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post #1681 of 3221
1/22/13 at 9:11pm
Quote:
Originally Posted by billybob_jcv View Post

Yes, any disruption of the smooth shape will increase drag. The drag caused by the shape of something is called "profile drag". However, there is also a "boundary layer" close to the surface of the shape that is caused by the air flowing over the leading edge. Within the boundary layer, the viscous effects of the air are high, which means the velocity of the flow is low. Have you ever noticed that they don't bother smoothing every rivet on the outside of an big passenger airplane? That's because the boundary on a big airplane is quite large, and the those little rivets are inside that B/L, therefore they don't add much if any drag. However, there is another type of drag called "friction drag" that accounts for that same viscous effect of the air being slowed near the surface. All of the different drag components are added together to determine the overall coefficient of drag (Cd) of an object.

I don't know how tall the boundary layer is at the center of the hood of a typical car, it depends on whether the flow is laminar (smooth) or turbulent at that point, and whole bunch of other factors. You really only need to consider the part of the scoop that is sticking above the boundary layer. The "NACA duct" you see on many cars is supposed to be the optimal shape for feeding outside air into the interior of the vehicle. You'll notice it doesn't stick-up at all, thus it is entirely within the boundary layer. You still have drag due to the other factors like the viscous drag, pressure drag, etc - but it's the best you can do.



Do you recognize that one? That's on a Veyron. tongue.gif

HOWEVER - all of that is probably just unimportant nonsense, because...

The question is NOT whether the hood scoop increases drag (it does) - the question is whether it is significant. If the drag increase is small compared to the overall drag of the vehicle, then it really doesn't matter - and that's certainly the case with an AC Cobra. Those big rotating tire meats and that open top probably create much more drag than the air scoop - and feeding air efficently to the engine will generate much more horsepower than any impact of the scoop drag.

Gee - that was fun - just like the old days of my former life... wink.gif


Good explanation, that explains the design of modern sportscar vents pretty well.

 

I read in a book somewhere, a car is even more complex to design than an airplane because it works in contact with two mediums. It runs on the road, and runs in the air. Hence, its a much more complex problem to solve because you don't want a car to behave like an airplane that flies off, and still want it to go as fast as possible with minimal power.

 

For comparison, I was amazed as a kid when I saw this video (I think it was discovery channel). A small plane, with a single cylinder engine per wing, could easily overrun a 12 cylinder sportscar....


Edited by proton007 - 1/22/13 at 9:16pm
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post #1682 of 3221
1/22/13 at 10:55pm
It's true that the car has rolling resistance to deal with - and "downforce" is really just negative lift. If you cut a Formula 1 or Indy car down the centerline and look at the shape of the cross-section, you will see a shape with a lot of similarities to an inverted airfoil. The total drag on a airplane at a low cruise speed can be lower than a car - but cruising in a car is also fairly easy - all the HP in cars is to get the car going and to keep it accelerating at the top-end. Cruising along at 60 MPH doesn't require a great deal of HP in either a car or an airplane.

On the other hand, the speed and properties of the air are MUCH more simple for a car than for an airplane. In a car (excluding the Thrust SSC and similar land jets/rockets), you don't need to deal with large variations in ambient pressure or air density - and you don't have to deal with transonic effects of flying at Mach 0.80+ like your average Boeing 737.

And then we could talk about stability & control and the difficulty of just keeping the plane flying straight & level. wink.gif
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post #1683 of 3221
1/23/13 at 12:24am
Thread Starter 
I did forget to mention, the shop bought a `69 Mach 1 to make an Elanor replica. Should be cool.
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post #1684 of 3221
1/23/13 at 12:27am

The way I see it, a high performance car like the F1 is a balancing act, two different types of interactions, because downforce needs to be increased while reducing drag.

A plane, well, its just improvement in one field. I agree, high performance planes are inherently instable systems that need constant control (hence computers and fly-by-wire), but modern cars are also similar, needing active suspension, adaptive aerodynamics and what not.

 

Oh, and the book (now I remember) was Tune to Win (Carroll Smith).


Edited by proton007 - 1/23/13 at 12:30am
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post #1685 of 3221
1/23/13 at 8:23am
Because so much speed is lost while cornering, and the time spent at top speed is relatively short, the downforce actually plays a much more important role in lap time than the drag reduction. F1 designers will give up drag to get more downforce - the only thing really limiting their downforce are the rules for the size of the wings, not being allowed to use moveable skirts, no vertical fans, etc. what this means is that they design for maximum legal (or sometimes mostly legal) downforce first, then make that car design as low-drag as possible.

