Brain Teaser

[Randy]

the plane is essentially remaining still do the wing create lift since there would be no air movement under the wings

No take off because as Randy pointed out, there needs to be air movement over the airfoil for lift.

[Assume small single engine aircraft.] If an airplane were standing still, without the wheels turning and you blew 110 mph air over the wings the airplane would take off (leave the ground) with zero forward speed. Even with the engine off. :blink: :wacko: True!

Alan

Actually I pointed it out Randy asked the question...

I saw that too, but, didn't want to break up the discussion.

 

[Just Plain Dave]

Sometimes what seems to make sense, upon closer examination, makes NO sense..

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Like White House policy and Bush's speeches.

You mean, kinda like the last line in the quote box above??? :bleh:

 

[twowheelnut]

The correct answer (question) is: Who cares? It's an improbable scenario anyway. Why would anyone build a runway as a conveyor belt? Now re-suppose the question moving the whole scenario to an aircraft carrier and hooking up an F16 backward on the catapult and I think you'll find the discussion much more interesting.

The Philosopher has spoken and has left the building...

:****:

(By the way, the radinator is correct. Word to yo mutha.)

 

[Just Plain Dave]

I quit. :blink:   You guys just aren't gettin it.  Classic teaser though, and it's working.

I got it, and you are right. I just hate to see this thread end. It's raining outside and I'm a fair weather rider.

By the way, If an asp in the grass is a snake in the grass, why is a grasp in the ass a goose???? :haha:

 

[FJRocket]

If the plane has enough thrust from a prop, there could be enough lift to get the plane off the ground. Believe me, when you tie some planes down, there is enough thrust creating prop wash over the wings that the plane will come off the ground. Not speaking from personal experience, of course....

If the plane is a jet, it could move the wheels beyond the speed of sound. But if the conveyer matches the speed of the wheels, the jet is essentially stationary. Jets don't blow over wings. No airflow over the wings= no lift. The jet won't move. <---- wrong. Thrust will ultimately move the plane forward and the wings will create lift.

The way I interpret the question, the plane is essentially stading still, therefore no air movement, and the wings (or lifting body, whatever) are not creating lift. It will not take off.<---- wrong. The thrust will overcome any wheel movement and the plane will move forward and the wings can create lift.

Now, if a bear shits in the woods, can anyone hear it? :blink:

(Edited for stupidity...)

 

[sparky3008]

The way I interpret the question, the plane is essentially stading still, therefore no air movement, and the wings (or lifting body, whatever) are not creating lift. It will not take off.

That's the thing. As explained to me and I understand it the plane actually will move forward as a plane does not use the act of pushing on the runway to move forward as we as humans a car or motorcycles does. It uses the thrust from the engine to the "air" to move forward... Hence brain teaser...

 

[Derek]

i've decided that the plane in question is a v-22 osprey or a harrier. and yes, i suppose that it will take off if it wants to get off that annoying coveyer belt.

derek

 

[Carnifex]

Yes, the plane would move - the conveyor would have essentialy no affect, other than to reduce/eliminate the friction between the wheels and the runway. It may even take off faster due to that.

(I think)

 

[Maddog]

It won't take off.

Think of a treadmill. No matter how fast your feet move you make no forward progress (forward progress is a requirement for lift).

Now substitute your feet for a powered wheelchair on a treadmill. No matter how fast the wheels on the wheelchair roll there is no forward progress.

Now put wings on the wheelchair and substitute the electric motor with your favorite Rolls Royce jet engine. The wheelchair will begin to roll when you hit the afterburner but the treadmill will match wheelspeed and no forward progress will be made.

You need to roll (i.e., forward movement relative to the surrounding air) before you fly unless you have a Harrier.

 

[Randy]

I'm no physics guy, but, from what the experts have been saying, the wheel speed is totally irrelevant and the thrust of the motor will propel the plane forward regardless of what the wheels are doing. The thrust is moving the plane forward - the wheels are just keeping the fuselage off of the ground until there is enough airspeed to create the lift.

Instead of focusing on wheels, think of a seaplane on skis. Regardless of how fast the water is moving under the skis, the engines are generating the thrust to propel the plane forward - the skis just keep it off of the water.

