will it take off?

[NewToy]

New Toy,

I understand what you are saying and I agree that if the plane moves, it takes off. In your scenario it moves. But to quote the original question:

"a plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction."

I interpret this to mean that the magic conveyor is propelling the plane backwards at an identical rate to its forward movement. So yes, your thrust is moving the plane forward but the "thrust" of the conveyor is exactly countering that. Again, this is just like the plane sitting there with brakes on.

Now take the winch example and apply it to a plane with its brakes on - what happens? Either the cable breaks or the winch burns out or the wheels slide on the ground. It is different from propeller thrust - the propeller can apply thrust when the wheels are braked and nothing may happen. So adding a winch or other mechanical pull to this scenario negates the whole thing.

The thrust is moving the plane forward resulting in the conveyor moving backwards at the same speed. The wheels will FREELY spin at 2x the speed of either the conveyor or the plane. The thrust has to move the plane forward in response to the rearward thrust. What becomes of the thrust energy in your scenario? The plane moves forward with the gear wheels going double time.
John

 

[RayH]

The thrust has to move the plane forward in response to the rearward thrust. What becomes of the thrust energy in your scenario? The plane moves forward with the gear wheels going double time.
John

Ok good, If the brakes are released and the wheels are free to move, how much rearward thrust is the conveyor producing? Keep in mind that the wheels are not locked, they are free to rotate as the conveyor moves backwards under them. There wont be much if any rearward thrust. Start up the engine and the prop will produce quite a bit more forward thrust than the conveyor has rearward thrust. The plane moves forward.

 

[DieselPower]

Well I thought I would ask a real pilot today. I was going to the gun store and thought I would stop by the local Fairfield Airport and asked this question. The 4 piolt's standing around me got a good laugh out of it if nothing else. They all agreed that the plane could not lift off if it had no movement through space. No movement through space, no air movement over the wings, no flight. All they did was verify what I said the other day. If the plane can not move forward in space due to the "magic" convyer there is no air movement over the wing's to create lift.

 

[dillo99]

okay - this is my last post on this topic.

I fully agree that if the plane moves it flies. It all comes down interpretation of the conveyor...if it is matching the planes speed (not the wheels...but the speed) then I would argue that it is stopping all movement of the plane. Naturally, this is impossible

So what happens to all the thrust if the plane isn't moving? Well what happens to it if the brakes are on? The thrust can't overcome the friction of the brake/tires.

So...if the thrust moves the plane and the magic conveyor is NOT able to exactly counter forward speed of the airplane then it will fly.

If this fictional and magic conveyor can exactly counter the speed of the airplane then the airplane does not move nor fly.

Based on all the posts here - I think everyone would agree with those last two statements - it all comes down to what this fictional conveyor is capable of!

 

[DieselPower]

Well one of the people (pilot's) I spoke with earlier today just gave me a call on the phone. Being the salesman that I am I had left a couple of business cards with them so they had my number. In any case they apparently changed their minds after I left. Here's why and it does make sense the plane WILL take off so I stand corrected.

He told me to check out this guy's webpage for the details. The Straight Dope: An airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off?

Here's the important part.

"First the obvious-but-wrong answer. The unwary tend to reason by analogy to a car on a conveyor belt--if the conveyor moves backward at the same rate that the car's wheels rotate forward, the net result is that the car remains stationary. An aircraft in the same situation, they figure, would stay planted on the ground, since there'd be no air rushing over the wings to give it lift. But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is, as you correctly conclude, make the plane's wheels spin madly."

So, in laymans terms it means that no matter how fast the convyer moves backwards the plane WILL move forwards due to it being drawn forward through space/air. No matter how fast the convyer goes and how fast the wheels spin the plane will gain forward movement through the air at it's normal speed and will take off.

I think my head hurts, fizzle, fizzle, pop!

 

[shaley]

Just one or two more questions so I can get this straight.
Have you ever tried to push your tractor by hand on level pavement? I know it took 6 guys just to budge mine. I'm guessing 300 Lbs of force. Arent these rubber tires just like the ones on aircraft.
How much force do you think it would it take to push a Cessna 150, DC3, or 727 on pavement?

