6,937 reputation
1424
bio website en.wikipedia.org/wiki/…
location United States
age 70
visits member for 2 years, 10 months
seen 12 hours ago

BS Mechanical Engr.
PhD CS(AI)
CS Prof (4yr)
Numerous consulting jobs.
15 yr at http://www.pharsight.com
Published book on CS & several articles
4 kids, 2 grand
Pilot(student)

P.S. The picture is a Beta-prime distribution. It shows the program speedup factors you can get if you see a problem twice in 2, 3, 4, and 5 samples.


Jun
30
comment Why airplanes fly: the final truth
@usumdelphini: FWIW, the Extra 300 has a symmetrical airfoil. It weighs about 1 ton and is stressed for +/- 10G. That's 10 tons of lift up or down. Since the stall speed is 100kmh, it has to be going about 300kmh $V_s\sqrt{10}$ to achieve that amount of lift. It can go much faster than that, so at high speed it is possible to break the airplane by pulling too hard.
Jun
30
comment Why airplanes fly: the final truth
@usumdelphini: The job of an airfoil is to deflect the air in the desired direction. That takes an angle of attack. Aerobatic planes (Extra 300), which must fly inverted as easily as upright, tend to have symmetric airfoils. A common airplane (C-172) is stressed for 1G +/- 3G, i.e. 4G to -2G. So its wing is capable of generating massive amounts of lift in either direction. It can generate more than that, but doing so "voids the warranty".
Jun
30
comment Why airplanes fly: the final truth
+1 @usumdelphini: mwengler is right. A couple points. 1) You're making a big deal out of something simple. Just stick your hand out the window of a moving car, at an angle, and you will experience the lift force. 2) Professors have little quality control on what they tell students. When he says that shape has no angle of attack, he's smokin'. If it deflects air downward, it has lift, and it has an angle of attack. Take a flying lesson - it's fun - and tell that prof to do the same. Airplanes have been flying for a century. It's not a mystery how they work.
Jun
27
comment Why isn't it allowed to use a flash when taking pictures in a certain place?
Last year I visited the historical museum in Iraklion, Crete. They've got colored artifacts from the Minoan civilization over 3000 years old - murals, figurines, clothing, etc. UV would be very damaging. They don't allow flash, and I wouldn't either.
Jun
25
comment What happens if an object has more kinetic energy than the Gravitational Binding Energy?
It slows down until its kinetic energy equals its initial kinetic energy minus the energy needed to climb out of the gravity well. Then it just keeps going at that speed.
Jun
20
awarded  Good Answer
Jun
19
comment Quickest “flights” across Atlantic and round the world
If air were not in the way, the fastest would be a circular orbit slightly above the ground. Since you gotta get through the air at both ends, you need to elongate the orbit enough to get above the air, but not lob yourself so high as to take too long.
Jun
19
comment Difference between propeller and fan in pulling force
@curious123: You can get a balsa wood glider with flat wings. The wings are airfoils, just not very good ones. In a turbofan engine, you bet those vanes are airfoils, and very good ones. BTW: the propellers on the first Wright flyer gave around 70-100 lbs of thrust. On a Cessna 172 it is about 400 lbs. On an F4U Corsair it is thousands of lbs.
Jun
19
revised Why does reentry from space tend to result in such great heat?
added 21 characters in body
Jun
19
answered Why does reentry from space tend to result in such great heat?
Jun
18
answered What are the prerequisites to studying general relativity?
Jun
18
comment Difference between propeller and fan in pulling force
Martin Beckett gave a better answer than I would have because he mentioned the high-vacuum case. At normal mach numbers, airfoils produce lift by momentum transfer to the fluid, so there is no difference.
Jun
17
revised What is the optimal design for a paper airplane? (Or, at least, how can you approach it?)
added 1 characters in body
Jun
17
revised What is the optimal design for a paper airplane? (Or, at least, how can you approach it?)
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Jun
17
revised What is the optimal design for a paper airplane? (Or, at least, how can you approach it?)
edited body
Jun
17
answered What is the optimal design for a paper airplane? (Or, at least, how can you approach it?)
Jun
2
comment Reading the Feynman lectures in 2012
Those are still among my favorite books.
Jun
1
comment Rotational speed of a discus
Let me suggest alternate wording, then you do what you want: The disk is an airfoil whose orientation is gyroscopically stabilized. As it follows its (nominally parabolic) arc, as it goes into the descending portion of the arc, its angle of attack increases. The rotational speed could have an effect of delaying the onset of aerodynamic stall, thus lengthening the trajectory. It also could have increased lift on the forward-moving side, causing precession that could also affect the angle of attack.
Jun
1
comment Rotational speed of a discus
@cdecker: don't forget, it's a gyroscope, and gyroscopes precess. So if there is a force to cause it to "pitch up", that will make it roll right or left, and vice versa. Which way it goes depends on the direction and rate of spin.
May
31
comment Rotational speed of a discus
Maybe I'm not used to the lingo, but "violently and quickly it displaces the air" and "absence or scarcity of air" doesn't sound like the kind of aerodynamics I'm used to. Want to give it another shot?