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Main wing profile

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foam-n-fibre
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Re: Main wing profile

Postby foam-n-fibre » Thu May 25, 2017 2:54 am

Here's a question about building these - when you refer to how much sanding you have to do on corecell, how are you getting your finished shape accurate? Are you using a hotwired bed in the right shape of the foil, or a CNC'd bed, or a full mold? Surely no one is sanding corcell and wrapping it in carbon and coming up with an accurate finished shape without something to make sure the shape is accurate in the finished product.

Is there a best method for doing this?? I should add that for the one foil I built, I had a flat bottom, vacuum bagged onto a curved sheet of plexiglass, and for my ability level it seemed to work just fine.

Thanks,
Peter

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Re: Main wing profile

Postby mopman365 » Thu May 25, 2017 7:23 am

Thanks for the comments everyone. I agree there's so many factors at play and probably the surface finish and front vs back wing AOA play a much bigger role in the final performance.

I'll probably stick to flat bottom foils like Clark Y or Aquila for now to make shaping and setup easier.
foam-n-fibre wrote:
Thu May 25, 2017 2:54 am
Here's a question about building these - when you refer to how much sanding you have to do on corecell, how are you getting your finished shape accurate? Are you using a hotwired bed in the right shape of the foil, or a CNC'd bed, or a full mold? Surely no one is sanding corcell and wrapping it in carbon and coming up with an accurate finished shape without something to make sure the shape is accurate in the finished product.

Is there a best method for doing this?? I should add that for the one foil I built, I had a flat bottom, vacuum bagged onto a curved sheet of plexiglass, and for my ability level it seemed to work just fine.

Thanks,
Peter
Peter, what I've done for a mast and last 3 wings is to print out the profile from the site mentioned before, stick it on some thin PVC sheet and cut out a 'stencil'. Then I start sanding the corecell from the one tip, sliding the stencil along as I go. Where the stencil gets "stuck" i mark with a pencil and sand down that area. This takes some time as you can imagine, but I end up with a fairly accurate core. I want to make a copy-carver jig for my Dremel router to speed up the profiling of the core, but that's another discussion...

Then layup the core with a couple of layers of UD and bi-ax 300 gsm carbon and vacuum the whole thing. When that's cured, add a thick layer of balloon filler both sides and later sand that down to get a smooth surface finish.

The carbon layer thickness is very uniform as i've seen on the mast after cutting it down, so i end up with a profile that matches the core - only a few percent bigger.
DSC_0273.JPG
DSC_0271.JPG
(Yes, that back wing AR ...)
DSC_1513.JPG
(First wing in the foreground ;))

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Re: Main wing profile

Postby kostantin » Thu May 25, 2017 8:43 am

Hi
I did a quick research on this Aquila profile and found a post in a german RC forum. I have copied the text into a google translator. This is the result. Funny but hopefully helpfull.

Poor is the high resistance at high speeds. Due to the deeply drawn-down nose, detachment occurs on the nose (on the underside of the surface) at small angles of attack. That is, if you do not want to fly particularly fast or on your back, no broken leg.

In the sailor is something unpopular. Since you cried "from eeeeeechts" and then it goes only down instead of forward.

Skip Millers Aquila (the first F3B world champion) had a modified profile

It is like I said previously, flat bottom profiles get nasty on higher speed. You will find air detachment and stall, flat bottoms create a lot of drag and have problems on higher aoa's. If I should make a building recommendation. Find someone that is able to cut styrofoam wings. Reduce the profile thickness from the thickness of your carbon.

You will get three pieces. Mold top, mold bottom and the wing. This is a good basis to start. Modern profiles are sensitive when it comes to building errors. By the way, don't vakuum bag styrofoam. It will look like toast afterwards.
Think about inserting carbon spars made out fo rovings.

tks

kosta

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Re: Main wing profile

Postby mopman365 » Thu May 25, 2017 9:59 am

kostantin wrote:
Thu May 25, 2017 8:43 am
...
It is like I said previously, flat bottom profiles get nasty on higher speed. You will find air detachment and stall, flat bottoms create a lot of drag and have problems on higher aoa's. If I should make a building recommendation. Find someone that is able to cut styrofoam wings. Reduce the profile thickness from the thickness of your carbon.

