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Jan 11, 2012, 10:54 PM
Just call me crash for short
Quick61's Avatar
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Using XFLR5 For Your Next Wing, or ...


Using XFLR5 For Your Next Wing
or
How I Learned to Stop Worrying and Learned to Love My Computer


Link to XFLR5 web site - http://www.xflr5.com/xflr5.htm
Window$ and Mac users can download the program here - http://sourceforge.net/projects/xflr...0v6.05%20beta/
Linux users - You know what to do.

After putting off teaching my self how to use XFLR5 for months, I finally ran out of excuses and just dove in. I started off by reading through, then following along with the tutorial written for Radio Controlled Soaring Digest dated 2008. It has worked OK but is a bit out of date as there are small differences in menu / button names and arrangement of sub windows, but with only a little thought, one can ride right on past that. I have not tried any of the other tutorials yet but when I do, I'll post back as to how useful I find them to be. A Google search for XFLR5 tutorial turns up a slew of videos that should be worth checking out as well.

I made the plunge into this software out of the desire to come up with a simple to construct thermal wing that would have a good chance of flying off the bench. For me, that wing looks to be a better designed Paoli Flying Wing. About the only thing I plan on keeping is the overall layout and dimensions. My main motivation is to build a test bed for applying the Dave Brown Skyloft covering. So a Flying Wing with a low parts count seemed to be a good choice to test out application, strength, and durability of said product.

Enter XFLR5. With this software, I can now try out different airfoils, wing twist - both amount and length of twisted section, be it full chord or some percentage from the tip, etc.. All without the need to build the wing, sling it into the air, and watch attempt after attempt re-kit themselves because I "guessed wrong". Diving in, I started with the e193 airfoil and thinned it down to 8.5% and as a starting point, gave it a full length twist of -4* I will use this as a starting point in compairing different twists, thickness, and airfoils. I hope to come up with something that will give me a reasonable speed range with focus on the float and reasonably slow stall. I know it's all going to be traidoffs but a happy middle is something to shoot for.

Below are a few graphs and 3D shots I got from XFLR5 with the wing described above. I understand the graphs, at least enough to be dangerous. What I'm not getting is the 3D output. I understand that the color is showing pressure, (right?), but I have seen outputs that show the pressure across the entire wing and I don't know how to get that output yet. So any hints or directions would be greatly appreciated.

I know I am just starting out with XFLR5 and I have a WHOLE LOT to learn. I hope this thread will encourage others that have not tried the software to do so as well as give a place for us noobs to ask questions and get answers from those with more experience. Together we should be able to figure out how best to use XFLR5 as one of the sharper tools in the kit when it comes to designing "The Next Wing"

Mark
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Jan 12, 2012, 03:11 AM
Deniable plausibility
Shedofdread's Avatar
I've only used Xflr as a GUI for Xfoil so I'll be very interested to see how this goes. BTW, which version are you on?
Jan 12, 2012, 03:46 AM
Just call me crash for short
Quick61's Avatar
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Quote:
Originally Posted by Shedofdread
I've only used Xflr as a GUI for Xfoil so I'll be very interested to see how this goes. BTW, which version are you on?
Me too (interested to see how this goes). I hope to be able to gather enough info and experience to make this XFLR5 program a goto tool for me when cooking up a new wing. I'v already noticed a few little funky things, I'll post on that tomorrow. Got to go snag a few winks, just hope that dog of mine ain't hoggen up all the bed again.

I'm currently using v5.01. I haven't bothered to run apt-get yet to update it, though I see the newest build is out there in a .deb package. Once I get a few more things nailed down, I'll update and be current.
Jan 12, 2012, 11:24 AM
Everything's A Composite
Knoll53's Avatar
This should be interesting Mark.

Can you do alternate configurations easily? Such as using a different airfoil? Inasmuch as this wing has little swept, it is almost a plank. I'd think that a more reflexed airfoil would make for a good comparison.

I'll be watching.

Kent
Jan 12, 2012, 12:10 PM
Just call me crash for short
Quick61's Avatar
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Quote:
Originally Posted by Knoll53
This should be interesting Mark.

Can you do alternate configurations easily? Such as using a different airfoil? Inasmuch as this wing has little swept, it is almost a plank. I'd think that a more reflexed airfoil would make for a good comparison.

I'll be watching.

Kent
Well Kent, that would depend on your definition of easy, , but yes, it is easy to run different airfoils once you have run the polars on them and built a data base. It's nothing more than a few mouse clicks to swap airfoils on the wing and run again.

Mark
Jan 12, 2012, 12:44 PM
Just call me crash for short
Quick61's Avatar
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Questions About the Numbers


Now that I have started using XFLR5 I have a few questions about the numbers.

1. INPUT - What would be a good range of Reynolds numbers to use when computing the polars for an airfoil? I am currently using 10,000 - 200,000 as per the tutorial from the 2008 RCSD article. Is this a good range?

2. OUTPUT - What graph and what range do I want to concentrate on to determine stability? Having a great glide with wide speed range ain't worth spit if the wing is unstable. I keep looking over a tutorial on it off the XFLR5 site, but it's just not clicking with me for the most part. (more than likely because I'm still new to this and have no reference point to start from)

Appreciate any help and pointers I can get.

