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plane question

#21
fr0me0 said:
i think whats interesting is how fast would the conveyor have to spin, to put enough friction on the axle's in order to negate the amount of thrust from the plane's engines. I really don't think theres a conveyor fast enough to do that
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it's hypothetical....is there even a conveyor belt big enough


better yet....why would someone try? it's a hypothetical question so whether or not there are things that can to do it are irrevevant....just the fact that it may be possible.....this opens up another can of worms on how the conveyor would work....but that's irrelevant to the question at hand....because it's hypothetical
 
#23
What's turning the wheels? The jet engine, it's not a car, so if the conveyor is keeping up w/ the jet engine ( the wheels), no thrust, no lift, no take off. Keeping up w/ the wheels is keeping up w/ the thrust of the engines.
If I understand right.
 
#24
i like tictacs said:
It will fly.

It will fly because it's pushing with the jet engines, not the wheels.

Doofuses.
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It doesn't matter how much thrust you have available or from what source. If there's no lift, it won't fly. Period. Pushing, pulling, jets, props, whatever. Likewise, if you're at maximum thrust and not moving (just spinning your wheels), you're not going to fly. Period. It's actually a pretty simple question once you clarify what specifically the question is. Regardless, there's a clear answer.
 
#27
ChenNuts44 said:
It doesn't matter how much thrust you have available or from what source. If there's no lift, it won't fly. Period. Pushing, pulling, jets, props, whatever. Likewise, if you're at maximum thrust and not moving (just spinning your wheels), you're not going to fly. Period. It's actually a pretty simple question once you clarify what specifically the question is. Regardless, there's a clear answer.
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But I think the point is with an airplane, if you have thrust, you have movement unless you tie it to something BIG and UNMOVEABLE. The treadmill can match it's forward speed, but that will not stop the plane from moving forward, just double the speed at which the wheels turn. My opinion is the plane will take off, but it will likely need slighly LESS absolute velocity (compared to a stationary point, not compared to the treadmill, we'll call that relative velocity) due to the tredmill's surface tension pulling more air toward the plane (kinda like a headwind...)

The only issue I could see that MIGHT interfere with this would be the chance that the treadmill was pulling more air under the wings than normal which would cause a reduction in lift (some people don't relize that the faster air moves, the LESS dense it is, so pulling MORE air under the wings will actually suck the wings downward. If you want to try it, hang 2 plastic balls from a string about 2 inches apart. Blow air inbetween them and they will move together, not apart) but I don't think this would be a significant amount to stop the plane from taking off.
 
#28
Lift occurs when when the pressure beneath the wing surface exceeds the wind pressure on the top of the wing. Bernoulli's Principle. It will not fly as suggested in the original post. Spinning wheels will just wear out the tires.

It has as much success of flight as travelling against the rotation of the earth would take you back in time.
 
#30
If it's stationary, it doesn't matter how fast the wheels are moving. It's all about airspeed. In your scenario, it's airspeed is still zero.

Another example: A plane taxis into position on the runway, and along comes a headwind of, say, 170 mph, or whatever the necessary airspeed is for takeoff, or V1. Assuming the flaps are set properly, all the pilot would have to do is activate the elevator (the rear horizontal control surfaces). The nose would lift and the plane would "take off." Granted, its relative land speed would be zero, but it would be flying.
 
#31
Essentially, the question is, will the drag from the conveyor belt be enough to make the thrust of the jet engines not enough to make the plane take off.

I'm fairly sure that the answer is no. Many people have already illustrated the main argument; that the sole effect of the of the conveyor belt would be making the wheels spin backwards. That would create trivial amounts of drag on the plane (unless the wheels aren't lubed or something) leaving the jet engines to do their job and thrust the plane into flight.

It is like asking "can a plane fly over a conveyor belt that is matching its groundspeed in reverse". This question makes it obvious that the conveyor belt and the planes flight are completely uncorrelated. The example with landing gear is simply more confusing, I think.

$0.03 dare I say?

~Alex
 
#33
Lift makes an airplane fly. How do you generate lift? Wind over the wings. Ray-man has it right. I've almost done it myself flying in very strong winds. You're sitting at the end of the runway and the airspeed indicator is showing 25 knots and the plane is stationary. The speedometer in an airplane is powered by ram air through a tube, called a Pitot tube. As Ray-man pointed out, if your rotation speed is 70 mph and you have have an 60 mile an hour wind on your nose, your airspeed indicator may show 70mph, but your ground speed will be 10 miles an hour. It's the wind over your wings generates lift.

Think about gliders. No engines on them.
 
#35
SnoBassMan said:
Ok, I have no physics knowledge....
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stop right there. :D :p

aerospace stuff isn't my thing, but gard's right. engines themselves aren't what cause planes to fly--it's relative motion between the planes and the air. the engines just allow for a sufficient rate of motion for flying.

and in any reference frame that's worth a damn, that plane on that hypothetical conveyor belt isn't moving an inch!


next problem!
 
#36
teej said:
I'm skeptical of all these "no" answers. A planes wheels are free-spinning - they aren't linked to the plane, so there is a possibility that with the massive amount of thrust a jet engine can deliver, that the plane would continue forward, while the wheels spin backwards.
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doesn't matter.

if you are running on a treadmill at 10mph and the treadmill is set at 10mph, are you moving forward? i hope not, cuz then you are going to hit that control-dealie at the front of the machine! :p

and wheels, like feet, don't "move" backwards (linearly)--they remain stationary with respect to the ground. the wheels spin backwards, sure, but there isn't any linear motion (unless they are slipping!).

(and if wheels weren't linked to the plane, then how come they don't stay on the ground whenever a plane takes off? that'd just be poor design! :p )
 
#39
Ray-man said:
If it's stationary, it doesn't matter how fast the wheels are moving. It's all about airspeed. In your scenario, it's airspeed is still zero.

Another example: A plane taxis into position on the runway, and along comes a headwind of, say, 170 mph, or whatever the necessary airspeed is for takeoff, or V1. Assuming the flaps are set properly, all the pilot would have to do is activate the elevator (the rear horizontal control surfaces). The nose would lift and the plane would "take off." Granted, its relative land speed would be zero, but it would be flying.
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Correct. This is exactly how birds appear to 'hover' or at least move very slowly, because they're flying into a strong wind and all they need to do is produce enough thrust to overcome the drag from the air passing over its body.

Obviously that 170mph wind thing is a hypothetical situation, that would be very dangerous indeed :/

Don't bother that poor physics teacher.
 
#40
Yep, there's airspeed and then there's groundspeed.

You can actually be in a strong headwind, so that you don't need forward thrust at all to stay in air, and then if the wind picks up so that the headwind exceeds the speed you need to stay stationary, you'd actually be moving backwards when looked from earth - birds sometimes do, although usually birds seem to have urge to move forwards rather than backwards so they start flapping to add some forward airspeed, so you won't see it all that often.

Air is not connected to ground in any ways, so that's why movement of ground doesn't matter. The earth could change spin direction for all the pilots in the air care.
 

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