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Old Jan 02, 2014, 10:56 PM
Su-27's Rule!
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symmetrical airfoil glider

ok i know there are some threads concerning this but i can't find the info i need,
now is a symmetrical airfoil with slight camber only at the ~1/4 of the TE work as a glider airfoil? thanks
Edit:
very similar to naca 2713 (tip) naca 1711 (root) but thickest point is 1/4 in from the LE
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Old Jan 03, 2014, 09:42 AM
Herk
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It's important to realize that an aircraft design doesn't start with an airfoil.

It's the purpose of the model - it's mission - that begins the process. Then other factors begin to come into consideration.

A symmetrical airfoil would be fine for an acrobatic glider designed for slope soaring. It's probably not the best airfoil choice for a high performance thermal sailplane. -- I hope you see what I mean.
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Old Jan 03, 2014, 12:05 PM
B for Bruce
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There are so many good glider airfoils available already I can't see why you want to try making a new one. Particularly something as odd as a shape with the thickest portion only 25% back.

If you insist on doing this the trick is to prove it by testing it against some other known good airfoils and see if you gain anything in the results. Programs like Xfoil and XFLR5 can analyze and produce performance graphs for most more or less smooth airfoil shapes.

You don't say what sort of glider this will be. A sloper or a thermal style. Or what you're considering making the wing from and why it has to have the oddball shape that you're considering.
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Old Jan 03, 2014, 02:05 PM
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Having seen stock Skeeters thermal out --- I decided to just go for a shape which was streamlined a bit for appearances sake - yet strong enough to hold it's shape-- works for me ---
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Old Jan 03, 2014, 03:08 PM
Su-27's Rule!
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haha ok sorry.. well let me explain i was looking at the ag15 airfoil and tried to make it out of foam board well the airfoil turned in to the ugly airfoils... so i was wondering how bad would the glide slope with the airfoils described on top..
also wondering if adding camber would make it have a better glide slope..
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Old Jan 03, 2014, 05:11 PM
B for Bruce
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If you look around at the various airfoils out there you'll see all manner of shapes. Some are very basic and do not do a lot more than a simple cambered flat plate while others are carefully shaped to gain the slimmest advantage.

The RG family certainly fits into the later category. So it doesn't matter what you started with if you ended up with an oddball then it's YOUR airfoil with no link to the RG inspiration.

With that out of the way it's actually quite surprising what you can get out of a "sort of" shape as long as you're not too demanding. Things you need to watch for are the leading edge radius and how the shape extends back from that. A nicely curved upper leading edge can, if not taken to extremes, act a little like some leading edge droop and promote higher angles of attack before a stall. Similarly a little "flap like" trailing edge curvature can extend the upper limit of the lift before the stall occurs.

But this is all very much a case of being "a little". If you go to great extremes and your airfoil does not look even remotely like anything with a known good history then likely you didn't just invent a new world beating airfoil.

As for YOUR airfoil? Again you are not giving us anything to go by. What you see and what we imagine from your description are going to be world's apart. It would be best if you can take an end on picture that shows the deformed airfoil and let us ponder if it is at all airworthy.

Adding camber to the airfoil will, all else being equal, allow it to reach a higher lift coefficient for a given angle of attack. Which means it should be able to lift harder right before the stall than the same airfoil with less camber.

As for it helping the glide slope? Well, that depends. If going for maximum duration adding camber will slow down the decent but typically steepens the glide slope. If going for the flattest possible ground eating glide or best level speed for the least power then a moderate camber that promotes a low drag value for the angle of attack for that speed is the way to go.

First thing is to feed us more info. What sort of size are you considering and what sort of performance do you reasonably expect and what conditions will it be flown in?

For example I see you posted in that Foamite thread of the guy that made the compact electic glider. Is that what you're considering? But maybe with a size increase?

