Which ones (if any) of these sentences are true?

[Vormulac]

1) Into a 50mph head wind
2) With a 50mph tail wind.

The difference in take off AIRspeed in these two examples is 0mph.
[Pete

This isn't true; assuming your take-off speed is 200mph (yes, airspeed - or the speed of the air flowing over the wings creating lift), the wings don't care if the airflow creating the lift is being delivered by a head wind or by the forward motion of the aircraft, a 50mph headwind means the aircraft will only need to generate 150mph of forward speed in order to achieve the 200mph take off speed required. A tailwind on the other hand makes take off significantly more difficult to achieve as it has the opposite effect, additional forward speed is needed by the aircraft to overcome it.

http://www.auf.asn.au/groundschool/umodule11.html

"Take-off into wind!

...
The ground (rolling) speed for take-off is lower. The airspeed during the ground roll equals the ground speed plus/minus the headwind/tailwind component. Thus, if the aircraft is rolling at 30 knots into a 10 knot headwind, the airspeed = 30+10 = 40 knots. If rolling at 30 knots with a 10 knot following wind, the airspeed = 30 –10 = 20 knots."

Anyway, back to our teaser; assuming a perfect frictionless treadmill, as the engines push the aircraft forward, the treadmill runs freely beneath it denying the aircraft any forward motion, so unless there is airflow over the wings being delivered by another source other than the forward motion of the aircraft itself, no lift is generated.

Or so it seems to me

 

[RogerS]

Mark

Are you winding us up?

Step One....

Imagine a long length of trackwork high up...bit like a gantry crane. With rails on. There is a trolley on these rails and this trolley can roll freely forward or backwards. Now dangle a plane beneath this trolley on a cable so that the plane is hanging in the air.

Now fire up the jet engines. What's going to happen? The plane is going to move forward under the thrust of its' engines. If you had a long enough set of rails then eventually the speed of the plane will generate enough lift from the airflow going over its' wings and it will fly.

Do you agree?

 

[Sportique]

Wrong - your "forward motion" above is ground speed, you are overlooking the fact that aircraft fly by virtue of their AIR speed not gound speed. It is accepted that, in this case, because the "ground" is moving with the aircraft then there will not be any ground speed, but the thrust of the engines will always generate AIR speed (regardless of how quickly or slowly the wheels are/not turning!) - unless the aircraft is tethered and this one is not tethered.

Dave

 

[Racers]

Hi,

Come on chaps the plane propeller/jet pushes aganst the AIR not the ground.

Pete

 

[PeterBassett]

Its better than 1:2. The odds if your change your mind are 2:3. The odds if you stick with your door are 1:3.

No. The odds are evens on the change of mind. Still better than 1:3 though.

These are the odds of winning the car if you Always choose to stick with your original door 33.3% (1:3) No disagreement here i think.

However these are the odds of winning the car if you Always choose to switch doors : 66.6% (2:3)

Note that these odds are of winning the car, not of anything else.

It's not intuitive at all and that is what makes it so good. Fields medal winners have been caught out by this. I was too until I wrote the above simulation.

 

[PeterBassett]

This isn't true; assuming your take-off speed is 200mph (yes, airspeed - or the speed of the air flowing over the wings creating lift), the wings don't care if the airflow creating the lift is being delivered by a head wind or by the forward motion of the aircraft, a 50mph headwind means the aircraft will only need to generate 150mph of forward speed in order to achieve the 200mph take off speed required. A tailwind on the other hand makes take off significantly more difficult to achieve as it has the opposite effect, additional forward speed is needed by the aircraft to overcome it.

Correct. Did you actually read my post? This is exactly what it says. I even capitalised AIRspeed

 

[bugbear]

Er - surely common sense does provide the answer?
Clearly the first choice has odds against being the car; is more likely to be goat. The second choice (after removing a goat) has even odds, equal likelihood and so is the better bet.
S'obvious!

http://en.wikipedia.org/wiki/Monty_Hall ... _not_1.2F2

 

[Vormulac]

Wrong - your "forward motion" above is ground speed, you are overlooking the fact that aircraft fly by virtue of their AIR speed not gound speed. It is accepted that, in this case, because the "ground" is moving with the aircraft then there will not be any ground speed, but the thrust of the engines will always generate AIR speed (regardless of how quickly or slowly the wheels are/not turning!) - unless the aircraft is tethered and this one is not tethered.