 

[BTDT]

Just one or two more questions so I can get this straight.
Have you ever tried to push your tractor by hand on level pavement? I know it took 6 guys just to budge mine. I'm guessing 300 Lbs of force. Arent these rubber tires just like the ones on aircraft.
How much force do you think it would it take to push a Cessna 150, DC3, or 727 on pavement?

I'm sure others will jump in here also, but this is nowhere close to the same thing. The airplane wheels are just tires and wheels on a spindle. The tractor tire/wheel is connected to axles, gears, heavy oil, etc. A closer analogy would be pushing a grocery cart and a riding mower. One just rolls, the other has drag due to mechanical makeup.

 

[MossRoad]

Mike,

Look at it this way...

The props pull the plane forward slower than the wheels are spinning on the conveyor. All the wheels and conveyor do is keep up with each other.

Another way to look at...

Model airplane and small conveyor. You start the conveyor and hold the model airplane in place with your finger. You can push it forward with your finger (airspeed). The only thing changing is the speed at which the tires and conveyor are spinning. Replace your finger with a prop or jet engine and they'll do the same as your finger did. The plane will move forward which will create airspeed.

Nope. As soon as you push the model plane forward with your finger, its wheels are turning faster than the surface speed of the small conveyor. You, in effect, have cheated.

 

[MossRoad]

For those having trouble with this, you should try tackling the flight pricipals of a helicopter.

Attached is a picture I took at the Henry Ford museum of a quote from Sikorsky regarding the first helicopter.... enjoy!

 

[MossRoad]

...

So...if the thrust moves the plane and the magic conveyor is NOT able to exactly counter forward speed of the airplane then it will fly.

If this fictional and magic conveyor can exactly counter the speed of the airplane then the airplane does not move nor fly.

Based on all the posts here - I think everyone would agree with those last two statements - it all comes down to what this fictional conveyor is capable of!

I agree completely.

 

[NewToy]

Just one or two more questions so I can get this straight.
Have you ever tried to push your tractor by hand on level pavement? I know it took 6 guys just to budge mine. I'm guessing 300 Lbs of force. Arent these rubber tires just like the ones on aircraft.
How much force do you think it would it take to push a Cessna 150, DC3, or 727 on pavement?

You must have a giant tractor or it has the brakes dragging. I can push mine (2600 lbs) easily by myself on level pavement. Never could get it going fast enough for takeoff though.
PS- The plane will fly.
John

 

[MossRoad]

So, in laymans terms it means that no matter how fast the convyer moves backwards the plane WILL move forwards due to it being drawn forward through space/air. No matter how fast the convyer goes and how fast the wheels spin the plane will gain forward movement through the air at it's normal speed and will take off.

I think my head hurts, fizzle, fizzle, pop!

The problem with this is, if the conveyer picks up speed to match the spinning tire, it never did its job of matching the speed in the first place. It was playing catch up, not exact match.

Splain that one, Lucy.

 

[MossRoad]

Just one or two more questions so I can get this straight.
Have you ever tried to push your tractor by hand on level pavement? I know it took 6 guys just to budge mine. I'm guessing 300 Lbs of force. Arent these rubber tires just like the ones on aircraft.
How much force do you think it would it take to push a Cessna 150, DC3, or 727 on pavement?

When I worked at the aiport, I frequently moved 10 passenger twin engine turboporps around the hangars by hand. I weighed 135 pounds. Airplanes do not weigh that much and have really good wheel bearings, as well as really smooth tread on their tires, usually just rain groves.

We could move a DC3 with an 18HP wheelhorse lawn tractor and a 727 with about 4 or five guys pushing each landing gear. The problem with a 727 is not getting it going, but getting it stopped when you get there! That's why you need a large tug with lots of weight and good brakes or the plane powered up to apply hydraulic pressure to the brakes. The mass gets rolling really easy and keeps on rolling.

 

[BillyP]

Nope. As soon as you push the model plane forward with your finger, its wheels are turning faster than the surface speed of the small conveyor. You, in effect, have cheated.

Nope, the wheels can't turn faster than the conveyor because the conveyor kicks into turbo boost overdrive and keeps up.