Here's a technical answer that was given by a guy with numerous astronomy, physics, engineering, aeronautical, and Astronautical degrees:

Plane will fly!
Thrust from engines acts on the air mass surrounding the plane, not on the treadmill, so the treadmill and wheels are basically a

frictionless surface under the plane. The air mass is the reference frame or coordinate against which to assess the action of the plane. Note that a plane with stationary wheels (brakes "on" or chocked) would fly if the air (wind) speed reached sufficient level to generate enough lift, kinda like gliders lifting off the top of a mountain or sand dune. In this case it'd be like taking off on an ice covered runway.

Thrust will move the plane forward relative to the air mass, and will result in air flow over wings sufficient to lift off. This problem can be represented with a classic mechanical engineering force balance, where forward force is produced by jet engines, and reverse force is from aerodynamic drag which increases exponentially as speed (velocity in the air mass) increases. There is no appreciable drag from the wheels, either on a conventional runway or on this mythical reactive/speed matched runway. Net force forward is difference between thrust and drag (T-D).

Newton says F=ma or a=F/m or a=(T-D)/m where m is mass of the plane (which decreases slightly with expenditure of fuel). This defines the acceleration rate of the plane with respect to the air mass.

Next, Newton's equations of motion say V=at, where V is velocity, a acceleration and t is time. If you know what air speed is necessary for lift to exceed weight of the plane (Vliftoff), you can solve above to learn how long it will take to lift off, t=Vliftoff/a or

t=Vliftoff(m/(T-D)).

Finally, you can determine how far the plane will move before take off (assuming a stationary air mass). Newton again says S=1/2 a t**2, (S equals one half a times t squared) where S is distance traveled with respect to the reference frame coordinates. You can substitute the above expressions for time to liftoff and acceleration in here and learn how far the plane will travel to lift off. S=(Vliftoff**2)m/(2(T-D)).

That's all well and good, but, here's something that's more along my level of understanding

 

[yanktar]

It won't take off.
Think of a treadmill. No matter how fast your feet move you make no forward progress (forward progress is a requirement for lift).

Now substitute your feet for a powered wheelchair on a treadmill. No matter how fast the wheels on the wheelchair roll there is no forward progress.

Now put wings on the wheelchair and substitute the electric motor with your favorite Rolls Royce jet engine. The wheelchair will begin to roll when you hit the afterburner but the treadmill will match wheelspeed and no forward progress will be made.

You need to roll (i.e., forward movement relative to the surrounding air) before you fly unless you have a Harrier.

I hope you don't teach physics... :blink:

If your concept was true, then NO plane could fly unless it was attached to the ground and pushing against it.....But we don't call that "flying"....

 

[Groo]

This thread is hillarious.... Randy, your last post is a classic.

The plane will take off. On take off, the wheels are not the driving force for the plane. The comparisons to a treadmill are false, because in those cases, your feet, or wheelchair wheels are propelling you.

Ignoring all the corollary physics issues, assume the wheels are spinning at any rate between zero and infinity. As was mentioned before, someone has a chain attached to the nose of the plane, and is pulling it with a huge motor from off the runway and in front of the plane. Those who think the plane won't take off are saying the motor will be unable to make the plane move forward. Those who are saying the plane will take off are saying the jet engines attached to the plane perform the same function as the chain/motor.

Another good way to think about it is raise the plane 1 foot off the runway (attach it to a big crane on a track), but still let the wheels spin at the same rate as they would as if still in contact with the runway/treadmill. Light the jet engines...no matter how fast the wheels spin or don't spin, the plan is going to move and/or fly.

 

[Maddog]

Yanktar

Luckily I don't teach physics; as I've demonstrated in my post above I know nothing about physics. But it was still fun trying to figure it out.

Actually the quote posted by Randy just above does make sense.

 

[fretbender]

The thing that makes the question such a stumper is that it's a bullshit question to start with. Look at the question.

"This conveyer has a control system that tracks the plane's wheel speed and tunes the speed of the conveyer to be exactly the same in opposite direction"

There's a contradiction here.

1) If the converyor belt runs just fast enough to match the wheel speed, then obviously the center of the wheel isn't making any progress as viewed by somebody on fixed ground, and we know that the center of the wheel moves with the same speed as the rest of the airplane. Soooo, if the conveyor belt is moving at a speed to match the wheel speed, the plane is not moving. Right? Right.

Next,

2) As radman keeps saying, the thrust of the engines is what makes the plane move. This is independent of wheel speed. Right? Right.

So. . .IF the plane is moving under the force created by engine thrust AND the wheels are not skidding, it is not possible for the conveyor belt to match the wheel speed.

If the conveyor belt matched the speed of the plane, in the opposite direction, then it's pretty obvious that the wheels just turn twice as fast to make up the difference, but that's not what the question is asking.

