A 500 pound fly (this is not a HW question)

[JaredJames]

I'm not sure if this is correct, but consider this:

Suppose a 500 lb man jumped straight up inside the plane. The plane is moving at 500 mph. The man lands exactly where he jumped from (the plane doesn't move out from under him). This is because the air inside the plane is part of the mass of the plane, as is the man (whether he is standing, or in the air). If the man were NOT part of the mass of the plane while he is not on the floor of the plane, then the plane would have moved out from under him.

When the man jumps, he is traveling at the same speed as the aircraft, so is the internal air. Thus, there is no resistance to slow him so he continues at 500 mph with the aircraft. Landing in the same place.

 

[zgozvrm]

When the man jumps, he is traveling at the same speed as the aircraft, so is the internal air. Thus, there is no resistance to slow him so he continues at 500 mph with the aircraft. Landing in the same place.

Right. There seems to be a lot of confusion as to whether the fly (or any object) is part of the mass of the plane if it isn't in direct contact with the plane.

My point was that the man is part of the mass of the plane when he is standing (or seated), which most people can agree. But, I submit that he is also part of the mass of the plane when he jumps. This is because the air inside the plane is part of the mass of the plane, so when he jumps, he becomes part of the mass of the air, which in turn is part of the mass of the plane.

Correct?

 

[DaveC426913]

Right. There seems to be a lot of confusion as to whether the fly (or any object) is part of the mass of the plane if it isn't in direct contact with the plane.

My point was that the man is part of the mass of the plane when he is standing (or seated), which most people can agree. But, I submit that he is also part of the mass of the plane when he jumps. This is because the air inside the plane is part of the mass of the plane, so when he jumps, he becomes part of the mass of the air, which in turn is part of the mass of the plane.

Correct?

He is part of the mass of the plane, and the internal mass of the plane is moving around. For the entire CoM to remain level, the external shell must move.

Let's remove one more confusing element: gravity and lift. Let's have the 500lb. weight go side-to-side.

If you rolled a 500lb ball from left wingtip to right wingtip, the CoM of the entire plane (including ball) would stay in a straight course, meaning the internal bits of the plane (500lb ball) would go right 49ft while the external bits of the plane (the metal shell) would go left 1ft.

OK so far?

Same with up and down. CoM stays level. Move the internal bits of the plane (500lb. weight) down 6ft, and the external bits (metal shell) go upward a few inches, leaving the CoM in level flight.

 

[zgozvrm]

If you rolled a 500lb ball from left wingtip to right wingtip, the CoM of the entire plane (including ball) would stay in a straight course, meaning the internal bits of the plane (500lb ball) would go right 49ft while the external bits of the plane (the metal shell) would go left 1ft.

OK so far?

Not really.

The way I see it, as long as something on the plane is touching the plane, it becomes part of the mass of the plane.

Imagine that there are passenger cabins hanging underneath each wing tip and there were some way for passengers to walk freely between the 2 cabins. Suppose the plane started out with equal weight (say 2000 lbs) in each cabin and began flying. Then, mid-flight, all the passengers migrated to the right hand passenger cabin, the pilot would have to adjust his ailerons to compensate for this weight shift, so the CoM must have shifted.

The same goes for a small fishing boat. If 2 fishermen stand on opposite sides of the boat, the CoM stays in the center of the boat and it stays level. However, if the 2 men stand on the same side of the boat, the CoM shifts toward that side of the boat and it begins to tip over.

 

[JaredJames]

Not really.

The way I see it, as long as something on the plane is touching the plane, it becomes part of the mass of the plane.

Unless you are in zero g or under free fall conditions, you are part of the mass of the aircraft. Unless the conditions are as above, you are applying a force to the aircraft to remain in that position, whether it is by hanging on a string or sitting on the floor or hovering with wings.

 

[DaveC426913]

Not really.

The way I see it, as long as something on the plane is touching the plane, it becomes part of the mass of the plane.

