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Author Topic: Getting Started in XFLR5 - Airfoil and Wing Analysis  (Read 3869 times)
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OZPAF
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« Reply #25 on: February 23, 2018, 02:34:24 AM »

Quote
Such as a panel method...
Sorry missed this - both XFLR5 and Lift Roll use a VLM - "Vortex Lattice Matrix " to achieve a 3d solution for the lift of the wing.
As Jon mentioned earlier - XFLR5 also has the option of using LLT - lift line calculation for the calculation of lift. I prefer to use their VLM option.
John
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Paul_BB
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« Reply #26 on: April 16, 2018, 01:34:50 AM »

Hi John,

Can you elaborate (explain) a little on what you have said in one of your posts :

"As a matter of interest this factor is often up around 97% for most modern plan forms and in my opinion the plan form efficiency needs to be balanced against good stall behaviour (ie as close to being within the first 1/3 of the semi span) in straight flight and checked in banking thermal flight."

Maybe here or somewhere else if you don't want to pollute Jon's thread?

Thank you.
Paul
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OZPAF
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« Reply #27 on: April 16, 2018, 05:29:35 AM »


Hi Paul - I well remember following the build of that nice French vintage glider - CB36?

Quote
"As a matter of interest this factor is often up around 97% for most modern plan forms and in my opinion the plan form efficiency needs to be balanced against good stall behaviour (ie as close to being within the first 1/3 of the semi span) in straight flight and checked in banking thermal flight."

If Jon doesn't mind I’ll enlarge a bit more here on this.

The Wing Span Loading is basically a means of representing the 3 dimensional lift distribution along the wing due to the shape - i.e. plan form of the wing, and thus it's area distribution. The area distribution together with any aerodynamic sweep (measured at a line running through the 25% chord points) and dihedral determines what the local CL will be to satisfy the local lift load.

I use the VLM spread sheet mentioned before to provide the local CL's and as mentioned also a so called plan form efficiency. This is a means of approximating the plan form to the ideal ellipse for which the CL would be constant along the span. This efficiency is calculated as the ratio of the average Cl/ the max. CL for the chosen AOA.   It is actually quite practical in a design sense as this plan form efficiency factor remains constant over the operating range of the wing (largely) and it can then be used to find the local CL's along the span for any point in flight - ie any average CL.
To use it - you need to first find the local CL's for the points along the span (aerofoil locations) for a given AOA from the graph generated by the programme. The relationship of the local CL’s to the average CL can then be found and then used for any other AOA.

As the efficiency increases the wing approaches a situation where with the CL almost constant along the span - it can stall anywhere between the root and almost the tip. Thus it becomes desirable to reduce the efficiency - usually by overloading the wing closer to the root, to move the stall point (the point of max. local CL) away from the last 1/3 of the wing. The Span Loading can be modified to include the extra loading from thermal turns (on the inboard wing panel particularly) and alterations made to the plan form and aerofoil selection as required. This can cause some surprises - particularly with large lightly loaded models such as modern F5J or F3J classes where 3.4m spans and wing loadings of 8oz/ft2 or lower, due to the tight - 30 deg banking turns used in RC.

When you are happy with the planform - then the aerofoils can be selected. I generally prefer to use a base aerofoil and then modify this to suit the local CL duty and Re No- optimising for minimum drag.

I hope this helps Paul. If you wish to know more than if you PM me with your email address, I can send a report I did on this subject based on the Avatar.
It's harder to write about than to actually do.

John
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Paul_BB
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« Reply #28 on: April 16, 2018, 06:33:15 AM »

Hi John,

Thanks for the reply. PM sent. I am slowly starting to design the airfoil for my next build, the CB 32, that will take place next winter (in the northern hemisphere, up above).

I will use XFLR5 to find out the Re numbers along the wind span. I will then design the airfoils for the respective Re numbers. Note: I will use the airfoil optimization tool XOPTFOIL to help me out do you know about this program?

But reading your posts in this thread made me worry about tip stalling, thus lift distribution. So I would like to incorporate this aspect to my airfoil design.

-Paul
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« Reply #29 on: April 16, 2018, 06:44:09 AM »

Hi John, where can I get a copy of Lift Roll please?

There was some work done by Kevin Caldwell on RCG into span efficiency in XFLR5 - if I recall he was able to manipulate it to provide odd results by increasing panel density in the wing tips. As a result I now tend to keep the wing tips simple approximations rather than over doing the panels to give exact tip shape.

None of this is hijacking in my opinion btw - it's all relevant and interesting Smiley

Paul, XOPTFOIL was developed by Dan Prosser - I've not looked into it yet but I use some of his other calculators and he knows his onions.

Jon
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dephela
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« Reply #30 on: April 16, 2018, 07:13:47 AM »

I have LiftRoll in my Dropbox, will keep it there for a short time for anyone:

https://www.dropbox.com/s/irod55ke35npwum/LIFTROLL.xls?dl=0
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Dennis
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« Reply #31 on: April 16, 2018, 07:38:34 AM »

That's excellent, thanks you!
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tross
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« Reply #32 on: April 16, 2018, 01:10:33 PM »

Thanks De.. Smiley
Straight forward and useful IMO.
Had the wash-out in there as a positive at first...... Roll Eyes Roll Eyes
You'll have that out here in the sticks. Grin
Tony
Attached files Thumbnail(s):
Re: Getting Started in XFLR5 - Airfoil and Wing Analysis
Re: Getting Started in XFLR5 - Airfoil and Wing Analysis
Re: Getting Started in XFLR5 - Airfoil and Wing Analysis
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Instructions: Step One...Assemble the pile of sticks shown in pic "A" to look like the model airplane shown in pic "B"........
OZPAF
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« Reply #33 on: April 16, 2018, 07:58:56 PM »

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I will use XFLR5 to find out the Re numbers along the wind span. I will then design the airfoils for the respective Re numbers. Note: I will use the airfoil optimization tool XOPTFOIL to help me out do you know about this program?

