Forum menu
That's the point. The plane will not take off if it's static and no air is passing over the wings but because the wheels of the plane are not driven and freewheel they don't stop the plane from moving forward. The plane is pulled through the air by it's propeller not driven by it's wheels
So pulling it along on the sheet was not like a treadmill then. I thought the whole point of the tread mill experiment was that the plane stayed static.
the wheel speed of the plane has nothing to do with whether or not it will take off. its the air speed which is being facilitated by the propellor. the propellor is pulling the plane forward, the fact that the 'ground' that the plane is standing on is moving in the opposite direction is entirely irrelevant. it will just make the wheels spin faster. but the plane will still move forward and create movement of air over the wings and will take off. the treadmill can be travelling at a million miles an hour in the opposite direction and the plane will still move forward and take off.
the fact that the 'ground' that the plane is standing on is moving in the opposite direction is entirely irrelevant. it will just make the wheels spin faster.
Ok, assuming no friction.
yeah assuming an entirely free spinning wheel. friction acting on the wheel will of course cause drag but it would have to be a hell of a lot of friction. i.e. braking.
it would have to be a hell of a lot of friction. i.e. braking.
or a fast conveyor
The old "Aircraft on a conveyor" argument is entirely stupid in all respects. To even consider it requires a complete ignorance of how an Aircraft actually functions.
And anyway, just turn the Aircraft around on the conveyor! 😉
no. assuming the conveyor can move that fast then so could the wheel. ground speed has no bearing on a planes ability to take off. its air speed over the wings
and what duffer says!
So,
If there was a really really strong tail wind, blowing at about normal take-off speed for a given plane, could that stop the plane from taking off / cause it to fall out of the sky?
Conversely, if there was a really really strong head wind, could a plane take off without moving (other than its elevons)?
Conversely, if there was a really really strong head wind, could a plane take off without moving (other than its elevons)?
you mean - like a kite? 🙂
Conversely, if there was a really really strong head wind, could a plane take off without moving (other than its elevons)?
Yep...
EDIT: plane take-off deniers should be forced to watch this video, Clockwork Orange style. The wheels are moving at exactly the same speed as the runway, and the plane takes off.
So,If there was a really really strong tail wind, blowing at about normal take-off speed for a given plane, could that stop the plane from taking off / cause it to fall out of the sky?
Conversely, if there was a really really strong head wind, could a plane take off without moving (other than its elevons)?
Why do think that, where practical, planes take off and land into the wind?
If there was a really really strong tail wind, blowing at about normal take-off speed for a given plane, could that stop the plane from taking off
As the plane's ground speed would be double its airspeed, such a scenario could lead to it running out of runway before its air speed was high enough to take off. Otherwise, no.
cause it to fall out of the sky?
No.
Conversely, if there was a really really strong head wind, could a plane take off without moving (other than its elevons)?
Yes - if it ran its engine or was tethered.
cougar, tailwind, no not really because the wind would also push the plane along but it would affect its ability to create lift yes. if airborne it wouldnt fall out the sky but if taking off it could run out of runway.
headwind, yes of course. if the wind speed is enough to generate enough lift then yes it will take off (by take off i mean leave the ground. realistically it would probably just flip over).
as mentioned this is why planes land and take off into the wind. it increases relative air speed.
Noooooooo.......
Quick a distraction!
Did anyone watch Stargazing the other night. I calculated the age of the universe using the Hubble Constant.
I LOVE SCIENCE!
Bouncy bouncy Fluid on Fluid action!
[img] 
[/img]
Levitating frog in a 10T field:
Done by Andre Geim - guy at Manchester who got the Nobel prize for graphene discovery a couple of years back.
Umm can I get a reference for that, coz everything I can find says otherwise. Also I'd have thought that people have more experience with shear thinning that thickening fluids (Toothpaste, Ketchup that sort of thing)
http://en.wikipedia.org/wiki/Non_Newtonian_fluid
The first paragraph covers it.
In a Newtonian fluid, the relation between the shear stress and the shear rate is linear, passing through the origin, the constant of proportionality being the coefficient of viscosity. In a non-Newtonian fluid, the relation between the shear stress and the shear rate is different, and can even be time-dependent. Therefore, a constant coefficient of viscosity cannot be defined.
To even consider it requires a complete ignorance of how an Aircraft actually functions.
