plane on a conveyor belt?

[Donzo]

It will fly.

Look at it this way:

Get rid of the engines and replace the thrust they would have produced by attaching a cable to the front of the plane.  

Now attach the cable to a big winch.  

Start it up.

Now once the winch starts pulling the plane, will it move forward?

Of course it will.  What's to stop it?  

The conveyor only causes the wheels to rotate faster.


Using the above analogy, think about putting one of the these conveyors under a F/A-18 being shot off an aircraft carrier.  Do you think that the conveyor is going to negate the catapult shooting the plane off the deck?

[APDrone]

Originally posted by eagl

You're forgetting that airplane wheels don't push the plane forward.  In fact, airplane wheels push straight down (weight) and actually "push" forward as they resist the rearward drag between the wheels and the ground.  The only force from the wheels that the engines must overcome on takeoff is that drag.  The wheels are simply not responsible for accelerating the plane forward so their only contribution to the whole thing is how much drag they're producing because the engines must push against that drag.

LOL.. no, I never implied that the wheels push the plane foward.

The plane cannot go forward if the wheels do not move forward. Ok.. it could move forward if the plane's engines were strong enough to overcome the friction and drag them along.  

If the wheels rotation is matched by a counter rotation, then the distance travelled by the wheels is zero.

Basically I'm seeing a plane that is sitting at the end of the runway and, relative to everything around it, it is not moving.  If that is truly the case, then it will not fly. Whether the wheels are spinning at 1 rpm vs. 1 million rpm.  If the plane is not moving in relation to the runway, the tower, the trees..etc.. then it isn't going to fly.

My understanding is that this contraption will negate the forward motion that would happen if the wheels could roll.  If that is indeed the case, then the plane will not be able to roll forward, thus not be able to move.

So.. I guess it boils down to one thing.  If you believe the contraption can prevent the plane from moving or not.

[JB88]

Originally posted by Kurt
BINGO!

what i was saying in the first place.

[Terror]

The plane WILL move forward on the conveyor.  Rolling resistance would never overcome the thrust of the engine(s) against the air.  So essentially the wheels would cover about twice the distance they would normally have had to cover for the plane to take off, but the plane itself would cover about it's normal takeoff distance.  (ie.  If a normal takeoff roll makes the wheels spin 10,000 revolutions, they would spin 20,000 revolutions during this hypothetical take off.)

It's why we use round wheels in the first place.  The energy lost to rolling resistance of a round object (the wheels in this case) is tiny compared to the energy required to start it and keep it moving.  

Terror

PS.  Even if the conveyor was moving WITH the plane and not against it, it would have very little effect on the take off.  The wheels would just spin half as much as the normal take off, but the distance would be about the same.

PSS.  I think people are thinking that the rolling resistance of the wheels add more to the energy equation than it actually does.

[Terror]

Or here is another hypothetical:

Assume a plane has no brakes on it's wheels.  The  plane is landing at 100 knots on a conveyor that is moving in the opposite the direction of the airplane at 100 knots, is it's landing distance changed?

My guess: not much.  Inertia will overcome the rolling resistance of the wheels and have to roll out the landing...

Terror

PS.  Rolling resistance does not increase proportionally with rolling speed...

F=CrrNf
where:
F is the resistant force,
Crr is the rolling resistance coefficient or coefficient of rolling friction (CRF),
Nf is the normal force.  (gravity)

Notice no reference to speed or revolutions.

[eskimo2]

Try this:

Get some roller skates or roller blades and go to the gym.  Level a treadmill, turn it on slow and skate on it; it will take a little bit of effort.  Another component would energy wasted by churning your legs.  In this situation you are working against wheel friction, but have no air friction.  Skating on normal ground would require more energy because you would be working against wheel and air friction (mostly air friction though).  Now turn it on fast: the air friction will be the same, wheel friction will go up a bit and your legs may work a bit more from zipping back and forth so fast.

Now grab onto those handles that they put on treadmills and relax.  Your legs don’t have to work anymore and they can just point the skates straight ahead.  There is only a little bit of wheel friction that can be overcome by a gentle grip on the handle.  One finger on the handlebar ahead of you will easily hold you in place.  Even if Tim Taylor rigged it so that it could go 100+ mph it wouldn’t be hard to hold yourself in place with a light grip on the handlebar.  (Keeping your skates pointing perfectly straight down the tread might be another matter).  If the treadmill was spinning slow, fast or 100 mph, you could easily pull yourself forward by gently pulling on the handle.  Why?  Because your skate wheels can easily freewheel until the bearings melt or the rubber wheel explodes from centripetal force (maybe at a few hundred mph).  The force of your grip on the handlebar is independent of what the treadmill may be doing.  

