Will it take off?

[TraceZ]

Kevin,

I do understand what it is you are getting at. And if it were possible to acheive what the question describes (perfectly matched acceleration it opposite directions), the plane would start moving backwards (because all of its weight is on the conveyor) at exactly the same rate it was thrusting forward, esentially zero forward motion. Such a system is in no way possible.

In this way the question is very illogical. It says the system will do something that is not possible. An imperfect human designed system could not accomodate this experiment. At any given moment either the plane or the conveyor would be slightly faster than the other. At that moment the question is invalid because they are no longer exactly the same speed.

Everyone who agrees the plane will fly is reading the question in a way that accomodates the only logical interpretation - that the plane moves first, and the imperfect monitoring system will all be trying to "catch up". Trying to "catch-up" once the plane is moving, would just be spinning the planes wheels and not hurting its forward progrees.

Can we at least agree the question is illogical?

The real question should be: Could you ever built a runway conveyor system (as described in the original question) that could prevent a plane from taking off? This answer is definately no.

Actually, even if the computer could react with an impossibly perfect 0.0000 second delay, the airplane would still move forward almost the same as if the ground was stationary.

the hypathetical-smart-treadmill-runway is powerless to stop it. It would have to literally reach up and grab the airplane to stop it from going forward. No matter how impossibly perfect you build the treadmill-runway it will never be able to even slow the airplane down significantly.

Have you ever held the tail of an RC airplane when a friend tuned the motor at WOT? If you have, you will know that it pulls forward pretty hard. I'm sure the wheels were resting on the grass while you did this WOT engine tuning, but imagine for one minute if instead they were on a regular exercise (running/jogging) treadmill. Say you crank the treadmill up to full speed. Is the airplane pulling any less? If you let go will it stay there?? Heck no!! If you let go it will go cruising forward off the end of the treadmill. You could crank the treadmill up to any speed you wanted and the little wheels would spin faster and faster.... BUT... it would still be pulling forward away from your hand just as hard. If you let go it would still go cruising forward.

[fastTa]

Actually, even if the computer could react with an impossibly perfect 0.0000 second delay, the airplane would still move forward almost the same as if the ground was stationary.

the hypathetical-smart-treadmill-runway is powerless to stop it. It would have to literally reach up and grab the airplane to stop it from going forward. No matter how impossibly perfect you build the treadmill-runway it will never be able to even slow the airplane down significantly.

Have you ever held the tail of an RC airplane when a friend tuned the motor at WOT? If you have, you will know that it pulls forward pretty hard. I'm sure the wheels were resting on the grass while you did this WOT engine tuning, but imagine for one minute if instead they were on a regular exercise (running/jogging) treadmill. Say you crank the treadmill up to full speed. Is the airplane pulling any less? If you let go will it stay there?? Heck no!! If you let go it will go cruising forward off the end of the treadmill. You could crank the treadmill up to any speed you wanted and the little wheels would spin faster and faster.... BUT... it would still be pulling forward away from your hand just as hard. If you let go it would still go cruising forward.

Ok, I can't resist. Are we comparing RC planes or a Jumbo Jet??? Let's get it straight because it makes an unmistakibly clear difference. An RC plane is blatantly obviously not going to experience anywhere near the resistance from sheer weight and friction that a passenger plane would encounter. Not a valid comparison by any stretch of the imagination.

Jeff and I had a good debate via AIM and it got me to thinking even with more clarity. The larger and consequently heavier the plane, the more this does not work. So we need to decide on a specific plane in which to reference. Let's use a 747.

the hypathetical-smart-treadmill-runway is powerless to stop it. It would have to literally reach up and grab the airplane to stop it from going forward. No matter how impossibly perfect you build the treadmill-runway it will never be able to even slow the airplane down significantly.

Powerless?? Powerless?? Really ...is that the word you are going to stick to? Just try to fathom the amount of pressure being exerted on the conveyor surface by the tires of the 747. A typical commercial 747 is listed at roughly 400,000 lbs empty and 800,000 full loaded. And you think for one second that there is no resistance That coefficient of friction is literally 10's of thousands of times moreso than that of your cable being pulled by human hand analogy!

The 747 would barely even move forward, if any at all assuming the perfection of the "system" that Trace describes. Simple physics guys.

