Can Plane Take Off

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#1
Not so much a picture post, but you guys should have some fun.

This has been all over the web, and has caused a lot of controversy.. and even a few lay offs..

Heres the scenario...

An airplane is on a conveyor. The conveyor is set to match the speed of the airplane in the backwords direction. However fast the plane moves, the conveyor moves just as fast.

Can the plane take off?

(yes, I know the answer to this, but a shocking amount of people do not)
 

Tuco

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#2
So if one were to stand beside the conveyor belt, they could stand next to the plane, and the plane would not be moving, yet the wheels of the plane would be moving at a speed to have the plane take off?

I think the answer is obvious, if you know the ideas of lift.

It"s yes, if the plane is a harrier, or some other plane with V-Thrust, no otherwise, imo.
 
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#4
the plane is not a harrier, or V-take off .. just a normal plane, can range from small to large, or even have a propellor or jet engine.
not giving answer till more polls are in.
 
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#5
no.

the wings are not moving through the air, so there"s no lift.
 
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#7
Amazingly you could argue both ways.

Technically, you cannot use a conveyor to hold a plane stationary that is trying to take off (Not unless you made it so incredibly fast that the friction created by the wheels turning is enough to negate the thrust of the engines. Which would probably be so fast that the rubber would tear apart. or the bearings would melt.)

So yes and no. If the plane truly stood still there would be no vertical thrust generated which would mean the plane could not take off. But then, there is no way you could use a conveyor to hold a plane still, thus it would accelerate nearly normally and take off. (the wheels would just be spinning significantly faster than a normal take off.)
 
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#11
Comes down to what definition of "matching the speed" you use really. Does it match the airspeed or the speed relative a stationary outside observer.
 
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#12
Here is a post that anyone can understand. This question was either created by someone that thought he was being tricky but screwed up or is a trick question to try and get people to focus on something that is irrelevent to the problem.

OK, let"s figure out why the airplane will fly.

Manfred"s in the airplane. Old Hack has the Army-surplus crane fired up and he"s picking up the J-3 and Manfred and carrying them over to Runway 27, which has been transformed into a 3,000-foot conveyor belt. It is a calm day. The conveyor drive is programmed so that if Manfred can start to move in the J-3, if he can generate any airspeed or groundspeed, the conveyor will move toward the east (remember Manfred and the J-3 are facing west) at exactly the speed of the air/groundspeed. Because the wind is calm, if Manfred can generate any indicated airspeed, he will also be generating precisely the same groundspeed. Groundspeed, of course being relative to the ground of the airport surrounding the conveyor belt runway. So, the speed of the conveyor belt eastbound will be the same as Manfred"s indicated airspeed, westbound.

Manfred does his prestart checklist, holds the heel brakes, hits the starter and the little Continental up front clatters to life. Oil pressure comes up and stabilizes and Manfred tries to look busy because the eyes of the world are upon him, but all he can do is make sure the fuel is on and the altimeter and trim are set, then do a quick runup to check the mags and the carb heat. He moves the controls through their full travel and glares at the ailerons, doing his best to look heroic, then holds the stick aft in the slipstream to pin the tail and lets go of the brakes.

Baron of the Belt

So far the J-3 has not moved, nor has the conveyor. At idle power, there"s not enough thrust to move the J-3 forward on a level surface, so Manfred starts to bring up the power, intending to take off. The propeller rpm increases and the prop shoves air aft, as it does on every takeoff, causing the airplane to move forward through the air, and as a consequence, forward with regard to the ground. Simultaneously the conveyor creaks to life, moving east, under the tires of the J-3. As the J-3 thrusts its way through the air, driven by its propeller, the airspeed indicator comes off the peg at about 10 mph. At that moment the conveyor is moving at 10 mph to the east and the tires are whirling around at 20 mph because the prop has pulled it to an airspeed, and groundspeed, of 10 mph, westbound. The airplane is moving relative to the still air and the ground at 10 mph, but with regard to the conveyor, which is going the other way at 10 mph, the relative speed is 20 mph.

Manfred relaxes a bit because the conveyor cannot stop him from moving forward. There is nothing on the airplane that pushes against the ground or the conveyor in order for it to accelerate; as Karen -- one of our techies here at the Lounge -- put it, the airplane freewheels. In technical terms, there is some bearing drag on the wheels, but it"s under 40 pounds, and the engine has overcome that for years; plus the drag doesn"t increase significantly as the wheel speed increases. Unless Manfred applies the brakes, the conveyor cannot affect the rate at which the airplane accelerates.

