ACHIEVING THE BEST REFLEXED AIRFOILS FOR FLYING WING USE IN THE SMALL PLANE CATEGORIES

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WINGITIS

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Jun 24, 2020
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138
Location
Wellington, New Zealand
Hello folks

After starting these discussions within the thread:

Flying wing as cheap and simple option for basic fun flying

We were taking up to much space discussing Airfoils and associated things so the intention is to bring the conversation here.

I will bring across some of the topics and data we have already discussed in the next day or two.

The premise is, can we gather the best of the FW airfoils(FOR RIGID WINGS) into one place where they can be tested against a uniform set of criteria for comparison and this may drive two outcomes:

1: Others may see the FW Reflexed airfoils that are available so they may perhaps choose one that best fits their application

2: Work can be done to perhaps progress to a better generation of FW airfoils as we get into the 100 year interval since the Horten and Munk M6 type of airfoils were promulgated.....

The idea being that XFLR5(ALTHOUGH NOT PERFECT) is used as the common test tool.

NOTE: You can use PNG2PDF on their website to take XFLR5 PRTSCREENS and combine them into PDF's to provide better quality viewing on this board.

Please provide the CL, CM, CD and LD images in that order in the PDF's.

Limit the number of airfoils tested in each series to 10, it can get messy after that.....

The base settings(WE HAVE TO HAVE SOME) to test airfoils against for comparison purposes are currently:

ANALYSIS = TYPE1
MACH = 0.1
AOA = -5 to +20
RN = 3,000,000
TE = 0.2 UNLESS IT IS ALREADY LARGER (SO NO POINTY TAILS YOU CANNOT BUILD THEM ANYWAY)
NCRIT = 9
TRIP LOCATIONS = CURRENTLY UNDER DISCUSSION

For your own purposes you can use what you want but please use these settings when posting results in here.



JUST A THOUGHT, what say you?

Regards,

Kevin Gill
Wellington
New Zealand

WARNING: YOU MUST NOT USE ANY OF THESE AIRFOILS ON ANY HUMAN CARRYING AIRCRAFT WITHOUT ENGAGING APPROPRIATE DESIGNERS AND ANALYSIS TOOLS TO COMPLETELY VALIDATE REAL WORLD USAGE(THIS MEANS ALL RESPONSIBILITY AND LIABILITY IS ON YOU AS INDIVIDUALS)

The comparison of airfoils with base known ones that are well documented in previously issued NACA papers does allow for conclusions to be drawn against other airfoils created but this is an academic activity as far as this thread is concerned and airfoils should only be tested on non-human carrying models, which can be largish ones if required using the available autonomous technologies in a regulated manner taking into account the laws of your country.

For people not familiar with the Reynolds Number and how it is derived and applies to your airfoil and usage I have attached this simple self explanatory calculator:
 

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Bille Floyd

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Sep 26, 2019
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242
Got into flying Hang gliders in 1977 ; flew Rigid wing since 1980.
With the rigids, (at the speed range i'm use to, 20 to 58) i noticed
that Twist works Really well in the lower speed range, and really
sucks at higher speeds ; but reflex works Quite well for the higher
speeds , and really sucks in the low end.

With my current rigid,-wing , the twist gets adjusted when i hit my
1/3 span flap ; but i have no control over my reflex at the tips, and
that's gonna get changed with some sort of mechanism, when i
place my engine on it.

To optimize performance , speed and stability ; both reflex
and twist, should and can be manipulated. Shouldn't be much
argument on that one ?

Bille
 

Vigilant1

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Kevin,
I'll be interested in the discussion. "Small aircraft" covers a lot of ground with a lot of competing attributes, so you may want to get specific pretty soon or your discussion could get very jumbled. Are we talking only about rigid wings, or the "best" airfoil for a rag-and-tube small aircraft? Assumed laminar flow or not? What typical Cl's /Cl ranges are we focusing on? etc.

I was intrigued by the stated desire to look at reflexed airfoils--any reason to immediately go there? I'm used to seeing them in tail-less designs, primarily. Tailless designs are interesting, but a small proportion of small airplanes.
 

pictsidhe

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North Carolina
XFLR5 is not ideal for airfoil development. It doesn't show the hysteresis at stall. The tendency for a wing to stay stalled until AOA is reduced several degrees. Some of the higher lift airfoils in exhibit this trait. It can lead to nasty stall behaviour. You can design around the issue by having the stall progress outwards from the root. This does mean you won't have a planform with ultimate CL performance.
The SWIFT glider used a laminar rooftop airfoil and I found no reports of a nasty stall. LR airfoils are some if the worst for that, which has hindered their uptake. The lift and drag figures are excellent.

Unfortunately, XFLR5 is probably the best we have...
 

plncraze

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What kind of hang glider were you flying when you found twist worked better at low speeds? Was it a straight or tapered wing?
 

