Skill's weekend teaser

[MrGecko]

We're all thinking that, Skill.

 

[Skill Leverage]

Touché

 

[Skill Leverage]

Mr Charts Ezreddy doesn't dispute the fact that the plane takes off, he makes a (somewhat valid) point that the wording of the question is incorrect. It is, however, completely irrelevant to the problem at hand, it's merely a paradox that is caused by the nature of the opposing motion of the wheels and the belt.

 

[Mr. Charts]

I hear what you are saying, SL, but I suspect ezreddy might return with some interesting points

Indeed, as raysor intimates, there are other factors not mentioned in the question which occured to me as soon as I read the question but didn't mention them as I thought they would appear to be facetious remarks.

So,

Originally Posted by Skill Leverage
Imagine a 747 sitting on a very large conveyor belt. The belt has the same dimensions as a runway at an airport*, and is set up to exactly match the speed of the plane's wheels, moving in the opposite direction. What will happen?

Same rules as before please; no explanation for a little while, no Googling, everyonerich you are banned from participating**.

SL

*To save confusion, the conveyor belt is the same length as the distance required for a 747 to take off from a normal runway; the frictional co-efficient of runway tarmac and the belt are identical, all other variables such as wind and atmospheric conditions can be ignored.
**I'm just kidding (but not really).

Are the planes engines running?

You didn't say the engines were running, SL.
The plane could have been pulled forward by a towing vehicle, for example.
In which case it was somewhat unlikely to take off.Hypothetical questions require carefully defined parameters, especially on t2w where there are many tools in the box which are much sharper than they might at first appear to be.........indeed some might be very qualified to answer.
Anyway, SL, please don't ask the mods to close the thread.
It might be time for the main protagonists to have a little R&R, but hey, I don't think the "discussion" is exhausted.
Richard

 

[Skill Leverage]

I hear what you are saying, SL, but I suspect ezreddy might return with some interesting points

Indeed, as raysor intimates, there are other factors not mentioned in the question which occured to me as soon as I read the question but didn't mention them as I thought they would appear to be facetious remarks.

So,

Originally Posted by Skill Leverage
Imagine a 747 sitting on a very large conveyor belt. The belt has the same dimensions as a runway at an airport*, and is set up to exactly match the speed of the plane's wheels, moving in the opposite direction. What will happen?

Same rules as before please; no explanation for a little while, no Googling, everyonerich you are banned from participating**.

SL

*To save confusion, the conveyor belt is the same length as the distance required for a 747 to take off from a normal runway; the frictional co-efficient of runway tarmac and the belt are identical, all other variables such as wind and atmospheric conditions can be ignored.
**I'm just kidding (but not really).

Are the planes engines running?

You didn't say the engines were running, SL. OK - but asking the question 'does the plane take off normally?' sort of implies that the engines are running.
The plane could have been pulled forward by a towing vehicle, for example. - I stated it wasn't a trick question, everything you needed to know was there.
In which case it was somewhat unlikely to take off.
Hypothetical questions require carefully defined parameters, especially on t2w where there are many tools in the box which are much sharper than they might at first appear to be.........indeed some might be very qualified to answer.
Anyway, SL, please don't ask the mods to close the thread.
It might be time for the main protagonists to have a little R&R, but hey, I don't think the "discussion" is exhausted.
Richard

.

 

[Mr. Charts]

"OK - but asking the question 'does the plane take off normally?' sort of implies that the engines are running".

You didn't ask 'does the plane take off normally'
You said "What will happen?"
Richard

 

[Skill Leverage]

Come on mate it's written in the poll. If you honestly have to ask whether the engines are running from the wording of that, then there's no hope for explaining the solution to you!

