How does the weight of airplanes affect atmospheric pressure?

[K^2]

Though I suspect the speeds in the vortex greatly exceed the speed of the downwash.

There would have to be regions like that to pull the condensate upwards as evident on the photo, yes.

 

[boneh3ad]

Well, true. You can look at it as downwash slightly greater than upwash, or you can look at it as one big vortex with net downward motion.

Either way, true, the net motion is downward. Though I suspect the speeds in the vortex greatly exceed the speed of the downwash.

For visualization purposes, sure. Really that isn't what goes on though. The wingtip vortices and the downwash are entirely different phenomena that just happen to be generated in the same flow field.

If you could somehow hypothetically remove the wingtip vortices from the picture, you would still get downwash or else your plane will fall from the sky. This is evident in the fact that you still get downwash (stronger actually) when winglets or raked wingtips are used to minimize the wingtip vortices.

A more accurage description is that the vortices are superposed onto the same flow field as the downwash. The vortices will not necessarily be moving down at the same rate as the downwash either, as they leave the wingtips largely level (relative to the wing) and end up bending downward under the influence of the downwash.

Yeah, they're both components of the same thing. But yeah, you can look a them independently. Which is what you were saying originally, though it was not apparent at first.

No they aren't. Wingtip vortices are a result of having a finite wing. Downwash is a necessary byproduct of lift. They are two fundamentally different phenomena generated for different reasons that just happen to coexist as a result of the operation of a finite airplane wing.

Another interesting distinction: downwash will die out after a while after the plane passes. The wingtip vortices persist for quote some time until eventually they are dissipated by the viscosity of air. Still, the can persist for miles behind a plane before this happens.

 

[A.T.]

Wingtip vortices are a result of having a finite wing. Downwash is a necessary byproduct of lift.

Aren't vortices a necessary byproduct of downwash (of a finite extend)? Can you accelerate air downwards locally without generating vortices? If a wing produces no downwash (symmetric airfoil with alpha=0), does it still create significant vortices?

 

[boneh3ad]

Aren't vortices a necessary byproduct of downwash (of a finite extend)? Can you accelerate air downwards locally without generating vortices? If a wing produces no downwash (symmetric airfoil with alpha=0), does it still create significant vortices?

Yes and no. In a practical sense, one will always come with the other. However, let's say you had an infinite, 2-D wing. You can certainly still generate downwash, but there will be no vortices generated. Generally, you can have the downwash without the wingtip vortices (in theory), but you can't have wingtip vortices without downwash since the wing must be generating lift to create the vortices.

They are certainly related phenomena, but not the same phenomena.

 

[sophiecentaur]

I'm not quite clear on this but some air must be displaced downwards, or at least there has to be a downward force on it to balance the lift force. This will produce a large vortex as air from above flows into follow it. By this time, the plane will have passed, of course, so it escapes the consequence. I imagine this could be why helicopters are less efficient when hovering because they get their own downwash (?).

In a global sense, I imagine the overall atmospheric pressure is increased to some extent by the weight of all the planes that are aloft at any time. Though, the degree of this effect would depend on their speeds (as in the case of planing boats).

 

[PYR]

For a NON Bernoulli explanation of lift you can also look at a 10 min video on you tube at:

how do planes fly - a non Bernoulli explanation




It explains how lift is due to the wing moving air downwards (action) and lift is the reaction of air on the wing (3rd law of Newton).

 

[rcgldr]

I imagine this could be why helicopters are less efficient when hovering because they get their own downwash.

When in a hover, the air has enough time to established a strong induced downwash above the rotor, requring the rotor impart even more speed to the air, which consumes more energy. Worse yet for some helicopters is to vertically decend into their own downwash. Aerobatic model helicopters don't have this issue becuase the thrust to weight ratio is 5 or greater.

In a global sense, I imagine the overall atmospheric pressure is increased to some extent by the weight of all the planes that are aloft at any time.

Consider the earth, atmosphere and every object in the atmosphere as a closed system. Assuming that the objects are not accelerating vertically, then the total average force applied to the surface of the Earth equals the weight of the atmosphere and the weight of all the objects supported by the atmosphere.

You can reduce this to a single model flying or hovering within a large box. As long as the center of mass of the system is not accelerating vertically, then the net downwards force on the box equals the weight of the air and the model. The downwards force is exerted via a pressure differential, less at the top, more at the bottom.