Frontal Area / Cd numbers

[Simspeed]

Wow...tough crowd tonight.   

Tortoise:  Yes, that is what we see from the equations where Cd lowers as the length of the tube increases for a given diameter. That's probably not true for all shapes I wouldn't know.  However for the SH shape it is true as I understand what has been studied and written about it.  Also it seems quite obvious to me that air flowing around a sphere of a given dimension will have a higher Cd that air flowing around a long pointy end tube of the same diameter.  Is that unrealistic in your estimation tortoise?

I don't think I admitted anything of the sort tortoise.  Wave drag is a component of transonic and supersonic speed as I understand it, but aerodynamic principles remain valid in subsonic and supersonic air flows where shape, length, and volume determine the Cd of a profile.  Far smarter people than me have determined the SH shape is one of, if not the least drag inhibitive aerodynamic shapes.  I think its common knowledge that well known and peer reviewed mathematical formulas generally produce accurate and dependable results tortoise...I think I'll stick with those rather than find some counterproductive fault based on...well, who knows what your thinking is based on.   

Interested Observer:  Independent yes, but not inconsequential given the nature of air flow over and around low Cd shapes which SH is without doubt one of the best.  Or is that statement suspect in your opinion?  Your suggestions I'm sure have value IO, I'm not so sure however that forgetting what I've researched an studied on your say so alone has any value to me in this context. 

I love naysayers...they are like finding metal filings in a pan of break-in oil.

[tortoise]

  Also it seems quite obvious to me that air flowing around a sphere of a given dimension will have a higher Cd that air flowing around a long pointy end tube of the same diameter.  Is that unrealistic in your estimation tortoise?

Not at all, but those are not the alternatives we are considering.

In forms like very good streamliners, where the flow stays attached all the way to the back, ending at a point, with little or no turbulent wake behind, the major source of drag is skin friction. The lower the surface area, (note I said surface area, not frontal area), the lower the drag. Smart guys like Costella, when they make their cars long for stability don't lengthen the fat parts, they splice in a skinny part in the middle to keep the surface area as low as possible.

By the way, the good airfoils for subsonic duty are pointy only at the back.

[Stainless1]

Well it's always a tough crowd.... but that is why we race... to prove our theories

I see you are not far from Denver... the lakester may be there in several weeks...  Will let you know

[Rex Schimmer]

Simspeed,

If you want to make a car using the  SH profile then go to it. Just remember the very first sentence from you link on this shape: "The Sears–Haack body is the shape with the lowest theoretical wave drag in supersonic flow, for a given body length and given volume." Just because it works in supersonic flow is doesn't mean anything at 300 mph. There are a number of unique streamliner shape that are running at Bonneville, Jacks needle nose belly scrapers, Poteet's flat bottom special, Rob Freyvogal's Carbinite liner which is based on "long runs of laminar flow" theory and the a number of the "standard" square section, long with a pointy nose and tails. All have had some success, some much more than others (Poteet and Jack come to mind along with Danny Thompson's "standard" style liner as cars that have run to their potential.)

It takes just as much effort to build a car to a non applicable design parameter as it does to build one that is based upon air flow at your targeted speed. My suggestion is to get a copy of Bruce Carmichael's "Personal Aircraft Drag Reduction" and looks at what it takes to go over 300 mph with less than 150 hp in an airplane, might change your thinking. I do like your engine selection as the Mazda rotary can make big HPs and is very small in cross section and the use of coupling to the drive wheels using the electric generator/electric motor combination is very out of the box thinking.

I am attaching a pic of a new streamliner that has appeared and has done some runs at El Mo that I would guess is pretty much based on some aero work done by Bruce Carmichael. I have not had a chance to see this car personally or talk with the owners, it is entered as the "Salt Shark" and driven by Tom Flattery from the Gear Grinders. Very unique shape.

Rex

[jacksoni]

I will leave the aero stuff to the guys who really know, having said my piece about my car.
To the cage issue. The rule book says 1.625x.120" wall mild steel or .095" wall moly. MINIMUM!!. Every time this comes up folks say more is better and particularly with high mph liners and such and suggest going to the 1.75"x1.34 wall stuff. More is better. Give up a mm or two for strength.  Weight is a non issue in all cases. Lot of steel wrapped around you. George Poteet put the Speed Demon on it's head at some big number and of course did fine and built a new and better. Your safety is of paramount importance, particularly in something that will be going really fast and take forever to stop etc etc. Have fun be safe.

[Dynoroom]

And has already been mentioned, enough clearance around the tires. Bob Dalton had an inner panel come adrift rubbing the rear tire causing this 425 mph "mishap" last year.
Talked to him last Friday night and his new cars is almost ready for the weld jig.

[Simspeed]

I see you are not far from Denver... the lakester may be there in several weeks...  Will let you know

Yes, please do Stainless.  I'm scheduled for a move to Houston in about 3 weeks so hope to see you before I move.  Thanks...

[Simspeed]

  Also it seems quite obvious to me that air flowing around a sphere of a given dimension will have a higher Cd that air flowing around a long pointy end tube of the same diameter.  Is that unrealistic in your estimation tortoise?

