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Post by memeengine on May 17, 2014 10:32:31 GMT
Are you sure you guys aren't secretly Americans - you are approaching this puzzle from a brute force perspective. I was merely addressing the problem that was posed, i.e. completely removing the dead-air behind the car. If you're merely looking to reduce the overall aerodynamic drag on the vehicle then there are a multitude of minor changes that could be made to a average family car, such as removing the wing mirrors. Each of these would contribute small improvements on their own and together they could have a noticeable effect on aerodynamics. To what degree would largely be down to how aerodynamic the vehicle was in the first place. However, these changes would all affect other aspects of the vehicle. In the case of the example above (removing the wing mirrors), another system would need to be rigged up to provide rear/side views. This could potentially be done with video cameras and internal screens. Of course, this additional internal system would add weight so you'd have to evaluate the trade-off between the small improvement in aerodynamic drag and the small increase in weight. Automotive design is all about that sort of compromise. Desirable feature A often compromises desirable feature B or introduces undesirable feature C. For example, increasing the passenger space (meaning the 'personal space' for each passenger) is a desirable selling point but that will usually mean an increase in the size of the vehicle. This, in turn, increases the cross-sectional area and so increases the aerodynamic drag. Small tweaks to a design aren't so straight forward either. For example, you could improve the aerodynamic shape of an element at the front end of the car and discover that change alters the airflow at the back of the car in a detrimental way. So all of the changes have to be considered in the context of the overall effect. If we're just after the easiest way to reduce the drag of the average family vehicle, then all you need to do is to convince the average family to drive a smaller car.
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Post by Cybermortis on May 17, 2014 12:04:19 GMT
Actually that isn't what I had on mind.
I think we are talking about two different things here. You are talking about equalising pressure at the front and rear of the vehicle. I'm actually talking about attempting to smooth the airflow at the rear of the car using, well, air.
At the speeds most cars can reach, and certainly at legal speed limits, the pressure difference between front and rear isn't all that big. If it were cars would be leaving vapour trails behind them. The bigger factor for drag is likely to be the vortices created from the (comparatively) flat rear end. In theory you should be able to use blown air to delay the formation of the vortices, and if they are then formed further from the vehicle that should reduce the amount of drag they cause.
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Post by the light works on May 17, 2014 12:31:03 GMT
Are you sure you guys aren't secretly Americans - you are approaching this puzzle from a brute force perspective. I was merely addressing the problem that was posed, i.e. completely removing the dead-air behind the car. If you're merely looking to reduce the overall aerodynamic drag on the vehicle then there are a multitude of minor changes that could be made to a average family car, such as removing the wing mirrors. Each of these would contribute small improvements on their own and together they could have a noticeable effect on aerodynamics. To what degree would largely be down to how aerodynamic the vehicle was in the first place. However, these changes would all affect other aspects of the vehicle. In the case of the example above (removing the wing mirrors), another system would need to be rigged up to provide rear/side views. This could potentially be done with video cameras and internal screens. Of course, this additional internal system would add weight so you'd have to evaluate the trade-off between the small improvement in aerodynamic drag and the small increase in weight. Automotive design is all about that sort of compromise. Desirable feature A often compromises desirable feature B or introduces undesirable feature C. For example, increasing the passenger space (meaning the 'personal space' for each passenger) is a desirable selling point but that will usually mean an increase in the size of the vehicle. This, in turn, increases the cross-sectional area and so increases the aerodynamic drag. Small tweaks to a design aren't so straight forward either. For example, you could improve the aerodynamic shape of an element at the front end of the car and discover that change alters the airflow at the back of the car in a detrimental way. So all of the changes have to be considered in the context of the overall effect. If we're just after the easiest way to reduce the drag of the average family vehicle, then all you need to do is to convince the average family to drive a smaller car. The point that was posed was that an urban legend had a teenager strapping rockets on the back of his car to win a drag race and whether such a lashup could have an advantage above the net thrust of the rockets due to aerodynamic improvement.