The most you will get me to agree to is that both high performance car and airplanes are incredibly complex machines. They have completely different design criteria. Of course, if you screw-up the car design or break-down in your car, you won't go spiraling down to a fiery death from 40,000 ft. You will just coast to the side of the road and wait for the tow truck... wink.gif
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post #1686 of 3221
1/23/13 at 9:42am
A buddy of mine had a Mach 1 'stang in high school. Unfortunately, it wasn't a '69 - it was probably the ugliest of all the old Mustangs - a 1972 (I think) Mach 1 with the anemic & smog-control choked 302. That thing was a pretty horrible car - it rattled and shook like a Wile E. Coyote robot and the visibility from inside the car awful - between the long hood, low roof, low seat position, high dash and giant roof pillars, you could barely see out of the thing. But - it was still cool... tongue.gif
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post #1687 of 3221
1/23/13 at 2:42pm
Speaking of F1 rules, this reminds me of one of my pet peeves about most forms of auto racing (or any pro sports) - rules. I think all forms of pro sports need to have an "unlimited" category where there are NO rules on design. Rules based on "safety" or to "slow the cars down" in the top classes like F1 (road racing) or Indycars (oval racing) makes no sense to me. If someone wants to take the risk - I say let 'em. If they want to run a nuclear-powered car using technology captured from visiting aliens from Beetlegeuse, fine - go for it.

And before someone says it - no - I am NOT talking about Death Race - no machine guns or other nastiness against your fellow racers - but I think there needs to be somewhere where the engineers are allowed to think completely outside the box. I guess the closest we have to that is a top-fuel dragster - but even there they had to create a separate class for jet cars. Other than land speed record cars, the only place I can think of that is pretty much "no rules" is unlimited tractor pulling. Those beasts are wild! Multiple turbine engines, 6 x supercharged Allisons or Merlins and hybrids with both! Unlimited tractor pulling is now VERY popular worldwide. When I was a kid, you only saw it at rural county fairs, and someone with 1 turbine engine or 1 V12 Allison was considered unbelievably exotic.

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post #1688 of 3221
1/23/13 at 3:46pm
Actually tractor pulling "unlimited class" has restrictions on it - at least in the US. There's restrictions imposed on weight and dimensions.
http://www.ntpapull.com/index.php?option=com_content&view=category&layout=blog&id=267&Itemid=100268

So for example you can't hook up a pair of GE CF6's "Pod Racer style" and go at it. There's also limitations on how they contact the ground (tire size), so you can't just roll up in an M1 and have at it. And this will eventually circle back to your "restrictions keeping engineers blocked" point - you can only get so much power transfer through the spec tires, and only so large of an engine into the 8000 lbs weight limit and size restrictions.
Edited by obobskivich - 1/23/13 at 3:59pm
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post #1689 of 3221
1/23/13 at 4:16pm
Yeah - I guess you are right. I remember in the old days the same vehicles would be in two weight classes, and they would just hang extra weights on the tractor for the higher weight class. But, that was before they just kept adding more engines...

Then again, I would probably be OK with another class above unlimited where pod racers and M1 tanks were allowed. NASA is probably looking for someone willing to buy this baby:


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post #1690 of 3221
1/23/13 at 4:26pm
Quote:
Originally Posted by billybob_jcv View Post

Yeah - I guess you are right. I remember in the old days the same vehicles would be in two weight classes, and they would just hang extra weights on the tractor for the higher weight class. But, that was before they just kept adding more engines...

Then again, I would probably be OK with another class above unlimited where pod racers and M1 tanks were allowed. NASA is probably looking for someone willing to buy this baby:



Isn't there a time limit on how long the thing can pull too? (I couldn't find anything about that on the NPTA website)

I'm also not sure how that thing's power would translate into pull strength - I mean they're big, but they're also super heavy (they weigh something like 3000 tons each), and super slow, and while they rely on electric motors for traction, their overall output is about in-line with a conventional pull tractor (think about this both ways).

I think you'd do better with a locomotive MU running on treads, or taking one of those absolutely MASSIVE Wartsila engines and throwing that on wheels. Like this big monster:
http://www.wartsila.com/en/engines/low-speed-engines/RT-flex96C

Here's its power plot:


For the Americans, that scale is roughly 75,000 - 108,000 hp.

The only problem is that it's designed to go in this:


And weighs around as much as the shuttle crawler for just the engine. Oh yeah, and it's around a third the length of a football field. Pretty sure the sled wouldn't know what hit it though. very_evil_smiley.gif

Oh, and just because this discussion is quickly on the way there already, and I need to honor Mr Taylor.

