Riddle me this batman

[frazil]

opcorn:

 

[civengPE]

From what I have read, MBs constructed a quarter mile conveyor belt and used an ultralight to test it out.

I was extremely disappointed by them not airing this in the airplane episode. :deadhorse:

 

[roadwreck]

From what I have read, MBs constructed a quarter mile conveyor belt and used an ultralight to test it out.
I was extremely disappointed by them not airing this in the airplane episode. :deadhorse:

I wonder how they achieved the "match speed" of the conveyor. That seems to be a difficult thing to do at full scale.

 

[civengPE]

I wonder how they achieved the "match speed" of the conveyor. That seems to be a difficult thing to do at full scale.

I don't know if matching speed is necessary to debunk the myth. If I was going to do it. Here is how I would set it up.

1. Determine the maximum airspeed of the aircraft.

2. Build a conveyor belt capable of achieving this (hardest part)

3. Place the aircraft on the conveyor belt and fasten it so it will not move.

4. turn the conveyor belt on full speed.

5. Apply full power to the engine and release the restraints.

6. If it can overcome this situation (and it will), then the other is, by default, debunked.

what happens if you are in a spaceship traveling near the speed of light and you turn your headlights on?

 

[Flyer_PE]

^Do you need to used max speed for the aircraft or just the rotation speed? Typical GA aircraft will lift off at less than 60kts. If you use a motor-glider or an ultralight, you can get the required treadmill speed down below 30kts.

Jim

 

[IlPadrino]

I'm on board with you, sray - hopefully there's nobody that disagrees with that.
taking it to the next step - I think the question is flawed. There are two ways to look at the "conveyor matches speed of airplane" aspect of the problem statement:

1: conveyor matches speed of the plane: plane gets up to 100, conveyor is going 100 the other way, and wheels are going 200. Plane takes off normally*. Would require a sufficiently long conveyor, though.

2: conveyor matches speed of wheels: obviously in this situation, the plane has no relative forward movement, and could not take off.

OK...

Fact 1: Any flying (not falling!) airplane requires sufficient air velocity over its wings to generate enough upward lift which counteracts the airplanes mass (which given gravity creates a downward force).

Fact 2: An airplane creates thrust independent of its wheels.

Ok... Fact 2 is so easy, let's move on! Any suggestions on how to phrase a conveyor fact? The moral is that the conveyor can impart very little force on the airplane.

 

[rdbse]

I see the conveyor as platform just under the plane. After reviewing the thread, I appears others see an infinite conveyor.

Also, I have excluded the AV-8B Harrier from problem.

 

[Guest]

^^^ I really haven't weighed in on this argument because I will be honest there is one point that I am not clear on.

I agree with your statement - the lift is generated by air flowing over the wing. So my follow-on thought is redirected to the issue of frame of reference, meaning that everyone has fixated on plane's position (and velocity) relative to the conveyor belt but wouldn't one need to think about the plane's position (and velocity) relative to the surrounding air mass?

My confusion arises because I am not sure how to draw the control volume for the air mass to represent the forces (pressures) that arise from the flow. The volume (and area) are not fixed so I am not sure how to represent it.

I hope you don't take this as a hijack or a jump beyond the simple first principles because I think this concept really is the critical linkage from your first point to follow through towards the conclusion. IMHO.

JR

 

[roadwreck]

I see the conveyor as platform just under the plane. After reviewing the thread, I appears others see an infinite conveyor.
Also, I have excluded the AV-8B Harrier from problem.

If the conveyor were a platform just under the plane (and once it moved off the conveyor it would fall?) then no plane could take off other the something with vertical lift capability. But the same could be said for a plane not on a conveyor. If the plane were only given a length of runway equal to the length of the plane itself then that plane could also not take off under it's own power (unless it again has vertical take of capabilities).

 

[cement]

the only way a plane on a conveyor could take off would be with a 100 mph headwind.

 

[frazil]

^ I agree!

 

[IlPadrino]

Fact 1: Any flying (not falling!) airplane requires sufficient air velocity over its wings to generate enough upward lift which counteracts the airplanes mass (which given gravity creates a downward force).

Fact 2: An airplane creates thrust independent of its wheels.

Now let's get to the interesting part...

