Frontal Area / Cd numbers

[Simspeed]

Does anyone have data or records of frontal area and Cd numbers from top contenders for unlimited wheel driven streamliners?  I'm interested in knowing how my body design compares with the best in class liners.  Here's an image of the largest cross-section area of the design I'm working with.   Thanks again...

[Simspeed]

Oops...I forgot to add the stabilizer fin cross-section.

[Stainless1]

Does it have Wheels.... and tires.... don't forget that racing tires are required and I don't see those...

[Simspeed]

Sure does Stainless but they don't show up in this widest cross section cut.  I've used the M/T 24.5 x 7.5 x 16 dimensional profile for all four tires in my design.  The spreadsheet showing needed hp for a given speed zeros out the tire Cd for streamliners, so I'm not trying to capture that drag under the body.  For now I'm only interested in the given inputs that reflect car weight, rolling resistance, frontal area, and body Cd to see HP required for a given speed.  Thanks to whoever made this and all the other spreadsheets available to forum users. So far the ideal data from the spreadsheet is projecting good numbers for my design given current records for unlimited liners.  Thanks for your comments Stainless...

[jacksoni]

Though your shape is not a square which would be less than 2 feet a side and your shape is taller than it is wide. Are you sure you can fit everything in there- like you (how big are you) and the tires already mentioned (tires grow at speed, need clearance) not to mention cage etc. Take off 3-4" from width to be the drivers compartment (minimum tube sizes x2 with some padding and etc) and you don't have a lot of space for shoulders. Since your picture indicates power is not some big honking V8 don't have that but how big is the power plant? Having built one packaging gets seriously tight when you start adding systems- fire, coolant and fuel tanks, running gear, electronics, blowers etc etc. Not having the luxury of computer systems and such when I built mine I built a frame from PVC and sat me and engine it it to check dimensions. Still screwed up the cage height (helmet clearance= 3" over the top plus padding on all bars that are in proximity.) So yours becomes a serious lay down driver position- Visibility from same? (over the tires that are 24.5" tall etc- more with growth) Stainless has experience with these issues I think he may comment on. Good luck. Your shape is more like Marlo's car. I don't know if they have posted any numbers. Mine was shaped more like Goldenrod with a bit more nose droop and with mid rather than front engine. Goldenrod was 8.53sq ft frontal with Cd of .1165. The car had negative lift by design so weight was not added. I can provide reference for this if you want.

[Simspeed]

Thanks Jacksoni that would be great.  Any info from or about those who have done it successfully would be appreciated. Yes this design is a very tight squeeze for someone of my height.  I designed everything around the engines (two turbo 4 rotor Mazdas) and a 6' 2" driver which happens to be my height.  The overall length is 36' which I copied from the team Vesco liner.  My first design was 22' but I discovered Cd drops with length for the Sears Haack shape which I'm using.  Calculated Cd dropped from .08 to .02 with the added length which is needed to fit everything inside.

The chassis I designed uses 1 .75" double hoops for the driver cage, 1.625 tubing for everything behind the firewall.  The front half and steering section will use thick wall oval tubing for added strength. So far everything you mentioned fits inside the shell and chassis.

The drive train consist of electric wheel motors to drive all 4 wheels so there's no need for a differential, drive shaft, or transmission to transmit power to the ground (salt actually).  The 4 rotor engines will spin axial PM generators that push electrical power to the wheel motors.  There will be no batteries on board in this design.  The rotary engines, generators and motors I've researched are capable of a combined 2,100 hp in this application which appears to be more than enough to set a few records God willing...

[jacksoni]

There are two SAE papers you might find interesting: The aerodynamic Design of the Goldenrod- to increase Stability, traction and Speed. Walter Korff. # 660390 from 1966.

Also: Aerodynamic Drag Characteristics

 of Land Speed Record Vehicles. Alex Tremulis 660387

If you can't get from SAE, PM me an email or fax # and I can scan or fax them.

I have some other stuff but more applicable to cars as opposed to streamliners and you are beyond that for sure.

[Interested Observer]

“Calculated Cd dropped from .08 to .02 with the added length..”

Strongly suspect there is something wrong with this statement or the basis on which it is concluded, in going from 22’ to 35’ length.  Would be interesting to see the analytical rationale that produced it.

[Peter Jack]

Before you go too far I would build the driver's compartment of the car. I'd be using 1 5/8 dom round tubing throughout the main portion of the car. It's going to have more than adequate strength when properly triangulated and you're going to need every fraction of an inch of space to meet the dimensions you're calling for. The next thing you need to invest in is all your safety equipment. Everything has to be within reach and operable when you're belted in TIGHTLY. Remember, helmets are huge these days, suits, boots and gloves are all large and bulky. When you've got all that sorted then you've got to be able to do the mandatory bailout in a minimum amount of time. It's a lot easier to modify the cockpit area before it becomes an integral part of the whole car and the dimensions you're calling out are definitely really tight.