You will get three pieces. Mold top, mold bottom and the wing. This is a good basis to start. Modern profiles are sensitive when it comes to building errors. By the way, don't vakuum bag styrofoam. It will look like toast afterwards.

tks

kosta
Thanks Kosta,

I think Styrofoam is too soft for main wing - there seems to be alot of force on the center section (makes sense considering the rider weight and load from the kite all on small connection area). My first wing failed as I used a lower density Corecell (still much higher compression compared to Styrofoam). The top skin failed due to the core buckling under compression. The bit of weight saving from Styrofoam vs Corecell is negligible even on a big wing. But yes, the shaping is a btch on Corecell.

Regarding the flat bottoms at higher speeds and AOA: Not sure about 30knots , but at lower speed (13knots = 1mil Reynolds number in water), the flat bottom Aquila seem to perform better at higher AOA compared to Eppler. Or am I reading these graphs wrong?

from http://airfoiltools.com/calculator/reyn ... =Calculate
Reynolds.JPG
>> Eppler (red) vs Aquila:
CaptureClCd.JPG
CaptureClCd.JPG (34.17 KiB) Viewed 772 times
CaptureCl.JPG
CaptureCl.JPG (34.73 KiB) Viewed 772 times

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kostantin
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Re: Main wing profile

Postby kostantin » Thu May 25, 2017 10:31 am

Hi,

the main problem with the styrofoam is that the material is very difficult when it comes to puncture. But you can build a wing extremely strong to prevent bending and worse torsion no matter what the type the foam foam is. The infill is only to get an accurate wing profile.
In the dark ages most of the RC models have been made out of styrofoam and veneer with spruce spars. Second step was carbon spars. Veneer is not possible in your case, because we look on a 3d lay out. Next step was molds to create a carbon foam sandwich, which is state of the art. Glider Pilots always look on weight. We don't have this problem. You can "over engineer" your wing to prevent braking.

I would go in this direction in your case. Divide the amount of carbon when it comes to thickness from the profile and chase someone that is able to cut out the templates for you. Maybe he is able to cut blue or green styrofoam for you.

This is what I would do in your case.

I stick on the statement of the aquila. Slow and the nose will make problems. There are much better profiles and if you go with the cut template you can have better profiles.

Have fun

Kosta

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Re: Main wing profile

Postby GrantL » Sun May 28, 2017 10:29 am

mopman365 wrote:
Tue May 23, 2017 2:14 pm
Hi,

Deciding on a profile for my next (3rd) main wing. Seems everywhere the EPPLER 817 profile is suggested as the best for low-speed hydrofoils. My first one was AQUILA 9.3% that has a flat bottom - so less sanding required for the Corecell core. Second one was the EPPLER 817 and it's a bit of a mission to shape (and also makes the allignment more difficult).

My question is, why everyone's using Eppler 817 when a simpler profile like AQUILA 9.3% seems to have better L/D performance? Am i missing something:

Profiles_airfoiltools.JPG

Blue line is Aquila that has better L/D from 2.5 degrees and more:
LiftDrag_airfoiltools.JPG

Lift_airfoiltools.JPG
I am only new also , built a foil late last year and really enjoying after the learning process. Over the last months I have been playing around on some software XFLR5 which allows you to analysis profiles in 2d and 3d. Even though Aquila9.3 will work it is not the most efficient profile. I have a number of images from the software generations which illustrate the differences. I have used Aquila9.3 , Tom Speers H105 and RG08 which is similar. The first set of images is only for a fixed polar at 2deg AOA.
Aquila9.3 fixed lift polar.png
H105 fixed lift polar.png
RG08 fixed lift polar.png

The next set of images are generated from a multi batch over a range of Reynolds Numbers .
Aquila9.3 multi batch.png
H105 multi batch.png
RG08 multi batch.png

I then added the 3 profiles into a 3d model all using the same rear stabiliser set at -3deg. Masses were added for rider and board weight. Analysis of model with 3 profiles. One of the parameters I was interested in is the moment of inertia. Around 2deg AOA is where the model is in balance.
4graph.png
Aquila9.3 model iso.png
H105 model iso.png
RG08 model iso.png
So the Graph cM-Alpha illustrates this with the negative sloping gradiant of each line. This is all in theory which is a starting point.

I think what ever profile you decide to use it is really important to have an accurate profile in 2d. What is also important I think is the AOA along your wing. That is a lot harder to get accurate.

Hope I have passes on something of interest.

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Re: Main wing profile

Postby TomW » Sun May 28, 2017 12:11 pm

So theoretically, which profile is best? The H105?
Can you explain numbers: efficiency, Cl/Cd. (V is velocity of course.)
And why those tip shapes?