Give us pictures and more info. Otherwise it's like you asking "how long is a piece of string?".
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Old Jan 03, 2014, 05:15 PM
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Actually it is possible to do very simple wing from foamboard which outperforms ag airfoils for some range of lift coefficient. Below see the image of 1-dollar foil from the readyboard from dollarama I make by just doing some cuts and breaks and very primitive sanding of leading and trailing edge. I have designed it to operate optimally in the range of lift coefficient 0.7-0.9. The breaks in the profile could seem not to alter performance for reynolds number less than 60K.

Covering this readyboard wing with thick paper with thin shiny covering from dollarama makes it very stiff (comparable with one covered by 2oz/m^2 fiberglass).
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Old Jan 03, 2014, 09:06 PM
B for Bruce
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Yep, that would be a good shape and roughly what I would sketch and suggest myself for a slower sort of park flyer style model.

What it lacks over the actual RG15 is the ability to work well at high speed. Looking at the Cl vs Cd chart it's pretty clear that if you put the nose down to try to punch out at higher speeds that it'll be draggy. But for a model intended to be flown at Gentle Lady like speeds that kinked flat board airfoil has much to recommend it.
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Old Jan 03, 2014, 09:57 PM
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Quote:
Originally Posted by BMatthews View Post
Yep, that would be a good shape and roughly what I would sketch and suggest myself for a slower sort of park flyer style model.

What it lacks over the actual RG15 is the ability to work well at high speed. Looking at the Cl vs Cd chart it's pretty clear that if you put the nose down to try to punch out at higher speeds that it'll be draggy. But for a model intended to be flown at Gentle Lady like speeds that kinked flat board airfoil has much to recommend it.
Yes. Also sometimes it is convenient to have higher drag at bigger speeds: it eliminates needs for spoilers. It is not good for discus launh gliders, but as you mentioned, it is ok for thermal duration or to use thermal glider on slope.

Actually, I have compared characteristics of wings with aspect ratio 10 with the above airfoil and with ag airfoils. This one shows better sink rate than ag, but ag shows about 5-10% better max glide rates (this is of course due to higher speed performance).
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Old Jan 04, 2014, 02:51 AM
Su-27's Rule!
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Quote:
Originally Posted by BMatthews View Post
If you look around at the various airfoils out there you'll see all manner of shapes. Some are very basic and do not do a lot more than a simple cambered flat plate while others are carefully shaped to gain the slimmest advantage.

The RG family certainly fits into the later category. So it doesn't matter what you started with if you ended up with an oddball then it's YOUR airfoil with no link to the RG inspiration.

With that out of the way it's actually quite surprising what you can get out of a "sort of" shape as long as you're not too demanding. Things you need to watch for are the leading edge radius and how the shape extends back from that. A nicely curved upper leading edge can, if not taken to extremes, act a little like some leading edge droop and promote higher angles of attack before a stall. Similarly a little "flap like" trailing edge curvature can extend the upper limit of the lift before the stall occurs.

But this is all very much a case of being "a little". If you go to great extremes and your airfoil does not look even remotely like anything with a known good history then likely you didn't just invent a new world beating airfoil.

As for YOUR airfoil? Again you are not giving us anything to go by. What you see and what we imagine from your description are going to be world's apart. It would be best if you can take an end on picture that shows the deformed airfoil and let us ponder if it is at all airworthy.

Adding camber to the airfoil will, all else being equal, allow it to reach a higher lift coefficient for a given angle of attack. Which means it should be able to lift harder right before the stall than the same airfoil with less camber.

As for it helping the glide slope? Well, that depends. If going for maximum duration adding camber will slow down the decent but typically steepens the glide slope. If going for the flattest possible ground eating glide or best level speed for the least power then a moderate camber that promotes a low drag value for the angle of attack for that speed is the way to go.

First thing is to feed us more info. What sort of size are you considering and what sort of performance do you reasonably expect and what conditions will it be flown in?

For example I see you posted in that Foamite thread of the guy that made the compact electic glider. Is that what you're considering? But maybe with a size increase?