Norman

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Grand Junction, Colorado
XFLR5 is not ideal for airfoil development. It doesn't show the hysteresis at stall.
It's worse than that. Panale codes (Xfoil, Javafoil, Eppler code, etc.) don't model CLmax and the stall accurately for any airfoil section (the cl and cd are usually optimistic after the cl over alpha curve begins to round off). This is why Harry Riblett was criticized by professional aerodynamists when he published and recommended airfoils that he had designed with the first Eppler code without any wind tunnel or real world testing. It's not that they considered the results of the program bad just unproven and recommending the resulting wing cross sections to amateurs was considered irresponsible at that time.

The tendency for a wing to stay stalled until AOA is reduced several degrees. Some of the higher lift airfoils in exhibit this trait. It can lead to nasty stall behaviour.
Fortunately the 3D stall is dominated by planform effects, not the 2D airfoil section stall. Lots of airplanes fly safely with NACA 5 digit airfoil sections because the wings are designed by people who take the 2D stall characteristics of those airfoils into account. This is usually done with washout but it can also be any one of several stall delaying techniques on the outboard panel.

You can design around the issue by having the stall progress outwards from the root. This does mean you won't have a planform with ultimate CL performance.
Wings without sweep should stall at the root first but swept wings should stall first near MAC because a root stall on a swept wing can cause a violent pitch down.

The SWIFT glider used a laminar rooftop airfoil and I found no reports of a nasty stall.
The Swift was designed with a very effective stall delaying device on the outboard panel.


Unfortunately, XFLR5 is probably the best we have...
The best free and fairly easy to use. Real CFD can give better predictions of CLmax and post stall behavior but the free CFD programs are not easy to use and the professional CFD software with nice interfaces ain't cheap, it's also pretty demanding of computer resources (the hardware to run it ain't cheap either).
 

Norman

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Here's the coordinates and data of a reflexed airfoil I've been working on and xflr5 data. It's not the one I'll use but it's the highest CLmax one of the family. I ran it at 4 Re# and once with forced transition at 80% on the lower surface to show the Cm with turbulence.
 

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WINGITIS

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Jun 24, 2020
Messages
138
Location
Wellington, New Zealand
Kevin,
I'll be interested in the discussion. "Small aircraft" covers a lot of ground with a lot of competing attributes, so you may want to get specific pretty soon or your discussion could get very jumbled. Are we talking only about rigid wings, or the "best" airfoil for a rag-and-tube small aircraft? Assumed laminar flow or not? What typical Cl's /Cl ranges are we focusing on? etc.

I was intrigued by the stated desire to look at reflexed airfoils--any reason to immediately go there? I'm used to seeing them in tail-less designs, primarily. Tailless designs are interesting, but a small proportion of small airplanes.
Hi Vigilant

I have added RIGID to the first page..

There are 1000's of non reflexed airfoils out there.......one for every occasion, just about.

It is not the same for Reflexed airfoils suitable for flying wings and the like....

LETS NOT LIMIT LAMINAR OR CL, OR ANY OTHER MAX PARAMETER..

The idea is to locate and/or modify a lot of the better ones to be even better...so that they can be used for more successful Flying Wing projects.
 

WINGITIS

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Joined
Jun 24, 2020
Messages
138
Location
Wellington, New Zealand
Here's the coordinates and data of a reflexed airfoil I've been working on and xflr5 data. It's not the one I'll use but it's the highest CLmax one of the family. I ran it at 4 Re# and once with forced transition at 80% on the lower surface to show the Cm with turbulence.
Thanks for the airfoil Norman, I have run it through XFLR5 at my setting and it shows more or less the same trends.

I have reduced MY panel count down to 299, modifying yours up to 299, because not everyone will have a version of XFLR5 that can do 500 panels and that takes longer on slower PC's, the results are mostly the same except when looking for minute details and weird configurations.

I see you use MACH 0.1 as the speed, shall we make that the standard for comparisons, I normally use 0.7 and 0.15 so that is in the middle somewhere?

May I suggest we use the RN of 3,000,000 for the comparisons as being a middle ground as well for these 1 person place aircraft.

In terms of the forced transition setting I am still divided on that, you can optimise the design of an airfoil to get the maximum reading at NON FORCED and then optimise a totally different one to get the MAximum reading with FORCED.

It is a dilema, what are you thoughts in terms of defining a testing standard for this thread?

Cheers
Kevin
 

cblink.007

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Jul 7, 2014
Messages
284
Location
Texas, USA
Hello folks

After starting these discussions within the thread:

Flying wing as cheap and simple option for basic fun flying

We were taking up to much space discussing Airfoils and associated things so the intention is to bring the conversation here.

