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XFoil and airfoil data

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Old 01-24-2003, 08:36 AM
  #1  
a088008
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Default XFoil and airfoil data

I have been playing with Xfoil tonight, but cannot get a good smooth looking CpX plot. Is there anything I can do to fix my airfoil data? I used the CADD command twice with 10 degree angle.

I've attached the plot below.

-Q.
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Old 01-25-2003, 06:27 AM
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Default XFoil and airfoil data

where can i get this Xfoil software?
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Old 01-25-2003, 07:10 AM
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Ben Lanterman
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Default XFoil and airfoil data

I would suggest a program which is called Profili 2.11 written by Stefano Duranti. It is freeware for the basic program and $15 for an upgrage on CD. It is basically an interface to the program Xfoil and makes it a lot easier to work with. The greater majority of the airfoils have been pre worked up and saves you the time of computing them. You can then make changes and see the results.

http://www.profili2.com/eng/default.htm

Highly recommended to look at data.

The original Xfoil is by Mark Drela and is a well thought out paneling method approach. It is just not as user friendly but is indeed the heart of Profili 2.11

http://cromagnon.stanford.edu/aa200b..._tutorial.html
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Old 01-25-2003, 09:07 PM
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Default XFoil and airfoil data

thanks alot!
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Old 01-27-2003, 09:06 PM
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Default XFoil and airfoil data

a088008, there's a whole Yahoo group dedicated to Xfoil and all it's foibles... Don't have the link but I'll bet you can get it at the Charles River club site...

www.charlesriverrc.org

PS: never mind, here ya go...

http://groups.yahoo.com/group/xfoil/
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Old 01-28-2003, 07:46 AM
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Default XFoil and airfoil data

What Ben said - Get Profili. It turns XFoil into a Windows program (without altering the XFoil code); much easier to deal with.
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Old 01-29-2003, 07:42 AM
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Default XFoil and airfoil data

Thanx for all the suggestions. I downloaded Profili and had a look at it. I was impressed and could see how it would be helpfull. I will at some point invest in a copy, but for now my immediate problem is to refine my airfoil co-ordinates so that XFoil can use them. I ahve become quite proficient in XFoil by now and the command line interface does not scare me.

The main reason why my co-ordinates are a problem is since I have no easy way to export them from a CAD program (where I use splines to define the shape) to co-ordinates. I wrote a program to take an .bmp (bitmap) file and convert the picture of the airfoil into co-ordinates. The plot I produced in XFoil is as a result of the output from my program. As you can see, it's a bit rough and XFoil has a hard time converting it into acceptable panels. I'm in the process of refining the program in hopes that I can get something usefull out of it.

I'll let you guys know if I have any success with my program.

Oh, by the way, am I wasting my time? i.e. is there a program that already does what I'm intending?

-Q.
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Old 01-29-2003, 09:11 AM
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wildblueyawner
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Default XFoil and airfoil data

Oh, by the way, am I wasting my time? i.e. is there a program that already does what I'm intending?
Profili has a raster, e.g. .BMP, file-importing feature, but haven't tried it yet, so can't comment on its utility. Go to the list of airfoils, look in the Import menu.
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Old 01-29-2003, 10:17 AM
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Default Wooopeeee!!!

Ok. I've managed to find out what was going on. The ordering of the points seems to be important. I had my x-axis goinf from 0.0 to 1.0 and then back again in an anti-clockwise fashion. I changed it to 1.0 to 0.0 and back again (anti-clockwise) and it worked!

I now have graphs! Yee Haw! Now I can finally find out what is going on with an airfoil I designed by eye alone.

I'll post some pics of the results.

-Q.
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Old 01-29-2003, 10:19 AM
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Default Cp vs. X plot

Here is the Cp vs. X plot.

-Q.
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Old 01-29-2003, 10:31 AM
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Default Polars

Here are some polars at various alpha and Re.

