Mythbusters to take on "the plane and the treadmill" conundrum?

[ScottMac]

Actually, when landing, the throttle is at, or close to, idle ... after you've touched down and cleaned up the airplane (flaps up, carb heat in, etc) then you go to full throttle / max RPM and take off again.

When doing carrier landings, the throttle is pushed to max / afterburner on contact with the deck in case the tailhook fails or fails to catch one of the three wires.

In either case, if the runway / deck suddenly started to move against the forward motion of the airplane, it wouldn't matter. You'd still keep pulling the plane through the air mass until you reached enough velocity to lift off.

Runways, like theoretical treadmill belts, have no effect on the airplane's movement through the airmass that contains it.

FWIW

Scott

 

[exdeath]

Originally posted by: ScottMac
Actually, when landing, the throttle is at, or close to, idle ... after you've touched down and cleaned up the airplane (flaps up, carb heat in, etc) then you go to full throttle / max RPM and take off again.

When doing carrier landings, the throttle is pushed to max / afterburner on contact with the deck in case the tailhook fails or fails to catch one of the three wires.

In either case, if the runway / deck suddenly started to move against the forward motion of the airplane, it wouldn't matter. You'd still keep pulling the plane through the air mass until you reached enough velocity to lift off.

Runways, like theoretical treadmill belts, have no effect on the airplane's movement through the airmass that contains it.

FWIW

Scott

Ah thanks for clarifying, I was second guessing as I was writing that and wasn't sure. I did know it was flaps down to increase lift as you slow down to land, then flaps full up after touch down to stay glued to the runway. I'll pay more attention to throttle next time.

Regarding carriers... it wouldn't work with a military jet due to the low wing lift and need for a catapult, forward speed of the carrier, etc, to make up for the short deck of a carrier and the long runway required by a jet with low wing loading...

But a small Cesna could take off from the back of a Nimitz carrier going backwards along the carrier while the carrier moves forward. That is exactly the scenario here with the treadmill, however the ability of the plane to do this on the carrier depends only on the runway length needed to take off that particular plane in a tail wind.

 

[KillerCharlie]

I am an aerodynamics engineer for a major aircraft manufacturer--I design airplanes. I know exactly how takeoff performance is calculated and how the landing gear is typically modeled. I've read enough of these stupid internet threads. Usually the problem is stated in a way that tries to do something that is already impossible. Most of the time the question cannot be answered because it was posed too poorly.

For example, sometimes the question says the treadmill is adjusted so the airplane holds its position. It is impossible to do this. The speed of the wheels DOES NOT significantly affect the force on the airplanes from the wheels.

If you state the problem 100% clearly, I GUARANTEE you I'll give you the right answer.

Unfortunately there are just way too many armchair aerodynamicists in here that don't know what they're talking about.


Mythbusters are going to butcher this. Although they are entertaining, they don't have the best grasp of physics. To the aerospace engineer it will be like nails on a chalkboard.

 

[DrPizza]

Originally posted by: BigDH01
LOL, this thread is hilarious. I've been spreading this question throughout the office and most get it wrong at first thought but quickly change their answer.

I just paraphrased the original question but I think I got the spirit. If a giant treadmill the size of a runway existed, and we placed a plane on that runway, and set the treadmill up so that it sped at the plane's velocity but in the opposite direction, would the plane take-off? Most people still thought the plane would remain stationary. A quick explanation and people realized that the plane would take-off. The most useful analogy that I've seen here is the space shuttle analogy. When I used that most people seemed to get the idea.

The office now firmly believes that the plane will take-off given the question that I asked.

Yeah, that's generally the case. The intuition of the majority of people tells them at first that the plane will not move. After a quick explanation, the vast majority realize that the plane WILL move, and thus will take off. The people who don't realize this are either absolute idiots or trolls. I'm finding it hard to tell the difference in this thread.

 

[KillerCharlie]

Originally posted by: DrPizza

Originally posted by: BigDH01
LOL, this thread is hilarious. I've been spreading this question throughout the office and most get it wrong at first thought but quickly change their answer.

