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Old Mar 20, 2012, 06:08 AM
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I need aerofoils please

Hi all

Was wondering if you could direct me or give me some high cambered flat bottomed aerofoils both symmetrical and assymetrical.
They must be high cambered and with a high thickness to chord ratio.

It's no easy searching on the databases as they don't categorise the aerofoils so you can't search for something other than the name if the aerofoil.

Please advise thank you all

Best regards

Billy
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Old Mar 20, 2012, 06:17 AM
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Billy


You write:
Was wondering if you could direct me or give me some high cambered flat bottomed aerofoils both symmetrical and assymetrical.

Do you need that aerofoil for a keel of a sailboat? For " flat bottomed symmetrical " you need the airfoil of the keel of a "Spanker", a Dutch type of sailboat I owned in the past.


(Or, in other words, can you give us a little bit more information because there are too much to show. Examples picture 2 and 3!)



Cees
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Old Mar 20, 2012, 07:19 AM
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Hi Taurus I need aerofoils for RC-aircraft of about 0.8m wingspan

Thank you
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Old Mar 20, 2012, 07:30 AM
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It's probably just my ignorance but I'm having real trouble working out how "high camber", "flat bottomed" and "symmetrical" fit together.

All the symmetrical aerofoils I know have zero camber. Isn't that part of the definition of symmetrical ? And surely if something is both flat-bottomed and symmetrical that means it's a plain rectangle, rarely a good shape for an aerofoil ?

Steve
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Old Mar 20, 2012, 07:47 AM
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Oh what I meant was if someone could provide me aerofoil names of various aerofoils with high thickness to chord ratios including well known aerofoils and not so well known.
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Old Mar 20, 2012, 09:04 AM
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Billy hi,

What has the plane to do for you, pattern, cargolifter, sport.
Why high camber, flat bottom, thick etc.

I do fly a Taurus, no symmetrical, not flat bottom but the wings are 19 % thick and these days I am also contructing a Simla 20 % thick symmetrical. These are pattern planes of the period 1961 - 1965 designed by Ed Kazmirski, THE pattern pilot of that period of the USA.
These thick wings were used to reduce the speed in pattern flying which can be a reason for "thick" so "high thickness to chord ratios".

Keep in mind there are thousands of airfoils, for example have a look:
http://www.worldofkrauss.com/foils/search

Type for : Maximum number to show: >>>> "ALL" for example!!


Cees
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Old Mar 20, 2012, 12:08 PM
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Well I have a wingspan limit of 0.8m & an already provided single prop motor of 107g (~650g thrust) and a ogival delta wing configuration (not flying wing! Just delta winged Wing configuration ... Think of a concorde with a prop at the front, with elevator and rudder) so I need an aerofoil to suit it.

It's been suggested to get a high thickness to chord ratioed-flat bottom aerofoil I.e. Clark-Y, I just needed other suggestions (not experimental ones like the KF set but ones with a lot of theoretical data).

I need my aircraft to do simple 360 circuits whilst abiding by the two factors of 107g motor and 0.8m wingspan.

I just need some suggestions of various types of aerofoils that are low cambered, high cambered, thin, thick, flat bottom, non-flat bottom ... So if anyone can provide me with suggestions for these 6 types of aerofoils then it's help so much!
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Old Mar 20, 2012, 02:32 PM
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You're still leaving out a lot of info that would aid us in better helping you. But here goes

For symetrical you can use the NACA 00xx series in whatever thickness you prefer. The "00xx" refers to 0% camber, 0% where the camber occurs and the xx denotes the thickness in % of the chord. Commonly used versions for models are 0010, 0012 and 0015. But any thickness can be used. If you google for "naca airfoil generator" you should find an online or downloadable utility which will draw these for you.

You already know of the Clark Y. Just don't get into the idea that ALL FLAT BOTTOM airfoils are "Clark Y's". There are others which share the flat bottom shape but perform much differently.

Two high camber airfoils used on a lot of models are the RAF32 and the NACA 6409. The Eppler 372 was also used on some older model gliders.

