Fun with physics!
SH...@slacvm.slac.stanford.edu
2) Left
3) The Bahamas
+---------------------------------------------------------+-------------+
| sig under construction | My opinions |
| | are exactly |
| ->Two wrongs do not make a right, but three lefts do.<- | that, MINE! |
| sh...@SLAC.STANFORD.EDU | MINE ALONE! |
+---------------------------------------------------------+-------------+
David Boll
1) If the 2nd law of thermodynamics is correct, are people who fan
themselves when hot just making themselves hotter?
2) If you're doing 60 mph in a car, and you hold open the driver side
door, which way will the car tend to swerve? (kids, don't try this
at home)
3) Where does the white go when the snow melts?
--
---------------
Dave Boll db...@hp-vcd.vcd.hp.com
"The speed of time is 1 second per second"
Michael K. Neylon
> 2) If you're doing 60 mph in a car, and you hold open the driver side
> door, which way will the car tend to swerve? (kids, don't try this
> at home)
Opening the door would have the same affect as moving the center of
mass of the car slightly left of center (assuming that it was already
balanced in the first place). This can also be done by asking
everyone in the car to lean left simultanoutly. Although, yes, the
car would severe left, this would be more like to cause the right
side of the car to lift off the ground, and if the door contained
oh, say 50% of the mass of the car, then it would most likely cause
the car to flip onto its left side (of course, the door is open and...
well you get the picture ;)
--
Michael K. Neylon |
Grad. Student | "Why do the action scenes slow down
Dept. of ChE, Univ. of Michigan | this movie?"
mne...@engin.umich.edu | Crow, MST3k
Matthew T. Russotto
}David Boll (db...@vcd.hp.com) wrote:
}: A couple of quickies for y'all:
}
}: 1) If the 2nd law of thermodynamics is correct, are people who fan
}: themselves when hot just making themselves hotter?
}
}moisture, cooling the person. The 2nd law doesn't work that way.
}
}: 2) If you're doing 60 mph in a car, and you hold open the driver side
}: door, which way will the car tend to swerve? (kids, don't try this
}: at home)
}
Left, unless you are the type of person who holds the bottom of the
wheel when you drive. (as you lean out to hold the door open, your
other hand on the wheel will turn the car left :-) )
--
Matthew T. Russotto russ...@eng.umd.edu
Some news readers expect "Disclaimer:" here.
Just say NO to police searches and seizures. Make them use force.
(not responsible for bodily harm resulting from following above advice)
Andrew Markham
: : 2) If you're doing 60 mph in a car, and you hold open the driver side
: : door, which way will the car tend to swerve? (kids, don't try this
: : at home)
: Left, unless you are the type of person who holds the bottom of the
: wheel when you drive. (as you lean out to hold the door open, your
: other hand on the wheel will turn the car left :-) )
It probably isn't politically correct to suggest that the driver side
door is on the left, but I won't tell anyone... ;^)
--
Andy Markham
mar...@bnr.ca
David Karr
>David Boll (db...@vcd.hp.com) wrote:
>> 2) If you're doing 60 mph in a car, and you hold open the driver side
>> door, which way will the car tend to swerve? (kids, don't try this
>> at home)
>
>Opening the door would have the same affect as moving the center of
Now, now, the mass of the door is trivial compared to the rest of the
car. It has a fairly large surface area, however, on which the
aerodynamic forces will be quite substantial.
Actually, from my experience in just sticking my *hand* out the window
of a speeding car, I don't believe that a person *can* open the door at
60 miles an hour.
But the question didn't say "open the door," it said "hold open the
... door." So I suppose you're sitting in the driver's seat of a car
going 60 mph with the driver's door open (held, perhaps, by a strong
cable), and someone says to you, "OK, I'm about to release the cable,
please hold the door open."
Then the direction the car swerves is whatever direction your right
hand turns the wheel as your extended left arm receives a compound
fracture from the door suddenly slamming toward you.
-- David A. Karr (ka...@cs.cornell.edu)
Geert-Jan Thomas
In article <2rv98i$b...@mercury.dur.ac.uk>, N M Miners <N.M.M...@durham.ac.uk> writes:
|> David Boll (db...@vcd.hp.com) wrote:
|> : door, which way will the car tend to swerve? (kids, don't try this
|> : at home)
|>
|>
EXCUSE ME !!! There are other countries in the world
than those 2. These kind of remarks i find, in random order:
RUDE
ARROGANT
&
IGNORANT
No offence.
__________________________________________________________________________
"And her eyes... they were mindless, two pools that reflected the moon.
No secrets, no truths, only despair." -Louis de Pointe du Lac-
Geert-Jan Thomas Email: tho...@tpd.tno.nl
__________________________________________________________________________
Jered Floyd
: A couple of quickies for y'all:
: 1) If the 2nd law of thermodynamics is correct, are people who fan
: themselves when hot just making themselves hotter?
No. Fanning moves the air past the person, allowing the air to pick up
moisture, cooling the person. The 2nd law doesn't work that way.
: 2) If you're doing 60 mph in a car, and you hold open the driver side
: door, which way will the car tend to swerve? (kids, don't try this
: at home)
Not sure offhand.
: 3) Where does the white go when the snow melts?
It never was. The crystals are clear, the white is just due to the
internal reflection/refraction of the light.
--
Jered Floyd
jjf...@vela.acs.oakland.edu
Geek Code: GAT d? -p+ c++++ l+ u++ e*@ m++ s/-- n--- h++ f? g- w++ t+++ r++
PGP Public key, Geek Code, picture, and assorted humor available by finger.
Isaac Kuo
David Karr <ka...@cs.cornell.edu> wrote:
>mne...@azure.engin.umich.edu (Michael K. Neylon) writes:
>>David Boll (db...@vcd.hp.com) wrote:
>>> 2) If you're doing 60 mph in a car, and you hold open the driver side
>>> door, which way will the car tend to swerve? (kids, don't try this
>>> at home)
>But the question didn't say "open the door," it said "hold open the
>... door." So I suppose you're sitting in the driver's seat of a car
>going 60 mph with the driver's door open (held, perhaps, by a strong
>cable), and someone says to you, "OK, I'm about to release the cable,
>please hold the door open."
Or maybe the person is only holding it open by a crack. That'd be easy.
