airfoil thickness
08-12-2004 | 06:47 PM
#27
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From: Bloomington, MN,
RE: airfoil thickness
ORIGINAL: adam_one
The chart below shows how a flat plate has more max lift than a typical "thick" airfoil at Re's <40,000.
The chart below shows how a flat plate has more max lift than a typical "thick" airfoil at Re's <40,000.
The chart is missing a curve labeled "thin airfoil". A properly selected thin airfoil, when compared to a plate of the same thickness, would have a higher maximum CL. I assume that phenomenon being represented in the chart is that, for low Reynold's numbers, a thick airfoil would require a pressure recovery that is not feasible, which would limit the maximum CL. An airfoil of the same thickness as the plate would probably not suffer from this problem. If this sounds wrong to you, try the following thought experiment. For a given thickness, optimize an airfoil to maximize maximum CL. Among your degrees of freedom is the chordwise thickness distribution. Do you think that a constant thickness (a flat plate) is the optimum solution? I don't think so. As I have mentioned before, there seems to often be some confusion between the thinness and flatness. If we properly separate those two characteristics of an airfoil, some of the confusion goes away.
banktoturn
08-12-2004 | 09:34 PM
#28
RE: airfoil thickness
I only wanted to know if the chart shown, is a rendition of someones actual testing .
I like good, qualified , simple charts -
Is it a provable piece of work?
Y/N
Thank you
I like good, qualified , simple charts -
Is it a provable piece of work?
Y/N
Thank you
08-13-2004 | 02:58 AM
#29
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From: Stockholm, SWEDEN
RE: airfoil thickness
I only wanted to know if the chart shown, is a rendition of someones actual testing .
I like good, qualified , simple charts -
Is it a provable piece of work?
Y/N
Thank you
I like good, qualified , simple charts -
Is it a provable piece of work?
Y/N
Thank you
The chart was "borrowed" from an article included in the last issue (no. 10, Sept 04) of the Fly RC Magazine.
The article's title is just: "Optimizing airfoils for small RC Models" based on Jef Raskin's research ([email protected]).
The chart is missing a curve labeled "thin airfoil". A properly selected thin airfoil, when compared to a plate of the same thickness, would have a higher maximum CL. I assume that phenomenon being represented in the chart is that, for low Reynold's numbers, a thick airfoil would require a pressure recovery that is not feasible, which would limit the maximum CL. An airfoil of the same thickness as the plate would probably not suffer from this problem.
It is stated in the article that the chart is to give a feel for how different kind of airfoils' lift changes dramatically at different Re's.
At very low Re <5000 a flat plate is best (usch), at higher Re a curved plate is best and somewhere between Re 50,000 and Re 100,000 conventional airfoils become superior.
As for how much thin is thin, one may assume that thickness between 1% to 3% of the chord may be considered a thin airfoil / plate.
The author has also come to the conclusion that an airfoil known as "44" was the best choice to his MiniStick (Bob Selman Designs with Re = 21,000).
After he has replaced the original 6% camber airfoil with his "44", which has 4% camber made of a curved sheet foam, the plane climbed faster, had better glide, penetrated the wind more effectively and maintained the altitude at a noticeable lower throttle setting (half throttle instead of 3/4 as with the original 6% camber).
The "44" airfoil has 4% camber made by two arc circles where the max thickness is at 40% from LE.
More details in the actual article...
08-13-2004 | 09:54 AM
#30
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RE: airfoil thickness
ORIGINAL: adam_one
Dick,
The chart was "borrowed" from an article included in the last issue (no. 10, Sept 04) of the Fly RC Magazine.
The article's title is just: "Optimizing airfoils for small RC Models" based on Jef Raskin's research ([email protected]).
Banktoturn,
It is stated in the article that the chart is to give a feel for how different kind of airfoils' lift changes dramatically at different Re's.
At very low Re <5000 a flat plate is best (usch), at higher Re a curved plate is best and somewhere between Re 50,000 and Re 100,000 conventional airfoils become superior.
As for how much thin is thin, one may assume that thickness between 1% to 3% of the chord may be considered a thin airfoil / plate.
The author has also come to the conclusion that an airfoil known as "44" was the best choice to his MiniStick (Bob Selman Designs with Re = 21,000).