Imagine that there are passenger cabins hanging underneath each wing tip and there were some way for passengers to walk freely between the 2 cabins. Suppose the plane started out with equal weight (say 2000 lbs) in each cabin and began flying. Then, mid-flight, all the passengers migrated to the right hand passenger cabin, the pilot would have to adjust his ailerons to compensate for this weight shift, so the CoM must have shifted.

The same goes for a small fishing boat. If 2 fishermen stand on opposite sides of the boat, the CoM stays in the center of the boat and it stays level. However, if the 2 men stand on the same side of the boat, the CoM shifts toward that side of the boat and it begins to tip over.

This is a unrelated issue. You are talking about whether the plane or boat becomes lopsided.

If you stand on a skateboard, and shift your mass side-to-side, the skateboard is going to move in the opposite direction to maintain CoM. That's all we're talking about for now.

 

[zgozvrm]

This is a unrelated issue. You are talking about whether the plane or boat becomes lopsided.

If you stand on a skateboard, and shift your mass side-to-side, the skateboard is going to move in the opposite direction to maintain CoM. That's all we're talking about for now.

So... if you have 2 planes that are identical in every way except that the outermost engine under the left wing of one plane is 1000 pounds heavier than all the other engines, you're saying that the CoM for both planes is at the same location on both planes?

 

[DaveC426913]

So... if you have 2 planes that are identical in every way except that the outermost engine under the left wing of one plane is 1000 pounds heavier than all the other engines, you're saying that the CoM for both planes is at the same location on both planes?

No. I'm not saying that at all. I'm saying that you're complicating the issue at-hand.

If you read over the thread, you will see that we are trying to assess why a 500 pound fly deciding to stop flying and land will affect the CoM of the plane.

 

[rcgldr]

Say that the bug or bird or whatever stops flying and drops to the floor. During that brief free-fall, would the apparent weight of the system change?

It depends if there is a corresponding reaction by the aircraft to the change in the action by the flying object inside the aircraft. Over time, if the center of mass of the system is accelerating upwards or downwards, then the apparent weight of the system will change (lower if acceleration is downwards, higher if acceleration is upwards).

During transitions, there's a finite time for changes in the impulse of the affected air at the flying object to reach the bottom interior surface of the aircraft. As just mentioned, over time these transitions will average out so that the apparent weight of the system has a constant average weight as long as it's a closed system.

The other issue as mentioned above, is that an aircraft generates lift depending on speed and angle of attack, and not the mass or weight of the aircraft and its contents. Any transitions from flying objects inside will result in an acceleration of the aircraft since the lift will not correspond to the "apparent" weight during transitions.

To eliminate the lift aspect, instead of an aircraft, imagine you have a large box resting on a scale. The system is a box that weighs 500 netwons, and it contains air that weighs 490 Newtons, and some object inside that weighs 10 Newtons. As long as there is no vertical component of accelration, the total weight of the system is 1000 Newtons. It doesn't matter if the 10 Newton object is resting on the bottom surface, or if the object is airborne within the box as long as there is no net vertical acceleration of either object or the affected air over time. If the object flies via a series of impulses, then the observed weight will cycle, but the average weight will remain 1000 Newtons. If the 10 Newton object is a helium filled model balloon hovering inside the box, the total weight still remains at 1000 Newtons. When the object is being supported by the air inside the box, it increases the pressure differential versus height within the box so that the net downforce of the air inside the box is the sum of the weight of the air and any object supported by the air inside the box.

 

[zgozvrm]

No. I'm not saying that at all. I'm saying that you're complicating the issue at-hand.

Apparently, this is a complicated issue.

Reading your comments, it sounds as if you're saying that a shift of passengers on a plane doesn't affect the CoM; that the CoM is maintained. What I'm saying is that the people on the plane are part of the mass of the plane, just like the engines are. So, if they move to one side of the plane, the CoM shifts in that direction.

