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#5 - theres no debate the power doesnt come from the wheels but fro…  [+] (33 replies) 07/22/2016 on Plane almost stops mid air... +50
#59 - erikths (07/23/2016) [-]
How does a plane fly?
Simple. There are four factors to consider when manouvering a plane.
1. Lift. the force pushing the plane up
2. Gravity. The force pulling the plane down.
3. Thrust. The force pushing the plane forwards
4. And drag. The force that slows the plane down

Now the lift is generated by air moving across the wings. The wings are shaped so that the air travelling above the wing has a longer way to go than the air going below the wing. This causes the air above the wing to get thinner. The air above the wing now has a lower pressure than the air below the wing. And high pressure will always move towards low pressure, but the wing is between the high pressure and the low pressure, so the wing itself will be pushed towards the lower pressure by the higher pressure.

The conclution is that you need a certain amount of air to pass the wing for the plane to take off.

pilots have two speeds to refer to when flying: Groundspeed and Airspeed. The airspeed is the planes speed relative to the air. Ground speed is the planes speed relative to the ground. It is iportant to refer to the airspeed when flying a plane, cause the air moves relative to the ground, and if you have tailwind and flying at low speeds following the groundspeed would cause the plane to stall eventually
TL;DR planes cant take off from a treadmill as long as the windspeed is lower than the planes minimum airspeed.
#34 - speedosnake (07/23/2016) [-]
The rules of the scenario prevent the plane fuselage from ever moving. Since the speed of the treadmill must match the speed of the tires, the plane could never accelerate forward, as that would require the wheel speed to exceed the treadmill speed. assuming that our reference point for "speed" is separate from both the plane and the treadmill The force being added by the propeller is meaningless because of this fact. The wheels/treadmill would just accelerate to infinite as soon as the prop began creating thrust. The only factor that could make the plane take off would be wind strong enough to lift it in place.
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#37 - oikake (07/23/2016) [-]
but even though the wheels are touching the ground the force of the plane is still technically pushing it though the air, the wheels are purely there as "training wheels" to protect the fuselage and allowed to free wheel, this means that even if on a treadmill the only thing the plane has to do is provide enough force to move its mass, regardless of treadmill speed...

stop thinking of the wheels are a point of acceleration and speed, thats what causes most confusion
#39 - speedosnake (07/23/2016) [-]
How are you defining speed though? If you choose to put the reference point the point which we define as stationary, and base all of our relative information on the ground (typically thought of as unmoving), the plane could never travel in any direction without the wheel speed and treadmill speed going out of sync. no shit the wheels don't power the plane, they freewheel on their hubs. We are on the same page there. They can freewheel all they want so long as their speed remains the same as the treadmill's. How exactly can the plane move forward without forcing the wheels to accelerate proportionately to the acceleration of the plane, and therefore disproportionately to the runway?
#42 - anon (07/23/2016) [-]
Hi, PIlot here. A plance moving on a treadmill is the exact same as sitting on the ground with your brakes on and your prop going full blast. The prop doesn't produce enough movement of air over the wings to generate lift, so you would never become airborn unless the plane was moving THROUGH the air. Its like trying to power a sailboat with a blow drier that's attached to the boat.
#49 - anon (07/23/2016) [-]
What the hell are you talking about? plane wheels are just for easy frictionless contact with the ground prior to liftoff. the rotational friction of the planes wheel bearings is waaaaaay to low to be the problem. Wheels will freewheel and while YES, initially the prop will just counteract the opposite movement caused by the treadmill (little to no friction is still friction so a plane would move back initially with the engine turned off.) but as soon as prop thrust would negate this tiny tiny friction it would take off normally because IT WOULD START TO MOVE FORWARD EVEN THOUGH THE "GROUND" MOVES BACKWARD. the wheels would have to rotate faster than normal but wheel hub friction is NOTHING compared to prop thrust. they are just freewheeling there. Do this: take your bicycle, put in on a treadmill and see how "hard" it is to hold it while it's freewheeling. it's next to zero force HEEDED TO HOLD IT ONCE IT STARTS TO GO AND YOU GAIN STABLE BALANCE. It would take off just fine. the wheels have nothing to do with the plane taking off. Neither does actual ground. Basic physics.

Besides mythbusters did exactly that, the exact myth and they busted it to oblivion in small scale and a large scale with an actual plane.

You simply (as most) misinterpret the problem itself. I'll make this esier to understand: the way the GROUND moves IS NOT important. it's the air movement what's important. and plane prop moves a plane IN AIR using AIR. ground has nothing, NOTHING to do with that.

