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MYTHBUSTERS 11/16: "Reverse Engineering"

Christopher

Writer
Admiral
Well, last week I was lamenting that we haven't seen the Alameda Naval Yards much lately on the show, and this week both myths were tested there!

Reverse engineering: I heard about this a long time ago, the idea that many car designs were more aerodymic in reverse. I read about it in Discover Magazine, so I figured it was true. Also, it was in reference to '80s cars, not '70s. Basically it was a claim made by a car designer who complained that good engineering was being ignored in favor of marketing -- designing cars that looked streamlined as opposed to ones that actually were.

The idea was that you compare it to an airplane wing, which is widest near the front and narrows toward the back. Cars tend to be the opposite of that, and the car they're using in this episode (they aren't naming it, but is that some kind of Corvette?) certainly looks that way in broad strokes. Although the wind tunnel tests showed it's more complicated; that dip created by the front windshield created a vortex in reverse much the same as the one behind the car going forward.

I'm not sure the speed and acceleration tests really showed anything definitive about the air resistance. I mean, for one thing, how do you know the accelerator is applying the same force, that all else is equal? In particular, with Adam in that much more hunched and uncomfortable position in the reversed car, how can they be sure that it wasn't his performance as a driver that was affected rather than the car's performance? Maybe his leverage on the gas pedal was less effective, his reaction time was slowed, etc. I would've thought the fuel efficiency was the most reliable measure, but apparently that's not necessarily the case.

The final test was the most credible -- just letting the two cars coast side by side so that drag was the only variable in play (assuming identical weight and identically low friction on the axles). The result seemed pretty clear, but I would've liked more analysis of why it worked out that way, why that shape produced those aerodynamic results.

It bugged me that Adam wasn't wearing a helmet in the reverse car. I guess he couldn't fit one in with the windshield being where it was. I suppose altering the shape of the windshield would've thrown off the aerodynamic comparison. Still, it seems unwise to drive at 100 MPH without a helmet.


Surf & turf: It was pretty obvious going in that the "surfboard as deadly weapon" thing from the movie was implausible. I'd imagine a surfboard is pretty lightweight and doesn't have a lot of momentum. Although it turns out the aerodynamics are more of a dealbreaker. Interesting that they're not designed to fly stably through a fluid, given that they operate in water. I know they're supposed to float atop it, but what about cutting through a wave, or whatever you'd call it?

Why did they even think that crashing an SUV into a smaller car would possibly bring it to a complete stop? They know enough physics to know that wouldn't work; they must've just been doing it pro forma to test the scenario shown in the film. But they should've pointed out that that busted it right there, because the larger vehicle wouldn't be completely stopped. They're just not talking about the scientific principles anymore.

Poor Tory. He built that whole really long track thing for the surfboard slingshot, and he only needed five feet!

Interesting result with the 85-MPH surfboard, though. When it hit the windshield, the nose angled up because of the slant of the windshield, and the back of the board had so much kinetic energy pushing it forward that it snapped the board like a toothpick. The slower board had time to change direction as a unit, for the transfer of momentum to be "communicated" from the front to the back of the board, but the faster board didn't, so the front and the back behaved separately and ended up going their separate ways.

They didn't mention it, but it looked to me like that "human-analogue neck" on the dummy was like "Meat Man" from a while back, stitched together from pig parts to accurately represent human tissue/bone/etc. I guess they've done all the basic things so often that they don't feel a need to show the setup anymore -- which is a problem because it doesn't consider the needs of new viewers. There is an increasing lack of attention to the basics on the show these days.
 
The car was a 1979 Porsche 928 (which didn't come out in the US until 1982 or 83.) One of the more classic Porsches ever made.

I'm in the middle of watching so I'll comment later but the Porsche myth is one of the best, and coolest, ones they've done in a long time.

The Junior Mythbusters getting saddled with yet another movie myth is tiring but fun none-the-less. (Guess what?! Hollywood exaggerates things!)
 
No surprises here. Now i may be talking out of my collective arse, but this episode was full of fail. I tend to agree with christopher the episode just felt wrong. Many things glossed over and more of the common sense approach stuff ignored for the sake of crashes.

I also think I know why the car in reserve and the surf board didn't do as they should and why they didn't catch on to it sot of makes me upset. The surf board fell cause of gravity, but the wind act as a downward force on the nose of the board causing it to fall faster. This never was properly shown in the wind tunnel cause the board they used was fastened at the end and not allowed to move.

The car is the same. If the car actually is shaped like a wing, as they said, pressure would create lift causing the car to be slower cause it would have less traction on the ground.

All simple to me, unless I'm really wrong.
 
The car wasn't shaped like a wing. Much like the "wing" of the improvised MacGyver ultralight the "face" of the wing was much to broad. And being "wing shaped" isn't the same thing as being "very aerodynamic."

The car handled poorly probably because, as Christopher said, Adam wasn't comfortable in the car once reversed (this was a case where a robotic system would've offered more control and consistency) and I'd argue some other things like maybe the front end (really the "rear") needed to be lowered more. But I really don't think the broadness of the Porsche really offers any aerodynamic advantage. Seems to me a broad face like that has to "push more" to cut through the air than it would if it was a "point."

