Skill's weekend teaser

[Skill Leverage]

if the conveyor has a speed of 180mph, and the engines are off, the planes speed is...wait for it... zero.

if the plane's engines are on, its speed is exactly the same as it would be on tarmac.

Look Bramble, I've said it twice already; a plane's wheels are merely there to stop the paint being scratched when it takes off. Stop focussing on the wheels, and what speed they match, it's irrelevant!

 

[Skill Leverage]

Don't ya wish you never started this thread and gone to pub instead. Very funny to read though - thanks. I too dumb to work it out so I leave you guys to argue!!

I was starting to, but I think I'm almost there with him... definitely the last of the weekend teasers though!

 

[MrGecko]

But Skill tells us the plane isn't moving forward as the exactly matching (but opposite) movement of the conveyor effectively cancels out any forward movement of the plane.

The conveyor belt cancels out any of the thrust forward the plane generates through the turning the wheels and the friction between the surfaces - none. The thrust forward is generated by the engines pulling the plane through the air; each kN of thrust will pull the plane one "air unit" in displacement forward, and the plane will accelerate forward until there are enough "air units" going over the wings to generate the lift necessary to take off.

 

[Skill Leverage]

Exactimondo. The plane is not generating any forward motion through the wheels, or anything connected to the ground; therefore, under frictionless conditions, the conveyor belt cannot affect its forward motion in any way.

Incidentally in reality the friction forces are negligible; the plane must overcome these before it can begin forward motion, but this is the same on tarmac. Since the belt is stationary when the plane is stationary, the plane will take off exactly as it would from the ground.

 

[MrGecko]

An interesting spin-off from the result is that you'd need a conveyor belt the same length (i.e. roller to roller) as a runway to get it to work. Right?

 

[Skill Leverage]

Yes, exactly! Well, not the length of a runway, but the length of the runway that's required for a plane to take off; I'm pretty sure the guys at Heathrow leave some slack either side just in case.

 

[TheBramble]

if the conveyor has a speed of 180mph, and the engines are off, the planes speed is...wait for it... zero.

if the plane's engines are on, its speed is exactly the same as it would be on tarmac.

Look Bramble, I've said it twice already; a plane's wheels are merely there to stop the paint being scratched when it takes off. Stop focussing on the wheels, and what speed they match, it's irrelevant!

Answer the questions as posed and you might find a glimmer of realisation.

If the conveyor is moving backwards (relative to direction plane is facing) at 180mph and that exactly equals the speed of the wheels on the plane, the plane has a realtive groundspeed (relative to ground under conveyor) of 0mph and an airspeed of 0mph.

If you were standing to the side of that conveyor in line with the plane - it would not be moving relative to your position. By your logic, that shouldn't matter and you'd both take off together I presume?

 

[Skill Leverage]

Haha sorry G-man, I knew exactly what you meant

 

[Skill Leverage]

Yeah bramble you're right...until the engines are turned on... What is it you're not getting?

Wheels dey do not affect de flight of de plane. Wheels spinny spinny, plane no movey. Jets they blow, airspeed increases, plane goes up in sky.

 

[Claudia123]

Even I get it now thanks to some well worded responses. How would one of those Harrier things cope though - or is that for next Sunday ??

 

[MrGecko]

If the conveyor is moving backwards (relative to direction plane is facing) at 180mph and that exactly equals the speed of the wheels on the plane, the plane has a realtive groundspeed (relative to ground under conveyor) of 0mph and an airspeed of 0mph.

Until the engines are switched on, then the plane has forward thrust.

 

[Skill Leverage]

Answer the questions as posed and you might find a glimmer of realisation.

If the conveyor is moving backwards (relative to direction plane is facing) at 180mph and that exactly equals the speed of the wheels on the plane, the plane has a realtive groundspeed (relative to ground under conveyor) of 0mph and an airspeed of 0mph.

If you were standing to the side of that conveyor in line with the plane - it would not be moving relative to your position. By your logic, that shouldn't matter and you'd both take off together I presume?

Until you understand that, due to the nature of a plane's construction and the fundamentals of its flight, it makes no difference whether the wheels are spinning or not, there is no hope for getting you to understand this problem.

 

[Skill Leverage]

Hmm, that's odd, Gecko posted the same answer as me at exactly the same time, seconds after you asked the queston... I wonder how that could be.

 

[TheBramble]

Yeah bramble you're right...until the engines are turned on... What is it you're not getting?

Wheels dey do not affect de flight of de plane. Wheels spinny spinny, plane no movey. Jets they blow, airspeed increases, plane goes up in sky.

OK mate. Next time you're on a 747 ask the Captain to forgoe the normal procedures for takoff and just stick the brakes on hard - wang the engines up to full thrust, wait till airspeed somehow magically gets to 180mph and the release the brakes. Should go up a treat I reckon.

Let me know. I want to be there...

 

[Skill Leverage]

Even I get it now thanks to some well worded responses. How would one of those Harrier things cope though - or is that for next Sunday ??

A Harrier has a 50/50 chance of taking off, I thought you knew that?

 

[new_trader]

Answer the questions as posed and you might find a glimmer of realisation.

If the conveyor is moving backwards (relative to direction plane is facing) at 180mph and that exactly equals the speed of the wheels on the plane, the plane has a realtive groundspeed (relative to ground under conveyor) of 0mph and an airspeed of 0mph.

If you were standing to the side of that conveyor in line with the plane - it would not be moving relative to your position. By your logic, that shouldn't matter and you'd both take off together I presume?

You do not explain how the wheels get to 180mph. I'm figuring that if the wheels are spinning at 180mph (keeping the plane stationary) it means the plane is using a minimum amount of thrust to keep it stationary against the drag of it's wheels. In otherwords, the wheels are spinning due to the conveyor not due to the engines thrust. I could be wrong, but that's the way I figure it.

 

[Skill Leverage]

OK mate. Next time you're on a 747 ask the Captain to forgoe the normal procedures for takoff and just stick the brakes on hard - wang the engines up to full thrust, wait till airspeed somehow magically gets to 180mph and the release the brakes. Should go up a treat I reckon.

Let me know. I want to be there...

hahahahaha....

The brakes on the wheels are there to slow the plane down on landing mate. If he put them on during takeoff, then you are introducing FRICTION, which is another force to overcome.

The plane would still take off though, albeit with some burnt rubber.

 

[TheBramble]

Until you understand that, due to the nature of a plane's construction and the fundamentals of its flight, it makes no difference whether the wheels are spinning or not, there is no hope for getting you to understand this problem.

Great idea. Rip off the wheels. Put a float under the mother.

Put it in a stream that can be made to flow against the forward thrust of the plane so that it exactly cancels out forward movement. No wheels. OK? Logically same teaser. OK with that?

Where does the airspeed come form?

 

[TheBramble]

The plane would still take off though, albeit with some burnt rubber.

Unbelievable.

 

[Skill Leverage]

You do not explain how the wheels get to 180mph. I'm figuring that if the wheels are spinning at 180mph (keeping the plane stationary) it means the plane is using a minimum amount of thrust to keep it stationary against the drag of it's wheels. In otherwords, the wheels are spinning due to the conveyor not due to the engines thrust. I could be wrong, but that's the way I figure it.

EXACTLY. And remember mate we are frictionless in this example, so there is no drag, and hence no thrust needed. But yes, even in real life you would still overcome this drag easily.