A jet plane on a large treadmill

I think I may be moving over to the fly side.
Its not real easy to figure though, so everyone calling everyone else an idiot for whatever theyri reasoning is isnt making much sense either. Its a complicated problem and Im still not so sure.

Everyone knows what makes an airplane fly- need air flow (or low pressure above the wing relative to the bottom). Thats fine everyone agrees.

The only thing people arent agreeing on is if the plane is moving forward (for some this whole post will be really redundant but Im trying to sort it out for everyone).

There are 4 forces in aerodynamics- thrust, drag, lift, and gravity.

For our purposes we can see that the legs/wheels of the plane are taking care of the lift AT THIS POINT WHILE ITS ON THE TREADMILL.
The opposing force is gravity, which is pulling the plane down. Here we have the equal and opposite reactions keeping the plane where it is (i.e. not going up or down)

The thrust is the jet engines pushing the plane forward. The drag is the air resistance (AND the treadmill, to a degree- Ill get to this later). For those who say the plane wont move forward, its because of the treadmill making the relative forward movement 0 right? The thrust is independent of the treadmill. The treadmill will spin the wheels in the direction it is moving but the plane, an independent system relative to the wheels, will continue forward.
If you put a plane on a giant ice rink (or a fucking frozen lake) and shot the engines off, it would move forward, despite no reaction between the wheels and the surface. The wheels are independent of the engines and forward motion. The wheels only hold it up in place.
So, I would say that the plane could move forward. My only thing is that THERE IS FRICTION in this world- otherwise...whatever. This all assumes a frictionless scenario which is stupid and hypothetical because without friction there isnt lift/drag/whatever anyway.
This friction is going to have some backward affect on the plane, between the wheels spinning etc, so Im just saying while it can move forwardusing the thrust of the engines, itll require a lot more power and Im not sure a plane can exert enough extra force to get it off the ground (to move forward fast enough-realistically). This is what I meant by the drag also being part of the wheels. Of course, if the plane CAN overcome this extra drag (using extra thrust) it will fly. Once in the air itll have normal drag.
Sorry for the apparent flip flop

Brock,

You got it, and to help clearify:

Rotational friction is independant of speed. The treadmill could be going the speed of light and would exert the same frictional force as regular ground. This is a common high school physics experiment where you drag a skateboard around with a load of bricks on it at different speeds and measure the tension on the spring. Friction does not increase with speed.

Quote:

---

Originally Posted by DJSapp

Brock,

You got it, and to help clearify:

Rotational friction is independant of speed. The treadmill could be going the speed of light and would exert the same frictional force as regular ground. This is a common high school physics experiment where you drag a skateboard around with a load of bricks on it at different speeds and measure the tension on the spring. Friction does not increase with speed.

---

Yup, Im just thinking the mass (Normal Force) of the plane will affect the friction force just in terms of sheer magnitude, and the thrust of a normal plane may not be able to overcome this enough to fly. It may be able to as well- I majored in physics, not aerospace engineering, and dont know the force needed to fly compared with mass.

Quote:

---

Originally Posted by Brock Landers

Yup, Im just thinking the mass (Normal Force) of the plane will affect the friction force just in terms of sheer magnitude, and the thrust of a normal plane may not be able to overcome this enough to fly. It may be able to as well- I majored in physics, not aerospace engineering, and dont know the force needed to fly compared with mass.

---

Wait, hang on. The treadmill doesn't make the plane heavier.

Rotational friction = u * M * g / r

Airplanes overcome this every day. This isn't aerospace engineering.

Youre right, it just seems like the friction at the wheels would be a little greater with the treadmill going in the other direction right? Maybe not, Im still thinking it moves forward...my friction thoughts are based more on intuition I guess which gets physical mechanics screwed up anyway

Quote:

---

Originally Posted by Brock Landers

Youre right, it just seems like the friction at the wheels would be a little greater with the treadmill going in the other direction right? Maybe not, Im still thinking it moves forward...my friction thoughts are based more on intuition I guess which gets physical mechanics screwed up anyway

---

In the real world, it does. There's heat loss, fluid friction from the grease on the bearings, and lots of other tiny stuff. When compared to the 380,000 lbs. of thrust a 747 can generate, those losses don't matter.

