Can the airplane take off?

[BrownTown]

OK, look there is no argument in this thread, we ALL know the right answer, DO NOT post here anymore. It is 100% clear that smack Down is either a TROLL or MENTALLY HANDICAPPED, either way it does not help anyone to keep posting.

 

[Xkaliber]

post

 

[Smilin]

Originally posted by: beansbaxter
Alright, this is NOT a homework question. In another thread on the Anandtech forums, I saw this mentioned deep within a thread but it was never addressed. I am still curious so I ask it here:

You have an airplane on a conveyor belt - when the airplane moves forward, the conveyor matches its speed in reverse. Can the airplane take off?

Yes.

"when the airplane moves forward" ... if it moves forward fast enough that the air moving over the wings generates sufficient lift it will take off.

"conveyor matches its speed in reverse" ... this just means the "ground speed" of the aircraft would be doubled. Ground speed of course has nothing to do with airspeed in this case.

Example:

Airplane has a rotation (liftoff) speed of 100 knots. If the airplane moves forward at 50 knots nothing will happen. If the conveyor is also moving at 50 knots in the opposite direction then the effective ground speed is 100 knots, but the airspeed is unchanged at 50 knots.

If the airplane then accelerates to 100 knots, the ground speed would be 200 knots...but again that's irrelevant. The airplane will lift off because it has achieved 100 knots of *airspeed*, not because it reached 200 knots ground speed (or 100, or 50, or 50,000).

Now... suppose something different. Suppose the airplane does not actually have a propeller but instead is being propelled by it's wheels (stick wings on a car for instance). Now 200 knots of speed measured by wheel rotation would actually be 100 knots of forward wheel movement but offset by 100 knots of the ground/conveyor going the opposite way. the net result would be 0 knots of airspeed and nothing would happen.


I guess in this whole thing it really depends on *where* you are measuring speed... by airflow over the airplane or by the rotation of it's wheels.

 

[Smilin]

By the way ... if anyone is still arguing about this .... you're dumb ;P just thought you should know. It's not a difficult one.


This isn't much harder, but stew on it:


Put an airplane on a treadmill. No brakes, wheels roll free. Put a large fan in front of the airplaine. Blow air at the plane at say 100 knots and roll the treadmill backwards at 100 knots. Spin up the propeller on the plane enough that it remains perfectly stationary against the fan and treadmill.

Does it take off?

Assume the takeoff speed of the plane is <100knots, but remember the plane is effectively stationary due to the fan and the treadmill.

 

[BrownTown]

yeah, it takes off, then gets higher then the fan, then falls back down and crashes into smack Down's head killing him and ending this debate once and for all.

 

[TaylorD]

so, what about an airplane on ice skates on a 45° incline?

 

[BrownTown]

the airplane slides backwards and crushes smack Down to end the thread?

 

[sweeper2]

i don't know... seems odd.. if the tconveyor is moving as fast as the plane is going... the plane will stay stationary ... am I correct on this..? .. so the plane is firing it's engines faster and faster ... but the converyor is moving faster and faster ... the plane is sitting there because of the "free spinning wheels" .. with no air under the wings to create lift... how in the heck can it take off... ??

a plane rolls down a runway roughly around 500 ft or so before it creates enough air speed to create "lift" ... no air speed... no lift... motionless air can not make an air craft take off...

why did i read this thread... uuugh... still trying to comprehend this... the craft is sitting on a conveyor that goes just as fast as the plane... remember.. the conveyor is moving backwards...., the tires are moving forwards.... just like someone running on a treadmill ... the aircraft is going to sit still... no air will be under its wings...

the only way it would try and take off was if the conveyor somehow went slower then the speed of the wheels... then the plane would actually start to move forward... but if it's matched speed for speed ... no way...

 

[sweeper2]

Originally posted by: kpb
That question doesn't make sense. A car which moves it's self foward using the wheels would be stationary relative to the ground if the conveyor was moving in the opposite direction at the same speed as the car is according to the speedometer. This is actually how they do a number of things on cars like testing horsepower and torque or how they figure out estimated mpg.

