Yes, yes yes…I already mined the SDBD threads, and the previous threads on this are SO polluted with copious gook, who can make any sense from it all? I just want to ask this: While mythbusters demonstrated a plane can take off from the conveyor (moving at equal and opposite velocity), their explanation made no sense whatsoever.
A wing needs air rushing over it. Even if the plane’s ground speed is zero, I assume it’s air speed is NOT zero. The plane IS moving forward, otherwise, the conveyor would move the plane backwards. Is this the crux of the explanation?
Please answer in 100 words, or less (unlike the other threads).
A normal conveyor belt has no means of preventing the aeroplane from moving forward in respect to the ground. The belt acts on the wheels but the aeroplane does not generate forward movement through its wheels, it gets it from thrust either from the propeller or jet thrust.
So, the aeroplane still moves forward relative to the ground, still gets airflow over its wings and still flies. The wheels will be spinning faster because the conveyor is moving rearward.
That is the Mythbusters and Cecil take on the question.
The question as originally posed, describes an impossible situation: the plane is applying forward thrust via its engines. The treadmill is attempting to prevent it from taking off by applying reverse motion to the plane’s wheels, matched so that it cannot move forward. This is impossible in the real world.
Once the plane applies any kind of serious thrust, it will move forward along the treadmill and either fall off the end, or if it’s an implausibly large treadmill, will take off. The plane’s speed relative to the surrounding air is really the only thing that matters. All else is smoke and mirrors.
Just one more thing, the aeroplane’s ground speed can not be zero. Assuming nil wind, the ground speed will equal the airspeed and so the ground speed must be sufficient to allow the aircraft to fly. The crux of the question is really that the wheels do not drive the aeroplane forward and so the conveyor belt is not preventing the aeroplane’s forward motion (relative the ground) by moving rearward.
If you want to delve deeper into the alternate interpretations of the question, you really need to read the many threads. I recommend Zut’s posts as covering everything accurately and in an easily understood manner. He even has some summary posts that lay everything out plainly.
Can someone summarize what actually happened on the Mythbuster’s episode? Jinx says that the conveyor was “moving at equal and opposite velocity,” while 1920s Style “Death Ray” says that the conveyor belt “has no means of preventing the aeroplane from moving forward in respect to the ground.” Those seem to be contradictory to me. So did they actually try at all to sync the treadmill to the wheel (or plane) speed, or did they just punt and dump a plane on some sort of treadmill? Or something else?
I haven’t seen it but gather the “treadmill” went backward at x mph relative to the ground, the aeroplane went forward at x mph relative to the ground and 2x mph relative to the treadmill.
Plane needs to reach a forward velocity of 50mph relative to the stationary ground and air to lift off.
What happens when treadmill is going opposite direction at 50mph?
Plane will still lift off when it reaches 50mph relative to the stationary ground and air but will be moving at 100mph relative to the treadmill.
It could indeed. The only reason it ran for so long here is that one or more people kept arguing that it was a magic treadmill, capable of increasing its speed so as to always keep the plane stationary relative to the ground and air.
Impossible in real life and therefore impossible to satisfactorily conclude a question with that parameter.
The overriding issue here is WHERE the thrust is applied. The thrust for an airplane does NOT come from the contact between the wheels and the ground. The amount of friction that is generated by the wheels moving over a treadmill going (nearly) any speed would not be enough to stop the plane from moving forward through the air. In fact, as the airspeed increases, drag on the wheels would be reduced, as the wings would begin to provide lift, and as friction=weight*FrictionCoefficient, when the weight applied is reduced, so is friction.
In fact, if the takeoff speed is 50MPH, and you point a plane into a 51MPH wind, it will lift off. If you have thrust being applied from the engine, it will continue to fly.
I heard Adam Savage on NPR’s news quiz show last week and it started me wondering if he was a doper. He mentioned the copious amount of chat in the internet about the plane/conveyor question and also mentioned that the results of his show didn’t move all people to agree with their conclusion. He MUST have at least seen the discussions on the dope, right? Is he a poster?
Mythbusters has its own message board, and I’ve participated in discussions with denyers elsewhere, too.
I believe the Dope is know to them, though. For one myth they were talking about the online research they did and some screen shots sure seemed to look like our boards.
I thought he very concisely put things to bed when he said, “The wheels are only there to keep the propeller from hitting the ground.” I think if you really think about this, it will explain why the airplane will take off.
You can do whatever the heck you want with the treadmill; it won’t affect the plane at all. No matter how fast the treadmill is going, the plane will still move forward and accelerate relative to the air and to the ground, just like it does on a runway, and when it gets going fast enough relative to the air, it takes off.
Going from memory here, but IIRC it was the OP who stated as part of the hypothetical that the treadmill would match the increasing speed. I think this stipulation is what contributed largely to the confusion.
The other problem was the wording of the OP stated that the belt speed matched the wheel speed, not the plane speed, which seemed to imply that a fixed spot on the wheel would, by definition, never make forward progress relative to the ground when measured after each revolution.
It took a long time to sort out (if it even happened), what the OP really meant by matched the speed of the wheel.
