Principles of flight?

[トラちゃん1]

Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?
Explain why it can, or cannot, take off?

 

[Sensuikan San]

It will sit there until hell freezes over!
Obviously, without movement of the 'plane there will be no airflow over the wings. Lift cannot, therefore, be generated.
Lift is generated by airflow over an aerodynamic surface like an airfoil (typical wing section). The air above the airfoil has to flow faster than the air below the airfoil, and creates an area of low pressure above the airfoil(wing). Essentially, an airplane is "sucked" into flight by low air pressure above the wing.
With no movement relative to the air, and no wind ... it will sit there 'til hell freezes over!
.... but there's still no guarantee that your baggage won't show up in Kualar Lumpar!
Next?
ﾆ淡ﾆ停?｡ﾆ停?

 

[トラちゃん1]

ジョンさん. I have left this thread for a day or 2 but I see it does not show alot of interest. I'm not sure where this will go but I hope you don't mind me firing a few questions at you?

What is stopping the plane itself from moving forward?

 

[Davey]

weight????

 

[トラちゃん1]

weight????

well, no. The wheels are ultimatly there to reduce friction between the plane and the ground. The engines have no worries moving it as long as there are a set of wheels beneath it.

 

[Sensuikan San]

well, no. The wheels are ultimatly there to reduce friction between the plane and the ground. The engines have no worries moving it as long as there are a set of wheels beneath it.

Well ... yes!

To reduce friction, true, but as long as the plane is sitting on its wheels there is friction! Thus, initially the plane is "tied" to the runway (conveyor belt).

Consequently the engines will have no trouble in moving the plane forward relative to the conveyor belt - but not relative to the surrounding atmosphere. What the plane needs is airspeed! Until it lifts off - it won't have any!

Consider the real analogy, and one that can actually happen: a flying boat or seaplane trying to take off, facing upstream in a fast moving river ......

This is one helluvan interesting question BTW!

ﾆ淡ﾆ停?｡ﾆ停?

 

[トラちゃん1]

Consider the real analogy, and one that can actually happen: a flying boat or seaplane trying to take off, facing upstream in a fast moving river

Yes, but the wheels on the plane have the ability to move faster ( if you bear in mind how speeds are calculated ) than the plane itself. Faster or slower.
Imagine, if instead of a plane, it was a hovercraft sitting on the conveyor belt.
As the hovercraft moved fowards at 20mph, the conveyor moved backwards at 20mph.
Since the hovercraft floats above the ground, it is irrelevant how fast the conveyor is going underneath it....
The propellers push against the AIR, not the ground....

Does the same thing happen to the plane?

 

[Sensuikan San]

Tricky, isn't it!
But IMO .... nope!
Your hovercraft is already airborne. It has no connection with the ground whatsoever. The ground becomes irrelevant.
But as long as the plane is in contact with the ground - notwithstanding the ability of the wheels to revolve - there will be friction (zero friction is an impossibility - if it wasn't, I'd tend to feel differently about this one). The plane will try to move relative to the surface upon which it sits - but that surface will move backwards, trying to carry the plane with it. Relative to the air ... the plane will remain stationary. It cannot reach take-off speed.
(.... I think ....! :erm: )

Who was the cruel bugger who thought up this question, anyway? The swine!

ﾆ淡ﾆ停?｡ﾆ停?

 

[トラちゃん1]

haha, I believe this is a question used in some universities. But the purpose of it is just to know if you have an understanding of how physics works.
And you my friend, would pass. Not so long ago I spent close to a solid hour discussing and debating with a friend of mine about this with no conclusion!

Scenario after scenario just couldn't find a definate answer. In relation to the hovercraft I ask, would there be any difference if a set of wheels were attached to the bottom?

 

[Sensuikan San]

haha, I believe this is a question used in some universities. But the purpose of it is just to know if you have an understanding of how physics works.

That figures! Yes, it is a bit of a teaser isn't it - now I'm not even sure I'm right either! (Hardly slept last night!)