Likewise, an airplanes “grip” on the air is independent of what its wheels are doing.  Airplane wheels are the same deal as skating on a treadmill; they will just spin as fast as they are asked to with relatively very little resistance.  The aircraft motor grabs the air, much like you can grab the treadmill’s handlebar.  


Another way of looking at it:

Imagine the same scenario as the conveyer runway except the runway is ice and the airplane has skis/skates.  You can pull that ice under the plane’s skate/skis as fast as you want, but the prop is still biting the air and will pull the plane forward.

[sluggish]

I can't believe the geniuses on this board are having a hard time grasping this...

[FBplmmr]

wow, if the navy knew about this they wouldnt have spent all that money on catapualts! lol

they would just park the jet on the edge of the carrier and lock down the wheels , spool up the engine and release.

the reason you tie down a plane at an airport is becuse wind creates airflow over the wing that will flip the little bastage up into the air.

if the plane on the conveyour belt has no forward moentum there is little airflow over the wing (probably more from a prop plane than a jet becuse of engine placement)

with little to no airflow ....


i hear they have alot of trouble with nextel cup cars getting airborne on the dyno lol:lol

[Donzo]

Originally posted by FBplmmr
wow, if the navy knew about this they wouldnt have spent all that money on catapualts! lol

they would just park the jet on the edge of the carrier and lock down the wheels , spool up the engine and release.

the reason you tie down a plane at an airport is becuse wind creates airflow over the wing that will flip the little bastage up into the air.

if the plane has no forward moentum there is little airflow over the wing (probably more from a prop plane than a jet becuse of engine placement)

with little to no airflow ....


i hear they have alot of trouble with nextel cup cars getting airborne on the dyno lol:lol

Put one of these fancy conveyors under a plane on the catapult.  What would happen at launch?

[FBplmmr]

i dont need a catapult we have majic no airflow thrust

[Donzo]

Originally posted by FBplmmr
i dont need a catapult we have majic no airflow thrust

Did you even think about the question I posed?

[Chairboy]

I'm a bit disheartened at all of the 'the plane can't take off because the conveyor belt is pushing against it' posts too.

I'm reminded of the New York Times who, in 1920, wrote:

That Professor Goddard, with his 'chair' in Clark College and the countenancing of the Smithsonian Institution, does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react - to say that would be absurd. Of course he only seems to lack the knowledge ladled out daily in high schools.

In 1969, after Apollo 11 was on the way to the moon, they retracted it:

Further investigation and experimentation have confirmed the findings of Isaac Newton in the 17th Century and it is now definitely established that a rocket can function in a vacuum as well as in an atmosphere. The Times regrets the error.

[FBplmmr]

Originally posted by Donzo
Did you even think about the question I posed?

yes briefly, my stance is that without air passing over the wings the plane can't achieve flight.


if the plane is in effect staionary(either by a belt or lockdowns) and the thrust being produced is parralell to the  ground the plane will remain stationary.  when released from eitherfrom  the belt or the lock downs the plane will begin forward and will not acheive flight untill sufficient airflow goes over the wings to allow it.

to be honest trying to visulize a conveyour under a jet on a catapault made my head bleed =)

[APDrone]

Originally posted by Donzo
Put one of these fancy conveyors under a plane on the catapult.  What would happen at launch?

The catapult is an external force in relation to the conveyor belt.  Anything attached to the catapult will be moved at the speed of the catapult, so if it is enough to provide the necessary airspeed for flight, then the plane will fly.  

This is not part of the original argument.

The original argument is that the plane uses its own power.

Now.. if you put the catapult on a device that moves in the opposite direction of the catapult at the same speed of the catapult, you have the same thing.  Net distance travelled by the object is zero.  Zero distance means no airflow.
No airflow means no lift.
No lift means no flight.

The wheels on aircraft are specifically put there to keep the plane off of the ground while it's airspeed is less than its stall speed.

If, at any time, you remove the wheels from the aircraft before it reaches stall speed you produce wreckage.

[majic]

I think what people are not getting is that the conveyor absolutely could not hold the plane in place.   The winch analogy was excellent.  (see Donzo's post)

The plane will move forward, not be held in place.