Research Day: I Get Questions
I do not typically take requests for Research Day, but I've recently been asked an assortment of interest-piquing questions in a variety of situations, and I might as well get them all with one fell swoop.
Question asked by The Queen during a commute: Why does this minivan in front of us have a spoiler? This question was already tackled over at Answer bag, a pretty neat website I just-this-second discovered. In short, the function of a spoiler on the back of a race car is the same as it is on an airplane wing: air exerts pressure upon it, thereby creating a downward force on the vehicle. For a racecar this is good, because it presses the back tires onto the pavement and provides more traction, but given that most street vehicles (a) weigh considerably more than a racecar, (b) go considerably slower than a racecar, and (c) have front-wheel drive, the spoilers you see on the freeway are strictly for show.
Question posed by my mother over dinner: I was once on a plane that got delayed, and the captain said it was 'because the tarmac is too hot for takeoff'. Was he just making that up? Research Day typically falls on the 15th of the month, but this one got pushed back two weeks while I tried to track down any evidence of truth to the "too hot to take off" claim. When I came up empty, I tossed the query over to the Seattle Public Library Ask A Librarian service. They responded three days later saying, essentially, they had found nothing. All of which makes me think that this particular pilot was full of what my dear departed grandfather would have called "baloney slices." But if any readers know otherwise, leave a comment.
Update: Several readers suggested that the pilot wasn't saying the hot tarmac itself prevented take-off, but that the hot weather necessitated more tarmac that the airport had available. In the words of Allan: "As temperature goes up air becomes less dense, so wings generate less lift and thus airplanes require more runway to take off." Two articles on the subject can be found at Salon.com's Ask The Pilot column and Why airplanes like cool days better. Thanks, y'all.
Coworker's musing during Seattle's recent heat wave: When we have a hot day in Seattle, I wonder why it stays warm until, like, 2:00 or 3:00 in the morning, whereas, in D.C. for instance, it starts to cool down right after the sun sets?: To get an answer, I wrote my local TV station's meteorology department. Here's what Scott Sistek, the KOMO Weather Producer had to say:
When it's 90 or more during the day, it's because we have an offshore wind blowing from the east. As the air comes down the western slopes of the Cascades, it sinks and warms. Overnight, that constant breeze sinking and warming has been known to hold up our overnight temperatures, whereas in the flat east, they don't have that problem (Although on warm humid days there, the humidity seems to make it feel a lot warmer at night than here).Thanks, Scott. Wow, I thought you guys just reported the weather -- I never realized you actually produced it.
Question left on my answering machine by a friend I've had since the third grade: Is there a word that means 'to be buried alive'?: I posted this query to the discuss forum of http://www.file- ummmm I mean a website I heard might maybe exist. Anyway, within moments someone replied with with the word vivisepulture which was also the winning word in the 1996 National Spelling Bee. (Actually, the word itself didn't win, some freakishly intelligent kid did.) Thanks guy from, um, some website!
Random email from some guy: Saw your website with the "I don't want to grow up...Toys R Us" words. Do you have the soundclip of that or any suggestion as to where to find it? Here you go, Squirt.
Posted on August 30, 2004 to Research Day
The "too hot for takeoff" may have been slang for "too busy for takeoff". It sounds like a military slang thing and most pilots are ex-military so this could be one explaination.
Or maybe he was just drunk.
Not 100% sure about the specifics, but in hot weather planes use up more runway to get into the air -- something about the air being heavier/denser. So it could be that it was so hot that the runway wasn't long enough to safely take off. Maybe someone can fill in the physics details here...
In my experience pilots usually don't outright lie about things, but they phrase everything in such a folksy, "aw shucks" tone that they aren't being 100% honest either.
Racecars need spoilers, sure, but I'm not sure that answers the question about the minivan. Or, say, these.
Too hot to take off? Where was this plane taking off from? The only place where this might the case is the surface of the SUN.
I have to think that there's something to Duane's first post and the Captain had a certain degree of facetiousness in that comment.
Otherwise how could places like Iraq and Phoenix, AZ ever sustain any sort of an airport if "it's too hot to take off" was truthful?
A choice quote from that link Jessica posted in her comment:
About Hamasaki Ayumi
She is Japanese singer.She sing a song very well.
Her face is very beautiful but small boob.
The phenomenon is "density altitude"; as temperature goes up, air becomes less dense, so wings generate less lift and thus airplanes require more runway to take off. hdc's comment about Phoenix is interesting: on occasion, flights at Sky Harbor are cancelled due to excessive density altitude--there isn't enough runway for airliners to take off.
