Naked Science Forum
General Science => General Science => Topic started by: diecastblue on 15/06/2010 07:02:11
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I was looking at DIY wind turbines, and stumbled across this video : It got me thinking, could this work for an aeroplane? If an airfoil has a lift / drag ratio greater than 1, can it pull enough energy to drive the plane, overcoming the drag of the blade? I'm just thinking out loud, what do you reckon?
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I was looking at DIY wind turbines, and stumbled across this video : It got me thinking, could this work for an aeroplane? If an airfoil has a lift / drag ratio greater than 1, can it pull enough energy to drive the plane, overcoming the drag of the blade? I'm just thinking out loud, what do you reckon?
The video shows a model boat with a whacking great wind-turbine (relative to the boat's small hull) mounted atop. The blades spin and push the boat by means of gearing to a small propeller directly in the direction of the wind (something traditional sailing boats can't do).
Now, due to the boat's prop transferring the work made available from the turbine's spin to a far more dense medium (ie. water) the forces below the water easily overcome the drag above (and in) it. So the prop is in a dense non-flowing medium with a small (low drag) cross-section, where as the turbine is in a low density flowing medium extracting energy with a large x-section.
Even ignoring how your 'plane' would remain up in the sky (I imagine a magic drag-free helium balloon), the blades of the turbine and the fan to drive the vehicle would have to be the same size - cancelling each other out.
The key to it working with a boat is the transfer of power from a low-density medium (air) to one of high density (water). Plus the 'free' buoyancy afforded by the displacement of the hull on the water.
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The essence of the problem is that, unlike a ship, a plane has nothing to push against.
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If it were possible to have a wing with a lift/drag ratio greater than 1 then we wouldn't need engines on our aircraft as all they would need to do is fly level to move forward. Such a wing would have to be made out of wishalloy.
The essence of the problem is that, unlike a ship, a plane has nothing to push against.
Umm... do you want to have a think about that?
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Sure, no problem.
OK I thought about it.
A ship, with a propeller in the water can push the water backwards and drive itself forwards.
I realise that an ordinary prop plane can do exactly the same.
However, think about a windmill, it can get energy from the wind because it slows the wind down a bit. In doing so it has to push back against the air and in order to do that it must also push against the ground.
A windmill on a balloon wouldn't work because the whole system would accelerate until it was travelling at the same speed as the air and then the windmill would be becalmed.
It's only because the water holds the ship back, that the windmill on a ship can continue to turn and gain energy to drive the ship.
A plane can't do that.
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Thanks for your replys.
@peppercorn: Yes I was not considering how to keep the plane up, just driving it forward, so say for the purpose it is in no gravity environment :)
@LeeE: According to the all knowing Wikipedia, modern sail planes have lift drag ratios upwards of 70. Lift from a wing is produced (roughly:) upwards not forwards.
@Bored Chemist: A windmill on a balloon, or on a boat will be pushed back with the same force in the same wind speed (Drag of the blades). To make either one move into the wind, you need to push it forward with more force than the drag creates. The water creates more drag than air, making it harder for the ship to move backwards, but to propel it forwards, you also need to push it harder to overcome the higher drag of the water too!
Thinking about this (in my dreams last night:), I think the key is that in the boat example, the driving propeller is in a fluid that is not moving relative to the boat, where as in a plane (or balloon:) the driving prop has to work in air that is already moving in the opposite direction. Just to keep it stationary, it would have to be driving the propeller at wind speeds (The boat in the video is not going anywhere near wind speeds) If you could have two separate chunks of air, one moving and one stationary, with the turbine and propeller in them respectively, I think this could would work. (oh and a third space with no fluid for the magic drag-free helium balloon:)
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Here you go!
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It's only because the water holds the ship back, that the windmill on a ship can continue to turn and gain energy to drive the ship.
Does that mean if the ship was out of the water and put on stationary wooden saw horses, that the windmill on the ship would stop turning?
Of course not. It would be just like a windmill on land. The saw-horses would hold it back.
@diecastblue.
From the vid that Geezer cited, you can see that windmill powered boats work. I bet you can't find a similar vid of a balloon.
