Physics Puzzle

[Agile]

Its amazing that so many people seemed unable to understand that a planes engines exert thrust and do not rotate the wheels to propell it forward.

The same people would be unable to explain how a seaplane can take off.

But then I suppose there are a large proportion of the population who have had no training at all in simple physics.

I cannot accept not knowing how things work!

Tony

 

[Crystal]

Agile wrote

"Its amazing that so many people seemed unable to understand that a planes engines exert thrust and do not rotate the wheels to propell (sic) it forward."

How have you come to this conclusion?

 

[johnny_t]

Which bit of Agile's answer are you questioning ? The fact that the engines don't propel the wheels or the fact that many people can't understand it ?

If its the first bit, then the wheels turning is a side-effect of the plane moving, because they happen to be in contact with the ground as the plane moves forward, but the point is (to the whole question) that it is not like a car where you apply power to the wheels to drag the whole thing forward, the wheels are just there to effectively reduce the friction barrier to moving forward that you would get from, say, having the plane sat on its belly on the runway.

The plane is propelled forward by the thrust from the engines - nothing else.

 

[empip]

For anyone who has a passing interest, but may like to read more.
A superb website - one for the favourites !

http://www.grc.nasa.gov/WWW/K-12/airplane/short.html


BTW One for the excessive 'wheel spin' ners.
As aircraft accelerates, lift is generated, therefore downforce on the wheels is constantly diminishing until at lift off, upward forces or 'lift' are greater than the forces holding the aircraft on the runway, gravity, drag etc.
I am betting that the aircraft tyres will be rated at around 240 knots, but there will be attributes which ensure that a 200 knot landing would be reasonably safe.... The stationary wheels suddenly smashing into the runway and accelerating in very short order to 200 Knots the severity would, I am sure, outweigh the much slower action of gradual spin up to double speed with attendant relaxation of downforce on bearings and tyres due to lift during takeoff.

 

[Diyisfun]

Hey what a good site I now know how much effort to raise a 737, sure I can find a use for it, or probably bore someone telling them about the site, Ive stored it in my favs.

 

[Crystal]

johnny_t wrote "Which bit of Agile's answer are you questioning ? The fact that the engines don't propel the wheels or the fact that many people can't understand it ?"

Neither jonny_t ! I am asking Agile what leads him to believe that "many" contributors to this thread posess the childlike naivety required to think that an aircraft is propelled by its wheels.

Having pondered further and for what its worth I think that Agile has entirely missed the point of much of the discussion.

It is the wording of the puzzle as originally stated that presents a conundrum.

At an early stage in the discussion ;

chilluk wrote "OK it's a major clue perhaps but the key in the question relates to the fact the conveyor matches the speed of the wheels not the aircraft itself....."

and JohnD "the wheels are not important in the aircraft taking off, unlike a car it does not apply traction through them. the thrust will push the aircraft along until it reaches liftoff airspeed. Ground speed is not important (c.f. aircraft carrier going into the wind)."

These two posts I suggest illustrate two separate interpretations of the wording of the question. In the former the convevor belt matches the circumferential speed of the wheels, the aircraft cannot proceed forward, cannot attain airspeed and therefore cannot take off.

The latter assumes that the conveyor matches the linear speed of the wheels, the aircraft will move forward, will attain airspeed and will take off.

I believe that the debate results from a difference of interpretation of the question and not because "many people" believe that an aircraft is propelled by its wheels.

(I take the view that question intended the conveyor to match the linear speed of the wheels (and therefore the aircraft), as the alternative presents strange outputs with the wheels absorbing all of the thrust generated by the engines or infinite conveyor speed etc.).

 

[Zampa]

So...is it yes or no...will it or wont it?

 

[Pensdown]

It's got to be a big yes....it will fly but it might need new wheel bearings before it can land

 

[empip]

Maybe not ... Landing places huge stress on tyres, wheels and bearings.. etc.
As I have mentioned, on the moving runway as airspeed increases so does lift, as the rotational stress increases, so the weight loading reduces due to lift. Perhaps a reasonable trade off.

Tyres are rated around 240 Knots - if that encompasses landing then spinning x2 with no real weight loading for a short period of time may not be a problem...

 

[dabaldie]

i'm not going to read all the millions of pages.....
but based on the initial statement.... forget the fact the plane starts stationary. if the wheels had any bearing (excuse the pun) the reaction from them is negated by the runway.. therefore you can remove the runway and the wheel from the equation.

ie..remove the wheels completely... ie drop the plane from a great height... start the engine... the plane moves forward.

hey viola..
all the wheels do is stop the plane hitting the ground.. tehy have no bearing on the forward motion of the plane.

apologies if its been said already

 

[Gary0]

I think it depends on how you interpret the question.

What is meant by "the conveyor matches the speed of the wheels" ? I take that to indicate that the belt matches the wheels so that the aircraft remains still, at a single location compared to all else (most importantly compared to the air).

If the aircraft is still there's no lift.

However I don't see this as being possible. The jet will push against the air and so has to move relative to it. No matter how fast you run the conveyor belt it isn't going to keep the aircraft in one place.

It's a nonsense question until the meaning is clarified better than it presently is.

 

[ban-all-sheds]

No - it's not a nonsense question, it's a question with a blindingly obvious answer of "Yes, of course it will take off - how can it possibly do otherwise?"

The engines, be they jet or propeller, work against the air, and as long as there is sufficient airspeed the plane will take off. Its speed relative to the ground is utterly irrelevant - it could be moving backwards or sideways, or even be stationary, but provided it reaches the right airspeed in a forwards direction it will take off!

All those who say otherwise, are also saying, in effect (if they could but realise it) the following:

1) In normal circumstances, a plane will crash as soon as it takes off because its wheels are no longer going round.

2) A person standing on this conveyor belt runway strapped into a hang glider would never leave the ground, no matter how fast the conveyor belt moved forwards, because he would not be moving relative to the runway.

FGS people - it beggars belief - I cannot begin to understand how anybody could think that the plane will not take off.

 

[Pensdown]

Could someone please reach over and turn the air conditioning back on in Bas's office, he seems to be getting a bit hot under the collar

 

[oilman]

No - it's not a nonsense question,..........

Yes it is. Where the hell are you going to get one of the specified conveyor belts?

FGS people - it beggars belief - I cannot begin to understand how anybody could think that the plane will not take off.

Then please write down the equation for the speed of the conveyor belt.

 

[ban-all-sheds]

No - it's not a nonsense question,..........

Yes it is. Where the hell are you going to get one of the specified conveyor belts?

OK - it's not a nonsense concept, or nonsense wording - I think we can ignore the practicalities of how it would actually work, whether the tyres would shred, or the wheel bearings melt.

FGS people - it beggars belief - I cannot begin to understand how anybody could think that the plane will not take off.

Then please write down the equation for the speed of the conveyor belt.

Assuming no wind, about the same as the airspeed of the plane, but I don't care - it is irrelevant.