will it take off?

[WhyNot]

Sounds like a question for Myth Busters

 

[MikeBurr]

Well, lets see.
A plane is flying just above a truck going down a highway at the same speed as the plane - will the plane fall from the sky? - relative to each other, there is no motion. Other than wind, you would be able to walk across the top of the truck and enter the plane in the same way you could if it was sitting still at an airport.

But if you were to stand at the back of the truck and jump up, you would have a brief moment to find out that both were moving away from you at a very great speed.

We must always be sure not to confuse our point of view with the real world...

 

[Iowachild]

If I understand what is being said, it's like a person running on a treadmill. Can you win the Boston Marathon running on a treadmill. You never leave the room the treadmill is in. The plane will not take off. There will be no air flow over the wings. That's what makes airplanes fly. Bernoulli's Therorem. The air must travel farther over the top of the wing causing it to speed up and the pressure drop. The wing is sucked up. (Take a 8.5" X 11" sheet of paper, hold it right under your mouth and blow across the top to see this in action.) If you really are interested in learning more about flight, read the book "Stick and Rudder"An Explanation of the Art of Flying by Wolfgang Langewiesche. Of course if you don't enjoy physics, you won't enjoy the book.

 

[755inNY]

If I understand what is being said, it's like a person running on a treadmill.

It is not like a person running on a treadmill because the plane's forward motion is not accomplished by a rotational force of the wheels against the surface of the movable runway. If it were a car on a movable road then it would be like a person running on a treadmill. But then it would not be the movable road matching the speed of the car but rather the rotational speed of the wheels. The only time the movable road and car would be at the same speed would be when the speed was zero.

Jeff

 

[BillyP]

OK, now what is your explanation for this? I mean, we are defying SOOOO many physical possibilities already, what makes it IMPOSSIBLE for a prop plane of any imaginary composition to do this? Now, don't get me wrong, I don't know if there is or isn't. My degree is in biology. I just love science a whole so this discussion is fascinating to me.

rback33, I don't have a degree in anything. All I can do is close my eyes an imagine and it just don't happen

 

[Iowachild]

It is not like a person running on a treadmill because the plane's forward motion is not accomplished by a rotational force of the wheels against the surface of the movable runway. If it were a car on a movable road then it would be like a person running on a treadmill. But then it would not be the movable road matching the speed of the car but rather the rotational speed of the wheels. The only time the movable road and car would be at the same speed would be when the speed was zero.

Jeff

The plane has no forward motion!

 

[hutch4472]

I dont know why but it's baffleing to me that people think the plane would fly. The plane isnt moving. It is using all of its applied power, to overcome the conveyour's movement, to stay in one place. It NEEDS air flow over the wings in an amount that no prop nor jet can provide alone. The engines just provide forward movement to move air over the wings and control surfaces via airspeed.

 

[MossRoad]

When I worked at the airport in Niles, MI, there was an old dude there that had a Cub with a STOL kit. He came out on a very windy day and said, "Watch this!". He taxied out to the start of the runway, ran up his engine, rolled about 50 feet forward and it popped up into the air. He went up about a hundred feet, drifted backwards about a hundred feet and slowly brought it forward and down exactly at the start of the runway. He flew backwards. In relation to the ground, he went forward 50 feet, backwards about a hundred feet, and then forward about 50 feet again. His airspeed said he was going 40 miles an hour, but he went nowhere.

Now, had this been a calm day, he would have had to move forward up to 40 MPH in relation to the ground, before he would have been moving through the air at 40MPH, the magic number he needed for lift. He has to overcome the rolling friction with the ground first, then gain the airspeed. If he isn't going anywhere in relation to the ground first, he can't go anywhere in relation to a point in the calm air, either.

Here's another example from that same airport...
A guy came out with his weiner dog. The weiner dog was named Snoopy and was wearing a leather pilots helmet like a WWI ace. They hopped into his airplane and tried to taxi out of the flight line. He had forgotten to untie his tail from the steel cable in the grass. He gave it lots of power. He pulled about a hundred feet of cable behind him, ripping out anchor after anchor. You would think that with a cable anchored to the ground a hundred feet behind him and his engine generating max thrust that he could just pull back on the stick and pop into the air. Well, he couldn't because he had zero airspeed. The same thing would happen with an airplane on a treadmill. He has to get up some ground speed in relation to a fixed point before he can get up some airspeed. I threw in the weiner dog part because he was cute. He was disappointed that they couldn't go for a ride that day.

 

[755inNY]

I dont know why but it's baffleing to me that people think the plane would fly. The plane isnt moving. It is using all of its applied power, to overcome the conveyour's movement, to stay in one place. It NEEDS air flow over the wings in an amount that no prop nor jet can provide alone. The engines just provide forward movement to move air over the wings and control surfaces via airspeed.

