Is there any wingtip vortex (by induced drag) at low angle of attack?
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Is there any wingtip vortex (by induced drag) at low angle of attack?
Hi,
As I know, induced drag is an unavoidable product of lift and is directly related to the angle of attack. To produce lift, the static pressure on the upper surface of the wing is lower than that of surrounding atmosphere. The static pressure on the upper surface tends to move in towards the fuselage, creating a spanwise flow. The static pressure on the lower surface is higher than atmospheric, and this creates a spanwise movement of air towards the wingtip. The spanwise flows on upper and lower surfaces are on opposite directions, and vortices form when where the two flows meet on the trailing edge of the aerofoil. The vortices takes energy out of the airflow around the aerofil and so increases drag. Induced drag decrease as the speed increases because the angle of attack is reduced, but it sill exists (according to the drag curve).
However, at small angle of attack, about 4 degree, there are static-pressure reductions over both the upper and lower surfaces, with the lift force generated by the pressure differential. If both upper and lower surface static pressure are lower then atmospheric pressure, how could wingtip vortices / induced drag form as shown in the drag curve?
Thank you so much.
Regards,
Issac
As I know, induced drag is an unavoidable product of lift and is directly related to the angle of attack. To produce lift, the static pressure on the upper surface of the wing is lower than that of surrounding atmosphere. The static pressure on the upper surface tends to move in towards the fuselage, creating a spanwise flow. The static pressure on the lower surface is higher than atmospheric, and this creates a spanwise movement of air towards the wingtip. The spanwise flows on upper and lower surfaces are on opposite directions, and vortices form when where the two flows meet on the trailing edge of the aerofoil. The vortices takes energy out of the airflow around the aerofil and so increases drag. Induced drag decrease as the speed increases because the angle of attack is reduced, but it sill exists (according to the drag curve).
However, at small angle of attack, about 4 degree, there are static-pressure reductions over both the upper and lower surfaces, with the lift force generated by the pressure differential. If both upper and lower surface static pressure are lower then atmospheric pressure, how could wingtip vortices / induced drag form as shown in the drag curve?
Thank you so much.
Regards,
Issac
If there is a differential pressure between upper and lower surfaces there will still be Span-wise flow. Therefore, there will be vortices.
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Hi Isaac,
you seem to be having a lot of basic aerodynamics questions. You will probably get better answers from your teacher/lecturer than from an internet forum. They may also be able to help you correct some of the misunderstanding which you seem to have, as evidenced by your posts.
As for your specific question, put very simply, a wing produces lift because there is a higher pressure on the bottom than on the top, and these are separated by solid barrier (the wing). Where there is no wing, air will flow from the high pressure area (bottom) to the low pressure area (top). This is what creates the vortices at the wingtips. The only way you can have no vortices is (i) no lift is produced or (b) an infinite wing.
I don't know why you refer to 4° AoA - there is nothing magical about 4° and any CL v. AOA will be a function of camber, profile etc.
Your statement that "If both upper and lower surface static pressure are lower then atmospheric pressure" is irrelevant. The absolute pressure (whether compared to what you call the atmospheric pressure or anything else) is completely irrelevant to whether the wing creates lift or not. The only thing relevant to the wing is the pressures on the wing surface and, if you want to have lift generated, the sum vector of those pressures around the wing surface needs to point upwards.
In reality, you will *mostly* find that pressure underneath the wing will be higher than what you call atmospheric pressure, and pressure on top of will be lower, but you could create lift even where both surfaces are at a lower pressure than ambient, as long as the differential is 'upwards'.
This :
Is all more or less wrong and/or presented the wrong way around. Pressures don't move anywhere, they "just exist". The spanwise airflows result from pressure differentials around a finite span and are not inherent in the lift creation. Vortices (which do not arise where the flows meet but arise where the flow "turns the corner" at the wingtips) do not 'take energy' from anything, if anything they add energy to the airflow - e.g. vortex generators are often used on leading edges to energise the airflow to ensure it stays attached to the lifting surface for longer. Even an infinite span aerofoil would produce lift-induced drag, without producing vortices. Induced drag does not exist because the drag curve says so... the drag curve shows what is there, not the other way around.
