will it take off?

[NewToy]

The speed of sound is proporsional to the density of the medium that it is travlling through. Example; water transmits sound faster than air and as you go further up in our atmoshere, the speed of sound gets slower as the atmoshere gets thinner until eventually sound will not travel in the vacuum of outerspace. I believe methane is a heavy gas ,more dense, so sound would travel faster, the same goes for smoggy NYC.

So, lead or gold would be a good conductor of sound waves? What is the speed of sound limited to in the most dense material? Does it eventually hit a wall? It travels 700 some mph in our atmosphere, is it's speed increase proportional to the density of the conducting medium? Not trying to be contrary, just curious.
John

 

[RayH]

Yes, I agree that the plane needs airspeed and not ground speed to take off and yes I understand that the wheels are not a source of thrust - they have nothing to do with POWERING the plane to take off. BUT the wheels do represent the speed of the plane while it is on the ground. So if the wheel is turning 5 rotations per second then the plane is moving forward at 5 rotations per second. If, you somehow magically counter that and move the plane backwards at an identical rate (5 rotations per second) then you effectively cancel out any movement of the plane. It is not moving. Yes, the propeller provided thrust to move it forward, but the ground moved the plane backwards an equal distance thus taking airspeed to zero.

To me, the conveyor is identical to the scenario of the brakes being locked. If the breaks are locked and thus the plane does not move, can it take off? No because it can't generate air speed. You point out that "all the wheels do is allow the plane to travel at the speed of the prop" - AGREED. And in this case, I interpret the conveyor to be preventing that movement. I guess it depends on your interpretation of the conveyor. If it truly can match the speed of the wheels instantly, then to me it is the same as the wheels not spinning which is the same as the brake being on. no movement = no airspeed = no flight

Ahhh! I see where youre coming from. The plane will still takeoff but I see where you're hung up.
For the plane to take off, the wheels will spin faster than the conveyor. Do you agree that the wheels are capable of spinning faster than the conveyor? If the wheels are spinning faster forward than the conveyor is moving backward, the plane has forward motion and will take off.

 

[dillo99]

You are failing to see that the thrust from the engine IS the same thing as a cable pulling the plane, you just can't see it. Why can't the conveyor match wheel speed if being pulled by a cable? It can, and will result in the wheels turning 2x at liftoff. The rearward force created by the thrust must result in the plane moving forward. Say the plane had a magnetic levitation (ala bullet train) landing gear. The thrust moves the plane forward an inch above the conveyor which is moving the opposite direction the plane is at the same speed, time etc. The conveyor isn't impeding the movement of the plane just like the wheels freely spinning would have no effect other than the negligible amount of energy lost in bearing and tire friction. Where do you propose all the thrust energy is going? It must propel the plane forward, just the wheels will be going twice as fast at LIFTOFF!
John

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.

 

[jwstewar]

"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."

Question for you guys that say it won't fly. Using the quote from above, if the conveyor belt matches the speed of the plane and you guys say the plane isn't moving, doesn't that mean the conveyor belt isn't moving as well? So what becomes of all that thrust the jet engines/prop are producing?

I'm just as confused at how they say the plane won't fly as they are of us saying the plane will fly.

 

[RayH]

So, lead or gold would be a good conductor of sound waves? What is the speed of sound limited to in the most dense material? Does it eventually hit a wall? It travels 700 some mph in our atmosphere, is it's speed increase proportional to the density of the conducting medium? Not trying to be contrary, just curious.
John

Sound is waves of pressure. Any material that can stop pressure waves will stop sound. The density effects how far sound is transfered as well as how fast, but in the opposite manner. Sound may not travel as far through dense material but it will travel faster for a shorter period. Does that make sense?
Example, in air, I can yell at you and you can hear it 500 yards away but if I yell at you underwater, you can only hear it 100 yards away.

 

[JimParker]

OK - I've really tried to stay out of this, because you can "prove" either viewpoint. Here's my answers:

Answer #1) If you use the assumptions in the original posting, and ignore reality, then the plane will NOT fly. A plane accelerates when THRUST (provided by the engine/propeller) exceeds DRAG. If the magic treadmill is capable of "infinite speed", it WILL produce enough friction drag to exactly equal the thrust of the propeller in order to keep it stationary. As long as THRUST = DRAG, acceleration is ZERO. The plane doesn't move, and therefore cannot develop either groundspeed OR airspeed. The one quibble I have with the people in this camp is that in order to produce enough friction (DRAG) to overcome the thrust, the wheels would be turning a whole lot faster than 2X liftoff speed. To get this much drag (again assuming no heat effects that would melt down the bearings, tires, and treadmill surface, and ignoring the centrifugal force that would tear the wheels apart), the treadmill might be turning the wheels at several hundred miles per hour!