Right? Right. Good one though.

 

[Rogue]

This one is soo obvious! It will not fly!

The conveyor will soon reach a speed so high trying to keep up with the jet that all wheel and conveyor bearings will seize, the conveyor will break, the planes wheels will stop turning (seized bearings), the tires will pop, the landing gear will collapse and the plane will explode killing all!

:bleh: :bleh:

 

[slapnpop]

The thing that makes the question such a stumper is that it's a bullshit question to start with. Look at the question.
"This conveyer has a control system that tracks the plane's wheel speed and tunes the speed of the conveyer to be exactly the same in opposite direction"

There's a contradiction here.

1) If the converyor belt runs just fast enough to match the wheel speed, then obviously the center of the wheel isn't making any progress as viewed by somebody on fixed ground, and we know that the center of the wheel moves with the same speed as the rest of the airplane. Soooo, if the conveyor belt is moving at a speed to match the wheel speed, the plane is not moving. Right? Right.

Wrong

As soon as the engine started pushing the plane, and the wheels rotated, the treadmill would move forward the same distance as the plane so the wheel wouldn't roll. The tread mill would actually be moving underneath the plane at the same speed, if it's job was simply to roll opposite what the wheels tried to roll. This would actually make it easier for the plane to take off, because the friction of the wheels rolling would then be eliminated, as the treadmill is moving along with the plane to keep the wheels from moving.

 

[dcarver]

Me thinks the guy with signature of:

"You say you're an electrical engineer and you can't even tell me how many amps are in a gallon??"

would HAVE to have the correct answer..

 

[ionbeam]

I can't believe I'm perpetuating this stupid thread <_<

Ignore the wheels, for a plane to fly it has to have airflow over the wings sufficient to generate enough lift to overcome the mass of the plane. A propeller does not provide the airflow for lift, a propeller is a rotating wing that works like a screw to pull the plane through the air until it is going fast enough to generate enough lift to overcome mass. A jet engine’s job is to push the plane through the air until the plane is going fast enough to generate lift to overcome mass.

The original teaser simply says, “The plane moves in one direction…” There is no mention of engines, airspeed, air movement, lift or any other flight controlling scenarios. The only thing the teaser focuses on is the speed differential of the conveyer and the wheels. Wheels/conveyer = apples. Plane moves/Will the plane take off or not = oranges. To answer the question of take off or not is an issue of airframe speed not wheel speed, the statement of “The plane moves in one direction” is insufficient and some people have read and extrapolated more into it than stated.

It would seem that any pilots in this forum have had the good sense not to get involved in this ‘discussion’. Perhaps Speedfreak would like to chime in?

:dribble:

Maybe I'm not too good with brain teasers but I have several hundred hours of piloting planes with a propeller (air screw) pulling me through the air. I have twice as many hours flying with no onboard propulsion system at all, Mother Earth’s gravity provides the free momentum.

 

[fretbender]

The thing that makes the question such a stumper is that it's a bullshit question to start with.  Look at the question.
"This conveyer has a control system that tracks the plane's wheel speed and tunes the speed of the conveyer to be exactly the same in opposite direction"

There's a contradiction here.

1)  If the converyor belt runs just fast enough to match the wheel speed, then obviously the center of the wheel isn't making any progress as viewed by somebody on fixed ground, and we know that the center of the wheel moves with the same speed as the rest of the airplane.  Soooo, if the conveyor belt is moving at a speed to match the wheel speed, the plane is not moving.  Right?  Right.

Wrong

As soon as the engine started pushing the plane, and the wheels rotated, the treadmill would move forward the same distance as the plane so the wheel wouldn't roll. The tread mill would actually be moving underneath the plane at the same speed, if it's job was simply to roll opposite what the wheels tried to roll. This would actually make it easier for the plane to take off, because the friction of the wheels rolling would then be eliminated, as the treadmill is moving along with the plane to keep the wheels from moving.

Ah, but no. I'm still right. The initial question states that the conveyor belt is moving from the front to the rear of the aircraft (backwards, not forwards), and it is this motion which, ahem, is what supposedly keeps the aircraft from moving (pshaw, as if. . . <_< ). Your statement has the conveyor belt moving in the same direction of the aircraft, which if it was, you would be right.

Right? Right! :clap:

 

[FJR&R]

The only way a jet is going to leave the ground is if it has enough air passing the wing to lift it -OR- you could tip the engine and use the thrust against the ground like a Harrier Jump Jet! ilot: