Ziggy
Topic Author
Posts: 173
Joined: Thu Jun 21, 2001 11:07 am

### What Makes Airplanes Fly?

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.

XFSUgimpLB41X
Posts: 3961
Joined: Fri Aug 25, 2000 1:18 am

### RE: What Makes Airplanes Fly?

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.

delta-flyer
Posts: 2631
Joined: Mon Jul 30, 2001 9:47 am

### RE: What Makes Airplanes Fly?

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
"In God we trust, everyone else bring data"

DE727UPS
Posts: 810
Joined: Wed Nov 22, 2000 10:55 am

### RE: What Makes Airplanes Fly?

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

delta-flyer
Posts: 2631
Joined: Mon Jul 30, 2001 9:47 am

### RE: What Makes Airplanes Fly?

hehe - you got me!

Money can make anything fly!
"In God we trust, everyone else bring data"

IMissPiedmont
Posts: 6199
Joined: Wed May 23, 2001 12:58 pm

### RE: What Makes Airplanes Fly?

Ever try to make a brick fly?
The day you stop learning is the day you should die.

aaron atp
Posts: 517
Joined: Sun Mar 19, 2000 1:17 pm

### Money

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

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

delta-flyer
Posts: 2631
Joined: Mon Jul 30, 2001 9:47 am

### RE: What Makes Airplanes Fly?

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?
"In God we trust, everyone else bring data"

delta-flyer
Posts: 2631
Joined: Mon Jul 30, 2001 9:47 am

### RE: What Makes Airplanes Fly?

Oops, I was having a senior moment above -- I was referring to Newton's 3rd Law (action-reaction) not second law (force-mass-acceleration).
"In God we trust, everyone else bring data"

Ikarus
Posts: 3398
Joined: Mon Jan 08, 2001 10:18 pm

### RE: What Makes Airplanes Fly?

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

Western727
Posts: 1459
Joined: Wed Jan 03, 2007 12:38 pm

### RE: What Makes Airplanes Fly?

Awww, here we go again...
Jack @ AUS

XFSUgimpLB41X
Posts: 3961
Joined: Fri Aug 25, 2000 1:18 am

### RE: What Makes Airplanes Fly?

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

delta-flyer
Posts: 2631
Joined: Mon Jul 30, 2001 9:47 am

### RE: What Makes Airplanes Fly?

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
"In God we trust, everyone else bring data"

airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

### RE: What Makes Airplanes Fly?

Everyone knows that paperwork makes airplanes fly.

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