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What Makes Airplanes Fly?  
User currently offlineZiggy From United States of America, joined Jun 2001, 178 posts, RR: 0
Posted (13 years 1 month 1 day ago) and read 1356 times:

Since right now their seems to be a lack of interchanging opinions, I thought I would try to start a intriguing subject. This question was broght up in my advanced aerodynamics class.
1. Is it Bernolli's law that effects lift
or
2. Is it Newton's 2nd law (for every action their is a opposite and equal reaction) i.e. The airfoil pushes air downward (downwash) pushing the aircraft into the air.

I would just like to hear some of the other points of view out there.  Smile

13 replies: All unread, jump to last
 
User currently offlineXFSUgimpLB41X From United States of America, joined Aug 2000, 4195 posts, RR: 37
Reply 1, posted (13 years 1 month 23 hours ago) and read 1312 times:

This was discussed quite in detail a few weeks ago. I believe it is a combination of the two at varying ratios depending on airspeed and such.


Chicks dig winglets.
User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 2, posted (13 years 1 month 23 hours ago) and read 1303 times:

Super, challenging question!

Both principles are correct, complementing one another.

Bernouli effect causes higher pressure under foil, lower above -- differential acting on foil area pushes up on airfoil, creating lift.

By Newton's 2nd law, the airfoil pushes down on the air around it with force equal to lift. If you map the air pressure below the foil, you will see a distribution of higher to lower pressure from the foil down to the ground. As you move away from the airfoil, the pressure decreases and spreads out in area. At ground level, there is a slight pressure rise above ambient (ie, ambient being any point far from the airfoil) covering a very large area. Integrating pressure x area over the ground yields the lift generated by the foil! (It's not really that surprising, is it?) That means that the ground is pushing up against the air, ie supporting the air, which is supporting the airfoil.

So good-old Newton is vindicated again.

I seem to remember solving this problem in aero engineering class in 1970 or so. Maybe one day I will try to find those notes.

Pete


User currently offlineDE727UPS From United States of America, joined Nov 2000, 814 posts, RR: 13
Reply 3, posted (13 years 1 month 23 hours ago) and read 1287 times:

You're all wrong....it's MONEY that makes an airplane fly.

User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 4, posted (13 years 1 month 23 hours ago) and read 1286 times:

hehe - you got me!

Money can make anything fly!


User currently offlineIMissPiedmont From United States of America, joined May 2001, 6287 posts, RR: 33
Reply 5, posted (13 years 1 month 22 hours ago) and read 1281 times:

Ever try to make a brick fly?


Damn, this website is getting worse daily.
User currently offlineAaron atp From United States of America, joined Mar 2000, 533 posts, RR: 2
Reply 6, posted (13 years 1 month 21 hours ago) and read 1283 times:

if it flies, floats, or f**ks it's going to be expensive...

>>>I thought I would try to start a intriguing subject <<<

try...it doesn't take much
start...nope
intriguing...yes
subject...try "argument" instead


http://www.airliners.net/discussions/tech_ops/read.main/18810/

everyone has their own opinion, and I promise not to post on this thread again; so if this starts a wildfire, just remember that it's too complex for a simple answer.



aaron




User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 7, posted (13 years 1 month 15 hours ago) and read 1263 times:

I said i my earlier post that Bernouli is the principle that generates lift from airflow around a foil. We definitely agree on that. But regardless of how a force s generated, Newton's 2nd Law ALWAYS holds whenever a force is present. I guess I did not quite get what was meant by the air hitting the foil, but that is irrelevant anyway.

Here's a problem: A man is drivng a covered truck full of birds. The truck weighs 10,000 pounds empty, the birds weigh 5,000 pounds, for a total loaded weight of 15,000 pounds.

He comes to a bridge that has a 10,000 pound weight limit. He scares the crap out of the birds to make them all fly (while still remaining inside) at once, and then attempts to cross the bridge.

Will the truck just weigh 10,000 pounds while the birds are in flight, and be able to cross the bridge safely?


User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 8, posted (13 years 1 month 14 hours ago) and read 1259 times:

Oops, I was having a senior moment above -- I was referring to Newton's 3rd Law (action-reaction) not second law (force-mass-acceleration).

User currently offlineIkarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2
Reply 9, posted (13 years 1 month 14 hours ago) and read 1259 times:

That truck question is incredibly interesting.....

If it is a closed truck, I say it still weighs 15000 pounds. It does not matter whether the birds are supported by the branches in their cages or the air in their cages. No loss of mass occurs, across the border between inside the truck and the outside world (unless it is a cage, and not a fully closed container), and so the mass of the system "truck" itself remains constant. Thr truck will fall into the river....

Regards

Ikarus


User currently offlineWestern727 From United States of America, joined Jan 2007, 746 posts, RR: 4
Reply 10, posted (13 years 1 month 14 hours ago) and read 1256 times:



Awww, here we go again...



Jack @ AUS
User currently offlineXFSUgimpLB41X From United States of America, joined Aug 2000, 4195 posts, RR: 37
Reply 11, posted (13 years 1 month 9 hours ago) and read 1238 times:

Does Denny need to pull out his metaphysics book so we can properly and logically ponder of this?


Chicks dig winglets.
User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 12, posted (13 years 1 month 1 hour ago) and read 1217 times:

Ikarus is right. Read my post near the top of this section.
You can look at it two ways. Ikarus looked at it as a control volume with fixed mass; therefore, the weight of the truck remains the same whether the birds are airborne or not, as long as gravitational field remains unchanged.

Another way is to look at how each bird affects the airflow inside the truck, determine the resulting pressure distribution over the floor of the truck, integrate it over the floor area, and thus obtain the force of the air acting downward on the floor. This will undoubtedly be 5000 lb, which is the weight of the airborne birds.

Now extend this problem to the whole world. Assuming the earth's surface to be the "floor" of the world, does it matter whether planes are parked or flying when considering how much "weight" the "floor" must support?

Pete


User currently offlineAirplay From , joined Dec 1969, posts, RR:
Reply 13, posted (13 years 3 weeks 6 days 10 hours ago) and read 1185 times:

Everyone knows that paperwork makes airplanes fly.
 Smile


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