XFLR5 help needed

[GaryO]

I'm having trouble getting started with this program. I'm doing something wrong, as I keep getting fatal error messages.

I really need something like "XFLR5 for Dummies", but there is nothing on the website. I've posted a request on their forum, but no answers yet.

Anyone have anything to offer?

[green66]

Not to sidetrack your efforts with XFLR5, but consider Profili (http://www.profili2.com/) which has XFoil built-in, ie does full CFD airfoil analysis, but is windows-based and way more user-friendly. Profili can't do 3D, ie full wing, analysis, but for that you might try Lift-Roll (http://www.geocities.com/jebbushell/COOKBOOK.htm) which is a spreadsheet implementation of the Vortex Lattice Method.

[GaryO]

I've got both Profili and Lift-Roll, but XFLR5 looks like it does a more comprehensive job of looking at the wing and plane. But those programs are a really good place to start.

[Stand]

I'd be interested in a short tutorial as well. I currently model some airfoils using Xfoil and Profili, but I can't even figure out how to load my airfoils into XFLR5!

SD

[GaryO]

Well, why not start a thread right here? We could also post on the XFLR5 forum, and I know some of the hlg guys are using this. Surely there would be enough knowledge to get something started.

I'll post on the hlg forum and see if I can get anyone to respond, but let's do the info thread here. Okay?

[GaryO]

Well, no one indicated interest, so we'll just have to muddle through on our own :(

[Joast]

As it happens, I downloaded XFLR5 the day before yesterday. And yes: I also have hardly any clue as to what I'm doing...

But maybe we can help one another? So far I've found out that in order to have polars available for wing analysis you first have to run a foil analysis over the expected flight envelope. This flight envelope you can determine with the standard formulae for speed and Re value. With Profili you can figure out what a likely range is for angle of attack. Also with Profili you can "prepare" the input .dat files for the foil analysis.

This far I've come. For me it's still a question mark as to how to interpret the results.

I'm first of all looking for:
- average cl value, as input for a more accurate calculation of the COG.
- lift distribution, to evaluate the wing efficiency. (Found, but how do I get the ideal elliptical distribution in the graph.)
- lift coefficient distribution, to evaluate stability.

I already found a Java tool that does the job for a single tapered wing:

www.rc-soar.com/tech/winganalysis.htm

but I want to go one step further. How far have you come over the last week?

[Joast]

Stand,

The first action is to prepare .dat files (or other regular formats?), the second to use the "load" option in the "file" main menu.

[CloudyIFR]

Is this something that would help with XFLR5?

Understanding Polars Without Math
A guide to getting the most
from polar diagrams
http://www.b2streamlines.com/books/UPwoM.html

Curtis
Montana

[Stand]

I haven't had time to go back and play with XFLR5, but I am still interested in learning how to use it effectively. I took a look at the wing analysis online Java program from Stanford. I don't really see that it's too useful as it doesn't appear to differentiate between say a brick and a NACA 65-010. So in my mind it can't be very accurate, can it? Or does that happen when you tune the CL to AoA factors?
It's been my experience that the leading edge greatly affects the stall of a wing; i.e., a sharp pointy leading edge can have nasty snapping characteristics, but still have the same CL for a given AoA up to a point. I guess I should actually take some Xfoil output and follow the instructions to see how it really works.

Stan D.

[CloudyIFR]

Would someone mind taking the time to show me how to find the Cmo of an airfoil?

i.e. airfoil moment coefficients

Thanks
Curtis

[CloudyIFR]

Perhaps I should clarify what I'm trying to do.

I'm working on a spreadsheet for flying wings that uses Dr. Panknin's twist formulas. The spreadsheet is complete and accurate.

However, to find the twist required for a swepte flying wing it requires the root and tip airfoils Moment of Coeffecients and Zero Lift angle.

If someone would like to review the spreadsheet with knowledge of Dr Panknin's work please email me as I'd appreciate the constructive review.

Curtis
Montana

[nmasters]

it requires the root and tip airfoils Moment of Coeffecients and Zero Lift angle.

I have “Understanding Polars Without Math”, it’s good. I haven’t seen the output from XFLR5 but those numbers are represented somewhere on any kind of polar diagram. Can you post an example of the output you’re getting? Maybe someone can spot it

--Norm

[CloudyIFR]

Here's two screenshots.

I've taken a SD-7037 airfoil and thinned it to 7.5% for my root airfoil, zero lift angle is about -3.0 but I'm not sure how to find the Cmo.


This is what Dr. Panknin program is asking for:

airfoil moment coefficients:
The moment coefficient of the whole aircraft must be positive for stability. While some of this moment is derived from the downforce generated by the wing tips, it is also greatly influenced by the pitching moments of the airfoils used. Sections used on contemporary swept wing tailless aircraft have near zero pitching moments.

design coefficient of lift:
The greater the design coefficient of lift, the more wing twist will be required. The design coefficient of lift should be the coefficient of lift at cruise speed so that trim drag is minimized. Some amount of up trim is expected to be necessary for thermalling.

airfoil zero lift angles:
The zero lift angles of the root and tip airfoils influence the geometric wing twist required.