Give us pictures and more info. Otherwise it's like you asking "how long is a piece of string?".
very well explained.. i'll just ditch "my airfoil" and build a new wing what i'm looking for is like a hotliner airfoil capable of fast speeds but also good at just gliding.. and if possible made out of foamboard.. thanks!
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Old Jan 12, 2014, 12:51 AM
G_T
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When looking at performance for a glider wing, don't be mislead by Type-I (fixed Reynolds number) analysis. One wants to do Type-II (fixed lift) analysis. If one doesn't, then it is extremely easy to believe performance improvement somewhere that the plane simply will not be flying.

Gerald
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Old Jan 12, 2014, 02:27 AM
B for Bruce
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Well, mostly the capable of fast speeds calls for fairly thin with a clean small radius leading edge and not much camber so it can still have low drag at the very low lift Cl's you'll be using at higher speeds. And some ballast helps to increase the wing loading to aid in getting the speed up there.

But good at just generally gliding around means slow to moderate speeds that you get from having a little more camber and a lighter wing loading.

And just like Goldilocks and that darn porridge you'll likely find that you're happy somewhere in the middle.

That being said if you build a clean airfoil that is between 8 to 11% thick and has around 1 to 1.5% camber along the mean line between the upper and lower surfaces and a fairly sharpish leading edge with nice progressive curves then you'll likely get what you want. It won't be perfect but as long as it looks fairly close to a few of the big "brand" names it'll likely do fine. Then build it so it's a little lighter than you want for good high speeds. Because you can add weight to get to where you're at the happy place where it flies fast enough and slows down enough. But if it comes out heavy it'll fly fast with no issue. But it's hard to find places to glue in the blocks of "lightness".
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Old Jan 12, 2014, 03:06 AM
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Originally Posted by BMatthews View Post
Well, mostly the capable of fast speeds calls for fairly thin with a clean small radius leading edge and not much camber so it can still have low drag at the very low lift Cl's you'll be using at higher speeds. And some ballast helps to increase the wing loading to aid in getting the speed up there.

But good at just generally gliding around means slow to moderate speeds that you get from having a little more camber and a lighter wing loading.

And just like Goldilocks and that darn porridge you'll likely find that you're happy somewhere in the middle.

That being said if you build a clean airfoil that is between 8 to 11% thick and has around 1 to 1.5% camber along the mean line between the upper and lower surfaces and a fairly sharpish leading edge with nice progressive curves then you'll likely get what you want. It won't be perfect but as long as it looks fairly close to a few of the big "brand" names it'll likely do fine. Then build it so it's a little lighter than you want for good high speeds. Because you can add weight to get to where you're at the happy place where it flies fast enough and slows down enough. But if it comes out heavy it'll fly fast with no issue. But it's hard to find places to glue in the blocks of "lightness".
Do you mean this to me or gt? Or both of us?
Quote:
But it's hard to find places to glue in the blocks of "lightness"
Wish i had some of those...
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Old Jan 12, 2014, 08:35 AM
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I think it is very important to remember that flying is all about managing energy. How you fly is going to change the usefulness of your aircraft.
something with a lot of camber & a bigger aspect ratio is not meant to fly fast, but you can achieve some serious lift! on the other hand if you streamline and go with something fast don't expect it to be a nice slow glider. as soon as your speed energy is gone its going to stall.
Remember most gliders have a huge aspect ratio and a pretty streamlined airfoil. Basically what ever you give up as far as width you are going to have to make up for in length.
Anywho i know that was all super basic but i think when you are designing its easy to lose sight of what is functional and go with what is cool.
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Old Jan 12, 2014, 01:33 PM
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Quote:
Originally Posted by G_T View Post
When looking at performance for a glider wing, don't be mislead by Type-I (fixed Reynolds number) analysis. One wants to do Type-II (fixed lift) analysis. If one doesn't, then it is extremely easy to believe performance improvement somewhere that the plane simply will not be flying.

Gerald
Do not exaggerate: For software which gives 10-50% error for drag estimate there will not be significant difference in fixed speed and fixed lift analysys in vicinity of projected speed.
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