How good of a rational thinker are you?

How good of a rational thinker are you?


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Roman

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You guys are going nuts about this... :?

Didn't read through all of it, only the first page and then I skimmed through.

flyingfridge said:
The speed of the wheel rotation doesn't matter shit to a plane. Airspeed is the answer. The plane will still accelerate because the ground has no bearing on airspeed.
The wheels could be spinning up to (infinite) speed and the engine's thrust would still propel the plane forward as soon as enough power was made to create motion.

Airspeed is the key to getting a plane airborne.

So yes it will move forward on the conveyor belt no matter the speed of it and it will take off. :)
 

oliB

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Of course the answer is yes!
OMG, that was soo easy! I can't believe you just had a 3-page-long discussion about it!

You don't have to study engineering (which I do BTW) to know this. I prolly could've answered that question correctly when I was 15 or 16 years old.

cya
Oliver
 

andyhui01

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monkeymax said:
Ah - it's just hit me what you're all saying. :oops: Right so the wheels can be spinning to maintain say 200m/s going back to my example with the plane moving at it's own speed of 100m/s and the belt moving at -100m/s...
I see what you're all saying and I suppose it is possible to create a system such that the aircraft would indeed take off in this case. Yup, okay I see.
would someone care to explain this?... I've studied abit of Thermodynamics... and I never understood the frictionless situation bit as well.
So the wheel is wheelspinning its way at 200m/s, but the real velocity of the plane is still 100m/s and therefore the conveyor belt would move backwards at -100m/s?
 

BerserkerCatSplat

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andyhui01 said:
would someone care to explain this?... I've studied abit of Thermodynamics... and I never understood the frictionless situation bit as well.
So the wheel is wheelspinning its way at 200m/s, but the real velocity of the plane is still 100m/s and therefore the conveyor belt would move backwards at -100m/s?
Yep that's pretty much it. Even if there is some friction in the wheels, the sheer power of the 747 will overcome it. The force of the jet engines will propel the plane forwards, regardless of coveryor belt speed. Thus, the wheels will be forced to spin at twice the notmal rate, but the plane will still take off as if it were on a normal runway.
 

andyhui01

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can you please explain why this won't happen in say... a fantasy 2000hp car that has the same power as the jet propelled engine but powers the wheel... won't it just wheelspin as well?
 

flyingfridge

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Andy, by changing the drive to the wheels, you are opening a whole new kettle. If the wheels were driven, then yes, the car would stay at one point. But a jet is not a car.
 

BerserkerCatSplat

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andyhui01 said:
can you please explain why this won't happen in say... a fantasy 2000hp car that has the same power as the jet propelled engine but powers the wheel... won't it just wheelspin as well?
The car is different because it is driven by the wheels. The car puts force on the conveyor, the conveyor moves backwards, the car's velocity is negated.

The airplane, however, is not driven by the wheels, it is driven by the jet engines pushing on the air behind it. The only function of the wheels is to keep the airplane's belly from dragging on the ground. Thus, the conveyor cannot stop the plane from moving, as it does not affect the plane's propulsion system in any way. If the plane floated on a cushion of air like a hovercraft, the conveyor would move the air a bit, but it could not stop the plane from moving. The wheels work on the same principle, but with a little more friction than the air cusion. They just keep the plane's bottom off the ground.
 

chaos386

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You fooled me at first (I didn't think the plane would take off either), but now that we've solved the ideal case, I've thought of some questions:

Can the wheels on the 747 withstand spinning at an equivalent of 200m/s?

How much friction do the wheels apply on their bearings? Sure, if we're talking about taxiing at 10km/hr the power lost through the wheels is negligible, but is that also true at 720km/hr?

What sort of turbulence does the conveyor belt create in the air near the ground? Is enough to create a headwind strong enough to negate any friction caused by the wheels?

What would be the effects of different windspeeds? Crosswinds?

Why doesn't the pilot just pick a different runway? :p

Is is possible for him to turn and get off the runway if the conveyor can move in both the x and y directions?
 

bihus

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^^ It's just a mental exercise...

No matter the speed of the wheels, it doesn't stop the plane moving forward...
 

Sony

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great topic ! im embarassed i voted no lol, its so logical :) . And stop complaining people about bearings and the weather. it is as said a mental exercise
 

Roman

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chaos386 said:
How much friction do the wheels apply on their bearings? Sure, if we're talking about taxiing at 10km/hr the power lost through the wheels is negligible, but is that also true at 720km/hr?
The 747 is built. tough. Believe me !!

Look at this video and pay attention to the outside left landing gear supporting the full weight of the plane before all other landing gears touch down.

Pretty amazing.

The video is a cross wind landing at the old Kai-Tak airport in Hong Kong.
 

flamingice

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Great question! Had me thinking for a while about it... It's just because in most other similar situations, eg a car, the object would not move relative to the ground... very clever, I like it. :D
 

Necx0

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What a great question. I experienced all thoughts but quickly came to the right conclusion. Pure luck, I have no engineering pedigree, for some reason the answer just popped into my head!!
 

SMN

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Wouldn't take off because there is no air floating around the wings at takeoff speed.
 

Necx0

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Actually I have reason to doubt this now. While I know some people will now go "But wheels are unrelated to airspeed etc. etc." consider this!!!

The plane will only take off if it gains enough velocity to create enough airspeed over the wings to create lift. HOWEVER for the plane to move forward the wheels have to move forward faster than they are moving backward, an impossibility considering the conveyer belt matches the wheel speed. While there is no drive to the wheels, for the plane to accelerate forward the wheels have to move forward as they are what connects the plane to the ground.

Maybe I am barking up the wrong tree, but can someone answer me this. How can the plane accelerate to take off velocity if the wheelspeed (the connection of the plane to the ground) is equal to zero thanks to the conveyer. Ok yes there is the thrust from the engines but that can only accelerate the plane while it is still on the ground if the wheels are moving forward.
 
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