Now if only I knew what all of this means. I think that one thing the data is telling me (CL vs. alpha graphs) is that my maximum lift co-efficient (CL) is at alpha = 10.0 degrees and is about 1.3, so that must be the stall angle. Am I way off base here?

I don't know what the Cl vs. CD and the Xlr / C graphs are telling me.
-Q.
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Old 01-29-2003, 10:43 PM
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Default graphs

You're right on the Cl/Cm vs alpha graphs; those are just the different values plotted at each alpha value. The Cl-alpha graph can also predict how violent the stall will be by how fast Cl falls off after stall. If it's a very shallow slope the stall will be mild and the plane should just mush through the stall; steep and your plane will fall like a rock.

The Cl vs Cd (drag polar graph) is in my (glider obsessed) mind the most important. It tells the designer over what range of Cl values the airplane is most effiecient. If you look at the graph, it shows that for a wide range of Cl, the Cd stays relatively low. Then, at some Cl, Cd starts increasing fast. That's called the drag bucket, and basically it says that as long as you stay between two particular Cls, then your plane is very efficient, but if you go over those, then you start incurring a huge drag penelty. Incidentally, this is not always where the airfoil starts to stall, it just happens that way with yours. Try running a symmetric airfoil from -15 to 15 degrees AoA, you'll really see the drag bucket form.

The other number that's also really important is the L/D, (Cl divided by Cd). It's the efficiency of the airfoil, and from it you can get the glide distance and duration. We glider designers will usually pick a Cl and alpha that maximizes either distance or duration and design around that.

As for the Xlr/C graph, I think it's showing Cl and Cm over the airfoil itself, with X/C=0 being the leading edge and X/C = 1 the trailing edge, although I don't know why or what that means. Personally, I don't think it's too important, I've never had to use it.

Hope this helps
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Old 01-30-2003, 02:01 AM
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Default XFoil and airfoil data

Originally posted by a088008
As you can see, it's a bit rough and XFoil has a hard time converting it into acceptable panels.
Try this...

LOAD airfoil_filename
MDES
FILT
! (repeats the previous FILT command)
!
.
.
EXEC

PANE
SAVE smoothed_airfoil_filename


Type "!" as often as needed to smooth the airfoil sufficiently. Looks like yours will need at least 10 repeats.
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Old 01-30-2003, 05:34 AM
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Default Re: graphs

That was exactly what I wanted to know. Thank you very much for the info.

One question. You mentioned that mushing occurs when the slope is shallow. Is this the case with my airfoil?

-Q.

Originally posted by Daniel Nelson
You're right on the Cl/Cm vs alpha graphs; those are just the different values plotted at each alpha value. The Cl-alpha graph can also predict how violent the stall will be by how fast Cl falls off after stall. If it's a very shallow slope the stall will be mild and the plane should just mush through the stall; steep and your plane will fall like a rock.

The Cl vs Cd (drag polar graph) is in my (glider obsessed) mind the most important. It tells the designer over what range of Cl values the airplane is most effiecient. If you look at the graph, it shows that for a wide range of Cl, the Cd stays relatively low. Then, at some Cl, Cd starts increasing fast. That's called the drag bucket, and basically it says that as long as you stay between two particular Cls, then your plane is very efficient, but if you go over those, then you start incurring a huge drag penelty. Incidentally, this is not always where the airfoil starts to stall, it just happens that way with yours. Try running a symmetric airfoil from -15 to 15 degrees AoA, you'll really see the drag bucket form.

The other number that's also really important is the L/D, (Cl divided by Cd). It's the efficiency of the airfoil, and from it you can get the glide distance and duration. We glider designers will usually pick a Cl and alpha that maximizes either distance or duration and design around that.

As for the Xlr/C graph, I think it's showing Cl and Cm over the airfoil itself, with X/C=0 being the leading edge and X/C = 1 the trailing edge, although I don't know why or what that means. Personally, I don't think it's too important, I've never had to use it.

