Frontal Area / Cd numbers

[robfrey]

I know you heard it before but in case you haven’t for a while, keep thinking of the game this way-
5 mile drag race on very wide dirt road.
Aero is important but it only has to be aero enough. Acceleration, thermal management, proper chute deployment, crash survivability and reliability all come before aero.
Take it from someone that has made this mistake before.
From my testing so far, I’m liking active aero.  I think it holds a bit of an advantage over 4x when it comes to the 6.5 mile FIA course but that has yet to be proven.
I’m sure Eddie is liking his 4x as his car is getting very successful!
This diversity is the coolest thing about our sport.


Rob Freyvogel
#496
AA/BFS

[Simspeed]

A project of this magnitude would have one goal: fastest wheel-driven car. Nobody'd much care what SCTA class it fit.

Exactly tortoise...that was my intention from the beginning of this design.  Feasible or not is yet to be determined, but from a systems standpoint there's nothing I've proposed so far that falls outside of known capabilities.  Aerodynamics and wheels are the greatest unknowns and I'm working on the aero limitations first because that's the least expensive project for me to tackle at this stage. Aero rules in my opinion.

Where we go from there depends on the Cd, Cp, and Cg values and their relationship as drawn or amended.  Thanks to everyone for their input and experience in helping a novice like me get up to speed on what it takes to design, build, and hopefully one day drive this design, or something near to it to its full potential.  If it's not me that makes it happen then i'd be more than happy for someone else to take the reigns and bring this to life.  For me its about seeing my vision fulfilled for that satisfaction alone.  I don't need to be the center of attention...the design will hopefully stand alone for better or worse. Thanks again...I am in your debt.

[Simspeed]

...This diversity is the coolest thing about our sport.

Rob Freyvogel
#496
AA/BFS

Thanks for your input Rob...I am a great fan of your design and effort.  Best of luck to you and your team on a successful chase to achieve your goals.

[Simspeed]

Today I printed a 30" scaled profile of the 36' body and glued it to a piece of Walmart foam poster board with some Super 77 I had on the shelf.  Cut it out cleanly with an Exacto knife and laid it across an triangle architect scale to find the balance point.  After some doing I pinpointed the balance center and transferred that to the computer model.  Turns out the theoretical Cp using this method is 10.278" to the rear of the theoretical Cg I came up with yesterday (simspeed 9 attachment ).  10 1/4" roughly which is only 2.38% of the overall 36' length.  

That's a pretty thin margin but it doesn't take into account the long narrow cross section at the nose relative to the fatter area from the roll hoop back past the real wheels. As Sumner said longer flatter areas should be more heavily weighted toward Cp which in this case I'd guess would probably move the Cp toward the real perhaps another 10".  It's all guesstimating but still there's some validity to the technique it seems.

I found an online aerodynamic modeling application at airshaper.com that I'm checking out.  Maybe just what I'm looking for to calculate the model's Cd numbers.  If I go that route I post the results to see what everyone has to say.

[jacksoni]

Aerodynamic stability is going to be very important in this design and will generate a lot of input I suspect. You are on the right track looking at it early in design considerations. But to another issue  that has been mentioned- packaging. There is not much space in here for the systems that are going to take up a lot of space. Look at the inside of ANY liner. There is no much available. One of those systems is going to involve heat management, not only from the engines but from the generator and electric motors. Granted, a run may be very short (a Speed demon video posted here suggested 70 seconds) longer if you can run on some longer surface (Bolivia for instance) but how are you going to cool this beast?

[robfrey]

Thanks for the compliment!
The question that too few ask.
How much further back does the cp have to be from the cg?
One of my hobbies is large model rockets. The basic rule of thumb is 1-2 calipers (body diameters).
Some say that is fine for rockets but this is cars. I contend that is one and the same. The road holding power that the tires have with the salt is just about next to nothing compared to the aero loads.
If you can get 1.0 calipers back, the car will be stable and cross winds will have negligible effect on weather vaneing.
In the rocket world, it is pretty cool when you get this right. Rocket pretty much goes right where it is aimed even in strong cross winds. It just gives a little wiggles indicated by smoke trail.
By the way, you might want to pick up Bruce Carmicheal’s book.
“Personal Aircraft drag reduction “.
You will have to search for it. It is out of print. It is very, very good for what we do.
Be aware that so far CFD does not do a great job with separation drag. So far no one has been able to ACCURATELY model what happens behind a semi truck. At least as far as Eric (Blue)and I know.
Remember the textbook equation for calculating aero drag.
Stagnation drag + Separation drag + Wetted area drag = Total Aero Drag
Cross section or displacement is not part of drag equation.
Optimum finess ratio (length to diameter) for subsonic design is a little over 3:1. We ended up at over 9:1 because of the tire choices available. It’s a long story. Lol. Anyhow, our car looks faster as we designed it but it would be much better shorter and fatter.
I’m excited for you! This is the fun part!
I look forward to following this build!
Feel free to reach out to to answer any questions. Tom Burkland did this for me. He called me up and said that he didn’t want to see me make the same mistakes he made. His input was invaluable and I saved and printed all our correspondence.  I can’t say enough about Tom.


Rob Freyvogel
#496
AA/BFS

[dw230]

I agree that the point of this exercise is being the fastest of the fast. Lets stop talking about recognized classes.

Being involved with a couple of cars using a front wheel set up similar to yours we discovered, actually Richie knew, you need Ackerman effect.

DW

[Interested Observer]

At this stage, this appears to be a fantasy CAD exercise with an interesting powertrain.  No provision for necessary ancilliaries or a realistic or competent structure.  It is doubtful that any kind of a one-off powertrain control system has been worked out.  With a razor’s edge nose, no suspension, scary steering linkage, and 9.5” track it would probably fall over in the first 200 yards, if that far.

