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Old May 27, 2011, 01:38 PM
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Higher Angle Of Attack Symmetric airfoil vs. High Camber airfoil wing

Hi guys,

Can someone explain the difference in flying a Higher Angle Of Attack Symmetric wing airfoil vs. high cambered (high lift) airfoil?

What are the benefits of a high cambered airfoil?

Z
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Old May 27, 2011, 04:37 PM
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For a given wing chord most airfoils at model size will stall at roughly the same angle. Using a thick, but not TOO thick, airfoil can delay the stall to a slightly higher angle. But we're only talking maybe a degree.

Adding camber to the airfoil makes it more able to bend the air more effectively. So it can reach a higher lift coefficient for the same angle of attack. And in fact such an airfoil is still producing lift with a zero degree angle of attack.

This is why cambered airfoils are used on models that are designed to lift heavier loads or where the flyer wants to enhance the slow speed flight end of the performance envelope.
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Old May 27, 2011, 04:49 PM
Grad student in aeronautics
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You can also think of it this way:
Deflecting a flap will shift the lift curve up - for a given angle of attack, you will get more lift (generally). Similarly, adding camber to an airfoil will do the same. Now note that a symmetric airfoil with a deflected flap is actually a cambered airfoil (be careful to correctly define the chord).
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Old May 27, 2011, 05:06 PM
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Thank you for the inputs, i am willing to build a custom aircraft myself, but cannot decide the airfoil shape.

So how do they compare for Gliding Distance for example?
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Old May 27, 2011, 05:15 PM
greg
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Quote:
Originally Posted by BMatthews View Post
This is why cambered airfoils are used on models that are designed to lift heavier loads or where the flyer wants to enhance the slow speed flight end of the performance envelope.
wouldn't drag be important? Can't you always increase the lift curve during take offs and landing, as DPATE said, using flaps.

I'm not sure, but I'm guessing that the minimum drag for a symmetrical airfoil is a an angle of zero, but it is at some non-zero angle for cambered airfoils. So for a heavy load wouldn't you choose a cambered airfoil because of its minimal drag at some positive AOA that you could choose either by airfoil shape or wing area?
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Old May 27, 2011, 05:47 PM
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Camber is used to set the coefficient of lift for minimum drag. It would seem reasonable to assume that symmetrical airfoils have their minimum drag at zero lift but that's not necessarily the case. I know that I've seen a few polars of symmetrical airfoil sections that showed a slight drag rise at zero lift (probably something to do with a bubble on both surfaces). Anyway DPATE and ciurpita wrote their replies while I was making this polar. As you can see all other things being equal as you increase camber the CL for minimum drag also goes up. This also shows why small flap deflections (less than 10 or 15 degrees) increase the lift to drag ratio. To find the CL of best lift to drag on the cl/cd polar you draw a line from the origin tangent to the curve.

--Norm
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Old May 27, 2011, 07:18 PM
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Yep, it's bubbles, This polar just shows how the curves smooth out with increasing Re but turbulators on both sides smooth it too but trips also increase drag

--Norm
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Old May 28, 2011, 01:19 PM
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Quote:
Originally Posted by ciurpita View Post
wouldn't drag be important? Can't you always increase the lift curve during take offs and landing, as DPATE said, using flaps.

I'm not sure, but I'm guessing that the minimum drag for a symmetrical airfoil is a an angle of zero, but it is at some non-zero angle for cambered airfoils. So for a heavy load wouldn't you choose a cambered airfoil because of its minimal drag at some positive AOA that you could choose either by airfoil shape or wing area?
He asked a simple question so I tried to keep the answer simple. But as you and Norm are pointing out the real world answer is anything but simple.

Z-matrix, as the later answers are indicating there's a LOT to designing and picking airfoils. On the other hand most of our models can get away with and perform well with the good ol' standby, the Clark Y.

The only time you need to look at a lot of other options is when you're doing a specific contest model intended to deliver every last fraction of a % of performance. But from the nature of your questions I'd respectfully say that you're a LONG way from that point on a number of fronts. Realistically you are not going to design and build a world beater on any of your first half dozen efforts.

In the meantime take some time to learn on your own the same way that many of us did. Study the designs of others that have built successful models of the same type and try to learn why they picked and shaped the way they did. And some healthy plagarism from those designs for your first efforts doesn't hurt either.
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Old May 28, 2011, 02:50 PM
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Quote:
Originally Posted by BMatthews View Post
He asked a simple question so I tried to keep the answer simple. But as you and Norm are pointing out the real world answer is anything but simple.

Z-matrix, as the later answers are indicating there's a LOT to designing and picking airfoils. On the other hand most of our models can get away with and perform well with the good ol' standby, the Clark Y.

The only time you need to look at a lot of other options is when you're doing a specific contest model intended to deliver every last fraction of a % of performance. But from the nature of your questions I'd respectfully say that you're a LONG way from that point on a number of fronts. Realistically you are not going to design and build a world beater on any of your first half dozen efforts.