In fact, because of Bernouli's principle, I'd expect it would be very
easy to open the door at 60mph. Just not all the way.
>Then the direction the car swerves is whatever direction your right
>hand turns the wheel as your extended left arm receives a compound
>fracture from the door suddenly slamming toward you.
Assuming the person uses both arms to hold open the door a bit,
and the steering wheel is precisely centered properly, then I'd
say most cars would turn right. On the one hand, the drag on the
door produces a counterclockwise torque. On the other hand, the
angle of attack of the door produces a force to the right. This
will produce a clockwise torque since the rear tires are pinned
while the front tires are free.
My bet is that for most cars, the angle of attack will be such that the
torque produced by the extra drag will be outweighed by the torque
produced by the rightward force. The moment arm is longer, and except
at great angles of attack, even an inefficient "airfoil" like a car
door will produce more "lift" than drag.
--
_____ Isaac Kuo (isaa...@math.berkeley.edu)
__|_>o<_|__
/___________\ "Moteru! Ore wa moteru!"
\=\>-----</=/ --Ranma, after being slapped silly by Ukyou
N M Miners
: In article <2rv98i$b...@mercury.dur.ac.uk>, N M Miners <N.M.M...@durham.ac.uk> writes:
: |> David Boll (db...@vcd.hp.com) wrote:
: |> : 2) If you're doing 60 mph in a car, and you hold open the driver side
: |> : door, which way will the car tend to swerve? (kids, don't try this
: |> : at home)
: |>
: |> Are we driving in the U.K. or the U.S.A. here?
: |>
: EXCUSE ME !!! There are other countries in the world
: than those 2. These kind of remarks i find, in random order:
: RUDE
: ARROGANT
: &
: IGNORANT
: No offence.
Sorry - I realise there are more than 2 countries in the world - I was
speaking figuratively, because ppl in the USA drive on the right (as do
the Dutch, the Germans, the French, the Italians... see the list goes
on) whereas ppl in the UK (as well as Oz, S. Africa and some other
countries I'm not sure about) drive on the left.
I shall now modify my statement to:
Are we using a left-hand or right-hand drive vehicle?
As you say, no offence intended.
Namarie
--
.88888888888888'"8888" "88" .o8888888o. "888888888o.
.88" .8888' "88'.8888' .88' .8888' ]888 .8888' "8888
" .8888' " .8888' .88'.8888' .8888 .8888' .d888'
.8888' .8888' .88'.8888' .8888' .8888' .o888"
.8888' .8888' .88' 8888' .8888' .88888888P"'
.8888' 8888' ,88' 888[ .8888" .8888'"888L
.o88888. 8888ooooo88" "888oo888P" .8888' "888b
"""""""""' `""""""""' `"""""" """""" """""'
N.M.M...@durham.ac.uk
N M Miners
: 2) If you're doing 60 mph in a car, and you hold open the driver side
: door, which way will the car tend to swerve? (kids, don't try this
: at home)
Are we driving in the U.K. or the U.S.A. here?
Namarie
_______ _ _ _______ ______
(_______) |_| |_| (_______) (____ \
_ _ _ _ _ ____) )
| | | | | | | | | | | __ /
| | | |___| | | |___| | | | \ \
|_| \_____/ \_____/ |_| |_|
Thomas Gray
In a previous article, db...@vcd.hp.com (David Boll) says:
> A couple of quickies for y'all:
>
> 1) If the 2nd law of thermodynamics is correct, are people who fan
> themselves when hot just making themselves hotter?
This question is ill posed. It is confounding the perceptual
experience of feeling cool with the physical effect of temperature.
>
> 2) If you're doing 60 mph in a car, and you hold open the driver side
> door, which way will the car tend to swerve? (kids, don't try this
> at home)
To the drivers side.
>
> 3) Where does the white go when the snow melts?
It goes to the same place where all phase dependent properies go.
>
>--
>---------------
> Dave Boll db...@hp-vcd.vcd.hp.com
> "The speed of time is 1 second per second"
>
--
What's the difference between DMS and SP1, they're both computers?
- software hacker (1974)
i.sinature
Buddha Buck
Geert-Jan Thomas <tho...@tpd.tno.nl> wrote:
>
>In article <2rv98i$b...@mercury.dur.ac.uk>, N M Miners <N.M.M...@durham.ac.uk> writes:
>|> David Boll (db...@vcd.hp.com) wrote:
>|> : 2) If you're doing 60 mph in a car, and you hold open the driver side
>|> : door, which way will the car tend to swerve? (kids, don't try this
>|> : at home)
>|>
>|> Are we driving in the U.K. or the U.S.A. here?
>|>
>
>EXCUSE ME !!! There are other countries in the world
>than those 2. These kind of remarks i find, in random order:
>
>RUDE
>ARROGANT
>&
>IGNORANT
>
>No offence.
OK, then rephrase: Are we driving in the U.K. or the Civilised world?
Matthew T. Russotto
}
}Actually, from my experience in just sticking my *hand* out the window
}of a speeding car, I don't believe that a person *can* open the door at
}60 miles an hour.
I think he could, but it would take cruise control, because he'd have
to use his feet. In that case, the car will move the direction which
it tends to "pull" due to road conditions or car conditions, because
he's going to have to take both hands of the wheel to do it...
DENNIS M. CROCKER
> In article <CqCzB...@news.tudelft.nl>,
> Geert-Jan Thomas <tho...@tpd.tno.nl> wrote:
> >
> >In article <2rv98i$b...@mercury.dur.ac.uk>, N M Miners <N.M.M...@durham.ac.uk> writes:
> >|> David Boll (db...@vcd.hp.com) wrote:
I have read your answers to this puzzle and laughed so hard I couldn't
hold the door open any longer. It immediately stopped swerving left
and the door slammed shut, fracturing my funney bone in one place.
Then I wondered aloud, though I was alone, what percentage of puzzlers
that post here were also the class clowns?
Dennis Crocker
GEORGE BROZE
>
>|> : 2) If you're doing 60 mph in a car, and you hold open the driver side
>|> : door, which way will the car tend to swerve? (kids, don't try this
>|> : at home)
>|>
>|> Are we driving in the U.K. or the U.S.A. here?
