How to create my own airfoil

[raptor22]

I know everyone is going to tell me to look for a proper pro-designed foil, or to modify one, and not to wanste my time here.

But, i don't really want to do that. Why? Who knows? I just want to design my own airfoil.

So my question is, where do I start? And what is the standard procedure for doing this? I at least know it would be a waste of time to make a bunch of random foils and simply hope to get something good.

--Alex

[slipstick]

Generally you start from deciding what characteristics you want, not forgetting strength and ease of construction as well as the purely aerodynamic. Then (sorry :() the standard procedure is to look for an existing airfoil close to your wishes and modify it so it's closer. I doubt if anyone has designed an airfoil without reference to the existing ones since early last century ;).

Of course it's very difficult to know it's genuinely better without either a wind tunnel and some expensive instrumentation or a lot of test models and an amazingly good feel for how a model is flying. Programs like Xfoil will give you some idea of the characteristics but they're far from perfect.

Alternatively you could do what most people and just draw round your shoe....that well-known airfoil "Nike size 9" ;). Even then if you checked you'd probably find the co-ordinates were almost identical to an existing "pro-designed" one :(.

Steve

[Tom Harper]

Raptor,

I understand the interest. Some pointers I've learned:

1. Raw lift is a function of the flat plate area of the wing, not the section shape.

3. Drag is a function of the section and planform.

3. There is no magic in airfoils. Any section will work. The advantage of published sections is that you can get some idea of their characteristics from the polars. You will notice that outside of laminar sections their characteristics are all very similar.

4. Years ago NACA noticed this similarity and offered a simple standardized process. The section is a curved (camber line) line with a symmetrical envelope wrapped around it. If the line is straight the resulting section will be symmetrical. If the line has the same curvature as the envelope the bottom will be flat.

5. The camber line determines where the lift force is felt along the chord and how it will change with angle of attack.

6. You can create envelopes by sketching or by inventing equations. Camber lines are the same. I have used stretched sine functions for reflex airfoils and they work great. They would also have worked fine if I'd just drawn a curved line.

7. An easy process is to sketch an envelope and camber line. Measure a few points with a ruler. Enter them into an Excel spread sheet. Add the coordinates together and have Excel plot them. If you know VB you can do it the same way.

Get a copy of Theory of Wing Sections (Abbott and Doenhoff). Even if you ignore the math it is fascinating reading.

[Ollie]

Deside starting the mean camber % for the max Cl. Wrap a streamline thickness %. Change the mean line max % fore or aft. Change the max thickness fore or aft. Vary results performance using X-foil.

[fhhuber506771]

Basics (some already covered above.. sort of..)

Thicker airfoils are better for low speeds. Thinner airfoils are better for high speed.

Blunter leading edges have a later stall. (higher angle of attack before they stall)

Undercamber is effective when used on a lower speed airfiol looking for maximum lift.

flat bottom is better or medium speed looking for maximum lift.

Semi symetrical does become better for high speed lifing airfoils (not just for aerobatics)

fully symetrical if you anticipate a lot of inverted flight.

*******
now for stuff that breaks the rules you THINK always apply...

the airfoil made when folding a paper airplane is suprizingly effective when scaled up. Experiments were done with this "checkmark" airfoil in wind tunnels and it was almost as good as some of the semisymetrical airfoils at all speeds.

You can have a combination of airfoil types mixd into one and get some interresting results. I did one that was symetrical the first 15% chord.. then transitioned semisymetrical to 45% then transitioned abruptly to flat bottom... it behaved like an underchambered at low speeds and at high speed almost matched a symetrical for inverted. (wierd lookng airfoil that has a very strange formula for plotting corectly at different scales because it s % thickness is scale dependant)

[Ollie]

"Thicker airfoils are better for low speeds."

fhhuber,

You know better.

[Sparky Paul]

I know everyone is going to tell me to look for a proper pro-designed foil, or to modify one, and not to wanste my time here.

But, i don't really want to do that. Why? Who knows? I just want to design my own airfoil.

So my question is, where do I start? And what is the standard procedure for doing this? I at least know it would be a waste of time to make a bunch of random foils and simply hope to get something good.

--Alex
.
Before you can design an airfoil, you need a grounding in why airfoils work.
This is best done thru self-education, using at least these two standard references:
"Model Aircraft Aerodynamics by Martin Simons
and
Basics of R/C Model Aircraft Design by Andy Lennon
Both available thru Amazon.
These will point you in the right direction, and possibly even dissuade you from reinventing a wheel (airfoil) which already exists. :)
There's just so many effective ways to go from the leading to the trailing edge, and most of these are already in the databases..
Some of the advice you will see on the public forums, ... take it or leave it, some of it is good, some is fantasy, and until you have a grounding in aerodynamics it will be difficult to seperate the fantasy from reality.
There's more dis-information available than information, sadly.

[raptor22]

I already ahve a good grounding in aerodynamics. i can understand pressure maps, polars, and alot of other things. i know how to choose a proper airfoil, and I know the proper airfoil exists for pretty much everything. I also understand the of basic airfoil features.

It's just that I knwo alot of people such as mark drela and martin hepperle design airfoils. I think going throught the process would be interesting. I don't really expect to necessarily come up with something better than Dr Drela, but I think going through the process of attempting it would be a good experience.

--Alex

[Sparky Paul]

Those are two good sources for the process.
But it will be long and laborious..
I prefer to use the products of Mark's and Martin's et als labors than try to work out something personally unique.
More time to do other things with the design, since I'm not pushing any envelopes with what I do anyway. :)

[raptor22]

What are good sources? mark and Martin themselves? Should I be sending some Pm's, maybe?

[Sparky Paul]

For the depth you want to go, Simons and Lennon aren't sufficient.
You want to get to the basic generation of a profile, one method is covered in "Theory of Wing Sections", by Abbot and Von Doenhoff.
They go into developing wing sections from cylinders using conformal transformations as developed by Joukowski.. the math is way daunting..
X-foil a program by Dr. Drela as contained in Profili, may let you design your profile.
(In any event, Profili is a must-have for anyone interested in aerodynamics..) (shameless plug)

[raptor22]

I'm not too worried about the math; I have a full year of college calculus. That is probably enough.

[raptor22]

I guess i'll have to purchase that book. I'm fooling around in xfoil right now, *stil* trying to work it w/out profili. Does anyone know what methods mark uses to design airfoils in xfoil?

--Alex

[HELModels]

GDES;QDES;MDES and not necessarily in that order. :D

[Ollie]

"I don't really expect to necessarily come up with something better than Dr Drela, but I think going through the process of attempting it would be a good experience."
Compare
http://www.charlesriverrc.org/articles/allegro2m/markdrela_allegro2m.htm
airfoils above with first case
http://www.charlesriverrc.org/articles/allegrolite2m/markdrela_allegrolite2m.
second case.
The AG16, -17, etc for the Re along span and Cl and twist and composite structure.
The second case AG-35, AG-36, etc similar except for build wood and different spar manufacture!
Also
http://www.charlesriverrc.org/articles/drela-airfoilshop/markdrela-ag-ht-airfoils.htm

First pick the relative functions of the plane. Then design the strength, stiffness and mass distribution for the structure. Then design the lift distribution along the span for planform, AR, twist, sweep, angle attack range, etc. Then pick (or design) the airfoils along the span of the wing.