Dave

Forgive my rather light-hearted example here, but surely then F-14s launching from aircraft carriers would spontaneously leap straight up in the air the moment their engines are pushed to maximum because the engines are 'generating airspeed'... hmm... I think they are actually given a huge dose of forward movement by a steam catapult. Engines produce thrust which drives the aircraft forward, this forward motion creates airspeed and lift, the engines do not of themselves produce lift.

 

[Vormulac]

This isn't true; assuming your take-off speed is 200mph (yes, airspeed - or the speed of the air flowing over the wings creating lift), the wings don't care if the airflow creating the lift is being delivered by a head wind or by the forward motion of the aircraft, a 50mph headwind means the aircraft will only need to generate 150mph of forward speed in order to achieve the 200mph take off speed required. A tailwind on the other hand makes take off significantly more difficult to achieve as it has the opposite effect, additional forward speed is needed by the aircraft to overcome it.

Correct. Did you actually read my post? This is exactly what it says. I even capitalised AIRspeed

Sorry, you're absolutely right, you were talking about the speed however it is achieved, I read it rather quickly and got the wrong end of the stick. Carry on!

 

[Jacob]

Er - surely common sense does provide the answer?
Clearly the first choice has odds against being the car; is more likely to be goat. The second choice (after removing a goat) has even odds, equal likelihood and so is the better bet.
S'obvious!

http://en.wikipedia.org/wiki/Monty_Hall ... _not_1.2F2

Right. I was beginning to wonder how the odds could be lower for one (1st choice) but evens for the other (2nd choice) as common sense also says they should be higher, if the other is lower!

 

[bugbear]

Er - surely common sense does provide the answer?
Clearly the first choice has odds against being the car; is more likely to be goat. The second choice (after removing a goat) has even odds, equal likelihood and so is the better bet.
S'obvious!

http://en.wikipedia.org/wiki/Monty_Hall ... _not_1.2F2

Right. I was beginning to wonder how the odds could be lower for one (1st choice) but evens for the other (2nd choice) as common sense also says they should be higher, if the other is lower!

There are gambling games where the house relies for its profit on the actual odds being a good deal worse for the punter than the "common sense" odds.

These games are, or were, normally offered on street corners with lookouts...

Another probability example, where the maths is fairly easy to follow, is the Birthday Paradox

http://en.wikipedia.org/wiki/Birthday_problem

It's not really a paradox, BTW, but it is surprising.

BugBear

 

[big soft moose]

...where the maths is fairly easy to follow,

hmmm so thats fairly easy to follow ...........................so long as you are a mathematical genius

 

[Anonymous]

my answer back from a physics professor, unfortunately not professor hawking.

my question:

if a plane is sitting on a rather large conveyor belt that is ready to take off, but as thrust is applied the conveyor counteracts the motion of the planes wheels, regardless of thrust imposed, will the plane take off.

Answer

if the conveyor ensues that the plane essentially stays still (which it does) there will be no airflow over the wings, its the flow of the air over the wings that gives it lift. No flow, No lift, No take off.

I am no longer going to argue this point regardless of the experiment that was conducted as it was inconclusive due to the (un)scientific conditions imposed.

 

[bugbear]

...where the maths is fairly easy to follow,

hmmm so thats fairly easy to follow ...........................so long as you are a mathematical genius

You're too kind ;-)

I googled around, and this was the best expressed explanation I found:

It turns out to be easier to compute the probability that no two people at the party have the same birthday, and then subtract the answer from 1 to obtain the probability that two people will share a birthday. For simplicity, let's ignore leap years. Thus, there are 365 possible birthdays to consider.

Imagine the people entering the room one-by-one. When the second person enters the room, there are 364 possible days for her to have a birthday that differs from the first person. So the probability that she will have a different birthday from the first person is 364/365. When the third person enters, there are 363 possibilities of him having a birthday different from both of the first two, so the probability that all three will have different birthdays is 364/365 x 363/365. When the fourth person enters, the probability of all four having different birthdays is 364/365 x 363/365 x 362/365. Continuing in this way, when 23 people are in the room, the probability of all of them having different birthdays is

364/365 x 363/365 x 362/365 x . . . x 343/365.

This works out to be 0.492. (It is when you have 23 people that the above product first drops below 0.5.) Thus, the probability that at least two of the 23 have the same birthday is 1 - 0.492 = 0.508, better than even.

