Ha! I was just hunting around here for a suitable spool for that experiment.
There is nothing like experiment, even if it just sticking your hand out of the car window, to separate the good theories from bad, and checkout arm chair hand waving arguments, like all that vortex, infinite-wing, ground effect nonsense.
Hmmm...I guess if you could wave you hands fast enough while sitting in an arm chair you might get the idea
Anyway, we seem to have two different debates here now. One is about how does a wing generate lift, the other about what is ground effect. Better to keep the separate I think.
Measure the amount power put into the motors of quad copter as it slowly rises from the ground.
My guess is there are 3 significant phases to this process:
1) At zero and small altitudes only a little power is required to start moving upward. Your altitude depends on the amount of power you apply.
Why? Because you are trying to compress a lot of air into a small space directly between the copter and the ground. That air pushes back and up you go. That air is escaping side ways of course, the higher you are the bigger "gap" it has to escape through. So more power more height.
2) At some point increasing the power does not increase your altitude so much as you expect anymore.
Why? Because the Bernoulli effect comes into play. The speed of that air running away from under the copter is lowering the air pressure beneath you and sucking you down. As your spool experiment demonstrates. You are stuck in a potential well, a balance between compressing air beneath you pushing you up and the Bernoulli effect sucking you down.
3) With enough power you rise further and escape that well. Now in free air you should continue to gain altitude even at constant power.
Why? Because if your thrust downward is greater than the force of gravity you have to be going up. You don't have to increase power to gain altitude. Increasing power will make your rise faster.
Anyone with a quad copter up for that experiment? I don't have one.
Somehow I thought my statements might lead to some objection. I'm adamant because I'm right
I quite happy to have one say that I'm wrong, I'd be happier if they would also offer a reason as to why.
Simply stated I believe:
1) One does not need to think about vortices to explain the lift of a wing.
Heater, I thought my comment about rockets would make it clear I thought the author of the vortex paper was wrong. As I reread my comment (made around 2 am) I can see how you thought I was saying you were wrong.
Ah yes, you mean "It seems like he should also be arguing rockets don't work in space since they can't push against the ground."
Seems we are in total agreement then
The article in question is a verbose, confusing mess. It has some good points but blows it by being wrong in major ways.
He is right in saying that many school books and other texts have misleading diagrams in them. Along with misleading explanations. The diagrams that show nice straight horizontal lines of air flow hitting a wing, then nice straight horizontal lines of air flow behind it.
This cannot be. Newton tells us that you need to throw some mass downwards to get some thrust upwards, f = ma and all that, like the rocket as you point out. In those diagrams nothing is propelled downwards so there can be no lift. Those diagrams are incorrect.
Then there is some waffle about whether lift is cause by the Bernoulli effect or Newton. Well, as the former can be derived from the latter that is just silly.
Then there is more waffle about vortices and ground effect that is totally irrelevant.
But now, I'm amazed by the wikipedia article that is telling me the typical school room demonstration of the Bernoulli effect is wrong. Which itself confuses many issues. Certainly those classroom demonstrations may not fit the strict mathematical definition of Bernoulli's principal. But they certainly demonstrate the physics of the idea. https://en.wikipedia.org/wiki/Bernoulli's_principle#Misunderstandings_about_the_generation_of_lift
Note: The round head of a shirt pin in the center of the card in line with the spool hole helps to prevent the card from sliding away. Otherwise you can secure the card by lightly obstructing the edges to hold it in place. The sliding off center is a result of the card not being exactly perpendicular to the airflow, and can also be seen in real life during take-off ... i.e. a slight list to a side as the craft ascends
It's interesting to think about that copter hovering in the midst of a lot of free air.
A rocket burns fuel which expands and gets ejected downward. Newton tells us about "every force has a reaction" and "f=ma" etc. So we see that throwing that fuel downwards at high speed propels the rocket upwards.
What about the copter? It has no such mass of fuel to eject. Instead it basically sucks air from above and blows in out below. Thus we are back to f=ma and we have lift.
But then what? The copter has removed air from above, creating a low pressure, and pushed air below creating a high pressure. That air will move in order to equalize the pressure difference. How does it do that? The only way it can, by creating a toroidal, doughnut shaped, vortex around the quad copter.