Answer #2) If you assume anything remotely resembling reality, the plane absolutely WILL fly. Given that there is INERTIA to the "rotating mass" of the treadmill, it cannot "instantly accelerate" to overcome the increased THRUST when the engine/propeller start up. Since THRUST > DRAG (for whatever short period of time it takes the treadmill to begin to react), the airplane would begin to accelerate. Once that accelaration begins, the end result is inevitable, and the airplane will eventually obtain sufficient airspeed to lift off. (At which point, the drag from the wheel friction would go to zero, and the plane would accelerate/climb very quickly!)

Background - started as an Aerospace Engineering major, switched to EE. Spent 8 years in the Army, most of it as a flight instructor, specializing in aerodynamics and principles of flight. And I agree with an earlier poster: "Stick and Rudder: An Explanation of the Art of Flying" by Wolfgang Langewiesche is the best "real world" text on how airplans fly that was ever written. Even more amazing, since we theoretically have learned a lot since the book was first published back in 1944...

My wife wants to know what any of this has to do with tractors?

 

[BillyP]

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.

 

[RayH]

My wife wants to know what any of this has to do with tractors?

Thats funny, my wife wants to know what any of this has to do with me.

 

[RayH]

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

 

[755inNY]

Yes, I agree that the plane needs airspeed and not ground speed to take off and yes I understand that the wheels are not a source of thrust - they have nothing to do with POWERING the plane to take off. BUT the wheels do represent the speed of the plane while it is on the ground. So if the wheel is turning 5 rotations per second then the plane is moving forward at 5 rotations per second. If, you somehow magically counter that and move the plane backwards at an identical rate (5 rotations per second) then you effectively cancel out any movement of the plane. It is not moving. Yes, the propeller provided thrust to move it forward, but the ground moved the plane backwards an equal distance thus taking airspeed to zero.

To me, the conveyor is identical to the scenario of the brakes being locked. If the breaks are locked and thus the plane does not move, can it take off? No because it can't generate air speed. You point out that "all the wheels do is allow the plane to travel at the speed of the prop" - AGREED. And in this case, I interpret the conveyor to be preventing that movement. I guess it depends on your interpretation of the conveyor. If it truly can match the speed of the wheels instantly, then to me it is the same as the wheels not spinning which is the same as the brake being on. no movement = no airspeed = no flight

A few members have taken this discussion from the speed of the airplane to the speed of the wheels. I recall, many pages ago, that the scenario stated the converyor exactly matched the speed of the plane.

First we struggled with the concept of how a plane moves (thrust from propeller vs. force applied to the wheels) now we are focused on wheel speed and the incorrect assumption that the wheel speed and plane speed have to be equal. They do not.

For example, hold the plane in position (tied to the ground) and run the conveyor up to 25 mph. The wheels will be spinning at a 25 mph rate but the plane's speed is zero. The force on the ropes to hold the plane in position on the conveyor will be minimal as the only force being resisted is the rolling resitance of the wheels. Increase the conveyor speed to 50 mph and the force on the ropes will go up a little due to the increase in friction at the wheels. The plane's speed will still be zero. The speed of the wheels is independent of the speed of the plane.

Another example. Place the plane on an smooth, ice covered runway with the brakes locked. Apply power to the engine (thrust from the prop). the plane will move down the runway picking up speed and it will take-off even though the wheel speed is zero. The speed of the wheels is independent of the speed of the plane.

One thing that has to be a given for this scenario is that the reference point for the speed of the conveyor and the plane is the ground (and still air). Logically it can''t be anything else to meet the requirement that the speed of the conveyor matches the speed of the plane. So with respect to the ground (air) the plane can be moving 50 mph in one direction and the conveyor moving exactly 50 mph in the opposite direction. The wheels take up the difference by spinning at the combined speed of 100 mph. The plane does need to have enough thrust from the propeller to overcome the increased resistance of the wheels spinning at 100 mph instead of 50 mph. That would not be very much.

The plane will take off.