>|>
>
>EXCUSE ME !!! There are other countries in the world
>than those 2. These kind of remarks i find, in random order:
>
>RUDE
>ARROGANT
>&
>IGNORANT
>
>No offence.
>
ANNOUNCER : "And from far left field, he's running, what's that in his hand?"
"is it an idea? Did he save? *CRASH* nope just a mistake.
The point of the UK vs USA question *might* have just been two
randomly picked countries where people drive on the right or the left
(unlike japan, where people drive on both)
You jumping on him like that is a bit unmerited.
Lamont "no defence" Lucas
Jonathan Haas
}David Karr <ka...@cs.cornell.edu> wrote:
}
}>But the question didn't say "open the door," it said "hold open the
}>... door." So I suppose you're sitting in the driver's seat of a car
}>going 60 mph with the driver's door open (held, perhaps, by a strong
}>cable), and someone says to you, "OK, I'm about to release the cable,
}>please hold the door open."
}
}Or maybe the person is only holding it open by a crack. That'd be easy.
}In fact, because of Bernouli's principle, I'd expect it would be very
}easy to open the door at 60mph. Just not all the way.
Just wanted to point out here that David Karr does not, in fact,
*believe* in Bernoulli's Principle.
--
__/\__ Jonathan S. Haas | Jake liked his women the way he liked
\ / University of Michigan | his kiwi fruit: sweet yet tart, firm-
/_ _\ posi...@eecs.umich.edu | fleshed yet yielding to the touch, and
\/ Finger for PGP 2.3 key | covered with short brown fuzzy hair.
Keeper of the Internet Automated Bidding Server. Send email with
BIDSERVER for the Subject and HELP in the body for info.
TM
: A couple of quickies for y'all:
: 1) If the 2nd law of thermodynamics is correct, are people who fan
: themselves when hot just making themselves hotter?
no
: 2) If you're doing 60 mph in a car, and you hold open the driver side
: door, which way will the car tend to swerve? (kids, don't try this
: at home)
i did this. The car didn't swerve at all. But there *is* a force
trying to move the car towardthe left. BTW, the window was open when
I did this, does that change the experiment?
: 3) Where does the white go when the snow melts?
when you grind up a diamond, the resultant powder is black. Cool 'eh.
: --
: ---------------
: Dave Boll db...@hp-vcd.vcd.hp.com
: "The speed of time is 1 second per second"
that means the speed of time is dimensionless. Incredible!!!! Time has no
speed? who'd ever think???
just trying to give your .sig a bad time
"TM"
John Wolford
> A couple of quickies for y'all:
>
> 1) If the 2nd law of thermodynamics is correct, are people who fan
> themselves when hot just making themselves hotter?
I'd answer that if i knew what the 2nd law is.
> 2) If you're doing 60 mph in a car, and you hold open the driver side
> door, which way will the car tend to swerve? (kids, don't try this
> at home)
Assuming you're able to keep the door open, i suppose it would tend to
swerve to the driver's side.
> 3) Where does the white go when the snow melts?
>
If by white you mean the rays of light that strike the snow, they don't go
anywhere, it's just that different rays of light are absorbed. If by white you
mean the quality of the snow that causes the light to be reflected in a way thatmakes it appear white, it is destroyed when the snow melts.
--
John
David Rysdam
>In a previous article, db...@vcd.hp.com (David Boll) says:
>> A couple of quickies for y'all:
>>
>> 1) If the 2nd law of thermodynamics is correct, are people who fan
>> themselves when hot just making themselves hotter?
>This question is ill posed. It is confounding the perceptual
>experience of feeling cool with the physical effect of temperature.
Are you saying that having air pass over a (possibly wet) body will not
actually lower the temperature? For shame! Read a chemistry textbook.
Look under "Evaporation."
--
-- David Rysdam's .sig of the day is:
"Why be a man when you can be a success?"
-- Bertold Brecht
Brice Wightman
In a previous article, tho...@tpd.tno.nl (Geert-Jan Thomas) says:
>
>In article <2rv98i$b...@mercury.dur.ac.uk>, N M Miners <N.M.M...@durham.ac.uk> writes:
>|> David Boll (db...@vcd.hp.com) wrote:
>|> : 2) If you're doing 60 mph in a car, and you hold open the driver side
>|> : door, which way will the car tend to swerve? (kids, don't try this
>|> : at home)
>
It seems to me that the principal forces are aerodynamic. I would say
that the door acts like a flap on an aeroplane (aka airplane) providing
lift and pushing the car away from the driver's side.
BTW, is not the Bernouilli force at this speed enough to pull the door out
of control?
--
========================================================================
Brice Wightman
Ottawa, Canada VE3EDR
========================================================================
David Karr
>Isaac Kuo <isaa...@lhasa.berkeley.edu> wrote:
>}David Karr <ka...@cs.cornell.edu> wrote:
>}
>}In fact, because of Bernouli's principle, I'd expect it would be very
>}easy to open the door at 60mph. Just not all the way.
>
>Just wanted to point out here that David Karr does not, in fact,
>*believe* in Bernoulli's Principle.
Just to be consistent, why not point out that I do not, in fact,
believe in gravity, or that 2 + 2 = 4?
Of course I *do* believe in Bernoulli's Principle. For example, it
allows Venturi tubes to be used as sensors for airspeed indicators.
What irks Mr. Haas is that I say that all his flight manuals are
snowing him when they give "Bernoulli's principle" as the explanation
for "how airplanes fly."
For a very nice, clear explanation of how airplanes fly (to be more
precise, how an airfoil generates lift), read "Bernoulli's Law and
Aerodynamic Lifting Force" by Klaus Weltner, in The Physics Teacher,
vol. 28 (Feb 1990), pp. 84-86.
(To summarize the debate, most popular explanations of the lifting
force say that the higher speed of the air flowing over the top of the
wing causes a drop in pressure by Bernoulli's principle. Weltner
concludes that indeed Bernoulli applies, but "the other way around."
Low pressure on the top of the wing, created by reaction forces,
causes the air to flow faster there.)
A car door, of course, is in many ways unlike a typical airplane
wing, so I wouldn't presume to apply the same arguments blindly.
