Helicopter Urban Myths
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Matthew, the reason a kite stays airborn is usually because it is not streamlined so the flow becomes turbulent in the wake (ie static pressure is not recovered on lee side). If air was inviscid then a kite would fall to the ground and also there would be no parasitic drag! Unfortunately the lack of coanda effect would mean our precious helis would not get aiborn either...
Assuming kite flies as an aerodynamic wing: The flow over the kite requires wind so the total stagnation pressure is higher than atmospheric (static + dynamic pressure). The kite induces vortices which by various methods (either magnus effect or vortex shedding) speeds up flow above the kite. This increased local velocity means an increase in dynamic pressure hence a reduction in static pressure to keep total stagnation pressure constant. This is why a wing produces more lift at higher airspeed.
Don't forget there is always a net change in momentum. I have a nice video somewhere of some Airbus wind tunnel work. Once the model flies through the smoke field the wingtip vortices are generated as expected. Afterwards the vortices slowly work their way to the ground, as each vortex causes a downwash on the other.
Assuming kite flies as an aerodynamic wing: The flow over the kite requires wind so the total stagnation pressure is higher than atmospheric (static + dynamic pressure). The kite induces vortices which by various methods (either magnus effect or vortex shedding) speeds up flow above the kite. This increased local velocity means an increase in dynamic pressure hence a reduction in static pressure to keep total stagnation pressure constant. This is why a wing produces more lift at higher airspeed.
Don't forget there is always a net change in momentum. I have a nice video somewhere of some Airbus wind tunnel work. Once the model flies through the smoke field the wingtip vortices are generated as expected. Afterwards the vortices slowly work their way to the ground, as each vortex causes a downwash on the other.
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Does this mean that 'weather' is a myth?
Does this mean that meteorologists can throw away their barometers and now think of 'bars' as a place to drink?
Pilots will definitely be happier, 'cause they no longer need to get a weather report.
Just joking.
Does this mean that meteorologists can throw away their barometers and now think of 'bars' as a place to drink?
Pilots will definitely be happier, 'cause they no longer need to get a weather report.
Just joking.
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Dave, weather is the result of a fluid trying to equalise warm and cold regions. The sun shines equally with 1.4kW/m^2 across the entire earths surface. However, the corners of the flat earth are unable to dissipate heat and so cause an accumulation of thermons. These thermons quantum tunnel through the air particles, causing no end of mischief. The reason that meteorologists concern themselves with isobars is this indictes regions where this mischief is likely to accumulate. In regions of low pressure there is a distinct absence of mischief, while high pressure leads to definate high jinx.
Now if i could just get the lid off my jar of bernoulli particles...
Now if i could just get the lid off my jar of bernoulli particles...
Last edited by Graviman; 29th Nov 2007 at 21:37. Reason: So that folks realise this is tongue in cheek...
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Matthew asked, "Graviman, would there be any pressure on the front of a kite? If not, why is it harder to hold the string when there is wind?"
Graviman answered, "...The flow over the kite requires wind so the total stagnation pressure is higher than atmospheric..."
Or more simply, yes.
Your answer is not why a kite flies. It is one way of describing it. There are many ways to describe these things.
Graviman answered, "...The flow over the kite requires wind so the total stagnation pressure is higher than atmospheric..."
Or more simply, yes.
Your answer is not why a kite flies. It is one way of describing it. There are many ways to describe these things.
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Mathew, stagnation pressure at infinity for a given condition is defined as:
Pstagnation = Patmos + 1/2RhoV^2, where v describes the wind velocity.
So it must be higher than atmospheric, but will be the same in all places for incompressible and inviscid flow. But, yes this is just one way to describe what is happening to the kite.
Pstagnation = Patmos + 1/2RhoV^2, where v describes the wind velocity.
So it must be higher than atmospheric, but will be the same in all places for incompressible and inviscid flow. But, yes this is just one way to describe what is happening to the kite.
