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Old Jul 21, 2011, 11:24 AM
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Airfoil Coefficient of Lift vs. Angle of Attack etc. Plots Books

Hi Folks:

I am looking for a site on line or in print where I can look at the wind tunnel results for airfoils such as a Clark y or the Dr. Seligís S3021. Yes Iím designing an electric powered airplane for a beginner or a person who needs a forgiving plane for easy flying.

Also, I know that airfoil polars compare the Cd vs. CL but I donít know the name for the plot that includes a sectionís CL & pitching moment vs. its angle of attack (α) at various Reynolds Numbers (Re). Please post the name in this thread.

Thanks.

Jim
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Old Jul 21, 2011, 08:28 PM
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Truly your best bet is to download and then pay for the unlocking codes for Profili 2. It comes with Xfoil bundled in but you can only access it with the unlock code. But it'll do all you want and then some for analyzing airfoils.

If you're doing a first trainer keep in mind that you actually do not want the slickest and most slippery airfoil. One that generates lots of drag at low angles of attack such as in a dive is actually a good thing. It avoids the model speeding up too quickly for your early flying skills.
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Old Jul 21, 2011, 08:52 PM
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XFLR5 can do more analysis tasks than Profili and it's free. Not quite as easy to use though
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Old Jul 22, 2011, 09:51 AM
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Originally Posted by BMatthews View Post
Truly your best bet is to download and then pay for the unlocking codes for Profili 2. It comes with Xfoil bundled in but you can only access it with the unlock code. But it'll do all you want and then some for analyzing airfoils.

If you're doing a first trainer keep in mind that you actually do not want the slickest and most slippery airfoil. One that generates lots of drag at low angles of attack such as in a dive is actually a good thing. It avoids the model speeding up too quickly for your early flying skills.
You can also get xfoil on its own for free...http://web.mit.edu/drela/Public/web/xfoil/

Just won't be quite as nice to use
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Old Jul 22, 2011, 10:58 AM
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Here is some of the Selig Wind Tunnel Data for the two that you mentioned.

The S3021 does not include tested data for Cm.
The Clark Y data does include Cm testing.
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Old Jul 22, 2011, 12:55 PM
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Originally Posted by zwtipp05 View Post
You can also get xfoil on its own for free...http://web.mit.edu/drela/Public/web/xfoil/

Just won't be quite as nice to use
Xfoil has a command line interface and doesn't do as much as XFLR5
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Old Jul 22, 2011, 02:09 PM
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I'd only add one thing, and that is that as a beginner, and one designing a trainer at that, are you going to build this in such a way that your wing will be 100% true to the section you select?

If its not 100% accurate, all those polar diagrams are somewhat irrelevant.

The more important factors here are basic design stuff, such as incidence, moment arms, areas, weights etc.
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Old Jul 22, 2011, 09:13 PM
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... as it would be if you are using an open bay multi spar style of construction with covering on the wing. Such an airfoil between the ribs has only a passing resemblance to what the airfoil at the rib is like.

And again since it is a trainer/sport flyer there just isn't the burning need to achieve an exact airfoil anyway. Instead something with a pleasant manner and a gentle stall is far more important than the slippery "go fast" performance of the 3021 you're asking about in the other thread. The Clark Y shape is a step in the right direction but even it can be more "slippery" than what you want on a trainer.

On any other style of model an airfoil which comes with a bit of a built in headwind would be a bad thing. On a trainer it aids in providing a more constant speed sort of model. And that gives newbie and low time pilots a lot more time to contemplate their next move. The needs of the new pilot turns what should be a failing into a feature.

By way of example here's an airfoil I used on a small high wing sport model some years back. Truth be known it was used on a rudder only .049 powered model to have some fun learning to fly "the old way". The airfoil is a basic TLAR shape with a high point slightly ahead of and thicker than a Clark Y. In use it turned out to be a real winner. A decent enough glide and a nice pre-stall mush which tends to lower the nose nicely and restore flying speed. Only if forced will it reach the point of a straight away stall. It builds speed in a dive and holds it well through a series of rudder only maneuvers. Maybe TOO well for a trainer. The structure was D tube back to the high point then open with cap strips back to a trailing edge.

