We made David Pogue wonder about planes on treadmills.

I’ve been a big fan of David Pogue for years now, back to his Macs for Dummies and Macworld days, so I thought it was nice to see that we apparently piqued his interest with our infamous Plane on a Treadmill thread.

I am not sure that this is a good thing.

Can the boards handle an additional 30 page trainwreck going over this nonsense AGAIN?

(Not to mention the disgrace of a tech writer getting the answer wrong.)

That question’s really made the rounds.

It was recently asked at another forum I frequent (which is owned by a former 'Doper, coincidentally)

Here is a post from that forum, and he did some searching of other forums to try and see what others thought.

So does it take off?

Quiet, you!

For those of us too ignorant and lazy to read the original thread, what was the general consensus that The Dope arrived at, if any?

The beauty of the piece of crap question is that it has the following characteristics:

  1. Really dumb people will guess that the planes wheels will still get spinning fast enough and it can take off in very little space therefore shrinking the necessary space for airports.

  2. Regular people will realize that the wheels spinning don’t mean a thing to a plane. It is the air over the wings that matter so it won’t take off.

  3. Smart people realize that the treadmill doesn’t really matter at all since the jet engines work against the air and the wheels simply follow no matter what so it will take off.

  4. Brilliant people realize that the question stipulates that the conveyor belt will instantly match the speed of the wheels and this implies that the whole system will quickly move up to infinite speed rendering the whole question impossible and irrelevant.

And correct people realise what? :smiley:

Well, on the plus side, I can offically say I’ve been quoted in the New York Times (sort of…I’m assuming that this is an ‘online only’ article).

Hey, at least he fixed the typos I made in the OP. :smack:
But now I feel dumb because I changed my position on the subject (ignoring the paradox of the plane’s wheels instantly going up to infinite speed, that is,) yet I’m quoted with my original theory that it woudn’t take off. What if some prospective emplyer asks this question, hoping for the right answer, but then reads this article or the thread and somehow connects my OP to me! Nooooo! :stuck_out_tongue:

People who understand how jet engines work realize that they don’t “work against the air”, they follow Newton’s Laws by throwing reaction mass out the back just like rockets do; the surrounding air is only a convenient source of oxidizer. :wink:

Fuck yeah. I was leaning to choice 4 but I never bothered to read the thread. Thanks for the synopsis!

i’m still waiting for mythbusters to focus an episode on this question… and wondering where they will stash the explosives on the full scale demo.

Well there’s also a number 3.5 where the wheels get spinning fast enough that the friction on the axle is enough to act as a brake.

Several answers are correct, in truth, depending on various interpretations and whatnot.

The air is also the thrown mass (or a considerable portion of it)

Allow me to throw in links to the Straight Dope columns for the small group of people who still might have some doubts on the matter:

An airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off? (03-Feb-2006)

“A plane is standing on a runway. . .” No, it’s not. Here’s why. (03-Mar-2006)

But as we can see from this discussion at digg.com , even at “technology” websites it is hard to convince people of the facts of the matter (a discussion on a video from some young canadian that settles the matter “once and for all”).


Now it’s on BoingBoing

And DailyKos. (That post references the SDMB!)

The original question was poorly phrased. Someone added the “The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.” condition, no doubt in an attempt to be helpful, but ended up making the scenario physically impossible (or at least added massive room for quibbling).
Phrase it someone differently, as in, “Allow the treadmill to achieve any physically reasonable speed, can the plane still take off?” and the answer is “Yes”.

Okay, I know this question is famous here on the board, but where did it come from? Did some curious Doper ponder it, or has this been a puzzle to mystify young engineering students for decades?

Quite right. The trend is toward higher bypass ratios (the ratio of air that flows around the turbine, as opposed to through it). This wiki article says:

With 8 times as much air flowing around the turbine as through it, and with some substantial part of the reaction from the turbine being due to accelerated air, it’s clear that air can be 90% or more of the reaction mass.