Isaac Kuo
>posi...@quip.eecs.umich.edu (Jonathan Haas) writes:
>>Isaac Kuo <isaa...@lhasa.berkeley.edu> wrote:
>>}In fact, because of Bernouli's principle, I'd expect it would be very
>>}easy to open the door at 60mph. Just not all the way.
>>Just wanted to point out here that David Karr does not, in fact,
>>*believe* in Bernoulli's Principle.
>Just to be consistent, why not point out that I do not, in fact,
>believe in gravity, or that 2 + 2 = 4?
>Of course I *do* believe in Bernoulli's Principle. For example, it
>allows Venturi tubes to be used as sensors for airspeed indicators.
>What irks Mr. Haas is that I say that all his flight manuals are
>snowing him when they give "Bernoulli's principle" as the explanation
>for "how airplanes fly."
(Warning:ranting immediately follows...)
I can't agree more! I can't stand the traditional "explanation" via
Bernoulli's principle of how airfoils produce lift either! For a good
example on how to dumbfound a person who blindly recites it, consider
a flat wing (barn door?) at a positive angle of attack. Both the top
and bottom are the same length...
(Ranting ends.)
Interesting how this little subthread turned out, maybe?
>For a very nice, clear explanation of how airplanes fly (to be more
>precise, how an airfoil generates lift), read "Bernoulli's Law and
>Aerodynamic Lifting Force" by Klaus Weltner, in The Physics Teacher,
>vol. 28 (Feb 1990), pp. 84-86.
Ooh, I always wanted a good reference on this!
>(To summarize the debate, [summary deleted]
>A car door, of course, is in many ways unlike a typical airplane
>wing, so I wouldn't presume to apply the same arguments blindly.
Yeah. I was talking about openning a closed door, and it just so
happens that Bernoulli's principle _is_ useful in this instance.
We know the airspeed on one side of a closed door to be zero and
the airspeed on the other side to be nonzero.
As soon as the door is openned, then the situation's a lot more
complicated, though...
Jonathan Haas
>
>I can't agree more! I can't stand the traditional "explanation" via
>Bernoulli's principle of how airfoils produce lift either! For a good
>example on how to dumbfound a person who blindly recites it, consider
>a flat wing (barn door?) at a positive angle of attack. Both the top
>and bottom are the same length...
Uh huh. Now, measure the airspeed you must move your barn door to make
it take off. It's pretty darned high. Now, give that barn door a slight
camber over the top and try it again. You'll find the airspeed necessary
to generate the required lift for takeoff decreases dramatically...
because, while the deflection of air helps create some lift, the
Bernoulli effect creates much more.
Your 'barn door', by the way, does not stall. It generates maximum lift
at a 45-degree angle of attack, then generates less and less lift as
angle of attack increases until it 'stalls' at vertical. As a pilot,
I can assure you that stalls occur at much, much lower angles.
George Galanis
> door, which way will the car tend to swerve? (kids, don't try this
> at home)
I can answer this from pracical experience.
I was trying to get rid of a large spider
which was on the drivers side door while
driving along the road. This was in Australia
so the drivers side door is on the right hand
side of the car. I opened the drivers
door in attempt to flick the spider out of
the car. The car swerved left and hit a tree
and rolled over.
So the answer is that the car swerves opposite
to the door that is open. However I was only
doing 40kmh at the time. Maybe a different
effect may be observed at 60 mph, maybe the
Coriolis effect would be dominant at a higher
speed giving a different result for different
directions of motion?
Mark Das
reading)
Thusly, the flight of a plane is caused by BP, however indirect it is.
Just like siphons and air pressure. Not that I've had my annoyances with
physics books. However, it seems like you are saying that it isn't
caused by BP, when it is, but not simply that.
Jonathan Haas
>Jonathan Haas responds to Isaac Kuo and myself without having read the
>article by Weltner (and without coming up with a counterargument other
>than the unsubstantiated assertions of some anonymous tech writers).
>Jonathan, do your homework!
I have attempted to, David, but I've been unable to locate a copy of
"Physics Teacher Magazine"... and I've tried.
I'll keep looking (I've just moved to a new and larger city, perhaps
the public library here will carry it), but in the meantime, may I ask
why I'm supposed to trust a single article in an obscure magazine as
the word of God, while you disdain the words that appear in college-level
aerodynamics textbooks?
For an expert in a field to take a stand that disagrees with all other
experts in his field is bold, daring, and risky. For a dabbler in a field
to take a stand that disagrees with all experts in that field is
foolhardy. You've been acting like a third-grader who, after browsing
his third-grade math text, announces that everything that everyone ever
knew about differential equations is false.
>Not only that, but many of the specific responses are misleading or
>wrong:
>>
>>Uh huh. Now, measure the airspeed you must move your barn door to make
>>it take off. It's pretty darned high. Now, give that barn door a slight
>>camber over the top and try it again. You'll find the airspeed necessary
>>to generate the required lift for takeoff decreases dramatically...
>
>Not all that dramatically. Yes, the curved thin wing generates
>somewhat more lift at a given angle of attack and velocity.
>(Actually, what the camber gives you is a negative angle of zero lift,
>or equivalently, a positive lift even at zero angle of attack.) The
>lift of the curved wing also maxes out at a steeper angle of attack.
>But it's not as if you have to double your takeoff speed. An increase
>of a few percent would make up for the difference.
Do you want to give any sort of proof for that statement, or even any
sort of empirical evidence, or did you just want to assert it _ex_cathedra_?
>>because, while the deflection of air helps create some lift, the
>>Bernoulli effect creates much more.
>
>This is a typical non sequitur of the "Bernoulli causes lift" school.
>We are still talking about very thin plate here. We have given it
>camber but not thickness. The top and the bottom are still the same
>length. Where did the Bernoulli effect suddenly spring from?
"Camber", as it relates to airfoils, means "the curvature of the airfoil
surface with respect to the wing chord." I specified camber "over the
top". Wanna tell me how you're going to give the surface curvature without
increasing the thickness of the airfoil?
>>Your 'barn door', by the way, does not stall.
>Define what you mean by "stall". Do you mean "loses lift due to
>separation of flow"? If so, then the barn door most definitely *does*
>stall, and at a much lower angle of attack than your cambered thin
>wing.
