Riddle me this batman

[FLBuff PE]

:deadhorse:

 

[IlPadrino]

Here we go again.

This would be easy to resolve if we just did a breakdown of the problem... The only ground rule is that you have to address *ONLY* the question being asked or reference already agreed upon answers. If anyone's game, I'll start:

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).

Anyone disagree or want to qualify the fact to make it more precise? But keep it simple.

 

[Hill William]

Read the whole thread. Don't make Sapper bust out his Free Body Diagram.

 

[IlPadrino]

Read the whole thread. Don't make Sapper bust out his Free Body Diagram.

I did read the whole thread... this ain't rocket science. But in my experience, when a problem is broken up into small enough pieces, it's easy enough to see where opinions diverge. Then it's a simple matter of debating a small piece.

Or not...

 

[roadwreck]

Read the whole thread. Don't make Sapper bust out his Free Body Diagram.

I want to see the Free Body Diagram!!

 

[rdbse]

The problem statement needs to address the amount of force generated by the jet engines, or simply what type of plane is this? I am sure there would be different results from military jets vs. cargo planes vs. small prop planes. In theory a light plane with large jet engines could achieve flight, but I have serious doubts about a fully loaded passenger plane staying up for more than five seconds. These planes need much more air velocity than the jets alone can generate, they need wind from the plane actually moving at a high rate of speed.

Again, Myth Busters will air there show in January.

 

[benbo]

This would be easy to resolve if we just did a breakdown of the problem... The only ground rule is that you have to address *ONLY* the question being asked or reference already agreed upon answers. If anyone's game, I'll start:
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).

Anyone disagree or want to qualify the fact to make it more precise? But keep it simple.

I already know the answer. I was wrong at first, but they convinced me.

But I'm curious where you're going with this. I'll agree just to see the next step. Sorry folks.

 

[Dleg]

It doesn't matter how much thrust it has, or even if it's a jet/prop engine. All that matters is that the engine is acting against the air, not the conveyor belt. There is no drive shaft to the wheels, they are free spinning. Therefore, the conveyor could be travelling backwards at 500 miles per hour, and the plane would still move forward and take off at it's normal takeoff velocity. A small amount of additional thrust to overcome the friction forces on the free-spinning wheels would be all that would be required.

Duh!

 

[rdbse]

Since I never got in on the "original" debate. An airplane achieves lift due to the wind velocity over the wing.

See link

http://www.allstar.fiu.edu/AERO/airflylvl3.htm

 

[DVINNY]

oh man. :brickwall: :brickwall: :brickwall:

OK, as much as I can't believe that I'm about to get into this again, I just can't help it.

The result will be the same with a jet plane, prop plane, as Dleg said. The wheels would never offer enough friction to slow the plane down enough for it to not take off. They are free spinning wheels for crying out loud.

Will a plane in the air still fly with the landing gear down? YES.

Then same plane will take off with the landing gear down.

conveyor belt means nothing in the equation. But most people think of how a car thrusts through the wheels, and from the wheels to the ground, but planes use thrust against the air, not the ground.

 

[rdbse]

I did not say anything about the wheels or friction. I agree the plane will move forward by the jets affect on the air, but not fast enough (nor far enough) to take flight. The only possibility I see is a military jet that can take off from an aircraft carrier. The others will crash off the end of the conveyor. And, this goes back to the type of plane.

 

[benbo]

I did not say anything about the wheels or friction. I agree the plane will move forward by the jets affect on the air, but not fast enough (nor far enough) to take flight. The only possibility I see is a military jet that can take off from an aircraft carrier. The others will crash off the end of the conveyor. And, this goes back to the type of plane.

You are the first person Ive seen agree the plane's wings move forward through the air, but that it will not take off.

I'm curious what you mean that the plane will crash off the end of the conveyor and will not travel far enough forward.

I have two questions -

How far forward does it have to travel, and how long do you think the conveyor belt is supposed to be? Because I don't see any length given for the conveyor. I just assumed it was inifinite in length. And if it does crash off, what exactly is there after the conveyor ends? I can't see the picture you have in your head, and where it comes from.

Let's put some numbers in and let's pick some ridiculous ones - lets say it takes one mile traveling forward for the plane to take off. THen lets say it's on a 50 mile treadmill. Now will it take off?

