why? concept is simple enough really.
I know, I’m trying to see where the question is confusing the answer…
Adam doesn’t have the answer wrong, he has the question wrong I think…
Mostly because the question is slightly ambiguous…
ok…here we go again…
This is where you are taking on the rest…
People are saying that because the speed of the converyor matches the speed of the plane, that the plane remains stationary with no air speed across the wing.
THE SPEED OF THE CONVEYOR HAS NO BEARING ON THE SPEED OF THE PLANE…DUE TO THE VERY FACT THE WHEELS OF THE PLANE HAVE NOTHING TO DO WITH THE ULTIMATE MOVEMENT OF THE PLANE.
You’re saying the plane will generate airspeed across the wing because of thrust exceeding the conveyor speed.
Right?
THE PLANE WILL GENERATE AIRSPEED ACROSS THE WING…DUE TO THE FACT IT WILL BE MOVING THROUGH THE AIR RELATIVE TO THE GROUND…AND EVENTUALLY WILL JUST TAKE OFF FROM THE CONVEYOR SOME DISTANCE DOWN IT…
i would surmize it would be the exact same take off distance it normally would have…in relation to a static ground object…slightly more taking into account any extra drag from the wheels due to this conveyor spinning them
I think what everyone is missing here is how an airplane wheel actually functions.
Think of a caster wheel.
My office chair has wheels on it. In theory, if the bearings were capable enough, you could put a conveyor under the chair traveling at 1,000mph in any direction and the chair would stay put.
The wheels freewheel.
So all the conveyor is actually moving is the wheels on the chair (plane) because the wheels’ only purpose is to spin and offer a frictionless surface for the plane to accelerate on. While sitting on the ground (not a conveyor) wheel speed would be governed by plane speed.
But in theory, even if you spin that conveyor at 100,000 mph, the plane shouldn’t move in a backwards direction.
As soon as you apply forward thrust, the plane itself will physically move forward, even if the wheels are rotating in the opposite direction.
The wheels are not directional.
Think of it this way.
If you put a RWD car’s front wheels on a treadmill, and its rear wheels on pavement, what would happen if you switched the treadmill on in reverse? Nothing. The front tires would spin backwards, and the car would stay put. Apply power to the rear wheels, and the car will move forward, with the front tires spinning backwards, until they come off the tread mill.
The planet is in constant motion correct? It rotates at approximately 1,600 km/h. Do you have to travel 1,601 km/h in one direction to move 1 km/h and -1599 km/h to travel in the other? No.
The wheels on the airplane are not it’s motive force, nor do they transfer it.
I may be just reiterating what ADAM has suggested, but what would I know? I dropped Physics one week into it. I didn’t agree with the ratio of cosign to launching model rocket modules.
simple experiment here…
take a toy car… place it on a belt sander… hold car stationary…it just stays in place…you can turn belt speed up or down…
NOW…apply thrust with your finger to the car…ohhh gee what happens…it moves forward…
better fix that…turn up sander speed more…hmmm no effect…you can still move the car forward
WHY…cause your finger is acting like THRUST…which is INDEPENDANT of the cars wheels…which guess what… is independant of the conveyor
guys… you are 100% wrong in your thinking…the plane will fly
Thank You Mr200
Adam,
A planes wheels will not leave the ground until it reachs a certain velocity…
I understand what you are saying about the wheels, but you have to understand, as the plane progressively accelerates, so does the conveyor speed. Which means the plane remains still because it will have no forward movement due to the conveyor matching its speed.
The only way the plane will take off is if it doubles the speed of the conveyor.
A plan needs a runway to take off, which means that the wheels are in contact with the ground until it generates enough thrust from forward movement…
Well if the conveyor constantly matches it’s speed from the beginning, the plan will never have a chance to achieve the forward movement needed to generate lift…
The wheels are still part of the equation until the force of lift exceeds the force of gravity.
