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# Leaving on a jet plane (or, how smart are you again?) the reprise

Discussion in 'Chit Chat' started by Turok, Oct 24, 2007.

1. ### Turok

Let me reword the question and make the scenario more specific...

I am in the cockpit and flying a simple Cessna 172. (single engine, takeoff speed ~75mph, max speed ~150mph).

You are in the control tower. In your hand you hold a dial control with which you control the speed of the entire north/south oriented "treadmill runway". This dial in your hand allows you to change the speed of the runway surface from 0 to 151 mph both forward and reverse, smoothly and quickly.

I pull out onto the south end of your runway and point my nose north. The winds are calm. I go through my normal checks, run-up the engine and release the brakes.

When you see me run-up and release the brakes, you decide you don't want me to leave. Can you stop me?

Can you stop me from moving down your runway? Can you make me go backwards on your runway. Do you have any control at all over where I go on your runway?

The above is the heart of, and the key to, the original question.

JB

2. ### Wayne Gibbous

No. As long as the plane motor/propeller can overcome the friction of the landing gear tires and bearings, the plane is essentially unaffected. The plane is being pulled through the air by the airscrew, not the ground wheels.

Think of a plane barely skimming the surface. The speed of the runway doesn't matter. Unless the plane is landing. Without thrust reversers, the plane could not stop if the runway was moving in the same direction.

3. ### Turok

Wayne's the man.

JB

4. ### Supermax

it was a trick question, too funny. if not for the PROP! (if it was a JET) the (true) answer is (STILL) a resounding NO!

requires specific knowledge of prop physics

dumbassIQ 'problem', imo (and even dumberass author of same)

5. ### Turok

Max:
>it was a trick question, too funny.

There's no trick involved.

>if not for the PROP! (if it was a JET) the (true)
>answer is (STILL) a resounding NO!

No difference -- prop, jet, ducted fan or rocket. All will takeoff in (almost) the exact normal distance.

>requires specific knowledge of prop physics

Nope. It's not prop specific, so no such knowledge is necessary. Cessna 172 to a Boeing 747 -- all just the same.

>dumbassIQ 'problem', imo

Since from the above it's clear you haven't yet grasped the problem, I understand why you think it's "dumbass".

>(and even dumberass author of same)

For the record, I didn't author it -- though I wish I had.

JB

6. ### stu

another perspective perhaps would be....
you get a model plane, you know, the ones that have those gas engines, jet or prop, and remote control. You have a conveyor. The plane engine is running but idling, not much power. Plane is standing on the ground. Your assistant has gotten hold of the wings - strides the conveyor and lowers the plane onto it. The wheels now start to spin. You have the remote and start to pump the power to what will achieve flight speed . You hear the engine pitch is ready but SuperZZzz is the one holding the plane wings, so you thrust the antenna up his ass - that is the only signal he can be trained to understand. He lets go the wings. Nothing has changed except the wheels are spinning - and the force from the prop or jet still hold the plane steady. Increase power. Takeoff achieved. Give SuperZZzzz a candy for not standing on the plane or falling over on the conveyor.

what was the question ?

8. ### Supermax

haha the plane does take off! i see it now!

the ground is irrelevant because no significant force is applied to the ground/belt

9. ### Turok

Exactly ... the wheels just coast along at whatever speed they need to.

JB

10. ### Turok

bump (because of references in original thread)

JB

#10     Nov 5, 2007
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