After he has replaced the original 6% camber airfoil with his "44", which has 4% camber made of a curved sheet foam, the plane climbed faster, had better glide, penetrated the wind more effectively and maintained the altitude at a noticeable lower throttle setting (half throttle instead of 3/4 as with the original 6% camber).
The "44" airfoil has 4% camber made by two arc circles where the max thickness is at 40% from LE.
More details in the actual article...
I only wanted to know if the chart shown, is a rendition of someones actual testing .
I like good, qualified , simple charts -
Is it a provable piece of work?
Y/N
Thank you
I like good, qualified , simple charts -
Is it a provable piece of work?
Y/N
Thank you
The chart was "borrowed" from an article included in the last issue (no. 10, Sept 04) of the Fly RC Magazine.
The article's title is just: "Optimizing airfoils for small RC Models" based on Jef Raskin's research ([email protected]).
The chart is missing a curve labeled "thin airfoil". A properly selected thin airfoil, when compared to a plate of the same thickness, would have a higher maximum CL. I assume that phenomenon being represented in the chart is that, for low Reynold's numbers, a thick airfoil would require a pressure recovery that is not feasible, which would limit the maximum CL. An airfoil of the same thickness as the plate would probably not suffer from this problem.
It is stated in the article that the chart is to give a feel for how different kind of airfoils' lift changes dramatically at different Re's.
At very low Re <5000 a flat plate is best (usch), at higher Re a curved plate is best and somewhere between Re 50,000 and Re 100,000 conventional airfoils become superior.
As for how much thin is thin, one may assume that thickness between 1% to 3% of the chord may be considered a thin airfoil / plate.
The author has also come to the conclusion that an airfoil known as "44" was the best choice to his MiniStick (Bob Selman Designs with Re = 21,000).
After he has replaced the original 6% camber airfoil with his "44", which has 4% camber made of a curved sheet foam, the plane climbed faster, had better glide, penetrated the wind more effectively and maintained the altitude at a noticeable lower throttle setting (half throttle instead of 3/4 as with the original 6% camber).
The "44" airfoil has 4% camber made by two arc circles where the max thickness is at 40% from LE.
More details in the actual article...
I should correct my 'definitions' from the previous post. There are three variables, or wing characteristics, that are relevant in this discussion. They are maximum thickness, chordwise thickness distribution, and camber. A section with a small value of maximum thickness (a relative characteristic) is 'thin'. A section with a constant thickness along the chord is what we have been calling a 'plate'. A section with zero camber could be called 'flat', although this term seems natural only for a plate, since a traditional section with no camber does not look flat. The terms 'thin', 'plate', and 'flat' are exclusive, and should not be confused. I think that much of the confusion on this topic results from interchanging these terms.
I take issue with the claim that 'flat plates are best' for low Reynold's numbers. Thinner sections (smaller maximum thickness) tend to be better for low Reynold's numbers, but for any given Reynold's number, the optimum section (say, for maximum CL) is not a plate, even if it may be thin, nor is it flat. I don't have access to the text of the article from which you borrowed the chart, but it sounds as though the '44' section is neither flat nor a plate. I would contend that a plate is seldom if ever best, aerodynamically, although it may be good enough that its simplicity makes it the 'best' choice for some applications.
banktoturn
08-13-2004 | 02:06 PM
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From: Stockholm, SWEDEN
RE: airfoil thickness
Banktoturn
Yes, some definitions should be clarified so we are able to understand each other…
Let's try:
The airfoil thickness as well as the camber, are normally expressed as % of the chord, right?
So, let's assume that an airfoil without camber (e.g. a flat plate) that has 3% thickness is considered thin.
If the wing has a chord of nine inches, the absolute thickness should in this case be about 1/4 inch.
According to the chart above, a thin flat plate is best at Re's <5000, so the actual wing chord at such a low Re is probably half the value mentioned above, so the absolute thickness would get down to 1/8 inch.
A cambered airfoil doesn't necessary have to be thicker than a flat plate, but if you have a camber of say 4%, the airfoil thickness may be more or less higher depending on what sort of airfoil, wing structure and covering material used.
E.g. indoors often use thin covering film over ribs between the spars, which make the absolute camber value almost equal to the absolute thickness.
As for the flat plate being the best airfoil…, I realise that you still are sceptical, but think that it applies only at very low Re's <5,000 where the airfoil's thickness starts getting closer to the size of the air molecules….
