ptxman

My Feedback: (2)

Just for kicks, I selected a group of known airfoils that were all 10% thick, but of any varying combination of camber, symmetry & high point position. I pulled them into xfoil, set the Re to the same value, plotted the Cl-Cd polars & visually compared the curves just to see how widely they varied & where the shapes deviated as a function of constrained airfoil shape. This got me wondering about how purpose-built airfoils come about by the original designers & other lofty questions…

If airfoil A and airfoil B are geometrically blended 50/50 into airfoil C, is it reasonable to assume the resultant polars for C would be right in the middle all other things considered equal?

Is there such a thing in the professional world as reverse engineering airfoil polars? In other words a Cl/Cd polar shape is first specified and then some algorithm/simulator computes a resultant airfoil shape (within limits of course) that best satisfies that polar requirement?

Lets say you need an airfoil with the the lowest possible drag bucket profile to predominantly occur within a target Cl range and/or weighted at a certain Cl value. Max thickness & Re are the only other constraints. How would you go about designing the best possible shape? Is there some independant equation that calculates the theoretical lowest Cd value that could ever be achieved?

davidfee

ptxman,
Here I am, digging up another old thread. Yes, I believe there are ways to do what you're aiming at. It takes a lot more knowledge than I have, however. With tools like XFoil, an unprofessional like myself can do some dangerous things. I plan to study the software carefully to avoid making bad assumptions.

One thing to keep in mind is that when you use a single tool to design and optimize an airfoil, you'll end up with something that looks really good in that software, and not so good with different software. And who knows what it will look like in a wind tunnel. So, it's nice to see quality wind tunnel data to compare to the software predictions... but that data is quite rare for these airfoils and meaningless when you make your own section. So XFoil is a pretty cool tool.

So now, to look at your question about blending airfoils, I took an airfoil which was custom designed (not by me) for F5D, a thinned RK40, and then geometrically blended the two. The polar does look like an approximate average between the two... but in reality, who knows?

-David

Tall Paul

"in reality, who knows? "
.
And that's the toss-up. Is it possible using normal construction techniques say, to construct a flying article with the selected profile to a degree of precision which excludes all the other variables such that only the profile change is evaluated?
Most likely not.
The variations in construction, to say nothing of the consequent flight reponses have too much human interaction to be free of biases.
The computer can come closest to an answer as to the differences, but these differences may be in fact too minute to be observed on a real object.
But then again, the computer output resembles reality only to the degree the programmer lets it.
It's more of an indication of what might happen in the real world, IF the program handles ALL the variables that a wind tunnel has, say.
To achieve a true definition of the differences would take a wind tunnel test of the models, contructed as close as possible to the test profiles and run thru the same test sequences.
Flying the profiles then -might- result in an agreement with the tunnel test results, but the number of variables outside a tunnel multiplies beyond control, so that an objective result can be overwhelmed by the subjective..
But that what makes horse races.. and Selig airfoils in lieu of NACA...

LouW

Full scale designers (of subsonic non military aircraft) typically go to published airfoil data and pick an airfoil with suitable characteristics. Up until about mid century airfoils were developed with little theoretical basis. In the thirties, NACA undertook a systematic investigation into airfoil design and produced the four and five digit series. The six series (laminar flow) airfoils (P-51,Piper Cherokee, etc) were one of the first where the desired pressure distribution was developed first then an airfoil developed to produce it. These airfoils have a pronounced drag “bucket� at low angles of attack, but only if a smooth surface is maintained in the laminar flow area.

These NACA reports still form the major source of subsonic airfoil data. However some very successful sections for which reams of data are available started as something drawn with a french curve with a little “TLAR�. During these long winter evenings, if you are interested, by all means study all the data you have available, and play with Xfoil. It will give you some insight into how changing the various parameters affect wing performance.

Full scale designers faced with providing maximum load carrying efficiency and range, and out maneuvering the enemy, do a good bit of such stuff. However when designing a model airplane for a fifteen minute flight consisting of yank and bank in circles around a small field, getting real “scientific� is really just a waste of time.