Not so long ago I spent close to a solid hour discussing and debating with a friend of mine about this with no conclusion!
Scenario after scenario just couldn't find a definate answer. In relation to the hovercraft I ask, would there be any difference if a set of wheels were attached to the bottom?

.... but now ... I think I've got it!
If the plane reaches normal take-off speed (relative to the conveyor) - then, as I stated - it cannot take-off. It is stationary relative to the air.
HOWEVER! ..... if it can keep accelerating to exactly TWICE its normal take off speed, then it will be travelling at it's normal take-off speed (relative to the air)!
Then, she could indeed take flight! (The tires would take a hammering, though!)
There! I changed my mind! Egg on face (sort of).
The hovercraft doesn't count, though. Nor would any vehicle with VTOL capabilities. It simply doesn't rely on forward motion to gain lift.
There ... I feel better now!
ﾆ淡ﾆ停?｡ﾆ停?

 

[Sensuikan San]

The thought just crossed my mind ...

Can you imagine landing that baby under these conditions ... putting on the brakes ... throttling back ..... and some passengers still have a cup of coffee in their hands ... !!!!

Oooooh!



ﾆ淡ﾆ停?｡ﾆ停?

 

[Sensuikan San]

Aaaaaaaaaargh!

Damn! Damn! Damn!

Back to my original argument I go!

.... This one is killing me, I swear!

- Of course, the conveyor will just accelerate too!

- We're still on the ground!

- We'll blow the hell out of our engines, shred our tires .... and still be on the ground.

Please resolve this somebody! This is a most cruel and unusual punishment!

ﾆ淡ﾆ停?｡ﾆ停??bawling:

 

[トラちゃん1]

Please resolve this somebody! This is a most cruel and unusual punishment!
ﾆ淡ﾆ停?｡ﾆ停??bawling:

I too, ask this. It is an impossible situation to set-up so I think we'll never know.
I will try and narrow it down a bit.

*The travelator speed and the wheel speed are only dependent on each other.
*The plane speed ( relative to the air around it ) can differ from the wheels.
*For argument's sake say the plane needs to be doing 200kmph to create lift. ( relative to the air around it )

Looking at Newton's law ''every action, has an equal and opposite reaction'' meaning, you thrust air one way, the plane should ultimatly move the other way.

I have thought of a similar situation; A roller blader set up on the same travelator, holding a rope suspended to a non-moving object just off the runway. The roller blader can pull himself along the rope ( represting thrust ), and the speed of the wheels or the travelator would have no effect on him pulling himself along. ( although it would make a tough task ).

I think this would work the same way with the plane. Although I'm sure alot of people would dis-agree.

ﾆ淡ﾆ停?｡ﾆ停?懌?堋ｳ窶堙ｱ what do you think?

 

[Reiku]

Does this really belong in the humor section?

Anyway, it depends on the type of plane.

Unlike a car, where the friction between the wheels and the road is what enables the car to move, a plane is driven by it's engines--the wheels merely serve to isolate the body of the craft from the surface.

You must remember that when landing, a plane cuts back on it's engines, and in the case of some jets uses thrust reversers. The wheels are not the sole braking mechanism, and locked wheels alone will simply not prevent an aircraft at full throttle from moving.

Whether the plane is actually able to become airborne would depend on many things. Offhand, and ignoring any outside factors such as wind, rain, or the presence of water on the belt, I can think of the following scenarios:

If the friction between the wheels and the belt is sufficient to prevent the wheels from moving, and the wheels and belt are considered to be indestructible, the plane might be forced to pivot on the wheel wells, and crash nose-first into the belt--depending on the number and placement of the wheels.

If the tires explode due to the strain, that moment of being airborne could be enough to allow flight--depending on how powerful the engines were, the falling rate of the plane, and the distance between the wheels and the ground after the tires blow.

Lastly:

If the conveyor belt surface gives before the wheels do, it's posible that the wheels will simply scrape over/through it until either takeoff speed is achieved or the engines burn out or run out of fuel--the "standing nosedive" is also possible in this situation.