Lots of the stuff I have written above can be pulled apart as well, as I am sure I have oversimplified and misrepresented; and I fail to mention Venturi, Bernoulli, Navier Stokes and Euler...
To quote a NASA website: "The real details of how an object generates lift are very complex and do not lend themselves to simplification." If you really want to understand this fascinating subject, internet forums are not really the right place to go to...
The NASA site a Glenn is a good place to start: https://www.grc.nasa.gov/www/k-12/airplane/guided.htm
B.
you seem to be having a lot of basic aerodynamics questions. You will probably get better answers from your teacher/lecturer than from an internet forum. They may also be able to help you correct some of the misunderstanding which you seem to have, as evidenced by your posts.
As for your specific question, put very simply, a wing produces lift because there is a higher pressure on the bottom than on the top, and these are separated by solid barrier (the wing). Where there is no wing, air will flow from the high pressure area (bottom) to the low pressure area (top). This is what creates the vortices at the wingtips. The only way you can have no vortices is (i) no lift is produced or (b) an infinite wing.
I don't know why you refer to 4° AoA - there is nothing magical about 4° and any CL v. AOA will be a function of camber, profile etc.
Your statement that "If both upper and lower surface static pressure are lower then atmospheric pressure" is irrelevant. The absolute pressure (whether compared to what you call the atmospheric pressure or anything else) is completely irrelevant to whether the wing creates lift or not. The only thing relevant to the wing is the pressures on the wing surface and, if you want to have lift generated, the sum vector of those pressures around the wing surface needs to point upwards.
In reality, you will *mostly* find that pressure underneath the wing will be higher than what you call atmospheric pressure, and pressure on top of will be lower, but you could create lift even where both surfaces are at a lower pressure than ambient, as long as the differential is 'upwards'.
This :
The static pressure on the upper surface tends to move in towards the fuselage, creating a spanwise flow. The static pressure on the lower surface is higher than atmospheric, and this creates a spanwise movement of air towards the wingtip. The spanwise flows on upper and lower surfaces are on opposite directions, and vortices form when where the two flows meet on the trailing edge of the aerofoil. The vortices takes energy out of the airflow around the aerofil and so increases drag. Induced drag decrease as the speed increases because the angle of attack is reduced, but it sill exists (according to the drag curve).
Lots of the stuff I have written above can be pulled apart as well, as I am sure I have oversimplified and misrepresented; and I fail to mention Venturi, Bernoulli, Navier Stokes and Euler...
To quote a NASA website: "The real details of how an object generates lift are very complex and do not lend themselves to simplification." If you really want to understand this fascinating subject, internet forums are not really the right place to go to...
The NASA site a Glenn is a good place to start: https://www.grc.nasa.gov/www/k-12/airplane/guided.htm
B.
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First of all, I would like to thank you for the answers.
At the moment I am a private pilot and self-studying for the CPL knowledge from a set of books. To be honest, I do have basic theory in aviation otherwise I won't be able to pass my theory exam and also the oral part of flight exam. I was an oversea student and I have now graduated and staying in my home country, therefore it is not a convenience way for me to contact an experienced flight instructor.
Even though I have gained much knowledge from my studies before, I love to clarify any questions I have in mind when studying. I do know induced drag is a by-product of lift, and wingtip voctices are formed because of the spanwise flow, but I want to know deeper to get a clear concept and even sometimes the books may giving wrong information, so I really appreicate everyone who helped or helping me here. The forum here is the best way for me to get assistance from professionals. While I was studying in Australia, I ran to my flying academy everytime I got a question even I don't have a car.
Anyway, let's get back to my question. What writing from the book is, the wingtip vortices are generated because the upper surface of airfoil has a relatively lower pressure than atmospheric pressure, so it moves towards the fuselage. On the other hand, the air at the lower surface has a relatively higher pressure than the atmosphere, therefore they move towards the wingtip. Because these two airflow are moving in opposite direction and meet up at the trailing edge, vortices are formed.
While a normal cambered airfoil at a small angle of attack, such as 4 degree, both surfaces have a relatively lower pressure than the atmospheric pressure i.e. both airflow would tend to move towards the fuselage, since they are moving in the same direction, how could wingtip vortices form?