Oryx

The airfoils designed by members of our research group (the Selig, Gopalarathnam, Giguere etc. airfoils) and the ones designed by Eppler and I believe also Martin Hepperle are all designed using inverse design. This consists of selecting the boundary layer development for the design Reynolds numbers, pressure distribution, certain geometric and other aerodynamic constraints, and these are then all fed into special computer programs that find the geometry to match those requirements. In our case, the resulting airfoils are then tested in a wind tunnel to make sure that the performance of the real-world airfoil matches the predicted/designed performance. It is therefore a bit more involved than just choosing what the polar should look like - it is done at a slightly lower level than that. These days it is also becoming more common to design the "airfoils" not in isolation, but to do the whole inverse design process on the actual 3D surface, so that the final wing cross-section actually varies all the way along the span. This has become especially popular in recent years in the transport aircraft industry to design so-called shock-free wings that fly at high subsonic Mach numbers, or when shocks are present to control their location and strength to some degree.

That said, there are still aircraft designers that use existing airfoils for their applications (especially in the homebuilt industry), but it is becoming less and less common. It does sometimes happen that your application is very similar to another application, and therefore you can borrow the airfoil from the other application. As an example, F3B model gliders all have very similar requirements, so many of them also use the same airfoils.

The inversely designed airfoils are also not only used on aircraft - some of the applications that use these custom designed airfoils include propellers/wind turbines, racing yacht keels and rudders, formula 1 racing cars, etc.

Finally, I also agree with Paul that airfoil selection doesn't mean much if you cannot build the wing accurate enough. Many of the applications I listed above are very serious applications and these people are willing to spend a lot of money to manufacture wings with extreme accuracy. However, you will be surprised how accurate some modelers can get their wings. We have tested some built-up wings here in the UIUC wind tunnel with phenomenal accuracy (each airfoil is measured on a coordinate measuring machine to verify its shape before testing).

Regards,
Bennie

Mike James

Peter,

What are you up to? Are you preparing to build something new?

I've always enjoyed seeing and reading about your work in the past, and am curious about what's next for you...?

LouW

(Just a general comment) As an academic exercise, a lot of investigation into airfoils is interesting and certainly helps develop a general understanding of how varying the several parameters effect performance. However if planning to build an actual model airplane, the biggest payoff for the effort expended would likely to be techniques for reducing weight to the practical minimum. Even with endurance models where a maximum lift/drag provides the minimum glide angle, weight determines the glide speed, which in turn affects sink rate. For the typical R/C model whose flight profile consists mostly of maneuvering flight, very low drag is not necessarily desirable, but every gram of excess weight reduces turning radius and increases minimum speed.

ptxman

My Feedback: (2)

Thanks for the compliment. Hmmm.. lets see, I backed off active pattern designs & competition to accomodate my young family & take it easy a bit. I tried a bit of Club20 winter racing with the local boys just for kicks. Then last summer I flew my very first electric plane, a little 400 model. That turned into trying out a nice all-composite Russian F5D racer (called an Avionik D99), but on a mild motor setup. That turned into getting pushedinto getting the real fast hardware & hooked into attending the Canadian F5D Team Trials. Now (gulp!) it appears I will be attending the WC's in York this August as part of the Team. What have I DONE!!? So much for taking it easy. Actually, Ive really enjoyed racing & electrics are very fascinating. The rules are about to change the airframe specs, so Ive been thinking... These models are top calibre though, so any mold effort is at a pretty high level. We'll see....

ptxman

My Feedback: (2)

This is fascinating stuff. Thanks, exactly what I was looking for. Do you have any links to websites or similar that goes into more detail about this process? Are the design programs you speak of proprietory self-standing code, or are they front end feeders that rely on/ feed xfoil in some iterative manner, or? Im particularly interested in the 3d aspect as that would bring all the shape/form dimensions into play.

ptxman

My Feedback: (2)

Nice job. Nice airfoil too!