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Post by memeengine on May 17, 2014 15:56:26 GMT
I think we are talking about two different things here. You are talking about equalising pressure at the front and rear of the vehicle. Ok, Silverdragon was certainly talking about equalising pressure at the front and rear of the vehicle, I was addressing the practical difficulty in doing so. At the speeds most cars can reach, and certainly at legal speed limits, the pressure difference between front and rear isn't all that big. If it were cars would be leaving vapour trails behind them. Actually, ordinary vehicles can leave vapour trails behind them at normal road speeds. The reason that you don't usually see them is because most cars are operated in places where the atmospheric conditions are not conducive to their formation. (http://contrailscience.com/ground-level-contrails/) The pressure difference wouldn't need to be all that great to have an impact on performance. Pressure is force per unit area. If we stick to the theoretical car I used before, it has a cross-sectional area of 4ft x 5ft or 2880 square inches. So a pressure difference of just half a pound per square inch, from front to rear, would apply a force of over half a ton to the vehicle. The bigger factor for drag is likely to be the vortices created from the (comparatively) flat rear end. In theory you should be able to use blown air to delay the formation of the vortices, and if they are then formed further from the vehicle that should reduce the amount of drag they cause. I'm not sure that vortex formation is the problem. After all, there are researchers that are using vortex generators on the trailing edge of the car roof to deliberately induce vortexes to reduce drag. These are probably worth a read - www.bnl.gov/isd/documents/78582.pdf geetaebook.com/pdf/aeronautical%20enginerring/ebooks/Aerodynamic.Drag.Reduction.by.Vortex.Generators.pdf(and in an amusing example of academic plagarism, the same paper produced by completely different authors, I'll let you decide who copied who.) vixra.org/pdf/1208.0177v1.pdf
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Post by silverdragon on May 20, 2014 7:46:44 GMT
I see your point, but...The answer is, The F1 car.... Seriously. The DRS and energy recovery systems may add an extra 20hp down the straight for overtaking, 5hp more may just give the "Edge" to make that overtake. Remember, they will spend about a Million (sterling) for 1,00th of a second... (per lap) 5hp may make that difference.
Even worse, I am a biker. There is no replacement for displacement.
And I am empathising the young boy racer attitude here that bolting on "something" will make it faster....
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Post by silverdragon on May 20, 2014 7:52:16 GMT
I was watching something over the weekend, it gave me serious problems, 'cos it makes sense.....
They addresses rear spoilers, and the ability to create a low pressure behind the vehicle that aids in drawing air over the vehicle thus removing drag. They referenced the "Golf-ball", in a way that I can only assume they are a Mythbuster watcher, as they also referenced the way a surface like that would aid in aerodynamics.
On a tangent to this myth, would sandpapering the car, even putting a vinyl roof with that golf ball dimple, actually 'work' here?.... (Maybe that needs more exacts, but do you get the principal?..)
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Post by the light works on May 20, 2014 14:34:09 GMT
I see your point, but...The answer is, The F1 car.... Seriously. The DRS and energy recovery systems may add an extra 20hp down the straight for overtaking, 5hp more may just give the "Edge" to make that overtake. Remember, they will spend about a Million (sterling) for 1,00th of a second... (per lap) 5hp may make that difference. Even worse, I am a biker. There is no replacement for displacement.And I am empathising the young boy racer attitude here that bolting on "something" will make it faster.... more difference as measured by percent improvement, not more difference by winning more money if it makes sufficient difference to win the race. I agree wholeheartedly with no replacement for displacement. every time a kid points out that you can turbocharge a Honda up to the same HP as a Chevy small block, I point out that anything you can do with an inline 4 you can do with a V-8, twice. - and you can supercharge the Chevy up to the same HP as FOUR Chevy small blocks.
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Post by the light works on May 20, 2014 14:35:57 GMT
I was watching something over the weekend, it gave me serious problems, 'cos it makes sense..... They addresses rear spoilers, and the ability to create a low pressure behind the vehicle that aids in drawing air over the vehicle thus removing drag. They referenced the "Golf-ball", in a way that I can only assume they are a Mythbuster watcher, as they also referenced the way a surface like that would aid in aerodynamics. On a tangent to this myth, would sandpapering the car, even putting a vinyl roof with that golf ball dimple, actually 'work' here?.... (Maybe that needs more exacts, but do you get the principal?..) I doubt sandpapering would be very effective, but a dimpled overlay might be light enough to have a net gain. (you recall the dimples alone had a net gain - with the weight remaining the same.)
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Post by silverdragon on May 21, 2014 6:18:22 GMT
Sandpapering, I am not sure, DeLorian had an idea with the stainless steel bodywork that a sandpapered finish "Did something"...?... But the sandpapering effect I am trying to suggest that a not smooth and polished surface may be better. Heck, they even went through this with the flow inside carburettors, until it was abandoned because fuel injection is better.
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Post by the light works on May 21, 2014 14:37:59 GMT
Sandpapering, I am not sure, DeLorian had an idea with the stainless steel bodywork that a sandpapered finish "Did something"...?... But the sandpapering effect I am trying to suggest that a not smooth and polished surface may be better. Heck, they even went through this with the flow inside carburettors, until it was abandoned because fuel injection is better. the main purpose for the "brushed" (actually die stamped) finish on the Delorean was to break up the reflectivity.