I just paraphrased the original question but I think I got the spirit. If a giant treadmill the size of a runway existed, and we placed a plane on that runway, and set the treadmill up so that it sped at the plane's velocity but in the opposite direction, would the plane take-off? Most people still thought the plane would remain stationary. A quick explanation and people realized that the plane would take-off. The most useful analogy that I've seen here is the space shuttle analogy. When I used that most people seemed to get the idea.

The office now firmly believes that the plane will take-off given the question that I asked.

Yeah, that's generally the case. The intuition of the majority of people tells them at first that the plane will not move. After a quick explanation, the vast majority realize that the plane WILL move, and thus will take off. The people who don't realize this are either absolute idiots or trolls. I'm finding it hard to tell the difference in this thread.

The interesting part is that since the wheel speed doesn't change the force on the airplane coming from the wheels (proportional only to normal force--weight minus lift), the plane wouldn't even know the difference. The plane will take off in the same distance.

 

[chrisms]

Originally posted by: KillerCharlie
I am an aerodynamics engineer for a major aircraft manufacturer--I design airplanes. I know exactly how takeoff performance is calculated and how the landing gear is typically modeled. I've read enough of these stupid internet threads. Usually the problem is stated in a way that tries to do something that is already impossible. Most of the time the question cannot be answered because it was posed too poorly.

For example, sometimes the question says the treadmill is adjusted so the airplane holds its position. It is impossible to do this. The speed of the wheels DOES NOT significantly affect the force on the airplanes from the wheels.

If you state the problem 100% clearly, I GUARANTEE you I'll give you the right answer.

Unfortunately there are just way too many armchair aerodynamicists in here that don't know what they're talking about.


Mythbusters are going to butcher this. Although they are entertaining, they don't have the best grasp of physics. To the aerospace engineer it will be like nails on a chalkboard.

Yes, you are the big shot aerospace engineer. Yet you can't answer the question and all of these "armchair aerodynamicists" can. Don't blame it on the way the question is asked, you can simply word the question how you see fit and give us an answer. Instead you say how smart you are and how you would give us an answer if only we were smart enough to ask you.

This question doesn't require an aerospace engineer to answer. It is really quite simple so spare us the bullsh1t, please.

 

[KingofCamelot]

Originally posted by: Citrix
because the treadmill is negating that forward movement.

go run on a treadmill and the treadmill has a sensor that changes the speed of the treadmill if you move forward of the dead center of the belt. if you all of a sudden sprint and move forward the belt goes faster to push you back, you are stationary to the ground. now bolt on some wings to your shoulder will you fly?

No.

Have someone walk up behind you, and push you in the back. Do you move forward? Of course you do, because an external force was applied. It doesn't matter if the system of your feet and the treadmill are in equalibrium, there is another system at work, you and your friend pushing you, which is not in equalibirum.

Its just like when a car is spinning its wheels in mud and not going anywhere. How do you get the car out? You push it. The engine on an airplane is like that push, while the wheels on the treadmill are like the wheels in the mud, which get no where.

 

[jagec]

Ha! I almost replied to someone in this thread, before realizing how long it was! History repeats itself, but ATOT does so much faster.

 

[WHAMPOM]

Originally posted by: KillerCharlie
I am an aerodynamics engineer for a major aircraft manufacturer--I design airplanes. I know exactly how takeoff performance is calculated and how the landing gear is typically modeled. I've read enough of these stupid internet threads. Usually the problem is stated in a way that tries to do something that is already impossible. Most of the time the question cannot be answered because it was posed too poorly.

For example, sometimes the question says the treadmill is adjusted so the airplane holds its position. It is impossible to do this. The speed of the wheels DOES NOT significantly affect the force on the airplanes from the wheels.

If you state the problem 100% clearly, I GUARANTEE you I'll give you the right answer.

Unfortunately there are just way too many armchair aerodynamicists in here that don't know what they're talking about.


Mythbusters are going to butcher this. Although they are entertaining, they don't have the best grasp of physics. To the aerospace engineer it will be like nails on a chalkboard.

