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airplane on convayor

AHA-D said:
This entire discussion is pointing to "fixed-mind-logic". Most people are thinking "car" and not "plane". A car needs ground and friction to move, a plane doesn't. (well, correction, it needs ground to be supported when the wings aren't doing so) When seeing a plane taxi towards the runway you might be tempted to think it's the same as a car driving, but it is not. There is no engine driving the wheels, the plane is moving forward on a relatively low amount of thrust from the jet engines.
Click to expand...


After this and other quotes, I have seen the error of my ways (its good to have a closed mind that gets opened).


I talked to a guy here who flies planes and you guys are right. The only relevent factor is the airspeed, NOT (as I was thinking) the ground speed.


Thanks


Kev
 
Did anyone consider the Earth's surface speed which is around 30km/s? This is irrelevant to whether the plane will take off in the same way that a moving conveyor will not prevent its take-off. If on the other hand, the air was like the conveyor belt with a tailwind always equalling the plane's forward (ground) speed then the aicraft would not take off.


Magneplaner: You could not have inifinite friction because no object is unmovable.


As Einstein once said: "Give me a lever long enough and I could move the World".


Incidentally, did anyone consider the Earth's surface speed which is around 30km/s? This never prevents aircraft take-off does it?
 
pjw of course the plane takes off (see my description in post #2 of thread). However, the conveyor belt's movement is not analogous to the Earth's surface absolute movement in the sense that it is parallel and independent in any way. The conveyor belt movement described in question is relative to the earth's surface, so its absolute motion has vector = vector of earth surface motion + vector for belt relative to earth. More pertinantly though, the earth's surface has zero velocity relative to the atmosphere, which of course is moving with the same angular velocity of the earth, so at the point of contact between earth's surface and the atmosphere there is zero velocity difference whereas there is a relative velocity difference between belt and atmosphere (force of the lift on a plane is appliedby the atmosphere).


Dammit I just made another overcomplicated post in an overcomplicated thread!


It's really vety simple: Planes are propelled by engines that are "pushing" on the air, not the ground. The wheels on planes are completely free.


Saying a plane wouldn't be able to take of because thesurfaceit is taking of from is moving is like saying a seaplane that didn't have propellers working underwater like a boat wouldn't be able to take off.


Post #2 tells exactly what would happen when plane under the circumstances described in the question.
 
well its been said several times. if you assume like i do that the wheels are free spinning on the airplane (most are on most airplanes) then the speed of the conveyor means nothing. we assume the friction of the bearings is far less than significant compared to the thrust of the airplane.

the plane moves as if its on a frictionless plane, the relative motion of a frictionless plane has no effect on the body resting on it. the plane takes off normally.

***the plane takes off without any effect from the conveyor******
 
Yes Kev you're right - it was Archimedes who said that.


2ms1 - I never said the plane didn't take off - all along I said it would. My point about the Earth's surface is valid. I was merely pointing out that the conveyor's relativevelocity to the plane was irrelevant in the same way that that the Earth's surfacespeed relative to the plane was irrelevent also(I admit there is relative velocity dfference between the conveyor and the atmosphere even though that too is irrelevent to whether the plane takes off or not).
 
in my previous post I got it...I was stuck on the weight at rest before. Then I realized, the wheels pose no real friction in either direction since they freely spin, so the thrust would be effective. The wheels are just along for the ride.

The plane will take off.

Now I will go beat up on my pride a bit and get some rest so maybe I will not be so slow next time hahah. note to self- no more posts at 3 in the morning

PJW- understood, my pride was "unmovable" for awhile there. I was thinking of infinite in the sense of a variable switch, which still has a min and max, but infinitely variable settings in between.

2ms1- are we driving you crazy? hehehe

this isn't my topic, but I do say thanks to everyone who posted. I hope everyone appreciates having an open forum with the freedom to state your mind.
 
Thnx guys for your thoughts, and explanations it's been very useful.
I must admit that here this rant goes pretty good, and civilizes, on other forums people star fighting each other, and spiting venom
smiley36.gif


As I said before, first I didn't believe that plane would take off, not because of wheels, but because of confusion between speeds, and measuring speeds.

The question is little tricky, and a bit philosophic, not quite engineering kind of question.

What I tried to achive with this thread is that someone would describe and solve this problem with sketches, formulas, equations, with real engineering explanation.

