Rundmaus said:
Aw come on...
I just spen the past half an hour posting to explain exactly why the wheel + conveyer belt situation would keep the plane stationary relative to the ground...
amazing huhRundmaus said:😱 😱 😱
Oh dear,
sorry no hope for this forum 😕 ...
18 Answers, 140 times read... and only 2 people who take time to really think about it for a moment...
I don't need to repeat it, as it was already mentioned 2 times, but again.. Has anyone of You EVER seen an airplane that powers the wheels of its landing gear in order to generate thrust for takeoff? Damn, I always wondered what these big, loud hairdryers beneath the wings were for...
I think I'll go to sleep now, this one has definitely grounded me for today
Greetings,
Andreas
i didnt see the question posted here (i read it in motorcycle forum), but i cant beleive u guys also!
of course the plain will take off no matter whats going on benieth it - the conveyor matching the speed of the wheels or gaing backwards at 1000 mph , no matter what- the plain will take off !!
i thought people dealing with electronics/science will easily answer that question , but i guess not 🙄
Guys, guys, guys (and possibly gals)...
STOP trying to isolate the jet engine + wings from the landing wheels and the conveyer belt! They are in the same system and must be analysed as such.
STOP trying to isolate the jet engine + wings from the landing wheels and the conveyer belt! They are in the same system and must be analysed as such.
eVITAERC said:
The only connection I see is the air flow created by the conveyor causing a slight flow past the foil. The rest I cannot see a connection. The foil needs a certain amount of air flowing over it to create lift. Now is that lift created mostly by the engine or the speed down the runway? I think by the engine.
Suppose you take one of you kids MatchBox cars. You know these small miniature cars kids play with. Tie a rope on the front and place the toy on the conveyer and make it turn the opposite way you want the car to go which is forward. Because there is friction of the wheels you (the motor) will have to pull the rope to keep the car steady. If you want the car to move forward, you’ll have to pull harder, no matter how fast the conveyer tries to pull the car backwards. These cars have good wheels and you feel you are perfectly capable of pulling the car forward. So fast that if the car had wings, it would take off.
/Hugo
/Hugo
Cal Weldon said:
If I may be so bold, the plane is always free to move forward in response to the thrust of the engines. The wheels can turn backwards, forward or remain static in response to the conveyor belt but the plane will still take off. The plane is always free relative to the rotation of the wheel because of the axle.
sorry, you didn't EXPLAIN it, because the belt simply WILL NOT keep your plane stationary relative to ground...
... your belt will work perfectly fine to keep a sports car stationary relative to ground, a train oder a motorbike as well... In these situations I agree with you... But your trick only works as long as the intended forward motion of the vehicle is created by powering the wheels... because the "forward" force created by wheels is perfectly cancelled by the conveyor belt... NOT cancelled is any other force... btw. take a good physics book (Tipler, for example) and read a bit about "conservation of momentum", ok?
rfbrw said:
Except that it can't move forward as the conveyor belt is counteracting the thrust of the engine and keeping it stationary.
I agree, but why will the plane take off if it is not being opposed by the normal "takeoff speed" air friction? The plane is remaining stationary to the rest of the earth.
I still believe the question is: What force creates enough air flow over the wing to create lift? I can't think the flow from the belt is significant so IMHO it's the engine or the air friction down the runway.
sorry mate , i'm tyred of answering this question miliion times on other forums 🙂Cal Weldon said:
i'll try to explain shortly
imagine the plain with ice cubes instead of wheels , will it take off on a normal runnway? sure it will , thats how water plains work for over a decade .
conclusion -plain dont really need wheels , they are there to support it while the plain is on the ground , they move freely (same as ice cubes)
whats pushing the plain forward is the engine , no matter what type - jet / propelled / rocket
so by saying that the conveyor is moving backwards and matching the wheel speed your mind automatically thnks that the plain will remain in the same place ...this only true if it was a car instead of the plain 😎
the conveyor can move backwards at constant speed , lets say 1000 mph and also lets say the plains top speed is 500mph it will still move forward ! and eventually take off .
conveyor matching the wheel speed and going backwards will make the wheels spin twise as fast compared to a normall runway .so if we talking about real life , where there is small friction between the wheels and the bearings , so making the wheels spin twise as fast compared to normal will slightly reduce the acceleration of the plain bvecause of added friction with ground , but thats all , eventually the plain will take off 🙂
Carl,
don't let yourself get confused...
