samnavy

iTrader: (7)

Ahhhhh, now I understand... it's a secret governement plane that moves on the mere brainwaves of the pilot and doesn't have an engine... Pilots aren't that smart, that's why they need NFO's.
Of course you must assume the engines are running.

Louis

Thrust will move the plane, but if the belt moves at an equal rate in the opposite direction, the plane will not move relative to the surrounding air.

Lift comes from the air moving over the wings, no air moving = no lift = no takeoff.

That is all, we now return you to your daily activities.

Philip

please explain why the plane won't move

Trent

iTrader: (11)

Because the plane is being piloted by a woman, and, as such, the parking brake is on.

olderguy

iTrader: (19)

"A plane is sitting on runway that can move" Nothing in the problem says there is actually any movement by the plane or the runway. It's all just sitting there looking pretty.

greenday3437

iTrader: (2)

It clearly states, WILL IT BE ABLE TO RUN UP AND TAKE OFF?

The answer if obvious if you would think about it.

Planes arent cars, they arent driven by wheels, they are driven by thrust created by engines fixed to the body of the plane, so therefor all it is doing is pushing it through the air while the wheels roll on the ground.

The only was the ground moving the opposite direction could possibly make the plane go any slower, is the SMALL extra amount of frictions on the bearings the wheels spin on caused by the extra speed, which will minimal.

samnavy

iTrader: (7)

Damn guys, this isn't that hard.
Forget about airplanes, let's go with something simpler!
Pretend this experiment is happening in your spare room.

Strap a rocket on your back and stand on a treadmill (pretend it's the fastest treadmill ever and can achieve the same speed as your rocket) wearing rollerblades. Lite off the rocket and the treadmill at the same time.... still think you're going to stay on the treadmill?
Really, just imagine yourself there on the treadmill hunched over with that rocket blasting away and the treadmill going at mach 4 and you NOT MOVING??? Come on guys, it isn't that hard.

For those of you saying "nobody said it was blah blah blah..." aren't sticking with the spririt of the thought process. It's designed to get you to realize that the wheels don't drive the vehicle and are independent of the motion of the ground.

How about this one: "If a plane crashes on the border of two states, which side do they bury the survivors"... HAHAHA, survivors don't get buried.
Then you say, well everybody dies sometime so naturally they'd get buried wherever the family thought was good and blah blah blah and ruin the joke.

DAMNIT, NOW YOU'VE GOT ME SECOND GUESSING MYSELF!!!!!!!!!!

samnavy

iTrader: (7)

****, could I be wrong about this? I've got a whole room full of pilots here talking about it and we're split 50/50.

olderguy

iTrader: (19)

I keep my treadmill in the bedroom so that I have a place to hang stuff.:gay:

Samnavy; you are perfectly right if it is a physics question.

But a "brain Teaser" is normally a logic question, which is why people look for other answers than first come to mind. My English teacher would have flunked me if I didn't first analyze the question. Oh, wait, she did.

Braineack

iTrader: (62)

this is why i let others figure things out, then take what the say, try to understand it, and pass it off like I've known all along.

Al Hounos

seriously, this isn't difficult at all, but everytime i see it it turns into a huge debate.

getsidewaysd1

My dad's a pilot, I'll ask him.

jayc72

iTrader: (14)

Let me ask you guys who are having trouble with this a question. Take a 747 and assume that at 200mph it will take off. Now change the normal wheels to rollers skate wheels, will it still take off at 200mph?

Take a random car with 20" wheels running the engine at 5000rpm, take the same car attach 13" wheels running the engine at 5000rpm. Which setup will be traveling faster?

Strap a rocket engine to the car and tell me how the wheels will effect the speed at which the car can travel under the power of the jet engine.

Joe Perez

iTrader: (8)

Prolly the car with 20" wheels, since you can't really find 13" tires in 185/400 profile. :gay:

Seriously though, this question cannot be answered without specific data concerning the materials involved.

This combination of the airplane and treadmill is a simple servo feedback mechanism. Same sort of thing that keeps the disc in your DVD player spinning at a constant linear velocity despite increasing track circumference, or adjusts the speed of a capstainless tape machine as the wrap around the reels grows and shrinks.

