Camber

[HankF]

What is the purpose of camber in an airfoil?

Hank

[Petri Nygren]

Minimizing the profile drag at required lift coefficient range.

The higher the Cl, the more air has to be deflected from free stream (more downwash). Profile drag is lowest when airfoil mean line - defining factor of camber - is adapted to the required amount of downwash.

Example: camber changing flaps. Increase the camber for high Cl and decrease it for low Cl. Cdp is minimized in both cases.

[Purdue Aero Man]

camber decreases the zero lift angle of attack, thereby shifting the lift curve of an airfoil to the left, allowing for a greater Cl at a lesser angle of attack, as well as a slightly increased(depending on the amount of camber) maximum lift coefficient.

[Ollie]

http://www.grc.nasa.gov/WWW/K-12/FoilSim/Manual/fsim001t.htm
http://142.26.194.131/aerodynamics1/Basics/Page4.html

[HankF]

Two popular airfoils. For the same Cl, the same Cd. How do you choose?

Hank

[Ollie]

For R.N. 3 million data, not for models 0.50 to 0.03 million R.N.

[Sparky Paul]

Just another troll. :(

[Ollie]

Paul,
Thanks.

[HankF]

Sounds like I crossed the "owners" of this forum.

Hank

[Sparky Paul]

It's a "straw man" question.
You know the answer as you demonstrated with the immediate posting of some data.
If the question is sincere, rephrase it.

[HankF]

The data simply points up my dilemma. I can't ask the question more simply. I can't seem to put a numerical value on the choice to help me decide.

I see airplanes that spend most of their lives right side up use cambered airfoils where airplanes that spend a goodly amount of time inverted tend to have semi to full symmetrical foils.

But the airfoil data that I have access to doesn't seem to support one or the other.

I see that to get a particular amount of lift, the cambered airfoil needs less decalage than the symmetrical although one shouldn't (in principal) be more draggy than the other except possibly in off design point operation thus generating more trim drag.

To me it's all just speculation.

Hank

[vintage1]

Well what Ollie may have been trying to say is that our Reynolds numbers the difference between a bit of plastic stuck on a frame and a NASA derived special section is not that great.

I don't fly contest sailplanes of pylon racers. Just sporty aerobats and scale, and frankly, the thing that makes the difference is wing loading, not the section.

[Ollie]

Hank,
Your Re 3 million data is wrong for models!

[Petri Nygren]

It´s very simple really. If your model spends time above Cl = 0.5, go with NACA4412. If lower including inverted, NACA0012.

But quite frankly; I would not use either. We have got quite a few more choices in 75 years.

[HankF]

Petri,

That's a very interesting graph. It shows that the maximum L/D for the 4412 (at Cl = 1.1) is almost twice that of the 0012 (at Cl = .7). Is that true? There's only a factor of 10 difference in RN from my data to yours. Can there be that big a difference in L/D?

Hank

[Petri Nygren]

That's a very interesting graph. It shows that the maximum L/D for the 4412 (at Cl = 1.1) is almost twice that of the 0012 (at Cl = .7). Is that true? ... Can there be that big a difference in L/D?
How about looking at it this way: ever heard of fully symmetrical profiles in high performance sailplane wings? If not, why could that be? After all, those planes are all about L/D.


There's only a factor of 10 difference in RN from my data to yours.
Things are far from linear in this world...

[markdrela]

That's a very interesting graph. It shows that the maximum L/D for the 4412 (at Cl = 1.1) is almost twice that of the 0012 (at Cl = .7). Is that true? There's only a factor of 10 difference in RN from my data to yours. Can there be that big a difference in L/D?


Your data is for finite-span rectangular wing of aspect ratio 6. Hence the measured CD is mostly swamped by induced drag. For example, at CL=1, the CD numbers for the 4412 are:

CL = 1.0 (measured)
CD = 1/14.5 = 0.069 (measured)
CDi = 1.0^2 / (pi*6*0.95) = 0.056 (estimated)
CDp = CD - CDi = 0.013 (measurement - estimation)
Even a small error in either the measured CD or the estimated CDi will produce a big change in the deduced profile drag CDp.

The wing will also have a nonuniform spanwise cl distribution, larger in the center and falling to zero near the tips. So the wing's profile CDp (even if it could be reliably estimated from the data) is at best a smeared average of the wing airfoil's profile cd over some range of cl's.

For these reasons, using finite-span wings to test airfoils has not been done since the 1940's. The resulting data is not very reliable or very useful.

[HankF]

Petri, Mark. Thank you much for straightening me out on source data. I rummaged around in my bookshelf and looked at the polars for the two airfoils I used in my illustration in my copy in Abbot and Von Denhoff's Theory of Wing Sections and the data is very different. They show an L/D for the 4412 of about 125 very near a Cl of 1.0 and an L/D for the 0012 of about 85 very near a Cl of about 0.9. These are at Re of 3 million. This agrees pretty well with what you've shown, Petri.

I'm still trying to digest your comment about above and below CL of 0.5. I'm trying to understand that operationally.

Hank

[HankF]

From what's been said, can it be generalized that cambered airfoils typically have greater L/D's than symmetrical one's do?

Hank

[markdrela]

From what's been said, can it be generalized that cambered airfoils typically have greater L/D's than symmetrical one's do?


Yeah, that's true in most cases. But the higher profile L/D will typically occur at the higher cl's, which means the wing Aspect Ratio must be large enough to exploit it. With a smallish A.R. like on a sport power plane, the big CDi will swamp the profile L/D benefit if you try to go there.

[HankF]

Mark, In practice how close can one come to the theoretical results of the airfoil programs?

I had a program (DOS!) called Airfoil-ii which I used several years ago and I remember it being pretty sensitive to the data being "smooth". In fact it had a special module for that task. It begs the question of how close one can come to reproducing a given "airfoil" in actual practice and how critical it is.

It's almost tempting to say that the old NACA data from the 30's is closer to reality in practice.

Hank

BTW, what airfoil computer programs are currently available for model use?

[HankF]

Never mind, I found the answers to my questions at:

http://www.mh-aerotools.de/airfoils/methods.htm

Very interesting stuff especially at low Re numbers. Looks like a crap-shoot!

Hank

[raptor22]

Mark, In practice how close can one come to the theoretical results of the airfoil programs?

I had a program (DOS!) called Airfoil-ii which I used several years ago and I remember it being pretty sensitive to the data being "smooth". In fact it had a special module for that task. It begs the question of how close one can come to reproducing a given "airfoil" in actual practice and how critical it is.

It's almost tempting to say that the old NACA data from the 30's is closer to reality in practice.

Hank

BTW, what airfoil computer programs are currently available for model use?

Profili, xfoil to name a few.

Markdrela wrote xfoil.

--Alex