The other thing you might want to think about is that as you age there is a tendency to expand. You might want to think about that a little. Unfortunately, I'm speaking from experience.  

Good luck with the project. It sounds fascinating.

Pete

[jacksoni]

X 4 or 5. Exactly what I was trying to say as well.

[jacksoni]

“Calculated Cd dropped from .08 to .02 with the added length..”

Strongly suspect there is something wrong with this statement or the basis on which it is concluded, in going from 22’ to 35’ length.  Would be interesting to see the analytical rationale that produced it.

I thought this a bit off as well. There are several folks on the forum here with obvious credentials with respect to your questions, IO and Blue come to mind. Robfrey as well with his liner. I hope they chime in. Me, I just read and copied best I could. But my car has been over 300 and was stable so something worked.

[Stainless1]

Are you planning to be at SpeedWeek to look at cars?  With any luck I will have Bockscar 2 there.... You will be welcome to sit in it.... it is 24 wide, 16 tall with 9 7/8 tall  x 17 1/2 tunnel on top for driver and all that excess motor room.  Since you need to be totally inside the frame rails and roll structure, with internal sheetmetal to ensure your parts can't get out if the external body is lost it seems to me you are allowing less than 19 inches for you...  I guess you won't rattle around much 
If not making Bonneville, you are welcome to drop by to fit.... if you decide to go with 1 5/8 tube I have a 7.5 inch CLR die for the JD bender you can borrow... if fact anyone on this site is welcome to borrow it if needed... I see it as the minimum size required hoop size based on current helmets and allowing a little for future helmet growth. 
You can view Bockscar 2 on the Build Diary section of this website.
Good luck with your project 

[Simspeed]

Jacksoni: Thanks for the SAE paper leads, I'll find them and study up.

Interested Observer:  I was surprised too when I ran the two different lengths and volumes through the Cd calculator at Fxsolver.  Here's the link to the Sears Haack body shape https://www.fxsolver.com/browse/formulas/Sears%E2%80%93Haack+body+%28Drag+Coefficient+related+to+the+Volume%29.

I haven't looked at my 22' body in a while, the Cd number should have read .0713 Cd; and the 36' body Cd is actually .027.  When you think about it the SH profile is a tube of a given diameter that smoothly tapers down to a needle point at both ends. So if we keep the diameter constant and increase or decrease the over all length the angular movement of air that flows from end to end decreases with length.  The Cd therefore grows smaller with length due to smoother air movement over and around the shape. The SH shape has the lowest theoretical wave drag in supersonic flow but may not be the best in subsonic flow.  I'm using it anyway because that's the shape I wanted to use before I even knew it had a name. 

Peter Jack:  Thanks for you logical recommendations; makes perfect sense.  I was under the impression however that cage hoops had to be 1 3/4 diameter.  Am I wrong? To your other point about size...I'm fairly tall but still under 200 lbs, and I've measured myself for fit which is doable.  On the practical side, if I'm able to build this I'll likely need a younger, thinner person to handle the driving chores.  Thanks for your encouragement. 

jacksoni:  300 mph is a mile stone for sure.  Congrats on your accomplishment.  I'll copy whatever works if applicable to my goals.

Stainless:  I looked through your build diary very impressive work.  Thanks for the invitation to visit your shop or look you up at SpeedWeek for a test fit.  Great comradery between the users here on the forum; I'm grateful to everyone who offers help and/or constructive criticism.  I'm the rookie here and not ashamed to admit it.  Thanks everyone...

[tortoise]

When you think about it the SH profile is a tube of a given diameter that smoothly tapers down to a needle point at both ends. So if we keep the diameter constant and increase or decrease the over all length the angular movement of air that flows from end to end decreases with length. Cd therefore grows smaller with length due to smoother air movement over and around the shape.

Here you're saying the SH equation demonstrates that increasing the length with constant diameter will lower C_d.  

The SH shape has the lowest theoretical wave drag in supersonic flow but may not be the best in subsonic flow.  I'm using it anyway because that's the shape I wanted to use before I even knew it had a name.

Here you admit the SH equation does not apply at subsonic speeds, and you're using the shape just because you like it. Hey, it's your car, no problem. I think, however, you are mistaken that lengthening the car and keeping the same diameter will lower the drag.

[Interested Observer]

From fxsolver:   “wave drag is a component of the drag on aircraft, blade tips and projectiles moving at transonic and supersonic speeds, due to the presence of shock waves. Wave drag is independent of viscous effects”

So, Cd based on “wave drag” is additive to the normal viscous and pressure drags that we non-supersonic and non-shock wave producing travellers must deal with.  May pertain to Bloodhound and others of that ilk.  Suggest you forget about that calculation and pay good attention to the info in the papers that Jacksoni has mentioned.