A beautiful thing. We could say the copter stays up because it is continually pumping air around that toroidal vortex loop and overcoming the friction involved in doing so.
This is where the whole "lift needs a vortex" confusion comes from.
The overarching reason an airfoil generates any force (through itself and anything it's connected to) is by pressure differences upon opposite surfaces. How those pressure differences are achieved depends on numerous factors such as airfoil shape, angle of attack, velocity (compressibility effects), altitude, etc. Vorticies are an effect of these pressure differences where air in the higher pressure region wants to move to the lower pressure region. These are generally undesirable and are mitigated by various techniques such as winglets or controlling the spanwise pressure distribution using tapered and elliptical wings.
Downwash can be generated by both the lower and upper surface of an airfoil. Consider a wing with an angle of attack > 0 degrees: the bottom surface will see an increase in pressure above ambient, and thus the air will naturally want to flow to any lower pressure regions, with the area below the wing being the most likely - though some will flow around the wingtips generating vorticies as mentioned above, and some will actually flow from the bottom of the trailing edge of the wing around to the top of the trailing edge).
That might seem pretty obvious, but how does the upper surface generate downwash? It's actually quite similar. The upper surface has areas where the pressure is below ambient (generating a pressure differential on the wing) , so the air above the wing wants to move downwards towards the surface. This downward flow continues behind the wing since it's moving, and so it effectively causes downwash. There have been some funky looking airfoils with high lift to drag ratios that mitigate downwash by carefully designing the pressure recovery section so that by the time the flow gets to the trailing edge, it's pretty much at ambient pressure and you no longer have a mass of air flowing downwards.
As for ground effect, air basically gets trapped between the bottom of the wing and the ground, which effectively increases the pressure on the bottom of the airfoil, causing a larger pressure differential between the two surfaces of the wings (or rotors - basically the same thing) thus increasing lift.
First, for a vehicle of this size you don't really need stabilization. If it were me, I would not want it. I would want the minimum between my control inputs and the response of the motors. It would behave like a small helicopter, negative stability but enough mass to make human control corrections practical. Though rate gyros could be helpful.
Second, all my experience with flying rotary wing craft has shown me that ground effect does occur. If you have the collective (throttle) in just the right spot the craft will find an altitude of equilibrium where it gently bobs up and down maintaining a certain average distance from the ground.
Lastly, after watching the video and seeing the behavior of the craft and listening to the motors, it looks to me like there is no weird effects going on. It is the pilot trying to manage the collective (throttle) while being careful not to go too high or to make abrupt changes. A skill that takes some time to master. Towards the end of the video you can already see improvement. I don't believe the distance above the ground is small enough for the Radial Momentum theory to apply.
Comments
Ha! I was just hunting around here for a suitable spool for that experiment.
There is nothing like experiment, even if it just sticking your hand out of the car window, to separate the good theories from bad, and checkout arm chair hand waving arguments, like all that vortex, infinite-wing, ground effect nonsense.
Hmmm...I guess if you could wave you hands fast enough while sitting in an arm chair you might get the idea
Anyway, we seem to have two different debates here now. One is about how does a wing generate lift, the other about what is ground effect. Better to keep the separate I think.
An experiment I would like to see goes like this:
Measure the amount power put into the motors of quad copter as it slowly rises from the ground.
My guess is there are 3 significant phases to this process:
1) At zero and small altitudes only a little power is required to start moving upward. Your altitude depends on the amount of power you apply.
Why? Because you are trying to compress a lot of air into a small space directly between the copter and the ground. That air pushes back and up you go. That air is escaping side ways of course, the higher you are the bigger "gap" it has to escape through. So more power more height.
2) At some point increasing the power does not increase your altitude so much as you expect anymore.
Why? Because the Bernoulli effect comes into play. The speed of that air running away from under the copter is lowering the air pressure beneath you and sucking you down. As your spool experiment demonstrates. You are stuck in a potential well, a balance between compressing air beneath you pushing you up and the Bernoulli effect sucking you down.
3) With enough power you rise further and escape that well. Now in free air you should continue to gain altitude even at constant power.
Why? Because if your thrust downward is greater than the force of gravity you have to be going up. You don't have to increase power to gain altitude. Increasing power will make your rise faster.