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BC: both air and water are considered to be 'fluids' in the context of aero/hydrodynamics. Both aircraft and ship propellers (or at least ship propellers that are more highly developed than a simple Archimedean screw) operate on the same principle as an aircraft wing i.e. not directly by diverted thrust but by creating relative low and high pressure regions on either side of the propeller blade surface. Air/water is thrust backwards as a result, but the vector of this thrust is not directly backwards and wouldn't provide sufficient thrust on its own, just as the downwash from an aircraft wing is not what keeps it in the air.
diecastblue: strictly speaking, yes: modern gliders can have a glideslope ratio (equivalent to their lift/drag ratio) in excess of 70:1, meaning that they'll sink one metre for every 70 metres of forward flight. However, I answered in the terms that the OP used, when they referred to a lift/drag ratio of > 1, instead of talking about a negative lift drag ratio.
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you can see that windmill powered boats work. I bet you can't find a similar vid of a balloon.
I suspect the reason it can work with a boat is because the water that is accelerated by the underwater propeller produces a greater reaction force (thrust) than the wind acting on the wind turbine. (BTW - this is a really old idea. I've seen illustrations of this invention from over 100 years ago.)
Soooo, I wonder if it might be possible to make it work with a balloon if the balloon carried a supply of water that could be accelerated by wind power to produce thrust? Obviously, even if it does work, there are some serious practical limitations with such an arrangement.
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Lee,
do you know that many planes can be flown upside down? The usual limiting factor is that the fuel pipes are at the bottom of the tanks. Most of what keeps a plane up is downwash.
No matter what the design of the propeller, Newton's third law says that the thrust that the propeller produces is exactly the same as the force that the prop exerts on the water.
Tommya300, On a really small planet where gravity wasn't a factor I could still get a windmill to work by bolting it to the ground. In practical terms gravity matters, but in principle it doesn't. Gravity pulls down and we are concerned about how the thing moves (or does not move) horizontally.
Geezer,
After a short while the balloon would be travelling at the same speed as the air round it. In that event the windmill would stop. It wouldn't be able to provide any energy to pump water.
Anyway, what would be the limiting speed of this plane driven by a windmill?
As it got faster the mill would catch more wind so it would be able to extract more power. That would make it able to go faster. That would, again, give it more energy so it would be able to get faster still.
It's airspeed would just keep increasing.
Now imagine that, from the point of view of someone on the ground, the wind stops.
The machine still feels the wind against its sails and carries on accelerating.
It seems to me that this "windmill driven plane" is a free energy machine and is not a valid topic of conversation on a scientific website.
I can run my house on wind power because the air is moving with respect to me. I can get more power if the wind is faster.
That windspeed is measured with respect to my house.
In the middle of some air, what do you measure the windspeed with respect to?
It doesn't have an absolute velocity so it doesn't have an absolute kinetic energy that you can tap into.
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After a short while the balloon would be travelling at the same speed as the air round it. In that event the windmill would stop. It wouldn't be able to provide any energy to pump water.
I was thinking the balloon might be able to fly into the wind, or at least at some angle to it just as the wind turbine boat can. If it was travelling in the same direction as the wind, there would be no point in ejecting water to produce thrust.
Admittedly, it would not surprise me to learn that there is something very dodgy about this arrangement, as it does seem that we might be getting something for nothing. I suppose the only way to find out is to do the math, but as the scheme is entirely impractical anyway, I'm not highly motivated [;D]
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@Geezer yes, this is a video by the same guy that made the video in the OP, I am not questioning that a boat will work. You can also find wind driven cars on Youtube as well, eg @Lee you lost me saying negative lift drag ratio (a brick has -L/D), [The boats] turbine blades must have >1 L/D, drag holding the blades back, lift creating torque to drive the boat, otherwise the boat would go backwards! I believe the sum vector of thrust must directly backwards, otherwise the board wold not go directly forwards! And as BC says, Newtons third law, equal opposite etc.
@Geezer The reason it can work with a boat is because the propeller produces a greater thrust than the drag of the wind acting on the turbine.
