Inspired by tonight’s Mythbusters involving tornado proof vehicles. They pitted ordinary cars as a control against a jet engine, and they pretty much always lost their hoods after 150mph winds and the whole car would float away by 250mph.
But most cars can get close to 150mph in driving conditions (not even counting headwinds), and quite a few can even get up to 250mph. But they don’t seem to be in any danger of losing their hoods, or going out of control from the windspeed.
Why is a blast of wind aimed at a car somehow more dangerous than the wind thrown at a car from it’s driving speed?
If the jets were aimed at the front of the vehicles there may have been more air going under the hood then over it. The hood may be dependent on a close to equal (or greater) amount going over as under the hood. The pressure over the top should work with the latch to help keep the hood down. In a real world application I’m guessing this is the case. I’d guess that the exhaust from a jet engine is fairly concentrated and if they were trying to flip the cars they probably had it directed downward.
Furthermore, there’s a difference between cars that CAN go 150 and cars that can “get close to 150.” Cars that can go 150 (and are designed to go that fast) may not have this problem with the hood (think about race cars). And just because they can go 150 doesn’t mean they’re going to survive it (for any length of time).
You might want to double check that. The Bugatti Veyron can get into the 240s, I think, but that’s about it. I think something came out recently that bettered the Veyron, and then Bugatti tweaked it a bit and took the record back, but I still don’t know if they’ve cracked 250.
Why, yes, I have been watching old episodes of Top Gear online lately.
Exactly - and furthermore, cars (at least high-performance ones) are designed so that the air flowing over them at high speed produces downforce which keeps them on the road. But only if you’re driving forwards.
They make sure the hood won’t fly off.
But seriously, I’m sure one some of the testing they do (again, think race cars) involves making sure body panels won’t rip off. One of the other important tests they do would be to make sure the air over the top of the car creates enough downward force to keep the car on the ground.
I think one of the the unusual situations of the Mythbusters test was that the flow from the jet engines would be highly turbulent. This differs significantly from driving thru calm air. Lots of bouncing and shaking forces were exerted.
And even a tiny sideways push can have drastic effects. It doesn’t take much of a sideways wind gust to make a car really dance around.
It is interesting how for both car hoods, they started to fold up before blowing off.
Just to clarify this one point, straight wind coming head-on to a car at x MPH is equivalent to the car driving into still air at x MPH.
That is the principle that allows vehicle companies to test their designs in a wind tunnel.
I didn’t see the MythBusters episode, so I don’t know how what they did is different than head-on straight wind. A tornado is not head-on straight wind.
Didn’t see it either, but a tornado is in aggregate circular in motion (hence the spiral), and at a single point on the circle, the direction of the wind is tangential so an argument can be made that testing in a single direction does not produce worthless data.
And yes, for completeness sake, you should probably add in some directional variability and test from all sides. (or at least front and back if you assume you’ll pass through one side and then the other)
The “tornado” used on the show were the engines on a 747. Depending on the distance from the engine and the thrust setting, they were running tests up to around 180 mph with the capability of going to 250 mph, IIRC. Hence my earlier comment on the turbulence from the engines. It’s not going to be all that uniform of a wind. But definitely in tornado range in terms of mph. They of course could not generate a spiraling wind or an updraft with this.
Was this a new episode? I’m assuming it must have been. Last time I saw them do car VS jet engine, they were trying to find out, specifically if a jet engine could flip a car. The myth coming from some airports shutting down roads perpendicular to the runway when certain airplanes take off for fear of flipping cars over.
Jamie and Adam designed and built a man-portable shelter that allows a person to withstand a 180 mph wind. They first tested it with Buster, and then Jamie, who has balls of steel. After the test (which was punctuated with flying chunks of ice as well as high winds), the usually phlegmatic and unperturbable Jamie was jumping up and down and hollering like Adam after something blows up. It was seriously cool.