Boy, I'm getting deeper into this than I thought I would. Those who feel the plane would not be moving are still thinking in terms of the wheels driving the plane forward. This is not what happens.

Let's use the example of the treadmill from Iowachild.

You are standing on one end of a long treadmill. You are holding on a cable coming from a winch at the far end of the treadmill (let's assume the cable has no weight so we don't have to worry about sag and stuff). For now, the treadmill is stationary but the cable is pulling at 3 mph. If you don't want friction burns from being dragged on the stationary belt, you will be walking at 3 mph. The cable keeps pulling you until you get to the other end of the treadmill.

OK, reset back to the other end of the treadmill. I flip the lever coupling the motor that drives the winch to also drive the treadmill. Gearing is such that the treadmill runs at exactly the same linear speed as the winch but in the opposite direction. Again, your goal is to not be dragged so you must start walking but this time to keep up with the 3 mph pull of the cable your legs must run at a 6 mph pace to keep perfect balance. You will get to the end of the treadmill in the same time as the previous run but a little more winded.

Now substitute the movable runway for the treadmill, the plane for your body, the propeller for the cable and the wheels for your legs. Why won't the plane move?

The propeller does not care what is happening between the wheels and the runway. It is pulling against the air (assuming a tractor configuration). Finally, I got tractor into the discussion!!!!

Jeff

 

[MossRoad]

One other story....

You try to stand behind an airplane with it's brakes locked directly behind the propeller or jet exhaust at full throttle and see if you can stand up... Nope, you get knocked on your keester.

How come you can stand by the length of the entire wing and not feel any breeze on that very same plane?

The plane can't go anywhere because the brakes on the wheels are locked. If the pilot releases the brakes, the plane starts moving forward. However, if the ground started moving backwards at the exact same time(like the treadmill scenario) the plane could not move forward in relation to the ground or the air. It would not fly.

 

[dirt clod]

It would fly. There is air around the plane and the conveyor. It's not the ground speed that is relative it is the air speed. In order for it not to take off there would have to be no air movement. i'm not talking about the prop wash but the simple fact is if you move the plane at say 100mph and the the conveyor moves at 100mph with 0mph wind you have 100mph wind blowing over the wings so you have lift. It is the same with take offs into the wind take off is easier not becouse you have more ground speed but becouse you have more air speed. Wind speeds for the weather is given at what the wind speed is against a none moving object. So regardless of how fast the plane is removing in regards a fixed point on earth if the plane if the plane is moving through the air at lift speed it will lift. It's like flying a kite in the wind if you have enough wind you can stand in one place and the kite will lift up. but if not enough wind running forward might be required again it's not the ground speed but the air speed of the object measured as the force against the object. since the prop screws through the air the plane would have to have a postive air speed to have any movement on the conveyor otherwise it would sit still. Air movement = lift.

 

[Iowachild]

Boy, I'm getting deeper into this than I thought I would. Those who feel the plane would not be moving are still thinking in terms of the wheels driving the plane forward. This is not what happens.
I am a pilot, I know what moves the plane forward!
Let's use the example of the treadmill from Iowachild.

You are standing on one end of a long treadmill. You are holding on a cable coming from a winch at the far end of the treadmill (let's assume the cable has no weight so we don't have to worry about sag and stuff). For now, the treadmill is stationary but the cable is pulling at 3 mph. If you don't want friction burns from being dragged on the stationary belt, you will be walking at 3 mph. The cable keeps pulling you until you get to the other end of the treadmill.
Get on a treadmill, don't move your feet, than start the treadmill. What happened?
OK, reset back to the other end of the treadmill. I flip the lever coupling the motor that drives the winch to also drive the treadmill. Gearing is such that the treadmill runs at exactly the same linear speed as the winch but in the opposite direction. Again, your goal is to not be dragged so you must start walking but this time to keep up with the 3 mph pull of the cable your legs must run at a 6 mph pace to keep perfect balance. You will get to the end of the treadmill in the same time as the previous run but a little more winded.

Now substitute the movable runway for the treadmill, the plane for your body, the propeller for the cable and the wheels for your legs. Why won't the plane move?
Put a toy airplane on the treadmill, start it up. What happens to the toy plane? Now tie a string to the toy plane and just hold it, don't pull it just hold it. Now what happens?
The propeller does not care what is happening between the wheels and the runway. It is pulling against the air (assuming a tractor configuration). Finally, I got tractor into the discussion!!!!

