A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).
The question: Will the plane takeoff?
this question usually starts long debates, but there is only one correct answer...hehehe.
(btw: I'm not entirely sure if this is the best place for it, but I think it falls under the joke/commentary category.)
Last edited by Tavel; July 25th, 2008 at 02:12 AM.
The plane wont take off because lift is generated by airspeed, not groundspeed. Now if there is a REALLY strong wind, lets say 65mph headwind on a small cessna, then yes it would still take off because there is enough airspeed generating the lift necessary to overcome gravity.
The plane wont take off because lift is generated by airspeed, not groundspeed. Now if there is a REALLY strong wind, lets say 65mph headwind on a small cessna, then yes it would still take off because there is enough airspeed generating the lift necessary to overcome gravity.
air speed comes from the air passing under the wing, but if the plane is moving on a conveyor it isnt moving in relation to the air surrounding the wings, therefore generating no lift. Thats why planes need to be tied down during storms, because the winds can actually be strong enough to generate enough lift to "fly" the plane, even though the aircraft isnt moving. It's also why aircraft take off and land into the wind, so they can have a relatively low groundspeed but get the additional lift generated by the wind.
I also said no. The plane maybe moving at 100 mph in comparison to the belt but it's real speed to the surrounding air is still 0. And without serious air movement underneath the wings, it can't take off.
It will take off because aircraft push against the air to gain forward momentum, not against the pavement like an automobile. The wheels are free spinning, so the mythical conveyor runway could be moving 1000mph in the opposite direction and the plane will still take off normally. that's why planes can take off from water, ice, tarmac, whatever.
TheEssigs is the only engineer.
edit: I just know someone is going to argue it, so I'll clear it up right now. The question never defines what the plane's speed is in relation too.
If it's in relation to the ground, the plane can take off normally because the belt only has to match the plane's speed. Plane moving West at 80mph, the belt moves East at 80mph. Simple. (the linear speed of the wheels is then 160mph, but it doesn't matter).
If it's in relation to the belt itself, then the belt can really create it's own speed because the wheels of the plane spin freely. The belt could even move at the same goundspeed as the plane, keeping the wheels stationary, and still satisfy the questin.
Last edited by Tavel; July 25th, 2008 at 11:32 PM.
the problem with your answer is you say momentum will give the plane lift, but that isnt true. It takes the change in pressure caused by the disruption in the airflow as the wing passes through the air to create lift. there must be "airspeed" in order to generate this change in pressure. I dont see how you would even generate momentum if the conveyor was neutralizing any forward movement anywayz. I took a 120hr compressed flight course to certify me for SUAV missions in Iraq. the suav is considered an ultralight plane and even though you dont need an actual license per se... to fly anything in Iraq you have to have FAA certification. The theory of flight was covered intensely and your answer just doesnt mesh with what I was taught.
the problem with your answer is you say momentum will give the plane lift, but that isn't true. It takes the change in pressure caused by the disruption in the airflow as the wing passes through the air to create lift. there must be "airspeed" in order to generate this change in pressure. I dont see how you would even generate momentum if the conveyor was neutralizing any forward movement anywayz. I took a 120hr compressed flight course to certify me for SUAV missions in Iraq. the suav is considered an ultralight plane and even though you dont need an actual license per se... to fly anything in Iraq you have to have FAA certification. The theory of flight was covered intensely and your answer just doesnt mesh with what I was taught.
you forget that everything is relative. planes operate with respect to the air, so their momentum is with respect to the air. Planes don't even have drivetrains to the wheels, therefore they put absolutely NO power to the wheels. It's not even possible for the wheels to move the plane, so why would it matter if the tarmac was moving or not? Pretty much how the windspeed doesn't really affect your car because cars operate with respect to the ground.
Think of it this way...If you hop onto a spinning merry-go-round, everything on the merry-go-round feels stationary while everything outside seems to be moving. Now, if you were to theoretically get caught up in a tornado, you would actually see everything around you in a fairly stationary arrangement (just as on the merry-go-round). You could interact with everything as if you were stationary. However, the things on the ground would appear to be moving dangerously fast...much like the things in the tornado are moving dangerously fast if you're on the ground.
planes caught in heavy winds must be strapped to the tarmac to keep them from flying off. Their airspeed is sufficient to induce flight, even though they're stationary with respect to the ground.