 

[Mr. Charts]

OK, Sl, fair point
But I admit I read the "teaser" long before I read the poll

One thing about t2w, SL, is that you will experience extreme frustration and will often feel you are banging your head against a brick wall. That just comes with the territory. Some people are brick walls, but many others are quite precise, analytical and intelligent people.
Even if that might not be immediately obvious
Richard

 

[the blades]

I hear what you are saying, SL, but I suspect ezreddy might return with some interesting points

I bet it's not at the exact same time as TheBramble:whistling

UTB

 

[Skill Leverage]

I bet it's not at exactly the same time as TheBramble:whistling

UTB

lol

 

[TheBramble]

I bet it's not at the exact same time as TheBramble:whistling

UTB

You cheeky bugger. Me and 'im, we're, we're just mates I tell you. Just mates.

 

[Skill Leverage]

Ok - so how about if you use a Harrier?

How fast are its wheels spinning?

 

[TheBramble]

The plane could have been pulled forward by a towing vehicle, for example.

Excellent. Rest and re-arm and you’ve given me the vehicle (pun pun pun) by which to hopefully achieve it this time.

Detach any concept of plane providing its own power. It’s going to be pulled by a tug (that’s the big thing that pulls planes around before they set off under their own steam). But a tug that can move in excess of 180mph while towing a fully laden (fuel & cargo) 747.

Forget friction, brakes, engine on or off, thrust and any sense of reality that might creep into this scenario. We are solving purely for what Skills has written in post #1. (The side issue about ‘opposite’ motion that ezreddy comments upon is valid but if we’re not being pedantic we can agree that Skills meant the conveyor would move at a speed and in a direction so as to keep the wheels, regardless of their speed of rotation, constant with respect to a fixed reference point).

If the tug pulls the 747 in excess of 180 mph it could theoretically generate sufficient lift for take off – it wouldn’t stay up for long of course, but it could take off. But not if the tug is also on this conveyor.

The one fixed constraint as Skills mentions in post #1 is that the speed of the wheels of the plane, HOWEVER THAT MOTION IS IMPARTED TO THEM, will be matched in an opposite direction by the conveyor. Even with the tug pulling the beastie, they, the tug and the plane and constrained by this given and Tony the Tug driver may well have the throttle jammed open, but nobody is going anywhere relative to the fixed reference point. It matters not a jot how this fixed relationship between conveyor and plane wheels could be achieved or that this could never (probably never) be achieved in reality. We are solving for a theoretical case.

 

[Skill Leverage]

No mate, this is wrong. If the plane is tugged quickly enough, it takes off; it is moving relative to the ground, no matter how fast the wheels of the plane/the belt are spinning.

This is the point you still fail to understand.

 

[Skill Leverage]

This is what you cannot understand:

it takes no more effort to move the plane when the belt is spinning at a million miles an hour than is required when the belt is stationary.

 

[Skill Leverage]

providing there is no friction between the wheels and their axels, of course. But we said that already.

 

[Mr. Charts]

"If the plane is tugged quickly enough, it takes off; it is moving relative to the ground"

Air speed and therefore flight would be limited without engine power.......

Hey Tony, I knew you'd be back soon. What's that old saying...you can't keep a bad man down"
I know they are called tugs but I reckoned some smart **** would say, "it's not on water you idiot"

Next week's fun filled teaser,
"If you land a jet liner at 140 knots on a river flowing in a direction opposite to that of the aircraft at 10 knots, and its angle of attack is 13.5 degrees, how many pieces of luggage will be lost by the operator if it's British Airways?"

 

[Skill Leverage]

No it wouldn't Mr Charts; if someone pulled on the plane hard enough they are exerting horizontal force, which is exactly the same as the thrust provided by a jet. If another plane towed this one off the runway with the same force its engines would provide, it would take off in exactly the same way; the plane would essentially be a glider, surely you've seen one of those getting off the ground?

 

[Mr. Charts]

My son has flown gliders and a glider behaves differently to a jet liner. Something to do with aerodynamics, weight, control surfaces and stuff like that. Stalling speeds of jet liners are also somewhat different........

 

[Skill Leverage]

Which is why you would need much more force to get a jet off the ground than a glider. The fact remains, it would still take off.