If you read over the thread, you will see that we are trying to assess why a 500 pound fly deciding to stop flying and land will affect the CoM of the plane.

If you read the original question, you'll see that we are trying to determine whether or not a flying (hovering) 500 pound fly is contributing to the weight of the plane (as opposed to a 500 pound fly that has landed on the interior of a plane, which obviously does). Also, the OP want's to know if there is any effect on the plane due to the hovering fly.

 

[JaredJames]

Apparently, this is a complicated issue.

It really isn't. As long as you understand the mechanics of the situation.

Reading your comments, it sounds as if you're saying that a shift of passengers on a plane doesn't affect the CoM; that the CoM is maintained. What I'm saying is that the people on the plane are part of the mass of the plane, just like the engines are. So, if they move to one side of the plane, the CoM shifts in that direction.

I don't think I can explain this better than DaveC already has.

If the CoM is to remain stationary as the ball moves (as per daves example) then the body of the aircraft must move in the opposite direction to the ball.

If you read the original question, you'll see that we are trying to determine whether or not a flying (hovering) 500 pound fly is contributing to the weight of the plane (as opposed to a 500 pound fly that has landed on the interior of a plane, which obviously does). Also, the OP want's to know if there is any effect on the plane due to the hovering fly.

We've answered this and I'd say hammered it to death.

 

[zgozvrm]

It really isn't. As long as you understand the mechanics of the situation.

Thanks for stating the obvious ... Nothing is complicated if you understand it!
Judging by the posts in this thread (mine included), not everyone completely understands the "mechanics of the situation."

We've answered this and I'd say hammered it to death.

So, since you understand the problem, those of us that have questions are supposed to stop asking?
I thought part of the purpose this forum was to help others understand. I guess not.

 

[DaveC426913]

Apparently, this is a complicated issue.

Yes.

Which is why we're trying to extract it bit by bit and examine the pieces ideally.

The original scenario concerns a mass involves the plane moving up and down - something that will not cause it to go lopsided (i.e. lopsidesness is not part of the scenario being examined!) Because up and down is closely tied to lift and weight, I removed the extraneous components (by showing the same thing happens side-to-side). If you then throw in the idea of lopsidedness, we're not getting anywhere.

Fine. Forget the plane. A skateboard will slew in response to you shifting your weight, because you and the skateboard are part of the same system. The you-skateboard CoM remains in the same place. Lopsideness is not an issue.

Likewise, if the fly slews its weight up/down or side-to-side, the plane will slew in the opposite direction so that the entire CoM of the plane/fly system does not move.

Reading your comments, it sounds as if you're saying that a shift of passengers on a plane doesn't affect the CoM; that the CoM is maintained.

That is correct. The CoM of the entire plane-passenger system is maintained. If an internal component shifts left, then the rest of the system will shift right. If the passengers all shift left, the plane's shell will shift right. CoM of plane-passenger is maintained.

What I'm saying is that the people on the plane are part of the mass of the plane, just like the engines are. So, if they move to one side of the plane, the CoM shifts in that direction.

CoM does not shift from a straight line unless you apply an external force. No internal reshuffling of the plane's masses will cause the CoM to shift.
(emphasis because it is the central principle of the entire thread, not because I am yelling at you)

The plane is flying, not in contact with any surface. If you move a 500lb. or 3 passengers or an entire engine from one side the of plane to the other, the CoM of the entire plane-passenger-fly-engine will continue in a straight line. For this to happen, the shell of the plane will have to shift left, just like the (frictionless) skateboard does.

 

[zgozvrm]

So ... I get it that you are saying that the if weight on the plane shifts one direction, the plane shifts the other, thus maintaining CoM.
But, does this only refer to the time during which the weight is shifting? I would think once the weight has stopped moving, the CoM would have moved, as stated in my question (post #37) about the 2 "nearly identical" planes, to which you seemed to agree.