And the tires are made of a super non skid resistant high performance guaranteed to have 100% traction Firestone 500 Finger Plus and made especially for model airplanes on a super responsive magical conveyor belt

 

[Steve_Miller]

Sense we were having so much fun debating the finner points of tire pressure in the other thread....

i thought this would be kinda fun...

a plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction.

the question is

will the plane take off or not?

(ps its been debated to death on other forums, its always fun to see how people present the theory behind there answer)

Answer me this one then, if the plane was doing 200MPH flying thru the sky with the landing gear down and you put this magical conveyor under the wheels, would the plane and conveyor fall out of the sky or would the wheels just spin at 400mph. Unless this magic conveyor is moving ground, sky, air, landscape or whatever else you desire it's going to keep flying until the propellent (jet, prop, rocket,etc) is removed from the equation.

a plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves

I guess my question is what exactly does the conveyor move? If someone can put a definition on this, the question will answer itself.

Steve

 

[DieselPower]

The problem with this is, if the conveyer picks up speed to match the spinning tire, it never did its job of matching the speed in the first place. It was playing catch up, not exact match.

Splain that one, Lucy.

That's just the point that had me originally thinking it would not lift off. Then the light bulb went off. There are two different points. The ground and air space. A plane is not made to move over ground (though it does) it is made to move through space. Doesn't matter what you put under it. The convyer plays no part in the plane lifting off. It is not propelled on the runway/convyer by driven wheels like a car. It is propelled through space by a propeller/jet engine. The plane moves through the air as a result of the reaction between the prop/jet and the "air" and not ground.

Think of it this way. If you had a crane 20,000 feet high and you lifted a plane up to this point and dropped it, it would fall to earth directly under the point it was lifted to. If you now start that plane up and drop it, it will now fly away because it is pulled through the air by its engine propultion system. Now, attach your magic convyer to the crane and lift it to 20,000 feet and drop it. The plane and convyer will fall to earth. Redo this but now with the plane's propultion system running. The convyer will fall to earth and the plane will be propelled off and forward of the convyer.

 

[755inNY]

Nope. As soon as you push the model plane forward with your finger, its wheels are turning faster than the surface speed of the small conveyor. You, in effect, have cheated.

And another
The problem with this is, if the conveyer picks up speed to match the spinning tire, it never did its job of matching the speed in the first place. It was playing catch up, not exact match.

Splain that one, Lucy.

MossRoad, You seemed to be stuck on the conveyor matching the speed of the wheels when the scenario states that the conveyor matches the speed of the plane. Splain that one.

 

[755inNY]

okay - this is my last post on this topic.

I fully agree that if the plane moves it flies. It all comes down interpretation of the conveyor...if it is matching the planes speed (not the wheels...but the speed) then I would argue that it is stopping all movement of the plane. Naturally, this is impossible

So what happens to all the thrust if the plane isn't moving? Well what happens to it if the brakes are on? The thrust can't overcome the friction of the brake/tires.

So...if the thrust moves the plane and the magic conveyor is NOT able to exactly counter forward speed of the airplane then it will fly.

If this fictional and magic conveyor can exactly counter the speed of the airplane then the airplane does not move nor fly.

Based on all the posts here - I think everyone would agree with those last two statements - it all comes down to what this fictional conveyor is capable of!

I don't agree with either of the last 2 statements.

 

[gizmo]

The conveyor is doing nothing but spinning the wheels. The prop will still pull the plane through the air regardess of how fast or even what direction the conveyor moves and the plane will fly.

Nothing complicated about this. Don't read too much into it and it makes sense.

I am a pilot and an engineer and I have argued this scenario about a dozen times now.

 

[NewToy]

The conveyor is doing nothing but spinning the wheels. The prop will still pull the plane through the air regardess of how fast or even what direction the conveyor moves and the plane will fly.

Nothing complicated about this. Don't read too much into it and it makes sense.

I am a pilot and an engineer and I have argued this scenario about a dozen times now.

The no fly contingent will have to have a conveyor to prove it to them. Since they believe the plane will not move the conveyor need only be large enough to accomodate the footprint of the landing gear. Also, it won't need all the magical speed matching properties, we can spin it at twice the speed the airplane could ever go and the plane would still take off.
John