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Feb
5
comment 2 airplanes same size and shap different mass
Not steeper, just faster. Both lift and drag are proportional to velocity squared, so their ratio is roughly constant, L/D. The tangent of the descent angle is just D/L. So if Weight is greater, Lift must be greater, so Velocity increases as the square root of Lift, and Drag increases linearly with Lift. So - same angle. For a small plane, the L/D ratio is about 9. For a jet airliner it is more like 25. For a sailplane, 30.
Feb
4
comment Adding cinnamon to my coffee makes it stop whirling much faster: Why is thats?
@HolgerFiedler; I'm with you. I suspect the coffee below is still spinning.
Feb
4
comment What is the difference between vortexing and centrifuging?
+1 The difference is in the angular velocity as a function of radius. In a vortex the angular velocity decreases as $1/r$, while in a centrifuge it is constant, so in a centrifuge the centripetal acceleration $\omega^2r$ increases with radius, so things more rapidly float/sink to their preferred depth.
Feb
2
comment Why is an airplane propeller so different from a boat propeller in shape?
I'm not sure I believe that other answer. This is just a comment because I don't really know. I can only point out that they are both rotary wings, and the lift/drag ratio of a wing is better if it has higher aspect ratio - long and narrow. Another factor is cavitation - water propellers cannot have too large of a pressure difference, to minimize steam bubbles, so they might have to make up for it with area. But I don't really know.
Feb
2
comment If the Earth is curved, why don't planes need to adjust attitude to stay parallel to the ground?
@Dargscisyhp: So you should know what I mean, having had your hands on the controls yourself. Aircraft power is not on/off, if it more/less. Aircraft are always gliding. Whether they glide down or glide up is just a matter of whether the thrust is less than the drag or more. If you want the plane to maintain constant height above the earth, that's one power setting. If you want it to follow a "straight line" and ascend, that takes a higher power setting. If you want to descend to the airport, you decrease the power, to idle if necessary.
Feb
2
comment If the Earth is curved, why don't planes need to adjust attitude to stay parallel to the ground?
@DavidHammen: Yeah, sure. Good planes, properly loaded, damp that out. (I'm talking about common aircraft. Aerobatics and fighters are a different story.)
Jan
28
comment How do the Americas Cup Yachts sails work?
@Carl: Glider speed record.
Jan
28
comment How do the Americas Cup Yachts sails work?
@Carl: The key is that 'other fluid'. Aircraft are built to resist sideways motion through the air, so you might call that a 'keel'. But as you say, they can't extract power from the wind. I take that back - if they are flying in an area of wind shear, they can. Some radio-controlled gliders have reached crazy speeds by doing vertical loops on the lee side of a hill.
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
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.