A few moments later, the roaring Continental, spinning that wooden Sensenich prop, has accelerated the J-3 and Manfred to 25 mph indicated airspeed. He and the airplane are cruising past the cheering spectators at 25 mph, while the conveyor has accelerated to 25 mph eastbound, yet it still has no way of stopping the airplane"s movement through the air. The wheels are spinning at 50 mph, so the noise level is a little high, but otherwise, the J-3 is making a normal, calm-wind takeoff.

As the indicated airspeed passes 45 mph, groundspeed -- you know, relative to where all those spectators are standing beside the conveyor belt -- is also 45 mph. (At least that"s what it says on Manfred"s GPS. Being brought back to life seemed to create an insatiable desire for electronic stuff.) The conveyor is also at 45 mph, and the wheels are whizzing around at 90 -- the groundspeed plus the speed of the conveyor in the opposite direction.

Manfred breaks ground, climbs a few hundred feet, then makes a low pass to see if he can terrify the spectators because they are Americans, descendants of those who defeated his countrymen back in 1918.

It"s All About Airspeed


(Don"t try this at home!)
(Don"t try this at home!)

While the speed of the conveyor belt in the opposite direction is superficially attractive in saying the airplane cannot accelerate, it truly is irrelevant to what is happening with the airplane, because the medium on which it is acting is the air. The only time it could be a problem is if the wheel speed got so high that the tires blew out.

Put another way, consider the problem with the J-3 mounted on a hovercraft body (yes, similar things were tried about 30 years ago). The hovercraft lifts the airplane a fraction of an inch above the conveyor belt, and so no matter how fast the conveyor spins, it cannot prevent the propeller -- acting on the air -- from accelerating the airplane to takeoff speed. It"s the same with wheels rolling on the conveyor belt. Those wheels are not powered and thus do not push against the belt to accelerate the airplane. Were that the case, the vehicle could not reach an airspeed needed to fly, because then the conveyor, the medium acted upon by the propulsive force, would be able to negate the acceleration relative to the air and ground.

I"m reminded of the New York Times editorial when Robert Goddard"s rocket experiments were first being publicized. The author of the editorial said that rockets can"t work in space because they have nothing to push against. It was laughably wrong, ignoring one of Sir Isaac"s laws of physics that for every action there is an equal and opposite reaction. Here the propeller is pushing against the air, as it does every time an airplane takes off. How fast the airplane is moving over the surface on which its wheels rest is irrelevant; the medium is the magic. On a normal takeoff -- no conveyor involved -- if there is a 20 mph headwind, Manfred and the J-3 will lift off at 45 mph indicated airspeed; but relative to the ground, it is only 25 mph. Should the wind increase to 45 mph and if Manfred can get to the runway, he can take off without rolling an inch. His airspeed is 45 and groundspeed is zero. It is not necessary to have any groundspeed to fly, just airspeed. Conversely, if Manfred has a lot of runway and nothing to hit, and takes off downwind in a 25 mph tailwind, the propeller will have to accelerate the airplane to a zero airspeed, which will be a 25 mph groundspeed, and then on to a 45 mph airspeed, which will have him humming across the ground at 70 mph. The speed over the ground, or a conveyor belt, when an airplane takes off is irrelevant; all that matters is its speed through the air, and unless the pilot sets the brakes, a moving conveyor belt -- under the freely turning wheels -- cannot stop the process of acceleration.

Things eventually calmed down as the number of "it won"t fly" folks dwindled as they began to understand that the airplane would take off. Old Hack looked at me and suggested we depart as the few holdouts showed no sign of changing their position. So, we headed out into the night to watch the guys take the conveyor out and reinstall the runway.
 

qxx_foh

shitlord
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#13
Come on, the idea of the converyer belt is to state its properties and we aren"t supposed to be taking its chemical and physical composition in mind. It"s the abilities of a converyer belt we"re after. Jesus. And, this is the most retarded "tricky but obvious" question I"ve ever read.
 
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#14
Those wheels are not powered and thus do not push against the belt to accelerate the airplane. Were that the case, the vehicle could not reach an airspeed needed to fly, because then the conveyor, the medium acted upon by the propulsive force, would be able to negate the acceleration relative to the air and ground.
Without knowing this, you can"t answer the question. This isn"t obvious.
 
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#15
You have to ask yourself what the point of the question was.

Was it to demonstrate to people not knowing how lifts and forces work that jets and props do not generate any vertical lift by themselves and thus a plane not moving relative to the air will not fly no matter how fast the props spin or how hard the jets blow? (though I am sure some small amount of vertical lift is generated by moving all that air around, but still not enough to fly?

Or was it a trick question to see how many people actually think a conveyor belt could hold a plane still relative to the ground interacting only with freely rotating wheels?
 
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#16
well sadly, I dont know the origin of the question since its so wide spread.

However, most people tend to belive (as the poll is starting to reflect) that a conveyor can be built that is able to hold a plane in place.