@BC "I have a free energy plane, I just need a few thousand more invested to get it to market and stick it to big oil" :) Don't be silly! the video of the boat(s) and cars are there to see working, they don't accelerate ad infinitum into the wind. In a still air, you couldn't push them and they keep going! There is power in the air because it is already moving, (giving by this formula btw Power (in Watts) = 1/2 * Air Density (kg/m^3) * Area (m^2) * Wind velocity^3 (m/s)) There is no need go relativistic, lets keep this Newtonian
For simplicity sake, imagine a no gravity environment, and a ship itself with no drag . It has connected to it a turbine above it, and a propeller below it. there are two (magically separated) fluids that it sits between, so that the turbine is in the top fluid, and the propeller is in the bottom.
If the top fluid is air and is moving towards the boat, and the the bottom water and is still, It can propel itself forward.
How about if they were both air, the top half moving and the bottom half still? (This is what I am proposing would work)
It cannot be the density difference air vs water, as if both fluids were water, the top moving, bottom still it would go even faster as there is more energy in the moving water than in moving air. (The reason I wish I had a river next to my house so I could build a water turbine rather than a wind one)
Also, Regardless of the fluid combination, if they are both moving towards the boat at the same speed, I don't think It could propel itself forward (?)
Ahh, thinking science instead of working. Brilliant.
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Diecastblue.
I know the videos of the boats etc are there.
If you read what I posted you would see that I know about them.
I said that you won't find a video of a windmill powered plane.
Until you find one, you are the silly one.
"There is power in the air because it is already moving,"
Moving with respect to what?
If you had two layers moving with respect to each-other then you could extract energy from that system but just a block of moving air (like you actually usually find- rather than calling on magic for help) cannot power a plane that is in that air.
Tommya300
"If gravity does not matter, then matter would not exist to create a small planet and the ground that you bolt the windmill to, will not be there. "
Why not? I could make a small planet from metal (like a spaceship) and bolt things to it. I realise I'd need to build a planet with a roof to keep the air in..
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BC: yes, I do know that aircraft can be flown upside down. Those that are intended to do so generally have symmetrical aerofoils and rely upon a positive Angle of Attack to generate lift. An aircraft with an asymmetric aerofoil can also be flown upside down but it would require an even greater AoA than one with a symmetric aerofoil, and if the AoA is too high then the wing will stall. If aircraft relied upon downwash for lift then delta winged aircraft, such as Concorde, wouldn't work.
If Newton's third law worked for fluids we wouldn't need to bother with all that complex CFD stuff.
diecastblue: Positive Lift to Drag ratios describe the loss of altitude in forward flight, so a gain of altitude would give a negative ratio. An LD ratio of 1, strictly speaking, would result in a 45° dive. A LD ratio of 0 is what would describe something that falls straight down, with no forward motion.
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Newton's 3rd law works with everything.
Building a planet with a roof is, in principle, perfectly possible. Magic planets are not.
The real problem with this craft is that how well it flies would depend on the observer in a rather odd way.
Imagine the inaugural flight.
** NEWS FLASH**
Disaster was narrowly avoided today at the launch of the new PWAWOI (Plane with a windmill on it) craft.
Conditions were perfect for takeoff with a steady 20 mile per hour wind blowing from the South.
Unfortunately, shortly after takeoff, a man on a bicycle with an anemometer and a mobile phone nearly crashed the PWAWOI.
He cycled along southbound with the anemometer at exactly the same speed as the wind; while doing so he videoed the anemometer reading of exactly zero and sent it to someone on board.
Once the plane knew that the windspeed was zero it couldn't generate any power and started to fall.
Fortunately for all concerned, a news crew rushing to film the impending disaster, also measured the airspeed from their car and were able to inform the PWAWOI that there was plenty of wind (from their point of view) to keep it airborne.
Seriously, Windspeed is not absolute; it depends on the observer so you cannot rely on it as a source of energy while it is blowing you about.
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OK, this is silly because it cannot possible make a difference to the practicality of a free energy machine, but here goes.