BumbleBees cannot fly. Aerodynamically. Practical answer to a plane on a treadmill. Before the palne could aquire lift, the spinning wheels would generate so much heat, the tires would blow or the bearings seize. After the loss of the spinning tire as an anti-friction device, the plane then nose dives and the moving treadmill tears it all to hell.

 

[smack Down]

Originally posted by: exdeath
Further proof...

When practicing landing it is common to perform what is called a 'touch and go' where you make a complete landing, all wheels make contact with the ground and begin to roll, but instead of backing off the throttle you keep it full power with the flaps full up holding you down (like a spoiler on a car), rolling down the runway. You can maintain this for the entire length of the runway, with the plane going forward and the ground moving backwards, and rotating the wheels the whole time. This has no effect on the planes forward velocity, and you can pull up and climb away as soon as you reach the end of the runway, or any time you want, with the wheels still freely spinning.

Being in a plane with a stall speed of 150 mph or 300 mph will change the speed at which you roll the wheels along the ground, but again, the speed the wheels are rolling, or that the wheels are even touching the ground, has no effect on the plane mainting its forward velocity under engine power.

I think you need to look up prof and idiotic examples in the dictionary.

Your example lacks the important effect of the treadmill that matches the speed of the object on top of it.

 

[smack Down]

Originally posted by: DrPizza

Originally posted by: BigDH01
LOL, this thread is hilarious. I've been spreading this question throughout the office and most get it wrong at first thought but quickly change their answer.

I just paraphrased the original question but I think I got the spirit. If a giant treadmill the size of a runway existed, and we placed a plane on that runway, and set the treadmill up so that it sped at the plane's velocity but in the opposite direction, would the plane take-off? Most people still thought the plane would remain stationary. A quick explanation and people realized that the plane would take-off. The most useful analogy that I've seen here is the space shuttle analogy. When I used that most people seemed to get the idea.

The office now firmly believes that the plane will take-off given the question that I asked.

Yeah, that's generally the case. The intuition of the majority of people tells them at first that the plane will not move. After a quick explanation, the vast majority realize that the plane WILL move, and thus will take off. The people who don't realize this are either absolute idiots or trolls. I'm finding it hard to tell the difference in this thread.

The vast majority of people will consider the treadmill as matching the speed of the aircraft relative to treadmill surface in which case the plane can't take off. (ie they think of the case of a car driving on a treadmill but staying put relative to ground). So if the question states the plane can't move then it can't take off. If you change the question to the treadmill matching the ground speed then the plane would take off and a car on the treadmill would also advance.

 

[loic2003]

Originally posted by: smack Down
I think you need to look up prof and idiotic examples in the dictionary.

Your example lacks the important effect of the treadmill that matches the speed of the object on top of it.

A real man would admit he was wrong instead or resorting to petty insults.

I will accept the above as admission that you realise you are indeed incorrect and have finally realised it.

Physics 1, Smack Down 0

 

[skace]

When I first saw this thread, I knew that it would never be able to stay on the topic of the show and devolve into the good ol argument . I'm glad to see the same old impossible scenario being used to keep the trolling going, oh you know, that the treadmill can match wheel speed when wheel speed would always be faster than it outside of standstill.

 

[Darwin333]

I am a new pilot in training so correct me if my analogy is wrong but I am confident I am right.

I fly in a Cessna 172, it has an approach speed of 60mph (flaps). You take an aircraft carrier in the sea with a 30mph wind. If the carrier turns INTO the wind and moves at 30mph itself that puts the wind speed over the deck at 60mph. If I am correct from my classes, the Cessna could appear to land like a helicopter (or a harrier) from the deck of the ship. It would not need much of a runway because relative to the ship it would have no forward motion however the Cessna would still have 60mph of wind going across the wings giving it the lift required for flight.


To the engineer here is a question. In the above situation shouldn?t the plane theoretically be able to go straight up and straight down (think VTOL taking off from the deck and landing with the wheels never moving) relative to the ship as long as there is 60mph of wind moving across the deck and the plane is faced into it with the proper throttle setting?

 

[exdeath]

Originally posted by: smack Down

Originally posted by: exdeath
Further proof...

