wouldn't let my driver operate under that name or build a back motored roadster either.  LOL

[ddahlgren]

Ah the infamous 'weather vane theory'.. I don't think you want to look up at the barn roof to make an important aero decision..
 Dave

[John Burk]

Hitz - If you haven't been to one of the Bonneville meets it might be good to not finalize your design till you've looked at some of the lakesters and talked to the builders - The CP of an air foil shape pointed into the wind is 25% back from the nose - For a lakester it no doubt is farther back - Sideways to the wind the CP is probably around the center of area - Look at what works for others and use a tail fin for insurance - John

[hitz]

John Burk, Jack D and all, Thanks for the info,the tail fin was on the drawing. I spent a couple of days at speed week this year but didn't talk to any lakester owners. My eyes and ears were enjoying it so much I forgot to ask questions.Not much of a chance the lakester being done by speed week this year but the basic design will be locked in by then.Will probably go help Jim Rotta in 2005. See you then.

[JackD]

Remember, anything that puts preassure on 1 side, sucks on the other.
 Ask anybody that has run canards in a cross wind.
 Ground effects are designed to suck down, until you get sideways with the wind, then they really suck when you fly.

[panic]

Let me guess: opinions here are given relative weight here in much the same manner as in the real world?
 "My car is faster than yours, therefore my explanation of how it works is correct".
 And Donald Trump is a financial genius.
 It's been fun...

[JackD]

I don't think the LAKESTER has been perfected yet.
 Things that work OK at 50 mph, won't do as well at 250 mph. Just ask the guy with the 250 mph CORVAIR.
 While rules and laws are said to be made to be broken, they can really be tough on your program.

[Rex Schimmer]

Hitz,
 I like to use a book called "Race Car Aerodynamics" by Joseph Katz as my reference for any aero questions that I have and Katz defines the Cp (center of pressure) as the point that a resultant single force can be applied that is equal to the total force generated by the aerodynamic pressure distribution, in side view, on the car. So the Cp in the side view is simialar in concept to the center of gravity. It is dependent on the aero shape of your car in plan view, top view, i.e. if your car has some aero shape in plan view such that it generates an aerodynamic force then the aero center of this shape is what the Cp is. Adding tail fins is a great way to move the Cp back as the fin is an aero shape and can generate a substantual aero force if the car starts to get side ways. And of course as JackD and Panic are correct that once the car gets really side ways the position of the Cp can and usually does move, and never in the direction you really want so the tail fin is a good plan.
 Rex

[JackD]

Some of the oldest cars have the biggest fins.
 That is same the reason they get to be old.
 Fighting a wrong car gets old too.
 The aero laws even apply to open bikes, but are less widely understood.
 
  <small>[ January 03, 2005, 06:00 PM: Message edited by: JackD ]</small>

[Rex Schimmer]

Hitz,
 Just another comment if you decide to do a tail fin on your car. I am also working on a lakester design and am planning to make my tail fin triangular in shape and with a sharp leading and trailing edge but the basic cross section shape would be a flat plane, i.e. it will not have an airfoil shape in cross section. I am doing this because this type of fin is low drag and generates "lift" or the aero force to push the car back on a straight line, by generating a high speed votex on the back side which generates both the aero force and a large increase in drag, both forces that are pushing the car back onto a straight course and help you slow down. Any comments regarding this idea from you "old salts" would be appreciated.
 Rex

[Ken Walkey]

Rex, question---
 What advantage does your fin design have over the basic design found on some very fast aircraft? Second, with regard to stability. I have always bought into the idea that you need stability when two to four wheels and tires come off the ground, and you are airborne. I also kind of like the idea of a dart, or an arrow where the mas weight is forward when the vehicle does go airborne. I have built both front engine, and rear engine vehicles, also both front and rear drive. I do believe a fin of any sort is a must on LSR vehicles and regardles of shape or contour, is a good idea.
 The rear engine cars "are fun to drive" because your always trying to keep that light frontend in front of you. The faster you go, the lighter it seems to get. A vehicle with the mas, or most weight in the front seems to handle better at high speeds for me (even at 35ft. of the ground, El Mirage,July '93)

[Rex Schimmer]

Ken,
 As I said in my first note, this type of fin design, one without an airfoil shape but a triangular shape if you look at it from the side and with sharp leading and trailing edges, generates lift or an aero force to help assist stabilizing the car by what Katz, (author of Race Car Aerodynamics) calls votex lift. Look at the wing and tail of a F-102 or F-106 these are vortex lift airfoils. That is as the sharp edge triangular shape is turned to the wind, given an attack angle, a high speed vortex is generated on the leeward side and as this is high speed air it is also low pressure so therefore lift is generated, also the Cd of this type of lift device is pretty high, so it generates a lot of drag when it is making lift, which I would think would help stabilize the car. Now this is just what seems "right" to me and I have not really sat down and done any real number grunching and probably won't. I will probably just make two rear stabilizers for the lakester that I am planning, one with this design and one with a NACA 6000 symetrical airfoil shape and the one that works best will stay on the car.
 
 I do agree with you completely that you need to keep the weight of the car forward, like a dart, to keep the center of gravity ahead of the center of pressure and adding a fin to the rear moves the center of pressure rearward.
 Rex