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Post by afkmatrix on Feb 12, 2008 13:52:21 GMT 1
Ok here is a little puzzle for you guys: "A plane is standing on a 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 the opposite direction). Can the plane take off?" Now once you have had a good think about it, go read this thread: forums.overclockers.co.uk/showthread.php?p=11086895&posted=1#post11086895Now if you go to page 5 and my post, then I am wondering if anyone does have a model plane with motor and a conveyer belt handy? ;D
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Post by eric on Feb 12, 2008 14:03:53 GMT 1
Yes, the plane will take off!
The thrust the engines produce are not connected to the conveyer belt, they thrust AIR out the back, the fact that the wheels are spinning round like crazy has little impact, only an increase in friction for the thrust to overcome.
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Post by nemesis on Feb 12, 2008 15:33:40 GMT 1
I would say no. Even though the thrust is at full pelt, if the conveyor belt is going at the same speed in the oposite direction, the aircraft is effectively stationary. If that is so, there is little or no airflow over the wings and so they're producing no lift.
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Post by Achtung!! on Feb 12, 2008 16:08:17 GMT 1
Thats what I though - honest (!)
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Post by Mike B on Feb 12, 2008 18:17:25 GMT 1
I agree with Nem. If you stand a pole on the ground next to the plane, the plane's position relative to the pole will not change, hence there is actually no forward airspeed, therefore no lift generated over the wing aerofoil. The thrust of the engine is immaterial.
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Post by markus on Feb 12, 2008 19:01:21 GMT 1
Yes, the plane will take off! The thrust the engines produce are not connected to the conveyer belt, they thrust AIR out the back, the fact that the wheels are spinning round like crazy has little impact, only an increase in friction for the thrust to overcome. fully agree with eric...plane will take off markus
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Post by nemesis on Feb 12, 2008 19:30:24 GMT 1
Looks like we got a fight on our hands, lol.
Have you ever seen film from WWII of aircraft taking off from an aircraft carrier and wondered how the heck they took off from the deck when they appeared to be moving so slow? It's because although they are moving slow in relation to the ground due to the short runway, the carrier is sailing into the wind to increase the air speed over the wings.
Theoretically, if you could get an aircraft carrier to sail fast enough, an aircraft could lift into the air with no forward movement along the deck.
It's the airflow over the wings that is the important factor, not the thrust from either a jet or props.
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Post by napoleon on Feb 12, 2008 19:58:15 GMT 1
Yes it would take off
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Post by swanrail on Feb 12, 2008 22:45:10 GMT 1
Surely the jet engine provides the thrust, so it would take off, as per a rocket, which has no wings at all!! If I remember correctly, a few weeks ago the BA jet crashed because it lost engine power, and the airflow over tiny wings was not enough to make it glide!!!
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Post by Mark on Feb 12, 2008 23:53:20 GMT 1
Looks like we got a fight on our hands, lol. Have you ever seen film from WWII of aircraft taking off from an aircraft carrier and wondered how the heck they took off from the deck when they appeared to be moving so slow? It's because although they are moving slow in relation to the ground due to the short runway, the carrier is sailing into the wind to increase the air speed over the wings. Theoretically, if you could get an aircraft carrier to sail fast enough, an aircraft could lift into the air with no forward movement along the deck. It's the airflow over the wings that is the important factor, not the thrust from either a jet or props. Eric & Don are spot on. The conveyor would be unable to hold the plane still at full throttle, so it would definitely take off. The wheels on a plane only serve to keep it off the ground when it's not flying Now here's another for you. You have a model helicopter in a closed box on a set of scales. It weighs 8 Kgs and the box weighs 2 Kgs. The scales obviously read 10 Kgs with the helicopter stationary. What do they read with the chopper hovering in the box. Let's make it an electric powered model so we can discount change in fuel weight, increase air pressure from the exhaust, etc.
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Post by nemesis on Feb 13, 2008 0:03:59 GMT 1
You are correct that the jet engine provides the thrust, Swanrail but that thrust is used to propel the aircraft through the air. That air flowing over the wings creates the lift enabling the aircraft to leave the ground.
But in the scenario we are talking about, the conveyor belt cancels out that forward movement by travelling in the opposite direction at the same speed. This means the aircraft is in effect standing still in the same way a person does when on a treadmill. No forward movement means no airflow over the wings so no lift.
Another way to look at it is this. Take the wings off an airplane and no matter how fast it travels down the road, it'll never fly because the wings are what provide the lift.
The rocket comparison is a different matter. A rocket doesn't rely on wings to provide it's lift. It relies on pure power to weight ratio to gain enough force to overcome gravity and lift it off the ground.
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Post by swanrail on Feb 13, 2008 0:04:27 GMT 1
The weight has got to remain the same as shown on the scales. Although one would think that the air being pushed down by the rotor blades might increase the weight, there is an equal amount of air circulating upwards and pushing the lid up to exactly counterbalance the downward effect!!! p.s. another example of wings not being used to lift, is of course the Harrier jump jet. Many moons ago, I took a group of apprentices (remember when they still existed??) to Bedford, where we saw the original "flying bedstead" (and it was!!). also while we there we saw the prototype scale models for the Concorde, the wind tunnel tests were in fact carried out by the model being taken up a tall pair of steps and being launched as per a glider!!! From this an initial design was initiated and full blown proper wind tunnel testing carried out!!! Wonder whatever happened to those dozen or so tests models, and the flying bedstead??
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Post by nemesis on Feb 13, 2008 0:12:34 GMT 1
Ooooh kay. The helicopter question, lol.......
The helicopter is going to need to produce a downward air pressure of at least it's own weight to hold it in a hover so, assuming the box didn't swing around alot, I would say the weight shown would have to remain at 10kg or above, although 8kg of that reading would actually be force pushing down on the box rather than actual weight.
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Post by nemesis on Feb 13, 2008 0:16:43 GMT 1
They're prolly in bits, Swanrail. Same as all the excellent models used in the making of the Battle of Britain. Only a couple of those models survived. Your harrier comparison is a cross between an aircraft and a plane. Vertical takeoff is, like the rocket, achieved by pure thrust, the wings are on the whole redundant at that point until it begins forward motion. This is a cool thread. Keep em comin BTW, Napoleon. Did you get my PM's, matey?
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Post by afkmatrix on Feb 13, 2008 0:24:20 GMT 1
Ok I agree that the plane will take off. Basically the power for the aircraft does not come from the wheels, these are free wheeling. Thus the plane will move forward as the engines are pushing it and eventually the plane will gain enough speed/have enough air over the wings to take off. Taking Nemesis't treadmill theory, think of a person on a skate board on a treadmill and his friend is standing next to him. The skateboarder will stay still on the treadmill as the wheels are free wheeling and will just spin faster the faster the treadmill goes. But the friend can still push him forwards and backwards (the friend being the planes engines) So no matter how fast the conveyer belt goes it will just make the plane's wheels spin faster. Here watch this mythbusters video, although their conveyor belt imo is naff lol uk.youtube.com/watch?v=tPFu7shIN3g
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