Airplane on a conveyor belt

[McGurk]

That's the ground speed vs. air speed. The ground speed indicates the aircraft speed relative to the treadmill. The air speed indicates the aircraft speed relative to an observer on the ground.

The question is flawed in that it doesn't provide enough information and leaves way too much open to interpretation.

Incorrect... airspeed, as the name suggests, is the speed of the aircraft relitive to the air.

It is posable (and can be done with some light aircraft) that the aircraft can be stood still on the ground, but if the wind is blowing fast enough(say 40kts), the aircraft can take off.
From the point of veiw of someone on the ground, it takes off verticaly, but as far as the aircraft is concerned, its moving through the air at 40kts

[pdcorlis]

Of course, he's "The Kilted Flyer"


The Seaplane takes off because the thrust is greater than the drag of the water/current moving the plane forward to increase airflow for lift, correct ? Same with a boat moving upcurrent. The thrust is overpowering the drag. If the current were stronger than the thrust you'd not be able to go upstream (memories of a long paddle boat experience).

So if the thrust is unable to overpower the ground drag it wouldn't be able to move forward, correct?

Its not really drag its friction that has to be overcome - the airplane in this example is sitting on wheels and those wheels have bearings - so just how much friction or drag - if you must - do you think has to be overcome by the thrust? Put on a pair of roller blades - attach a rope to the wall and stand on a moving treadmill. You will be able to pull yourself forward with the rope with just a bit more effort than if you were standing on the floor.

By the way, if you want to see airplane propellers overcoming a moving conveyor belt - just drop by youtube. There are several examples there.

[Pleater]

See that's my point. To me, the question indicates that the speed of the aircraft is it's ground speed, otherwise it wouldn't be on the treadmill. So, if the ground speed of the plane matches the speed of the treadmill, the net forward speed to an observer on the ground is zero, thus no moving air and no lift.

I used to work with a man who had been on various UK Navy ships which had spotter aircraft.

When the ship was steaming into a headwind the plane would lift off the deck and fly along a foot or so in the air. It had to be tethered carefully so it was held level and did not dance about. Ground speed was zero, the plane flew.

To reclaim the aircraft the ship steamed alongside the plane, which then gradually moved over its deck to the right spot, the handlers then snagged it and pulled it into place, strapped it down and the pilot could turn off the engine.

The retrieval system was suposed to be a lot more complex, but once it dawned that the ship could steam upwind faster than the plane could fly it was abandoned for the simpler method.

[ccga3359]

Yes, any helicopter, Osprey and Harrier. Perhaps also an autogyro and probably a pelican but definately not an albatross..

[The Wizard of BC]

As a teacher of physics and a pilot I have been reading this with some interest.

It's amazing how this subject comes around every year or so on the internet. It also comes up each year with new students. This question and the one about firing a bullet from a moving vehicle are the classic misunderstandings of the laws of physics.

Let me give an example that may help the understanding.

If you are sitting on a bicycle on a conveyor belt and have a rope in your hands that is tied to a fixed point and the conveyor starts to move you can stay in the same spot relative to the outside world by simply holding on the the rope. The bicycles wheels will spin at the same speed as the conveyor but you remain motionless.
Now start to pull on the rope. You will start to move. You simply must overcome the friction of the wheels and the static mass of you and the bike.
If you don't hold onto the rope when the conveyor starts you will begin to move backwards. You can still overcome the movement by overcoming the friction and now the inertia of yourself as a moving body.
No matter how fast the conveyor moves you can stop yourself or move forward or back at will as long as your arms can overcome the small friction or inertia.

Now look at an airplane. If an airplane flies low over a moving conveyor with it's wheels up will it magically fall out of the air? No. The airplane does not care what is on the ground underneath it. It only cares that a certain velocity and density of air is passing over its wings. Even it the case where the conveyor is moving backwards taking the plane along with it the airplane must only start to move forward till it's speed is equal to its take-off speed. In fact it is easier for the airplane to start moving in this case because there is air now trying to push it from behind and helping it along.

If the airplane is moving backwards with the conveyor it will simply take more time for it to achieve take-off speed. Similar to an aircraft attempting to take off with a tail wind. The only factor is fuel.

OK, now on to the real tough question. If you are on an airplane and fire two guns, one pointing forward and one pointing backwards. Assume the aircraft is flying at the exact speed equal to the muzzle velocity of the bullets.
What happens to the two bullets?

[The Wizard of BC]

The only real conundrum we have in Physics is:

If a man speaks in the woods and there is no woman around to hear him, is he still wrong?

[ccga3359]

The only real conundrum we have in Physics is:

If a man speaks in the woods and there is no woman around to hear him, is he still wrong?

Hell yeah, my wife has powers of telepathy!

[ccga3359]

OK, now on to the real tough question. If you are on an airplane and fire two guns, one pointing forward and one pointing backwards. Assume the aircraft is flying at the exact speed equal to the muzzle velocity of the bullets.
What happens to the two bullets?

Interesting but we are not allowed to discuss it!

Now aren't we sitting on a conveyer belt spinning at 17,000 mph or something?

[Yaish]

Thanks Wizard of BC! A good simple, easy to understand explanation.

[McGurk]

The only real conundrum we have in Physics is:

If a man speaks in the woods and there is no woman around to hear him, is he still wrong?

My woman says... Yes.
I'm still uncertain.