We have the airplane engine producing thrust against the atmosphere, which we'll assume is still. This will tend to cause the airplane to accelerate forward. Then, we have the treadmill, which senses the absolute movement of the airplane and starts reeling backwards.

Since the airplane is coupled to the runway only by its wheels, which are non-driven, then the only mechanism by which force can be transmitted from the treadmill to the airplane is through the wheels. And since the wheels are freely mounted to the airplane through bearings, only the friction in the bearings can account for force being transmitted from the surface of the treadmill to the airframe.

If the treadmill is able to move fast enough to produce sufficient drag within the wheel bearings such that the force transmitted from the treadmill surface to the airframe is sufficient to equal the thrust of the engine, then the airplane will remain stationary. And a millisecond later the wheel bearings will sublimate, assuming the tires haven't long since exploded.

On the other hand, if the airplane's wheel bearings are not perfectly frictionless, then neither is whatever mechanism drives the treadmill/runway. It is unlikely that a structure massive enough to move a runway could be designed to have a lower internal friction then a wheel bearing, in which case the treadmill would be unable to move fast enough to exert sufficient force upon the airframe.

Joe Perez

iTrader: (8)

Actually, you know what? I just re-read the original question, and realized that I mis-read it the first time. It's actually very simple. Let me quote the original poster:You catch that? The conveyor isn't trying to keep the plane stationary, it's merely moving the surface backwards at a rate equal to the absolute velocity of the airplane. It's not accelerating wildly trying to keep the plane from moving, it's just moving backwards at a 1:1 rate. So the wheels of the airplane are going to be turning twice as fast as they would be during a normal takeoff, but the airplane will still launch with no problem whatsoever. Just like taking off with a 100 MPH tailwind.

Don't ask a pilot, ask an engineer.

bripab007

If the conveyor belt is keeping the plane stationary, relative to the surrounding ground, then it has no airspeed...therefore no air molecules flowing over the wings to let them do their whole lift-generating thing...

It doesn't matter if it's powered by engines driving the wheels, jet engines or a nuclear reactor, if it has no airspeed across the wings, it's not taking off.

Put another way, if the jet had equally powerful jet engines attached in the opposite direction of it's usual ones, and they were both set to full power, would the plane just hover stationary above the ground?

Of course not!!

Joe Perez

iTrader: (8)

If the airplane had two engines, one rear and one forward, then you'd be correct. But that's just the thing- the airplane is rigidly coupled to its own engine, but it is not rigidly coupled to the ground. The airplane *will* move forward, despite the fact that the ground is moving backwards beneath it, because the engine is acting upon the *air*, which is stationary. It's not like we threw an anchor out of the airplane, we're just adding a bit more drag.

As the ground begins to roll backwards, it will simply cause the wheels to spin faster as the engine acts upon the air to propel the airplane forward. By the time the aircraft reaches Vr, the runway will be scrolling backwards at -Vr, and the wheels will be spinning at a rate equivilant to 2Vr, but the airplane will still be moving forward relative to the atmosphere. Ergo: airflow over the wings.

In order to keep the plane from moving forward, you'd have to move the whole atmosphere, not just the ground.

Let me put this in more everyday terms. Say you're in a swimming pool. You are swimming forward, and yet you are somehow managing to drag one toe on the bottom of the pool. (you're a really good trick swimmer.) At the same time, the bottom of the pool begins to move backwards at a rate equal to your forward progess through the water. Will this cause you to stop? Of course not. Your arms are exerting far more force upon the water than the floor of the pool is exerting upon your toe, and since the *water* is not moving backwards, you will continue to swim forward.

Snowsurfer03

Im goin with this one BROTHERS!

The HULKSTER says, "No airflow = no flying yo!"

Why do they use big ol' fans on the dyno??? Because there is no airflow!

But does a jet really need airflow? Hummm...uhhh I give up, just tell me!!!

jayc72

iTrader: (14)

Kill me now.

Snowsurfer03

U Got a problem with the Hulkster BROTHER!? LOL

I'm j/p BROTHER!