[FroSSty]

....What a blood bath lol.....Another thing to consider is gravity. The amount of gravity pulling down on the rc plane against the surface or the conveyor is much less than that of the force pulling down against the larger airplane. I didnt take physics in school so I don't know if this would matter...The other thing I see is that you cant compare the "ground speed" of the conveyor with the "air speed" of the plane. Seems like apples to oranges to me... Anyways, I would like to see mythbusters do a program on this exact subject. I'd like to see all of the angles of this experiment. Oh and I am NOT going to look in this post again. haha

EDIT: HAHA Kevin you were thinking the same thing at the same time as I was... You just like to type and I don't. TRUST ME do like me and never look in this thread again.

[TraceZ]

Ok, I can't resist. Are we comparing RC planes or a Jumbo Jet??? Let's get it straight because it makes an unmistakibly clear difference. An RC plane is blatantly obviously not going to experience anywhere near the resistance from sheer weight and friction that a passenger plane would encounter. Not a valid comparison by any stretch of the imagination.

Jeff and I had a good debate via AIM and it got me to thinking even with more clarity. The larger and consequently heavier the plane, the more this does not work. So we need to decide on a specific plane in which to reference. Let's use a 747.



Powerless?? Powerless?? Really ...is that the word you are going to stick to? Just try to fathom the amount of pressure being exerted on the conveyor surface by the tires of the 747. A typical commercial 747 is listed at roughly 400,000 lbs empty and 800,000 full loaded. And you think for one second that there is no resistance That coefficient of friction is literally 10's of thousands of times moreso than that of your cable being pulled by human hand analogy!

The 747 would barely even move forward, if any at all assuming the perfection of the "system" that Trace describes. Simple physics guys.

The scale changes nothing. As the airplane gets larger, so do all the forces including the thrust of the engine(s). The proportions remain unchaged for the most part. I recall the last time I flew on a large passenger jet being very impressed with the force that was pushing me back into my seat when we lifted off.

When a 747 taxis, it uses a very minimal amount of throttle. The engines barely have to even try to move that large airplane across the ground... it is nearly effortless for them. When it hits WOT for takeoff, watch out, all hell breaks loose. the runway could spin as fast as it wanted to, it wouldnt hardly even slow it down.

Also, a small consideration is the laws of the conservation of energy. An object at rest tends to stay at rest. If the plane is stationary and the runway tries to move it backwards the plane will want to stay put due to its mass. the wheels will turn and the runway will move, but the airplane will stay put, or move very little. If the engines are running, and just cracked open a little the runway will spin and the plane will stay in one place or move forward.

here is a hypothetical for you...
secure a cable to the tail of your 747. secure the other end of the cable to a force gauge bolted to very solid stationary object off the end of the smart-treadmill-runway.
bring all the engines on the 747 to full throttle.
Measure the force on the cable
now start the treadmill-runway spinning at 50 mph
measure the force on the cable
100 mph
measure the force on the cable
150 mph
measure the force on the cable

The force would be nearly identicle in all 4 measurements, minus some minimal parasitic loss from the wheels spinning freely. If you cut the cable at any time the airplane would go cruising down the runway and take off just as if it were on solid ground.

[TraceZ]

The answer:

http://www.avweb.com/news/columns/191034-1.html

[Jeff_in_Atl]

The answer:

http://www.avweb.com/news/columns/191034-1.html

"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. "

[Red20170]

Wow what a thread... The above statement is correct. If this were a car the wheels provide forward motion, The car would stay in the same place. An aircrafts forward motion is provided by the thrust of an engine. Lift is provided by the wings. Aircraft wheels spinning on the conveyor plays no part in forward motion.

[Christopherrr]

we can all agree that it takes lift to get a plane in the air. regardless if it's a turbine or prop...that's the only way planes fly.

imagine this...

there was treadmill big enough to fit your car...you accelerate to 10-20-50-100 mph...while you're doing that, the treadmill is matching your speed exactly. roll down your window...can you feel any wind? of course not...

no wind = no lift.

you ain't going no where...