I can make a big hollow sphere from metal. I's hollow so it's not very heavy. The effect of gravity is small.
I can make a very big spherical bag and place it round the metal sphere.
I can use a few long ropes to hold the bag centred on the sphere.
I can fill the bag with air.
I can bolt a big fan to the surface of the sphere and connect it to a big battery so as to generate some wind (remember, we are not concerned about what drives the wind on Earth so I can come up with any source of wind I like for this scenario).
I can put the "plane with a windmill on it" in that moving air. (for example I can climb up one of the ropes with it.)
Then I can watch the plane fail to work because it's a free energy machine.
No magic- just rather tricky engineering.
Then I can build an ordinary windmill on the surface of the sphere and bolt it down.
It would turn because of the wind. I could draw power from it. This power would come indirectly from the battery so it's not a free energy machine.
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OK, actually have to work at the moment, so I'll just post a quick reply :)
@LeeE "If Newton's third law worked for fluids we wouldn't need to bother with all that complex CFD stuff." <- That is just funny, and very wrong.
"Positive Lift to Drag ratios describe the loss of altitude in forward flight" - also wrong, you are probably thinking of glide ratio. Lift drag ratio is, well the ratio of lift to drag, lift divided by drag. Lift is always greater than drag for an airfoil (that is not a brick). Glide ratio is the distance a plane will travel forward / the distance it will drop when traveling at a constant speed, and is equal L/D for a whole plane, and is still always a positive number. Negative LD = airfoil that pushes downwards, negative glide ratio = a brick, or a magic plane that gains altitude while gliding.
@Bored chemist. I still don't think you can look at this in a relative type of way. Air moving at 1m/s has a certain amount of kinetic energy, no matter where you are observing it from, if you are stationary or moving past it, it has the same kinetic energy. A stationary brick has no kinetic energy, it does not gain kinetic energy if you run towards it, you do. If this was not the case the boat in the video would also be a "free energy machine" and accelerate into the wind!
Refer to the equation in my previous post, that is the power available in the wind (no matter how you observe it). If your windmill swept area / wind speed left you get 10 Watts of energy from the wind, that is all you can get, any extra energy you get from traveling faster into the wind is just a (poor) conversion of the ships own kinetic energy into torque.
I believe I am wrong, and kinetic energy is based on your frame of refrence, that annoys me as my primitave monkey brain cannot think relatively very well. I will have to ponder this more, however, as the plane and boat work off the same principal, and the boat defiantly does not accelerate into the wind, this will be the same for a plane. I have tried to more clearly explain my question in the next post.
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Ignoring defining a plane, glide ratios, balloons, magic steel micro planets etc, I have made this picture that better communicates my original contemplation:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fimg576.imageshack.us%2Fimg576%2F2277%2Fboat2.jpg&hash=f95ddc1fbf6a9b596f4fd38e27a81590)
We know that, if fluid 1 is air, and moving towards the boat, and fluid 2 is water and stationary, the boat will move forward. Agree?
So answer these 5 questions, what will happen if:
1) fluid 1 is air moving towards the boat, fluid 2 is water moving the same speed and direction as the air?
2) fluid 1 is water moving towards the boat, fluid 2 is also water and is stationary?
3) fluid 1 is air moving towards the boat, and fluid 2 is stationary. What happens as we slowly decrease the fluid 2 density from that of water to that of air?
4) fluid 1 is air moving towards the boat, fluid 2 is also air and is stationary?
5) both fluids are air moving towards the boat?
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LeeE are you relating to these 3 condition with respect to airflow and airfoil
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Hmm... That diagram is not quite correct.
The lift vector is not directly upwards but is actually angled slightly forwards, towards the direction of movement. This was something that the Wright brothers discovered with their experiments using their tethered gliders and were it not for this fact, they knew that they wouldn't have been able to build an engine powerful enough for their Flyer 1.
I'm not sure about describing 10° α as 'medium' either, whatever they mean by it. A flat plank of wood will stall somewhere between 10-12° whilst in a commercial airliner, an α of around 10° would be uncomfortable for the passengers and you'd normally only experience it whilst on the glideslope during landing*. Modern fighter aircraft can effectively change the degree of camber of the wing, via the leading and trailing edge flaps, to suite the flight regime and can quite happily fly at > 40° α, albeit at low speeds.
