Mike Dunlavey
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 Jan 28 comment How do the Americas Cup Yachts sails work? @Carl: Well, they sort of do, actually. Jan 28 comment How do the Americas Cup Yachts sails work? ++ for DWFTTW :) The car can go directly downwind, but the sails (the rotary wings) are going at an angle. In fact, I think it can also go upwind faster than the wind. Jan 28 revised How do the Americas Cup Yachts sails work? added 2 characters in body Jan 28 revised How do the Americas Cup Yachts sails work? added 2 characters in body Jan 28 answered How do the Americas Cup Yachts sails work? Jan 26 comment Aircraft Thrust to weight ratio contradiction with energy conservation You don't need a giant airplane to understand this. Make a paper airplane and throw it. Once you let go, it has zero forward thrust. Even so, it glides and maintains its speed, though it loses altitude gradually. Once an airplane has been accelerated to flying speed, all the engine thrust (minus drag) does is determine the angle of ascent or descent. Jan 26 comment Aircraft Thrust to weight ratio contradiction with energy conservation Lift is proportional to velocity squared. It is also proportional to angle of attack, for small angles. Jan 24 comment Why are four-legged chairs so common? There are 1-legged chairs. Jan 24 comment How much energy would it take to take down a 747 using vibrations? Look up aeroelastic flutter. This is a problem that aircraft engineers take seriously, and they have various methods to minimize it, such as putting weights in the control surfaces. Jan 23 awarded aircraft Jan 22 revised How to sensor Jerk=$d^3{\bf r}/dt^3$, or higher derivatives (4th, 5th, 6th order) when applied in the equation of motion of a ballistic missile? deleted 5 characters in body Jan 22 comment How to sensor Jerk=$d^3{\bf r}/dt^3$, or higher derivatives (4th, 5th, 6th order) when applied in the equation of motion of a ballistic missile? @DukeofSam: It's difficult to take derivatives of physical signals because the noise is amplified. The reason accelerometers are so useful is that you can integrate them, once to get velocity, and twice to get position. They do need to be calibrated to offset constant error, and they do drift, as gyroscopes do, so they occasionally need correction. But there's no good way to take numerical derivatives without heavy smoothing. Jan 22 revised How to sensor Jerk=$d^3{\bf r}/dt^3$, or higher derivatives (4th, 5th, 6th order) when applied in the equation of motion of a ballistic missile? deleted 1 character in body Jan 22 answered How to sensor Jerk=$d^3{\bf r}/dt^3$, or higher derivatives (4th, 5th, 6th order) when applied in the equation of motion of a ballistic missile? Jan 21 comment Why is an Aircraft Runway NOT like a Teaspoon? @Jonathan: Curved runway. I'm impressed. Jan 21 comment Why is an Aircraft Runway NOT like a Teaspoon? @Jonathan: Right. When I fly, the object is not to achieve upward velocity. The object is to reach sufficient airspeed for the wings to support the weight. Aircraft carriers have a problem that often the aircraft does not reach that speed until after leaving the deck, during which time they sink. The ski jump idea gives them a little more surface clearance. Jan 18 comment rocket vertical landing @numbynumb: Automatic control systems were very sophisticated in the 60s. The Apollo Lunar Module was programmed not only to land on the moon at a specific place, but to rendevous in orbit. Here's a good first-hand account. That's not to say it's a slam-dunk, as the SpaceX people are showing. Jan 9 comment What shape is a vortex? I'm not sure you're answering the question, which was what does it look like in a horizontal cross section. A rotating furnace has a surface that is a balance between gravity and centripetal force, given angular velocity independent of radius. What is the similarity between that and a vortex in air or water? Jan 9 comment 2D - What is the next point given a start point, velocity, yaw rate, acceleration, and time change? @StephenT.Robbins: I just realized, it $d\theta/dt$ is constant, that means $dr/dt$ is constant, so $dv/dt$ is constant, and that's your constant acceleration. So yes, the spiral is Archimedean. Jan 8 comment How does an anti-g overall work? @EdYablecki: There's a lot going on inside the cabin when a car collides. What does a head weigh - 12lbs? That's a 500lb force being applied for a fraction of a second. Ouch. And that doesn't count the rest of the body. Without air bags and seat belts, it's a "life changing event".