A jet plane on a large treadmill

[The AD]

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.

[Brock Landers]

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

[bucbanzai]

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.

[summit]

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

[Shepherd Wong]

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.

[The AD]

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.

[BurnHard]

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

[Roo]

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.

[The AD]

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.

[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.

[BurnHard]

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.

[focus]

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.

[DJSapp]

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?

[The AD]

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.

[Telenater]

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.

[The AD]

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.

[bagtagley]

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.

[Dantheman]

Holy.

Fuck.


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

[bagtagley]

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.

[Telenater]

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.......

[huck4bucks]

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?

[Cirquerider]

Holy.

Fuck.


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

Well put!

[f2f]

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.

[focus]

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???

[Telenater]

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.