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Old Nov 16, 2010, 12:53 PM
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femer's Avatar
Sverige, Östergötlands Län, Linköping
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Flying wing build, proportions, layout, etc.

Hello everyone, I'd very much like to build a flying wing but it has been put off due to lack of a foam cutter. However, right now I have access to a cnc cutter and just about an endless supply of bluecore. So I guess it's time to realize the project.

However I need help choosing design and parameters such as wing profile, chord length etc. since I'm no expert in these. Some rules of thumb would be duly appreciated.

I've drawn up a quick sketch of what I'm thinking of, but I guess this may be too complicated to cut? Maybe a Ritewing style airframe would be a better choice?

If so I can make an parametrized model of this and would be happy to share with you if it turns out A-ok.

Oh, and the maximum width of the cutter seems about 50cm, so this is quite limiting. My preferences are for carrying capacity and flight time, since I'd like to create a small fleet of fpv airframes.

Also, is bluecore (DOW) suited for direct use w/o glassing? Would be great if I can use it just adding reinforcements, equipment and paint.



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Old Nov 17, 2010, 02:47 PM
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Colorado
Joined Aug 2006
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I'm not sure that you could cut that that center section unless you have a cnc mill since it's all compound curves (no straight lines, like the surface of a ball). The curved edges in the plan view are the cause of the compound curves. A large chord on the center is good both for cargo space and aerodynamically because it reduces induced drag. Nothing wrong with compound curves, they're just hard to build, but you can get nearly the same benefits with polygons instead of rounded leading and trailing edges. The Nurflugel program can do a lot of the math work for you. You can increase your speed range quite a bit with landing flaps.

--Norm
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Old Nov 17, 2010, 04:01 PM
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Sverige, Östergötlands Län, Linköping
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Just realized I'm in waaay over my head. To be able to quickly draw something TLARR (That Looks About Right) really is'nt the same as designing a whole airframe if its supposed to have any good properties.

So, I've downrated my request to plans of wings or .dxf files that I can use to program the cutter. I'll only have access to the cutter for one more month, so time is not in abundance. I'm currently waiting for plans of the Tinamou design kindly offered by a member here. Let's see if I can take it from there to something flyable..
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Old Nov 17, 2010, 05:27 PM
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Tinamou looks good in the videos that I've seen. The flaps could be tweaked a bit though. Read the landing flaps thread I linked to especially the post by Don Stackhouse on page 4. You mentioned that carrying capacity and flight time are important to you and that you're interested in FPV: That sounds like you want a powered plane with plenty of space in the center section for equipment. If that's the case you may need to design a center section to fit between the Tinamou wings with more depth to house your stuff. Possibly with a pod because the motor will make balancing it more difficult. Don't worry it's not a big problem and as you can see there are plenty of people here ready to give advice. Some more experienced than others but all ready with their 2 cents worth.

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Old Nov 18, 2010, 01:40 PM
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Sverige, Östergötlands Län, Linköping
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Hi,

good suggestion with the pod. I'll certainly look into this. I realized too when I read about the Tinamou that it was'nt powered, but thought I could probably tweak this by adding a pod of some kind after the wings been cut.

Been looking into cutter programs and the cutter itself tonight. It seems to be quite an old machine using the Step Four hardware and program. The software seems really difficult to draw in and it does'nt contain any library of airfoils. The Tinamou requires two of these, an SD7037 7.5% and an SD8020 6%. To be cut in three segments with different washout.

I've demoed Step Fours Wing Designer software and was able to draw the whole wing as it should be cut. It also had several great functions i.e. the function to create a mirrored wing half and have these cut at the same time. Best of all, after it was drawn it could be exported directly into my software/hardware configuration. Only not in the demoversion.

Buying this software isn't worth it. I'm only going to do this one wing. If only someone could offer to do the wing in their WD and send it to me...
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Old Nov 19, 2010, 07:28 AM
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United States, Mt, Helena
Joined Apr 2002
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You could make a removable pod.
The motor most definitely should be placed in the nose as you'll need the nose weight for sure! Have the front of the pod slip over the top of the wing, then extend on the bottom all the way back to the skeg. Then just have a pin run through the pod and the skeg to hold it in place. Then the battery pack and speed controller placed in the pod and moved forward/aft to balance.

This idea wasn't mine, it came from HerkS.

As far as the airfoils go you could cut the cores so that you have the SD7037 at the root and taper to the SD8020 at the tip. I'd use 5 degrees for a faster soarer and 8 degrees for a more thermal model.

Curtis
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Old Nov 19, 2010, 08:45 AM
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United States, OH, Bradford
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A few comments:

It's not entirely clear to me exactly what you're planning to do with this model, but I get the impression that there might be a lot of experimenting with on board equipment, which probably means that some flexibility in C/G requirements would be useful. Also, for things involving cameras in airplanes, something with gobs of dynamic stability (the ability to damp out oscillations) would be highly desirable. A flying wing layout, with its typically narrow C/G tolerance, and weak damping in both pitch and yaw, is likely to be less than optimum for both of those requirements.

Be careful with pods, or any sort of bulge that puts lateral area ahead of the C/G, will probably cause some grief in the yaw stability department. Even small additions there can have profound effects. For example, on the tests of the Horten H-IV sailplane at Mississippi State, someone thought it would be a good idea to add a fairing over the nose skid. The additional lateral area messed up the plane's yaw stability to the point that they had to do the tests with a much further forward C/G just to have enough yaw stability to make it safe to fly. This totally messed up the lift distribution on the wing, which significantly hurt the plane's induced drag.

They also made some other significant changes, such as a bulged aft cover for the cockpit to make room for a test pilot who was physically larger than the cockpit had been designed for. The greater slope on the aft portions of the cover caused flow separation over about 30% of the cover, and increased the parasite drag. There were also some repairs done to the wing that may have distorted the airfoils and twist.

The net result of all this was that the plane delivered substantially lower performance in the MSU tests than it had in the original tests back in Germany. The test report and many of the later discussions of those performance discrepancies glossed over or ignored the role of those modifications.

As far as putting the motor up front, there certainly is merit to that from a C/G standpoint, and tractor props are definitely more efficient than pushers in nearly all applications. However, props mounted ahead of the C/G are destabilizing in both pitch and yaw, while both are increased by a prop mounted aft of the C/G. On a flying wing arrangement, with its inherently weak yaw and pitch stability and damping, a pusher prop usually makes more sense. Unfortunately, mounting the motor in the nose, driving a pusher prop behind, will probably require a long driveshaft of some sort. This opens a whole "Pandora's box" of torsional vibration and whirl-mode instability issues. Given that our motors today are relatively light, an aft-mounted motor and prop, with the battery mounted up front just behind the leading edge, is probably a safer approach.
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Last edited by Don Stackhouse; Nov 19, 2010 at 08:51 AM.
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