Fact 3: Given an airplane sitting stationary on any movable surface (e.g. a conveyor belt, a trailer, etc.), a force equal but opposite to the rolling resistance (i.e. rolling friction) of the airplane wheels will keep the airplane stationary despite the moving surface.

We're getting close if no one disagrees with Fact 3.

 

[benbo]

Fact 1: Any flying (not falling!) airplane requires sufficient air velocity over its wings to generate enough upward lift which counteracts the airplanes mass (which given gravity creates a downward force).Fact 2: An airplane creates thrust independent of its wheels.


Now let's get to the interesting part...

Fact 3: Given an airplane sitting stationary on any movable surface (e.g. a conveyor belt, a trailer, etc.), a force equal but opposite to the rolling resistance (i.e. rolling friction) of the airplane wheels will keep the airplane stationary despite the moving surface.

We're getting close if no one disagrees with Fact 3.

I don't know if I disagree or not. I don't really understand what your saying. I mean, obvviously, if we tie the plane in a stationary position it won't move regardless of what the treadmill does. But that's irrelevant to the problem. It is impossible to hold the plane stationary with just the thrust of the jets or prop, and the treadmill, because the treadmill has zero effect on the motion of the plane. I would restate the Fact -

Given enough thrust and a sufficiently long treadmill the plane's wings will move forward through the air regardless of what the treadmill does.

This is the critical part of the problem.

I assume you believe the plane will take off.

 

[roadwreck]

I don't know if I disagree or not. I don't really understand what your saying. I mean, obvviously, if we tie the plane in a stationary position it won't move regardless of what the treadmill does. But that's irrelevant to the problem. It is impossible to hold the plane stationary with just the thrust of the jets or prop, and the treadmill, because the treadmill has zero effect on the motion of the plane. I would restate the Fact -
Given enough thrust and a sufficiently long treadmill the plane's wings will move forward through the air regardless of what the treadmill does.

This is the critical part of the problem.

I assume you believe the plane will take off.

I think his point is that if the wheels are allowed to move freely and the conveyor starts to move but no force is applied to the plane via the engines the wheels should turn, but the plane should remain stationary despite what the conveyor is doing. Newtons first law of motion, a body will stay at rest unless a force acts upon it. Since the wheels rotate freely, no force should be transfered to the body of the plane, the plane shouldn't move and the conveyor will just cause the wheels to role.

 

[benbo]

I think his point is that if the wheels are allowed to move freely and the conveyor starts to move but no force is applied to the plane via the engines the wheels should turn, but the plane should remain stationary despite what the conveyor is doing. Newtons first law of motion, a body will stay at rest unless a force acts upon it. Since the wheels rotate freely, no force should be transfered to the body of the plane, the plane shouldn't move and the conveyor will just cause the wheels to role.

Oh - in that case I agree.

Carry on!

 

[civengPE]

First the obvious-but-wrong answer. The unwary tend to reason by analogy to a car on a conveyor belt--if the conveyor moves backward at the same rate that the car's wheels rotate forward, the net result is that the car remains stationary. An aircraft in the same situation, they figure, would stay planted on the ground, since there'd be no air rushing over the wings to give it lift. But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is make the plane's wheels spin madly.

A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily.

As you point out, one problem here is the wording of the question. Your version straightforwardly states that the conveyor moves backward at the same rate that the plane moves forward. If the plane's forward speed is 100 miles per hour, the conveyor rolls 100 MPH backward, and the wheels rotate at 200 MPH. Assuming you've got Indy-car-quality tires and wheel bearings, no problem. However, some versions put matters this way: "The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation." This language leads to a paradox: If the plane moves forward at 5 MPH, then its wheels will do likewise, and the treadmill will go 5 MPH backward. But if the treadmill is going 5 MPH backward, then the wheels are really turning 10 MPH forward. But if the wheels are going 10 MPH forward . . . Soon the foolish have persuaded themselves that the treadmill must operate at infinite speed. Nonsense. The question thus stated asks the impossible -- simply put, that A = A + 5 -- and so cannot be framed in this way. Everything clear now? Maybe not. But believe this: The plane takes off.

 

[civengPE]

I found this explanation on another website. I hope it clears things up for you non believers!!

:holyness:

 

[civengPE]

 

[civengPE]

FBD

 

[roadwreck]

The plane takes off.

You jumped ahead of us by a few steps, but we were getting there.