drkohler, normal engines in planes these days (jet engines) don't create airflow over a plane's wings by themselves (and in the case of a plane with propellers mounted centered on the wings, it would not create nearly enough airflow for take off). I don't think they're talking about building some strange new plane to do this.

epsilon72 said: drkohler, normal engines don't create airflow over a plane's wings by themselves. I don't think they're talking about building some strange new plane to do this.

Jetengines, no, but propellers actually do (which isn't the intention of a propeller) but they do as an added bonus. An actual experiment to check the myth will most likely fail anyways with any nornal plane geometry because the engine(s) cannot suck enough air to generate the power required.

 

drkohler said: epsilon72 said: drkohler, normal engines don't create airflow over a plane's wings by themselves. I don't think they're talking about building some strange new plane to do this. Jetengines, no, but propellers actually do (which isn't the intention of a propeller) but they do as an added bonus. An actual experiment to check the myth will most likely fail anyways with any nornal plane geometry because the engine(s) cannot suck enough air to generate the power required.

 If that were true a propeller plane would start lifting off the ground as soon as its props spin up to full speed. The lift is generated by the apparent wind flowing around the wings and the propellors generate forward movement which results in an apparent wind.

blaydcor said: they're using an "Ultralight flying machine", not a jet, if anybody would take the time to read the OP. A lot of you are arguing using pseudo-physics, anyway. Personally I'ld be amazed if they got the plane to take off.

 

completly of topics---the mime would care--after all you never said he/she dies

If that were true a propeller plane would start lifting off the ground as soon as its props spin up to full speed. The lift is generated by the apparent wind flowing around the wings and the propellors generate forward movement which results in an apparent wind.

Correct, but no existing propeller plane generates enough airflow over its wings for that. Therefore noone has ever been able to see that happen. There are propeller planes explicitly contructed to be able to take off on ultrashort runways but it still take 20-30 meters for that. The only propeller planes that actually can do the trick are called helicopters and do no\'t count in our (theoretical) myth.

"*"ç% what the hell happend with the quotes?

@drkohler,

I see what you are getting after now.

I would like to point a few things out, lets see if we can work this out: First I am going to talk about the wheels a bit, as I think this is a point of misscommunication.  Then I am going to move into your example and explain a bit more about why I brought up this topic again.

If the wheels are free-spinning, which is standard for all planes, the plane will be able to maintain a stable position with little effort.

For this situation lets assume the plane has already become stationary on the treadmill. Now the only external force being exerted on the plane in this situation is the treadmill\\\'s friction on the wheels. Whenever you have a surface tangent to a free-spinning wheel and there is friction as they move across one another the result will be that the vast majority of energy from friction is expended in spinning the wheel.

The rest of the energy is transfered through the wheel axle and into the frame of the aircraft which will result in \\"acceleration\\" in the same direction that the treadmill is traveling. But this acceleration is minor compared to the amount of energy being lost in spinning the wheels.

The result is that it actually doesn\\\'t take but a small amount of extra energy to maintain a given speed on the treadmill versus off. The plane won\\\'t need to expend a lot of energy to attain a stationary position since all it needs to do is overcome the small amount of force that actually makes it to the frame and any extra thrust it produces will counteract the starting momentum and it will \\"move forward\\" and will continue to do so until it not only passes a \\"stationary\\" position/speed but until it is traveling forward fast enough as to produce wind resistance that offsets its \\"extra thrust\\".

 

The reason I point this out has to do with your idea of a plane designed to produce airflow over the wings. The plane clearly cannot take off without first attaining a "stationary position", I think we are on the same page with that part but I am stating for clarity. So this is where what I was just talking about comes into play. If I am reading you correctly you are saying it will be able to produce enough airflow on its wings while stationary to take off. But I think that the amount of thrust it will be producing to create that airflow will have it moving forward long before it takes off for the reasons I stated above.

I think an aircraft designed to take off purely from the airflow it produces and nothing more is possible, but I don\'t think it can take off from a stationary position soley with the aid of a large treadmill.

PS - I am oversimplifying everything to make sure we are on the same page every step of the way, it also makes it easier if we have different understandings of a concept so that we can sort that out before dealing with the larger issue with a compatible set of terminology.

We totally agree. However, there is one minor point you must consider: In order that the plane stays stationary on the belt (as per myth), the speed of the treadmill has to be increased (by the mythbusters) when the engine power is inceased by the pilot to increase airflow und consequently uplift (otherwise the plane would indeed start to roll forward off the belt).

I'm pretty sure constructing a special plane that generates enough uplift with its propellers alone would way overcome the (financial and aironautical) capacity of the mythbusters, so in their real test with an ultralight plane (which won't do it just because it is 'ultralight'), they will have a hard time to even achieve a stationary plane as this situation actually requires an active feed-back system to constantly cancel out any plane/belt speed differences. Without a feed-back system, all the pilot will do is frantically power up/down the engine(s) to stay on the belt due to all those litlle influences that separate theoretical physics from the real world physics..

 

*ç%*ç what happened with the quoting?? Everytime a correct a misspelling in my text, the quotes turn black..

The site is undergoing some big changes right now so there are some new bugs that need to be ironed out. ioi should hopefully have the more annoying stuff fixed by the end of the week.

On Topic: The idea of increasing the speed of the treadmill is an interesting one and will technically work. But the amount of extra speed required to offset the immense amount of thrust this thing will be producing has me cringing at the thought of building such a treadmill. Since only a small fraction of the treadmill's power is actually turning into deceleration for the plane it will take a huge amount of belt speed to counteract the accelerating plane.

I won't hazard an exact guess but it seems like the belt could be required to approach mach speeds to accomplish this task. It really depends on what the actual percentage of treadmill energy is lost in spinning and the percentage that ends up in the frame. It won't be a constant percentage over a range of speeds either, which complicates things. And iirc the faster the wheels are spinning the less energy is actually transferred into spinning and more is lost in heat and in the frame, so that aspect at least is working in our favor.

Let me know if I am missing something.

Sqrl said: The site is undergoing some big changes right now so there are some new bugs that need to be ironed out. ioi should hopefully have the more annoying stuff fixed by the end of the week. On Topic: The idea of increasing the speed of the treadmill is an interesting one and will technically work. But the amount of extra speed required to offset the immense amount of thrust this thing will be producing has me cringing at the thought of building such a treadmill. Since only a small fraction of the treadmill's power is actually turning into deceleration for the plane it will take a huge amount of belt speed to counteract the accelerating plane. I won't hazard an exact guess but it seems like the belt could be required to approach mach speeds to accomplish this task. It really depends on what the actual percentage of treadmill energy is lost in spinning and the percentage that ends up in the frame. It won't be a constant percentage over a range of speeds either, which complicates things. And iirc the faster the wheels are spinning the less energy is actually transferred into spinning and more is lost in heat and in the frame, so that aspect at least is working in our favor. Let me know if I am missing something.

Agreed. The only solution I see is to actually test the myth is a rotating drum and a fast feed back system to keep the plane (on auto pilot, no real pilot is that suicidal) on top of the wheel. You can pretty much slowly speed up the drum to senseless speeds, but then at some point even the best tyres will probably explode before the take-off speed is reached. The more I think about it, the less likely the experiment becomes a real possibility.

 

"If a plane is traveling at takeoff speed on a conveyor belt, and the belt is matching that speed in the opposite direction"

this whole concept is a little iffy. Is it traveling at takeoff speed relative to the conveyor belt, or the air?