The picture below is just to ilustrate some terms despite you may already know them well..
Yes, some definitions should be clarified so we are able to understand each other…
Let's try:
The airfoil thickness as well as the camber, are normally expressed as % of the chord, right?
So, let's assume that an airfoil without camber (e.g. a flat plate) that has 3% thickness is considered thin.
If the wing has a chord of nine inches, the absolute thickness should in this case be about 1/4 inch.
According to the chart above, a thin flat plate is best at Re's <5000, so the actual wing chord at such a low Re is probably half the value mentioned above, so the absolute thickness would get down to 1/8 inch.
A cambered airfoil doesn't necessary have to be thicker than a flat plate, but if you have a camber of say 4%, the airfoil thickness may be more or less higher depending on what sort of airfoil, wing structure and covering material used.
E.g. indoors often use thin covering film over ribs between the spars, which make the absolute camber value almost equal to the absolute thickness.
As for the flat plate being the best airfoil…, I realise that you still are sceptical, but think that it applies only at very low Re's <5,000 where the airfoil's thickness starts getting closer to the size of the air molecules….
The picture below is just to ilustrate some terms despite you may already know them well..
08-13-2004 | 03:23 PM
#32
RE: airfoil thickness
I know that most of you guys don't fly/try the current rage 3D type models .
I do and here is a bit of "cut and try" info.
I am doing tests on a particular 3D kit which is about 1100 sq in and has a progressive airfoil being same absolute thickness at tip as root -- chord has a taper ratio of 2-1 with a tip thickness of over 18% of chord.
The plane is extremely easy to fly ( very low wing loading of 20 ozs/ft)
The interesting part to me, is that doing very tight flat power off turns --the inside wing stalls as easily or maybe even quicker than a very similar wing but with a very thin wingtip setup . I tried this a few times - just to be certain.
The old saw of a thicker foil has better lift - -just does not appear to be correct-
I never thought it did .
I only mention this as some feel that a thicker tip prevents "tip stall".
On models -I have never found this to be true.
Sounds good tho ---
I do and here is a bit of "cut and try" info.
I am doing tests on a particular 3D kit which is about 1100 sq in and has a progressive airfoil being same absolute thickness at tip as root -- chord has a taper ratio of 2-1 with a tip thickness of over 18% of chord.
The plane is extremely easy to fly ( very low wing loading of 20 ozs/ft)
The interesting part to me, is that doing very tight flat power off turns --the inside wing stalls as easily or maybe even quicker than a very similar wing but with a very thin wingtip setup . I tried this a few times - just to be certain.
The old saw of a thicker foil has better lift - -just does not appear to be correct-
I never thought it did .
I only mention this as some feel that a thicker tip prevents "tip stall".
On models -I have never found this to be true.
Sounds good tho ---
08-13-2004 | 03:40 PM
#33
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From: Bloomington, MN,
RE: airfoil thickness
ORIGINAL: adam_one
Banktoturn
Yes, some definitions should be clarified so we are able to understand each other…
Let's try:
The airfoil thickness as well as the camber, are normally expressed as % of the chord, right?
So, let's assume that an airfoil without camber (e.g. a flat plate) that has 3% thickness is considered thin.
If the wing has a chord of nine inches, the absolute thickness should in this case be about 1/4 inch.
According to the chart above, a thin flat plate is best at Re's <5000, so the actual wing chord at such a low Re is probably half the value mentioned above, so the absolute thickness would get down to 1/8 inch.
A cambered airfoil doesn't necessary have to be thicker than a flat plate, but if you have a camber of say 4%, the airfoil thickness may be more or less higher depending on what sort of airfoil, wing structure and covering material used.
E.g. indoors often use thin covering film over ribs between the spars, which make the absolute camber value almost equal to the absolute thickness.
As for the flat plate being the best airfoil…, I realise that you still are sceptical, but think that it applies only at very low Re's <5,000 where the airfoil's thickness starts getting closer to the size of the air molecules….
The picture below is just to ilustrate some terms despite you may already know them well..
Banktoturn
Yes, some definitions should be clarified so we are able to understand each other…
Let's try:
The airfoil thickness as well as the camber, are normally expressed as % of the chord, right?