Quote:

---

Originally Posted by DJSapp

In the real world, it does. There's heat loss, fluid friction from the grease on the bearings, and lots of other tiny stuff. When compared to the 380,000 lbs. of thrust a 747 can generate, those losses don't matter.

---

Sounds good, I didnt know how much extra "doesnt matter" stuff a plane taking off could take. I know theres a ton of thrust needed just to get it off the ground but it looks like theres enough to overcome this extra stuff, thanks for the insight/help/whatever

As I point out in the other thread, the speed required by the treadmill based upon those small difference to get enough frictino to hold the plane stationary (in the dubious supertreadmill interpreation) would result in enough treadmill interaction with the air that it would probably generate wind... which would generate lift... which would decrease the normal force... which makes the treadmill go waaay faster... and then the plane takes off (assuming no mechanical failure of the wheels/tires/treadmill)

plane might stall when it gets out of the generated wind ground effect area if it doesnt accelerate enough... but it will take off!

ETA the lower the aircraft's takeoff speed (lighter the weight) the sooner it happens
ETA if you switch to another aircraft whose wings are lower to the ground, you get a more significant wind effect sooner too

Quote:

---

Originally Posted by Summit

As I point out in the other thread, the speed required by the treadmill based upon those small difference to get enough frictino to hold the plane stationary (in the dubious supertreadmill interpreation) would result in enough treadmill interaction with the air that it would probably generate wind... which would generate lift... which would decrease the normal force... which makes the treadmill go waaay faster... and then the plane takes off (assuming no mechanical failure of the wheels/tires/treadmill)

plane might stall when it gets out of the generated wind effect area if it doesnt accelerate enough... but it will take off!

---

I see your point, I just dont think even at ridiculous speeds would the treadmill cause enough wind/airflow over and under the wing to cause lift. Maybe though- Ive never been exposed to a supertreadmill first hand, this is just my "supertreadmill JONG" opinion. And not to nitpick, but itll only affect the net normal force- the normal force is always m x g, provided the m is normal to the ground. If the lift were to occur, it would increase lift, not decrease normal force... SEMANTICS, Im a nerd

Quote:

---

Originally Posted by Brock Landers

And not to nitpick, but itll only affect the net normal force- the normal force is always m x g, provided the m is normal to the ground. If the lift were to occur, it would increase lift, not decrease normal force... SEMANTICS, Im a nerd

---

isn't normal force the force thats exerted by the surface on the aircraft? i think N=m*g - F(lift)

anyways, friction forces become less

Quote:

---

Originally Posted by Brock Landers

I majored in physics, not aerospace engineering, and dont know the force needed to fly compared with mass.

---

Of course you do. The upward component of the force (lift) must be greater than m*g.

Quote:

---

Originally Posted by The AD

Of course you do. The upward component of the force (lift) must be greater than m*g.

---

I meant specifics. Like I dont know how much a plane weighs or how much thrust it has

How about this..... have the treadmill turn the other way. Would the plane take off.....NO. The wheels on the plane would spin backwards while sitting still. It is the force of the thrust of the jet engines that moves the plane.

I think the example of the plane on a sheet of ice is a good one.

Quote:

---

Originally Posted by BuccaneerBanzai

How about this..... have the treadmill turn the other way. Would the plane take off.....NO. The wheels on the plane would spin backwards while sitting still. It is the force of the thrust of the jet engines that moves the plane.

---

If thetreadmill moves at 150kt and the pilot hits thhe wheel brakes ;)

Quote:

---

Originally Posted by PearlJam09

Its simple bernoulli principle. The same reason why a frisbee flies. The shape of the wing causes the air going over the top to go faster, thus lowering the pressure. This pressure difference causes force to be applied to the underside of the wing, creating lift. If there is no air going past the wing, there is no lift. This plane cannot, will not ever take off.