I can see the graph for Cmo and in the foil analysis it gives the Cmo in the bottom middle for the selected AoA, but which AoA should I use?

I have the Understanding Polars without Math book on my Holiday wish list for my wife, so hopefully I'll have before the end of the year, if not I'll purchase it myself after the Holidays!

Thanks so much for the help.

Curtis
Montana

[nmasters]

Okay, normally, you find the best lift to drag ratio on the cl/cd curve and then take that cl over to the cl/alpha curve to find the AoA for best glide. Unfortunately it looks like XFLR5 isn’t using enough data points so we can’t pull that out of the cl/cd graph. The cl/cd/alpha (usualy not on polars) graph seems to show that the maximum L/D is 55 at 6 degrees. Notice that all the lines have a kink at about 1.5 degrees; that’s the minimum drag (CDmin). Any design CL between Cdmin and best L/D will be good. Then trim for lower speed with the elevons.


I can see the graph for Cmo and in the foil analysis it gives the Cmo in the bottom middle for the selected AoA, but which AoA should I use?

Cm0 is only one point on the cm curve. It’s the Cm at zero lift. For this airfoil that looks like about -0.045 at about -1.5 degrees AoA


I have the Understanding Polars without Math book on my Holiday wish list for my wife, so hopefully I'll have before the end of the year, if not I'll purchase it myself after the Holidays!

Different labs format their polars differently. The book goes into that. Sometimes they’ll actually print the number of Cm0 next to the cm curve so you don’t have to guess.

--Norm

[CloudyIFR]

I may be getting there, thanks for the help.

However, how do I deteremine what the Zero Lift Angle is of an airfoil?

Thanks
Curtis
Montana

[nmasters]

Zero lift is where the curve crosses the X axis on the cl over alpha graph

--Norm

[CloudyIFR]

Is it possible to find the Zero Lift angle in this view?

This way it'll then give me the Cm as at zero lift as well.

http://static.rcgroups.com/forums/attachments/9/0/5/0/t1572991-216-thumb-foil.jpg

I really appreciate the help.

Curtis

[raptor22]

To get the CM at zero lift, right click on one of the graphs and go to Graphs>variables.

Then, set the Y variable to CM and the X variable to Cl. Then, wherever the line intersects the X axis is Cm0.

--Alex

[nmasters]

The red lines in that pressure distribution chart are for AoA=-1.38 degrees. That’s pretty close to the -1.5 that I estimated from the cl/alpha graph so I guess that’s the true zero lift angle.

--norm

[flyonline]

I've done a little with XFLR5, though I catergorically state that I'm no expert and may even be doing it wrong, however the little I've seen come out of it seems to make sense. What issues are you having with it? It took me a while to work out how to load the foils into the program to use. I'll have a play with it tonight (I was going to anyway, but will try and take some screen shots to post here) as I'm also looking to work out the PM b/n two different airfoils (a thinned e374 and a Don Ayers D7517).

Steve

[zitron]

Guys, XFLR5 is a very useful tool, but it's not really something for beginners in aerodynamics. Alot of the displays and options will make more sense if you try to do some basic reading on things like pressure distributions, lift distributions, low Reynolds number aerodynamis in general, and if you are really nuts, lifting line theory and panel method.

Remember, garbage in, garbage out!

-Z-

[Francesco]

If someone is still interested, an article I wrote about XFLR5 model analysis has just been published in the Feb 08 issue of R/C Soaring Digest.

[CloudyIFR]

I'm having trouble getting started with this program. I'm doing something wrong, as I keep getting fatal error messages.

I really need something like "XFLR5 for Dummies", but there is nothing on the website. I've posted a request on their forum, but no answers yet.

Anyone have anything to offer?

Have you seen the February issue of Radio Contolled Soaring Digest?

Quote: XFLR5, a “virtual wind tunnel” allows a detailed investigation of the aerodynamics of a specific airframe. Francesco Meschia demonstrates all of the procedures necessary to evaluate the X-Models 1.9 m Blade V-tail sloper. End Quote.

If not go here: http://www.rcsoaringdigest.com/

By the way RCSD is a phenomenal publication and it's Free, however, I highly recommend Making a Donation to help keep it going.

Curtis
Montana

[The Tellurian]

Bravo Francesco

Richard

[Francesco]

Bravo Francesco

Richard
Thanks Richard.

[kcaldwel]

Yes, great article, thanks Francesco!

The lift distribution of the glider you analyzed in your article with that swept back tip shape looks quite poor, if something near an elliptical distribution is considered optimum. Is there something that I'm missing? Is this mimicking a winglet effect?

I find it strange the max L/D speed is so close to minimum sink speed for a glider designed for low drag and high speed as that glider appears to be? It would be interesting to know if an elliptical lift distribution planform would show a bigger spread out to max L/D speed.