Not at all, but those are not the alternatives we are considering.

In forms like very good streamliners, where the flow stays attached all the way to the back, ending at a point, with little or no turbulent wake behind, the major source of drag is skin friction. 

Now that makes sense tortoise...I'll have to read up on the effects of skin friction dealing with Cd...good point.

[Simspeed]

Simspeed,

My suggestion is to get a copy of Bruce Carmichael's "Personal Aircraft Drag Reduction" and looks at what it takes to go over 300 mph with less than 150 hp in an airplane, might change your thinking. I do like your engine selection as the Mazda rotary can make big HPs and is very small in cross section and the use of coupling to the drive wheels using the electric generator/electric motor combination is very out of the box thinking.

I am attaching a pic of a new streamliner that has appeared and has done some runs at El Mo that I would guess is pretty much based on some aero work done by Bruce Carmichael. I have not had a chance to see this car personally or talk with the owners, it is entered as the "Salt Shark" and driven by Tom Flattery from the Gear Grinders. Very unique shape.

Rex

Thanks for the lead on Carmichael's book.  I didn't find it in print but I read a review of the book by Piolenc that gave good reason to keep trying. http://archivale.com/weblog/?p=102.  

The Salt Shark looks like on wild idea on aerodynamic drag brought to life.  I'd love to see that one in person or read about its success or failure whichever ultimately turn out to be the case.  Also thanks for your input on my posts and what the realities may be on the SH design.  Like I said I'm a rookie in these areas so my comments should be taken with a grain of salt...or maybe I'm the one who needs salting..   cheers.

[Simspeed]

The rule book says 1.625x.120" wall mild steel or .095" wall moly. MINIMUM!!. Every time this comes up folks say more is better and particularly with high mph liners and such and suggest going to the 1.75"x1.34 wall stuff. More is better. Give up a mm or two for strength.  Weight is a non issue in all cases. Lot of steel wrapped around you. George Poteet put the Speed Demon on it's head at some big number and of course did fine and built a new and better. Your safety is of paramount importance, particularly in something that will be going really fast and take forever to stop etc etc. Have fun be safe.

Thanks for clearing the tubing issue that up jacksoni.  I always used 1 5/8" tubing both mild and chrome moly on drag cars I built and never saw the need for larger tubing.  Two cars had big wrecks and both cages held up well with only shakeup injuries to either driver.  I'll go back and change out the 1 3/4 for 1 5/8 in my design.  That little bit will make a difference.   Thanks...

[Simspeed]

And has already been mentioned, enough clearance around the tires. Bob Dalton had an inner panel come adrift rubbing the rear tire causing this 425 mph "mishap" last year.

Wow...big wreck Dynoroom.  Thanks for sharing that.  I plan to weld in fitted sheet metal panels to the chassis tubes wherever removable panels aren't need for systems access.  I always enjoyed doing the bright tin work on drag cars but welded metal last longer and protects better where weight savings is not a big issue.  Silicon bronze rod works great on headers and sheetmetal.  Thanks...

[Rex Schimmer]

My son, Duke, and I were pitted by Bob Dalton at the WOS meet and I will say that his car is not the greatest aero design (my opinion only) but he makes up for it by a really STOUT motor. He is not afraid to "tip the can" when needed. We were in line behind him on his "last" run and he definitely had a load in it for that run. He had qualified at around 450 an I am sure that if he did not have the tire blow he would have easily gone over his qualifying speed. We looked at the car after they brought the remains back to the pits and it certainly was built stout, which is just what you want at 450 mph! Good guy.

Rex

[Eddieschopshop]

I will leave the aero stuff to the guys who really know, having said my piece about my car.
To the cage issue. The rule book says 1.625x.120" wall mild steel or .095" wall moly. MINIMUM!!. Every time this comes up folks say more is better and particularly with high mph liners and such and suggest going to the 1.75"x1.34 wall stuff. More is better. Give up a mm or two for strength.  Weight is a non issue in all cases. Lot of steel wrapped around you. George Poteet put the Speed Demon on it's head at some big number and of course did fine and built a new and better. Your safety is of paramount importance, particularly in something that will be going really fast and take forever to stop etc etc. Have fun be safe.

Weight is an issue for certain cars.  Speed demon 2.0 went on a big diet from the first version.  Weight is a factor on any car that is still accelerating at the 5. If your speed flattens out before that than aero is the biggest problem.  If you are still accelerating at the 5 (read fast streamliner) than weight is a big factor in how hard you are accelerating to that point.

[jacksoni]

I take your point and of course you are correct. Rolling resistance and power to accelerate the mass go up with increased weight. What was in my mind when I said that is the traction problem that pretty much all high power cars have and the ballast often added to aid that issue.

[tortoise]

Weight is a factor on any car that is still accelerating at the 5. If your speed flattens out before that than aero is the biggest problem.  If you are still accelerating at the 5 (read fast streamliner) than weight is a big factor in how hard you are accelerating to that point.

If you have the power to get to big speeds, then you probably have enough power at lower speeds so that the limiting factor in acceleration is wheelspin, not power to weight. You need aero downforce for that.  Variable aero downforce, ideally.