Planes can take off from wheels on the land, skis on snow and ice, or floats on water because they don't use the wheels/skis/floats to generate forward arispeed. (airspeed replacing the term "momentum" for ease of argumentation).
ground speed, relevant to either the belt or the ground, is absolutely irrelevant. the belt could theoretically move a MILLION miles per hour and the plane would still operate normally because it pushes on the air for momentum...momentum relevant to the air. the belt doesn't neutralize the plane's airspeed because the plane doesn't push against the belt for airspeed, it pushes against the air.
planes take off into the wind to generate greater airspeed over a shorter distance. they'll still take off just fine if they go with the wind, but it will take a greater runway length to reach take-off speed. however, and here's the real kicker, the actual time required to reach takeoff speed into the wind and with the wind are exactly the same because it's all relevant to the air...not the ground.
Aircraft operate with respect to the air, not the ground. just as boats operate with respect to the water, not the ocean floor.
honestly, this is the correct answer. it's not up to debate, its pure physics.
this is a classic example of why knowing how to operate some machine doesn't indicate you know the physics behind said machine. just like you don't have to know chemistry to cook. flight school is about as relevant to aviation physics as culinary school is to organic chemistry.
That's why it's the "are you an engineer" question, not "are you a pilot".
Last edited by Tavel; July 26th, 2008 at 04:01 AM.
The tornado example you use is a variation of einsteins theory of relativity.
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the actual time required to reach takeoff speed into the wind and with the wind are exactly the same because it's all relevant to the air...not the ground.
i knew that the time was the same, thats something we were taught.
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the belt doesn't neutralize the plane's airspeed because the plane doesn't push against the belt for airspeed, it pushes against the air.
and this is the part that explains your point very well. its a difficult concept to get your mind around, and this is the part that I was missing. The plane is indeed creating enough airflow over the wings even though it isnt moving, so lift is being generated without forward propulsion. a very interesting point. Im studying to be an engineer, although i havent transfered to the engineering department yet, still finishing up my prereqs... its been a long time since i took calc and I really need to refresh.
YOU opened your mind, not me. Most people stick cloth in their ears and yell "No NO No NO". You at least considered that the world doesn't work the way you think it should, and that's a very hard thing to do (my first physics class was mind blowing, man). There might be hope for making you into an engineer yet
Sorry I over-explained it though, I just wanted to make sure the points got across to anyone reading it.
btw: what kind of engineer are you looking to become? I'm going into electrical engineering because that stuff STILL blows my mind.
Quote:
Originally Posted by David C
The tornado example you use is a variation of einsteins theory of relativity.
i knew that the time was the same, thats something we were taught.
and this is the part that explains your point very well. its a difficult concept to get your mind around, and this is the part that I was missing. The plane is indeed creating enough airflow over the wings even though it isnt moving, so lift is being generated without forward propulsion. a very interesting point. Im studying to be an engineer, although i havent transfered to the engineering department yet, still finishing up my prereqs... its been a long time since i took calc and I really need to refresh.
edit: found the episode on youtube (great thing). Honestly, I'm really surprised this answer is not stunningly obvious to everyone...especially the pilot. My jaw actually dropped when i heard the pilot say "I won't be able to take off".
I guess as I said before, knowing how to operate a plane and knowing how a plane operates are two entirely different things.
I guess it's also why this question separates the engineers (or those with engineering aptitudes) from the rest; only engineers can detach themselves from their surroundings and conceptualize a complex system like that.
I also love figuring out why various perpetual motion engines and generators won't work. Like this new one. people have been trying this for hundreds of years, but they think they have something "revolutionary"...
not a chance it works, and the news coverage made me chuckle. They didn't realize that the University was making fun of them when they said "we'd have to rewrite the thermodynamics texts if it works."
Last edited by Tavel; July 26th, 2008 at 06:19 PM.
i havent chosen my engineering specialty yet, but it will probably end up being either structural engineereing or something with alternative energies like wind turbines and the such. ive started to obsess over wind generators (living in ks with all this wind) and the potential energy involved. im trying to get designs for a new style turbine and build one for myself to see how it works. theres no better teacher than your own two hands.
i havent chosen my engineering specialty yet, but it will probably end up being either structural engineereing or something with alternative energies like wind turbines and the such. ive started to obsess over wind generators (living in ks with all this wind) and the potential energy involved. im trying to get designs for a new style turbine and build one for myself to see how it works. theres no better teacher than your own two hands.
too true. i'm also ultimately hoping to get in on the alternative energy thing too, there's no doube I'll be a part of it in electrical engineering. though, not so much wind in illinois.
sure we have the windy city, but i don't think a bunch of long winded politicians are the cure to our energy crisis