Keep in mind that if you're using a wide chord delta like planform that the higher the camber value the stronger the pitching moment of the airfoil. Wide chords require big tail areas or long tail moment arms to deal with a "pitchy" airfoil and make the model stable. When you describe your design as a "Concorde with a tail" I think you'll find that what you need is going to be thin and symetrical. Using much in the way of an airfoil with a positive camber, let alone the highly cambered options like the RAF32 or 6409, will result in the tail not being able to stabilize the wing with any reasonable CG location. Using the wrong airfoil could easily result in a totally non flyable model where using the right one could produce a real winner.
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Old Mar 24, 2012, 03:37 PM
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Hi again all,

Sorry Bruce what other information did you need, I literally included my design specification in my previous post my friend.

1) What other flat-bottomed airfoils are there apart from the Clark-y (so I can compare them as you said their performances were different)
2) You had said that the highly cambered options like the RAF32 etc would not be able to stabilise But does this mean that the Clark-Y will have a strong enough pitching moment to destabilise my design?
3) At what camber thickness does this pitching moment become strong enough to destabilise the design on a delta?
4) Would you please provide some sort of reference (books/online studies) regarding how high cambered options result in tail unable to stabilise the wing? Or are there theoretical calculations I can do to determine the best airfoil for my design - think of it as a sort of airfoil tradeoff

I have so far looked at various CL's at a specifies AOA of 4 degrees but the CL values from these airfoils don't mean much to me apart from the fact that I know one lifts more than the other (the higher the CL the greater the lift) but this just doesn't seem enough in terms of proving why one airfoil is better than the other in terms of theory/calculations

Please advise because it's giving me many sleepless nights and getting me Ill from the stress of all this stuff, nevertheless I am still interested in it just need your guidance because you have given very amazing advice in the past too!

Thank you so much I look forward to hearing your advise, oh and I'm sorry to be a bother :-/
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Old Mar 24, 2012, 04:01 PM
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Billy sent me the following questions in a PM. But they are really so general or aimed at this thread topic that I've taken the liberty of posting them here so we can all help out;

Quote:
Originally Posted by Billy88
1) What other flat-bottomed airfoils are there apart from the Clark-y (so I can compare them as you said their performances were different)
2) You had said that the highly cambered options like the RAF32 etc would not be able to stabilise But does this mean that the Clark-Y will have a strong enough pitching moment to destabilise my design?
3) At what camber thickness does this pitching moment become strong enough to destabilise the design on a delta?
4) Would you please provide some sort of reference (books/online studies) regarding how high cambered options result in tail unable to stabilise the wing? Or are there theoretical calculations I can do to determine the best airfoil for my design - think of it as a sort of airfoil tradeoff

I have so far looked at various CL's at a specifies AOA of 4 degrees but the CL values from these airfoils don't mean much to me apart from the fact that I know one lifts more than the other (the higher the CL the greater the lift) but this just doesn't seem enough in terms of proving why one airfoil is better than the other in terms of theory/calculations
Let's clarify what I think you're looking at for a planform before we go much further. When you say "Concorde with a tail" I think of something like the Boeing SST contender shown here;



If I'm way off you'll need to say one way or the other or show a picture of what you're considering building.

1) Flat bottom variations are found in a lot of model designs. But not many of them have proper coordinates since they are sketched freehand by the designers. A couple which do come with "brand names" are the NACA 2412 I believe it is that has so slight a lower side curve that it is realistically a flat bottom shape. The free flight world is full of flat bottom/curved top shapes. Some of them have been coordinate'ized and show up in the airfoil library of such programs as Profili II ( www.profili2.com ) or in the airfoil collection of Michel Selig which you can find at
http://www.ae.illinois.edu/m-selig/a..._database.html
In the part below the first area most of the listed coordinate file names also have a .gif image file.

Some notable flat or near flat bottom options are the AG35, Eppler 205, Gottingen goe795, and Selig SD3021. But like I mention, there's been literally many hundreds of flat bottom airfoils simply drawn by the modeler themself based on past experience which work decently but which do not have any coordinate tables avalable.

2)ANY of the airfoils with a moderately strong negative pitching moment will prove to be unstable on a "Concorde with tail" style of look. The Clark Y most certainly is one of those. This is because of the very wide wing chord you have with a Delta wing shape where the aspect ratio is well under 1. The aspect ratio being the ratio of the wing span to the Mean Aerodynamic Chord. The MAC being roughly the same as the geometric average for the wing's chord.