Sorry, I have to say this, but if you're really a pilot and you have succh poor understanding of why planes fly I just hope I don't live in an area where you fly.
#50 - anon (07/23/2016) [-]
To be fair to the prior anon, the pilot in that very episode of Mythbusters said that he himself didn't think that the plane would fly. It's extremely unlikely that the prior anon has had to take off from a moving platform, so I think his misunderstanding here is more about a lack of physics intuition rather than a lack of flight skills.
#36 - sentinyl (07/23/2016) [-]
You would be right if there were any treadmill that could accelerate infinitely fast to infinity and magically knew how to match the plane's wheels speed, but the reason the plane won't be able to take off isn't because planes can't take off from treadmills, it's because this unholy monster of a treadmill you hypothetically created would just melt the wheels to slag almost instantly and then do god-knows-what with the rest of the plane once the wheels are destroyed beyond all recognition. In real life planes can take off from treadmills no problem because they're based off of real life physics and not some random fucker's imagination.
#38 - speedosnake (07/23/2016) [-]
The rules are the limitation here just as much as the physics of the devices described. In order for the treadmill speed and wheel speeds to remain identical, that is what would have to happen. They never explain how any of it would work in the situation described. Obviously friction, heat, and technological limitations would prevent the experiment from ever being properly carried out in real life. for example, the mythbusters experiment doesn't follow the rules because the wheel speed of the plane accelerates disproportionately to that of the treadmill If we choose to place our reference point for "speed" on the plane however, it would be able to take off just fine without violating any of the rules, regardless of the treadmill speed...
#58 - anon (07/23/2016) [-]
Think of it this way. You have a wheel attached to a stick, say, and you're standing next to a treadmill with the stick in your hand and the wheel on the treadmill. The wheel is spinning. Now, you walk forward, and the wheel stick moves with you. This is because your walking is providing the impetus for motion, not the wheel on the stick. The same thing with the plane, except the impetus is the spinning prop. Wheels do not affect forward motion whatsoever except to hinder it slightly due to friction. Emphasis on slightly. And here's another emphasis on slightly.
#41 - sentinyl (07/23/2016) [-]
Your reference point doesn't matter, Newtonian physics doesn't give a single shit if you're looking at the plane or at the treadmill or the ground or a dog's anus halfway around the world. Planes have no problems taking off from treadmills because no treadmill can move fast enough for the force of friction on the wheel hubs to completely counteract the maximum thrust the engine can put out, especially not without destroying the plane, If the question is "can planes take off from treadmills" the answer is yes, if the question is "can a hypothetical plane take off from a hypothetical impossible treadmill that has a very specific set of hypothetical impossible qualities that make it impossible for planes to take off from it" then shit you got me. Planes cannot take off from hypothetical impossible treadmills that have a very specific set of hypothetical impossible qualities that make it impossible for planes to take off from it. It's very similar to asking "can a man lift a 100kg weight if the constant of gravity in our universe were high enough that no man could ever possibly lift a 100kg weight?" The answer is no because the question is designed so the answer can't be anything other than no, there's nothing to think about at all.
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#81 - lean (07/25/2016) [-]
Thrust from prop > rolling friction of tires. Simple as that. Speed of treadmill is irrelevant.

In an example, a 1500 kg car with .5 meter dia tires at 40lbs pressure on asphalt has 441 Newtons of rolling friction at 100 mph. This does increase along a parabolic curve, but in practical purposes you are going to have to let the plane sit on the treadmill until it reaches the neighborhood of tens of thousands mph before the prop thrust produces less force than the rolling friction.

For all practical applications, a moving track is irrelevant to plane takeoff. Mythbusters did it.
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#21 - therealtjthemedic (07/23/2016) [-]
the propeller moving doesn't make the plane fly, buddy
it's the air going past the wings
if it's on a treadmill, it's sitting still relative to the air
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#51 - spacexplain (07/23/2016) [-]

The prop does provide the thrust. It's not just some fan that blows on the wings for a bit. According to that logic, pusher planes like the Wright Brothers prototype wouldn't work.
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#57 - therealtjthemedic (07/23/2016) [-]
if the plane is on a treadmill, then it's not moving relative to the air
if you stood next to the treadmill you'd see a plane with the propeller on full not moving at all
no air would run over the wings
the wings would provide no lift
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#82 - lean (07/25/2016) [-]
Thrust > rolling tire friction bud.
The thrust will pull it forward regardless of the track speed.
#62 - anon (07/23/2016) [-]
Let's analyze your assumption. Why do you think the plane will not move if it's on a treadmill? You know the impetus for forward motion is not provided by the wheels, but by the prop creating thrust. You know the prop does not interact with the ground. You know the wheels are not powered. You know (or should know, at least) that the prop's thrust is strong enough to overcome any friction generated by the wheels unless the brakes are on (in a traditional prop plane, at least). As seen in the GIF, you also know air speed and ground speed are not necessarily the same thing. So, why do you assume the plane will not move if it's on a treadmill?
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#63 - therealtjthemedic (07/23/2016) [-]
Because, the air speed is zero.
The plane is not moving relative to the air if it's just on a treadmill moving forwards as the treadmill moves backwards..
That's it.
#65 - anon (07/23/2016) [-]
You keep saying the same thing. If the plane is not moving if it's on a treadmill, obviously its airspeed is zero. I ask, "Why do you think the plane will not move if it's on a treadmill?" and you respond with, "The plane is not moving relative to the air if it's just on a treadmill moving forwards as the treadmill moves backwards.." I understand that's your assumption. I want you to tell me why.
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#66 - therealtjthemedic (07/23/2016) [-]
Isn't that the base of the question? Whether a plane on a treadmill going the same speed as the treadmill in the opposite direction could take off?
That's how it was told to me.
#72 - anon (07/23/2016) [-]
I'm specifically looking for what physical laws/phenomena you believe are in effect that prevent the plane from lifting off.

