Center of Pressure, tip plate location

[Mac3170]

Does it make any major difference in center of pressure location if the tip plates on a streamliner are installed near the outside of the body sides, or, if they are inset about 12inches on each side? I do not want a tail or a wing, just tip plates.

For mounting strength & ease of removal for transport, I want to install them inset.  Our design will need them & I do not want to give up any significant benefit by insetting them.  I have not found any discussion or information on this particular subject. 

[interested bystander]

There's a whole bunch of us semi-knowledgeable aero people out here and some that may actually know something, but I'm not clear as to what you mean- tip plate with wing or do you mean tailfin? If wing,where's the wing located?etc.

With a little clarification I think you can get a useful answer from this group.

[Glen]

I agree with IB, give us more information to work with. As far as a tail in should be considered if you plan on going fast. Is the liner front or rear wheel drive or both. More info on your part.

[Sumner]

Does it make any major difference in center of pressure location if the tip plates on a streamliner are installed near the outside of the body sides, or, if they are inset about 12inches on each side? I do not want a tail or a wing, just tip plates.

For mounting strength & ease of removal for transport, I want to install them inset.  Our design will need them & I do not want to give up any significant benefit by insetting them.  I have not found any discussion or information on this particular subject. 

I didn't know what they were either, but looks like they are what most of us call "spill plates":

http://performance.alternatefuelsracing.com/wings.html

I would say they are going to be more effective out by the body sides.  So are you going to use a wing for down-force??

c ya,

Sum

[Mac3170]

I do not want to use a wing.  The design requires rear bodywork to move the CP farther back. In my racing world we call them "tip plates", or "spill plates".  I guess it would be logical to refer to them as "tailfins".  I want to inset them about 12 inches for a variety of reasons, but cannot find information on the effect that has compared to mounting them at the outer body line.

I would appreciate any information from "semi-knowledgable areo people", or heck, even opinions with reasons.

[Sumner]

I do not want to use a wing.  The design requires rear bodywork to move the CP farther back. In my racing world we call them "tip plates", or "spill plates".  I guess it would be logical to refer to them as "tailfins".  I want to inset them about 12 inches for a variety of reasons, but cannot find information on the effect that has compared to mounting them at the outer body line.

I would appreciate any information from "semi-knowledgable areo people", or heck, even opinions with reasons.

If they are set in will they see the air if the car starts to get sideways or will the body disrupt the air going to them??  You might want to look at this page:

http://purplesagetradingpost.com/sumner/bvillecar/bville%20-%20LSR%20Thoughts-4.html

I think it is going to depend on the shape of the body as seen from the side,

Sum

[Jonny Hotnuts]

Does it make any major difference in center of pressure location if the tip plates on a streamliner are installed near the outside of the body sides, or, if they are inset about 12inches on each side? I do not want a tail or a wing, just tip plates

I think I have a fairly good idea what you are asking.

Answer:
No

The spill plates can be inset and still have the same affect providing they are not so physically close that the adjacent plate doesn’t provide air to surface contact (basically the air would see 2 spill plates as one, for instance when you are parachuting and want to F with you buddy so you go fly under him and stall the air causing him to free fall for a moment…..always good for a laugh). They would have to be really close before this would become a problem if they were the size of the standard spill and even less if there were no other physical obstructions around them.


-JH

[Mac3170]

Thanks to all who have responded, I now have the info I need to proceed....I think. We are making the spill plate mounts today.

Jonny Hotnuts commented "if they were the size of the standard spill", which leads to another question.  Is there a practical limit on the size of spill plates when two (mounted 2 ft apart) are used? I plan to make each approximately 4 ft x 4 ft, and mount them at the extreme rear of the car, essentially to extend the bodyline back.  Will this size work?

The rear deck height of the body is 30 inches, and the width is 40 inches.  It is being built by two old guys in Indiana who have never built such a car, and good info is hard to obtain. 

I really appreciate all the responses. I feel that I can turn to this forum to quickly get the information I need as I encounter questions.   Our goal is to end up with a practical & safe car.

[Stainless1]

Thanks to all who have responded, I now have the info I need to proceed....I think. We are making the spill plate mounts today.

Is there a practical limit on the size of spill plates when two (mounted 2 ft apart) are used? I plan to make each approximately 4 ft x 4 ft, and mount them at the extreme rear of the car, essentially to extend the bodyline back.  Will this size work?

The rear deck height of the body is 30 inches, and the width is 40 inches.  It is being built by two old guys in Indiana who have never built such a car, and good info is hard to obtain. 

I really appreciate all the responses. I feel that I can turn to this forum to quickly get the information I need as I encounter questions.   Our goal is to end up with a practical & safe car.

Mac, everyone that built their first LSR car had never built such a car, welcome to the club...
Do you have pictures? 
How do you plan to support your 32 square feet of material, what thickness do you plan, why not a fin... 
I can tell you that while flexibility is a good trait for racers, it is not a good thing for race cars....

[Mac3170]

Stainless, thank you for the welcome. We have so much to learn.

We are taking pictures as we progress, but all of our time is now being spent on construction.  Perhaps in a few weeks we will be able to take time to create a build diary.

Our support will be a chromoly tube support structure, bolted directly to the main frame rails.  We will have cross bracing between the  fins, or whatever they are called.  Thickness of the fins will be 1/4" or 3/8" (yet undetermined) honeycomb covered with aluminum skin.  We selected this material to prevent flexing.

We are still uncertain as to the appropriate size of the fins.  Is 32 sq ft too large?  What size is reccommended for a 1,000 lb motorcycle powered liner?

[Sumner]

......................We are still uncertain as to the appropriate size of the fins.  Is 32 sq ft too large?  What size is recommended for a 1,000 lb motorcycle powered liner?

I don't think anyone is going to be able to tell you that without info about the car.  Did you read the link about Center of Pressure and Center of Gravity??  Do a search on here also as it has been talked about before.  The key will be where is the Center of Gravity of the car and then with a cardboard cutout of the side profile of the car you can determine roughly where the center of pressure is.  You can then change the size and shape of the plates and determine if they will move the Center of Pressure where you want it.  I will not figure the size of the tail fin or if I even need one till about the very end of the build.  By then I'll have the cars weight and where the CG is and can then figure to fin size, shape and location to get the CP where I think I want it.

I agree with Stainless on the fin.  In most cases one fin will do the same as two of your tip plates on both sides of the car.  The one fin will have less drag and it is all about drag.

Taking a few hours to take and post some pictures for comment might save you lots more time than that in build time.

Good luck with the car,

Sum

[Jonny Hotnuts]

Once the vehicle is aerodynamically stable (cg in front of cp) by body design and weight, any spill plates larger than then they need to be will create unneeded drag.

Depending on the setup and weight the vehicle it may not need any spill plates at all to be aerodynamically stable at speed.

Pics would helpful.

-JH

[panic]

The added drag from more spill plate, fin, blah than needed is larger than its cross-sectional frontal area × drag coefficient, partially because air passing the sides will not always be normal to the fin axis, so you have surface drag as well.