Here's another article that describes "why airplanes like cool weather better."
There actually is a reason (although a questionable one) for spoilers on things like minivans, as I understand it.
Because the back end of the van is flat the air going over the top suddenly has nothing between it and the lower-pressure air at the back, causing a minor vaccuum effect, the spoiler "spoils" the air stream coming off the top of the van, which can reduce this vaccuum effect, thereby slightly increasing gas milage. Spoilers on compact cars on the other hand...?
I once had a buddy who had an ultralight airplane, you know, basically a hang glider with a fan on the back. He wanted to show it off for me, but the day that he chose to show me was about 98 degrees and he was unable to get his 250 pound ass into the air. His older, but smaller, brother was able to get airborne, but even he had a tough time gaining any altitude. All they really did was smash all the plants in their alfalfa field. Dragging their wheels through half an acre of alfalfa probably didn't help them gain any speed, either.
From my husband, amateur pilot and son of former chief pilot of major airline: what all the answers said--the air is too dense. How do planes fly regularly from Phoenix and Iraq--they have longer runways and load the planes differently. If there's a problem with this, blame my husband. I'm just the messenger.
Matthew, what a charming post (obviously I'm a sucker for question and answer-kind of science kind of thing).
Some flat-backed cars (including minivans) have spoilers on the back to deflect the high-speed (highway) air down the back window to help keep it cleaner. Normally, all kinds of crap gets blown across the top of the car and sticks to the rear window. The deflected high-speed air flings the crap back down on the road for the next succah. At least, That's What I've Been Told.
"I didn't realise you actually produced [the weather]"
Heard a couple of days ago on the evening news:
"The weather bureau has cancelled showers for tomorrow"
"When it's 90 or more during the day, it's because we have an offshore wind blowing from the east."
How is it that offshore winds blow in from the east on the west coast? Isn't "inland" to the east? I assume I can blame my ignorance of Seattle geography, but doesn't the wind generally come from the west anyway?
"As the air comes down the western slopes of the Cascades, it sinks and warms."
Air sinks and warms? I thought warm air rose. I guess the tops of the mountains are cool and the bottoms are warm, but as the air warms, why wouldn't it rise again?
I'm sure there are logical scientific explanations for these things, but right now I feel like an ignoramus.
Ken is a little confused is all. On our lovely West Coast, there are occasionally winds that blow from the east out to sea, hence "offshore". Generally, the winds are onshore, from the ocean onto land. These offshore winds (Santa Ana winds) are great here in SoCal because the wind literally holds the waves up for an extra moment when you surf them. It works out nicely.
Oh, and the cold air does sink out of the mountains to the warmer, lower altitudes, then warms up, then rises.
(Okay unless Todd Johnson writes the weather updates, in which case it's BORING)
Is there some kind of story about the www.file-... reference that I am missing? What site is Matt referring to?
Yeah, what's up with the www.file-? I feel like the crowd in Life of Brian when he won't finish his sermon. What is this www.file-? Is it the secret of eternal life? Tell us, master!
Comments about spoilers on Answerbag.com notwithstanding (none of 'em are entirely correct), a few people in these comments have gotten some of the answer right.
1) The factory spoilers you see on cars/minivans are there to help with airflow... smoothing the air flow over the rear of the vehicle cleans it up (reduces turbulence) and helps with gas mileage. Yes, they're also there for looks.
2) Some factory-supplied spoilers ALSO help with rear downforce, i.e., those on the Subaru WRX/STi and Mitsubishi Lancer Evo (and others), and can supply a decent bit of rear downforce at speeds as low as 70MPH.
3) On race cars, the type and effectiveness of the spoiler depends on the type of car... a rear wing on a Formula 1 car is vastly more important than the rear spoiler on a car in the SPEED Touring Car series (e.g.).
4) OTOH, my race car has a rear spoiler on it, but it's a factory mostly-for-looks dealie and doesn't do anything for my car other than clean up the air flow a little... it's too low to do much of anything else.
Finally, while some of those giant "whale tail" spoilers you see on 4-cylinder imports might actually be providing some downforce, what a significant percentage (I'd bet) of those drivers don't realize is that rear wings (can)also provide a healthy amount of aerodynamic drag, sucking even more speed out of a car that's not exactly overly horsepower-endowed.
Well, the too hot for takeoff bit has some truth and some myth to it. I'm a pilot, if thats worth anything . . .
