|Jul 03, 2012, 11:49 AM|
Frans (or anyone else with an informed opinion)
Most of your wing designs for the MiMi are polyhederal designs - do you perfer the poly wings to straight? If so, why?
At first glance it would seem that a straight wing design would be stronger and lighter than a poly design.
I'm intrested in the wing that will give me the best chance at finding and staying in thermals for the longest duration possible.
Speed, range and launch heigth do not matter to me as much.....
|Jul 03, 2012, 01:43 PM|
Joined Dec 2003
If you look back through the thread, I think the general consensus was the poly wing launched higher and handled somewhat better. I know your post said launch height etc was secondary but more height on launch generally means a better "chance" of finding lift. Look for the posts by Bob McGowan, he was getting 160+ ft launches with a mildly modified poly version. Of course honing your flying and air reading skills afford the best chance of finding and flying in lift....which higher launches and better handling also make possible.
|Jul 03, 2012, 04:24 PM|
Just like Brian wrote, the poly wing is better than the "normal" single dihedral wing. Launches are higher and it turns better. Both the poly and single wing have the same number of panels. So both have a joint half way each side, not much difference in strength here.
Perhaps the build-up wing is the strongest. This wing has a standard spruce spar.
Of the three wings, the poly version is the best performing, both solid and semi-solid are equal.
Important is that the wing has sufficient torsional strength.
Worth a try, i should say.
A dutch builder constructed a mini-wing that has top and bottom sheeting.
Works great. To get a more accurate airfoil shape you need extra ribs in the D-box
|Jul 03, 2012, 05:02 PM|
Thank you Frans & Brian, poly wing it is.
Do you have any suggestions for how to make the foam core strong enough at the poly breaks for discus launch?
I was planing on 2mm CF rod spars, top and bottom running the length of the wing with 1/2" wide, 6 - 8" long CF tow across the middle portion of the wing above and below the spar and a shorter length of tow (4"?) at the poly breaks.
3/4 oz fiberglass across the whole wing with strips of 1.4oz fiberglas at the center and poly breaks.
My fear is at the trailing edge of the poly breaks in the wing may not be strong enough in compression for the launch forces.
|Jul 04, 2012, 12:39 PM|
I had plans to make a bagged wing for my MiMi, somehow I never came around building one.
This is what I had in mind for the ploy breaks.
Bag wing half in one go.
Over all 50 gram/meter glass.
As spar, carbon UD 80 gram band, 20mm or so wide. top and bottom.
Only at the bottom place a 1 inch strip/band of kevlar (50 gram or so) centered over the break. After bagging cut the poly break leaving the kevlar intact. Sand the diheral.
Bend the tip panel up using the kevlar as a hinge (make sure the cut is perfect parallel to the root). At the UD spar make a hard point by cutting small pockets between the top and bottom UD. wide as the UD and 8mm or so deep. Mix epoxy, microballoons and finely chopped UD carbon fiber (2mm) and cook for 10 minutes on alow fire
Forget the cooking part.
Use this mix to fill the pockets, put some epoxy on the rest of the break and fold the tip up to the correct hight. Wipe off excess epoxy.
Place a 15 mm wide band/strip (50 gram glas) over the top seam.
I know of a pilot who builds his own 1.5meter wings and only uses the hardpoints (2x) to stick wings halves together, no extra band/strip.
He is a good launcher and never had a break.....
So you could leave out the kevlar at the bottom and glas on top.
|Jul 05, 2012, 02:04 PM|
For what it is worth.
I took Blue Polystyrene (fff blue-core) doubled, so about 11mm thick at the maximum thickness. This is a 6.5 thick section or so at the cord, and closer to 9 at the tips. Spar had much to do with thickness at tips. Turned this into a very nice semi-semetrical airfoil. The airfoil actually took off almost 1/2 of the original foam weight of my blanks. This is the MiMi wing planform at 42" (1,066.6mm) span. (66.6mm longer span than the MiMi). Estimated 256 SqIn*2 of area (0.16052 sq m*2 if my conversion is correct).
One 1/4" Poplar Spar, weight of spar 18 grams.
Before any finish or coatings I think I was at 74g.
Took 1.5 ounce glass (42.5g) and used Z-Poxy to glass top an bottom of center section only 5.5" wide. Starved the glass quite proper.
Then I covered with an iron on mylar known as TopFlight Econocote (sticks to foam) but did a light spray of 3M Super 77 spray glue first (like slope soaring EPP type construction). With some color details added (also Econocote) my wing came out at a whopping 156g
That is more than some MiMi weigh ready to fly.
Wing is very strong as it is. Substitution of CF spar would make little differance in all up weight. In my opinion the foam is much heavier than built up balsa, and I don't think 2 ounce (50g) glass would weigh much less than mylar, especially if you went to paint the final wing.
All this said, I will put this on a motor glider I am almost done with because it is too heavy for a MiMi DLG. I live in high wind area, few slopes, but think I will have a fun ship to penetrate some of this windy weather and so some more hot liner type aerobatics or such. We will see.
I do not know how much more of a diet this current wing could go on, but I am sure not too much. I am guessing the balsa wing is a much better way to go for weight factors. If my wing was 1.5m like Frans' friend, I think weight would be dispersed better into the larger area. Anyway something to think about on a foam MiMi wing.
Any comments welcome.
|Jul 06, 2012, 07:23 AM|
Thanks for the information guys! I'm going to use hard points and CF rod as Frans explained above, build it light and see what happens.
I'm using this project as way to become familiar with cutting foam cores and bagging wings so if the finial product is too heavy I'll discard it in favor of the solid balsa wing.
I cut some cores yesterday but I'm not happy with them. Since the wing is flat on the bottom I only cut the top portion of the airfoil. The resulting core is very nice but it ended up warping afterwords ?!!! The dark edge under the photo of the tip shows how it's warping.
I'll try cutting both top and bottom of the airfoil next and see how they come out so far the total foam weight of the entire wing is 24 grams.
|Jul 07, 2012, 01:42 AM|
Watching your progress.
|Jul 07, 2012, 06:41 AM|
|Jul 08, 2012, 10:15 AM|
Frans: I received the plan today, thanks a lot !!!
I īve reading the plans but I donīt understand how the elevator work (the spring), are there any sketch? or photos.
|Jul 08, 2012, 10:28 AM|
Paul, check out these posts:
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