Last edited by Graviman; 30th Nov 2007 at 21:02. Reason: Typo: Pstat should have read Pstagnation.
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Ground effect explained
Ground effect explained
Misinformed Instructors who propel the pressure bubble myth should be asked to do a quick Yahoo search using "induced drag ground effect" so that they can learn how to tell their students the truth, and not convenient myths. We would be all the better for it. Pressure bubblers, please note:
Induced drag explained:
http://galileo.phys.virginia.edu/cla...ero/node5.html
http://www.grc.nasa.gov/WWW/K-12/airplane/induced.html
Ground effect explained:
http://www.faatest.com/books/FLT/Cha...oundEffect.htm
http://www.pilotfriend.com/training/...g/aft_perf.htm
"Many pilots think that ground effect is caused by air being compressed between the wing and the ground. This is not so. Ground effect is caused by the reduction of induced drag when an airplane is flown at slow speed very near the surface."
The quiz for today, for the bright student:
Define induced drag
define aspect ratio
explain how aspect ratio affects induced drag
explain how proximity to the ground affects aspect ratio
Misinformed Instructors who propel the pressure bubble myth should be asked to do a quick Yahoo search using "induced drag ground effect" so that they can learn how to tell their students the truth, and not convenient myths. We would be all the better for it. Pressure bubblers, please note:
Induced drag explained:
http://galileo.phys.virginia.edu/cla...ero/node5.html
http://www.grc.nasa.gov/WWW/K-12/airplane/induced.html
Ground effect explained:
http://www.faatest.com/books/FLT/Cha...oundEffect.htm
http://www.pilotfriend.com/training/...g/aft_perf.htm
"Many pilots think that ground effect is caused by air being compressed between the wing and the ground. This is not so. Ground effect is caused by the reduction of induced drag when an airplane is flown at slow speed very near the surface."
The quiz for today, for the bright student:
Define induced drag
define aspect ratio
explain how aspect ratio affects induced drag
explain how proximity to the ground affects aspect ratio
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I thought induced drag had two components, the first as described in the article and the second being the vector consequence of tilting the wing relative to the airflow to produce lift. In other words I thought induced drag was defined as all the drag consequences of lift production, as opposed to one particular part of it.
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Ah well, I confess I was thinking of rotor drag rather than drag. Does the argument change for rotor drag ? ie. is the vector consequence inline with the rotor plane of increased angle of attack actually defined as part of profile drag ?
In the big scheme of things it probably doesn't much matter, but even for something relatively arcane I'd rather not be the source of further myths !
In the big scheme of things it probably doesn't much matter, but even for something relatively arcane I'd rather not be the source of further myths !
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To get the hover ball rolling away...
Using non-mathematical terminology, to start the discussion. 
Define induced drag:
The drag which is produced from the tip vortices produced by a finite size of wing, and is directly proportional to useful lift.
define aspect ratio:
The wing span to average chord, with >10:1 being required for an efficient wing.
explain how aspect ratio affects induced drag:
By reducing the strength of the tip vortices, the induced drag goes down, so the wing becomes more efficient.
explain how proximity to the ground affects aspect ratio:
Although there is no physical alteration of aspect ratio, the ground restricts the downwash and hence the strength of the tip vortices. The effect on the flow field is as if the wing aspect ratio had been increased.
Hmmm, good links.
Define induced drag:
The drag which is produced from the tip vortices produced by a finite size of wing, and is directly proportional to useful lift.
define aspect ratio:
The wing span to average chord, with >10:1 being required for an efficient wing.
explain how aspect ratio affects induced drag:
By reducing the strength of the tip vortices, the induced drag goes down, so the wing becomes more efficient.
explain how proximity to the ground affects aspect ratio:
Although there is no physical alteration of aspect ratio, the ground restricts the downwash and hence the strength of the tip vortices. The effect on the flow field is as if the wing aspect ratio had been increased.
Hmmm, good links.
