Are you serious, or are you just trying to get me going?

A plane absolutely does not push against the ground when it is flying. A plane flies because as the wings move through the air, the air above the wing has to move faster than the air under the wings due to the curved shape if the upper part of the wing having a longer length than the bottom part of the wing. The faster moving air on the top side of the wing has lower pressure than slower moving air on the bottom of the wing. This differential pressure is what we call lift. Google Bernouli for more tech, but trust me, the ground has nothing to do with it. The reason you hear a plane has nothing to do with it's lift and everything to do with the engines that just happen to make noise. They even make noise just idling when the wings aren't creating lift.

Have a great day.

Mike

 

Hmmm. Maybe I should. Or maybe I should rely on my aeronautical engineering degree and my 15 years as an airplane engineer (most of them in airplane noise control).

If the air is pushing up on the plane, then the plane must be pushing down on the air, right? So what happens to that downward push?

Consider a boat. When it floats on the water, it pushes down on the water. If you put a boat into a small tank, you can see the water level rise.

So what happens when the water level rises? It means the pressure at the bottom of the tank goes up a bit. In other words, the fluid spreads the weight of the boat out over the entire bottom of the tank.

Are you starting to see the similarity? The weight of the airplane has to be supported by something, and that something is the atmosphere. The atmosphere is a fluid (a compressible one, unlike water). It can't just support the weight of the airplane magically -- it has to transmit it somewhere. Where that weight eventually goes is into an increase in pressure over the surface of the earth. A very, very, very small increase, considering the surface area of the earth.

You have a great day yourself,

-Mike

 

Actually, that's not quite correct either. It's the common explanation people receive, but there are holes in that reasoning. It's really because of the wing diverting air downwards.

 

Yes, that's a more general answer. However, I was deliberately trying to stay out of the ever-popular momentum v. pressure debate, because of course they are mathematically equivilent anyway.

 

I'm glad you'rean aeronoautlical engineer, but you're still not right. Bernouli works at any angle and doesn't require a solid object as a foundation. You can speed up the airflow on one side of anything and that object will want to go in that direction. Since you are an engineer, I assume you did labs in school. Remember making coins on edge tip over by blowing a hairdryer on one side? Or maybe you made a cardboard tube roll towards the moving airstream? Are you going to tell me that the other side of the coin or the toilet paper roll was reacting against the far wall of the classroom? Did the coin suddenly start exerting more force against the wall that caused it to tip over? How could it, there is nothing going on between the calm side of the coin and the wall, only the side of the coin with the moving air is affected. It's just a pressure differential thing, not a reaction against the world. Go try these experiments. Keep the hair dryer on the no heat setting so we don't get Boyle's law involved (which will actually make the coin fall more easily or the tube roll faster anyway if heat is added).

A plane creates lift, a boat floats due to buoyancy, its not the same. Airplanes are not bouyant in air, they fly because they create lift. Boats do not create lift (at leat sitting still, there are hulls that do create lift when moving), they are just less dense for their volume than the water they are sitting in.

 

Yea, but what does the rest of the world believe?


Mistress:
What is it that allows you to hear that noise? Things don't "just happen". It's pressure on your ear drums making that noise. I'm not going to argue how it relates to the wings, just wanted to clarify that things don't just happen .


Wings work by diverting air downard, and that pushes against whatever is underneath the plane in every direction. You don't feel it because it's spread over the entire earth, as already stated.

 

Mike, lift is a force. A force has to be reacted against something, or else it causes an acceleration. So the air is accelerated downwards. Due to viscosity, the downwards velocity of the air is eventually converted to a general increase in static pressure.

And by the way, labs we did at MIT involved wind tunnels and laser sheet flow visualization, not blowing hair dryers on cardboard tubes.

Buoyancy is also "just a pressure differential thing", by the way. Calling something "lift" does not grant it any mystical properties.

 

How come when I see a picture of a wing in an air tunnel I never see the smoke trails going downward towards the ground under the wing?

 

 

Look, this is simple. First off, what you are seeing in the pane picture is the turbulence (wake) from the plane making a hole in the sky, which the air needs to backfill. This is basically a vortex that eventually dissipates. See all the pretty curly cues, Notice they don't go down? If there was a cloud above the plane, you'd see the same thing. Here's a question for you. If you take your airplane wing and turn it upside down and put it on a racecar, does that wing push the air up? If so, what does that air push against, Mars? The moon? Since there really is nothing for it to push against, it must just go into space. We should very quickly ban race cars with wings before we loose our atmoshphere! We also better ban roof exhaust fans, etc. You basically saying that Bernouli's principle is a sham. So, we're all victims of an elaborate ruse where teachers have been making us do really hard math when all they need to do was tell us that wings push air down. Even if some air happens to get deflected downwards, the friction of the air moving against itself would turn the kinetic energy of motion into heat, so still the force would still not reach the ground. You're an aeronautical engineer, right? So, notice how skin temps go up do to friction when a plane is flying? That's kinetic energy being transferred to heat energy. Want even another example? How does a sailboat sail almost directly into the wind? Surely the wind isn't pushing it upwind. It happens because the sail acts as a wing and the low pressure (upwind) side literally pulls the sail through the air. If this didn't happen, all sailboat trips would be one way. There is no reaction on the forest at the end of the lake. It is simply high pressure air trying to get where the low pressure air is because pressures try to equalize all the time. Same thing with a wing. The high pressure air wants to get where the low pressure air is, the wing is caught in the middle, so it goes up. But, the pressure under the wing does not change all that much relative to what it before the plane got there. In fact, since it is the plane that is really moving, and not the air, you could theorize (it's close but not exact) that there is effectively no change in the air below the wing, while the air above the wing is accelerated by the arc of the wing moving it upwards. That's a little simlified, but that's how it works. Look Mike, I will never convince you that you are wrong, and you will always be wrong. Please, before you hurt someone, fold your degree back up and put it back in the Cracker Jack box.

Oh yeah, and buoyancy is NOT a pressure differential thing, it's a density thing.

Can someone please post a new riddle for us to solve? That was much more fun.