No, what I mean is "ceases to generate lift". And if you think the
"barn door" stalls, try this experiment:
Extend a perfectly flat piece of thin wood out the window of a car,
perfectly horizontal. (You could use your hand, but your hand has
camber and that may bias the results.) If you've got it perfectly
horizontal, there shouldn't be any lift. Now, tip it up... slightly.
You'll instantly feel both lift and induced drag. Not a *lot* of
lift (try affixing a heavy weight to your wood and you'll see what
I mean), but some. Tip it up a bit more. You'll feel both more lift
and more induced drag. Keep increasing the angle of attack, and feel
how lift increases. You'll find that lift keeps increasing until 45
degrees... the angle of maximum lift for a noncambered airfoil. (Drag
keeps increasing right up to 90 degrees).
>>It generates maximum lift
>>at a 45-degree angle of attack,
>
>Absolutely incorrect. The maximum lift occurs at a relatively shallow
>angle of attack, maybe 10 or 15 degrees. (Less than the angle of
>maximum lift for a curved plate, remember.)
Again, did you want to support that at all, or just assert it?
>>then generates less and less lift as
>>angle of attack increases until it 'stalls' at vertical.
>
>This part is more or less true, except that the stall (as noted above)
>occurs much earlier.
As noted, but not supported with any evidence at all, theoretical or
empirical.
David Karr
Jonathan Haas responds to Isaac Kuo and myself without having read the
article by Weltner (and without coming up with a counterargument other
than the unsubstantiated assertions of some anonymous tech writers).
Jonathan, do your homework!
(An earlier version of Weltner's article appears in Amer J Phys, vol.
55 p. 50, but the later version seems better presented.)
Not only that, but many of the specific responses are misleading or
wrong:
In article <2si0r1$3...@zip.eecs.umich.edu> posi...@quip.eecs.umich.edu (Jonathan Haas) writes:
>Isaac Kuo <isaa...@lhasa.berkeley.edu> wrote:
[...]
>>a flat wing (barn door?) at a positive angle of attack. Both the top
>>and bottom are the same length...
>
>Uh huh. Now, measure the airspeed you must move your barn door to make
>it take off. It's pretty darned high. Now, give that barn door a slight
>camber over the top and try it again. You'll find the airspeed necessary
>to generate the required lift for takeoff decreases dramatically...
Not all that dramatically. Yes, the curved thin wing generates
somewhat more lift at a given angle of attack and velocity.
(Actually, what the camber gives you is a negative angle of zero lift,
or equivalently, a positive lift even at zero angle of attack.) The
lift of the curved wing also maxes out at a steeper angle of attack.
But it's not as if you have to double your takeoff speed. An increase
of a few percent would make up for the difference.
>because, while the deflection of air helps create some lift, the
>Bernoulli effect creates much more.
This is a typical non sequitur of the "Bernoulli causes lift" school.
We are still talking about very thin plate here. We have given it
camber but not thickness. The top and the bottom are still the same
length. Where did the Bernoulli effect suddenly spring from?
>Your 'barn door', by the way, does not stall.
Define what you mean by "stall". Do you mean "loses lift due to
separation of flow"? If so, then the barn door most definitely *does*
stall, and at a much lower angle of attack than your cambered thin
wing.
>It generates maximum lift
>at a 45-degree angle of attack,
Absolutely incorrect. The maximum lift occurs at a relatively shallow
angle of attack, maybe 10 or 15 degrees. (Less than the angle of
maximum lift for a curved plate, remember.)
>then generates less and less lift as
>angle of attack increases until it 'stalls' at vertical.
This part is more or less true, except that the stall (as noted above)
occurs much earlier.
>As a pilot,
>I can assure you that stalls occur at much, much lower angles.
Yes, at about 18 degrees for a Clark Y profile, for example. The
coefficient of lift (CL) in this case is about 1.55. As the angle
increases beyond 18 degrees, the CL falls off gradually to about 1.25
at 25 degrees, then about 1.05 at 30 degrees (from Fig. 115, p. 160 of
Von Mises, Theory of Flight).
Andy Germain
ka...@cs.cornell.edu writes:
> What irks Mr. Haas is that I say that all his flight manuals
are
> snowing him when they give "Bernoulli's principle" as the
explanation
> for "how airplanes fly."
I'm glad you said that -- I too have been annoyed at the
bernoulli explanation for flight. It seems to me that the
wing, moving forward with an angle of attack causes air to be
PUSHED DOWN. To exert this downward force on the air, an equal
upward force must be exerted on the wings. Pretty simple,
really.
I believe that a plane with a wing which was flat on the
bottom, and kept horizontal, would run off the runway before
getting off the ground.
Yes, the curvature of the wing is nice - it reduces drag.
In evidence, I offer these two examples:
1) How is a plane's climb or descent controlled? By adjusting
its angle of attack!
Within normal operating limits (I have to put this clause in or
some nit-picker will beat me up), the climb or descent rate of
an airplane is controlled by changing the angle of attack of
the wing. With a constant throttle setting in a level cruise,
pulling back on the wheel (slightly) causes the elevator to go
up, pushing the tail down. The wing thus now has a higher
angle of attack, and develops more lift, resulting in the
airplane starting to climb.
Since this is the normal way to control the plan, it seems
reasonable to me that the lift due to this factor must be more
significant than any bernoulli effect.
2) Some planes have SYMMETRICAL wings!
The Pitts special, for example. Since it is intended primarily
for acrobatics, it was designed to fly upside-down about as
well as right side up. So its wings, although curved, are
exactly
symmetrical. How can Bernoulli fly this one.
But with angle of attack, it is easy. It either case (normal
or inverted) the tail is kept somewhat low - this applies some
angle of attack to the wings. And provides lift!
Comments?
#include "disclaimer.*" Andy Germain
Jonathan Haas
> David Karr, ka...@cs.cornell.edu writes:
>> What irks Mr. Haas is that I say that all his flight manuals
>are
>> snowing him when they give "Bernoulli's principle" as the
>explanation
>> for "how airplanes fly."
>
>I'm glad you said that -- I too have been annoyed at the
>bernoulli explanation for flight. It seems to me that the
>wing, moving forward with an angle of attack causes air to be
>PUSHED DOWN. To exert this downward force on the air, an equal
>upward force must be exerted on the wings. Pretty simple,
>really.