Edited by obobskivich - 1/23/13 at 4:54pm
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post #1691 of 3221
1/23/13 at 5:03pm
Quote:
Originally Posted by billybob_jcv View Post

Speaking of F1 rules, this reminds me of one of my pet peeves about most forms of auto racing (or any pro sports) - rules. I think all forms of pro sports need to have an "unlimited" category where there are NO rules on design. Rules based on "safety" or to "slow the cars down" in the top classes like F1 (road racing) or Indycars (oval racing) makes no sense to me. If someone wants to take the risk - I say let 'em. If they want to run a nuclear-powered car using technology captured from visiting aliens from Beetlegeuse, fine - go for it.


The problem with F1 has been that its too fast for the driver. And F1 hasn't been the most difficult of them all, with their semi-auto gearboxes, smooth tracks and emphasis on safety.

 

If you really want a taste of what driving on the edge is like, take a look at Group B Rally. Its been popularly know as 'too fast to race'. Little restrictions, purpose built rally cars with close to F1 levels of performance.

 

 

Then there's group C (the likes of Porsche 956) that still holds the record at Nurburgring, equally fast. Some say it was killed because it became too popular, started competing with F1 in terms of viewership.

 

 

The same thing happened with FIA GT, when Mercedes came up with the CLK GTR. They absolutely obliterated everyone, and no one wanted to compete anymore because the CLK was miles ahead in terms of performance.

 

 

So I guess its two things. First, learning the hard way that brute force wasn't the way to go. The F1 and WRC cars today achieve similar lap times as the cars of those era, albeit with less power and more control. I think its good for designers to work in constraints, because then the boundaries to push are well defined.

 

Secondly, politics and money comes in the picture. Some teams have a stupendous amount of money to spend, and if it isn't for these restrictions, they would keep making better cars than the rest of them and keep winning. (Ferrari + Schumacher).


Edited by proton007 - 1/23/13 at 5:08pm
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post #1692 of 3221
1/23/13 at 5:06pm
Quote:
Originally Posted by obobskivich View Post


Isn't there a time limit on how long the thing can pull too? (I couldn't find anything about that on the NPTA website)

I'm also not sure how that thing's power would translate into pull strength - I mean they're big, but they're also super heavy (they weigh something like 3000 tons each), and super slow, and while they rely on electric motors for traction, their overall output is about in-line with a conventional pull tractor (think about this both ways).

I think you'd do better with a locomotive MU running on treads, or taking one of those absolutely MASSIVE Wartsila engines and throwing that on wheels. Like this big monster:
http://www.wartsila.com/en/engines/low-speed-engines/RT-flex96C

Here's its power plot:


For the Americans, that scale is roughly 75,000 - 108,000 hp.

The only problem is that it's designed to go in this:


And weighs around as much as the shuttle crawler for just the engine. Oh yeah, and it's around a third the length of a football field. Pretty sure the sled wouldn't know what hit it though. very_evil_smiley.gif

Oh, and just because this discussion is quickly on the way there already, and I need to honor Mr Taylor.


Haha. Those engines...a person could comfortably stand in one of those cylinders, they're huge.

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post #1693 of 3221
1/23/13 at 5:53pm
My problem with the race rules are that it has become easier to get something outlawed than it is to engineer something new and better. That's a recipe for stagnation.

obob: consider the weight of the shuttle and launch platform that the NASA vehicle has to move. It is moving that weight, from a standstill, in the worst possible conditions - all the weight is right over the tracks. A tractor pull sled moves the weight forward to put more and more weight on the tractor's wheels. It actually has an easier time because it has some momentum going before the weight reaches it's maximum. According to Wikipedia, the latest upgrade increases the lifting capacity (meaning it can lift it then move forward) to 18,000,000 pounds! Now - does being able to lift and move forward with 18M pounds on your back correspond to being able to drag 18M pounds? If the surface was frictionless, I think it would, but it isn't. If I was still half an engineer, this would be an easy problem - but it's been too long and too many years in management has killed far too many brain cells... redface.gif
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post #1694 of 3221
1/23/13 at 6:45pm
Thread Starter 
Never was into tractor pulls or Gravedigger type stuff, I've always been a speed freak. I can appreciate the engineering that goes into Monster trucks, bu the sport itself doesn't do much for me. redface.gif


It seems that the Mustang the shop bought was a 70, not a 68. Way to read the title, guys. rolleyes.gif The good news is, it was originally a Boss, though it's kinda hard to tell.

H|?

Okay, yeah, I can see how it could be hard to identify this...
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post #1695 of 3221
1/23/13 at 7:32pm
Yikes. They might have been better off starting with a Dodge Charger and making a Eleanor from that... tongue.gif

Hmm - as I recall, the original Eleanor was a long-hood mid 70's Mustang (maybe a '72 or '73?), and the Nick Cage Eleanor was a '67 Fastback. What are you going to do with a '70? How about the Trans-Am version of the Boss 302? The Parnelli Jones 1970 Boss 302 would be perfect!




Check this out to see the size of the cajones on Parnelli Jones...

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