When practicing landing it is common to perform what is called a 'touch and go' where you make a complete landing, all wheels make contact with the ground and begin to roll, but instead of backing off the throttle you keep it full power with the flaps full up holding you down (like a spoiler on a car), rolling down the runway. You can maintain this for the entire length of the runway, with the plane going forward and the ground moving backwards, and rotating the wheels the whole time. This has no effect on the planes forward velocity, and you can pull up and climb away as soon as you reach the end of the runway, or any time you want, with the wheels still freely spinning.

Being in a plane with a stall speed of 150 mph or 300 mph will change the speed at which you roll the wheels along the ground, but again, the speed the wheels are rolling, or that the wheels are even touching the ground, has no effect on the plane mainting its forward velocity under engine power.

I think you need to look up prof and idiotic examples in the dictionary.

Your example lacks the important effect of the treadmill that matches the speed of the object on top of it.

Your rebutal lacks the important effect of the treadmill not making a world of difference since it cannot act directly on the body of the aircraft in a direction opposite of engine thrust, let alone act in a force of the required magnitude.

I think you need to pick up a book on physics... no wait... you'd probably come back here arguing that F=ma is invalid... on second thought don't ever open a physics book.

If you can show that a single roller bearing in one of the wheels can generate the 5,000+ lbs of friction resistance at 180 mph wheel speed, and that the tires can transfer that force through to the treadmill without sliding, and that the treadmill belt can hold that force without tearing, I might change my views. To refresh, assume 40 bearings total = 2 on each side of each of 20 wheels, 4 wheels per 5 landing gear truck on the 747, which produces about 220,000 lbs / 1,000,000 Newtons of thrust. Each bearing would have to produce 5,500+ lbs of force against the axle, in order for all 40 bearings to contribute 220,000 lbs of opposing force against the engines. Again, assume the tires will grip the treadmill surface 100% without sliding, and focus on just the bearing forces.

Until then, stick to flipping burgers.

 

[MercenaryForHire]

Originally posted by: exdeath
Until then, stick to flipping burgers.

If a burger was on a grill with an infinitely greased surface, and it's rotating at a speed of 5mph, and smack Down flips it with a spatula of 25 square inches, how much does he make per hour doing it?

- M4H

 

[junkiefp]

Originally posted by: exdeath

Originally posted by: junkiefp

Originally posted by: Jeff7181
Just skimming through this thread I'm amazed how many people don't know how a plane gets airborne.

Indeed. Without moving air which creates lift on the wings the plane would not move up.

Planes do not 'move air' or 'blow air across the wings'. Planes push themselves against the air with thrust far less than the weight of the plane, relying on the forward motion of the wings through the air to generate lift. It's like sweeping your hand through the water at the right angle; with only movement in the horizontal plane your hand feels an upforce due to principles of fluid dynamics (of which aerodynamics is a specific class of fluid dynamics that deals with air and flight, particularly in the Earth's atmosphere).

There are few exceptions, such as the F-15 which has a thrust to weight ratio so high that it can pretty much fly like a rocket on thrust alone, even accelerating straight up. It doesn't really even need wings other than for directional control. An F-15 has almost 60,000 lbs of thrust and the unarmed plane weights 30,000 lbs with 2,000 lbs of JP8 in the tanks. Compare that to a 747 which is 800,000 lbs and generates about 220,000 lbs of thrust. This is why the 747 has such large wings and relys on wing lift rather than engine power. You could not fly a 747 straight up or perform a loop, it would start to stall out and lose air speed and level itself out.

If you've heard of the term 'stall speed' this has nothing to do with the engines, but with the speed at which the lift generated by the wing design is no longer greater than or equal to the weight of the plane. The engines only need be powerful enough to overcome air resistance drag encountered by the plane as it keeps the planes forward velocity greater than or equal to the stall speed. As the wings attached to the plane are forced through the air by that forward motion, they manipulate the air in a way that produces lift in the same way as the hand in water example. Variable geometry such as flaps, and angle of attack also come into play as much as the shape of the fixed portion of the wing.

In this case the treadmill's movement simply slips against the wheel bearings and rotates the wheels faster, the plane is still able to push itself against the surrounding air with it's engines and gain the required forward speed for the wing design to work. On a 747, the wings generate enough lift to reach equillibrium against the 800,000 lb weight of the plane with only 180 mph of forward movement. At this point there is no weight on the landing gears.