Jeff

 

[spanky8N]

My guess is no flight. As fast as the prop could potentially move the plane the "treadmill"would negate that forward motion exactly. If you were standing next to this I am guessing you would hear a loud plane engine and a fast moving "treadmill" and the plane would have no forward motion. No forward motion means no lift. Thats is my guess.
Dan

I would like to add that I would never attempt this stunt without the ROPS up and ear protection in place!!

 

[HTWT]

The plane will lift off when it has enough air speed.
When the plane is stationary the belt does not move, it only moves when the plain moves forware. It will require the wheels to turn twice as fast but the belt moving in the opposite direction will create air movement that will inhance the lift for the plane. The plane will move forward it only has to overcome the friction in the tires.

 

[755inNY]

Then you should know that using feet against a treadmill is not a good comparison for what moves a plane through the air. I am a pilot too.
I think we gotta find ourselves a moving runway .

Jeff

 

[sparky2terra]

A plane on a conveyor belt - Wikipedia, the free encyclopedia

But, anyone could have written this.

 

[Tom_Veatch]

The airplane will fly.

The scenario is described in such a manner that one is led to intuitively conclude that the aircraft does not move relative to a fixed observer. Thus, no airspeed and no lift. But that is an invalid conclusion. The airframe/engine will move through the air completely independently of the conveyor whether they are separated by one foot or one mile as long as the connection between them can be assumed to be frictionless or nearly so.

To get an understanding to this, perform the following thought experiment. Imagine you have a strut with a wheel on the end of it. Stand on the ground beside the conveyor and, holding the strut, place the wheel in contact with the conveyor. Walk forward allowing the wheel to roll on the conveyor. As you begin to move forward, the conveyor begins to move aft at the same speed you're walking forward. Does your motion stop? No. Conveyor movement has no effect on your movement. The wheel is spinning twice as fast as it would be if the conveyor were stationary, but the wheel rotation doesn't cancel your forward motion.

The situation with the airplane is identical. The aircraft accelerates forward along the conveyor for the same reason you are able to walk forward alongside the conveyor. The force accelerating you (your feet and legs thrusting against the ground) and the force accelerating the airplane at takeoff power (the engine propelling a quantity of air and/or exhaust gasses to the rear) are completely independent of the conveyor. In both cases, you and the airplane move forward without regard to the movement or non-movement of the conveyor. Aircraft do not derive their motion as a consequence of contact of their wheels with the ground. The wheels are purely and simply idlers and transmit little or no horizontal force between the aircraft and the ground. (That is obviously a false statement while the aircraft is braking or steering, but is generally true in a still air takeoff.)

Of course the takeoff distance of the aircraft may be slightly shortened because of a small headwind induced by the conveyor's boundary layer (What do you know, Reynolds Number does have an effect! ). Likewise, it may be slightly lengthened because of increased power loss to rolling resistance of the wheel, bearing friction, tire deformation, etc, due to the higher than normal rotational speed of the tires. But those are higher order effects and can be justifiably neglected in this example.

 

[Iowachild]

Then you should know that using feet against a treadmill is not a good comparison for what moves a plane through the air. I am a pilot too.
I think we gotta find ourselves a moving runway .

Jeff

Of course it is, it's still because of friction. A propellor pulls an airplane through the air the same way threads pull a screw into wood. If this conveyor belt is moving backwards at 60 mph, friction between the belt and the tires of the plane will move the plane backwards at 60 mph. It will take the same thrust to keep the plane from moving backwards as it would take to move the air plane forward at 60 mph on a non-moving runway. To move forward the thrust would have to increase beyond that point. What's your ground speed when your indicated airspeed is 120 mph and you are into a 60 mph headwind. How about your ground speed with the same ISA of 120 mph and a tailwind of 60 mph?
If you find that moving runway, I'm taking all bets and then getting myself that new tactor!

 

[Duke4AZ]

Sorry guys, Jeff and Tom are dead on. The plane will take off based on the assumptions they are describing. That is of course unless someone forgot to mention the tailwind that exactly matches, but in the opposite direction, the speed of the conveyer belt, which matches, but in the opposite direction, the speed of the plane which now negates the airflow over the wings resulting in an airspeed of zero and not only is the airplane not going to take off, I want my money back so I can drive my tractor to NY instead of flying.

Sorry, Jeff, had to throw that in there. Lets see what pain that will inflict on the old cranial mass.

 

[hutch4472]

It doesnt matter if the drive power is going to drive wheels or to create thrust (props or jet). All the drive power is doing is allowing the object (aircraft) to stay in one place instead of moving backwards with the conveyour belt!

When I run on the treadmill (rarely) there is no wind in my face like when I jog along the street (also rarely). If I hold a kite in my hand and jog on the street it will get lift, if I jog on the treadmill with a kite in my hand it hangs limp by my side. NO LIFT. Hutch