Anyone with a quad copter up for that experiment? I don't have one.
Heater, I thought my comment about rockets would make it clear I thought the author of the vortex paper was wrong. As I reread my comment (made around 2 am) I can see how you thought I was saying you were wrong.
Ah yes, you mean "It seems like he should also be arguing rockets don't work in space since they can't push against the ground."
Seems we are in total agreement then
The article in question is a verbose, confusing mess. It has some good points but blows it by being wrong in major ways.
He is right in saying that many school books and other texts have misleading diagrams in them. Along with misleading explanations. The diagrams that show nice straight horizontal lines of air flow hitting a wing, then nice straight horizontal lines of air flow behind it.
This cannot be. Newton tells us that you need to throw some mass downwards to get some thrust upwards, f = ma and all that, like the rocket as you point out. In those diagrams nothing is propelled downwards so there can be no lift. Those diagrams are incorrect.
Then there is some waffle about whether lift is cause by the Bernoulli effect or Newton. Well, as the former can be derived from the latter that is just silly.
Then there is more waffle about vortices and ground effect that is totally irrelevant.
But now, I'm amazed by the wikipedia article that is telling me the typical school room demonstration of the Bernoulli effect is wrong. Which itself confuses many issues. Certainly those classroom demonstrations may not fit the strict mathematical definition of Bernoulli's principal. But they certainly demonstrate the physics of the idea.
https://en.wikipedia.org/wiki/Bernoulli's_principle#Misunderstandings_about_the_generation_of_lift
A rocket burns fuel which expands and gets ejected downward. Newton tells us about "every force has a reaction" and "f=ma" etc. So we see that throwing that fuel downwards at high speed propels the rocket upwards.
What about the copter? It has no such mass of fuel to eject. Instead it basically sucks air from above and blows in out below. Thus we are back to f=ma and we have lift.
But then what? The copter has removed air from above, creating a low pressure, and pushed air below creating a high pressure. That air will move in order to equalize the pressure difference. How does it do that? The only way it can, by creating a toroidal, doughnut shaped, vortex around the quad copter.
A beautiful thing. We could say the copter stays up because it is continually pumping air around that toroidal vortex loop and overcoming the friction involved in doing so.
This is where the whole "lift needs a vortex" confusion comes from.
Downwash can be generated by both the lower and upper surface of an airfoil. Consider a wing with an angle of attack > 0 degrees: the bottom surface will see an increase in pressure above ambient, and thus the air will naturally want to flow to any lower pressure regions, with the area below the wing being the most likely - though some will flow around the wingtips generating vorticies as mentioned above, and some will actually flow from the bottom of the trailing edge of the wing around to the top of the trailing edge).
That might seem pretty obvious, but how does the upper surface generate downwash? It's actually quite similar. The upper surface has areas where the pressure is below ambient (generating a pressure differential on the wing) , so the air above the wing wants to move downwards towards the surface. This downward flow continues behind the wing since it's moving, and so it effectively causes downwash. There have been some funky looking airfoils with high lift to drag ratios that mitigate downwash by carefully designing the pressure recovery section so that by the time the flow gets to the trailing edge, it's pretty much at ambient pressure and you no longer have a mass of air flowing downwards.
As for ground effect, air basically gets trapped between the bottom of the wing and the ground, which effectively increases the pressure on the bottom of the airfoil, causing a larger pressure differential between the two surfaces of the wings (or rotors - basically the same thing) thus increasing lift.
First, for a vehicle of this size you don't really need stabilization. If it were me, I would not want it. I would want the minimum between my control inputs and the response of the motors. It would behave like a small helicopter, negative stability but enough mass to make human control corrections practical. Though rate gyros could be helpful.
Second, all my experience with flying rotary wing craft has shown me that ground effect does occur. If you have the collective (throttle) in just the right spot the craft will find an altitude of equilibrium where it gently bobs up and down maintaining a certain average distance from the ground.
Lastly, after watching the video and seeing the behavior of the craft and listening to the motors, it looks to me like there is no weird effects going on. It is the pilot trying to manage the collective (throttle) while being careful not to go too high or to make abrupt changes. A skill that takes some time to master. Towards the end of the video you can already see improvement. I don't believe the distance above the ground is small enough for the Radial Momentum theory to apply.