Of course the plane will only take off it it reaches the normal "take off speed" relative to the surrounding air. As long as the plane remains stationary to its surrounding, it will not make any attempt to take off 😀
The real point is that as long as you don't use wheel brakes or anything else on the landing gear, the plane simply WILL NOT remain stationary to ground or surrounding atmosphere. Because its forward motion is not realized by forces on the wheels. Why do planes have landing gears? Yes, because friction between runway and plane will create heat and bad-looking marks on the belly when trying to take off or land without landing gear. Now, lets forget the wheels and use a good lubricating oil between plane and conveyor instead... will the belt still "keep the plane stationary"? NO, it will not.. So why should it do this when replacing the lubricating oil by wheels to reduce friction?
Andreas
don't let yourself get confused...
Of course the plane will only take off it it reaches the normal "take off speed" relative to the surrounding air. As long as the plane remains stationary to its surrounding, it will not make any attempt to take off 😀
The real point is that as long as you don't use wheel brakes or anything else on the landing gear, the plane simply WILL NOT remain stationary to ground or surrounding atmosphere. Because its forward motion is not realized by forces on the wheels. Why do planes have landing gears? Yes, because friction between runway and plane will create heat and bad-looking marks on the belly when trying to take off or land without landing gear. Now, lets forget the wheels and use a good lubricating oil between plane and conveyor instead... will the belt still "keep the plane stationary"? NO, it will not.. So why should it do this when replacing the lubricating oil by wheels to reduce friction?
Andreas
Netlist said:
Yes, except during the time that the car moves forward with respect to ground, the conveyer is NOT moving as the same "speed" as the toy car (I'm talking about the tangential speed of both the car and the conveyer).
Simple physical experiments like this doesn't work very well in this case because here the conveyer belt as described in the question is a frigg'n magical item.
Rundmaus said:
sorry, you didn't EXPLAIN it, because the belt simply WILL NOT keep your plane stationary relative to ground...
Hmm.... conservation of momentum obviously comes into play here but not in the way that you think. All I can do is throw your suggestion back at you and read up on "Rolling Motion" and "Rotational Kinematics". Out of a actually good physics text though, like Resnick et al. Tipler is a self-indulgent mind-masturbating tool that doesn't really know how to write about physics.
That's only entirely true if you assume that the wheel is lossless. If there are any losses in the wheel, then the engines will lose some power merely overcoming those losses, in which case it will always be possible for the conveyor to completely negate all force from the engines by moving fast enough. Ok, so it will have to move at insane speeds, but a conveyor belt as large as a runway is already pretty amazing.rfbrw said:
Regardless of the disagreements, I think we can all agree that there are multiple plausible solutions depending on how you parse "match the speed of the wheels" and how many assumptions you make about friction etc.
Cal Weldon said:
The conveyor belt can do no more than rotate the wheels. As the wheels are free to rotate, the plane is free to respond to the thrust of the engines. If the conveyor belt matches the acceleration of the plane the wheels will remain static. If, in the misguided notion it has any influence over the plane, the conveyor belt speeds up, the wheels will just rotate in response to the direction of the conveyor belt but the plane will take off.
Cal:
Try not to get distracted about the wheel and the conveyer belt. The information as posed in the quesiton will make sure that there is no forward motion of the plane. You have the right method of thinking and (as best I can tell) is focusing on the right question: How much thrust comes from just the engine running, drawing air past the wings? This is currently my stand as well!
Is your real name Carl? If not so, don't trust anyone who reads your name as such 😀
Try not to get distracted about the wheel and the conveyer belt. The information as posed in the quesiton will make sure that there is no forward motion of the plane. You have the right method of thinking and (as best I can tell) is focusing on the right question: How much thrust comes from just the engine running, drawing air past the wings? This is currently my stand as well!
Is your real name Carl? If not so, don't trust anyone who reads your name as such 😀
Well....... what if the expediential increase in the rotation means that the bearings overheat and the gear colapses before there is sufficient air speed?
rfbrw said:
That is just wrong.
Allright, allright, time for thought experiment
Imagine you're out on a spaceship, in such a mostion that the earth is stationary to you but you can see it rotate. Now imagine you watching a plane cruising on a runway somewhere on earth.
Is it possible that the plane may be going fast enough so that it looks stationary relative to you? Of course, it just has to move at the right speed.
Is it possible to keep the plane stationary to you as it speeds up, no matter how fast it's running relative to the earth? Of course, just "speed up" the earth by the exact same amount.
Now come back to the airport grounds and translate your results. Your "fixed point of refrence" becomes the airport grounds and the earth is now the conveyer belt. The rotational machenics of the wheel and something spinning underneath it is still the same, nothing changed, except maybe you "shrunk" the earth to a conveyer belt. Same thing.
Mr Evil said:
The bog standard jet engine, not to mention the monstrosities planned for the A380, can pretty much overcome any frictional losses. The bearings would have to seize in order to stop the plane taking off, a feat the belt would have to achieve within seconds of the start of the take off run.
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