 

[JaredJames]

So, since you understand the problem, those of us that have questions are supposed to stop asking?
I thought part of the purpose this forum was to help others understand. I guess not.

It's not extra questions I'm talking about, it's the same question asked over and over with a few words changed.

 

[rcgldr]

The CoM of the entire plane-passenger system is maintained. ... CoM does not shift from a straight line unless you apply an external force.

That's the issue with a plane, there is an external aerodynamic force, which generates lift depending on speed and angle of attack, and will generate an oppossing side force to any side movement of the aircraft (assuming rudder trimmed for straight ahead flight).

forget the plane ... skateboard

Assuming frictionless wheels, this eliminates external forces in the direction of the skate board.

A better example, might be a large box on a bunch of massless and frictionless marbles that provide no resistive forces to any horizontal movement. Then the large box and marbles could be sitting on a large scale, that displays the amount of vertical force between the box and the scale. This would eliminate any CoM changes in the horizontal direction.

If the box were in outer space free of all external forces, then the CoM would be fixed and moving at some constnat velocity (or zero depending on frame of reference).

I think this going beyond the point of the OP, where the flying object inside the system exerts a downwards force on the air inside the closed system, which in turn transmits that downwards force onto the interior surface of the system.

 

[DaveC426913]

So ... I get it that you are saying that the if weight on the plane shifts one direction, the plane shifts the other, thus maintaining CoM.
But, does this only refer to the time during which the weight is shifting? I would think once the weight has stopped moving, the CoM would have moved, as stated in my question (post #37) about the 2 "nearly identical" planes, to which you seemed to agree.

Yes. Once the internal movement stops, so does the external movement.

That's the issue with a plane, there is an external aerodynamic force, which generates lift depending on speed and angle of attack, and will generate an oppossing side force to any side movement of the aircraft (assuming rudder trimmed for straight ahead flight).

Yes but how the pilot/plane compensates is a separate issue. The OP simply wants ot know if the fly falling makes the plane weigh less. The answer is yes.

Assuming frictionless wheels, this eliminates external forces in the direction of the skate board.

A better example, might be a large box on a bunch of massless and frictionless marbles that provide no resistive forces to any horizontal movement. Then the large box and marbles could be sitting on a large scale, that displays the amount of vertical force between the box and the scale. This would eliminate any CoM changes in the horizontal direction.

If the box were in outer space free of all external forces, then the CoM would be fixed and moving at some constnat velocity (or zero depending on frame of reference).

But if it were free from all external forces - including gravity - then the fly can't hover and the fly can't land - so it's a completely different scenario.

I think this going beyond the point of the OP, where the flying object inside the system exerts a downwards force on the air inside the closed system, which in turn transmits that downwards force onto the interior surface of the system.

That's the first part of the equestion. The second part is whether the fly stopping flying (i.e. landing) changes anything. The answer is yes (fly drops, plane rises, CoM stays level), it's just a matter of having everyone see that.

 

[rcgldr]

I think this going beyond the point of the OP, where the flying object inside the system exerts a downwards force on the air inside the closed system, which in turn transmits that downwards force onto the interior surface of the system.

That's the first part of the equestion. The second part is whether the fly stopping flying (i.e. landing) changes anything.

The OP (who never posted again in this thread), only asked the first question. Danger asked about the fly stopping in post #24.

Yes but how the pilot/plane compensates is a separate issue. The OP simply wants to know if the fly falling makes the plane weigh less.

The downforce related to gravity is less if the fly is falling, since the center of mass of the system would be accelerating downwards if not compensated for (if the air craft remained in level flight). As you pointed out previously, if the generated aerodynamic downforce remained constant, the center of mass of the system would not move vertically, with the plane accelerating upwards at a fraction of the flys rate of acceleration downwards. The side issue is once the plane starts climbing, the pilot would need to compensate so that the vertical component of forces between aircraft and air remain constant.

 

[DaveC426913]

Agreed.


Quick! Lock the thread!