The diagram does show though, how little downwash there is at normal flight attitudes.
*And of course, during take off too.
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It is possible to power an aircraft from the wind.
One form is usually called a 'kite' and involves attaching it to the ground with a cable.
Far less obviously, but it's also possible, you could use a cable to make use of wind shear. For example if you have the aircraft in the jet stream and lower a cable down out of the jet stream then you can make use of the very high windshear to power the aircraft, even drive it upwind, or downwind faster than the jet stream (yes, I know it sounds impossible or even ridiculous, but it definitely can be done- and it doesn't violate conservation laws, similar things are done with land yachts).
It's also theoretically possible for a glider to make use of wind shear to maintain altitude by dipping in and out of the different air masses, but it's very difficult to make it work.
What you can't do is get energy if there's no wind shear; there must always be a variation in speed.
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How do you get the craft to advance forward into a head wind?
Can you deflect the drag force tangential to the line of thrust and drag?
Isn't it all a combination in a vector sum at given angles?
It is possible to power an aircraft from the wind.
One form is usually called a 'kite' and involves attaching it to the ground with a cable.
Far less obviously, but it's also possible, you could use a cable to make use of wind shear. For example if you have the aircraft in the jet stream and lower a cable down out of the jet stream then you can make use of the very high windshear to power the aircraft, even drive it upwind, or downwind faster than the jet stream (yes, I know it sounds impossible or even ridiculous, but it definitely can be done- and it doesn't violate conservation laws, similar things are done with land yachts).
It's also theoretically possible for a glider to make use of wind shear to maintain altitude by dipping in and out of the different air masses, but it's very difficult to make it work.
What you can't do is get energy if there's no wind shear; there must always be a variation in speed.
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If you think about it, that's what a kite does when you first launch it. Kite's make use of the difference in speed between the ground and the air.
There's fiddly ways to do it with turbines, but I can't be bothered to go into it.
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Someone get that kite to fly into a head wind as it is anchored to the ground with a cable
Maybe they will witness an unexpected tail spin.
If you think about it, that's what a kite does when you first launch it. Kite's make use of the difference in speed between the ground and the air.
There's fiddly ways to do it with turbines, but I can't be bothered to go into it.
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@wolfekeeper Yes someone not over complicating things. I agree with you
What you can't do is get energy if there's no wind shear; there must always be a variation in speed.
, this is the conclusion I came to as well in post 5 Thinking about this (in my dreams last night:), I think the key is that in the boat example, the driving propeller is in a fluid that is not moving relative to the boat, where as in a plane (or balloon:) the driving prop has to work in air that is already moving in the opposite direction.
, and what I was trying to explain with my image, as long as there is a difference in speeds of fluid between the turbine and prop it is theoretical possible to push the ship with some force forward.
Also I was just browsing Youtube trying to find a good remote control sail plane / glider slope soaring video, there are many of them if you want to look Bored chemist, they will quite happily stay in the air / gain altitude for as long as the wind is blowing, no free energy there, just energy from the wind.
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Yes, but that's slightly different, there the wind is blowing upwards up a slope and is lifting the aircraft, you don't need wind shear for that. But nevertheless that is just powered by the wind.
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Here i one picked semi at random
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So I need a plane with 10 km very low drag pole on it to extend the turbine into a jetstream, simple I'll cook up some Einsteinon in the microwave tonight and give it a go.
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Several folks have already nailed this one: kinetic energy can be extracted only when there is relative motion.
Generally it would be hard to find a shear layer within air or water that is distinct enough to allow useful extraction with only one craft -- though it is done in lee rotors with Rc planes ( Also as others have mentioned, it is theoretically possible however to tether two distant craft capable of differing speeds together (say one in the jet stream and one in surface winds), extracting and transfering energy from one to the other via the tether and use this energy to propel the pair.