Hope this helps
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Old 01-30-2003, 05:35 AM
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Default Oooohhh!

I tried the method you suggested. It definately helped. I also used the PANE, then PPAR command to reduce the number of nodes, which speeded up computation and also helped get rid of little imperfections.

I've attached an image of the latest results. Much, much better!!!

Thanks again. You have been a great help!!!

-Q.



Originally posted by drela


Try this...

LOAD airfoil_filename
MDES
FILT
! (repeats the previous FILT command)
!
.
.
EXEC

PANE
SAVE smoothed_airfoil_filename


Type "!" as often as needed to smooth the airfoil sufficiently. Looks like yours will need at least 10 repeats.
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Old 01-30-2003, 08:16 AM
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Default Symmetric airfoil Cp vs. X plot

I have included a CP vs. X plot of a symmetric airfoil of my own design. It's going to be flown in my next design, which is nearing completion. This airfoil gave me even more problems to get it smooth, but I have tweaked my program to better plot co-ordinates and it worked.

-Q.
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Old 01-30-2003, 08:18 AM
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Default Symmetric airfoil Cp vs. X plot (Lift Bucket)

Here is the polar plots of the symmetric airfoil for my new design.

Is this what is meant by a list bucket?

-Q.
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Old 01-30-2003, 03:47 PM
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Default XFoil and airfoil data

Mark,

Do you happen to know whether anyone has compiled XFOIL for Macintosh OSX? It's been problematical for me to do compiles on my Mac, so it would be handy if someone had already built it.

Thanks,

banktoturn
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Old 01-31-2003, 02:22 AM
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Default Drag buckets

Yup, that's a classic drag bucket. It's called that since it kinds looks like a 2-D bucket on it's side.

In this case, the drag this airfoil creates will hardly increase untill you get to a stall, then it skyrockets. One thing, XFoil is a 2-D program, there are other forms of drag that affect the airplane that it can not measure.

Pretty steep Cl descent after the stall. Is this airfoil going on an aerobatic airplane?
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Old 01-31-2003, 04:06 AM
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Default Re: Drag buckets

Yip. The airfoil will be for something aerobatic, but not what you might be used to. It's design is quite different from most planes in that the body produces a canard effect, so I'm not too concerned with stall as it will mostly be compensated for by the canard stall effect (i.e canard stalls before the wing does and the nose drops).

Having said that I also anticipate that the stall characteristics (steep CL drop-off) might actually enhance the wilder side of aerobatics e.g. snap rolls, etc. Is that why you ask?

-Q.

Originally posted by Daniel Nelson
Yup, that's a classic drag bucket. It's called that since it kinds looks like a 2-D bucket on it's side.

In this case, the drag this airfoil creates will hardly increase untill you get to a stall, then it skyrockets. One thing, XFoil is a 2-D program, there are other forms of drag that affect the airplane that it can not measure.

Pretty steep Cl descent after the stall. Is this airfoil going on an aerobatic airplane?
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Old 01-31-2003, 04:56 AM
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Ben Lanterman
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Default XFoil and airfoil data

Keep in mind that if you look at the front of the airplane that the lift of the wing carrys across the fuselage in an ellipse (or something roughly like it). A fuselage wide enough to produce a significant amount of lift toward the front will have the pressure distribution reasonably messed up by the relatively large pressure influence from the wing. You end up with something that resembles a wing with a long square lex.

A few years ago a design came out in the model press with a wing that had an aspect ratio of about 5 and a rectangular planform. A fuselage that was about 20% of the span wide was added to the front of the wing and had roughly the thickness of the wing in depth. The nose was a cylindrical section. The tail was mounted on two booms back fron the edge of the fuselage with the stab between them. It was interesting but nothing more was ever mentioned about it.

Basically canards work well because they are separate surfaces with their own wing carryover effects. Think about the real high aspect ratio that Rutan's designs use.

