8,677 reputation
1730
bio website en.wikipedia.org/wiki/…
location United States
age 71
visits member for 3 years, 10 months
seen 2 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.


May
15
comment Numerical modelling of a step function in time in a hydrodynamic system. (Runge Kutta fourth order)
@Floris: The models we make are always idealized at some level - we take that as a given. Since we're trying to fit a model to rather sparse data, we have to keep the model simple enough that the data can say something about it, so the exact mechanism for things that take almost no time is pedantic.
May
14
comment How do eagles fly slowly for a long time?
@Jubobs: It's not that soaring is different from flapping. All birds flap their wings to put energy into their flight, and all birds glide when they don't need any additional height or speed. All birds are able to soar, which is just gliding in updrafts. Some soar more than others, depending on what they need to do.
May
13
comment Numerical modelling of a step function in time in a hydrodynamic system. (Runge Kutta fourth order)
It's not a deep concept. It happens all the time in pharmacometric modeling, like here. Here's something that may be a bit much. Basically, any pharmacometric model expressed as a set of differential equations has to receive doses at points in time. So the ODE solver runs up to that time and stops. Then the dose is given which, for example, bumps the amount of drug in blood plasma. Then the ODE solver is started up again, up to the next event time, like an observation.
May
13
comment Why don't we build helicopter based space shuttles?
Can airplanes fly in no atmosphere? Of course not. There's nothing for the wing to react against. A helicopter rotor is just a wing that goes in a circle. (They are called "rotary wing aircraft".)
May
12
comment If atoms are mostly empty space, why doesn't light pass through everything?
True, if you keep in mind that where an electron "is" is uncertain - it is a wave function that is spread out. So is a photon.
May
11
comment Why is paper (or any tailless) airplane pitch stable?
If you see it nosing up and stalling, that means move the weight further toward the front. It needs to be forward of the center of lift, wherever that is (but not too far forward).
May
11
comment Why is paper (or any tailless) airplane pitch stable?
@Cleric: Actually you can choose any point you like as the fulcrum, it doesn't matter. If you choose the CG, the lift of the main wing produces an up torque, and the tail a down torque. (In a vacuum there is no lift.) Go back to the simple aircraft. Now lengthen the back of the main wing until it touches the horizontal stabilizer. Now you have a single-wing aircraft, but it still works, because at the back there is less lift than at the front (or negative lift). The differential lift between front and back produces the torque. A simple paper airplane can illustrate this perfectly.
May
11
comment Why is paper (or any tailless) airplane pitch stable?
@Cleric: The speed-dependent torque due to differential lift doesn't go to zero. The weight in the nose produces its own torque that counteracts the speed-dependent torque, so it's stable when the total torque is zero.
May
10
comment Control surface effect
What you're asking is really basic aircraft controls. There's no calculation required. Center of mass should be forward of center of lift. Ailerons control rate of bank, and bank controls rate of turn. Elevators control speed, and throttle controls rate of ascent or descent.
May
10
comment Why is paper (or any tailless) airplane pitch stable?
@Wet: Thanks for edits, and yes, quaternions are basic "rocket science".
May
8
comment Why is paper (or any tailless) airplane pitch stable?
You're seeing "decalage" in action. Here's some basic stuff to read.
May
6
comment Does my (too low) tire pressure, affect the speedometer of my car?
I agree with this answer, on the assumption that the steel belts determine the outer circumference of the tire. If the tire is very flat, the tread gets squeezed tangentially as it moves through the contact patch, because it it taking a shorter path. This is not like a caterpillar tractor tread, unless the tire is allowed to slip tangentially along the rim, which I assume it is not. It will experience tangential stress, which might cause some slipping. Evidence of this would be the filler tube departing from being perpendicular to the rim.
May
4
comment Is a Super-luminal Laser spot possible?
Why fool with seconds? Suppose you see a supernova at one point in the sky, and one year later you see another at a different point. They are separated by millions of light years. Nothing "traveled" between them, except maybe our imagination.
May
3
comment How does a hovercraft hover, if it has low pressure underneath it?
I suppose they call it "low pressure" because it's lower than "high pressure".
Apr
29
comment Why is the water in a cup not inclined (opposite the cup) when the plane takes a turn?
Actually, airplanes can make a wings-level turn, just by pressing a rudder pedal, and in that case, you and your coffee do feel a sideways force. But it's slow, uncomfortable, and very inefficient, compared to just banking into the turn.
Apr
27
comment How to make a structure of which water flows out at a constant rate?
What you need is constant pressure. Check out carburetor.
Apr
27
comment How to make a structure of which water flows out at a constant rate?
The key is the float-operated valve. Carburetors have had this for a century.
Apr
23
comment Do rotating non-circular projectiles also experience Magnus lift?
Almost any method of inducing circulation produces lift by deflecting the airstream. Simple example: take a strip of paper 1/2" wide by a few inches long. Hold it the long way between your hands and drop it, with backspin. It keeps backspinning and glides quite nicely.
Apr
23
comment A moving brush on a vibrating surface
I suspect where the bristles touch the surface not completely vertical there is a ratchet action.
Apr
22
comment Why is the water in a cup not inclined (opposite the cup) when the plane takes a turn?
Because planes don't turn the way cars do, they turn the way bicycles do.