---

No it's not. It's the angle of attack that causes most of the pressure gradient between the top and bottom creating lift. That's why stunt planes with symmetrical wings can take off! Paper airplanes fly pretty well also and their wings are straight.

:eek:

Quote:

---

Originally Posted by Roo

All the people who think that the plane will not take off must also think that if you were on said treadmill with rollerblades on that there is no way you'd be able to pull yourself forwards along the treadmill by using the handrails.

Think about it.

---

The flaw here is your example is a conventional treadmill, not one that adjusts it's speed based on your speed. Think about trying to pull yourself forward on a treadmill that instantaneously started accelerating when you starting pulling on the handrails.

Credentials: I'm an Aerospace Engineer AND Mathematician.

Solution: The airplane WILL take off.

Explanation: Thrust is provided by the jet engines. Thrust is needed to overcome Drag. Drag is made up of aerodynamic drag, and friction drag on the wheels. The friction drag on the wheels/bearings will be *marginally* higher on the spinning treadmill, but will continuously diminish as the airplane gains speed and the lift increases, thus reduction wheel friction.

Source of confusion: The problem statement is slightly confusing because it says something about the treadmill spinning at the same speed of the aircraft. This could induce some people to believe that the two velocities cancel each other out, whereas the truth is that the wheels will simply spin twice as fast (remember, the wheels of the aircraft are free spinning).

- B

Quote:

---

Originally Posted by The AD

The flaw here is your example is a conventional treadmill, not one that adjusts it's speed based on your speed. Think about trying to pull yourself forward on a treadmill that instantaneously started accelerating when you starting pulling on the handrails.

---

Ah, but the wheel/treadmill speed is an irrelevance. Your arms will always pull you forward.

Quote:

---

Originally Posted by Roo

Ah, but the wheel/treadmill speed is an irrelevance. Your arms will always pull you forward.

---

Not true. It depends on your arms' ability to overcome the friction in the wheel bearings. Imagine a treadmill going 1 mph. You'll have to hold on to the rails a lot "less tightly" than on one running at 100 mph.

Fun stuff. Picture a seaplane taking off against a fast moving tidal current. Even with the friction of the moving water, the plane will eventually plane up on the water and overcome the frictional force of the water moving in the opposite direction. The plane on the treadmill has negligible frictional force applied through the wheels relative to the engine thrust to move forward.

This would make for a good thread killer.

To give some numbers:

Let's assume the airplane weighs 50,000 N (VLJ type aircraft).
Let's assume the airplane has 5,000 N of thrust.
Let's assume the airplane has a wing surface area of 20 sq.m.
Let's assume the rolling wheel-friction coefficient is 0.01
Let's assume the wing coefficient of lift is 1.5.

Lift = (1/2) * (airdensity) * (speed^2) * (coefficient of lift)

VTo = 1.2 * Sqrt( (Weight / Surface) * (2 / (density*(coefficient of lift))) ) = 60 m/s = 220km/h = 140mph

Thus, the aircraft has to reach 140mph AIR SPEED to achieve take off.
The roll friction drag is going to be negligible compared to the thrust of the engines, and thus it will not make any difference whether the airplane is on a rolling threadmill or on a static runway.
The airplane will have to accelerate to 140mph AIRSPEED, which will take the same time on either type of runway. Airspeed is entirely independent of ground speed.

Consider the following scenario: The treadmill starts rolling with the jet engines off. The airplane starts rolling backwards. Now, the pilot engages the jet engines, but with barely enough thrust to keep the airplane stationary. This will be ~1-2% available thrust.
In this case, the airplane isn't moving w/ respect to the air, but it is rolling on the threadmill. Even if the threadmill accelerates, the friction drag is going to be approximately the same, and no thrust increase is necessary.
Now, the pilot pushes the throttle to 100% available thrust. The jet engines easily overcome the friction drag from the treadmill, and the jet accelerates with respect to the rolling runway, and eventually takes off.

