MythBusters:

When I saw this episode I was very surprised about this. I'm without any aeronautical background but I thought a plane needed airflow on the wings to gain lift?
And the engine of the plane just moves the plane fast enough forward over the runway to get this done?

Obviously they did not match the speeds and the plane was clearly moving forwards. Otherwise , the plane would just hover in the air since it has no forward momentum? No of course not ... the plane would absolutely not fly if they matched the speeds because there would be no air moving over the wings , like the pilot described.

if they want to prove/disprove this myth, why are they using airplanes that have insanely high power to weight ratio to the point where they dont really need much or any forward momentum to take off ..?? Toy planes are basically helicopters in the shape of planes. The power to weight is so high they can take off straight up, hover in the air, etc. They are not taking off like a traditional airplane, relying on the wings to create lift. The engines alone make them fly

why not use a glider. Pull a glider on wheels on a conveyor belt, and if it is not moving forwards, it will not take off. Otherwise it would just be floating in the air...

As you can see the airplane is still moving forward (because the engines act on the air and not not on the ground) so the airflow on the wings is the same as when there would be a 'normal' runway.


It doesn't matter how fast the conveyor belt goes. The motion of the plane will always be the same, even if the runway would be made out of ice

the motion of the plane will not always be the same, if the belt is moving slower than the plane, the plane will move forwards along the belt as it did in the video

i agree that the engines are acting on the air and it doesnt matter how fast the ground is moving , the ground moving is not creating lift.. i agree, thats not what im saying though.

the plane is moving fowards, while still on the ground

the point of the experiment was -- IF WE MATCH SPEEDS-- and they did not match speeds, the plane is moving forwards on the belt before it lifts off

Because glider != airplane? The fact that the airplane has its own source of thrust that is independent of its wheels is meaningful.

I think we all are saying the plane needs forward motion to gain enough airflow on the wings for lift to take off.
I didn't thought about the fact that the plane will always gain forward motion because of engine/air effect.
Is it actually possible to just match speeds for some timespan (keep stationary position)?

I don't think it's possible to match the speed of the plane and the belt. The plane engine 'pushes against the air' so the plane will move forward relative to the air. If we ignore friction in the weels the belt could move 1000 miles an hour backwards, but the plane would still be moving forward. The only way you can keep the plane at the same position is by having a wind equal to the speed of the plane, but this would generate enough lift to make it airborne. It just wouldn't be able to move forward.

...

You're asking a different question: Can an airplane take off from a moving runway if the relative groundspeed of the plane is equal to the speed at which the runway moves?

i said PULL a glider .. to match the speed of the conveyor belt, which is what the experiement was about.. not , can this engine create enough thrust to lift an ultralight plane with a very small run up ... they said in the video that plane can climb straight up

i dont either otherwise they wouldnt use runways at airports

Can you cite the time at which they said that?

http://www.metacafe.com/watch/107597...r_belt_part_2/


the pilot says it at 1:18 ,


"it will take off straight up, 1200 feet a minute "

It's extremely useful when referring to a video other than the video that has been posted to link that video.

But that aside, are taking "take off straight up" as strict literal fact? Because it's not strict literal fact.

The wording of the problem is explicit and clear. If you want to argue about something else, that's fine. But maybe you should start your own thread because it's not this problem.

By the way, the answer to this is yes. It is possible for this to happen if the windspeed is sufficiently high.

If you understand this, the fact that the plane engines work against the air, and that the wheels of the plane spin freely, then you'll understand why a plane can take off from a conveyor belt. The ground has very little bearing on the situation.

This is so stupid, all this does is make the wheels spin twice as fast as they normally do, the plane still has the same takeoff speed relative to the runway but twice that of the conveyor belt.

Agree 100%. Pretty much the wheels on a plane do is stop the fuselage scraping along the ground.

How about running the belt the other way at the same speed as the plane, and stopping the wheels turning completely? Same thing.

What IS interesting is doing this with a sea-plane on a river. However the dynamics are completely different as the "ground" and "wheels" actually do cause a massive drag on the plane until it starts getting lift.

Several hundred post thread in OOT about this

So the trick here is thinking this means the treadmill moves so that the plane's speed in relation to the treadmill matches the plane's normal groundspeed during takeoff. That would only be possible if the drag from the wheels at takeoff speed matched the plane's thrust - something that might happen, as mentioned above, for a river plane trying to take off against the current. As it happens in the video, with negligible wheel drag, such speed matching is not possible. In the video the plane's speed in relation to the treadmill is about twice its takeoff airspeed.


PairTheBoard

Come on now why is that even a myth buster issue, this is elementary result of the fact that all that matters eventually is the relative speed of the air and the plane and of course the geometry of the plane wings and the fact its engine is working both combine together to get the lift. The fact is even if the airplane had no engine, just a glider say or a very small weight plane with tiny engine you would still experience take off.

Read more here.

http://en.wikipedia.org/wiki/Bernoulli's_principle

http://en.wikipedia.org/wiki/Navier%...okes_equations

http://en.wikipedia.org/wiki/Lift_%28force%29


My point is you could put the plane on a huge truck and start moving fast with the small plane on top of a platform carried by the truck and eventually it would take off. Same thing. It doesnt have to be that the plane is responsible for its own motion with respect to the air. If the wind is ridiculously large in speed (ridiculous weather say) the plane will take off similarly while sitting on the runway (eg wind tunnel) (how do you think they test wing geometries by the way).

Think about it this way too. A plane with no engines left can still fly for a while if it has large enough speed. What does this tell you about who is responsible for it flying?

It's 100% lolworthy that so many people continue thinking the plane won't take off after dozens of posts explaining it.

You sound like you've never seen the show. If you have, you obviously don't get it.

I have seen some episodes that actually make sense to test and watch like landing on moon effects to disprove conspiracy theories or one with a water slide to test a viral jump video etc. Those actually were more interesting.


One would imagine that people would understand that its the relative speed with the air that produces the lift or that they have seen on documentaries things like wind tunnels or heard about them or simply that they have bloody taken their hand out of a car's window in the wind as kids while the car is moving fast in the highway and noticing the hand experiencing a lift depending on the shape/angle you create for it etc. Its a lot less hard to visualize than some other things they have filmed. So what is it that i am not getting?

The show is mostly for building ridiculous contraptions and playing with them while talking about some "myth" that may or may not only be confusing to a complete layperson.

FYP

The point of the show is entertainment. Although there are some educational aspects to what they do, that's not what it's about. As a result, it's not really marketed towards people who are really smart to appeal to their intelligence.

They tested this myth because it generated a lot of interest on their fan webpage (and in on the internet in general), not because it was a particularly difficult thought experiment to understand. And they did it with a real plane because a theoretical explanation for a thought experiment makes bad TV.

One would only imagine this if that person doesn't know that many people, or all the people that person knew were of a narrow demographic of highly educated persons.