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Ground effect explained
Nick I don't really understand : "Many pilots think that ground effect is caused by air being compressed between the wing and the ground. This is not so. Ground effect is caused by the reduction of induced drag when an airplane is flown at slow speed very near the surface."
all your links refer to fixed wings not helicopters
What about an helicopter at zero speed ? so why at zero speed the ground effect is different on grass or concrete?
all your links refer to fixed wings not helicopters
What about an helicopter at zero speed ? so why at zero speed the ground effect is different on grass or concrete?
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Froggy, the rotor system can be seen as a wing in a rotating reference frame and the fixed wing in a linear reference frame. The fixed wing may be easier to visualise, and is also discussed more on the web, but the basics of flight apply equally to rotorcraft... 
Puntosaurus, in answer to your question i would say that the change of lift vector is just as valid for rotorcraft as it is siezed wings. In both cases the cause is the downwash across the wing/rotor, which results from the unbounded tip vortex.
Puntosaurus, in answer to your question i would say that the change of lift vector is just as valid for rotorcraft as it is siezed wings. In both cases the cause is the downwash across the wing/rotor, which results from the unbounded tip vortex.
Last edited by Graviman; 1st Dec 2007 at 02:53.
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I wasn't making a fixed/rotary distinction, just trying to confirm my understanding of the definition of induced drag (Since Nick asked the question).
So I went back to Prouty, and he defines it (Fig 1.2) as the 'horizontal' component of the lift vector. Since he was talking about the hover we can avoid arguments about what horizontal means. Wagtendonk seems to be working along the same lines (Fig 4-6).
My point is that as defined by Prouty, induced drag is simply a mechanical consequence of the definition of lift (ie. that force which is perpendicular to the local velocity) and the wing being tilted out of the horizontal plane.
Now I know the stuff about tip vortices and I'm sure that's an additional souce of lift induced drag, but coming back to my original point, the tip vortices (as I understand it) are not the only cause of induced drag.
To put it another way if you had no tip vortices (eg. by extending the wing through the walls of the wind tunnel) would an aerodynamicist still say that there is induced drag on a lift producing wing ?
So I went back to Prouty, and he defines it (Fig 1.2) as the 'horizontal' component of the lift vector. Since he was talking about the hover we can avoid arguments about what horizontal means. Wagtendonk seems to be working along the same lines (Fig 4-6).
My point is that as defined by Prouty, induced drag is simply a mechanical consequence of the definition of lift (ie. that force which is perpendicular to the local velocity) and the wing being tilted out of the horizontal plane.
Now I know the stuff about tip vortices and I'm sure that's an additional souce of lift induced drag, but coming back to my original point, the tip vortices (as I understand it) are not the only cause of induced drag.
To put it another way if you had no tip vortices (eg. by extending the wing through the walls of the wind tunnel) would an aerodynamicist still say that there is induced drag on a lift producing wing ?
Last edited by puntosaurus; 1st Dec 2007 at 04:25.
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Induced drag is a consequence of producing lift. Lifting line theory defines this in terms of circulation. But in simple terms, if you are producing lift, then you are producing downwash, you can't escape from that.
When looking at an aerofoil section, the apparent angle of incidence is increased by the downwash, and so a tiliting of the lift vector backwards.
So what's all this about vorticies then? At the tip of an aerofoil, the air rolls round from higher to lower pressure (in simple terms). This produces an increase in the local downwash towards the tip adding to the induced drag effect. In a low aspect ratio wing, this is proportianally much higher than in a high aspect ratio wing to the total lift/drag. When not interested in optimising for compressibility effects etc. the aim is to produce a uniform downwash across the span. This represents the minimum induced drag any given aerofoil can achieve.
When looking at an aerofoil section, the apparent angle of incidence is increased by the downwash, and so a tiliting of the lift vector backwards.
So what's all this about vorticies then? At the tip of an aerofoil, the air rolls round from higher to lower pressure (in simple terms). This produces an increase in the local downwash towards the tip adding to the induced drag effect. In a low aspect ratio wing, this is proportianally much higher than in a high aspect ratio wing to the total lift/drag. When not interested in optimising for compressibility effects etc. the aim is to produce a uniform downwash across the span. This represents the minimum induced drag any given aerofoil can achieve.