Quote:

---

Originally Posted by Cirquerider

Fun stuff. Picture a seaplane taking off against a fast moving tidal current. Even with the friction of the moving water, the plane will eventually plane up on the water and overcome the frictional force of the water moving in the opposite direction. The plane on the treadmill has negligible frictional force applied through the wheels relative to the engine thrust to move forward.

This would make for a good thread killer.

---

y'all are still on the wrong track. Rollerbladers, seaplanes, and etc..... The forces are not working in complete opposition. The problem has nothing to do with relative forces. NOTHING.

Quote:

---

Originally Posted by The AD

Not true. It depends on your arms' ability to overcome the friction in the wheel bearings. Imagine a treadmill going 1 mph. You'll have to hold on to the rails a lot "less tightly" than on one running at 100 mph.

---

Ummmm, no. Go put your rollerblades on and go to the gym. Try it for yourself. You will be suprised.

edit: 25 pages? If we put this thread on a treadmill, would it ever stop?

Quote:

---

Originally Posted by DJSapp

Ummmm, no. Go put your rollerblades on and go to the gym. Try it for yourself. You will be suprised.

---

My gym doesn't have treadmills that go up to 100 :)

I think you're wrong about this.

Quote:

---

Originally Posted by The AD

My gym doesn't have treadmills that go up to 100 :)

I think you're wrong about this.

---

We think you're wrong about this.

Get a hanging scale (like for weighing big fish). Put on your rock climbing harness and roller blades. Tie one end of the scale to the treadmill and the other to your harness. Record the force exerted on the scale by you on your rollerblades at different speeds. It'll go up slightly as the speed goes up, but not proportionately to the speed of the treadmill (unless you have really crappy bearings/wheels).

And be prepared to defend your science experiment from liability minded gym employees.

Quote:

---

Originally Posted by Telenater

It'll go up slightly as the speed goes up

---

So you agree with me then. I don't know what the relationship is. It probably isn't linear, but all I'm saying is the force needed to hold you stationary does go up as the treadmill's speed goes up.

Quote:

---

Originally Posted by The AD

Not true. It depends on your arms' ability to overcome the friction in the wheel bearings. Imagine a treadmill going 1 mph. You'll have to hold on to the rails a lot "less tightly" than on one running at 100 mph.

---

Dude, if you can pull yourself on rollerblades at 100 MPH, you're missing out on some sort of amazing fruit booter opportunity of a lifetime. If nothing else, it would be pretty sweet to see you pulling yourself around a roller rink that fast.

Holy.

Fuck.


You might as well have put the horse in a blender at this point.

Quote:

---

Originally Posted by Dantheman

Holy.

Fuck.


You might as well have put the horse in a blender at this point.

---

I see it like this. If not for the longevity of this thread, I never would have gotten to visualize The AD swirling around a skating rink like some cheesy sit-com fast forward scene.

Quote:

---

Originally Posted by The AD

So you agree with me then. I don't know what the relationship is. It probably isn't linear, but all I'm saying is the force needed to hold you stationary does go up as the treadmill's speed goes up.

---

No, I'm saying that the airplane's forward speed will continue to increase until such time as the resistance from air and wheel friction equals the thrust of the jets. With good bearings the friction for rotation is essentially constant (with some very slight increase in bad bearings/other friction that also not dependant on speed). Essentially the resistance from the wheels becomes insignificant compared to air resistance (which we already know can be overcome by the engines).

Hey, this was reply #469.......

Quote:

---

Originally Posted by Bernardo

To give some numbers:

Let's assume the airplane weighs 50,000 N (VLJ type aircraft).
Let's assume the airplane has 5,000 N of thrust.
Let's assume the airplane has a wing surface area of 20 sq.m.
Let's assume the rolling wheel-friction coefficient is 0.01
Let's assume the wing coefficient of lift is 1.5.