BugBear

 

[Anonymous]

...where the maths is fairly easy to follow,

hmmm so thats fairly easy to follow ...........................so long as you are a mathematical genius

You're too kind ;-)

I googled around, and this was the best expressed explanation I found:

It turns out to be easier to compute the probability that no two people at the party have the same birthday, and then subtract the answer from 1 to obtain the probability that two people will share a birthday. For simplicity, let's ignore leap years. Thus, there are 365 possible birthdays to consider.

Imagine the people entering the room one-by-one. When the second person enters the room, there are 364 possible days for her to have a birthday that differs from the first person. So the probability that she will have a different birthday from the first person is 364/365. When the third person enters, there are 363 possibilities of him having a birthday different from both of the first two, so the probability that all three will have different birthdays is 364/365 x 363/365. When the fourth person enters, the probability of all four having different birthdays is 364/365 x 363/365 x 362/365. Continuing in this way, when 23 people are in the room, the probability of all of them having different birthdays is

364/365 x 363/365 x 362/365 x . . . x 343/365.

This works out to be 0.492. (It is when you have 23 people that the above product first drops below 0.5.) Thus, the probability that at least two of the 23 have the same birthday is 1 - 0.492 = 0.508, better than even.

BugBear

The above maths is very easy to follow if you know what values p and n are.

 

[PeterBassett]

if the conveyor ensues that the plane essentially stays still (which it does).

How? The conveyor *can't* stop the plane from moving.

 

[RogerS]

Mark

Are you winding us up?

Step One....

Imagine a long length of trackwork high up...bit like a gantry crane. With rails on. There is a trolley on these rails and this trolley can roll freely forward or backwards. Now dangle a plane beneath this trolley on a cable so that the plane is hanging in the air.

Now fire up the jet engines. What's going to happen? The plane is going to move forward under the thrust of its' engines. If you had a long enough set of rails then eventually the speed of the plane will generate enough lift from the airflow going over its' wings and it will fly.

Do you agree?

Mark...do you agree with the above?

 

[Racers]

Hi, Mark

Plane wheels aren't driven so they are free to turn, the plane pushes AIR backwards and since every action has an equal and opposite reaction the plane moves forward, and takes off.


Pete

 

[Dibs-h]

Wow - this plane thing isn't half generating some typage! :lol:

IIRC - the shape of an aerofoil is such that with airflow over it, a pressure differential is created between the upper surface and the lower surface. The lower pressure area above and the higher pressure below - resulting in a net upwards force. At an airspeed above X the lift exceeds the craft's weight and obviously drag and the craft is airborne. No air over the wing - no lift.

Now looking at the treadmill - if it is perfect, i.e. zero friction, any thrust provided by a jet engine, would entirely counteracted by the belt like treadmill - so where is the forward motion and air over the wings?

For a prop type plane - there would be some airflow over the wings, generating some lift, but it would be hard to say whether this would have any significant affect, i.e. whether you need an infinite long treadmill for the air over the wing & lift to get the plane airborne. But zero friction is one thing, infinite treadmill & infinite time to launch are probably equatable to never.

Don't mean to teach anyone to suck eggs - just coming at it from a basic principles thing.

The plane on a gantry\wires things isn't analagous.

Dibs

edit - found this very interesting, http://www.airplaneonatreadmill.com/

 

[Anonymous]

nooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo.

i can't take it anymore!!!!

i don't know what it is you don't get?

I have had my answer clarified by a physics major, a professor someone who does this kind of thing for a living.

and you are still arguing the point.

the conveyor counteracts thrust through motion in the wheels of the plane. the point you are missing here is the wheels are the major effect.

if the wheels were braked the plane would take off as the plane would skid across the conveyor in order to take off.

But the wheels are free.

If you are running on a treadmill you don't go anywhere do you if the speed is set correctly.

the propeller or engine of the plane transfers its energy into the wheels (whilst it is in contact with the ground) to get it moving forward, but if the conveyor matches this energy newton for newton the wheels of the plane just spin in the opposite direction equally as fast so the plane cannot go anywhere!!!

I don't understand what is so hard to understand about this?

I am not arguing for the sake of it.

How much more clarification do we need? I have response from a physics major, I know i'll get in touch with NASA maybe they will shed light on it?