Comments on: Plane on a conveyor belt

By: builders merchants

builders merchants — Fri, 19 Nov 2010 16:06:03 +0000

Interesting thing - and I've just confused my office with the plane on a conveyor belt question. I think we'll all be discussing it for the rest of the afternoon. I can't see that the plane would move because surely for something to move the force pulling it has to be greater than the force pushing it in the opposite direction? Surely it would just sit in one place?

By: Roy

Roy — Tue, 05 Oct 2010 06:11:38 +0000

Honestly it was economics and not physics that made me pretty sure something like what happened in mythbusters would happen. I didn't understand about the freestanding wheels, but I did think that, if a plane could take off while not moving relative to the ground, then no airport would ever pay a bloody fortune for land for runways. Obviously they do, so obviously that can't be done. So I thought it wouldn't take off, but I didn't think about the fact that the treadmill basically can't keep the plane stationary because the plane doesn't depend on its wheels. The Mythbusters subtly changed the conditions to the treadmill going backwards at the same speed as the plane was going forwards..of course the plane just accelerated to twice the speed relative to the treadmill and took right off. If you designed a plane which took off by using a car engine driving the wheels to get up the necessary speed, then the treadmill could prevent it from taking off.

By: Triffid

Triffid — Sat, 02 Jan 2010 12:10:00 +0000

When I first watched this episode I admit I thought the challenge involved an aircraft rendered stationary by a moving treadmill, and I thought the experiment was flawed when I saw the plane moving forward. True, I am NOT a pilot, nor do I claim to have a firm grasp on the laws of aerodynamics, but I should not have jumped so quickly into such an erroneous conclusion. I soon realized my mistake when I read the phrase ” A treadmill CANNOT prevent a free-rolling plane from moving forward. The propellers PULL the plane through the air, thus creating lift. The plane WILL take off!”

The analogies of attaching a rope to the back of the aircraft, or holding a toy car stationary with your hand do not apply. THE PLANE WILL MOVE FORWARD AND TAKE OFF! Luckily, I realized my mistake before I posted any letters to the contrary.

I cannot call it a trick question, like the rooster laying an egg on top of a barn, but at first glance it can mislead.

Kind of reminds me of this one…If a space ship traveling at the speed of light turns on its’ headlamps, how fast does the light from the headlamps travel!

Well, maybe not.

Anyway…Naysayers, for a short time I was one of you. YOU ARE INCORRECT !

By: ML

ML — Sun, 20 Apr 2008 16:18:13 +0000

Okay, so, I just saw this episode and was a bit disapointed.

Let's take this even more theoretical. What if the airplane was bound to the treadmill. So, the airplane was not moving with respect to the treadmill surface. What is your prediction then?

This removes (or perhaps furthers the idea) of the friction being infinite and allows one to remove the worry about skidding reducing the friction at some point

By: Jason

Jason — Tue, 05 Feb 2008 01:01:11 +0000

With the impossible or near impossible conditions that the conveyor belt needs to meet, I think the best compromise to test the true intention of the question is to have the conveyor belt move in the opposite direction at the ground speed equivilant to the airspeed required for a particular aircraft to take off. So if a particular aircraft needs to reach 100 knots indicated airspeed to attain flight, and not factoring in windspeed or direction, to have the conveyor belt move 100 knots in the reverse direction. I don't see how anyone could argue against that as a fair demonstration, because the speeds will be matched at the critical point. But I am sure you will find that the aircraft would still take off.

By: Kevin

Kevin — Mon, 04 Feb 2008 19:16:27 +0000

At first I was disappointed by how the Mythbusters handled this test because I, like many of the posters here, wanted to see the plane sit still relative to the ground and take off somehow (which I doubted would be possible because there wouldn't be enough air flowing over the wings to cause lift). However, after reading these comments, I understand that there is no way a conveyor belt could match the plane's speed and force it to sit still because the wheels are nearly meaningless when it comes to the plane's acceleration. No matter how fast the conveyor belt goes, the plane can always move forward relative to the ground, thus causing air to flow over its wings, followed by flight. This was confirmed by the Mythbusters.

I think the problem is that the other posters haven't come to the realization that the conditions of the myth itself are impossible. If the INTENTION of the myth was to ask if a plane could take off without moving relative to the ground (and without the assistance of wind), then putting a plane on a conveyor belt does not prove anything. A better test would be what another poster mentioned: tie the plane to a wall so that it CANNOT move forward, and see if it lifts off of the ground. However, if the intention of the myth was to test whether enough friction can be exerted on the wheels of a plane to keep it from taking off, then it has been busted by the Mythbusters.

By: Jason

Jason — Sun, 03 Feb 2008 18:40:43 +0000

The fact of the matter is the original question does not state the aircraft is held stationary. It only states that the conveyor belt is moving at the same speed of the wheels meant to imply the question "Will it keep the aircraft stationary?" The 4 basic forces affecting an aircraft are Lift, Gravity, Thrust, and Drag. The conveyor belt is assumed to be contributing to Drag, which is the opposing force to thrust. I don't have numbers to back up what I say, but I have plenty of experience with flying, including 1100 hours as a crewman in an H-60 Seahawk and witnessing at least 5000 aircraft carrier take offs and landings. I can almost promise you that no amount of friction in the bearings of the wheels will ever create enough "drag" forces to overcome the power of thrust. Depending on the aircraft itself, some could even take off with the brakes locked and dragging the tires arcoss the runway. You will not impart any more friction in the direction of drag than that scenario on the wheels.

By: Sammy

Sammy — Sat, 02 Feb 2008 14:04:10 +0000

OK, the premise put forward by the MythBusters was that the conveyor belt would be moving backward at the same speed as the plane was moving forward, and the plane would just sail right off into the air - or not. It's obvious that they didn't accomplish this, as one could easily see that the plane was moving forward, relative to the cones they had positioned alongside the tarpaulin. In reality, their premise would not be significantly different from that of a plane revving its engine & propellor to full flight speed with its brakes fully locked. If the thrust of the propellor is all that's needed for flight, we'd have no need for a runway - planes could sail right off into the sky from a stationary point. But they don't - you've got to have forward movement, with air moving over the airfoil of the wings, generating lift. Bernoulli's Principle and all that, you know...

By: Rich

Rich — Fri, 01 Feb 2008 21:57:46 +0000

bvbellomo: Regarding your interpretation - you are assuming that the conveyor belt can continue to put ever-increasing force on the plane. This is inaccurate. Certainly we can let it run at an arbitrary speed, but at a certain point, the force will exceed the coefficient of static friction of the wheels, and they will skid, which reduces the force that the conveyor belt imparts on the plane, and allows it to take off. I feel like a broken record, but this is the key point that people seem to be disregarding.

By: Dave

Dave — Fri, 01 Feb 2008 17:14:38 +0000

I actually thought the MythBusters proved it conclusively in the first few minutes of the show. Their little model airplane flew off the treadmill. Instead of having a plane moving at one speed and a conveyor belt moving at the same speed in the opposite direction, they had a treadmill moving at one speed and a plane not moving at all. Since the plane was able to move even though the ground beneath it was moving backwards, it proves that the plane would take off no matter how fast the treadmill or conveyor belt was moving.