Very simple. Now calculate the force exerted on the wing by that air.
(You go ahead, I'll wait here). I'll bet you a million billion dollars
that you'll find the lift generated by a wing is much more than that.
Oh, and how about helicopters? If what you say is correct, the blades
of a helicopter are driving air downward with enough force to hurl
a multiton object into the air. Anyone standing underneath the blades
would be crushed to pulp in an instant.
And don't you think the designers of these aircraft perform empirical
testing on the wings, to maximize performance? If aircraft depended
on the reaction force of driving air downward for lift, the most efficient
shape for helicopter blades would be exactly like the blades of a fan.
If this were true, don't you think Bell and Sikorsky would have discovered
that by now, and implemented it?
>I believe that a plane with a wing which was flat on the
>bottom, and kept horizontal, would run off the runway before
>getting off the ground.
Beliefs are great, aren't they? Trouble is, they're often wrong... like
this one.
>Yes, the curvature of the wing is nice - it reduces drag.
Sure does.
>1) How is a plane's climb or descent controlled? By adjusting
>its angle of attack!
Partly. Mostly by changing its power.
>Within normal operating limits (I have to put this clause in or
>some nit-picker will beat me up), the climb or descent rate of
>an airplane is controlled by changing the angle of attack of
>the wing. With a constant throttle setting in a level cruise,
>pulling back on the wheel (slightly) causes the elevator to go
>up, pushing the tail down. The wing thus now has a higher
>angle of attack, and develops more lift, resulting in the
>airplane starting to climb.
This is correct so far... except for refering to the yoke or stick as
a "wheel", which no self-respecting pilot will do... but a more or less
accurate description of how the controls of a plane operate.
>Since this is the normal way to control the plan, it seems
>reasonable to me that the lift due to this factor must be more
>significant than any bernoulli effect.
This is where the nonsequitur comes in. Yes, increasing the angle of
attack increases the lift from the reaction force... which is present,
but usually small compared to the Bernoulli force, except under certain
special circumstances. However, increasing angle of attack ALSO increases
the Bernoulli force (up to the critical angle). The upper surface of the
wing deflects air further, resulting in lower pressure above the wing,
resulting in increased lift.
>2) Some planes have SYMMETRICAL wings!
True. Aerobatic planes, primarily.
>The Pitts special, for example. Since it is intended primarily
>for acrobatics, it was designed to fly upside-down about as
>well as right side up. So its wings, although curved, are
>exactly
>symmetrical. How can Bernoulli fly this one.
It doesn't, which is why planes with symmetric wings (aka zero total
camber) require extremely high speeds, and extremely light airframes.
You'll never, never see a passenger jet with symmetric wings...
the reaction force just doesn't generate enough lift to make a jumbo
jet lift off.
>Comments?
/* Sure. */
>#include "disclaimer.*" Andy Germain
This is a compile-time error, btw.
John Wolford
Can you be sure that the car's swerving wasn't due to another
cause, like for example your arm moving the steering wheel? After
all, it sounds like you might have been under quite a bit of stress if
you deem it important enough to open the door at 40 mph to try &
flick a spider (albeit large, i'm sure) out.
John
Isaac Ji Kuo
Jonathan Haas <posi...@quip.eecs.umich.edu> wrote:
>Isaac Kuo <isaa...@lhasa.berkeley.edu> wrote:
>>I can't agree more! I can't stand the traditional "explanation" via
>>Bernoulli's principle of how airfoils produce lift either! For a good
>>example on how to dumbfound a person who blindly recites it, consider
>>a flat wing (barn door?) at a positive angle of attack. Both the top
>>and bottom are the same length...
>Uh huh. Now, measure the airspeed you must move your barn door to make
>it take off. It's pretty darned high.
Not really. Try making a paper airplane with completely flat wings.
The easiest is to take a straw and put a paperclip on one end. Then
tape two cylindrical wings to it (made from strips of paper about an
inch wide). Make the front cylinder smaller than the rear one.
This thing not only flys, it flys _well_. I found the design in some
paper plane book when I was little and expected it to fall like a
rock, not producing any lift at all. Yes, it was because I thought
curved wings produced much better lift than a flat wing.
Try it. It's actually a very fun plane to fly. And it glides
wonderfully. To a firm "believer" of curved airfoils (like I was) the
first flight is like magic.
>Now, give that barn door a slight
>camber over the top and try it again. You'll find the airspeed necessary
>to generate the required lift for takeoff decreases dramatically...
>because, while the deflection of air helps create some lift, the
>Bernoulli effect creates much more.
Look, a flat wing at an angle does not violate Bernoulli's principle at
all. The airspeed above the wing _is_ greater than the airspeed below
it. But I defy anyone to explain how Bernoulli's principle causes it.
At high angles of attack, there is a very pronounced region
on the bottom near the leading edge where the air actually goes forward
in order to "flip" around the leading edge. Go to the bookstore and
look at the covers of the fluid dynamics books. I bet at least one
has an image of this on the cover.
I defy you to explain how the Bernoulli effect _causes_ that.
>Your 'barn door', by the way, does not stall. It generates maximum lift
Yes it does stall, and it stalls much worse than a curved airfoil.
--
_____ Isaac Kuo (isaa...@ocf.berkeley.edu)
Isaac Ji Kuo
Jonathan Haas <posi...@quip.eecs.umich.edu> wrote:
>Andy Germain <Andy.G...@EOS.NASA.gov> wrote:
>>I'm glad you said that -- I too have been annoyed at the
>>bernoulli explanation for flight. It seems to me that the
>>wing, moving forward with an angle of attack causes air to be
>>PUSHED DOWN. To exert this downward force on the air, an equal
>>upward force must be exerted on the wings. Pretty simple,
>>really.
Yes, but the air most affected is actually the air _above_ the wing,
for any wing which has not stalled. That's why a stall (separation
of the airflow above the wing) results in such drastic loss of lift.
You can think of it as air being "pulled" down rather than pushed.
>Very simple. Now calculate the force exerted on the wing by that air.
>(You go ahead, I'll wait here). I'll bet you a million billion dollars
>that you'll find the lift generated by a wing is much more than that.