The problem with a plane is that the movement is generated by the jet engine or propeller and is not dependant on the wheels at all. They just support it and allow it to move foward until lift allows it to take off. So even if the conveyor belt was moving backwords the plane would still end up moving forwards and gaining speed. The wheels would just be turning twice as fast. The plane would either reach then end of the conveyor belt and crash or what ever or take off once it got enough speed. The conveyor belt wouldn't effect things at all really. The type of plane wouldn't change this at all because all planes must generate thier forward motion using an engine and not the wheels or they'd start loosing speed as soon as they took off =)

"the conveyor matches its speed in reverse"

 

[gsellis]

Cliffs - A car applies its drive force to the treadmill, so it stays in place. A plane applies its drive force to the 'air' which is independent of the treadmill. It moves regardless of the treadmill EVEN if the treadmill were to go 2x, 3x of the planes groundspeed.

The bar is this high. If you are not this tall, you cannot ride this ride.

JEEZ FREAKING LOUISE... It just dawned on me and this should be SIMPLE ENOUGH for even Smack Down to understand. Newton's Law of Motion... Simplified, for every action there is an equal and opposite reaction. THAT is how a treadmill will keep a car in place. Because the wheels of a car are on the treadmill, the reaction of the treadmill CANCEL any forward progress of the car. For an airplane, the action is on the AIR, not the treadmill. Since the treadmill does NOT affect the air, it cannot counter that action. The airplane will move in spite of any reaction by the treadmill which it is attempting to react to. It only reacts to what is sees on itself, and not the air.

 

[Gibsons]

Originally posted by: sweeper2
i don't know... seems odd.. if the tconveyor is moving as fast as the plane is going... the plane will stay stationary ... am I correct on this..?

Nope.

 

[Chaotic42]

I'm bored, I'll dive in.

The whole argument that the conveyor belt will impart a force to the airplane that is significant to completely counter the thrust doesn't work because the wheels of the plane are free spinning. Yes, they do have physical contact with the plane, but they are designed in such a way that they can rotate when an angular force is applied to them without affecting the aircraft. You could turn the plane upside down and spin the wheels as fast as you want, either forward or backwards, and the plane would be stationary.

This doesn't work in a car because the engine that powers the car is attached to the wheel. The wheel cannot move on the car without the engine moving (Unless it is in neutral).

Hope this makes sense, I'm about to go to bed. :beer:

 

[hellokeith]

Ha, this thread again! One would think the answer would be "no" since the poor plane hasn't made it off the conveyor belt for as long as this hypothetical situation has been discussed.

There are some friction components which will push against the landing gear arm/assembly and thus be pushing against the forward motion of the plane, but they are quite small and practically negligible.

*ANSWER* - It's all about the tires (materials and design).

Depending on how the conveyor system is setup, specifically:

1. Conveyor is spinning the belt at the same velocity in the opposite direction of as the plane's forward motion, relative to the ground.
2. Conveyor spins the belt up to maximum/infinite velocity in the opposite direction of as the plane's forward motion, relative to the ground.

Airplane tires are not rated for very high speed and will self destruct due to heat. Planes can and have crashed on takeoff due to tire failures.

Assuming case #1, a small prop plane like a Cessna or Piper would likely take off without a tire failure event, because those planes weigh little and need little lift, and the tires would not be spinning faster than their rated design. A heavier plane like a jet requires more lift, but they usually have more expensive tires, so it would basically be an engineering issue as to how much lift the plane needs vs. how fast the tires are spinning before reaching that lift velocity.

Assuming case #2, it's doubtful any plane would make it off the belt in anything less than a fiery mess, and if it did, it would not be able to land safely.

edit: few clarification edits

 

[Smilin]

Originally posted by: smack Down

Originally posted by: dkozloski
It makes no difference what the conveyor belt does. It is irrelevant. The belt can go forward, it can go backward, it can start and stop, it can alternate back and forth, and it makes no difference other than being an annoyance.

That is simpley wrong. By definition if the treadmill is going backwards at the same speed the wheels are turning then plane can't go anywhere.

This is true for a CAR where the force applied by the engine via the wheels is what makes it move.

For an aircraft, the propeller/turbines are providing the force so it can still move regardless of what it's wheels are doing.

Think of a guy on roller skates standing on a backwards rolling conveyor but holding onto a rope that's attached to a wall.. His ground speed measured by the rotation of his roller skate wheels will be whatever speed the conveyor is moving at. Despite this "ground speed" he himself would be stationary relative to the rest of the room. Now take the rope he's holding onto to keep himself still and attach the other end to a rocket instead of a wall....