 

[roadwreck]

^^

it would seem that rdbse sees the conveyor as having a finite length. A length that is not long enough for a plane to take off. reasonable assumption I suppose. I mean it would be quite an achievement to make a conveyor long enough for a real plane to take off on. Not to mention one that "exactly matches the speed of the plane". And even if you could construct such a conveyor, it really would beg the question, why would you want to try and make a plane take off from a conveyor?

 

[Dleg]

WOW. I just now, for the first time ever, read this entire thread (it pre-dated me by a few months). Now I see where civingPE's 500 posts came from.

Good to see that it all worked out, and that MA and benbo saw the light!

:laugh:

BTW, a jet engine does kind of act like a rocket engine - it does not pull air in and push it back out to create its thrust, like a propeller inside a tube or something. Nor does it draw air across the wing surfaces like some sort of giant vacuum machine or fan. The air drawn into the front of the engine is simply the oxygen needed to support the combustion of fuel, compressed by the fans to allow the burning of even more fuel, which creates thrust by expelling the expanding exhaust gases out the rear. Just like a rocket. And yes, I know that modern jet engines also harvest some of that exhaust gas power to turn larger "fans" in the front to also provide some thrust through the propeller-in-the-tube method. (around the outside of the primary jet combustion chamber, though).

 

[IlPadrino]

This would be easy to resolve if we just did a breakdown of the problem... The only ground rule is that you have to address *ONLY* the question being asked or reference already agreed upon answers. If anyone's game, I'll start:
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).

Anyone disagree or want to qualify the fact to make it more precise? But keep it simple.

You guys are fucking this up... it only works well if we stick to small facts taken one at a time. Does anyone disagree with Fact 1?

 

[Hill William]

Would a plane take off of a sheet of ice???

Same concept.

 

[ODB_PE]

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.

so which camp are you in?

IMHO, situation #1 is what will happen, but situation #2 is what the problem implies will happen. Since the plane provides thrust to the air, however, #2 is impossible-until, of course, you consider friction. Imagine the plane just sitting there with the conveyor going 100 the other way. In a frictionless world, the plane would remain at rest with the wheels spinning away but in reality the plane would need a little bit of thrust to keep it stationary relative to the ground.

* Normal is not quite accurate, because a little extra thrust would be required to offset the added rolling resistance

I think of the problem this way: imagine walking through an airport at 5mph pulling a bag with wheels. You walk adjacent to a conveyor going 5mph the other way, and put your bag on the conveyor while you continue to walk on the stationary ground at 5mph. You and the bag continue to move forward at your walking speed, although you will have to use a little more effort to pull the bag due to the added rolling resistance.

 

[Capt Worley PE]

oh man. :brickwall: :brickwall: :brickwall:
OK, as much as I can't believe that I'm about to get into this again, I just can't help it.

The result will be the same with a jet plane, prop plane, as Dleg said. The wheels would never offer enough friction to slow the plane down enough for it to not take off. They are free spinning wheels for crying out loud.

<snip>

But most people think of how a car thrusts through the wheels, and from the wheels to the ground, but planes use thrust against the air, not the ground.

Oh! I see it now! I feel like a real dope!

:brickwall: :brickwall: :brickwall: :brickwall:

 

[civengPE]

Now I see where civingPE's 500 posts came from.

What are you trying to say Dleg? ldman:

 

[benbo]

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.

so which camp are you in?

I'm in camp one.

Since this is what the problem says -

"This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

I can't really see how you can be in any other camp. It says the conveyor speed is the same as plane speed but opposite. It doesn't say anything about wheel speed. To me, plane speed means plane speed not plane wheel speed. I think like most hypotheticial problems we just assume no friciton.

And since the motion of the plane and the motion of the treadmill add to give the speed of the wheels, I don't see how the speed of the wheels could equal the speed of the treadmill anyway unless the planes engine is turned off and it is held in place with the treadmill spinning underneath. That is a litte far fetched interpretaiton of hte problem for me. But of course, in that case it won't fly.

And I say this as someone who had a complete mental block about this problem - I actually had to roll a little toy airplane on a treadmill to convince myself. I am anxious to see how they frame the question on Mythbusters.