That's a million billion dollar IOU, no matter what. The only thing
which exerts any force on the wing is the air around it and the
fuselage (which is pulling it down, obviously). It is true that the
angular deflection of the air above and below the wing of an airplane
is not much, but that's because the force is spread among all of the
passing air (and there's lots of it when you're going 200+mph).
>Oh, and how about helicopters? If what you say is correct, the blades
>of a helicopter are driving air downward with enough force to hurl
>a multiton object into the air. Anyone standing underneath the blades
>would be crushed to pulp in an instant.
A helicopter spreads the force of its weight among a large area. The
larger the area, the more efficient this is. That's why a helicopter
can hover much more easily than a Harrier jump jet. And yes, you'd
take a serious blow if you got too near a Harrier nozzle.
>And don't you think the designers of these aircraft perform empirical
>testing on the wings, to maximize performance? If aircraft depended
>on the reaction force of driving air downward for lift, the most efficient
>shape for helicopter blades would be exactly like the blades of a fan.
Untrue. Different power levels and expected airspeeds have a drastic
effect on the shape of an airscrew--compare a propellor to a helicopter's
rotors.
>>1) How is a plane's climb or descent controlled? By adjusting
>>its angle of attack!
>Partly. Mostly by changing its power.
This is a statement from fantasy-land. Extra power is needed in a
climb to keep it at a constant speed, but saying an adjustment in
the throttle caused the airplane to climb is like saying you get on
the highway by speeding up to 65mph. Let's ignore the elevators
at the rear of the plane, shall we?
>>Since this is the normal way to control the plan, it seems
>>reasonable to me that the lift due to this factor must be more
>>significant than any bernoulli effect.
>This is where the nonsequitur comes in. Yes, increasing the angle of
>attack increases the lift from the reaction force... which is present,
>but usually small compared to the Bernoulli force, except under certain
>special circumstances. However, increasing angle of attack ALSO increases
>the Bernoulli force (up to the critical angle). The upper surface of the
>wing deflects air further, resulting in lower pressure above the wing,
>resulting in increased lift.
As if there were some magical difference between the "reaction force"
and the "Bernoulli force." The Bernoulli principle applies at all times,
except for very bizzare circumstances. It's even valid in supersonic
flight. Given the surface airspeed and static pressure at a given
point, one can use Bernoulli's principle to find its dynamic pressure.
The reason this principle is so useful for subsonic flight is that
the static pressure of the atmosphere around a subsonic airplane is
_very_ constant. Air simply isn't getting compressed significantly.
Therefore, in a wind tunnel, one can use streamlines of air to determine
airspeed (since the air isn't compressed anywhere), and in turn one
can use this to determine the pressure at any point (since the static
pressure is constant).
It's much cheaper to set up an array of smoke streamers than to put
pressure sensing transducers on the surfaces of every airfoil being
tested...
>>The Pitts special, for example. Since it is intended primarily
>>for acrobatics, it was designed to fly upside-down about as
>>well as right side up. So its wings, although curved, are
>>exactly
>>symmetrical. How can Bernoulli fly this one.
>It doesn't, which is why planes with symmetric wings (aka zero total
>camber) require extremely high speeds, and extremely light airframes.
>You'll never, never see a passenger jet with symmetric wings...
>the reaction force just doesn't generate enough lift to make a jumbo
>jet lift off.
Planes with symmetrical wings produce lift in the same way as planes
with assymmetrical wings. They're less efficent, but not by an order
of magnitude like you suggest. And you see symmetrical airfoils on
passenger jets all the time--the rudder is the obvious example, but
the elevators are often symetrical as well.
Charles Bryant
posi...@quip.eecs.umich.edu "Jonathan Haas" writes:
...
> Oh, and how about helicopters? If what you say is correct, the blades
> of a helicopter are driving air downward with enough force to hurl
> a multiton object into the air. Anyone standing underneath the blades
> would be crushed to pulp in an instant.
When a helicopter is hovering, since there is no vertical acceleration,
the net vertical force must be zero. Also we know that the gravitational
force is equal to its weight. How else can it exert an equal downward
force except by driving air downward?
--
Charles Bryant (c...@chch.demon.co.uk)
Jonathan Haas
>Jonathan Haas <posi...@quip.eecs.umich.edu> wrote:
>
>>Uh huh. Now, measure the airspeed you must move your barn door to make
>>it take off. It's pretty darned high.
>
>Not really. Try making a paper airplane with completely flat wings.
>The easiest is to take a straw and put a paperclip on one end. Then
>tape two cylindrical wings to it (made from strips of paper about an
>inch wide). Make the front cylinder smaller than the rear one.
>
>This thing not only flys, it flys _well_. I found the design in some
>paper plane book when I was little and expected it to fall like a
>rock, not producing any lift at all. Yes, it was because I thought
>curved wings produced much better lift than a flat wing.
You're seriously comparing the aerodynamics of paper planes with real
ones? Very well... a Cessna 150, at its 1600-pound gross weight,
can achieve a power-off glide ratio of about eight to one... meaning
that for every foot it sinks, it will glide forward eight feet.
(source: "Model 150 (1972) Owner's Manual", Cessna Corp., fig. 6-5).
I'll be generous and say that your little plane is only 1% the size
of a C-150, even though the ratio is closer to 4% and larger in
wingspan. So to make it fair, affix a 16-pound bowling ball to your
little toy, *then* let me know how well it glides.
Chris Best
> of a helicopter are driving air downward with enough force to hurl
> a multiton object into the air. Anyone standing underneath the blades
> would be crushed to pulp in an instant.
----------
You probably don't understand how a bed-of-nails works, either.
Isaac Kuo
Jonathan Haas <posi...@quip.eecs.umich.edu> wrote:
>Isaac Ji Kuo <isaa...@OCF.Berkeley.EDU> wrote:
>>Jonathan Haas <posi...@quip.eecs.umich.edu> wrote:
>>>Uh huh. Now, measure the airspeed you must move your barn door to make
>>>it take off. It's pretty darned high.
>>Not really. Try making a paper airplane with completely flat wings.
>>The easiest is to take a straw and put a paperclip on one end. Then
>>tape two cylindrical wings to it (made from strips of paper about an
>>inch wide). Make the front cylinder smaller than the rear one.
>You're seriously comparing the aerodynamics of paper planes with real
>ones?