So, let's assume that an airfoil without camber (e.g. a flat plate) that has 3% thickness is considered thin.
If the wing has a chord of nine inches, the absolute thickness should in this case be about 1/4 inch.
According to the chart above, a thin flat plate is best at Re's <5000, so the actual wing chord at such a low Re is probably half the value mentioned above, so the absolute thickness would get down to 1/8 inch.
A cambered airfoil doesn't necessary have to be thicker than a flat plate, but if you have a camber of say 4%, the airfoil thickness may be more or less higher depending on what sort of airfoil, wing structure and covering material used.
E.g. indoors often use thin covering film over ribs between the spars, which make the absolute camber value almost equal to the absolute thickness.
As for the flat plate being the best airfoil…, I realise that you still are sceptical, but think that it applies only at very low Re's <5,000 where the airfoil's thickness starts getting closer to the size of the air molecules….
The picture below is just to ilustrate some terms despite you may already know them well..
I don't think we have yet succeeded in identifying common terminology. You make the comment that for a wing made of a thin film, absolute camber is almost the same as absolute thickness. This is not true, because thickness and camber are completely independent. Thickness is the distance between the upper and lower surface at a particular chordwise location, and the maximum thickness is the maximum value that occurs anywhere on the chord. Camber is the maximum distance between the chord line and the mean line. Consider a constant-chord wing made of an infinitesimally thin sheet of material, curved to, say, 10% camber. This wing has 0% maximum thickness, and 10% camber. Incidentally, it is also a plate, since the thickness is constant, zero, all along the chord.
Having said that, I agree that a cambered airfoil need not be thicker than a flat plate, but I don't know what point you are trying to make.
I am indeed still skeptical that a flat or curved plate is the 'best' airfoil at any Reynold's number. If you want to convince me, try to construct an intuitive argument that the chordwise variation in thickness somehow degrades the performance of an airfoil at low Reynold's numbers. I have tried to do this for myself, and failed. It seems very unlikely to me that the optimum airfoil section would suddenly be a constant thickness section, at some low Reynold's number. As I have mentioned before, I think that the confusion is caused by the fact that 'optimum' airfoils for lower Reynold's numbers do tend to be thinner, and thinner airfoils do begin to bear a visual resemblance to plates (curved or flat). In other words, the maximum thickness of the 'best' airfoil approaches zero as the Reynold's number approaches zero, but that does not mean that the thickness is constant over the chord at any given Reynold's number. In any case, this has nothing whatsoever to do with the ratio of the wing thickness to the size of the air molecules. Reynold's number effects are completely unrelated to the size of the molecules.
banktoturn
08-13-2004 | 04:15 PM
#34
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From: Stockholm, SWEDEN
RE: airfoil thickness
I don't think we have yet succeeded in identifying common terminology. You make the comment that for a wing made of a thin film, absolute camber is almost the same as absolute thickness.
Anyway, I think that our definitions are the same.
If you want to convince me, try to construct an intuitive argument that the chordwise variation in thickness somehow degrades the performance of an airfoil at low Reynold's numbers.
The air molecules "see" a thin flat plate airfoil with a positive AOA at very low Re's as it was a cambered airfoil, if you add a camber you may spoil the ideal airfoil at that low Re...
08-13-2004 | 10:05 PM
#35
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From: Bloomington, MN,
RE: airfoil thickness
ORIGINAL: adam_one
I admit that this part was not well formulated, I was thinking about the spars that support the thin covering would add some thickness to the entire section… but I give you a point here…[8D]
Anyway, I think that our definitions are the same.
I've tried in my previous posts using the argument that the air molecules don't "see" a thin flat plate airfoil the same way we see it.
The air molecules "see" a thin flat plate airfoil with a positive AOA at very low Re's as it was a cambered airfoil, if you add a camber you may spoil the ideal airfoil at that low Re...
I don't think we have yet succeeded in identifying common terminology. You make the comment that for a wing made of a thin film, absolute camber is almost the same as absolute thickness.
Anyway, I think that our definitions are the same.
If you want to convince me, try to construct an intuitive argument that the chordwise variation in thickness somehow degrades the performance of an airfoil at low Reynold's numbers.
The air molecules "see" a thin flat plate airfoil with a positive AOA at very low Re's as it was a cambered airfoil, if you add a camber you may spoil the ideal airfoil at that low Re...
