Originally Posted by
scoow
I (sorta) understand the concept of lift on the wing being what really allows the plane to fly. And, this makes sense:
But, I have a question about this:
Without forward motion, would the plane ever generate the necessary airflow over the wing? I'm not arguing whether the motion is created by the wheels or engines.
Yes, you are, that is exactly what you're arguing, you just don't realise it
But, if the conveyor (or brakes, etc) cause the plane to sit still rather than move forward, it won't take off, right?
Absolutely, if the conveyor stopped the plane from moving it would not take off. This however has no relevance to the discussion because it isn't
If I understand this, the plane in gt_croz's example would not be able to take off. Right?
If there is no airflow over the wings the plane will not take off. However as the plane is quite happily moving forward there is. So it does
I know it's not the wheels that are important (or float planes would never take off). But, any force stopping the forward motion has to be overcome before the plane can fly. Right?
Yes, but the force stopping the plane isn't the conveyor belt as there are wheels. Which spin. (Assuming frictionless bearings) if the engines were off and the conveyor belt was moving the plane would sit still with the wheels turning. Turn on the engines and the plane would move forward.
If it was a car, with the drive going through the wheels, and the conveyor belt matched the wheel speed then it would sit still. Can you see why whether the engines or wheels are propelling the plane forward is so important. If you drive the wheels then the conveyor belt would be the opposite force. However the engines don't drive the wheels, they push the aircraft itself forward and the wheels simply turn to negate what the conveyor belt is doing.
Let me put it this way. If I gave you rollerblades with frictionless bearings and put you on a treadmill at turned it on you would be standing still with the wheels turning. If I then shoved you in the back would you stay standing still or go facefirst into the control panel.
I do believe planes fly (or the airlines have some really cool magic that gets planes safely around the world).
So, without wheels, the bottom of the plane would be messed up because it has to scrape along the ground to gain the requisite speed of airflow over the wings. On landing, the cause of damage is a bit more obvious... and the "stop" is likely to be rather abrupt.
A kite can "fly" when you stand still because the natural wind is sufficient to generate the requisite lift. A small plane / ultralight or a plane with superior wing design will be able to take off at a slower ground speed than a typical commercial aircraft. And, on a REALLY windy day, this small plane might generate enough lift when it is barely moving. But, a large plane has to be moving to generate enough lift via air flowing over the wing. Am I missing anything?
Yes, your missing the fact the plane is moving, you're overcomplicating things without addressing the massive mistake in your base assumptions