Airplanes do lose performance in hotter weather, but this is most pronounced in airplanes which use smaller piston engines. Its not so much the fact that the air over the wings is less dense as it is the fact that the air entering the cylinders in the engines is thinner. This results in less compression and poorer engine performance. Its the same reason that people put nitrous in their cars to super-cool the air going in so they get better compression.
The little pipers and cessnas you see buzzing about have small eninges and need to be cranked to full just to take off, so a hot day and a soft field to roll down can spell trouble.
Your mother was most likely flying on something a little larger, which had a jet, or at the very least, turbine powered prop. The decline in performance due to hot air is moot in such engines.
Also, large commercial airports have very large runways. They are built to acommidate error and to be landed on as well as taken off from.
Air is thin when its hot and when it is high. Jets can use their abundance of power to cruise at high altitudes, just as they would to take off on a hot day.
While it is possible that the thin air could have made a take-off unsafe, it would have to have been in an airplane which was at the very limits of the airport's facilities, which the FAA doesn't allow very often. If you knew what airport or even what kind of plane it was, you could further evaluate this tale for baloney.
Hey, just wanted to let you know the weekly News Quiz on NPR featured your Morning News "Tricks of the Trade" article over the weekend. The host offered Dr. Ruth (I'm not making this up) a profession, and asked which of the following three possibilities were actual "tricks of the trade." I was listening on Saturday morning while on a 2 hour drive, and exclaimed to my sister "I read that guy's blog!" Why I was proud of this, I haven't been able to determine.
Anyway, credit was attributed to the Morning News, but I'm afraid you weren't mentioned. Nevertheless, Dr. Ruth was asked questions from your article. That is freaking awesome!
"Air sinks and warms? I thought warm air rose. I guess the tops of the mountains are cool and the bottoms are warm, but as the air warms, why wouldn't it rise again?"
Yes hot air does rise - but air also heats up as it is condensed. As I understand it, when there's an offshore flow, cold & less dense air from the top of the mountains (where atmospheric pressure is low) flows down into the valleys (where atmospheric pressure is 2 times 'heavier') and heats up as it is condensed. Thus replenishing heat that would otherwise be lost in flatter regions when the sun goes down.
"too hot for takeoff"
As an ex-aircraft dispatcher for a small airline that flew into even smaller airports I think I can explain what the pilot was trying not to say. He wanted to say, "Folks, it's so hot out here that, with the aircraft loaded to the max the way it is, we need more runway then we have available to get this plane into the air. Now, we could take-off right now if we remove all the baggage and 90% of you passengers but we know you would not stand for that so we will just wait until the sun goes down and try again."
BTW, we flew nine-seat turboprops.
All you smart people confuse me.
Dispatch for a small airline la peregrina? That actually sounds like a lot of fun, and 9 seat turboprops loaded down with people and their overstuffed bags would definitely have their performance degraded by heat.
As far as big jets getting grounded by the heat, although they can still fly there are all those crazy FAA rules that say they have to be able to take off with a lot of runway to spare, in case a takeoff needs to be aborted. I still cannot imagine a scenario where a 737 or larger gets grounded by heat at a major American airport.
I usually fly a 4cylinder piston Piper Warrior in Massachusetts. One November I flew a warrior in cold-ass massachusetts and only a week later flew the same type plane out of Sedona, Arizona near mid-day. I was shocked to feel how sluggish the Arizona takeoff was, I felt stuck to the runway and took a LONG time to get to my takeoff speed. But then again, even in the dead of winter, I need to keep the throttle fully open just to climb in this little plane, it has no more horsepower than it absolutely needs.
Yet a 737 has a goodly sized abundance of power. Listen the next time you take off in one. 10 seconds after you leave the ground you can hear the pilot retard the throttle to avoid exceeding the 250kt speed limit below 10,000 feet. They'll probably have the engines at only 75% through most of the climb as well.
The aopa and salon articles people mentioned before are great, but they don't mention type of plane, just that they are airliners, which could also include 5-50 seat props.
I have a flying lesson on saturday. I'll get my instructors' 2 cents on if something as big as a jumbo jet could succumb to heat.
"Too hot for take off"
Happens in Phoenix, AZ every couple of years... My father-in-law (Air Force Colonel) tells me that most commercial airliners are not tested above 130F (ambient at take off), so it comes down to liability.
That sounds logical.
[http://www.filepile.org is good and I wish I had an account]
i can second the phoenix airport closures. some rare summer days in the city can reach higher than 140 degrees and the airport closes. and i remember two reason - that the airplanes weren't tested for those high temperatures and that the tarmac would soften. though that tarmac softening thing does sound wonky.