piku wrote:If this whole mental exercise is confusing you, just replace "aeroplane" with "man holding onto hang-glider, running on treadmill". Quite obviously he wouldn't take off. People running on treadmills feel no wind on their face because they aren't moving through the air. No airflow, no lift. No lift, no flight.
The running on treadmill analogy doesn't work since a runner pushes himself forward through his legs which are in contact with the treadmill itself therefore the motion of the treadmill has a direct 'countering' effect on the 'forward force'. This is not the case with a plane.
It's like putting a pushbike on a treadmill set to 5mph then sitting on said bike and pedalling on it at 5mph, you don't go anywhere! Contrasted against standing on the ground, off the treadmill, and pushing the bike forward at 5mph, in this case, despite the speed of the treadmill the bike moves forward at 5mph, it's just your wheels will spin twice as fast.
piku wrote:If this whole mental exercise is confusing you, just replace "aeroplane" with "man holding onto hang-glider, running on treadmill". Quite obviously he wouldn't take off. People running on treadmills feel no wind on their face because they aren't moving through the air. No airflow, no lift. No lift, no flight.
QED
Well that seems blindingly obvious to me too, hence the original thread title. How the hell this is 14 pages long is a mystery to me.
The problem is Jedburgh's first post, he's read the linked scenario and posted an incorrect interpretation of it.
I didn't try it, because I know it would work. It would work because the pressure of the air below the paper would push it up when the pressure of the air above it was reduced by Bernoulli's principle.
piku wrote:If this whole mental exercise is confusing you, just replace "aeroplane" with "man holding onto hang-glider, running on treadmill". Quite obviously he wouldn't take off. People running on treadmills feel no wind on their face because they aren't moving through the air. No airflow, no lift. No lift, no flight.
QED
Well that seems blindingly obvious to me too, hence the original thread title. How the hell this is 14 pages long is a mystery to me.
The problem is Jedburgh's first post, he's read the linked scenario and posted an incorrect interpretation of it.
If the plane is stationary, then yep, no lift, but if you cannot negate the engines thrust as Kalamar correctly pointed out a few pages ago, then you are fighting a loosing battle as the thrust of the plane will easily overpower any retardational force you can apply to the plane via its contact patch with the ground...
I didn't try it, because I know it would work. It would work because the pressure of the air below the paper would push it up when the pressure of the air above it was reduced by Bernoulli's principle.
Mmmm... A planes wing generates lift by accelerating air over the top of its surface thus creating a low pressure area over the wing into which the wing is pulled,
Right, so I am joking why?
Because in the first of those you talk about the wing being pulled up by the low pressure.
This isn't the case.
It's always regions of high pressure that push. If it was a pull, then the lowest possible pressure would be unknown, because there is no way of knowing what the greatest possible pulling energy could be. However, since it's a push, we know that sea level air pressure limits the greatest pressure difference that can possibly exist, on Earth.
"My way is the way of not fighting":- the book of Enter the Dragon.
Mmmm... A planes wing generates lift by accelerating air over the top of its surface thus creating a low pressure area over the wing into which the wing is pulled,
Right, so I am joking why?
Because in the first of those you talk about the wing being pulled up by the low pressure.
This isn't the case.
It's always regions of high pressure that push. If it was a pull, then the lowest possible pressure would be unknown, because there is no way of knowing what the greatest possible pulling energy could be. However, since it's a push, we know that sea level air pressure limits the greatest pressure difference that can possibly exist, on Earth.
There is not an area of high pressure under the wing, the pressure of the air under the wing is pretty much the same as the rest of the air surrounding the aircraft. The air under the wing is at a higher pressure than the air over the wing, because of the air being accelerated over the wing, and the venturi effect that the wing shape is having on this accelerated air.
I will agree that I should have said the wing moves into this area of lower pressure, rather than the wing is pulled, as you are correct, the wing is pushed up by the difference in pressure under the wing. But this is quite a minor point of wording
The plane will roll backwards against the resistance of it's wheel bearings and end up at the arse-end of the treadmill, or behind it. In which case the airflow over the wing will be the wrong way around.
Why are we still trying to find an answer to something that's fucking obvious? Let's have more pixies, those at least have comedy value.
Well, it is, and it isn't. I know the difference, and it seems you know the difference, but apparently a few pages ago piku didn't. He's training to be a teacher, so will be in a position to pass on any misunderstazndings.
Physics teaching has been trying to lose Centrifugal force and Suction as theoretical forces that are used in conversation by the general population since before I took O level physics in 1976. They are still confusing people, to this day, and the concepts shouldn't be propagated.
Anyhow, glad we can agree.
"My way is the way of not fighting":- the book of Enter the Dragon.