It would be fun to stick this into Xfoil and see what the charts look like. I can supply DXF output to anyone that may want to try it.
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Old Jul 25, 2011, 06:47 PM
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Originally Posted by MCarlton View Post
I'd only add one thing, and that is that as a beginner, and one designing a trainer at that, are you going to build this in such a way that your wing will be 100% true to the section you select?

If its not 100% accurate, all those polar diagrams are somewhat irrelevant.

The more important factors here are basic design stuff, such as incidence, moment arms, areas, weights etc.
MCarlton:

Thanks for your comments. I have books by Howard Chevalier, Model Airplane Design and Performance for the Modeler, Andy Lennon's Basics of R/C Model Aircraft Design, Martin Simons' Model Aircraft Aerodynamics, and Carlos Reyes' Model Airplane Design Made Easy. The second two are currently lost in moving boxes some where. I am going to careful about that "basic design stuff" based on what I can glean from these books and RC Groups.

Jim
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Old Jul 25, 2011, 06:53 PM
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... as it would be if you are using an open bay multi spar style of construction with covering on the wing. Such an airfoil between the ribs has only a passing resemblance to what the airfoil at the rib is like.[...]
Thanks Bruce.

I have been thinking hard about making this wing a 'D' tube design for just this reason. Weight is an issue to this might make the airplane too heavy. I'll see.

Your airfoil sounds like what I'm after.

Jim
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Old Jul 27, 2011, 05:40 PM
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Here is some of the Selig Wind Tunnel Data for the two that you mentioned.

The S3021 does not include tested data for Cm.
The Clark Y data does include Cm testing.
The other thread HerkS mentioned is Airfoils Related to the S3021.

Jim
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Old Jul 28, 2011, 01:52 PM
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...Your airfoil sounds like what I'm after.

Jim
Maybe yes, maybe no. In my case it worked out nicely for the size and speed I'm flying. But is it the best? Not by a long shot.

My point is that sometimes and for some cases "close enough" is good enough. Look at how many successful RC tainers have been built over the many decades of RC flying. Few of them use actual tested and charted airfoils. And those that do tossed out the wind tunnel validity by using open structures with lots of covering sag in most cases.

A good airfoil for a trainer is one that has a nice soft mushy stall so that it gives the pilot lots of warning before letting go. It should also have a quick build up of drag so that the model doesn't tend to speed up a lot in a short time when it goes into a dive. A quick speed build up for most students equates to the controls becomeing extremely sensitive from the speed. And with that added sensitivity comes the likelyhood of overcontrol and a rather nasty recovery or even total disorientation due to the rapid response "yanking" the model's perspective out of the mind's eye of the student pilot.

This is likely why the most successful RC trainers have typically had some of the oddest looking and inefficient, by normal standards, airfoils of any type of model used in RC flying. Examples of this include the Carl Goldberg Eagle from many years back and the ever popular Telemaster and Senior Telemaster. These all used what I like to call "Florsheim" airfoils since they used a flat bottom with a top shape that looks like it came from tracing the outsole of the designers shoe.... Yet the needs of a good trainer turn the failings of such airfoils into desireable features.

Anyhow, it's some food for thought to further complicate your airfoil selection..
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Old Aug 09, 2011, 06:11 PM
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Bruce I would be interested in checking out you're airfoil
if you're still interested in posting a DXF file.

Dale.
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Old Aug 22, 2015, 02:03 AM
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I realize this thread is old, but if anyone comes across it looking for info,
i'm using the gottengen 398, and it has great characteristics for electric powered flight of long duration, (4+ hrs electric flight no thermals)with gentle stall and does not build speed too quickly. it's nice and thick, and glides better than it should. i love it, and it's simple to build.
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