Recent content by SDEric

Understand. However, you can measure lift on an airplane wing in a wind tunnel, supported by thin rods or wires, and measure the force on those rods or wires. The wing does not deflect in the wind, yet you can measure the force imposed by the lift and drag. So it is definitely not necessary that...

It seems to me that requiring the hill to have the working fluid underneath it to generate lift is unnecessary. I understand that there is no such thing as "negative pressure", only less pressure than somewhere else. We can work around that requirement, however. The question is not whether the...

Understand and appreciate the discussion. The thing is, you don't need the bottom to have higher pressure for the top to have lower pressure. You can demonstrate this with various flat-bottomed airfoils; with the right (mildly convex) curvature of the bottom, you can have net lift of the...

I think I devised a thought experiment that completely isolates the effect being discussed. Say you have a hill that rises up out of a flat plain. The beginning of the hill has radius of curvature R. It smoothly rises up through π/2 radian, from the plain; 1/4 of a circle; then the surface of...

I am afraid I am not smart enough to understand even the beginning of that paper. I don't understand the significance of a coordinate system that rotates and follows the streamlines. But I think I finally understand the pressure difference as a result of curving the airflow. For the airflow...

The hill has an element not present in the airfoil: flat ground surrounding it. The air flowing over the hill is deflected yet again after it passes the hill, so afterward it is again flowing parallel to the ground. The flat ground in front of the hill similarly participates in deflecting the...

Ah! That is fallacy #1. The air flowing over the top of the wing has NOTHING to do with the air flowing under the bottom! Assuming that the two flows are somehow connected leads one to the "equal transit time" fallacy. I am not sure. The simulations, and supposedly some wind tunnel data, say...

Thanks. I think the general question is, for frisbees, hills, and semi-circular cross section airfoils, how does an object, symmetrical front to rear, and oriented parallel to the air flow, generate lift? It just begs for the (incorrect) longer path explanation. The problem I see with this...

According to one UC Davis M.S. thesis, a frisbee generates some lift at a zero angle of attack. See figure 2-3 in http://morleyfielddgc.files.wordpress.com/2009/04/hummelthesis.pdf. I'd love to have an intuitive understanding of this lift. According to Foilsim, there is lift on a...

Thanks for the informative post. (Aside: So an airfoil would not generate lift in liquid helium?!?) I chose a half cylinder for the thought experiment, but really it is a general question: Is lift generated by an airfoil that is (1) symmetrical front to back, and (2) at a zero angle of attack...

Thanks! I can easily see how a slice of a cylinder would create lift if it were at a non zero angle of attack. The angled bottom would deflect air down, and the curved top would turn the upper air stream down. The net effect would absolutely be to push air down. Probably the optimum angle of...

Could one of you physics gurus answer this one. There was an answer to this question posted early in this thread, but I'd like to make sure it reflects the consensus opinion. If you had an airfoil that was half a cylinder, with the flat bottom at a zero angle of attack (that is, parallel to...

Thanks! I think I understand Kirchoff's Law pretty well; it's a necessary result from the conditions of thermal equilibrium. I also understand both (1) and (2). My question is regarding what happens when not at thermal equilibrium; I think you hinted at the answer, and to understand this I need...

Thanks. I can see why emissivity has to be equal to reflectance, from a thermodynamic perspective: If they weren't, you could have an object that is good at absorbing and bad at emitting, and vice versa, so if you put those two objects in a room together, heat would flow from one to the other...

Everything I have read about emissivity simply says that in thermal equilibrium, emitted radiation has to be equal to absorbed radiation. That's a glib explanation. What about an object that is not in thermal equilibrium with its surroundings? What about an object that is in a vacuum that...