Being on wheels, a plane is technically "always" on a conveyor. The wheel speed has nothing to do with the forward momentum. (as they have no gears and spin freely)

So the plane will simply accellerate forward despite the wheel speed untill its moving fast enough to take off.

The whole conveyor thing is nothing more than an illusion.
 
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#17
the plane will take off

say the plane moves at 250 miles per hour at takeoff. it"s using jet engines which generate thrust against the non-moving air around it

on a normal (non-conveyor belt) runway, the wheels will spin at the speed of the plane (250 mph) and the plane will accelerate forward up to 250 mph until it achieves sufficient foward speed to take off

if the runway beneath it moves backward at 250 mph, the plane"s wheels will simply spin at 500 miles per hour, because the plane is creating 250 mph forward thrust against the air. The conveyor belt is not producing thrust against against the plane or the air, but only against the wheels, which (assuming little or no friction) spin on their bearings twice as fast because of the two competing forces. The plane however, doesn"t rely on friction created by the wheels with the ground to create thrust, but instead creates forward thrust because of the friction caused by forcing air at extremely high pressure through it"s engines (in the case of a propeller) or in the case of a jet it creates thrust by Newton"s third law of action and reaction. A gas, or working fluid, is accelerated by the engine, and the reaction to this acceleration produces a force on the engine pushing or accelerating it forward through the air.

so if you could stand right outside the plane (ignoring jet thrust, the conveyer belt, etc), the plane would appear to move forward at 250 MPH and take off. as it took off, the wheels would spin at 500 mph.

this of course all ignores the friction on the highly vulnerable surfaces of the wheel and the bearings inside it, which is obviously impoosible, i.e. trick question
 
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#18
LISeru said:
Without knowing this, you can"t answer the question. This isn"t obvious.
The wheels don"t have to be powered, and no thrust needs to be directed toward them.


Newton"s First Law said:
A body at rest remains at rest and a body in motion continues to move until acted upon by an external force.
Newton"s Second Law said:
A force acting on a body gives it an acceleration which is in the direction of the force and has a magnitude inversely proportional to the mass of the body.
Newton"s Third Law said:
Whenever a body exerts a force on another body, the latter exerts a force of equal magnitude and opposite direction on the former.
Aye, there is no need to consider the plane"s chemical composition. The answer to this question lies in these three universal laws.

The result would be the same with either a jet or prop plane. The engine or propeller pushes against theair, not the conveyor belt. As a result, the air pushes back against the airplane (Newton"s Third Law).

Due to the shape and design of the plane, this creates vertical thrust, despite the plane"s standing completely still.

There is a caveat here in that the plane"s lifting off would require a higher terminal air velocity than if it were moving. Anyone who has ever flown understands why. When a jet is taking off, the first thing that the pilot does is burn the engines to create an initial vertical lift. Some runway is needed to lift off because the breaks on the landing gear are not strong enough to absorb the force required for take off.

In this case, the only caveat to taking off with both horizontal and vertical acceleration without eating up any runway or coveyor is the conveyor"s ability to negate the plane"s movement relative to the ground. The reason for the higher required terminal air velocity is that energy and momentum being transfered from the air to the plane is, in turn, being transfered from the plane to the conveyor in some small fractions, and that energy is being used.

To contrast, energy transfered to the ground would be much more dampened by the Earth"s mass if the conveyor belt were not there, and thus without the belt, the system would not bleed energy the way that this one does.

The important fact here, though, is that the plane is still being pushed on by the air.

The actual math for this is fairly non-linear, as the mass of the plane is changing along with everything else.

A scaled down model of this experiment would make an awesome toy IMHO.
 
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#19
Galiem, There are a few little videos of this.. i"ll try to find a link if I can hunt it down.

I been talking to a few of the guys from mythbusters and trying to get them to do this myth on air.

Its a rather heated topic over there. In fact, it was so big they removed it all together from the fan club site. Only available to the members and science guys atm. .. fortunately, im one of them
 
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#20
TheWand said:
Galiem, There are a few little videos of this.. i"ll try to find a link if I can hunt it down.

I been talking to a few of the guys from mythbusters and trying to get them to do this myth on air.

Its a rather heated topic over there. In fact, it was so big they removed it all together from the fan club site. Only available to the members and science guys atm. .. fortunately, im one of them
It"s a fairly interesting experiment, to be honest. I"ve made conjectures based on Newton"s Laws, but the escence of physics lies in systems behaving differently than our intuition would lead us to expect.

Since I don"t know enough about planes to sketch the math for this, or even a realistic enough free body diagram, I"m left only with the laws themselves.

I would absolutely love to see those guys tackle this one, even if their budget only allowed a scaled down version of the experiment. The principles would be the same with a model plane, I"d assume, so long as all the plane"s thrust originates from jets or props.