[dw230]

Yep, a lot of paper race cars here. I remember a prop deal from a ways back.

DW

[robfrey]

Interested Observer- Must you be so negative?
He is just getting started on design!
Btw, YOU are the reason I almost quit posting!
As far as I know, no one ever built a monument to a critic.
Everybody has to start somewhere.

Btw, you were not always wrong but your tone definitely frustrated me.

[tortoise]

Stagnation drag + Separation drag + Wetted area drag = Total Aero Drag
Cross section or displacement is not part of drag equation.

Can you give us a rough idea of the percentage of the three components of drag on the Carbiliner?

Optimum finess ratio (length to diameter) for subsonic design is a little over 3:1. We ended up at over 9:1 because of the tire choices available. It’s a long story . . .it would be much better shorter and fatter

The long story about how tire availability made the skinny shape necessary would interest a lot of us, I think. If I remember rightly, many of the "critics" found that hard to believe about short fat bodies, with their higher frontal area.
Your kindness in indulging us in these theoretical discussions  is much appreciated.

[Simspeed]

I said my piece on naysayers early on but just know that I recognize the need for every voice in a conversation from cheerleaders to doubting Thomases.  How you want to contribute to the discussion and/or be viewed by your peers is 100% your choice.  This project is important to me succeed or fail; so I welcome input from wherever and whomever elects to do so.  I know just enough to get myself in trouble without the support of people who have been there and done this in their careers. So like others I may get aggravated by someone's tone but I appreciate your perspective and what you can bring to the table of ideas.  I can't do any of this with out help positive or questioning...so thanks to everyone again.

[Simspeed]

Aerodynamic stability is going to be very important in this design and will generate a lot of input I suspect. You are on the right track looking at it early in design considerations. But to another issue  that has been mentioned- packaging. There is not much space in here for the systems that are going to take up a lot of space. Look at the inside of ANY liner. There is no much available. One of those systems is going to involve heat management, not only from the engines but from the generator and electric motors. Granted, a run may be very short (a Speed demon video posted here suggested 70 seconds) longer if you can run on some longer surface (Bolivia for instance) but how are you going to cool this beast?

Right you are jacksoni...packaging is critical in a design this small. It may well make or break my approach if what's absolutely necessary won't fit in the shell.  I see heat management in the conventional sense within the envelop job #1. My intention is to design cooling systems for the IC and motor/generators to use pressurized fluids used by Nascar today. The brief run time is the only saving grace on that front.  The gen/motors for this application have cooling circuitry built into the housings using either oil or automotive type coolants. The short run time, and scaling power curve to peak output which like you say is measured in seconds rather than minutes, offers a pretty good window for heat soak and cool down time for IC and electrics both.  Casings for the rotary engines and gen/motors will both be machined billet aluminum.  No cast iron.

On board cooling capacity has to be limited because of the lack of space so my plan is to stage the systems for peak power through shutdown, and then use an external system in the chase truck to plug high pressure cooling lines into the car when it arrives to help lower the temps as quickly as possible.  The greatest on board cooling capacity will be to cool the intake charge downstream of the turbos in the fuel/coolant tanks on top of the rotor housings.  I'm wondering if there's a more effective cooling system base on compressed gas expansion rather than heavy and space consuming ice and water?  Spraying Nitros Oxide for example dramatically drops the temp of surrounding metal and air in those systems.  I believe I've read where NOS had been use in the past in some LSR cars.  Anyone know more about that?

[jacksoni]

Gary Eaker (owns the Aerodyne and A2 wind tunnels in NC) ran a firebird  to 298 mph in 1989 with N2O blowing onto intercoolers for the turbos. Though running on gasoline, because of all the N2O floating around I think he was put in a Fuel class. So yes can and has been done. CO2 could also be used. There are some street setups that do this.

[Simspeed]

Thanks for the compliment!
The question that too few ask.
How much further back does the cp have to be from the cg?
One of my hobbies is large model rockets. The basic rule of thumb is 1-2 calipers (body diameters).
Some say that is fine for rockets but this is cars. I contend that is one and the same. The road holding power that the tires have with the salt is just about next to nothing compared to the aero loads.
If you can get 1.0 calipers back, the car will be stable and cross winds will have negligible effect on weather vaneing.

Remember the textbook equation for calculating aero drag.
Stagnation drag + Separation drag + Wetted area drag = Total Aero Drag
Cross section or displacement is not part of drag equation.
Optimum finess ratio (length to diameter) for subsonic design is a little over 3:1. We ended up at over 9:1 because of the tire choices available. It’s a long story. Lol. Anyhow, our car looks faster as we designed it but it would be much better shorter and fatter.
I’m excited for you! This is the fun part!
I look forward to following this build!
Feel free to reach out to to answer any questions. Tom Burkland did this for me. He called me up and said that he didn’t want to see me make the same mistakes he made. His input was invaluable and I saved and printed all our correspondence.  I can’t say enough about Tom.

Rob Freyvogel
#496
AA/BFS

Thanks for you input and encouragement Rob.  Like others who have offered their advice and assistance I value and appreciate your and their openness and kindness.

the widest diameter cross section of this body is roughly 22".  So having the Cp location between 22" and 44" back of the theoretical Cg is considered ideal for a real world design?  I'm guesstimating its at 20" now so that's good to hear.

I understood that cross section (frontal area) did factor into aero drag along with the others you mentioned. Body displacement or volume, along with length factor into Cd based on what I've read on the subject and the formulas I found on fxSolver.