Kevin

[Francesco]

Yes, great article, thanks Francesco!

The lift distribution of the glider you analyzed in your article with that swept back tip shape looks quite poor, if something near an elliptical distribution is considered optimum. Is there something that I'm missing? Is this mimicking a winglet effect?

I find it strange the max L/D speed is so close to minimum sink speed for a glider designed for low drag and high speed as that glider appears to be? It would be interesting to know if an elliptical lift distribution planform would show a bigger spread out to max L/D speed.

Kevin

Thanks Kevin,

what do you think is wrong with the lift distribution? If you look at figure 18, the distribution shape is quite close to the optimum elliptical arc... perhaps you were looking at the Cl distribution (figure 17), weren't you?

I agree with you, the best L/D and minimum sink points are close, but not particularly. 10% difference is common even for full-scale gliders. What I find puzzling is that the L/D and minimum sink, for the unballasted glider, are very low in my opinion. I think this is because the Blade wing is an offspring from an F3B glider, but now people have found more exciting way to fly it, such as DS, and prefer to trade some efficiency for speed. I have flown the 3.2 m bigger brother, the Superblade, and in comparison it is much more efficient (but less exciting).

[kcaldwel]

Thanks for the reply Francesco!

No, I was looking at Fig. 18. It looks to have far too much tip loading to me, far from an elliptical distribution. This of course also shows up as huge Cl's at the tip in the Cl plot as well.

It would be interesting to see the elliptical line on Fig. 18, as it is always hard to judge by eye. I have seen 2 panel wing profiles that are within a few percent of an elliptical distribution, and this many-panel wing looks to be much further off.

Your analysis does not include any estimate of fuselage or interference drag as far as I can see? I would have thought the spread between min sink and best L/D would have been larger than 10% with only the lifting surface drag - some full size sailplanes are 12 to 14% with a big person carrying fuselage, and real production gaps, etc. Of course they have Re on their side.

Kevin

[Francesco]

Thanks for the reply Francesco!

No, I was looking at Fig. 18. It looks to have far too much tip loading to me, far from an elliptical distribution. This of course also shows up as huge Cl's at the tip in the Cl plot as well.

It would be interesting to see the elliptical line on Fig. 18, as it is always hard to judge by eye. I have seen 2 panel wing profiles that are within a few percent of an elliptical distribution, and this many-panel wing looks to be much further off.

I am attaching two pictures of the loading curve for zero and 5 deg AoA superimposed to a pure elliptical arc. I personally don't see much tip overload, what's your opinion?

Your analysis does not include any estimate of fuselage or interference drag as far as I can see? I would have thought the spread between min sink and best L/D would have been larger than 10% with only the lifting surface drag - some full size sailplanes are 12 to 14% with a big person carrying fuselage, and real production gaps, etc. Of course they have Re on their side.
I think you're right. Unfortunately I could find no easy way to model the fuelage influence in XFLR5, other than modeling it as a part of the wing.
Cheers,
Francesco

[raptor22]

I hope that XFLR eventually gets a utility to model fuselages. From what I understand, uses AVL code to do the vortex lattice portion of the program, and AVL has a provision for fuselages using slender body theory.

--Alex

[Francesco]

I hope that XFLR eventually gets a utility to model fuselages. From what I understand, uses AVL code to do the vortex lattice portion of the program, and AVL has a provision for fuselages using slender body theory.
I support, that would be a welcome addition!

[kcaldwel]

Yep, they look real close! Thanks for doing that. I guess I was superimposing expectations from what I was seeing on the Cl plot onto lift distribution plot.

It still seems strange to have a Cl distribution that demands such high Cl from the low Re tip airfoils.

I've never understood this tip shape that seems so prevalent. It is very surprising the real glider doesn't have tip stall problems, which seems to indicate there is something unusual going on that I don't understand, and I've never seen any real explanation for.

Kevin

[rofujiyama]

Here's a planform to consider, instead of long slender wings with swept back,pointy tips, try a short wing with only swept back tips , for the wing. = Delta-ish planform, and as the i recall , delta shape wing can maintain a pretty high angle of attack,and still remain airborne. :)
Those wings are cool in the previous post. :D

[lostrider]

I've just started to try XFLR5 and i'm getting some errors and then this message appears:

"Point is out of the flight envelope"

Which point?? i cant see the entire analysis report to see the errors. its just appears real fast in analysis process and then disappears.

[Francesco]

I've just started to try XFLR5 and i'm getting some errors and then this message appears:

"Point is out of the flight envelope"

Which point?? i cant see the entire analysis report to see the errors. its just appears real fast in analysis process and then disappears.
You can recall the log output via the "View Log file" command in the "Operating point" menu.

[lostrider]

You can recall the log output via the "View Log file" command in the "Operating point" menu.
Thanks. View log file from operating point menu... just like shutdown from start.. Whats the meaning of user interface.. :P