This all is a very simple concept. I myself have been guilty of not getting something simple because some little portion of it failed to click. SmackDown, if this is the case with you that's cool... no worries, no shame Once the 'click' happens you'll get the very satisfying 'aha!' moment.

However, if you really do get it and are just kinda trolling for arguments that really wouldn't be cool. A lot of people have very nicely tried to explain this and it would be a shame if you were just pulling a trick.

If this thread keeps progressing, please keep in mind that the correct answer has been explained several times so try not to be overly defensive or let any flames erupt because when this does 'click' with you it should be a fun moment, not an embarassing one I also hope that this brief lecture isn't interpreted as talking down to you in any way. The smartest person on the planet (me of course ) has made similar mistakes in the past.

 

[sweeper2]

"Smilin" ... that makes sense.... hmmmm ... never thought of that... !! guess it would move forward... now if it was a car .. it would stay stationary...

damn... good analogy ...!!!

 

[Bill Brasky]

How about this question. When was the last time a highly technical thread made it into hot topics?

Been said a million times, but the plane takes off when the lift overcomes the force of gravity on the plane.

 

[Smilin]

Originally posted by: hellokeith
Airplane tires are not rated for very high speed and will self destruct due to heat. Planes can and have crashed on takeoff due to tire failures.

Aircraft tires are incredibly tough. They are rated for extremely high speeds and designed to withstand very hard impacts and a great deal of friction that results when a stationary tire first impacts the rough concrete of a runway at high speed.

For instance. The aircraft I worked on in the Navy had tires that were filled to THREE HUNDRED psi with nitrogen. Nitrogen was used because the tires would reach unbelievable temperatures and if one did rupture while full of regular air the oxygen would be so compressed and hot it would explode and ignite both the rubber and any magnesium or aluminum components nearby.

Remember, brakes stop the tires but it's the tires that stop the aircraft. If you want to smack a 30,000lb plane down at 1500feet per minute then slow it from 150mph to zero in several hundred yards you are going to have to dissipate a LOT of energy. Aircraft tires are very much up to the task.

They have many many layers of rubber, steel belting and cords. I would speculate if you used a small calibre gun like say a .22 and fired at the tread (not sidewall) it would not penetrate the tire. Even so, the tires only lasted for a handful of landings before the rough concrete of the runway tore inches of diameter off of them...tire changes were frequent and given the cost of the tires, they were always retreaded instead of discarded.

The only tire failure I ever saw: F-18 catapult launched with it's parking brake still set. The mains were ok, but the nose tires buckled under the added weight of the forward pitch and friction. The plane lifted off just fine (the catapult probably didn't even notice). The landing was the problem. The nose tires blew immediately on landing. Since carrier landings require full throttle at touchdown, the shrapnel from the nose gear FODed the engines. Black smoke, 20ft spark streaks, metal fragments and probably a kitchen sink came blowing out the back of that plane. Scary ****** to be near. The turbines never let loose and shot out the side thank God.

 

[Erydin]

The plane will not take off or lift off the ground. An airplane flies because of the "lift" (this being an important word) it creates. "Lift" is created by the air going OVER the wings not under. Airplanes, jets, etc... all have a lift capacity based on their design, not push capacity . Wing curvature: this plus the size of the wing, decides how much lift an aircraft has. Why an aircraft is able to fly is due to the difference in distance that the air going over the top of the wing has compared to the difference of the air going under. Easy test to show this is take a plane piece of paper (light weight printer paper works fine). Hold the piece of paper in your hands so that short side is between your hands and the paper is draping over the top of your hands with the end you are not holding away from you. Blow hard over the TOP of the paper. The end that is draped over the top of your hands will lift itself and "fly" as long as you blow hard enough to keep the difference of the air going over the top of the paper vs. the air under to provide enough lift to counterweight the end of the paper. Our hypothetical plane will not take off since it is not moving on this hypothetical treadmill. But why it does not take off is due to air not going over the top of the wings at a sufficient velocity to create enough lift to get it into the air. If you have a stationary plane that is sitting in the middle of a runway not moving, and some good healthy straight line winds come along... the wind can cause the plane to lift into the air without any movement from the plane itself for this same reason... this does happen even sometimes , so remember those grounding cables are not only to ground the aircraft, it serves to keep the smaller ones on the ground also and where the pilot left them in case of a windstorm also. This also is why pilots have "flaps" to assist in the landing (flaps increase the lift by changing the curvature to a greater amount hence more lift which is good since an airplane is near a midair stall when its landing) and why fighter jets wings tend to be very near flat and hence have to come in at higher speeds than other aircraft (they travel at high speeds so once in the air do not need as much curvature and also can not have to much curvature or they can not travel at the high speeds they do without tearing the wings off). Aircraft designs for the lift portion are balancing, wing strength vs. curvature vs. speed. To much curvature plus to much speed = strength failure due to friction = wings flying right off of the aircraft. To low curvature, or to low speed, or combination of the two = not enough lift, aircraft falls. Ask any pilot, he will tell you the same.