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Puntosaurus, i have wondered the same thing about the change of lift vector in an unbounded wing in a wind tunnel. I'll just consider this, rather than rewrite Waspy's excellent post.
Basically in all flight conditions the induced drag is always the result of the angle of attack revectoring lift rearwards. Don't forget AOA is defined from the datum of tip to tail, so even a cambered aerofoil will produce no lift at zero AOA (they will be optimised for say 8'). Put another way zero AOA can be defined as the attitude of the wing section to produce no lift.
When the wing section is in a wind tunnel, increasing the AOA both increases the lift vector magnitude (since Cd increases with AOA) and revectors it rearwards. The drag is thus the result of the induced drag, and a parasitic term associated with the increasing turbulence as you approach stall. If the same wing section is operating in free air the tips spill off the circulation as vortices. These vortices mean that the section is now effectively operating in a local downwash. Thus the AOA must be increased a little more, to produce the same lift, and thus the induced drag goes up some more.
So yes, if you had no tip vortices an aerodynamicist would still say that there is induced drag on a lift producing wing.
Basically in all flight conditions the induced drag is always the result of the angle of attack revectoring lift rearwards. Don't forget AOA is defined from the datum of tip to tail, so even a cambered aerofoil will produce no lift at zero AOA (they will be optimised for say 8'). Put another way zero AOA can be defined as the attitude of the wing section to produce no lift.
When the wing section is in a wind tunnel, increasing the AOA both increases the lift vector magnitude (since Cd increases with AOA) and revectors it rearwards. The drag is thus the result of the induced drag, and a parasitic term associated with the increasing turbulence as you approach stall. If the same wing section is operating in free air the tips spill off the circulation as vortices. These vortices mean that the section is now effectively operating in a local downwash. Thus the AOA must be increased a little more, to produce the same lift, and thus the induced drag goes up some more.
So yes, if you had no tip vortices an aerodynamicist would still say that there is induced drag on a lift producing wing.
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Many thanks for that. I agree because at least it is internally consistent. Whereas from the NASA site
The emboldening is mine, and encouraged by your support I rather disagree with it. The Virginia article also focuses exclusively on the tip vortex issue, and ignores the mechanical issues.
To quote again from the NASA article
Maybe someone who has endured the tedium can shed some light on this. Tip vortices, revectoring, or both ?
This additional force is called induced drag because it faces downstream and has been "induced" by the action of the tip vortices. It is also called "drag due to lift" because it only occurs on finite, lifting wings and varies with the square of the lift.
To quote again from the NASA article
The derivation of the equation for the induced drag is fairly tedious and relies on some theoretical ideas which are beyond the scope of the Beginner's Guide.
Last edited by puntosaurus; 1st Dec 2007 at 11:04.
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Puntosaurus, don't forget the wing or rotor is only required to generate enough lift to lift the aircraft. For a bounded wing in free air it would need to be infinately long, so each segment would be required to produce zero lift. So for a practical wing it is fair to comment about the induced drag being a function of the vortex strength. As for the equation being unfathomable - that first link put up by Nick is well worth a peruse for some of the equations.
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Was this a trick question?
Yesterday Nick asked:
"Explain how proximity to the ground affects aspect ratio."
The correct answer is that extreme proximity reduces it. (You knock your wingtips off)
"Explain how proximity to the ground affects aspect ratio."
The correct answer is that extreme proximity reduces it. (You knock your wingtips off)
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Pressure bubble
Nick
What is the most common FORMAL definition of induced drag ?
Why am I asking this apparent simpel question?
When calculating the detailed dynamics for my simulator, I needed a formal definition to aggregate the precise data so as to obtain a precise global figure (=integrated over one revolution) for induced drag.