Travel down the roadway sometime, and we've all done this, and make a broad-facing with your hand (either by making a fist or putting your palm up) you're meet a LOT of resistance verses if you lay your hand flat (letting it "cut through" the air.)

Now, the problem is if we "reversed" the wing so the "slant" was forward we'd solve half the problem by having the narrowest surface being forward but now the broadest surface is in back which is going to create a large wake which will create a bit of a vacuum that'll "pull back on" the car." So the ideal shape for a car, really, would be something slightly diamond shaped in profile. It "cuts through" the air in front and lets the air form almost no wake out the back.

The airfoil design isn't about aerodynamics for fuel efficiency in fact, I believe, planes are pretty fuel inefficient! The wing-design is all about breaking up the air to create high air-pressure on the bottom and low air-pressure on top to "push up on" the plane. This is such an effective design the spoilers on very high-end sports cars (the ones that need spoilers/wings on them) have upside down wings to reverse this effect so the car can keep traction. Otherwise the airfoil effect would lift the car off the ground. A spoiler causes "lift" to "act backwards" and push the car to the ground.

Now, there may be cars out there with designs that are "more efficient" either for speed or fuel-efficiency driving backwards than forwards. But it's likely to be a design that has a less broad back than it does front and a flat-facing on the back/front is going to be as aerodynamic as a brick. Which was, mostly, the case here. The back of the car put up to much resistance so it had to push harder against the air.

This one will likely see a revisit probably with other car designs.
 
I don't know, this one felt like an excuse for the Mythbusters to dick around and not much else. I mean that's part of the fun of the show but it didn't seem like there was much meat to go with the treat. I mean why start with the wind tunnel if you're just going to discount it and go ahead anyway?
 
No surprises here. Now i may be talking out of my collective arse, but this episode was full of fail.

:confused: How can one person have a collective anything? Are you a hive consciousness occupying multiple bodies? ;)

I tend to agree with christopher the episode just felt wrong. Many things glossed over and more of the common sense approach stuff ignored for the sake of crashes.

That's not actually what I was saying, so you're not really agreeing with me. Some things were glossed over, but it wasn't a bad episode overall, at least in the context of the current season.


I mean why start with the wind tunnel if you're just going to discount it and go ahead anyway?

:confused: That's what they usually do -- test a principle in small scale first, then ramp it up to full scale. After all, physics doesn't necessarily scale perfectly -- what works in miniature doesn't always work the same way in full size. Things like bench tests and wind-tunnel tests are just preliminary data-gathering, to help them figure out what they need to look for in the full-scale.

Plus it's also for the sake of TV; even if they could get scientifically valid results from a small-scale test, they'd still demonstrate them in full scale for the sake of entertaining the audience -- as well as verifying to the audience that the small-scale results really are reliable.
 
^^ Maybe it's happened but I don't remember seeing them getting completely unpromising results in small scale and going forward. Usually, it's like "maybe there's something to this" or whatever.
 
We had this episode last week, and the tablecloth one this week...

Both episodes merely confirming for me that there absolutely positively *has* to be a Mythbusters/Top Gear crossover sometime, and the sooner the better! (And I mean UK Top Gear, with Adam, Jamie and the Juniors vs Jezza, Hamster and Captain Slow...)

I must get an online campaign for that started...
 
Plus it's also for the sake of TV; even if they could get scientifically valid results from a small-scale test, they'd still demonstrate them in full scale for the sake of entertaining the audience -- as well as verifying to the audience that the small-scale results really are reliable.
Bingo! Those with an aero degree (like myself) can tell you that most wind tunnels aren't very big because the test results *can* be accurately scaled up. But of course, that doesn't make for good TV! :D

There's one thing I think they left out of the surfboard test (unless I missed it while dealing with the kids), is an ideal calculation. They estimated the SUV was going 40mph and it was about 40ft away, right? I think you can also conservatively estimate the initial height of the surfboard to be 6ft. At that speed, the surfboard (in a vacuum, no drag) would hit the ground ~36ft away from the SUV, not even reaching the car. Assuming the car's windshield is about 3 ft off the ground, the SUV would need to be going ~63mph for the surfboard to hit its target. Of course, that doesn't take into account any aero effects.

Both episodes merely confirming for me that there absolutely positively *has* to be a Mythbusters/Top Gear crossover sometime, and the sooner the better! (And I mean UK Top Gear, with Adam, Jamie and the Juniors vs Jezza, Hamster and Captain Slow...)
You do realize that the US version of Top Gear premieres this Sunday, right? But it's going to be on the History Channel (:wtf:), so different owners.
 
I don't know, this one felt like an excuse for the Mythbusters to dick around and not much else. I mean that's part of the fun of the show but it didn't seem like there was much meat to go with the treat. I mean why start with the wind tunnel if you're just going to discount it and go ahead anyway?

The scientific method, which Jamie and Adam often follow, is to test something in small-scale and then go large scale. They've tested myths in small-scale like this from the beginning to get an idea of what to expect.
 
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