To get a canard effect it is easier to use a canard, lift is best produced by wings, fuselages are good to wrap equipment around but are not too good at lift generation (knife edge taking a lot of sideslip angle)
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Old 01-31-2003, 06:27 AM
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Default XFoil and airfoil data

Hi Ben.

I'll take your advice into consideration, but I wish I could show you the design (it's top secret at the moment, so that's not going to happen). It's not quite a fuselage by any definition. Yes, it wraps equipement in it, but that is where the similarity ends. I can't wait to show you guys the end result and tell you how it flies. Think of the SR-71 Blackbird and then take it to the extreme. Also throw in the silver spaceship (I could not remember it's name) in Star Wars Episode 2, for good measure.

-Q.

Originally posted by Ben Lanterman
Keep in mind that if you look at the front of the airplane that the lift of the wing carrys across the fuselage in an ellipse (or something roughly like it). A fuselage wide enough to produce a significant amount of lift toward the front will have the pressure distribution reasonably messed up by the relatively large pressure influence from the wing. You end up with something that resembles a wing with a long square lex.

A few years ago a design came out in the model press with a wing that had an aspect ratio of about 5 and a rectangular planform. A fuselage that was about 20% of the span wide was added to the front of the wing and had roughly the thickness of the wing in depth. The nose was a cylindrical section. The tail was mounted on two booms back fron the edge of the fuselage with the stab between them. It was interesting but nothing more was ever mentioned about it.

Basically canards work well because they are separate surfaces with their own wing carryover effects. Think about the real high aspect ratio that Rutan's designs use.

To get a canard effect it is easier to use a canard, lift is best produced by wings, fuselages are good to wrap equipment around but are not too good at lift generation (knife edge taking a lot of sideslip angle)
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Old 01-31-2003, 11:30 PM
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Default XFoil and airfoil data

So, you're talking about an aerobatic model with a wide, flat nose section, with a symmetric airfoil. Will the wing have any taper, or is it rectangular? What's the stabilizer configuration; is it standard or some kind of delta wing?

The way you describe the fuse, it sounds like some sort of lifting body. In this case, I don't think you'll get any carnard effect in a stall. A traditional carnard stalls before the main wing because it acts as an elevator; to change the pitch it has to move, therefor to enter a stall it has to be at a higher alpha then the main wing. But a fixed lifting body fuse is at the same alpha as the main wing, so I don't think it will stall first.

Of course, if you have over a 1:1 thrust-weight ratio this really doesn't matter; you can just power out of the stall at will.

Sounds interesting, please keep us informed.
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Old 02-01-2003, 02:05 AM
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Tall Paul
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Default XFoil and airfoil data

At the Re in your plot, 100000, there is a "seperation bubble" as indicated... what does the plot look like at higher Re's?
100000 is very low for an aerobatic airplane of common size... 5 to 6 foot span..
(color inverted for easier viewing)
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Old 02-01-2003, 05:29 AM
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Default XFoil and airfoil data

The airfoil is for a model plane that I'm currently working on in seceracy (for fear of someone else copying and kitting the plane). The first prototype will have a 47" wingspan, with a larger one (60" - 70") to follow after some flight tests have been carried out.
I plotted Re = 100,000 as an example to show the seperation bubble and where it is located. At higher Re = 250,000 the bubble does reduce sygnificantly.

I want to ask you guys what Re means in my case. I know it is the "stickiness" of the air but that is not quite scientific enought for my brain. Does low Re = 100,000 indicate slow flight characteristics and Re = 250,000 equate to higher speed. I have a feeling I have got it reversed, but then what do I know. (really! - my knowledge of Re is dangerous)

I've included a plot at Re=250,000.

-Q.



Originally posted by Tall Paul
At the Re in your plot, 100000, there is a "seperation bubble" as indicated... what does the plot look like at higher Re's?
100000 is very low for an aerobatic airplane of common size... 5 to 6 foot span..
(color inverted for easier viewing)
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