In more detail:
First consider Plank, linear aerodynamics:
Given a certain flow we have ONE force on the airfoil, this force is "arbitrarily" decomposed to drag and lift with respect to some "conventional" reference frame. This is the frame that is conventionally taken with reference to some points of the airfoil profile. Rotating this reference frame gives different meanings to drag and lift (without changing the resulting total force). This of course will also change the definition of induced drag which is the drag caused by the lift because it is projected in a different (rotated) coordinate system.
Putting a wing at a rotated reference with respect to the plane may alter this reference and thus the precise NUMBER of induce drag.
Having the plane fly at a different pitch angle may again change the reference system and thus the notations of lift, drag and induced drag.
Puntosaurus used for instance "horizontal", can he give a precise definition ?
Now go to the heli
We can take the hub plane (that is perpendicular to the physical rotor shaft (this is what I took, because its the most natural one to calculate moments and torques around the rotor axis)
or the effective rotor plane or disk plane (once it starts flapping), which looks more natural when analysing all flow components around a particular blade element, or the direction the body goes (plane of motion)
So even the "induced drag" line of reasoning may not be as simple, question of references.
Back to the pressure bubble.
Again Nick I fully endourse your arguments (conditional to theabove caveat), but reiterate that this is not the only possible "angle". We can talk pressure, angles, molecules... all lead to the same: less power needed. I guess it is up to your personal preference.
May be the analogy to electricity does not appeal the the rotary world, but take for instance a car: why does it accelerate
- because the driver hit the pedel
- because the engine gives more power
- because carb pressure increase
- because sheer force between ground and tyres increases
I think all are correct...
d3
What is the most common FORMAL definition of induced drag ?
Why am I asking this apparent simpel question?
When calculating the detailed dynamics for my simulator, I needed a formal definition to aggregate the precise data so as to obtain a precise global figure (=integrated over one revolution) for induced drag.
In more detail:
First consider Plank, linear aerodynamics:
Given a certain flow we have ONE force on the airfoil, this force is "arbitrarily" decomposed to drag and lift with respect to some "conventional" reference frame. This is the frame that is conventionally taken with reference to some points of the airfoil profile. Rotating this reference frame gives different meanings to drag and lift (without changing the resulting total force). This of course will also change the definition of induced drag which is the drag caused by the lift because it is projected in a different (rotated) coordinate system.
Putting a wing at a rotated reference with respect to the plane may alter this reference and thus the precise NUMBER of induce drag.
Having the plane fly at a different pitch angle may again change the reference system and thus the notations of lift, drag and induced drag.
Puntosaurus used for instance "horizontal", can he give a precise definition ?
Now go to the heli
We can take the hub plane (that is perpendicular to the physical rotor shaft (this is what I took, because its the most natural one to calculate moments and torques around the rotor axis)
or the effective rotor plane or disk plane (once it starts flapping), which looks more natural when analysing all flow components around a particular blade element, or the direction the body goes (plane of motion)
So even the "induced drag" line of reasoning may not be as simple, question of references.
Back to the pressure bubble.
Again Nick I fully endourse your arguments (conditional to theabove caveat), but reiterate that this is not the only possible "angle". We can talk pressure, angles, molecules... all lead to the same: less power needed. I guess it is up to your personal preference.
May be the analogy to electricity does not appeal the the rotary world, but take for instance a car: why does it accelerate
- because the driver hit the pedel
- because the engine gives more power
- because carb pressure increase
- because sheer force between ground and tyres increases
I think all are correct...
d3
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Well that's why I stuck to the hover, I wanted to isolate the problem for discussion without getting bothered with other stuff, and in the hover there's a little less debate about what horizontal is.
But this all comes back to the definition issue. Clearly the boys who worked out the formula for induced drag had a view about what was included and what frame of reference they were using. I was rather hoping my last post would smoke someone out who could explain that !
But this all comes back to the definition issue. Clearly the boys who worked out the formula for induced drag had a view about what was included and what frame of reference they were using. I was rather hoping my last post would smoke someone out who could explain that !