Erydin

 

[uberman]

I'm diving in. I can't take this anymore. I only read about 4 pages of the thread. Lift is created by low pressure on top of the wing. This happens because air is traveling around the wing. If the conveyor belt matches the planes speed; the plane is stationary. A stationary plane has no air traveling around the wing. There is no lift. The plane does not fly.

Did I miss something? I thought this was a no brainer.

 

[Oblivionaire]

Originally posted by: uberman
I'm diving in. I can't take this anymore. I only read about 4 pages of the thread. Lift is created by low pressure on top of the wing. This happens because air is traveling around the wing. If the conveyor belt matches the planes speed; the plane is stationary. A stationary plane has no air traveling around the wing. There is no lift. The plane does not fly.

Did I miss something? I thought this was a no brainer.

Yeah you are missing something. The airplane isn't stationary. Why would it be? The conveyor belt doesn't stop it from moving forward. All the conveyor belt does is make the wheels spin faster. That's it. The airplane moves forward because the engines are providing thrust against the AIR. The conveyor belt plays little or no role in the plane's movement.

 

[randay]

Originally posted by: uberman
Did I miss something? I thought this was a no brainer.

Indeed.

 

[TheoPetro]

the last posts by Erydin and uberman tell me its time for some IQ cleansing. Grab your guns boys were hunten stubborn idiots!

The plane takes off. It takes off for almost every reason previously posted. The jets push the plane. The plane moves forward. Just because its wheels are spinning doesnt mean sh!t. The plane does move relative to the air. And it moves foreward. Some people need an actual experiment to realize this because their math/analytical skills arnt up to the task. Dont get mad its true. If you actually sat down with actual numbers and modled this scenereo you would come to the conclusion that the plane takes off. If you wish to invest a year in math and physics courses so you can do this go for it. (well youll probably need 2 years of math to get through calc 1-3 and ODE)

 

[Oblivionaire]

Originally posted by: TheoPetro
the last posts by Erydin and uberman tell me its time for some IQ cleansing. Grab your guns boys were hunten stubborn idiots!

The plane takes off. It takes off for almost every reason previously posted. The jets push the plane. The plane moves forward. Just because its wheels are spinning doesnt mean sh!t. The plane does move relative to the air. And it moves foreward. Some people need an actual experiment to realize this because their math/analytical skills arnt up to the task. Dont get mad its true. If you actually sat down with actual numbers and modled this scenereo you would come to the conclusion that the plane takes off. If you wish to invest a year in math and physics courses so you can do this go for it. (well youll probably need 2 years of math to get through calc 1-3 and ODE)

Yeah next thing you know the buffoons will be trying to convince us that 0.999=1

 

[beansbaxter]

Here is the answer to this question:

The airplane will fly.

 

[TheoPetro]

Originally posted by: Oblivionaire

Originally posted by: TheoPetro
the last posts by Erydin and uberman tell me its time for some IQ cleansing. Grab your guns boys were hunten stubborn idiots!

The plane takes off. It takes off for almost every reason previously posted. The jets push the plane. The plane moves forward. Just because its wheels are spinning doesnt mean sh!t. The plane does move relative to the air. And it moves foreward. Some people need an actual experiment to realize this because their math/analytical skills arnt up to the task. Dont get mad its true. If you actually sat down with actual numbers and modled this scenereo you would come to the conclusion that the plane takes off. If you wish to invest a year in math and physics courses so you can do this go for it. (well youll probably need 2 years of math to get through calc 1-3 and ODE)

Yeah next thing you know the buffoons will be trying to convince us that 0.999=1

im AT makes me cry sometimes, I know yorue kidding....im still crying