Aircraft Physics Question

I don't believe in over-complicating simply to arrive at the wrong conclusion.

 

Here's a video showing a RC helicopter. You can see the downforce equals the weight of the heli. It's interesting that when the "top plate" is applied, the downforce is about 1/2. I'm guessing the other 1/2 is being pulled on the top plate. So still a net downforce equal to the weight of the heli.

[ame=http://www.youtube.com/watch?v=DWDdh1-B4lo]RC Helicopter Hovering In Ground Effect Over Scale - YouTube[/ame]

Not birds, but the same idea.

 

Your guess is correct. The force pulling the top plate toward the rotor plus the force on the scale always equal the weight of the helicopter (And they don't have to each be half, but the total will always equal 100 percent of the weight. So, if the scale showed 60% the top plate would show 40%, etc).

As was mentioned before, there is no free lunch. And the forces acting on the plane as a result of the weight of the birds inside can't be made to disappear.

 

Of course they do. That's like saying a cruise ship doesn't weigh anything because its floating in the ocean.

Yes, it would. The mass of that steel ball bearing can not suddenly vanish. Try this with a small kitchen scale: Fill a glass half way with water. Weigh it. Drop 5 or 6 big ice cubes in it. Weigh it again.

 

Uhmmm...perhaps you do not understand my dry sense of humor.

My adopted cat understands me completely.

 

Sounds good to me.

 

Sounds like incredibly horrible physics to me.

But then, perpetual motion used to sound good to me - when I was 7 years old.

 

then how about just YES!!!

:=)

 

teak360's container analogy: Imagine that the container has no top. When the birds all fly within the container,its weight drops by about 5000 pounds. The air from their wings would exert a relatively tiny downward force.

The helicopter analogy: The scale is measuring the downward force of the air from the rotor. You can do the same thing by blowing on the scale.

In OP's question,the weight drops by about 5000 pounds. Imagine that the birds were all coasting at the same time,not flapping their wings;do you think that they would still add 5000 pounds of weight to the plane?

 

The birds must exert exactly what they weigh otherwise they wouldn't stay aloft. 5000 remains

Yes, the scale is measuring the force of the rotors air. Just like blowing on it. That's how the birds stay aloft, just like blowing on it.

If they birds were coasting, not flapping their wings. Dynamic situation, but still averages to the same weight. As the birds flap their wings to climb high enough to glide, they exert more force, countering the glide portion. Like if you were to jump in an airplane. As you're pushing to jump up, the airplane is pushed down (heavier). When your in the air, it's(lighter). But that ends up averaging the same. In this case it's temporarily lighter while you're in the air. BUT, you're not sustaining flight.

Do we all agree that anything aloft must generate enough lift to counter it's weight?

 

Forget the birds......

Here's a better one.

Would a miniature helium filled balloon weighing say 5 lbs floating in a 747 alter the aircraft weight? What if you popped the balloon?

No change, right?

 

I'm not very good at this stuff but from what is in my old head is acceleration and the measuring thereof. The measurement of weight is an expression of acceleration. If the acceleration has not been arrested , or has not met something to arrest its acceleration , then there is no measurement of its force, or weight. So, I think that if those birds are flying around inside that cargo area, they are still accelerating and have no weight because they have not been arrested and measured. The reactions of their flying are related to them and the fluid in which they are flying and has no effect on the cargo deck that would arrest their acceleration where it could be measured.
I have no idea of what I just said but it sounds High tech, doesn't it?

 

agreed lift needs to equal weight

fyi... a liter of std air weighs 1.29g at STP or .0807 # per cu ft or a 10'x10'10' room/space/container (1000 cu ft ) would have ~80.7# of air in it...

 

you're good...

 

the floating balloon does not add weight, it is neutral, in reality it replaces the volume of air equal to its weight... at the moment it deflates, the plane becomes lighter until the volume of air equalizes at which point the plane becomes heavier by the weight of the balloon...
do the math, I posted the working weight and volume of air

 

So the balloon displaces enough air (5 lbs) in order to float. Makes sense. Where does the displaced air go?

 

But the question remains....Will the plane take-off of the conveyor??

 

as Bob said - remember weight is just the effect of acceleration upon mass. In the balloon scenario, the total mass does not change, therefore the weight does not change.
w= C * (Mass of plane) + (Mass of cargo inside of plane)

 

Newton's Third Law of motion says otherwise.

 

I dont have the math, but not sure it is needed if you think about this a little bit.

Let's make a small change to the situation in the original post. Let's fill the airplane half full of water, and have a bunch of humans on it.

According to your theory, if the humans sank to the floor (made contact with the floor) the plane would weigh more than if the humans were floating on top of the water. That obviously isn't the case.

There is no difference between water and air in this example.

 

I got it!

And the bird flies because forces are exerted on that fluid, and in turn from the fluid to the floor of the aircraft...the net force being 5,000 pounds. Again: no free lunch.

Whoa!!! Black magic again.

 

there are very big differences..the mediums are not the same, their density is very different...making comparison between them is comparing apples and oranges totally different... there is a great difference between the mediums and how they respond with respect to gravity...

 

really ??? what does it say that is in disagreement

 

A key point in all of this is that the airplane we have been discussing is assumed to be a closed container of fixed volume. If the bird takes off in an open cockpit plane, then yes, the plane will weigh less. In a closed airplane cabin (fixed volume container), aircraft weight (mass) remains the same.

It's good for me to be forced to think about this, for mass, gravity, and gas behavior laws all come into interaction.

 

I see it as the weight of the plane will not change, only the resultant lift. If all birds take off from the floor at the same time there will be a downward force due to acceleration on the plane. The pilot might notice an attitude change dependent on CG. When the birds stop accelerating that loss of lift returns. As their wings oscillate through +/- motion lift of the aircraft also oscillates but averages out. Lift is changing not weight. Weight is not mass, F=ma. Anybody agree or already said this in another way?