Think about a plane, with rockets instead of turbofans, out in space. Spin the wheels up as fast as you can in either direction at any speed you want. Even give the plane a push backwards so that it may continue backwards indefinately. Ignite the rocket engines, and the plane will slow down, then reverse direction and start heading forward, regardless of what the wheels are doing. A turbofan jet engine pushes on the surrounding air in an enclosed atmosphere in the same way that a rocket pushes itself on the rocket exhaust byproducts.

What the wheels are doing or what they are in contact with absolutely do not matter, provided they are rolling or not snagging on something, in which case they would just be torn off by the jet thrust. The wheels free spinning on any surface at any speed have little effect on the acceleration of the plane in the horizontal plane. The constant force of the engines will eventually accelerate the plane to its stall speed and beyond and cause the plane to lift off the ground. If the plane was kept still and started going backwards on the conveyer (no rolling of the wheels occuring), it would just take longer and take more runway, because with constant acceleration, it would have to go from -180 mph to +180 mph instead of 0 to 180 mph. It would still be the same force, the same power output, just applied over a longer time to overcome rearward inertia first.

Spinning the wheels faster does not impart any significant drag to the plane in opposition to the engines, that is what the axles and bearings are for. In fact, the faster the plane begins to move foward, the less drag there is on the axles because as the wings build up lift, the weight of the plane decreases and the normal forces involved in friction decrease.

Hey smart ass. Where do you see me say that planes move air across the wings? Engines propel plane forward, air rushes past the wings creating lift...

 

[BigDH01]

Originally posted by: Darwin333
I am a new pilot in training so correct me if my analogy is wrong but I am confident I am right.

I fly in a Cessna 172, it has an approach speed of 60mph (flaps). You take an aircraft carrier in the sea with a 30mph wind. If the carrier turns INTO the wind and moves at 30mph itself that puts the wind speed over the deck at 60mph. If I am correct from my classes, the Cessna could appear to land like a helicopter (or a harrier) from the deck of the ship. It would not need much of a runway because relative to the ship it would have no forward motion however the Cessna would still have 60mph of wind going across the wings giving it the lift required for flight.


To the engineer here is a question. In the above situation shouldn?t the plane theoretically be able to go straight up and straight down (think VTOL taking off from the deck and landing with the wheels never moving) relative to the ship as long as there is 60mph of wind moving across the deck and the plane is faced into it with the proper throttle setting?

Yeah, you can do some fun things in a Cessna. I've had a negative ground speed before when facing a stiff wind.

You could actually do what you are describing but it wouldn't work well. You'd still have to have the engines running and when you actually lifted up from the ground you'd start to accelerate. I guess the only way to test this would be to have chalk immobilize the wheels and pull back on the stick with engine running. As soon as the wheels lifted off the ground you'd start to accelerate forward. You could also try to hover above the carrier for landing and set it down. Sounds dangerous to me though.

 

[jjzelinski]

Originally posted by: Darwin333
I am a new pilot in training so correct me if my analogy is wrong but I am confident I am right.

I fly in a Cessna 172, it has an approach speed of 60mph (flaps). You take an aircraft carrier in the sea with a 30mph wind. If the carrier turns INTO the wind and moves at 30mph itself that puts the wind speed over the deck at 60mph. If I am correct from my classes, the Cessna could appear to land like a helicopter (or a harrier) from the deck of the ship. It would not need much of a runway because relative to the ship it would have no forward motion however the Cessna would still have 60mph of wind going across the wings giving it the lift required for flight.


To the engineer here is a question. In the above situation shouldn?t the plane theoretically be able to go straight up and straight down (think VTOL taking off from the deck and landing with the wheels never moving) relative to the ship as long as there is 60mph of wind moving across the deck and the plane is faced into it with the proper throttle setting?

