Plane on a conveyor belt
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I just thought of an explanation/analogy, let me know what you guys think of it.
Reading a couple of the posts here, people like to try to think of this in terms of a treadmill, so we will use that.
Imagine standing on a treadmill wearing a pair of rollerblades. Now, someone is standing behind you (off of the treadmill). Turn the treadmill on, and have the person behind you push you forward. Do you start moving forward in relation to the ground? Hopefully, the answer is yes.
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Phats wrote:
For the plane to move forword, the wheels must move forword, which means the tradmill moves the other way.Unless u can kill my theory.
With the assumptions I am making about the wheels of the plane and everything else the conveyor could be actually moving in the direction the plane wants to go or the opposite as in this case at ludicrous speed (or infinite velocity) and the plane would still not move. A body at rest will stay at rest, unless it is acted upon by a force. In my magic frictionless scenario the only force on the plane is its own thrust and I guess of course aerodynamic resistance as it accelerates which is needed to fly.
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Ok, time to put this to rest
Think about it this way if you are driving a motorcycle with a side car on it at 50MPH and the conveyor is ONLY on the side car you can still drive the motorcycle at 50MPH, however the sidecars wheel(s) will be traveling 100MPH. Same principal applies here. The planes motors providing thrust would be the motorcycle in above analogy and the planes wheels would be the side car. It is very possible for the plane to take off.
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I think I know why Kirkeide quit.
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Phats wrote:
So, Dubbsy
Explain too me why the foreword thrust wouldn't move the tires more.Plane has thrust on it's body, pushing the plane a 50mph, the wheels, attached to the plane, spin at 50. The "Treadmill" spins at 50.
The plane has thrust pusing it's body too 100mph, that means the plane needs too acelerate, forewords, because we need too go forwords to generate lift, we all know that.
so, if the plane goes forword, the wheels turn forword, the Treadmill matches the wheels.Which means it matches the plane.
Which means that it doesn't matter what speed the Jets are pushing the plane, the wheels are going that fast too, and as long as they are, the treadmill is matching.
I understand what u are saying, but, it's not possible too me.
If u can explain it, please do.
You're looking at it wrong...
I think there's a very big thing here that everyone is totally looking past, but I can't figure out how to explain it so people will understand it...Imagine that the wheel bearings are 100% efficient and produce no friction. The plane would stay in the very same spot no matter what direction the conveyor moved...forward or back. Now the actual efficiency of the bearing isn't that bad so they are free to roll as they wish, however they will drag the plan with them a bit... BUT, it's not enough to keep the plane in one spot.
look at chucks analogy because it just about perfectly describes this..
if the motorcycle has a speed of 50mph, the conveyor is spinning 50mph the opposite direction. The wheel on the side car however is spinning at the combined speeds of the conveyor and the motorcycle...
Now to bring friction into this.. the motorcycle will have to work a bit harder to reach 50mph than if they were on a static surface because they have to fight the little bit of friction in the side-car wheel (although, I did read that the friction in the wheel bearing doesn't change from 50 to 100mph (or any other speed), so this may not be applicable).now throw that over to the plane. The Jet engines are the motorcycle.. The motorcycle pushes the side-car, the jets push the plane. The motorcycle force is acting on static ground while the jets are acting on the static air above the conveyor.
The only time that thrust is applied AND the speed of the plane is zero is when the brakes are applied. **It defies the limits of the question **to say that the conveyor is moving and the plane is staying in one spot, because if the plane is sitting in one spot then it's speed is zero. The question says the conveyor matches the plane's speed, so therefore the conveyor would not move. If neither are moving, then what's holding the plane in one spot with thrust applied? brakes?
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I win.
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Sweet-WRX-Lovin wrote:
I think I know why Kirkeide quit.No, I think she actually liked me Schulzy. The school board on the other hand.......:rolleyes:
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I in no way meant you. Too bad they couldn't work something out, she was the best.
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Everyone who have posted on here are right, unless there are proves to prove them wrong.
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I CAN prove you wrong and did with my analogy. Its not much different than if a plane flew over a moving conveyor belt that was moving the opposite direction at the same speed as the plane...only difference would be that the wheels aren't touching the conveyor belt. You could do a touch-and-go with a plane on a moving conveyor belt as well.
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Nobody gave me feedback on my rollerblades on treadmill analogy, it must not be very good
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dubbsy wrote:
Mitch, if I"m reading this right...everything about it is wrong...#1. Air travels faster over the tops of wings.. This creates a low pressure that in turns sucks the wing up (bring the plane up with it)
#2. The plane DOES have to physically move. The engines do not move enough air around the plane to create lift. ...but no matter what, when thrust is applied, the plane will move in relative space and the plane will take off just like there was no conveyor there in the first place.
alright hang on a second... the shape of a plane's wings are such that air moves over the top and accelerates downward AFTER it crosses the back right? so doesnt the air move more quickly under the bottom? it just accelerates as the air washes over tail end of the wing?
thats how i understood it?
now for shits and giggles... how does tunneling the air through a series of passages under a car hold it to the ground, because that would mean air traves more quickly over the top and would lift the car into the low pressure area created wouldnt it?
if i have this backwards id really like to know
somebody speak up cause im confused now -
Quit it guys. All this thinking hurts. I actually believe that the plane would take off and even remove itself from the conveyor because the thrust from the turbines alone would overcome the resistance of the conveyor itself. I don't believe it would take off from the very spot it was on though. Just that the conveyor wouldn't be able to keep up with the thrust.