The intersection of mediums however offer more typical opportunities and allow for some feats that many feel violate the laws of physics. An example of this is the Blackbird, a vehicle which can travel directly downwind, faster than the wind - something even the fastest ice-boats and land-yachts cannot do.
http://www.wired.com/autopia/2010/06/downwind-faster-than-the-wind/
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So I need a plane with 10 km very low drag pole on it to extend the turbine into a jetstream, simple I'll cook up some Einsteinon in the microwave tonight and give it a go.
No, you put the aeroplane at the top in the jet stream, and then at the bottom of a tensile cable you put the lower section (in the simplest case it's just a drogue chute.) And then it basically just works like a kite.
In the more complicated case the aeroplane has a windmill on it, the cable is an electrical cable, and the drogue is replaced with a propeller that is powered from the windmill via the cable and pulls the whole system upwind.
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No, you put the aeroplane at the top in the jet stream, and then at the bottom of a tensile cable you put the lower section (in the simplest case it's just a drogue chute.) And then it basically just works like a kite.
In the more complicated case the aeroplane has a windmill on it, the cable is an electrical cable, and the drogue is replaced with a propeller that is powered from the windmill via the cable and pulls the whole system upwind.
The sort of strange but cool thing about that arrangement is that the airplane is facing and flying backwards relative to it's intended path over the ground. An around the world record might be possible with something like this 'backing' it's way around the globe.
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@ThinAirDesigns Thanks for the two links, will make for some fun reading (when I'm not meant to be working at work :) I like things like DDWFTTW, that on the surface don't seem to make sense, it certainly dosn't make sense to me yet, think my favorite is the Monty Hall problem, but that has nothing to do with wind.
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Yes essentially wind shear or wind-ground shear is where you can get energy from. The fastest model aircraft is I think a glider where the record sits at getting on for 400mph. They work in an area of high wind shear looping between the slow and fast moving air.
With high lift to drag ratios you can go a lot faster than the speed difference of the fluids
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Alright, now lets say you have a remote controlled plane with a brushless motor, eventually the planes battery will die. Now look at a wind turbine, it generates its own power, so if you get what I am saying then you don't have to keep reading. In order to keep a wind powered plane in the air the propeller would need to generate its own power. That power goes to a generator which then goes to a motor and powers the plane.(WARNING: Do not try this on a real plane, these statements need to be tested on a home-made remote-controlled plane, DO NOT TRY)
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Alright, now lets say you have a remote controlled plane with a brushless motor, eventually the planes battery will die. Now look at a wind turbine, it generates its own power, so if you get what I am saying then you don't have to keep reading. In order to keep a wind powered plane in the air the propeller would need to generate its own power. That power goes to a generator which then goes to a motor and powers the plane.(WARNING: Do not try this on a real plane, these statements need to be tested on a home-made remote-controlled plane, DO NOT TRY)
Sorry to burst your balloon, but that would be what is otherwise known as a perpetual motion machine, which violates the laws of physics...
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To start with do you mean "auroplane" or "aircraft".
Then what do you mean by "powered".
If you mean "propelled" and you are happy with "aircraft" then the answer is its called a balloon.
But if you mean "propelled" and you really mean "aeroplane" then you have a problem.
An aeroplane is kept up by air flowing over its wings but for it to go forward under the impulse of wind it would need the air to be flowing in the opposite direction.
If the aerofoil was reversed to accomodate for this then forward motion would cancel the lift.
Perhaps Magnus rotors instead of wings would work. That would enable wind to generate lift even if its from behind (exploited for propulsion). But Im not a physicist and can only speculate that forward motion would impart a Magnus effect in the reverse direction and cancel the lift.
Of course, you could put gigantic sales on a VTOL airplane in hover mode but then, its wings will do no work and technically its not an "aeroplane" (its lifted by thrust).
So, yes you can have a wind propelled aircraft....BUT, no, it would not be an "aeroplane".
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...and before anyone points out I wrote "sales" not "sails"...I should point out that "gigantic sales" were indeed put on a VTOL aircraft by the British MOD which sold its entire Harrier fleet to the US Marine Corps for spare parts at scrap value.