will it take off?

[Egon]

Re: Mythbusters/airplane/treadmill

Me, being kinda slow, have trouble trying to figure out how the plane wheels can turn. Seeing as the plane is sitting on a belt but is unable to move forward and has no system to rotate the wheels the conveyor belt don't move either. Kinda a conundrum ehh. everything is sitting still.

 

[ray66v]

Re: Mythbusters/airplane/treadmill

Trying to understand....How can there be rotational acceleration of the wheel mass when it is in contact with the conveyor? The conveyor would work against that happening would it not? Even with rotational acceleration, I don't see any forces that would transfer to the plane stopping it from accelerating and taking off. All I see are 2 separate forces, the lower one gets canceled out by the wheels leaving the forces from the prop to move the aircraft forward.
But, I am listening.

 

[ray66v]

I bow to the genius who conceived this, LMAO

 

[Egon]

As per the original question.

a plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves but in the opposite direction. The conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction.

Under these conditions the plane remains stationary relative to the surrounding countryside. Would that not indicate the wheels are not turning as they have no mechanism to make them turn! Now if that is reasonable and the planes wheels do not turn the magical conveyor belt does not move either! It would seem to be a static situation that has been transformed into one of illusions of motion.

With a plane that doesn't move and a stationary runway only vertical takeoff or a heck of a strong wind that will allow vertical takeoff will ever let the plane fly!

 

[patrick_g]

I am going to quote myself from page one on Nov 8, 2006 just to say "Told you so".


It is simple physics and it amazes me that so many people don't get it.

THE WHEELS DO NOT PROPEL THE AIRCRAFT, IT DOES NOT MATTER HOW FAST THEY ARE SPINNING ON A CONVEYOR BELT. OBVIOUSLY IF IT WAS A CAR, THE CAR WOULD NEVER MOVE.

Along the way the practical was abandoned for the theoretical where the conveyor belt was given magical powers and was supposed to be capable of prodigious feats. Among these was the ability to accelerate very fast (at speeds significantly below the speed of light if you wish)

Since the wheels and tires have mass and therefore inertia the conveyor has to transfer energy into the spinning wheels which results in a counterforce in the direction of travel of the conveyor. (Simple physics.)

The conveyor would accelerate as required for this retarding force to equal the thrust of the prop and the acceleration og the plane would be zero.

At some point this rapid acceleration would definitely produce relativistic velocities and the wheels and tires if not the bearings would have long since failed in the real world and and plane would be carried backwards on the conveyor.

Clearly if the conveyor was restricted to real world properties the retarding force on the plane would be such a small percent of the prop's thrust that the plane would take off and a casual observer would not detect a difference in the take off run.

Therein lies the HUGE difference between the thought problem we fussed with here vs the real world test on the "Myth Busters" TV show. Jamie's pickup was incapable of the massive acceleration required to spin up the planes wheels with sufficient jerk (third derivative of distance with respect to time or the rate of change of acceleration, again simple physics) to generate a credible retarding force on the planes forward motion.

The most surprising result was NOT that the plane flew (again simple physics) but that the bozo pilot actually thought the plane would be inhibited from taking off and he was surprised to find it took off just fine with no problem.

Pat

 

[RobJ]

Re: Mythbusters/airplane/treadmill

BUT, if somehow the plane was driven by the wheels, then the senerio would be totally different. If the takeoff speed was matched by the conveyor, then the plane would just be sitting still ( with the wheels spinning) but no air would be going under the wings to acheive flight because the only thing moving on the plane IS the wheels.

This would be called a car...not a plane.

The Myth was busted when the plane moved forward on the treadmill. The treadmill matched the planes takeoff speed and it moved forward. After that it was just TV entertainment.

The expanding foam bit was cool to!

 

[Egon]

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!

 

[ray66v]

Since no matter how fast the wheels turn from the belt the forces are canceled out by the wheels. It is not possible for the conveyor to match the speed. Because, the aircraft {propeller} has an advantage over it, the aircraft will always accelerate. So, it is a moot point.

 

[SPYDERLK]

Re: Mythbusters/airplane/treadmill

With the thrust forces involved, both that of the prop moving the plane forward and the belt pulling backward, the ammount of energy needed to accelerate the wheels to a VERY high RPM is inconsequential. It just dosn't take that much force to spin up a lightweight aircraft wheel with good bearings.

Put the rear end of your car up on jackstands. Start the engine and stomp on the gas and see how long it takes to get to redline. Now remove the wheels and repeat the experiment. Even with large heavy steel or alloy auto wheels, the difference in engine acceleration with and without the inertial load of the wheels is pretty small...

We are seeing a matter of degree - HP to wheel mass. The point is it does make a difference in how long it takes the engine to rev out when rotationally accelerating the wheels. 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. As you say, the lighter the wheels the easier they spin up. That just means they have to be accelerated faster to counter the plane HP, leaving none available to accelerate the plane. You run into the limits of materials very soon, but until then the plane sits still. Even totaly frictionless bearings would not prevent the conveyor resisting thrust in this way.
larry

 

[rback33]

So, it is a moot point.

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