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What a silly question. It wouldn't take off, it can't. As a few people said before, air is needed under the wings. The movement of the wheels is inconsequential and in truth the only reason we have wheels on a jet plane is because we have yet to find something better..
Jet engines are pointed backwards so downward thrust is nill. Notice that rockets point down...
As for the car/wind tunnel scenario, faster air makes for lower pressure according to simple physics. So yes, if the bottom of your car were completely flat then it would create upward lift. This is the reason for ground effects and air diffusors, as well as air spoilers (wings, lips, scoops). The reason why dragsters have a giant upsidedown wing and a front spoiler. The aero effect doesn't occur until excessive speeds that many cars will never reach.
2005 Mercedes-Benz C240 4Matic
1993 Mazda Rx-7 Twin Turbo (sold)
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you guys are looking at this all wrong. Airplanes do not accelerate using their wheels, they are used as a way to reduce friction (as opposed to dragging the belly of the plane) with the ground until the airplane has reached the speed in which it will lift. The planes jet engines/propellers/whatever provide forward thrust to get the plane to the speed at which it will build lift. The airplane doesn't care how fast its WHEELS are being driven in the other direction, just how fast the plane itself is moving. If the plane is traveling 100mph in one direction and the conveyor belt is traveling 100mph the other direction, the only thing affected are the wheels and the wheel bearings (which would be spinning at 200mph) and a tiny amount of friction. As soon as the airplane built enough thrust to counter the gravity/friction holding the plane to the conveyor belt it would start moving forward regardless of what speed the conveyor belt is moving. In other words you guys are reading the question wrong. If the question stated could a stationary airplane generate lift while not moving you would be correct, however the question asks what happens if it is placed on a belt that is moving the opposite direction of the plane at the same pace as the plane. Again, see my sidecar analogy that I posted earlier in this thread. Exact same principal....screw it, I'll post it again:
Think about it this way if you are driving a motorcycle with a side car on it at 50MPH and the conveyor is ONLY on the side car you can still drive the motorcycle at 50MPH, however the sidecars wheel(s) will be traveling 100MPH. Same principal applies here. The planes motors providing thrust would be the motorcycle in above analogy and the planes wheels would be the side car. It is very possible for the plane to take off.*
By your guys theory, the motorcycle in the above situation is not capable of moving forward either.
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AcesHigh wrote:
As for the car/wind tunnel scenario, faster air makes for lower pressure according to simple physics. So yes, if the bottom of your car were completely flat then it would create upward lift. This is the reason for ground effects and air diffusors, as well as air spoilers (wings, lips, scoops). The reason why dragsters have a giant upsidedown wing and a front spoiler. The aero effect doesn't occur until excessive speeds that many cars will never reach.see.. what you are saying, as well as I am saying.. is that the air moves more quickly under the wing to create lift, correct? That is exactly why diffusers ect are used on cars. That is why the top of the wing is shaped and the bottom is flat? the air washing over the tail accelerates and creates lift?
the plane could take off and in theory if wind was moving fast enough (either by nature or by engines) it would lift off the ground standing still regardless of what two little round pieces of rubber are doing below it. Chuck how does a a harrier (sp?) take off? it creates enough thrust or lift downward to lift it off the ground. In theory, a plane could lift off the ground if air were moving quickly enough?
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tjamz wrote:
you guys are looking at this all wrong. Airplanes do not accelerate using their wheels, they are used as a way to reduce friction (as opposed to dragging the belly of the plane) with the ground until the airplane has reached the speed in which it will lift. The planes jet engines/propellers/whatever provide forward thrust to get the plane to the speed at which it will build lift. The airplane doesn't care how fast its WHEELS are being driven in the other direction, just how fast the plane itself is moving. If the plane is traveling 100mph in one direction and the conveyor belt is traveling 100mph the other direction, the only thing affected are the wheels and the wheel bearings (which would be spinning at 200mph) and a tiny amount of friction. As soon as the airplane built enough thrust to counter the gravity/friction holding the plane to the conveyor belt it would start moving forward regardless of what speed the conveyor belt is moving. In other words you guys are reading the question wrong. If the question stated could a stationary airplane generate lift while not moving you would be correct, however the question asks what happens if it is placed on a belt that is moving the opposite direction of the plane at the same pace as the plane. Again, see my sidecar analogy that I posted earlier in this thread. Exact same principal....screw it, I'll post it again:Think about it this way if you are driving a motorcycle with a side car on it at 50MPH and the conveyor is ONLY on the side car you can still drive the motorcycle at 50MPH, however the sidecars wheel(s) will be traveling 100MPH. Same principal applies here. The planes motors providing thrust would be the motorcycle in above analogy and the planes wheels would be the side car. It is very possible for the plane to take off.*
By your guys theory, the motorcycle in the above situation is not capable of moving forward either.
You are my hero, excellent explanation. PPl don't understand that the plane would still move.
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