Lift = (1/2) * (airdensity) * (speed^2) * (coefficient of lift)

VTo = 1.2 * Sqrt( (Weight / Surface) * (2 / (density*(coefficient of lift))) ) = 60 m/s = 220km/h = 140mph

Thus, the aircraft has to reach 140mph AIR SPEED to achieve take off.
The roll friction drag is going to be negligible compared to the thrust of the engines, and thus it will not make any difference whether the airplane is on a rolling threadmill or on a static runway.
The airplane will have to accelerate to 140mph AIRSPEED, which will take the same time on either type of runway. Airspeed is entirely independent of ground speed.

Consider the following scenario: The treadmill starts rolling with the jet engines off. The airplane starts rolling backwards. Now, the pilot engages the jet engines, but with barely enough thrust to keep the airplane stationary. This will be ~1-2% available thrust.
In this case, the airplane isn't moving w/ respect to the air, but it is rolling on the threadmill. Even if the threadmill accelerates, the friction drag is going to be approximately the same, and no thrust increase is necessary.
Now, the pilot pushes the throttle to 100% available thrust. The jet engines easily overcome the friction drag from the treadmill, and the jet accelerates with respect to the rolling runway, and eventually takes off.

---

You're thinking like a scientist. However, this is an imaginary treadmill that can spin at any speed necessary to keep up with the plane. Then what?

Quote:

---

Originally Posted by Dantheman

Holy.

Fuck.


You might as well have put the horse in a blender at this point.

---

Well put! http://ic3.deviantart.com/fs6/i/2005..._by_livius.gif

Quote:

---

Originally Posted by huck4bucks

You're thinking like a scientist. However, this is an imaginary treadmill that can spin at any speed necessary to keep up with the plane. Then what?

---

are you saying the threadmill can spin at a speed larger than the speed of light? i'll tell you what then: nothing. and everything.

stop "thinking" and start thinking.

Quote:

---

Originally Posted by f2f

are you saying the threadmill can spin at a speed larger than the speed of light? i'll tell you what then: nothing. and everything.

stop "thinking" and start thinking.

---

The plane will take off long before it ever reaches the speed of light. I'm sure my car would achieve lift-off before it reached the speed of light (unless it had a flux capacitator). What do you guys think take-off speed is, anyways???

Quote:

---

Originally Posted by huck4bucks

You're thinking like a scientist. However, this is an imaginary treadmill that can spin at any speed necessary to keep up with the plane. Then what?

---

The treadmill never moves faster than the plane(in the negative direction). Therefore, if the plane has a stall speed of 400mph (and postulating that the plane actually does move), the maximum speed that the treadmill would ever go is 400mph.

Quote:

---

Originally Posted by f2f

are you saying the threadmill can spin at a speed larger than the speed of light? i'll tell you what then: nothing. and everything.

stop "thinking" and start thinking.

---

I can't think anymore. Sell the treadmill to the girls' track team and give the plane back to Boeing.

Personally, I think it's a great explanation on the thread next door. That makes sense to me. The question was never worded correctly and specifics are purposefully left out to create mass hysteria, which we've seen here over the past 20+ pages.

w00t!!! page 20!!!! Plus another 5 in a second thread!!!

Rolling friction does not increase with speed. The rolling friction on a 747 is 78,000 lbs. 78,000 lbs of resistance is all the treadmill can possibly provide no matter what speed the treadmill is spinning at. It could be going twice the speed of light and it would only provide 78,000 lbs. of resistance.

The 747's engines provide 380,000 lbs. thrust.

380,000 is bigger than 78,000

THEREFORE:

There is a net force (read: acceleration) of 302,000 lbs. on the airplane.

If you don't understand this, open a fucking physics book and get a fucking education.

DJSapp
You're one persistant fellow to keep goin at this

everyone seriously
just look at the stupid motorcycle picture, its the same thing

imagine what would happen if you had a tow rope sucked into the turbine and shot out the other end. the plane would move along the tow rope and the wheels would just be spinning twice as fast. (ie im saying just imagine the wings\turbines were attached to a tow rope like at a ski resort)

thats my final post here

How about a helicopter? Could it take off if sitting on a treadmill?

^ you can't win against a cold, calculating, malevolent and omnipotent threadmill.