Airfoil properties (relationship between L/D and CL)

[MadMatt]

I am in the process of finding good airfoil profiles for motorsport application, and as everybody knows there is no "perfect" profile, or if there is, it won't be shared anyway. So I came across a website that classifies the airfoil profiles by their L/D, but this is all at 0° angle of attack.

Can I assume that if a wing X is better than a wing Y at 0°, it will be the same at let's say 10°? Is it the kind of assumption that is realistic? I am not so much interested in the L/D ratio but more in the CL so basically I plan to take the best L/D ratio wing and start from here, even if I know that an efficient wing can produce less amount of lift than a less efficient wing. That is just giving me a start for my investigation.

From what I know the slope of the lift curve VS angle of attack is pretty much linear up to the stall point so this assumption would make sense, but I want confirmation (or other answer).

[Tommy Cookers]

Can I assume that if a wing X is better than a wing Y at 0°, it will be the same at let's say 10°? Is it the kind of assumption that is realistic?

definitely not
the L:D ratio at zero lift is a misleading way of categorizing performance

also, Re is important

[Jersey Tom]

Can I assume that if a wing X is better than a wing Y at 0°, it will be the same at let's say 10°? Is it the kind of assumption that is realistic?

definitely not

Agreed.

Not that I'm much an aero guy but before you go looking for airfoils, I'd think you would need to know specifically what you're trying to achieve and what's appropriate for the platform. I'd think the aero needs for a Formula Vee would be a good bit different than a Star Mazda or F3, and then different yet for Indycar at the '500, different still for F1 at Monaco.

[Kiril Varbanov]

Help yourself - 3D Foil Sim. It needs Java, be aware.

[MadMatt]

Yes I've been using this software, as well as JavaFoil, but there is such an immense database of profiles that I need to start somewhere. Reynolds is in the region of 1'000'000. I'm trying to achieve maximum downforce, drag is not so much an issue in my case. I know that I would have to use multi-element wing, but for simplicity I will focus on a dual element wing, with probably a Gurney flap.

L/D is not really what I am looking for, but I haven't come across a wing profile classification depending only on the CL.

[NoDivergence]

http://airfoiltools.com/

There used to be tons of these websites. Most of the data that it uses is from the UIUC airfoil database, but those are simply coordinates for the airfoils. This site uses XFOIL and plots across angle of attack for varying Reynold's Numbers.

L/D and max CL do not directly correlate. I made two designs, one for efficiency and lower drag and one for just max downforce. You will see a lot of separation for the max downforce case with wake bursting (especially without enough iterations to get slot gaps right, as in my case).

For F1, the airfoils are extremely cambered. More than you will find in 90% of airfoil databases. You can specify your own NACA 4 digit airfoil with that camber >14%, but it will not necessarily have the performance of some of these researched airfoils. I personally like the Eppler 420 quite a bit. Gave very decent results for my wing design.

[MadMatt]

Yes, E423 gave also good results. The website you've put is the one I was talking about in my first post, but they only allow classification of the profiles depending on the L/D, hence the first question I had. I came across McBeath's own profiles, called 183 and 203, will try to find the XY coordinates, but I was hoping to find something more scientific to base my choice on!

[NoDivergence]

I don't know what you mean, the site clearly shows CLalpha plots. In reality, it all goes to sh!t when you do multiple elements anyways since the slot gaps and airfoil position (fore aft, deflection angle, etc) messes up a lot of the design that goes into these airfoils. You may want to look at default mutliple element airfoils. They have max Cl in the 3.4-3.6 range. I think I was close to that or around there with my own design. But my design was with F1 regulations and I included analysis with DRS usage.

Btw, E420 outperforms E423 in every way, L/D and max Cl and stalls later This is because of the increased camber and thickness

[MadMatt]

Yes I mis-explained myself, I meant that there was no way of classifying the profiles depending on the maximum CL, just L/D. And you are right, with multi-element it gets more tricky, not to mention car interaction when put in real situation. But yes I will continue to investigate, I have contacted Simon McBeath regarding his profiles, who knows, he looks like to have a pretty open-mind policy so we will see.

[NoDivergence]

You can see the maximum Cl, it's on the chart plotted against the angle of attack... It's labeled Cl/alpha. Just look at the max value of that graph, that is the maximum Cl as well as the angle of attack for that max. For example, for the E420, the max Cl is ~2.23 at 1 million Re.

You can download the geometry and run it in xfoil to get the exact value at the exact Reynold's number you want anyways, but honestly it doesn't matter that much. Just ballpark it and try setups.

[MadMatt]

I know mate, I know! Again, there is the possibility to classify the profiles depending on their L/D but not directly on their CL and we know that a wing with good L/D might not produce enough downforce. But its fine I will have to do it this way

[NoDivergence]

Ahhhh, I get what you are saying

I personally looked for the most camber i could find then looked for the best performance (Cl)
I know that F1 airfoils are even more cambered

Then i chose a flap that had the characteristics that i wanted. Low drag or high downforce

[MadMatt]

Makes sense! Thank you!

[old grumps]

I too am interested in the aerofoils used in F1. They are not, IMO particularly efficient and certainly incur too much drag.

The problem appears to be that the designers are using plate aerofoils. Now that might be a part of the design specifications, laid down by Powers that be. (I've been down that road in another sport)

Plate aerofoils are not very efficient. May I suggest you return to the Foilsim site, as suggested by another poster. NASA are now on V.III and it is much more user friendly and comprehensive than Foilsim II.

Select "Negative Camber" and play with the chord and angle ratios for maximum negative lift and minimum drag.

Next select "size" and move the cursors to 3.1ft length and 1ft chord. Speed defaults to 100mph and can be adjusted to whatever you want, for analysis.

"Flight" allows you to play around with a variety of parameters, which may be of interest, if not much practical use in designing F1 aerofoils.

Have fun!

[MadMatt]

Yep, this is a very good website too, unfortunately no "weird" profile testing. I played recently with JavaFoil and multi-element wing, and came with this setup, using 2 E420 profiles:



Gave a CL of 3.7 @ 100[mph] for a CD of 0.99, with an aspect ratio of 5 and air density of 1.1845[Kg/m^3]. I like JavaFoil but you have to be careful with it. Last time I made some airfoil profiles experiments with it, values were more optimistic than CFD analysis, probably due to the viscous effects not taken in consideration. Anyone using JavaFoil experienced the same "optimistic" behavior?