Yes I know that this isn't necessary, that is from simply explanation clear that plane would take off, but then there are some people that are so stubborn, and for them only pure mathematic and physic would force them to change their mind.
 
Right. Let's all remind ourselves of Einstein's Theory of Relativity, shall we? Don't take the viewpoint of being on board the aircraft, take it as an observer alongside the "conveyor" (By the way, I wish some of you would learn how to spell).


The aircraft is not moving. It is, relative (there's the operative word) to its surroundings, stationary (not stationery - unless it's a paper plane). It therefore has no forward moment. No groundspeed (irrelevant). No indicated airspeed (vitally relevant). No sufficient indicated airspeed, no lift. No flight. No nothing, just a waste of fuel.........


The next time you're at an airport, try walking the wrong way along amoving pavement, and carefully match your speed to that of the belt. How fast are you going, relative (there's that word again) to the advert for Tax Free perfume alongside you? Flap your arms all you like, you'll never get off the ground (an attempt at humour, that last bit).


If all contributors are, as I suspect, CAD Engineers - ergo designers - and responsible for the future of manufacturing here in the UK, Heaven help us..........


Still, it's only meant as a bit of fun, isn't it?....... It is, Isn't it??


OK. Let's all get on with some work.............
 
bjagflyer:


If the aircraft is sat there on the runway with throttle at full thrust, then where is all that energy going? It's sure as hell being lost as heat through the brakes, bearings or tyres.


Think about it.


pjw
 
I think the key word here is that the conveyor changes speed to match the planes forward momentum. I understand at the beginning it doesn't matter what the conveyor is doing because the wheels will just spin freely. But you introduce trust which in turns provides forward momentum but the conveyor changes to match that forward momentum, how can the plane move forward? The wheels on the plane are still making connect with the ground.
 
As you say the wheels just spin freely, and
the thrust gives the aircraft a forward velocity and hence the momentum you
describe. However, this thrust acts on the air and has nothing to do with the
velocity of the aircraft relative to the ground. The wheels
effectively make the aircraft and the ground independent in the longitudinal
direction.
 
Perhaps I can help some that may not have the benefit of seeing a plane up close with a different way of explaining, as I see it:

imagine this:

you- wearing roller skates, the kind with four wheels positioned in the corners that spin freely.
your friend- ( to provide thrust)

you step onto a conveyor that is not moving so you are standing still, on roller skates, on the conveyor.

the conveyor turns on as your friend starts pushing you forward. You and the conveyor keep increasing speed. your friend would still be able to push you forward and even pick up speed. Since the roller skate wheels can spin freely they provide little resistance to your friend pushing. If you had wings, and your friend pushed really hard, you too would take off and fly. In fact, your friend could hold you in place while the conveyor spun up and your roller skates wheels would spin like crazy. If they let go you would be standing there with the wheels spinning matched to the conveyor. And if he then pushes you forward you would still be able roll forward.

the plane is just like you standing on roller skates, it can roll forward, or backward, it doesn't really matter. The thrust is like your friend pushing you.

don't get tripped up like I did with the weight at rest , or the friction of the bearings. Those are all extras that might happen as a breakdown in a failure mode.

just imagine, roller skates...friend pushing, the conveyor cant stop you because there is very little resistance in the wheels

come to think of it, you could do this on a big sphere too
 
I'm not buy it. you have to move the plane forward at 80not relative to the ground. If the ground is move, do don't get that movement. Your logic is one that the plane is moving independent of the ground as if the plane was already in the air. This is impossible.
 
I like the analogy, the demonstration would probably get you in less
trouble then flying a model aeroplane around an airport. You could add to the
realism and leave your friend to get the drinks ready by holding a beefed-up
hairdryer under your arm.

Sam
 
I guess, depending on the age or gender of the pilot, this plane might not take off.


Picture the driver of that car in front of you who's brake lights keep flickering...


"I'm not buy it. you have to move the plane forward at 80not relative to the ground"


Relation to the ground is of no matter. It only matters what the relationship with the air. Air gives the plane lift...
 
SW-

or just eat alot of baked beans and provide your own thrust HAHAHAHH

Phoxeoy- when I was taking lessons my instructor always said to me to "never stop flying the plane until the plane is parked". ( crosswinds, turbulence from other aircraft, they all make for a long explanation why the plane is upside down in front of the flight school) So in a way, you are flying while taxiing, just too slow to take off. Unless you taxi like me hahah

the conveyor is not really the ground because its moving, and the wheels are free to spin.
 
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