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Re: Main wing profile

Postby GrantL » Mon May 29, 2017 11:03 am

TomW wrote:
Sun May 28, 2017 12:11 pm
So theoretically, which profile is best? The H105?
Can you explain numbers: efficiency, Cl/Cd. (V is velocity of course.)
And why those tip shapes?
Good Question. T Speers has optimised his H105 for low speeds <40knots.
The RG08 is very similar , possibly as good as the H105.

My conclusions I make from the analysis I have done.
2d wing profiles Cp (pressure - X ).
Aquila9.3 has a very high pressure gradiant at the leading edge , more likely to cavitate at tip . The graph demonstrates this with the green arrows length and direction of force.



The 3d models show steady state speed required at 2deg AOA to create lift with mass of around 95kg total
AquilA9.3 21knts
H105 23knts
RG08 20,8knts
model speed.png
Efficiency , Lift/drag ratio correction factor for 3d model.
Cl/Cd , the ratio lift/drag.
Cm , Moment of Inertia of model about COG.

Wing tips on model are not complete, need more panels in model to produce a smooth radius. The wing shaped I used in Model is only a test model.

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Re: Main wing profile

Postby mopman365 » Tue May 30, 2017 10:03 am

Great info thanks GrantL! Will have a more detailed look through the screen-shots, but this looks exactly like the information I was after.

"Aquila9.3 has a very high pressure gradiant at the leading edge , more likely to cavitate at tip . The graph demonstrates this with the green arrows length and direction of force."

The green arrows (top graphs)? Is there anyway to see from your software at what angles/speed cavitation is likely?

I can also see from the graphs I posted earlier, that the Eppler is only more efficient at AOA > 2.5 degrees. From 0-2 the Speer/Eppler profiles are better. So your 3D model is showing the cruising angle is around 2 degrees?

Interested in the XFLR5 software. Do they have a trial period before buying?

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Re: Main wing profile

Postby GrantL » Wed May 31, 2017 2:06 am

mopman365 wrote:
Tue May 30, 2017 10:03 am
Great info thanks GrantL! Will have a more detailed look through the screen-shots, but this looks exactly like the information I was after.

"Aquila9.3 has a very high pressure gradiant at the leading edge , more likely to cavitate at tip . The graph demonstrates this with the green arrows length and direction of force."

The green arrows (top graphs)? Is there anyway to see from your software at what angles/speed cavitation is likely?

I can also see from the graphs I posted earlier, that the Eppler is only more efficient at AOA > 2.5 degrees. From 0-2 the Speer/Eppler profiles are better. So your 3D model is showing the cruising angle is around 2 degrees?

Interested in the XFLR5 software. Do they have a trial period before buying?
Good question about cavitation. The first graphs are only the 2d profile. All you can calculate from inputs Reynolds Number and a single or range of AOA. Graphing illustrates pressure curve over top and bottom surfaces. Profile image illustrates boundary layer (in red) and Cm (large green arrow). If you run software with a range of AOA's you can animate the profile to show the changing of the boundary layer and pressure.

Yes 2deg AOA. I think you will find that most foiling setups will have a "Cruising" AOA of around 2deg. So on my last set of graphs Cm verse Alpha is really important to determine if you have stable flight. If you look at this graph when Cm =0 , the angle it passes on the Alpha axis determines when you have stable flight. This is a result of the main wing AOA , stabiliser AOA (sum of wings + mast + fuselage pitching moments), and COG of the model assembly + rider mass. The resultant moment of inertia determines at what angle you will have stable flight. The line (curve) on the graph must pass through Cm=0 otherwise the model is unstable. So I have 2deg as my stable AOA , if I had at 6deg then my model would be cruising with the board pointing up in the air at 6deg but would be stabe also.
In the software you can easily see how the mass of the rider affects the Cm=0. But I have found that regardless of position on COG of the rider you can't always move the curve to (in my case) 2Deg. What does make a significant difference is the size of the stabiliser and as we all know the AOA of the stabiliser.

The software is free! Well supported with videos and a forum. http://www.xflr5.com/xflr5.htm

I agree also as others in your thread have noted , graphs are only a starting point. But if you are going to undertake a build it is nice to at least get the starting point in a good range. Any profile will work but how well stable it is is really important. Home building a prototype 1 off is always trickly . That is why version 2 and 3 should be better!
Cheers
Grant


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