The force that causes the plane to move forward does not depend on its interaction with the ground, unlike a car or bipedal motion. If the treadmill is moving back at a rate of 2v (twice the velocity of the airplane), and the airplane is moving forward at 1v, the wheels will spin at a rate proportional to 3v. The plane will move forward at 1v because the prop is thrusting against the air, not the ground. The air (assuming a calm day) is stationary, unlike the ground. In a car on a treadmill, for example, the car is thrusting against the ground, which means the car's velocity relative to stationary ground will have to be greater than that of the treadmill's in order to achieve motion. The same restrictions don't apply to the plane because the force inducing motion is acting on a different medium.
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#73 - therealtjthemedic (07/23/2016) [-]
To counteract the movement of the ground would require a force from the plane. You're assuming the wheels are frictionless, I guess?
#75 - anon (07/23/2016) [-]
That's what I'm saying: there is a force from the plane, provided by the prop. The wheels are just coasting there.

Think of it this way, in a reverse situation. Let's say the wind is blowing, and you're walking into it. You can do this because your forward motion is provided by your feet interacting with the ground. Sure, you'll be resisting the wind, and the plane will be resisting friction from its wheels (a small amount, but it's there nonetheless), but you can still move forward. Similarly, the thrust provided by a prop--which is acting on the air, not the ground--is easily able to overcome the friction from the spinning wheels.

Here's another example. Ocean currents can be dangerous because they'll sweep you out to sea, and they do this because the swimmer is resisting the same medium that's moving. In walking in the wind and taking off from a treadmill, you/the plane are resisting mediums that are stationary, even while a nearby medium is not.
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#76 - therealtjthemedic (07/23/2016) [-]
The wheels aren't really coasting, though. Consider a plane attempting to move from one spot on the ground to another, it has to apply a constant force due to friction from the wheels.
If you take that reference frame and put it in motion, you'd have the treadmill problem.
Wherin, the plane would have to apply thrust to counteract the motion of the treadmill.
#77 - anon (07/23/2016) [-]
And that constant force is provided by the prop. Friction does not increase with speed. The equation for friction force is F=mu*N, where N is the normal force, the Greek letter mu is the coefficient of friction (determined by the materials interacting with one another), and F is the force created by friction. There is no velocity term in the equation. The force provided by the prop does increase with speed: as the prop spins faster, the force increases. The friction from the wheels is going to be small. When I took flying lessons (back when I was able to afford it), the pilot would pull the plane out himself. There's not a lot of resistance there. So, no matter how fast the wheels are spinning, friction remains constant. Obviously, if the wheels spin much too fast, they can heat up, which can create physical changes, thereby changing the coefficient of friction, but that's beyond the scope of this discussion.
#78 - anon (07/23/2016) [-]
This anon is going to head out. If you still have your doubts, there's probably some Khan Academy-esque video about it.
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#69 - spacexplain (07/23/2016) [-]
The way I understand the situation, the treadmill moves at a constant rate. If this treadmill is going let's say 5mph backward relative to the plane, it could still take off, since it's essentially just simulating a very slight tailwind. The plane still picks up speed and takes off no problem, since the treadmill isn't interacting with the prop, just the freely spinning wheels.
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#70 - therealtjthemedic (07/23/2016) [-]
well yeah, all you have to do is ignore the treadmill and look at the airspeed
if the plane could move faster than the treadmill and have an actual airspeed then definitely, the plane could take off
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#71 - spacexplain (07/23/2016) [-]
Exactly, so as soon as the plane breaks even with the treadmill, the takeoff process is as normal.
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#74 - therealtjthemedic (07/23/2016) [-]
Of course.
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#32 - oikake (07/23/2016) [-]
prop provides the thrust, the wheels have no use other than to allow free wheeling when the prop gets going...
#29 - waffies (07/23/2016) [-]
I guess hypothetically a prop engine does send air over the wings? I mean no plane is designed to generate that much lift that way, the idea is that the props pull forward and that wings pass through the standing air, but TECHNICALLY a super overclocked prop plane COULD do it if it was made of diamonds and didn't rattle itself apart running at ungodly speeds?

I mean its meant to be troll physics but.... KINDA?!? I know i heard the treadmill argument once before i worked on planes, and now im really theorycrafting here...
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#109 - **oikake used "*roll picture*"** **oikake rolled image ** …  [+] (6 replies) 07/22/2016 on roll for your new parents +60
#336 - jacodpwns (07/23/2016) [-]
#257 - xiiisabers (07/22/2016) [-]
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#131 - knoppers (07/22/2016) [-]
to good to be rolled
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