747 Conveyor Belt (again)
Imagine a wine bottle lying sideways on a checkout conveyor.
When the belt starts to move the bottle shows a very strong tendency to remain exactly where it is by starting to roll. Indeed if we could ignore the pesky complications of rolling friction and rotational inertia it will never reach the cashier. The faster the belt goes the faster the bottle will roll.
A 747 with its engines not running would show a very similar characteristic. Nomatter how fast the belt goes it will just sit there - the wheels rolling faster and faster to match the belt speed (don't forget we are ignoring rolling resistance of the tyres or the rolling inertia of the wheels).
So now we start the engines. The engines generate reactive thrust by accelerating air through themselves. That thrust is applied directly to the airframe - nothing happening with the ground, the wheels, or the belt have any bearing on that force. So the aircraft begins to accelerate forwards. When it has accelerated to take of airspeed we can rotate and away we go.
When the belt starts to move the bottle shows a very strong tendency to remain exactly where it is by starting to roll. Indeed if we could ignore the pesky complications of rolling friction and rotational inertia it will never reach the cashier. The faster the belt goes the faster the bottle will roll.
A 747 with its engines not running would show a very similar characteristic. Nomatter how fast the belt goes it will just sit there - the wheels rolling faster and faster to match the belt speed (don't forget we are ignoring rolling resistance of the tyres or the rolling inertia of the wheels).
So now we start the engines. The engines generate reactive thrust by accelerating air through themselves. That thrust is applied directly to the airframe - nothing happening with the ground, the wheels, or the belt have any bearing on that force. So the aircraft begins to accelerate forwards. When it has accelerated to take of airspeed we can rotate and away we go.
(don't forget we are ignoring rolling resistance of the tyres or the rolling inertia of the wheels).
Fly a lot of wine bottles, do we?
These can be ignored because their magnitude is tiny compared to engine thrust. For some strange reason aircraft are designed to have low rolling resistance - something to do with the twin desires to (a) take off, and (b) not have the wheels melt in the process.
Of course it can make it more difficult to stop when landing, but the designers mitigate this problem by adding features like brakes and thrust-reversers.
Heck, most 4-engined jets won't even stand still in the asphalt with even two engines running at ground idle unless the brakes are applied - that gives an indication just how small the rolling resistance really is.
Of course it can make it more difficult to stop when landing, but the designers mitigate this problem by adding features like brakes and thrust-reversers.
Heck, most 4-engined jets won't even stand still in the asphalt with even two engines running at ground idle unless the brakes are applied - that gives an indication just how small the rolling resistance really is.
You ignore them because it is a thought experiment (in fine Einstein tradition). It has no practical application but is intended to demonstrate the distinction between the ground and air frames of reference when considering aircraft dynamics. So you only consider the primary factors and ignore the practical limitations.
Once you start to include all of the practical limitations then you end up going nowhere other than arguing about what all those practical complications are.
Once you start to include all of the practical limitations then you end up going nowhere other than arguing about what all those practical complications are.