Many (millions upon millions) moons ago, there was a flying pterosaur by the name of quitzoquatlas (butchered spelling but I litteralyl can't get close enough to even spell check) that was the largest flying animal of all time. It was so big that it relied on the VTOL you described just to get off the ground. I'm sure you've also seen the same effect with birds on a windy day. It would seem actual movement in space time is not whats important in flight, it's simply one method of generating enough wind speed to provide lift for the wings.

EDIT:

Quetzalcoatlus

 

[exdeath]

Originally posted by: MercenaryForHire

Originally posted by: exdeath
Until then, stick to flipping burgers.

If a burger was on a grill with an infinitely greased surface, and it's rotating at a speed of 5mph, and smack Down flips it with a spatula of 25 square inches, how much does he make per hour doing it?

- M4H

$5.15 /hr

Skill is not relevant in this problem and has absolutely no effect on the income. The burger will cook no matter what.

 

[Darwin333]

Originally posted by: BigDH01
You'd still have to have the engines running and when you actually lifted up from the ground you'd start to accelerate. I guess the only way to test this would be to have chalk immobilize the wheels and pull back on the stick with engine running. As soon as the wheels lifted off the ground you'd start to accelerate forward. You could also try to hover above the carrier for landing and set it down. Sounds dangerous to me though.

But would you accelerate away from the spot you just took off from? Flying into a 30mph wind with a ground speed of 30mph puts 60mph of wind over the wings which is above the stall speed (with flaps). So in theory you would match the acceleration of the carrier and therefor not seem to move forward or reverse of the spot you took off from, right?

Of course a sudden decrease in wind speed would definitely ruin your day, but I am just curious if my theory is correct.

 

[randay]

Originally posted by: exdeath

Originally posted by: MercenaryForHire

Originally posted by: exdeath
Until then, stick to flipping burgers.

If a burger was on a grill with an infinitely greased surface, and it's rotating at a speed of 5mph, and smack Down flips it with a spatula of 25 square inches, how much does he make per hour doing it?

- M4H

$5.15 /hr

Skill is not relevant in this problem and has absolutely no effect on the income. The burger will cook no matter what.

The burger will not cook. With no heat, there can be no cooking. no heat = no cook. The grill torques the burger and causes a buildup of kinetic energy, resulting in an angular velocity equal to the fat content of the burger. Its newtons third law, look it up some time!

 

[pnad]

Originally posted by: smack Down
The vast majority of people will consider the treadmill as matching the speed of the aircraft relative to treadmill surface in which case the plane can't take off. (ie they think of the case of a car driving on a treadmill but staying put relative to ground). So if the question states the plane can't move then it can't take off. If you change the question to the treadmill matching the ground speed then the plane would take off and a car on the treadmill would also advance.

This is where you get it wrong. If the object on the treadmill is a vehicle driven by the wheels (a car) and the treadmill was able to match the forward ground speed (same as wheel speed) of the car then the car would NOT move forward in space.

The treadmill can't hold the airplane in one place. The treadmill contacts the airplane in one place - the wheels. Therefore the treadmill can only have an effect on the wheel speed. Wheel speed has no effect on airspeed of the plane.

From your past posts it appears you believe the plane will not move because you think the question/problem STATES that the treadmill keeps the plane stationary. I do not read the problem like that. The treadmill ATTEMPTS to keep the plane stationary by matching it's speed.

If the treadmill tries to match the plane's airspeed (150?) then the treadmill will move backwards at 150mph, the plane moves through the air at 150mph and the wheels turn at 300mph. The wheels turning at 300mph do not produce enough rolling resistance to slow the plane down at all.

If the treadmill tries to match the plane's wheel speed the treadmill would quickly accelerate to it's maximum speed because no matter what speed the treadmill turns - the wheels will be going that speed PLUS the airspeed of the plane. If you can imagine a treadmill with unlimited speed potential, then I can imagine an airplane wheel that has the same rolling resistance at any speed. The wheels spinning at any speed do not slow the plane down at all.

I don't think you are stupid smack Down. I think you are just reading the problem wrong.

 

[Ramma2]

What if the plane is loaded with rock salt?

 

[AbsolutDealage]

Originally posted by: Ramma2
What if the plane is loaded with rock salt?

What if you had an alternator-sized hunk of bulk beef in the backseat?