But reading your posts in this thread made me worry about tip stalling, thus lift distribution. So I would like to incorporate this aspect to my airfoil design.

Lift Roll is quick and easy to use but will not handle many panels. I have no experience with XOPTFOIL but after Jon's comments will take a look at it. I use Profili as a means of running XFOIL for airfoil optimisation. However it is not free and although good I think it is becoming too expensive, and I think I would recommend XFLR5 as a first option for airfoil analysis. Whatever programme is used I would recommend that it should only be an interface to XFOIL.

Quote
There was some work done by Kevin Caldwell on RCG into span efficiency in XFLR5 - if I recall he was able to manipulate it to provide odd results by increasing panel density in the wing tips. As a result I now tend to keep the wing tips simple approximations rather than over doing the panels to give exact tip shape.

Yes the few times I have used XFLR5 - I did the same thing, and use a minimum of panels to represent the curved planform of the wing.

Thanks for your offer Dephela .

That's a wild plan form Tony(WW1?).

Quote
None of this is hijacking in my opinion btw - it's all relevant and interesting Smiley

Thanks Jon - I was getting a bit concerned.

John from the Southern Highlands of NSW




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Paul_BB
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« Reply #34 on: April 17, 2018, 12:25:04 AM »

I have LiftRoll in my Dropbox, will keep it there for a short time for anyone.

Thank you dephela.

And thank you Jon for your hospitality. Smiley
And John for your help. You've already helped me a lot with the CB36 a few years ago.

I've had a look at XOPTFOIL. You need some time to get used to it (many parameters), and I found that the hardest thing to do is to define the operating points that match your requirements.
For example, if you want to maximize the power factor of the airfoil for a given Re number, you just can't specify one operating point - say max power factor for alpha = 5°. You will end up with a crooked power factor curve (vs alpha) and a crooked airfoil. You must specify a few operating points (eg max power factor for alpha = 3 and 5 and 8° with respective weightings). But a solution computed by XOPTFOIL can be a good starting point for further shaping using XLFR5 and splines.

But I must say that I'm new to this program.

-Paul
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OZPAF
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« Reply #35 on: April 17, 2018, 01:48:05 AM »

Paul I'm more familiar with using Profili to modify airfoils as it has a very easy to use XFOIL mod interface. However I need to look into XOPTFOIL.
When looking into the performance of the foil - you should be able to plot Power factor against Alpha for any Re on XFLR5. Thus the optimisation process of the foil requires a change to be analysed and then either accepted or further modified depending on the graphical results.

John
« Last Edit: April 17, 2018, 01:59:32 AM by OZPAF » Logged
tross
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« Reply #36 on: April 17, 2018, 09:54:16 AM »

You don't miss much John. Smiley WW1 it is.
Even with this wild plan form, the program indicated a noticeable difference between wash in and out at the tips, as the AOA was increased.
As the AOA drops to zero and under, it's what I expect to see in the field. Shocked
Would it be more useful for what you are doing if we added a couple of panels?
I don't use controls of course, so a lot of the calcs are for others to appreciate.
This is a very informative thread Jon, one thing leading to another as it is.
Thank you. Smiley

Tony.
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Instructions: Step One...Assemble the pile of sticks shown in pic "A" to look like the model airplane shown in pic "B"........
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« Reply #37 on: April 17, 2018, 12:36:30 PM »

Thank you very much Dennis for the Dropbox paper.
Now I have something else to spend my time instead of building models  Grin

Urs
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OZPAF
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« Reply #38 on: April 17, 2018, 08:27:20 PM »

Tony - it's also interesting to see the effect of the overloaded centre section due to the cutout for the pilot at the root. Lots of induced drag indicated by the large local CL. However it look safe from a stall point of view.

More panels in "Lift Roll" would be nice - however it is till handy as it is and back when it was originally generated 3-4 panel wings were the norm in RC Glider.

That's not your Nieuport top wing by any chance?

John
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tross
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« Reply #39 on: April 19, 2018, 12:02:20 PM »

Right John. Nieuport top wing.
The better average (less local) when entered this way.
Probably wouldn't have a good day set up like this. Roll Eyes
It's just for fun, but I'll be making my way through Jon's (very clear) procedure above.
I hope all is well, new digs? Smiley

Tony
Attached files Thumbnail(s):
Re: Getting Started in XFLR5 - Airfoil and Wing Analysis
Re: Getting Started in XFLR5 - Airfoil and Wing Analysis
Re: Getting Started in XFLR5 - Airfoil and Wing Analysis
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Instructions: Step One...Assemble the pile of sticks shown in pic "A" to look like the model airplane shown in pic "B"........
OZPAF
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« Reply #40 on: April 20, 2018, 12:38:40 AM »

I moved last September from the old place near Mascot Airport. I'm now about 120k (11/2 hrs) South of Sydney in Moss Vale NSW. I've gone up in the world Smiley - about 2,000' or so Smiley

John
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