We have online "CG calculators" (google for the quoted words to find them) which you fill in a bunch of measurements and then the calculator tells you where the overall neutral point is located and where the balance needs to be to fly in a stable manner. But these calculators do not include a spot to fill in the airfoil's pitching moment. For most conventional designs this isn't a big deal. Or we allow for it by simply increasing the desired amount of % of stability. But on something as extreme as a "Concorde with a tail" you're going to find that it's not going to work.

You can play with this using a small sheet balsa or sheet foam test glider model. Make three of them using your design plan. The first will have a flat sheet wing with no camber at all. The second will have a negative pitching moment style simple arched camber for the wing with roughly a 3% camber value. The third will have a positive pitching moment S shaped camber airfoil with around 3% camber in the forward half of the airfoil and a reflex shape in the rear of the airfoil.

Go play with the balance and tail angle for each to find where they balance for a stable glide that isn't TOO stable. You'll find that the flat plate version will balance and fly well with the balance point located back somewhere near 25% of the MAC. The simple cambered version will balance well forward of this. But even then when thrown hard you'll likely find that it wants to nose down instead of up. This being due to the strong pitching moment overcoming the relatively small stabilizer.

3)What happens is that the tail volume coefficient is simply not high enough to deal with the speed related change in the wing's pitching moment with extreme examples. And they don't come much more extreme than the Boeing SST style or a Concorde with a tail.

The problem is that for our models we simply do not include a spot for the wing airfoil pitching moment in the usual calculations. This is because for MOST conventional layouts the pitching moment is a small concern. But when we shift to designs with very low aspect ratios or with very small tail sections on very short tail lengths this becomes far more important. There likely is some rather fancy books that take ALL the factors, the pitching momemt included, into accout. But frankly at that level of math my mind blanks out and I decide to go cut the grass. What I do to get around this and still make it work is I treat this style of design more as a flying wing that happens to have a small tail. And a "flying wing with small tail" only works with airfoils that have a neutral or positive pitching airfoil.

Now a delta wing is a little different. It's sort of a swept wing that happens to have a radical taper ratio. So with an airfoil which has a smaller camber value it could be possible to still fly well despite the small negative pitching moment IF you severly washout the wing towards the tips by something like 5 to 6 degrees leading edge down at the tips. Adding this span wise reflex to the planform would take a lot of the work load off the small tail mounted on the short tail moment arm. In effect the stabilizer would become more of a control and less of an actual stabilizing surface. Depending on the airfoil chosen and the amount of wing washout twist you could well end up not even needing the tail while still using a cambered airfoil.

How much camber can you get away with? Not a lot. If you study the airfoil performance charts of a wide variety of airfoils you'll see that the pitching moment is closely related to the amount of camber. The more the camber the more the pitching moment. Airfoils with very high maximum lift coefficients come with strong pitching moments. You won't be able to easiy stabilize such an airfoil on a Boeing SST style planform. But something like the low pitching moment airfoils from Martin Hepperle intended for use on swept flying wings such as the MH64 from Martin Hepperle should be reasonably practical to stabilize well on a delta wing tail using a combination of some wing washout and a decently sized and spaced back tail. With a ClarkY not so much unless the wing twist is extreme and the tail size and moment arm somewhat extreme as well. With an RAF32 or NACA 6409 a snowball in hell would have more chance of a long life.

4)As I mentioned my mind glazes over when I look at deeper math with lots of squiggles in the equations... So I'll leave it to the more serious folks to suggest those sort of texts on stability.

In the meantime I would highly recomend that you experiment with small test gliders to show you how all this works similar to what I suggested above. The math is all wonderful but you can learn a lot from a hour at the bench and some time in the garden watching and testing how the stuff actually flies.

For example. Make up a simple plank style flying wing with a cambered airfoil of around 3% arc. Try to make it trim out right side up. You won't be able to. It just won't happen. Now flip the model upside down and try again. You'll find that it flies in a very nice and stable manner with very little trouble. This is what I mean by the idea that our stability calculators really need to take the pitching moment into account for the more extreme designs. At some point the pitching moment becomes far more significant and MUST be included.

If you look up the stability factors for flying wings as written by Walter Panknin the airfoil's pitching moment is a big factor as it well should be. You can read about this at http://www.b2streamlines.com/Panknin.html
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