OmCheeto said:
Really? A plane can fly in a purely turbulent state? Perhaps I don't know what turbulence is then.
Yes, it absolutely can and they often do. Based on this statement I suspect you don't actually understand turbulence. Many people think about the airline pilot coming on over the intercom and talking about buckling your seat belts because they are heading into a turbulent patch of air. That is a very narrow scope of what turbulence is, though. Really, it is just chaotic motion of air characterized be increasingly small eddies in a flow with some energy dissipation mechanism. A simple experiment to show laminar and turbulent flow is to light a match and blow it out and watch the smoke rise. The smoke is initially smooth as it first comes off the match head and then starts to twist and turn and mix itself around until you can't see it anymore. That is an example of the transition to turbulent flow. You can see a similar application when you just barely crack your faucet on and see the smooth stream of water coming out and then open it some more and watch it turn "bubbly".
It is usually discussed in the context of a fluid boundary layer. In the case of fastening your seat belts, it is usually often the large turbulent structures associated with the atmospheric boundary layer that cause the plane to buffet. These large eddies are much larger in scale than the wings of the plane though, so the plane will fly just fine through them as long as the pilot is competent.
When talking about flow over the wing, however, far more relevant to its function is the turbulence in the wing boundary layer, which on most modern planes, is turbulent nearly 100% of the time.
sophiecentaur said:
Exactly. I wish people would see that this is very relevant and stop trying to adopt one stance or another.
There's a great example of the relevance of this when you compare a displacement boat with a planing boat of the same weight. They are both being supported by the water but the planing boat produces a much smaller wash because the amount of water it needs to displace is spread out over many boat lengths.
sophiecentaur said:
I have a feeling that the quantity at issue with flight and lift is more Momentum than KE. That would tie in with the idea of efficiency and deflecting as much air as possible and at a slow velocity. On the runway, the plane needs no energy to maintain its lift because the tarmac goes nowhere.
sophiecentaur said:
I wouldn't disagree and it's not an either/or. But the lift is surely due to momentum change and the support from the runway is not due to any friction. Better wheel bearings and tyres are the
equivalent of an efficient wing with a good lift / drag ratio. The more lift that can be generated without turbulence (just air deflection), the more efficient the wing.
So let's talk about energy here for a moment. Think about the force on an airfoil in level flight and the implications for energy. An airfoil moving through the air is going to have two major forces on it: lift and drag. Drag retards the motion, so there is certainly work being done on the airfoil in that case since it is the drag force applied over some distance in the same direction as the drag force. However, lift is orthogonal to the direction of motion, so there is no work being done by the lift. In other words, you can get a complete picture of lift in level flight without considering the energy. It is only when trying to get accurate drag results that energy becomes a factor, and that is the reason why in the early days of fluid mechanics drag was so much more difficult to predict theoretically, and even is still today.
Of course, you can't have lift without drag, so there is still some role of energy there, but you can predict lift without considering it. Momentum is the more important quantity for determining these things since really change in momentum constitutes a force, which is our end goal in the first place.
russ_watters said:
Agreed. Most of what they say is "wrong" is at worst incomplete, a simplification or not completely applicable. Which of course means that by saying it is wrong,
they are oversimplifying! One of the more annoying is their criticism of the applicability of the Venturi tube concept. A wing is
basically an inside-out Venturi tube. They criticize the analogy essentially for being an analogy (paraphrase): a wing isn't a Venturi tube because Venturi tubes aren't inside out.
ORLY? Then why is NASA developing an inside-out rocket engine?:
http://en.wikipedia.org/wiki/Aerospike_engine
http://www.nasa.gov/centers/marshall/news/background/facts/aerospike.html
I disagree completely. If one part of a theory is wrong, no matter how right other parts are, then the theory is wrong. The NASA sites do a good job of taking those wrong theories and explaining which parts are correct and then discussing how to make the theory fully correct on the two pages about how to actually explain lift.
And no, an airfoil is not an inverted Venturi tube. Venturi tubes rely on the area change through the duct to accelerate the flow due to conservation of mass. This is
not how an airfoil works. In a Venturi tube, there are solid walls which constrict the flow, requiring more velocity in order to pass all the mass coming into the section of the duct where the area is lower. This is not true of an airfoil, which is an open flow that can easily deflect around the airfoil out to infinity if necessary. There is nothing constraining the flow to see it as a smaller area.
With an aerospike engine, you have a couple fundamental differences. First and foremost, you have the fact that this is a compressible flow where the exhaust is coming out of the nozzle supersonically and with a vastly different pressure from the ambient air, so you have what is called a slip line between the exhaust jet and the ambient air. Those work a lot better as a virtual centerline than do random streamlines out at infinity. Even then, this is simplified because the slip line will not be straight and the Venturi effect does not actually hold for supersonic flows anyway, as the relationship between area change and flow rate changes completely.
Second, trying to explain an aerospike engine on a page aimed at the general public in its full physical glory would be fruitless for all but those with a background in compressible gas dynamics. As such, those landing pages are certainly watered down a bit and describe the engines in a way that makes it easy to visualize but leaves out the finer details.
sophiecentaur said:
Is the air under the boat supported on skyhooks? It surely rests on the water. Of course the pressure may be low but you can hardly argue it's not there.
We seem to have the big and little endians at work in this thread. "My theory, right or wrong".
sophiecentaur said:
Of course not,but what has that got to do with the fact that the supporting force is spread over a bigger area when planing? With a purely displacement boat, it's Archimedes at work and no hydro / aerodynamics. When it's planing, just because there's some fluid flow involved doesn't mean there is not enough force acting on the water to keep the boat from sinking.
I can't imagine another topic that would have people implying that reactionless forces actually exist.
Describing planing in terms of spreading the force out is a bit fishy in my view. Hydrodynamically speaking, it has to do with the boat moving much faster so the momentum it is changing in deflecting the water downward is going to be much greater per contact area of the boat than at a low speed, so that will tend to lift the boat up until it comes to an equilibrium where less area of the boat is in contact with the water.
True, the air pressure absolutely would play some role to hold the boat up while planing. It would almost certainly be orders of magnitude less than the role the water plays, however. The main effect it would have is if the nose comes up too much, allowing the effect of the air to grow enough to generate appreciable lift, and then you see those crashes.