[Joe 1320]

This is such a vague and debateable question to begin with, it reminds me of a question used to stir debate. I can sum it up plainly . For a plane to take off, wind speed is required. The movement of air above and below the wing is of different speeds and that is what generates lift. For an airplane wing to generate lift, there must be enough wind speed passing over the wing. There are a few ways this can happen. The plane could sit on a runway facing hurricane force winds..... it would lift up and thrust over on it's back and tumble the instant it was released, sit in a wind tunnel and released..... ending in the same result, or be moving fast enough relative to a fixed position so that an approapriate speed can be achieved for the wing to do it's job. For an airplane to take off and climb, it must exert a force to overcome drag and gravity. Either that or something else must exert the force similar to a glider being towed into the air by another airplane. Gliders work great once you gain altitude to ride nature's thermal elevators. Taking off is another matter. A Harrier jet can accomplish verticle take off and landing to to thrust vectoring so that the wing is not need to generate lift upon takeoff, so that one is an exception. A helicopter uses the same wing design but the theory uses the rotors traveling in a circular and thus one direction generating the lift mechanically instead of via high rate of foreward motion. A very high powered prop plane or jet can do this also, but it's not practical. That's why the Navy has catapaults on an aircraft carrier, to move the airplane fast enough relative to a fixed position so the wing is already generating lift by the time it clears the deck. If the catapault fails and the airplane tries to take off without it, there won't be suffecient air moving across the wing to generate lift and she goes into the drink. Heck, a Volkswagon even flies for a while when the catapault works. Goes for about a mile I believe. Simple physics. without a wing to generate lift, it will still fall at 9.8 meters/sec², so it starts falling the instant it leaves the flight deck. It just happens to be moving fast enough in a foreward direction that it travels a mile before reaching the water line.

[Dave M]

we can all agree that it takes lift to get a plane in the air. regardless if it's a turbine or prop...that's the only way planes fly.

imagine this...

there was treadmill big enough to fit your car...you accelerate to 10-20-50-100 mph...while you're doing that, the treadmill is matching your speed exactly. roll down your window...can you feel any wind? of course not...

no wind = no lift.

you ain't going no where...

Read the link above. An airplane does not drive with it's wheels, it is pulled or pushed through the air by a prop or engine. The tires have no effect of where the plane goes. The engine will still push/pull the plane through the air no matter what the ground below is doing because the wheels are not connected to anything. They don't drive the plane like a car's wheels does. The plane will fly.

[Christopherrr]

Read the link above. An airplane does not drive with it's wheels, it is pulled or pushed through the air by a prop or engine. The tires have no effect of where the plane goes. The engine will still push/pull the plane through the air no matter what the ground below is doing because the wheels are not connected to anything. They don't drive the plane like a car's wheels does. The plane will fly.

I know that...what I was trying to get to is...regardless of what used for propulsion...if there isn't forward movement, then there isn't any wind generated...thus no lift.

[Red20170]

Like I said The prop or turbine is providing forward motion. Not the wheels like a car. When I would take A Harrier to the high power pit and chain it down. The plane could careless at what speed the wheels where turning. Their was enough force at full power to over come the weight of the aircraft that if the chain holding it down snapped. It was going forward at a rapid rate.

[Red20170]

if there isn't forward movement, then there isn't any wind generated...thus no lift.

But their is forward motion because the wheel contact on the conveyor plays no part in forward motion.

[Joe 1320]

The plane will fly.

As long as the plane is not restrained, it will fly. The wheel bearings and the tires may melt if the conveyor belt were as long as a runway. The theory would be if the belt could move fast enough to impart enough drag on the tires and bearings, the airplane's engine could not overcome the drag to gain suffecient speed for the wing to generate lift.

[Dave M]

I know that...what I was trying to get to is...regardless of what used for propulsion...if there isn't forward movement, then there isn't any wind generated...thus no lift.

But there will be forward movement! The conveyor has no effect on the plane other than to cause the wheels to spin twice as fast. The plane will go forward at 50 mph (or whatever speed), the conveyor will move under at 50 mph (in the opposite direction), the tires will spin at 100 mph. The wheels free-wheel and have no effect on the airplane. It's the thrust from the engine, which is not connected to the conveyor in any way, that makes the plane move forward. The wheels, the only part contacting the conveyor, plays no part in the forward movement of an airplane.