will it take off?

[ray66v]

Re: Mythbusters/airplane/treadmill

That 100 hp put into the wheels for a second or so to spin them up to 120mph is the same 100 airplane hp that could be cancelled by the conveyor quickening at a rate that accelerates the plane wheels that quickly.
larry

But, it is not possible for the forces from the airplane to be canceled by the belt, because the wheels cancel the belt leaving the propeller forces with the advantage. Right?

 

[BotaBigot]

I'm fascinated by the type of discussions that generate long threads (currently 78 pages)... No, I didn't read them all...just the first page, and then here. Yes, I know the answer...but won't spoil it for participants...

 

[SPYDERLK]

Re: Mythbusters/airplane/treadmill

Ok, If I see your point correctly, are you looking at the gyroscopic effect that the wheels could eventually develop if enough speed is applied???
Because rotational acceleration as I know understand it, I'm having trouble applying to this example.

This situation would not be affected by the gyroscopic properties present in the spinning wheels because the wheels' plane of rotation is not being changed - the airplane is going straight so the wheels spin in a vertical plane. If you try to lean or twist the wheel the gyro effect wakes up.

The way that a free spinning wheel can affect the plane thrust is by its resistance to having its spin speed changed. This has nothing to do with bearing friction. The plane pulls the axle forward - the earth holds the tire still - the top of the tire moves at twice the axle speed - the wheel rotates. Greater force applied across the axle/earth pivot results in greater rotational acceleration - it spins faster more quickly. A plane taking off on a normal runway rotationally accelerates its wheels until it leaves the ground. The energy that goes into accelerating the wheels to 1000rpm or so has been supplied by the engine. The wheels are a tremendous convenience, but this storing of rotation energy delays takeoff slightly. Something that didnt rotate -like skis - and had sliding resistance the same as the wheels rolling resistance would have a quicker takeoff. Moving to the conveyor idea we have a 'runway' that moves back the same speed as the plane moves forward. This means that the wheels spin twice as fast at takeoff. It takes a little more time to reach takeoff airspeed because a higher percentage of engine energy output is being stored rotationally - therefore, the translational energy required for takeoff takes longer to accumulate. This situation follows from the wheels being rotationally accelerated at a little less than twice as fast as seen in in a normal takeoff. Now lets extrapolate to a condition where the conveyor can accelerate the wheels at so great a rate that it stores rotation energy in them as fast as the plane engine can produce energy. Theres nothing left to accelerate the plane.
larry

 

[ray66v]

Seriously?!?? Have you read this thread entirely? There is no such thing in regards to "will it take off."

Sorry for the oversight, I will try to get "with the no moot points program".

 

[ray66v]

Re: Mythbusters/airplane/treadmill

But, that is not possible, right?
Is this another one of those "moot point doesn't matter" discussions?

 

[rback33]

Sorry for the oversight, I will try to get "with the no moot points program".

Noting your lack of smilies I must ask.. You did realize I was just giving you a hard time right? there are certainly moot points around here, but there was some pretty creative bull in that thread... I even produced some of it myself...

 

[ray66v]

All smiles here

 

[rback33]

All smiles here

Good... and I must agree to that the helicopter one is downright hilarious too!

 

[ray66v]

I think I will run the Helicopter one by my flying club's think tank, at the meeting tonight. We have several engineers and chopper drivers, whom I'm sure will enjoy a lively discussion over it.

 

[Iplayfarmer]

The planes wheels do not turn. The place sits in a steady state with no forward movement. The wheels have no motive power. Thereon lies the misconception of spinning masses and moving conveyor belts.

It's a simple play of words leading to presumptions of motion.

And this is confusing us all!

The wheels never have any motive power in an airplane. They just roll freely like a trailer's wheels. It's the prop or the turbines that provide the motive power.

It's one of those trick questions meant to make the answerer feel silly for even stressing about it. It's just like these two:

1) If a rooster lays an egg exactly in the middle of a roof peak , which side of the roof will the egg roll off?

2) If a plane crashes exactly on the border between USA and Canada. Where do you bury the survivors?

Now let's spend another 750 posts giving magical powers to genetically engineered roosters so that they actually can lay eggs and further traumatizing unfortunate survivors of a tragic plane crash by planning to bury them alive in whatever their native country may be.

What if we re-framed the question to refer to a 20 foot flatbed trailer holding a brand new John Deere 5025 sitting on our theoretical conveyor and hooked it up to Mr. Jimi's new F-550 (with which I am genuinely and fully impressed, by the way) such that the truck is not on the conveyor but the trailer is? If Mr Jimi put the beast in granny gear and floored it, would he drive away?

I'll bet we'd get a lot more concensus from the question framed like this.