SimScale CAE Forum

L/D-data for DAE-11 Airfoil


#1

Hello,

I hope it is okay to ask these kinds of questions here, since it is not specifically about the software and more about airfoils and CFD in general.

I’m working on a ultra light airplane project and is currently searching for suitible airfoils.
One of the airfoils that has caught my eye is the DAE-11 airfoil, which seems to be suitible for the slow flying airplane that I’m designing.

On airfoiltools.com the maximum L/D-ratio is at least 135.8 (angle of attack is 7.25 degrees and Ncrit=9). But when I look at this page: LINK the L/D is “only” 47.05 (at an angle of attack at -0.5 degrees).

In my preliminary simulations of tha airfoil in at 7.25 degrees AOA, here on simscale, I get nothing close to a L/D ratio of 135.8.

My questions:

  1. Why is there a large difference in maximum L/D-ratio between the two sources?
  2. Is there any other good way of finding reliable data for L/D-ratios on airfoils?

#2

Hi @Filiptheking, I did a lot of work in this area not long ago. I posted this:

There are a few settings you need to ‘tweek’ from the defaults to get accurate drag values otherwise they may be much higher (and thus reduce drastically your L/D ratio. So give it a read and let me know if you have any other questions.

Best,
Darren


#3

Also @Filiptheking, just noticed this was a general question about foils, not sure if you have used xflr5, its quite good at getting fairly accurate L/D ratios, and has a GUI.

Best,
Darren


#4

Thank you for your answer!

I’ll read the texts in your project and check your settings.

I have not used xflr5, but I’ll check it out.
Since you say that xflr5 gets the the L/D failry accurate, I guess the L/D values on airfoiltools might be pretty close to reality too (xfoil preditions). But they seem almost too good to be true to me.


#5

Yes but I imagine that they ran a fairly robust analysis on all foils as a batch, if you are more careful, keeping convergence tight I think you will get better results. It will be interesting to see how it compares.

Best,
Darren


#6

I just realised that the L/D ratios from airfoildb are just for Re=100 000. If you look at the L/D values for Re=100000 on airfoil tools, they seem to be similar to the ones on airfoildb.