Hmmm, this is an interesting point. I mean I think I know how an aircraft functions, and the conveyor question seems trivial. Clearly the relevant part is the airspeed over the wings. But what are people's misconceptions which makes the question problematic? For me, I thought the assumption was that the air speed of the plane was zero so flight was not possible.
But just to be sure, can you clarify how an aircraft actually functions?
the shape of an aircraft wing is such that the air passing over the top of it has to travel further than the air passing under it. to do this is has to travel faster. this creates low pressure above the wing and that in turn creates lift. how much air is needed and at what speed is dependant on the shape, size and orientation of the wing. to create the movement of air over the wing the aircraft needs to move. this is facilitated by the engine, be it a jet or a propellor. the engine itself does not blow air over the wing it simply provides enough thrust to give forward movement to gain sufficient air speed to create lift.
friction between the aircraft and the ground will prevent forward motion of the aircraft so to reduce this they are generally on wheels. the wheel only turns because the plane is moving or because some idiot put it on a treadmill. it matters not how fast the ground/treadmill is moving. as soon as there is sufficient airspeed over the wing it will take off.
the shape of an aircraft wing is such that the air passing over the top of it has to travel further than the air passing under it. to do this is has to travel faster. this creates low pressure above the wing and that in turn creates lift. how much air is needed and at what speed is dependant on the shape, size and orientation of the wing.
ok, that starts to make sense... is that why aeroplanes cannot fly upside down?
ah ha! this is a wind up. 🙂
wind up, airplane down. right?
i think we've all learned something here today.
What about helicopters?
No, really if you think about a helicopter's rotor, on one side the rotor is moving into the direction of travel and on the other it is moving with the direction of travel, so the lift being generated would vary as the rotor goes round so it will be wanting to flip over the whole time
dont even get me started on helicopters.
ooops to late
you see the interesting thing about helicopters is the way that they compensate for different rates of lift during forward flight.
how do they do that?
cant remember.
Dissymmetry of lift
One cannot begin to talk about the mechanics of helicopters until the problems associated with rotary wing aerodynamics are understood. When the first rotary wing pioneers started trying to make a helicopter fly, they noticed a strange problem.
The helicopters rotor system would generally work just fine until one of two things happened: Either the aircraft began to move in any given direction, or it experienced any sort of wind introduced into the main rotor system. Upon either of these events, the rotor system would become unstable, and the resultant crash would usually take the life of the brave soul at the controls. The question then was; Why does this happen? The answer is what we refer to today as "Dissymmetry of lift".
What "Dis-Symmetry of lift" means is, when the rotor system is experiencing the same conditions all around the perimeter of the rotors arc, all things are equal, and the system is in balance. Once the system experiences a differential in wind speed from any angle, it becomes unbalanced, and begins to rotate. Take for instance forward flight. Imagine a two bladed rotor system spinning at 100 MPH.
The blade moving toward the forward end of the aircraft is going 100 MPH forward, and the blade moving toward the back of the aircraft is travelling at 100 MPH in the other direction. This is just fine when the aircraft is not moving or is in a no wind condition. It is experiencing 100 MPH of wind in all directions, so it is totally in balance. Once the aircraft moves forward, it begins to change this balance. If we travel 10 MPH forward, then the forward moving, or advancing rotor blade, is experiencing 110 MPH of wind speed, and the rearward, or retreating blade, is experiencing only 90 MPH of wind speed.
When this happens, we get an unbalanced condition, and the advancing blade experiencing more lift wants to climb, while the retreating blade experiences less lift and wants to drop. This is where we get the term "Dis-Symmetry of lift". The lift is not symmetrical around the entire rotor system.
How do we compensate for this situation? We compensate by allowing the rotor to flap. By allowing the advancing blade to flap upward, and the retreating blade to flap downward, it changes the angle of incidence on both rotor blades which balances out the entire rotor system. As you can see in this simple graphic, there are a few ways to allow for blade flapping.
One is to allow the blades to flap on hinges (Articulated rotor system). Another way is to have the whole hub swing up and down around an internal bearing called a trunion (Semi-rigid rotor system). Unfortunately, we can not compensate completely for dis-symmetry of lift by using blade flapping. Once the aircraft gets to a certain airspeed, and the rotor had flapped as much as it possibly can, then "Retreating blade stall" may be experienced. In retreating blade stall, the retreating blade can no longer compensate for dis-symmetry of lift, and the outer portions of the blade will "Stall".
