747 on a treadmill conundrum..

[StealthySkeptic]

From what little understanding I have of aerodynamics and forces acting on a plane in flight, even supposing that a huge conveyer belt could create enough drag to counteract even a part of the plane's forward thrust, there isn't be enough weight (gravitational force) exerted that isn't already there that won't allow the plane to pass air above and below the wings and generate enough lift to push up and off the conveyer belt.

[ignoramus]

Huge thrust will want to push the mass forward only.
The effect of liftoff is made from air pressure under the wings only.

So, no it won't ever take off in this theoretical scenario.

[SteelCurtain]

My degree is in aeronautical engineering. Fluid dynamics is my shizz.

This is a very easy thought experiment.

The treadmill has a negligible effect on the plane. The wheels on an aircraft are free spinning, they do not translate any thrust whatsoever to the aircraft. Ignoramus you are quite wrong about the "effect of liftoff" being made from air pressure under the wings only. Lift comes from the iterative accumulation of the pressure differentiation over the chord length of the airfoil's wingspan. Basically the plane gets sucked up into the air from above, not pushed up from beneath.

All of this is moot, though. The plane's wheels have effectively zero effect on the plane's ability to create thrust and move forward on the runway. If the treadmill is always set to match the forward velocity of the aircraft, then the 747's wheels will always be rotating at 2x the indicated airspeed. The plane will take off just fine.

[Anomalocaris]

Huge thrust will want to push the mass forward only.
The effect of liftoff is made from air pressure under the wings only.

So, no it won't ever take off in this theoretical scenario.

Uh, Where do you think the air pressure under the wing is coming from if not from the forward movement of the mass of the airplane? From the spinning of the wheels?

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Why not just take the wheels off and stand the plane up on its "landing pegs". Isn't that the same thing? Could a plane take off just sitting on pegs?

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Wait... Nevermind. The engines would keep increasing thrust and it'd tip over.

I am beginning to think you seriously overestimate your own intelligence.

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If the plane's engines can generate enough air resistance without needing to move forward on the ground then surely it could just lift off like a helicopter and then fly off like a rocket as soon as the wheels weren't touching the treadmill...

I don't know enough about 747 engines to know if it is specifically possible in this case but I'd imagine it is possible in theory.

In reality I don't think it would ever be possible to exactly match the wheels of the plane with the speed of the treadmill as you'd need a large treadmill with a feedback loop that goes up to some insane speeds.

And even then the plane's wheel has to move before the treadmill can adjust its speed.

The engine generate thrust, not resistance, unless it is stalled.

No matter how fast the treadmill is rolling backwards, the plane can roll forwards because the landing gear wheels are free spinning. So the presence of the treadmill will have almost no effect on the behavior of the plane once it's engine is started. The plane will accelerate forward, causing airflow over the lifting surfaces.

The shape of the lifting surfaces is so designed as to ensure airflow moving over the top surface of the wing momentarily travel faster than airflow traveling under the bottom surface of wing. By Bernoulli effect, this result the air flowing under the bottom of the wing excreting a higher upward pressure on the wing than the downward pressure generated by the air flowing over the top wing. The excess of upward pressure over the downward pressure on the wing is called lift.

When the forward speed of the aircraft increases, the differential speed of airflow over and under the wing increase, as does the differential pressure, or lift, on wing. When the lift exceeds the weight of the plane, the plane lifts off.

The pilot can control just how much the lift the wing generates at a given speed by pitching the wing slightly up, or down. When a wing pitches up compared to the direction of the incoming air (higher angle of attack), lift increases, as does drag. Pitch the wing down, lift decreases, as does drag. So at the instant of take off the pilot rotates the aircraft to pitch the wing up in order to increase the lift, this allowing the pilot to exactly control when the lift will exceed the weight of the plane, and thus the exact moment of takeoff. If he does not rotate the plane, the plane will simply continue to accelerate straight down the runway, the plane will still shortly start to float up off the ground as its forward speed continue to increases.

In none of this principle of flight, does the backward movement of the ground under the wheels of the plane excert any significant effect.

So the treadmill is an extraneous factor introduced into the scenario to pull the leg of people who doesn't know much about aerodynamic flight.

[ignoramus]

I get it now guys... my bad...