In the meantime take some time to learn on your own the same way that many of us did. Study the designs of others that have built successful models of the same type and try to learn why they picked and shaped the way they did. And some healthy plagarism from those designs for your first efforts doesn't hurt either.
The goal is the best gliding efficiency, lowest drag, with a BLDC motor and LIthium batteries.
i would like to get gliding distance >17.

I will probably end up building more prototypes to get wing loading and airfoil type, i don't expect it to be perfect at first.

i already knew high cambered airfoil gives greater lift at 0 angle of incidence vs symmetric airfoil.
And so a symmetric airfoil needs to have a larger angle of incidence to allow the fuselage to be "flat" to the airflow in normal cruising flight.

My question still would be what is the difference in efficiency and maneuverability.
Will a symmetric or high cambered airfoil take me greater distance with same mAh?

Haven't analysed the graphs Norm gave me yet, it may have the answer i will get back to that later.

Z
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Old May 28, 2011, 04:14 PM
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For greatest distance you would want neither a symmetrical or a highly cambered airfoil. A slightly or moderately cambered airfoil (about 2 -4%) would likely be the best if distance is the objective... You would be looking for an airfoil with good lift/drag figures.
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Old May 28, 2011, 05:45 PM
greg
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have you looked at any of Mark Drela's designs at http://www.charlesriverrc.org/articles.htm

he doesn't just design/select an airfoil for a particular aircraft, but for each span position considering the Reynold Number and goals of the aircraft (thermalling, hand-lauch)
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Old May 29, 2011, 02:33 AM
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Originally Posted by JetPlaneFlyer View Post
For greatest distance you would want neither a symmetrical or a highly cambered airfoil. A slightly or moderately cambered airfoil (about 2 -4%) would likely be the best if distance is the objective... You would be looking for an airfoil with good lift/drag figures.
Exactly. Just like the kid's story the best porridge is the one that is neither too hot nor too cold. Your two options of symetrical or high camber are like the bear's porridges being superheated and at somewhere down near absolute zero.

Modern competition sailplanes are even opting for airfoils with camber values from 1.5 to 2%. Anything more than that seems to be seen as "floaters" these days. There's no doubt that the focus has changed from low sink rate to best L/D and the ability to use this to cover a lot of ground in a short time in an efficient manner.

Which suits you the best will depend on how fast you need to cover ground and under what sort of conditions it needs to be done and what you're going to use for power.
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Old May 29, 2011, 06:09 PM
G_T
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I would suggest doing a little more reading and studying before designing your first wing. Chances are the results will be more to your liking that way. I'll provide a few starting points here, and I apologize that they are to threads that are either mine or are related to some of my work. The details in these threads are not all that important; what is worth considering is the methodology, the tools, the thought process, the analysis...

http://www.rcgroups.com/forums/showthread.php?t=1110049
http://www.rcgroups.com/forums/showthread.php?t=1378637

http://www.rcgroups.com/forums/showthread.php?t=1366021 - where I helped someone go through development of their own plane.

Gerald
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Old May 29, 2011, 09:45 PM
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Thank you for the replies,

So what i'm looking for is a conservatively cambered airfoil with optimal wing loading, thickness, and wingspan, that has the least drag capable of producing required lift at x speed

something like NACA 4312

Z
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Old May 29, 2011, 10:02 PM
G_T
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No, still not quite that simple. One can design a wing and then find it to be completely unflyable by a mortal in real life atmospheric conditions.

First study up a bit on how to read polars and various ways to analyze the data.

Then get some practice in with an analysis tool, such as XFLR5 or Profili, that can generate good results for low Reynolds numbers wings.

Then enumerate your design goals - make a list.

Then determine your limitations and expectations. Span limit? Weight limit? Powertrain options? Desired airspeeds? Control characteristics; maneuvering characteristics?

Then get busy. Create a template wing, using a single airfoil family, and get it somewhat optimized. See how it does for meeting your requirements and goals. This then becomes your first benchmark wing.

Then take that template wing, and substitute a different airfoil family. See if it does better.

Iterate on this until you have chosen the foil family you want (and if none of those, then you have by now some idea of what you want the series to do that you will have to design).

Now that you have chosen a foil family for the wing, go back and spend some more time really optimizing that wing design.

Then design the rest of the plane to support that wing. This includes tail design, etc.

Expect to take a half a year the first time around to produce a decent design. There is quite a bit of a learning curve. That is before anything at all has been done with actually creating the plane.

If one doesn't go through a procedure such as this, then one ends up designing a plane that ends up being in the "that looks about right" school of "engineering". But without the experience to know what looks about right, odds are greatly in the direction that it will not perform as desired.

Translation of all the above - if you want performance, engineer the design as a serious effort. If you just want to play at it, that is fine too! Nothing wrong with that! This is a hobby! Just don't set your hopes high. Others are putting in the effort.

Gerald
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