Bouyant

 

I don't know much about sailing, but I'm pretty certain sailboats never sail 'almost directly into the wind'. They usually tack 30-45

 

I don't think anyone has the whole picture here. I'm not sure I do either, but here are a few of my thoughts:

1) Lift generated by a wing is a combination of both the air diversion and longer-distance explanations people have mentioned above. You can take an airfoil shape with a perfectly flat bottom, blow air over it, and generate lift. No air is being diverted downward. You can generate MORE lift by giving it some angle of attack, so that air is both being accelerated over the top and diverted downwards. You continue to get more lift as AoA increases, thus diverting more air down, until the flow over the top separates and the wing stalls. I have not seen a rigorous mathematical description of this all put together, and I'm not smart enough to combine Bernoulli with flat plate theory (is that what it was called?) all on my own.

2) I think mistressmotorsports is correct on the wing-pushing-on-the-earth thing. In an incompressible fluid, local pressure differences are in fact felt everywhere. It's just like the hydraulic effect. Apply pressure to the top of a water tank, and every point on the walls feels that pressure. In a compressible fluid though, this is not the case. The potential energy generated by a pressure gradient is dissipated by friction/viscosity effects in the fluid and changes of state/temperature.

3) There is no real difference between a "pressure differential thing" and a "density thing" here. Pressure and density are inextricably related in a gas...remember Pressure = density * gas constant * temperature.

 

[QUOTE=mistressmotorsports,Dec 8 2005, 07:10 AM]Look, this is simple.

 

Many are using airplanes as an example, whereas a helicopter (which I fly) demonstates a much more apparent example.

When an airplane flies over your head, you will feel a downwash affect. Ever see that video of Allan Decadene doing a documentary about the Spitfire, and one comes from behind and scares the crap out of him? Watch the grass below the plane.

Now a helicopter.....uses the same principle of lift as aicraft do. Rotor blades are essentially shaped the same way. You can get HUGE down wash effects from helicopters. The key is wing loading. On an airplane, you have a large, wide lifting surfaces that lift the aircraft into the air. On a helicopter, the blades are narrow, and not necessarily as long.

So if you have to aircraft, one plane and one helo, that weight the same, the rotor blades will have a much higher wing loading than the plane will, because the skinny rotors have to work that much harder, thus producing downwash. Someone will argue that the rotors only produce the downforce when you are hoving or flying around ie: when there is pitch in the blades, or angle of attack. Wrong, even at FLAT PITCH, there is a small amount of downwash, which is very apparent in a snowy day.

As far as supersonic rotor blades go...yes, at times, the blade TIPS will break the sound barrier - that's the WOP WOP WOP you hear sometimes (which is also caused by a blade hitting the turburlence of the blade ahead at slower aircraft speeds)

But its just the tip....the lifting portion of the blade doesn't not go supersonic. This is called advancing blade compressiblity stall, where the advancing blade approaches supersonic speeds and stalls out, causing the helicopter to pitch and roll. This is one of the main factors that limits a helicopters overall top speed.

 

1) Actually, even in your "flat bottom" case, you will find that if you are getting lift, you are also imparting a downward velocity into the air. The one goes with the other. If you had seen the "rigorous mathematical description of this all put together" you would have seen this.

2) Compressible simply means that the density changes along with the pressure. It doesn't mean that the force represented by the pressure doesn't have to be reacted against by something.

3) We were talking about boats in that case. In water, density is pretty much unrelated to pressure. But you are correct that in a gas density is related to pressure. Both are also related to velocity, which is what Bernouli is actually about.

 

No, he's right about a sail being a wing. Mostly. A spinnaker acts more like a parachute, but most sails create thrust by acting as wings.

 

Not majority of its thrust, but some from lift. This is why windsurfers can sail faster than the wind that's blowing them

 

OK, I'll buy that.

 

Yes, it is simple:

http://www.aa.washington.edu/faculty/eberhardt/lift.htm