I'm not talking about adding camber, I'm talking about a varying thickness along the chord. I don't believe that a plate, with or without camber, is the optimum section. What makes a plate a plate is the constant thickness along the chord.
banktoturn
08-14-2004 | 02:39 AM
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From: Stockholm, SWEDEN
RE: airfoil thickness
I'm not talking about adding camber, I'm talking about a varying thickness along the chord. I don't believe that a plate, with or without camber, is the optimum section. What makes a plate a plate is the constant thickness along the chord.
The same argument applies if you vary the thickness along the chord.
As the Re (and the wing chord) gets smaller, the air density and viscosity become more dominant factors to how the air behaves along the wing's surface.
As I mentioned before, when the air approaches the LE of a wing with a positive AOA, it starts sensing the high pressure under and the low pressure above and some of the air just below the LE manages to sneak over the top.
This causes the shifting of the stagnation point to a spot somewhat below the wing's LE. And even if this natural behaviour is not unique to airfoils that are flat and thin, it has a special significance at very low Re's with airfoils that are flat and thin.
As the air molecules hooks its way back around the LE and over the top, it rounds the shape as it goes, so the upper surface flow gets a long, curved path from the stagnation point to the wing's TE.
By varying the thickness along the chord you may spoil the airfoil at such a low Re.
08-15-2004 | 01:43 PM
#37
RE: airfoil thickness
Perhaps someone could build a half dozen "flat plate wings" for little 5 oz wing loading planes - aerobatic models--not gliders .
I would bet money that if all were say 3% thick --one could smooth LE/TE/ make gentle curves / etc.. on and on -- and IF all were subjected to the most proficient fliers -- No one could tell the difference.
From many years back - I always believed in researching first - then trying out the premise.
Without the actual flight test - it all meant little.
My first pattern planes - 30 years ago - followed the accepted designs but as I kept trying different wing setups -I found more and more of the rules simply did not apply to these planes.
Progressive airfoils , special tips , etc., all were put forward as "better " for acrobatic setups but the only real improvements I could ever find was simply from reducing wing loading and increasing power loadings.
What is funny , is that this is the same as found in full scale aerobatics.
I can't think of any real exceptions --
just rambling--
I would bet money that if all were say 3% thick --one could smooth LE/TE/ make gentle curves / etc.. on and on -- and IF all were subjected to the most proficient fliers -- No one could tell the difference.
From many years back - I always believed in researching first - then trying out the premise.
Without the actual flight test - it all meant little.
My first pattern planes - 30 years ago - followed the accepted designs but as I kept trying different wing setups -I found more and more of the rules simply did not apply to these planes.
Progressive airfoils , special tips , etc., all were put forward as "better " for acrobatic setups but the only real improvements I could ever find was simply from reducing wing loading and increasing power loadings.
What is funny , is that this is the same as found in full scale aerobatics.
I can't think of any real exceptions --
just rambling--
08-15-2004 | 02:10 PM
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From: Stockholm, SWEDEN
RE: airfoil thickness
How about this:
Aerodynamic advantages of bumpy leading edges
After two decades of experimentation, including wind-tunnel testing, professor Frank E. Fish has come to the conclusion that the bumps confer significant aerodynamic advantages… the flippers with bumpy leading edges generate as much as 32 percent less drag and as much as 8 percent more lift than similarly sized flippers with a conventional smooth leading edge…
Scientific American magazine, August 2004.
Aerodynamic advantages of bumpy leading edges
After two decades of experimentation, including wind-tunnel testing, professor Frank E. Fish has come to the conclusion that the bumps confer significant aerodynamic advantages… the flippers with bumpy leading edges generate as much as 32 percent less drag and as much as 8 percent more lift than similarly sized flippers with a conventional smooth leading edge…
Scientific American magazine, August 2004.
08-15-2004 | 04:57 PM
#39
RE: airfoil thickness
That's a whale of a report!
Actually I wasnn't fishing for that kind of info but due to the carping and some crabbing I have had from various bank fishermen I guess that some casual blubbering is to be expected----
Actually I wasnn't fishing for that kind of info but due to the carping and some crabbing I have had from various bank fishermen I guess that some casual blubbering is to be expected----
08-16-2004 | 03:13 AM
#40
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From: Stockholm, SWEDEN
RE: airfoil thickness
Hi Dick,
This may seem out of topic and rather controversial but:
Have you ever wondered why golf balls have dimples?
