Mike Dunlavey
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 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 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 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". Jan 8 comment 2D - What is the next point given a start point, velocity, yaw rate, acceleration, and time change? @StephenT.Robbins: I'm tempted to say Achimedean spiral where radius is linear in $\theta$, but I'm not sure. Jan 8 comment How does an anti-g overall work? It makes a difference if the person is sitting or supine. If sitting, the need is to prevent blood flow away from head to lower extremeties. If someone is floating in water, and the person + water is subjected to high-g, they can tolerate more. Jan 8 comment 2D - What is the next point given a start point, velocity, yaw rate, acceleration, and time change? OK, this is like a car. Now the constant rate of turning. Acceleration comes into play. Do you mean the steering wheel is held at a constant angle, so the front wheels are at a constant angle, so there is a fixed center about which the car is turning, in which case the curve is a circular arc? Or do you mean that as the speed increases the steering wheel is turned back toward 0 so as to keep the yaw rate constant? Just trying to clarify the question. Jan 8 comment 2D - What is the next point given a start point, velocity, yaw rate, acceleration, and time change? Is this vehicle like a car with wheels, so it cannot slide sideways, or is it like a rocket, that can go sideways? Jan 5 comment Can the Arcaboard fly? @SamuelWeir: I'm skeptical too, but control does not look to me like an issue. You can just tilt it with your feet, to turn it or let it accelerate horizontally. Jan 4 comment How Vision Works Good place to start. Jan 2 comment Why does moving air have low pressure? Here it's more explicit. The air being deflected downward is not just the air below the wing, but also the air above the wing. In particular, anything that disturbs the flow on top of the wing, like a thin layer of frost, kills lift badly. Big cause of accidents. Also this. Note emphasis on top of the wing. Jan 2 comment Why does moving air have low pressure? Ummm... Last sentence? Try reading this. Dec 31 comment Driving on the moon ++ Yes. But if your runway took the form of a helical loop which then exited upward, you could do it :)