THE FRIENDLY SKIES...

Lt. Sgt. Jackus writes...

Okay, here's something I overheard two guys talking about on the El here at school, and had to get off the train before I could hear a full explanation. The general question is something like this: "An airplane is on a giant runway that is actually a conveyor belt. The Belt moves in the opposite direction the plane is facing. It is rigged up to always move the same speed backwards that the wheels of the plane move forward. If the plane tries to take off, will it?" One of the guys on the train said his teacher said it would take off, so I'm inclined to believe it. But this guy didn't seem to buy what the teach was saying. The teacher was trying to convince him the professor was right. But how?

Well Mr. Jackus, I think I understand the question and if so, can explain the answer.

If I understand the premise correctly, the plane is using a conveyor belt for a runway. That belt is moving backward at the same speed the wheels are moving forward. The thought is that the plane would be standing still and of course, if it was standing still, there would be no airflow over the wings to create lift. If we were talking about a car with wings, this would be true. The wheels are pushing against the road (conveyor belt) and the road is moving backward at the same speed the wheels are pushing forward, ergo no forward movement.

On the other hand, the airplane does not drive on it’s wheels, it pushes against the air. So the airplane would move forward no matter how fast the wheels were moving. Whether or not it will fly is a matter of how fast it is moving compared to the air. As soon as the airplane surpasses its stall speed, it would take off.


What would keep the airplane from traveling forward would be the air speed. If it were to fly into a wind that was as fast as its air speed, it would have no forward motion, though it would still take off. Physics doesn’t care whether the wings move into the air or the air moves into the wings. It only matters that the air does move over the wings fast enough to create enough lift to overcome its weight. We run into this every time we fly. If we have a tailwind, we get there faster. If we have a headwind, it’s going to take longer.

jordansmall's photos

Okay, I know I have been neglecting Ask Jim lately, but it has been for a good cause. I've been busy tying flies and working on Bluebelle.

Anyway, if anyone were to Google Adam Saraceno, they would find these photos...
























And we don't even want to talk about the chicken fried steak incident!