This situation, when not immediately recognized can cause a severe loss of aircraft controllability. This is a major airspeed limiting factor for helicopters. For many years, aeronautical engineers have tried to figure ways to eliminate this problem and increase the forward airspeed for single rotor helicopters. Although many breakthroughs have been made, the manufacturers of single rotor helicopters are usually not willing to change the entire design on their products because of the extra costs involved for little airspeed payoff. Most have resigned themselves to slower airspeeds for their aircraft, at a lower cost and less maintenance.
Did you just copy that? 'cos that flapping bit sounds too vague, the amount it would have to 'flap' would need to change with airspeed.
yes i did but its pretty much there.
The first paragraph covers it.In a Newtonian fluid, the relation between the shear stress and the shear rate is linear, passing through the origin, the constant of proportionality being the coefficient of viscosity. In a non-Newtonian fluid, the relation between the shear stress and the shear rate is different, and can even be time-dependent. Therefore, a constant coefficient of viscosity cannot be defined.
I know what a non newtonian fluid is, although it has been a long time since I studied them at Uni, my question was related to the statment you made that water is non-newtonian in nature as there is nothing that I can find that indicates any sort of time or shear rate viscosity variation.
Can the airplane/conveyor people get their own thread? Or, better, do some Googling, reading and thinking?
sorry mike
Can the airplane/conveyor people get their own thread? Or, better, do some Googling, reading and thinking?
Or you could just explain it to us
thats what ive been trying to do.
my question was related to the statment you made that water is non-newtonian in nature
I thought water was one of the few Newtonian (or near as damnit) fluids
thats what ive been trying to do
I know, and you've done very well, it's just that there are still some big gaps in my understanding. But given that mike asked us to go our own way and I guess he's the thread owner or boss or something, we'll have to leave it for now
The first 1.5 pages of this thread were great.
i didnt intend to ruin it. its just been a very slow day.
Or you could just explain it to us
It takes off, because of science.
I know what a non newtonian fluid is, although it has been a long time since I studied them at Uni, my question was related to the statment you made that water is non-newtonian in nature as there is nothing that I can find that indicates any sort of time or shear rate viscosity variation.
Pump the water through a pipe, add a heat source to the pipe, does the viscosity of the water change along the pipe? Yes, therefore it's non-Newtonian as you've introduced a time dependance to the viscosity .
You make an assumption that a fluid is newtonian to make the maths simple, pretty much all liquids are going to thin with a change in temperature. In reality it will be non-newtonian to a degree. Just like the ideal gas law is applicable to most cases even though we know it doesn't model the behavior of real gasses.
Pump the water through a pipe, add a heat source to the pipe, does the viscosity of the water change along the pipe? Yes, therefore it's non-Newtonian as you've introduced a time dependance to the viscosity .
Err, no. You haven't introduced a time dependancy you've introduced a temperature dependancy. Variation in viscosity due to temperature is nothing to do with whether or not a fluid is Newtonian or not. A time dependant non newtonian fluid will change viscosity after a period of time at constant temperature.
Err, no. You haven't introduced a time dependancy you've introduced a temperature dependancy. Variation in viscosity due to temperature is nothing to do with whether or not a fluid is Newtonian or not. A time dependant non newtonian fluid will change viscosity after a period of time at constant temperature.
du/dt is non newtonian then surely you can see that if du/dT is applicable to most fluids, and a heat source gives dT/dt then du/dT * dT/dt = du/dt (where u is 'mu' for viscosity). The water is less viscous after some period of time.
the shape of an aircraft wing is such that the air passing over the top of it has to travel further than the air passing under it. to do this is has to travel faster. this creates low pressure above the wing and that in turn creates lift. how much air is needed and at what speed is dependant on the shape, size and orientation of the wing.
I thought the Bernoulli effect had been proven to be a minor contributor to the lift generated by an aircraft wing and everyone was now taught it was newtons 2nd law.
It takes off, because of science.
Good science or bad science?
[IMG] 
[/IMG]
And more to the point - if plane takes off and someone switches the conveyor belt off - and so the plane crashes - where do they bury the survivors?