That’s the only way I can explain it amigo…
hmmm model rockets…
i built a bugger of one this year…a 3 stager…thrust fixes everything…
its rated for 3000ft…
though i lost the ones that went to 1000ft…so if i launch the bugger i am sure it will be gone…
anyway…
TM…
the plane will move forward across this conveyor regardless of what or how fast this conveyor spins the planes wheels …as it moves forward it will pick up air speed and eventually take flight as it normally does at whatever airspeed it normally takes to make it fly
simple as that… I cant state it any simpler
gonna go home soon…
MR200 I leave the sanity up to you…
continue the fight…and see how long these guys will bicker against reality…
firstly im not bragging about my education, im simply reaffirming that i’m not some kid who’s talking out of his ass. secondly how do you know what im capable of? dont judge someone if you dont know them.
so onto this debate…
let’s say the engine produces no thrust:
- now we all agree that ground speed of fusalage/wings/tail = tangential velocity of wheels = 0 mph
- speed of belt = 0 mph
now let’s say the engine produces 10000 lbs of thrust (a force):
- F=ma…plane weighs say 1000 lbf, then a = 10 ft/s^2. Let’s say the plane accelerates for 10 secs (not realistic but to keep the numbers simple) so the velocity = 100 ft/s
- as stated, V (plane) = V (tangential of wheels) = 100ft/s
The way the question is asked assumes ideal conditions so let’s ignore any frictional losses. The question also says that the belt matches the speed of the wheels.
so,
-V (belt)= V(tangential of wheels)===> -V(belt) = V(plane)===> 0=V(plane)+V(belt) = net velocity
No matter how much thrust you apply, (since we’re talking about a plane taking off from the ground) the forward velocity of the plane is translated into motion via the wheels which spin freely along the ground. If the velocity of the wheels is cancelled out by the matching velocity of the belt, V(wheels)= 0 ft/s.
but like we already said V(wheels) = V(plane) so V (plane) = 0ft/s as well.
Thrust and weight act in different directions in our situation and do not effect each other. however the airflow over the wings create a certain amount of lift force which acts in opposition to weight, so it is the direct effect of lift alone that will overcome weight, not thrust.
ADAM, your analogy is incorrect, because you holding that car in place is the thrust force. If the belt speeds up, you have to apply more force to keep the car in place. This works inversely, so if you apply more force, the belt will correspondingly speed up.
if that were true solarian, then the parasitic drag normally associated with an aircraft would never allow it to take off…
suffice to say…an aircraft has MORE than enough excess thrust to overcome any drag induced by wheel/bearing/aerodynamic drag…
if it didnt…it would not be able to take off or fly in the real world.
loius…
if you calcualte you has 100ft/s based on thrust and weight/acceleration…then guess what… the plane would be moving across the conveyor at 100ft/s …and eventually either take off if the conveyor distance was long enough…or reach the end of the conveyor and still take off if there as a viable surface to keeping taxing down…
you guys are not understanding that the thrust is independant of what the wheels are doing or not doing…
for shits and giggles…
lets take a rocket…and add some 18" bling bling set of wheels to it…
we place the rocket on the conveyor…light it off
bam…the conveyor instantly goes up to mach 3 to compensate for the wheels…
too bad…cause the rocket still blasts down the conveyer like it was not there…
why…you say…
cause its rocket engine is moving it forward has nothing to do with the wheels we attached to it…
same with a plane…except somewhere during all this moving forward…it will gain enough lift from the air passing over the wings to lift off…where as the rocket is just happy flying on its thrust alone…
Okaaaaay! I’m doing a 180 now. Pretty embarassed, but I’m gonna agree with ADAM now.
There is an unbalanced force on the plane caused by the thrust, which will cause an acceleration, since the frictionless (or even frictioned) wheels cannot provide an opposing force since they are freewheeling. The only thing the treadmill will do is spin the shit out of your wheels. Plane takes off.
Furthermore, this question is retarded.
Fixed. :hsugh:
THANK YOU
this is really a VERY simple question…and now you have it