?

Log in

No account? Create an account
Plane flys off conveyor belt. Now we have it on video. - Adventures in Engineering — LiveJournal
The wanderings of a modern ronin.

Ben Cantrick
  Date: 2008-01-31 14:43
  Subject:   Plane flys off conveyor belt. Now we have it on video.
Public
  Tags:  reddit

http://www.youtube.com/watch?v=IbRcg3ji_Pc

Thank you MythBusters for doing the actual experiment and getting it all on video tape!

Also - IN YOUR FACE, "it won't fly"ers!!
Post A Comment | 8 Comments | | Link






Coinneach Fitzpatrick
  User: scarybaldguy
  Date: 2008-01-31 21:52 (UTC)
  Subject:   (no subject)
FAIL.

The argument is that the treadmill is moving at THE SAME SPEED AS THE AIRPLANE. In this demonstration, THE TREADMILL IS NOT MOVING AT THE SAME SPEED AS THE AIRPLANE. Airflow is being generated by the airplane's forward motion!
Reply | Thread | Link



MegaZone
  User: zonereyrie
  Date: 2008-01-31 22:00 (UTC)
  Subject:   (no subject)
It doesn't matter - as long as the planes wheels can free-wheel you can *never* get the treadmill to match the plane. Period. The plane is pushing against the air, it is moving relative to the *air*, not the treadmill. Speed up the treadmill and you'll just increase the speed of the planes wheels - NOT keep it in place. The speed of the wheels over the treadmill will always be (speed of treadmill) + (airspeed of plane). The faster you move the treadmill, the faster the wheels spin - but the plane will keep moving forward - as long as the wheels can free wheel effectively.

If the plane is going 100mph and the treadmill is going 100mph - the wheels sill simply go 200mph and the plane will move forward.

Now, there is some friction in the bearings, so there is going to be some motive forced on the aircraft, but just a fraction of the full speed.
Reply | Parent | Thread | Link



Coinneach Fitzpatrick
  User: scarybaldguy
  Date: 2008-01-31 22:03 (UTC)
  Subject:   (no subject)
That was exactly my point: it's relative airspeed, not groundspeed, that generates lift, and that's the entire point of the experiment.
Reply | Parent | Thread | Link



MegaZone
  User: zonereyrie
  Date: 2008-01-31 22:06 (UTC)
  Subject:   (no subject)
OK... I don't understand. There is simply no way for a treadmill to keep the plane from flying. It sounds like you were saying the experiment was bad because the treadmill wasn't going fast enough - is that not what you meant?
Reply | Parent | Thread | Link



Coinneach Fitzpatrick
  User: scarybaldguy
  Date: 2008-01-31 22:15 (UTC)
  Subject:   (no subject)
I'm saying the original experiment posits zero forward motion, which is the point that everyone misses.
Reply | Parent | Thread | Link



MegaZone
  User: zonereyrie
  Date: 2008-01-31 22:21 (UTC)
  Subject:   (no subject)
That's now how I've heard it. I understood it to be the treadmill moving at the same speed as the aircraft - which in a car would be zero forward motion, but not in a plane. It is just completely illogical posited as 'zero forward motion' because the treadmill *can't* do that with a plane. So it is a bogus experiment when presented that way.
Reply | Parent | Thread | Link



  User: nickhalfasleep
  Date: 2008-02-01 04:07 (UTC)
  Subject:   (no subject)
Which is impossibly impracticable due to the massive power imbalance in imparting motion to the body of the plane via a direct propulsion (the propeller) or the weak pull of a treadmill on a freely rolling wheel.

Hooray for the mythbusters having plenty of silly experiments on the internet to toy with.
Reply | Parent | Thread | Link



Ben Cantrick
  User: mackys
  Date: 2008-02-05 05:35 (UTC)
  Subject:   (no subject)
Now, there is some friction in the bearings, so there is going to be some motive forced on the aircraft, but just a fraction of the full speed.

This is a side point to the main argument, but it's worth explaining because it may help some people understand why the plane does take off:

There was a guy who was studying mining carts. He wanted to know if it was cart speed or cart load weight that made it hard to move mining cars. His name was Robertson, and he came up with something called Robertson's Contrivance.

Basically, it was just a big wheel that had railroad tracks running around the outside. He put an empty mining cart on the wheel, and attached a force measuring gauge to it. Then he ran the wheel at different speeds, measuring the force necessary to keep the cart stationary. Basically he was measuring the force of friction between the cart and the wheel.

He found that there was basically no difference in the amount force necessary to move the wheel at different speeds. It took almost exactly the same amount of force at 5 MPH as 25 MPH.

Then he threw a bunch of rocks and dirt in the mining cart, to make it heavily loaded, and spun the wheel again. It now took a lot more force to make the wheel spin, because the cart was much heavier. But again, no matter what speed he made the wheel go, the new larger amount of force didn't change. It was the same no matter what the speed was.

The conclusion was inescapable: for a vehicle with free-spinning wheels (like mining carts or airplanes) an increase in wheel speed does not result in an increase in friction. The amount of force required to make the cart move depended only on the cart's weight, not on its speed.

This is why ground-speed doesn't matter. The runway can be moving backward at 10x the plane's takeoff speed, but the plane is still going to be able to roll forward. The amount of friction between the plane and the runway does not depend on the plane's speed, only on the plane's weight. And the plane's weight will not increase as it rolls down the runway.
Reply | Parent | Thread | Link



browse
May 2015