Just look at the leading edge of any Learjet and you'll find little machined diamonds attached nearby.
It is claimed to create counter-rotating vortices at either side of each leading-edge bump.
The vortices "inject momentum into the fluid flow, which keeps the flow attached to the upper surface rather than allowing it to separate".
Don't be surprised if you see some models with knobbly-knotty leading edges in the future…
Here's further source of information:
http://www.ascribe.org/cgi-bin/spew4...=2004&public=1
http://scitation.aip.org/getabs/serv...cvips&gifs=Yes
This may seem out of topic and rather controversial but:
Have you ever wondered why golf balls have dimples?
Just look at the leading edge of any Learjet and you'll find little machined diamonds attached nearby.
It is claimed to create counter-rotating vortices at either side of each leading-edge bump.
The vortices "inject momentum into the fluid flow, which keeps the flow attached to the upper surface rather than allowing it to separate".
Don't be surprised if you see some models with knobbly-knotty leading edges in the future…
Here's further source of information:
http://www.ascribe.org/cgi-bin/spew4...=2004&public=1
http://scitation.aip.org/getabs/serv...cvips&gifs=Yes
08-16-2004 | 09:12 AM
#41
RE: airfoil thickness
No -I know why they have dimples -but I wonder about other dimples .
They look good on kids cheeks but bad on the posterior of older women.
A "sandpaper finish is also better in some cases for best drag.
The new LEXAS cars have a special finish on the underside to aid airflow, as I recall reading recently.
They look good on kids cheeks but bad on the posterior of older women.
A "sandpaper finish is also better in some cases for best drag.
The new LEXAS cars have a special finish on the underside to aid airflow, as I recall reading recently.
08-16-2004 | 11:15 PM
#42
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From: Bloomington, MN,
RE: airfoil thickness
ORIGINAL: adam_one
banktoturn,
The same argument applies if you vary the thickness along the chord.
As the Re (and the wing chord) gets smaller, the air density and viscosity become more dominant factors to how the air behaves along the wing's surface.
As I mentioned before, when the air approaches the LE of a wing with a positive AOA, it starts sensing the high pressure under and the low pressure above and some of the air just below the LE manages to sneak over the top.
This causes the shifting of the stagnation point to a spot somewhat below the wing's LE. And even if this natural behaviour is not unique to airfoils that are flat and thin, it has a special significance at very low Re's with airfoils that are flat and thin.
As the air molecules hooks its way back around the LE and over the top, it rounds the shape as it goes, so the upper surface flow gets a long, curved path from the stagnation point to the wing's TE.
By varying the thickness along the chord you may spoil the airfoil at such a low Re.
I'm not talking about adding camber, I'm talking about a varying thickness along the chord. I don't believe that a plate, with or without camber, is the optimum section. What makes a plate a plate is the constant thickness along the chord.
The same argument applies if you vary the thickness along the chord.
As the Re (and the wing chord) gets smaller, the air density and viscosity become more dominant factors to how the air behaves along the wing's surface.
As I mentioned before, when the air approaches the LE of a wing with a positive AOA, it starts sensing the high pressure under and the low pressure above and some of the air just below the LE manages to sneak over the top.
This causes the shifting of the stagnation point to a spot somewhat below the wing's LE. And even if this natural behaviour is not unique to airfoils that are flat and thin, it has a special significance at very low Re's with airfoils that are flat and thin.
As the air molecules hooks its way back around the LE and over the top, it rounds the shape as it goes, so the upper surface flow gets a long, curved path from the stagnation point to the wing's TE.
By varying the thickness along the chord you may spoil the airfoil at such a low Re.
I'm afraid nothing that you've said provides any rationale for the view that a plate (constant thickness along the chord, with or without camber) is the optimum airfoil for low Reynold's numbers. Saying that a non-constant thickness distribution 'may spoil the airflow' does not shed any light for me. I remain unconvinced.
banktoturn
08-17-2004 | 03:02 AM
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From: Stockholm, SWEDEN
RE: airfoil thickness
Banktoturn,
Let's recall one of your own explanations:
So, the bigger variation in pressure over the chord as you mentioned above is not only because the airfoil is thick but also because the airfoil is far from being a flat plate.