I'm seriously suggesting you go and _try_ this thing. I didn't suggest
building a Piper Cub sized prop plane with flat wings for two reasons:
1. You probably won't do it. I never have. Does that surprise anyone?
2. A flat wing a couple millimeters thick can't support the fuselage.
I don't have to explain why this is so, do I?
> Very well... a Cessna 150, at its 1600-pound gross weight,
>can achieve a power-off glide ratio of about eight to one... meaning
>that for every foot it sinks, it will glide forward eight feet.
>(source: "Model 150 (1972) Owner's Manual", Cessna Corp., fig. 6-5).
>I'll be generous and say that your little plane is only 1% the size
>of a C-150, even though the ratio is closer to 4% and larger in
>wingspan. So to make it fair, affix a 16-pound bowling ball to your
>little toy, *then* let me know how well it glides.
Perhaps you should learn a little about Reynolds numbers and how
aerodynamics scale before claiming this. There's a reason why small
insects don't look like miniature birds. Smaller craft have worse
_theoretical limits_ on glide ratios. So to be perfectly fair,
scale down that Cessna to the size of a paper airplane. Actually,
this is very _unfair_ because the shape of the Cessna is optimized
for its size...
Try making this airplane. Try making a better one with identical
proportions except for cambering the wings a little bit. Really.
In my experience, trying to improve a paper plane by cambering its
wings almost always spoil the aerodynamics, and at best equals it.
Even if the resulting paper plane is still stable, the camber usually
just increases the frontal area increasing drag. For small paper
planes, there's truly no point--look at insect wings.
--
_____ Isaac Kuo (isaa...@math.berkeley.edu)
DENNIS M. CROCKER
door theory". Guess what? The door was torn off the
car. You owe me a door.
Dennis Crocker
Brian Pickrell
: >of a helicopter are driving air downward with enough force to hurl
: >a multiton object into the air. Anyone standing underneath the blades
: >would be crushed to pulp in an instant.
: A helicopter spreads the force of its weight among a large area. The
: larger the area, the more efficient this is.
Right. Take, for instance, a hypothetical 6000-pound helicopter with
a 20-foot blade span. These blades have to push down a column of air
with an area of 314.16 sq. ft., with a total force of 6000 pounds.
That's a pressure of 20 lbs per sq. ft., or 1/7 pound per square
inch: 1/100 of standard atmospheric pressure. This is less than hte
pressure difference you experience when a cold front blows in, hardly
enough to crush anyone to pulp.
[...]
: >>1) How is a plane's climb or descent controlled? By adjusting
: >>its angle of attack!
: >Partly. Mostly by changing its power.
: This is a statement from fantasy-land. Extra power is needed in a
: climb to keep it at a constant speed, but saying an adjustment in
: the throttle caused the airplane to climb is like saying you get on
: the highway by speeding up to 65mph. Let's ignore the elevators
: at the rear of the plane, shall we?
Whoops! A pilot DOES put a plane into a climb by adding power.
You usually pull back on the yoke at the same time, but if you add
power and do nothing else, the airplane will start to climb. This
is because the extra speed creates more lift, which causes the
nose to rise. It will "seek" an attitude in which it
will fly at the original airspeed.
In fact, the wings of an airplane in a constant-speed climb generate
LESS lift than in level flight.
: >increasing the angle of
: >attack increases the lift from the reaction force... which is present,
: >but usually small compared to the Bernoulli force, except under certain
: >special circumstances.
"Bernoulli force" and "reaction force" are really the same thing.
The laws of aerodynamics are just a statistical expression of the same
laws of motion that apply to everything. Every action has an equal
and opposite reaction: this applies to air molecules exactly
as it applies to planets, billiard balls and aluminum boxes full
of people.
Brian Pickrell
Richard A Reitmeyer
newsgroup---but I would like to do what I can to clear up what looks
like a misunderstanding of aerodynamics.
Fundamentally, camber has nothing to do with lift.
Camber exists to reduce drag; a flat plate generates just as much lift
as a wing, but the flow will separate from a flat plate at any angle
of attack, whereas a cambered wing will not separate for 10-15 degrees
from the zero-lift-line.
Someone pointed out something about Reynolds Number, and I'd like to
confirm the importance; at low Reynolds Numbers, separation is not as
serious an issue. Insects have flat wings, and early airplanes had
little or no camber, due to the low airspeeds (as well as ignorance).
Good luck all.
Richard
--------------------------------------------------------------------------
Richard A. Reitmeyer aero/astro grad gor...@leland.stanford.edu
--------------------------------------------------------------------------
Charlotte: Have you got a favorite book?
Henry: _Finnegans_Wake_
Charlotte: Have you read it?
Henry: Don't be silly.
-- Tom Stoppard, 1937-, ``The Real Thing''
Newsgroups: rec.puzzles
Subject: Re: How things fly (was: Fun with physics!)
Summary:
Expires:
References: <1994May25.1...@cs.cornell.edu> <2si0r1$3...@zip.eecs.umich.edu> <2spiq6$m...@agate.berkeley.edu> <2sstjk$l...@zip.eecs.umich.edu>
Sender:
Followup-To:
Distribution:
Organization: Stanford University, CA 94305, USA
Keywords:
Cc:
I didn't catch all of this thread---I'm not a frequent reader of this
newsgroup---but I would like to do what I can to clear up what looks
like a misunderstanding of aerodynamics.
Fundamentally, camber has nothing to do with lift.
Camber exists to reduce drag; a flat plate generates just as much lift
as a wing, but the flow will separate from a flat plate at any angle
of attack, whereas a cambered wing will not separate for 10-15 degrees
from the zero-lift-line.
Someone pointed out something about Reynolds Number, and I'd like to
confirm the importance; at low Reynolds Numbers, seperation is not as
serious an issue. Insects have flat wings, and early airplanes had
little or no camber, due to the low airspeeds (as well as ignorance).
Good luck all.
Richard
--------------------------------------------------------------------------
Richard A. Reitmeyer aero/astro grad gor...@leland.stanford.edu
--------------------------------------------------------------------------
Charlotte: Have you got a favorite book?
Henry: _Finnegans_Wake_
Charlotte: Have you read it?
Henry: Don't be silly.
-- Tom Stoppard, 1937-, ``The Real Thing''