I will bring across some of the topics and data we have already discussed in the next day or two.

The premise is, can we gather the best of the FW airfoils(FOR RIGID WINGS) into one place where they can be tested against a uniform set of criteria for comparison and this may drive two outcomes:

1: Others may see the FW Reflexed airfoils that are available so they may choose one that best fits their application

2: Work can be done to perhaps progress to a better generation of FW airfoils as we get into the 100 year interval since the Horten and Munk M6 type of airfoils were promulgated.....

The idea being that XFLR5 is used as the common test tool.

JUST A THOUGHT, what say you?

Regards
Kevin Gill
Wellington
New Zealand
Oh man, I would love to share the geometry and data for the airfoils we shaped for our flying wing, but my group is bound by restriction, as the shapes are IP. I can say, however, that the tip airfoil is a symmetrical NACA shape, and the root airfoil was shaped by the Horten method. My apologies up front.
 

WINGITIS

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Joined
Jun 24, 2020
Messages
138
Location
Wellington, New Zealand
Thanks for the airfoil Norman, I have run it through XFLR5 at my setting and it shows more or less the same trends.

I have reduced MY panel count down to 299, modifying yours up to 299, because not everyone will have a version of XFLR5 that can do 500 panels and that takes longer on slower PC's, the results are mostly the same except when looking for minute details and weird configurations.

I see you use MACH 0.1 as the speed, shall we make that the standard for comparisons, I normally use 0.7 and 0.15 so that is in the middle somewhere?

May I suggest we use the RN of 3,000,000 for the comparisons as being a middle ground as well for these 1 person place aircraft.

In terms of the forced transition setting I am still divided on that, you can optimise the design of an airfoil to get the maximum reading at NON FORCED and then optimise a totally different one to get the MAximum reading with FORCED.

It is a dilema, what are you thoughts in terms of defining a testing standard for this thread?

Cheers
Kevin
I also always set the TE setting to 0.2C to mimick the fact that we will never be able to construct a sharp TE. Which I see you did as well, so that should be the standard then?
 

WINGITIS

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Messages
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Location
Wellington, New Zealand
Oh man, I would love to share the geometry and data for the airfoils we shaped for our flying wing, but my group is bound by restriction, as the shapes are IP. I can say, however, that the tip airfoil is a symmetrical NACA shape, and the root airfoil was shaped by the Horten method. My apologies up front.
Thats the same for many of us CBLINK we may not be able to post the ORDS of all our airfoils, but as long as we compare to the same standards and post the RESULTS including polar data using the export polar feature to a Excel file we can compare apples with apples without seeing the airfoil form.

In terms of the Horten types there is nothing to protect, I have already indicated on another thread that the Horten 12% 2.15 camber is a reasonable place to start.

Cheers
 

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WINGITIS

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Messages
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Location
Wellington, New Zealand
Hi Folks

OK here are the first 8 airfoils compared, set to Forced Transition = 0 for now.

I suggest the filename format we should use is "Initials-HBA-Your sequence number-Title" hence, KG-HBA-1-FIRST 8 FOR VIEWING

You can see there is a wide variety there but they all attempt to have a positive CM at least in part of the AOA range, which is what we are looking for for flying wings.

Ideally we want the CM to be positive at all times whilst maintaining the greatest lift and having the best area under the LD curve we can get.

If this all coincides with the CRUISE AOA, CLIMBOUT AOA and MAX AOA we are aiming at all well and good.

The LD curve can be further left or right depending on what the total flight envelope needs to be, CRUISING versus COMBAT/AEROBATICS for instance.

Theres more to it than that but that is a start...

Whether we attach the individual airfoils Polars(Excel) at this stage is optional, its an extra step but some people may want to read the data....?

Cheers
Kevin
 

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WINGITIS

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Messages
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Location
Wellington, New Zealand
Here is a sample for us to reflect on the Forced Transition settings of XFLR5.

My "SDN" algorithm can generate the optimized airfoil to a set of parameters to achieve the highest results, as input to ANY analysis tool.

Another tool may have a different method and achieve different results but the "SDN" will deliver the best airfoils for that tool, based on the requested inputs.

In this case I specified a ZERO CM aim point and a FULL ENVELOPE MANEUVERING capability.

The two airfoils look vastly different and generally are VERY GOOD performers......IN THE TOOL with the two settings.

Clearly they give two different capabilities but they are both airfoils that suit a flying wing.

Obviously the one with 2.2 CL would be DESIRABLE although it has a bit more drag at cruise, it would easily outperform the other on any aircraft it is put on.

The question then is, WHICH IS THE CORRECT SETTING TO USE, 0 or 1???????????????

I do not not know the answer myself but do know which I would prefer if it was attainable...

Members thoughts??

Cheers
Kevin
 

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