As the Re gets smaller the ideal airfoil gets thinner and also more like a flat plate.
Now we may say that a flat plate is made of straight lines, but a straight line is in fact an arch from a circle, which radius is close to infinitum…
Let's recall one of your own explanations:
...The higher the Reynold's number, the more likely the air is to achieve pressure recovery without separating. Thicker airfoils have a bigger variation in pressure over the chord, and so the pressure recovery is more extreme. For this reason, wings operating at lower Reynold's numbers cannot be as thick as wings operating at higher Reynold's numbers without separating. This is why 'good' low Reynold's number airfoils are thinner, and tend to resemble flat plates, and also why flat plates compare more and more favourably to traditional airfoils as the Reynold's number gets small. Having said that, it is the thickness that relates to Reynold's number, not the flatness.
As the Re gets smaller the ideal airfoil gets thinner and also more like a flat plate.
Now we may say that a flat plate is made of straight lines, but a straight line is in fact an arch from a circle, which radius is close to infinitum…
08-17-2004 | 10:47 AM
#44
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From: Bloomington, MN,
RE: airfoil thickness
So, the bigger variation in pressure over the chord as you mentioned above is not only because the airfoil is thick but also because the airfoil is far from being a flat plate.
As the Re gets smaller the ideal airfoil gets thinner and also more like a flat plate.
Now we may say that a flat plate is made of straight lines, but a straight line is in fact an arch from a circle, which radius is close to infinitum…
As the Re gets smaller the ideal airfoil gets thinner and also more like a flat plate.
Now we may say that a flat plate is made of straight lines, but a straight line is in fact an arch from a circle, which radius is close to infinitum…
It seems to me that, as I have mentioned before, the confusion is between 'thin' and 'flat'. The severity of the pressure recovery increases with maximum thickness, not as a result of thickness variation. This is why airfoils with smaller maximum thickness tend to be more favorable at low Reynold's numbers. This in no way implies that a constant thickness is favorable for low Reynold's numbers. The point I was making in the text that you quoted is that a thin wing bears a visual resemblance to a flat wing, since the curvature on the top and bottom surfaces is less visually obvious than it would be on a thicker wing. I conjecture that this visual resemblance is part of the reason for the apparently common confusion between thin wings and flat wings.
My contention is quite simply that the 'optimum' airfoil, even for low Reynold's numbers, is one with varying thickness along the chord, even if the maximum thickness is small. If someone can explain to me why performance of a constant thickness airfoil is optimum in some range of Reynold's numbers, I'd certainly change my thinking.
banktoturn
08-17-2004 | 08:22 PM
#45
RE: airfoil thickness
well I can't explain -because I can't find any demonstrable differences in fussing with LE and trailing edges on these little flat plates.
these are NOT gliders - they are models which fly over a wide range of attitude and speed so I really can't see how some minute shift in leading edge thickness, roundness, etc., could make a difference - I rounded some - simply--- nothing appeared different in flight - -
From your past work -I can see that you feel the improvements there must pass on down - I simply can't find that these make any differences at these sizes and under these flying parameters.
these are NOT gliders - they are models which fly over a wide range of attitude and speed so I really can't see how some minute shift in leading edge thickness, roundness, etc., could make a difference - I rounded some - simply--- nothing appeared different in flight - -
From your past work -I can see that you feel the improvements there must pass on down - I simply can't find that these make any differences at these sizes and under these flying parameters.
08-25-2004 | 07:34 PM
#46
My Feedback: (2)
RE: airfoil thickness
My own personal observation of how different thickness airfoils behave is this: Thick sections will stall at a LOWER Angle Of Attack then a thin section, but the thin section will stall more abruptly. I suspect this is due to the fact that air does not like flowing around curves, especially in a flow field where the pressure is increasing, ie. top of the wing behind the thick point. (Adverse Pressure Gradient) The thick section typically has MORE curvature in this case then the thin section. When the thin section does finally depart, it does however with less indication and more suddenness... (This is admittedly a subjective opinion/observation)
http://www.rcuniverse.com/forum/upfi...27/Ca81759.jpg
maybe this one Dick?? LOL
http://www.rcuniverse.com/forum/upfi...27/Ca81759.jpg
maybe this one Dick?? LOL