I understand about the plane being sucked up but it still needs the relative higher pressure below the wing to cause this effect in the first place doesn't it?
That's what I meant.

[naimless]

Huge thrust will want to push the mass forward only.
The effect of liftoff is made from air pressure under the wings only.

So, no it won't ever take off in this theoretical scenario.

Uh, Where do you think the air pressure under the wing is coming from if not from the forward movement of the mass of the airplane? From the spinning of the wheels?

---

Why not just take the wheels off and stand the plane up on its "landing pegs". Isn't that the same thing? Could a plane take off just sitting on pegs?

---

Wait... Nevermind. The engines would keep increasing thrust and it'd tip over.

I am beginning to think you seriously overestimate your own intelligence.

---

If the plane's engines can generate enough air resistance without needing to move forward on the ground then surely it could just lift off like a helicopter and then fly off like a rocket as soon as the wheels weren't touching the treadmill...

I don't know enough about 747 engines to know if it is specifically possible in this case but I'd imagine it is possible in theory.

In reality I don't think it would ever be possible to exactly match the wheels of the plane with the speed of the treadmill as you'd need a large treadmill with a feedback loop that goes up to some insane speeds.

And even then the plane's wheel has to move before the treadmill can adjust its speed.

The engine generate thrust, not resistance, unless it is stalled.

No matter how fast the treadmill is rolling backwards, the plane can roll forwards because the landing gear wheels are free spinning. So the presence of the treadmill will have almost no effect on the behavior of the plane once it's engine is started. The plane will accelerate forward, causing airflow over the lifting surfaces.

The shape of the lifting surfaces is so designed as to ensure airflow moving over the top surface of the wing momentarily travel faster than airflow traveling under the bottom surface of wing. By Bernoulli effect, this result the air flowing under the bottom of the wing excreting a higher upward pressure on the wing than the downward pressure generated by the air flowing over the top wing. The excess of upward pressure over the downward pressure on the wing is called lift.

When the forward speed of the aircraft increases, the differential speed of airflow over and under the wing increase, as does the differential pressure, or lift, on wing. When the lift exceeds the weight of the plane, the plane lifts off.

The pilot can control just how much the lift the wing generates at a given speed by pitching the wing slightly up, or down. When a wing pitches up compared to the direction of the incoming air (higher angle of attack), lift increases, as does drag. Pitch the wing down, lift decreases, as does drag. So at the instant of take off the pilot rotates the aircraft to pitch the wing up in order to increase the lift, this allowing the pilot to exactly control when the lift will exceed the weight of the plane, and thus the exact moment of takeoff. If he does not rotate the plane, the plane will simply continue to accelerate straight down the runway, the plane will still shortly start to float up off the ground as its forward speed continue to increases.

In none of this principle of flight, does the backward movement of the ground under the wheels of the plane excert any significant effect.

So the treadmill is an extraneous factor introduced into the scenario to pull the leg of people who doesn't know much about aerodynamic flight.

This mysterious treadmill is not an extraneous factor my friend.

As for planes, My Great Uncle Ruseman built the first aerodynamic flight.

[FreeTony]

In any normal sort of scenario the aeroplane gets airborne with the treadmill at any sort of finite speed. However consider the workings of the treadmill:

Wheel speed and treadmill speed start at zero.

Plane starts to accelerate, from 0 to deltaV. Wheels are spinning at deltaV. Ttreadmill speed is zero.

Treadmill reacts, by going from speed 0 to deltaV, as it must match the wheel speed. This means that wheels are now spinning at 2*deltaV. But now the treadmill must react by increasing it's speed to 2*deltaV as ist must match the wheel speed, which increases wheel speed to 3*deltaV.

This cycle would go on until infinity under idealised scenarios. It would all depend on the sampling rate of the treadmill. If deltaT (the inverse of the sampling rate) were very small, the tyres would explode and the plane would get sent backwards and crash.

(I think)

[Chas]

The wheels and treadmill are irrelevant.

Assuming the air is still, the engines provide thrust and will move the airplane relative to the air, thus providing lift.

[FreeTony]

They're not irrelevant if the wheel speed reaches 200+ knots and the tyres explode.

[Thumpalumpacus]

What's the airflow over the wing doing?