Lift is produced by the pressure difference between upper and lower surface, so induced drag is also generated. However in this case, wingtip vortices do not seem to be exist.
So, my two questions are:
1) Are wingtip vortices exist in the situation mentioned above i.e. 4 degree AoA.
2) If vortices were not formed, where was the induced drag coming from?
Thanks again, and I really do appreciate from the bottom of my heart.
Issac.
At the moment I am a private pilot and self-studying for the CPL knowledge from a set of books. To be honest, I do have basic theory in aviation otherwise I won't be able to pass my theory exam and also the oral part of flight exam. I was an oversea student and I have now graduated and staying in my home country, therefore it is not a convenience way for me to contact an experienced flight instructor.
Even though I have gained much knowledge from my studies before, I love to clarify any questions I have in mind when studying. I do know induced drag is a by-product of lift, and wingtip voctices are formed because of the spanwise flow, but I want to know deeper to get a clear concept and even sometimes the books may giving wrong information, so I really appreicate everyone who helped or helping me here. The forum here is the best way for me to get assistance from professionals. While I was studying in Australia, I ran to my flying academy everytime I got a question even I don't have a car.
Anyway, let's get back to my question. What writing from the book is, the wingtip vortices are generated because the upper surface of airfoil has a relatively lower pressure than atmospheric pressure, so it moves towards the fuselage. On the other hand, the air at the lower surface has a relatively higher pressure than the atmosphere, therefore they move towards the wingtip. Because these two airflow are moving in opposite direction and meet up at the trailing edge, vortices are formed.
While a normal cambered airfoil at a small angle of attack, such as 4 degree, both surfaces have a relatively lower pressure than the atmospheric pressure i.e. both airflow would tend to move towards the fuselage, since they are moving in the same direction, how could wingtip vortices form?
Lift is produced by the pressure difference between upper and lower surface, so induced drag is also generated. However in this case, wingtip vortices do not seem to be exist.
So, my two questions are:
1) Are wingtip vortices exist in the situation mentioned above i.e. 4 degree AoA.
2) If vortices were not formed, where was the induced drag coming from?
Thanks again, and I really do appreciate from the bottom of my heart.
Issac.
1. Yes, of course they will at any time where the lift coefficient is non-zero
2. Vortices also form along the length of the trailing edge because the spanwise flow component creates upper and lower flows that are moving in different directions. When these flows meet again at the TE the direction difference causes the flows to shear, producing vorticity.
PDR
2. Vortices also form along the length of the trailing edge because the spanwise flow component creates upper and lower flows that are moving in different directions. When these flows meet again at the TE the direction difference causes the flows to shear, producing vorticity.
PDR
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The only wing that does not produce wing tip vortexes are Hump Back Wale flippers the best wing design in the world with millions of years of research and development behind it.
The wing/flipper has leading edge vortex generators sticking out all along the leading edge which keep the water flow attached at high angles of attack and soft stalls. This design beats the best human straight leading edge design by a reduction in drag of 8% and can be applied to all aerodynamic surface of an aircraft. Designers do not seem have woken up to this performance increase as yet. The Hump Back can pull 30ft diameter turns and can weigh upwards of 30 tons. food for thought. See company called Wale Power that designs wind turbine blades based on this concept.
The wing/flipper has leading edge vortex generators sticking out all along the leading edge which keep the water flow attached at high angles of attack and soft stalls. This design beats the best human straight leading edge design by a reduction in drag of 8% and can be applied to all aerodynamic surface of an aircraft. Designers do not seem have woken up to this performance increase as yet. The Hump Back can pull 30ft diameter turns and can weigh upwards of 30 tons. food for thought. See company called Wale Power that designs wind turbine blades based on this concept.
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The only wing that does not produce wing tip vortexes are Hump Back Wale flippers...
See company called Wale Power...
But really, these whales have developed a marvelous method for creating high lift at low speed with excellent efficiency and low drag. There will certainly be technical applications for that principle (e.g. fans and wind turbines). But aircraft need also be able to generate low lift at high speed for which this design is not suited well. So I doubt that we will see humpback inspired wings on normal aircraft.