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Post by silverdragon on May 22, 2014 7:39:04 GMT
Ah, no, it wasnt done that way...... The panels were "Brushed". This was done after they were cut out, and shaped, because that was the only way they could get the front-to-back lines. I watched a restoration of one recently, they still make Delorians, there is a 'Scouse guy (Liverpudlian) who bought the whole lot from the closed down factory, and shipped the lot to America, he has enough parts to make about 3,000 more, but is selling parts. He does do one or two assemblies of "New" cars per year, but his order books are difficult to get on to, as most of his production is pre ordered many years ahead. (According to that program) The panels come pre brushed, but to repair a panel, the original way, as done in the factory, is with a "Flapper wheel"..... Tricky work. (And something I have done myself)
Pre-Brushed steel makes cutting the panels out very tricky and wasteful, as you have to cut them "In line", bending the steel also crushes the lines you get, so they did the brush work after the panel was formed.
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Post by silverdragon on May 22, 2014 7:43:21 GMT
We are drifting WAY off topic here, but, I think we can tie this in with just how DO you get a Delorian to 88 mph?....
I have driven one... (It was a specialist transport job, I had to load it up, you know the score...) I know that fast as they were, 88mph is quite a target, and in the film, the "Run up" to get that speed was, well, shall we say, artistic licence?... As in, maybe with a following wind on a good day with a very much lightened vehicle.... And perhaps twice the distance they showed?.... Either they lied about the speed they got in such a short distance, or.... And this is the tie-in.... They had a couple of rockets strapped to the back bumper......
Hows THAT for diverting back on topic?... any prizes here?....
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Post by the light works on May 22, 2014 15:20:28 GMT
Ah, no, it wasnt done that way...... The panels were "Brushed". This was done after they were cut out, and shaped, because that was the only way they could get the front-to-back lines. I watched a restoration of one recently, they still make Delorians, there is a 'Scouse guy (Liverpudlian) who bought the whole lot from the closed down factory, and shipped the lot to America, he has enough parts to make about 3,000 more, but is selling parts. He does do one or two assemblies of "New" cars per year, but his order books are difficult to get on to, as most of his production is pre ordered many years ahead. (According to that program) The panels come pre brushed, but to repair a panel, the original way, as done in the factory, is with a "Flapper wheel"..... Tricky work. (And something I have done myself) Pre-Brushed steel makes cutting the panels out very tricky and wasteful, as you have to cut them "In line", bending the steel also crushes the lines you get, so they did the brush work after the panel was formed. actually, no. the repair site I looked through on the odd occasion when I looked at a DeLorean, specifically said that the brushed finish is actually embossed into the metal, and Delorean bodywork men have repair dies specifically for repairing the pattern - and that if you are looking at a DeLorean that shows signs of having had either steel wool OR a wire brush/flapper wheel used on it, you are not to pay above poor condition prices for it, because it will be expensive to repair the finish at best. (and the one I looked at had swirl marks from steel wool all over it)
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Post by the light works on May 22, 2014 15:22:11 GMT
We are drifting WAY off topic here, but, I think we can tie this in with just how DO you get a Delorian to 88 mph?.... I have driven one... (It was a specialist transport job, I had to load it up, you know the score...) I know that fast as they were, 88mph is quite a target, and in the film, the "Run up" to get that speed was, well, shall we say, artistic licence?... As in, maybe with a following wind on a good day with a very much lightened vehicle.... And perhaps twice the distance they showed?.... Either they lied about the speed they got in such a short distance, or.... And this is the tie-in.... They had a couple of rockets strapped to the back bumper...... Hows THAT for diverting back on topic?... any prizes here?.... US cars of that era had top speeds in the 75-85 MPH range. which would make a little bit of boost from a rocket potentially effective.
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Post by silverdragon on May 23, 2014 6:54:24 GMT
I am going to have to correct whoever told me a pack of lies then..... Actually, thinking on, the makers of that program that gave me that bad set of facts, to say they wouldnt lie is saying the sea is above the sky. But I thought they would get even that bit right?....
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Post by the light works on May 23, 2014 13:35:47 GMT
I am going to have to correct whoever told me a pack of lies then..... Actually, thinking on, the makers of that program that gave me that bad set of facts, to say they wouldnt lie is saying the sea is above the sky. But I thought they would get even that bit right?.... I did see youtube hits come up that implied people were using flappy wheels on deloreans; so it is entirely possible someone has sent out bad information. - particularly in the light that there are stainless steel and aluminum finishes that are applied with sanding techniques after shaping. (on other things)
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