Long rod question

Grrrrr said:
Why does this make a difference?

It honestly feels like you're all in on a joke to make me look silly. I struggle to believe that so many of you believe the plane would take off. It beggars belief really!
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Its makes all the difference.

The plane pulls against the air (or pushes if its a jet), it doesn't use its wheels to push against the ground to generate motion.
 
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EffBee said:
No, you've missed the point - its about relative speed of plane to treadmill surface and treadmill surface to surrounding environment NOT effect of treadmill on the tyres of the vehice.
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OK, how about this...

A car and a plane are on a smooth frictionless surface. Both start their engines.

The car doesn't move, because no matter how much force it applies to the wheels, they can't grip the surface and the wheels just slide.

The plane's engines apply force to the air, not the ground, so the plane has something to push against. It moves forwards. It generates lift. It takes off.

Now, let's consider simulating the smooth frictionless surface with a treadmill...
 
Ok i think i get what you guys are saying (not that i think you are right).

A jet engine works by 'pulling' air through the engine and spitting it out the back at high speed. You are all saying this movement of the air generates the lift?
 
Dr.EM said:
Can I just be really annoying and throw this conundrum in :D. A HELICOPTER on a horizontal platform spinning the heli in the opposite direction and at matched RPM of it's blades. Will it take off?

(please, I hope everyone has figured out the answer after the plane one, but you never know!)
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Well sounds to me like the blades are standing still relative to the air mass and the helicopter body is spinning furiously - so no airflow over the rotor blades so no lift so no takeoff.
 
Grrrrr said:
Ok i think i get what you guys are saying (not that i think you are right).

A jet engine works by 'pulling' air through the engine and spitting it out the back at high speed. You are all saying this movement of the air generates the lift?
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no

the movement of the air makes it go forwards but not up. going forwards makes it go up (when wings hit the air)

you just need to see that the treadmill cannot stop the plane moving forwards no matter what speed the treadmill is spinning at.

to do this i advise holding a skateboard/roller skate on a treadmill and turning it on.
 
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Grrrrr said:
Ok i think i get what you guys are saying (not that i think you are right).

A jet engine works by 'pulling' air through the engine and spitting it out the back at high speed. You are all saying this movement of the air generates the lift?
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That is precisely and exactly not what I am saying.
 
Do you all agree that air needs to pass over the wings to cause the plane to take off?

Let's do this step by step. Yes or no to the above
 
Dr.EM said:
Can I just be really annoying and throw this conundrum in :D. A HELICOPTER on a horizontal platform spinning the heli in the opposite direction and at matched RPM of it's blades. Will it take off?

(please, I hope everyone has figured out the answer after the plane one, but you never know!)
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No, because the wind is moving over the blades in the wrong direction. Helicopter blades only work in one direction, reverse the airflow over them and you're not going to have a very fun day.

Grrrrr said:
but because the treadmill is counteracting the 'motion' it doesn't actually move. Therefore no lift
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Aeroplane wheels are free-wheeling, unless someone puts the brakes on.
 
EffBee said:
Well sounds to me like the blades are standing still relative to the air mass and the helicopter body is spinning furiously - so no airflow over the rotor blades so no lift so no takeoff.
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At matched RPM. If the body is doing 2000 RPM and the blades are doing 2000RPM in the opposite direction, it would still lift. (The body doesn't negative lift).
 
Katoom said:
At matched RPM. If the body is doing 2000 RPM and the blades are doing 2000RPM in the opposite direction, it would still lift. (The body doesn't negative lift).
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No it wouldn't because the blades would bit hitting the air on the reverse edge, helicopter blades only work in one direction, like plane wings really.
 
I would have though the speed would be governed by the elasticity of the metal. Seeing as the bonds in the metal (I assume its steel) are rigid, then it will be instant. It probably takes longer for the light to hit your eyes.
 
Katoom said:
At matched RPM. If the body is doing 2000 RPM and the blades are doing 2000RPM in the opposite direction, it would still lift. (The body doesn't negative lift).
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It's all relative. The blades need to be 2000 rpm relative to the air mass its sitting in. If the body is spinning one way at 2000 rpm, and the blades are spinning the other way at 2000 rpm, relative to the air mass the blades are not moving, though they are moving at 2000 rpm relative to the helicopter body. If the blades were 4000 rpm in the opposite direction to the body then you would get lift off.
 
Ok i might have to eat my pride and say i might be coming round to what you are saying...

If a car had wings and you stuck it on a treadmill it wouldn't take off, there would be no movement relative to the air. The comparable experiment with plane would be sticking it in a wind tunnel with the wind blowing from behind, in this case also there would be no movement relative to the air.

I think
 
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