Mark
Jan 12, 2012, 10:24 PM
Just call me crash for short
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Been working a bit more with XFLR5 and as Kent wanted to see, I did a comparison of a few airfoils. As a reference pont, I loaded a s3014 then loaded a couple of airfoils from John Yost that were designed for swept wing tailless aircraft. These were the EH 1.5/9.0 and EH 2.0/10.0. Putting them into the wing model, and running them through the analysis I quickly lost interest in the s3014, then the eh2010, and focused on the eh1590. played around with the CG a bit. Then I got the idea of trying a thinned down version that i am calling the eh1580, (8% thickness) What I ended up with (remember, I'm trying to keep it simple) is the eh1580 wing with 4* liner washout. The graphs don't look to bad to my untrained eyes, but maybe someone with more knowledge could point out where they might not be?

At any rate, attached is my latest output. I'm going to look at some more airfoils, but so far, this thinned down eh1590 foil is about the best so far for this wing planform that fits the design criteria of this exercise.

Mark
Jan 13, 2012, 01:32 AM
TheyreComingToTakeMeAway!
derk's Avatar
sorry, had to remove this, its been a while since i calculated larger models...
most of the micro models i build are under 50,000 some even lower. at really low numbers, conventional aerodynamics start changing as it becomes more fluid than gas.
Last edited by derk; Jan 13, 2012 at 01:40 AM.
Jan 13, 2012, 01:47 AM
Just call me crash for short
Quick61's Avatar
Thread OP
Quote:
Originally Posted by derk
sorry, had to remove this, its been a while since i calculated larger models...
most of the micro models i build are under 50,000 some even lower. at really low numbers, conventional aerodynamics start changing as it becomes more fluid than gas.
Sorry I didn't get to see what was posted. Are you a more experienced type user? Maybe give a stab at a few of my questions? The one on telling what is showing stability is the one that is still eluding me
Jan 13, 2012, 02:44 AM
TheyreComingToTakeMeAway!
derk's Avatar
sorry, i haven't used this, but i plan on trying.... i have done Rn calculations before to help design a few models though.
Jan 13, 2012, 03:01 AM
Just call me crash for short
Quick61's Avatar
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Quote:
Originally Posted by derk
sorry, i haven't used this, but i plan on trying.... i have done Rn calculations before to help design a few models though.
OK ,shiny. I think I might be about to answer my own question on the stbility but I have some more reading to do before I can say for sure.

I toyed with the program off and on a few times for some months but when I decided to just go for it, I followed the examples and it all started to come together. Don't put it off if you want to learn, just jump in!! Once you know what it wants, it's not that hard to get working.

Mark
Jan 13, 2012, 10:25 AM
Everything's A Composite
Knoll53's Avatar
Quote:
Originally Posted by Quick61
I did a comparison of a few airfoils.
Mark
You gotta like the output. Those graphs are great. Thanks for posting.

Kent
Jan 13, 2012, 11:34 AM
Just call me crash for short
Quick61's Avatar
Thread OP

Stability


Sure thing Kent.

After doing some more reading last night, I got the answer I was looking for as to what shows stability. The answer is in the graph that compares the Total Pitching Moment Coefficient, (CM) to Alpha.

In the graph below, there are 4 lines that represent the model with 4 different CG locations. The first one, RED, is at 15% of MAC, GREEN is at 20%, BLUE is at 25% and lastly, YELLOW is at 30%. Notice that the lines move in a negative direction for 15, 20 and 25%, all positions of CG known to be stable, with 15% a good place to start testing a new model and 25% on average being maybe a bit twitchy on most wings. As the CG moves more rearward, the slope of the lines decreases and at 30% the line increases. The straightness of the line is also of importance. With the line being for the most part linear, it is showing that CM is null, (glides in balance) at the range of Angle of Attack. The downward slope denotes that the wing will want to return to it's orignal Alpha if it were disturbed by gusts or turbulent air. On the other hand, the upward sloping line shows that disturbances will want to push the nose up and increase the AoA.

Of course, this is not an end all, be all determination of any given wings stability, but is a useful tool to determine if your on the right track.

Mark
Jan 13, 2012, 04:22 PM
Just call me crash for short
Quick61's Avatar
Thread OP
Doing some more mucking about with XFLR5 trying different wing twists. Made sone more improvement with both the glide and stability. Given that at some point one needs to move on to further the project and for me, that would be now.

Attached is the plots from last post with the wing twist plots added. The one that I am moving forward with is the plot highlighted with points. Also, the wing section layout showing the amount of twist per section and the foil being used.

Next up is the addition of control surfaces, (elevons) to the mix.

Learning while doing, the only way to fly.

Mark
Jan 13, 2012, 05:41 PM
Everything's A Composite
Knoll53's Avatar
Quote:
Originally Posted by Quick61
What would be a good range of Reynolds numbers to use when computing the polars for an airfoil? I am currently using 10,000 - 200,000
Mark
I would think that you'd be on the upper end of that range. Maybe 250,000.

Kent


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