|May 13, 2012, 06:47 PM|
OSG pimped for the Slope #1
Looking for a slope soarer, I was pointed to this wonderful and easy to build plane designed by Springer: http://www.rcgroups.com/forums/showthread.php?t=1138106
This plane was designed to be a gentle soarer with motor to help her gain height, and as a such she performs beautifully!
However, the requirements on a sloper are a bit different. What I learned from talking to Springer, davereap and Don Stackhouse from DJAerotech (check http://www.djaerotech.com/, there is a GREAT information section in "Ask J & D", these guys build beautiful planes), there are a few important design considerations to follow when building for a slope. THANK YOU VERY MUCH guys, I learned (and am still learning) a lot from you!
The most important points are:
1. Stress / Load:
- Hang gliding in stronger winds impose rather high loads on the structure. Build the plane accordingly.
- Landing without a motor also sometimes turns out to be a pretty hard one. Make sure the fuse will survive it.
2. Visibility. On the slope, you can get pretty far in search for thermals. Interestingly, it's mainly the chord length and not that much the wing span which aids the visibility of a plane!
3. Wing thickness. Don't go over 7-8% of the chord length, that will induce high drag. I experienced it myself, sloping in some 30-40km/h winds with my Floater Jet - where other planes (real, dedicated slopers) were penetrating the wind as if it were nothing, my poor FJ hovered in place like a helicopter (it was a question of honor not to get out of that misery by using the motor ). That was not bad at all, actually - I had a lot of time to make my maneuvers, just turn the nose into the wind and stop to think.
4. Airfoil. A sloper needs a good L/D ratio. While KFM wings are GREAT for what they are, they might not be the first choice when making a performance glider.
5. Weight. Don't build it too light, I'd say at a *minimum* 1kg should be the target (and with that you'll be thrown around pretty hard in turbulences). I hope my OSG will be somewhere between 1 and 1.5kg. Many slopers actually provide the means to attach additional ballast when the wind picks up.
Here is how I modified the OSG to turn it into a better slope soarer:
1. Enlarged the original plans by factor 1.18, to get to 70".
2. Enlarged the chord even more, in order to reach that 7% target without having to build a 3mm thick wing. My chord is now 200mm, wing span 1800mm.
3. Due to the over-proportional enlargement of the chord, I also extended the fuse by 4cm. That was a "feeling". As it turned out, I'm afraid that was not really enough. I'll have to see on the maiden. I suppose I'd go for additional 8-10cm instead, if I were to build the fuse again.
4. I opted for a S6063 profile, EPS core with 6mm CF tube, wood LE, wood somewhere in the middle of the wing (still to be done), and covered with 1mm balsa, instead of the KFM2/3/9 which was what other people used for the OSG. Reaching 7% with a 6mm depron KFM and still have a wing stiff enough for the rough slope conditions would be very hard without enlarging the chord even much more, and building with 3mm depron didn't appeal to me. Besides, I wanted the L/D to be as good as I can get it, and KFM wings, while good lifters and gentle "stallers", are not famous for a good L/D.
5. Probably due to scaling, my tail volume went all wrong. I had to scale the horizontal and vertical stabs accordingly, leaving the "take the OSG measures and multiply by a factor" path. Just google for "Tail volume coefficient"
to get more info, or visit this post: http://www.rcgroups.com/forums/showthread.php?t=122785
At this point, I must admit that I'm not an experienced builder by any stretch of imagination! Also my tools are not that great, I have only a hand-held hot wire cutter. The S6063 is way more challenging a build than what I experienced with the KFM wings so far, but it's also a very interesting learning experience. So far, I'm coming along quite well, but the profile will not exactly be the S6063, due to my own inefficiency as a builder. I hope I won't destroy the characteristics of the foil *too* badly, it would be a pitty to invest so much time and to have no gain from it.
Long story short, here is how far I came.
Nothing exciting here, just a straight build.
I like making a cut on the inner side where the bend should occure. Helps greatly in further build, just watch out not to cut through and to have the outer side well taped with packing tape.
I also like adding all sorts of "helper steps" to hold my top/bottom fuse covers and similar.
One side attached. I use UHU Por, no need for clamping and waiting, but also no second chance to slide the parts in position - once a contact is achieved, that was about it.
That's it, fuse finished.
Tail finished, servos mounted (I had to see that tail wiggle )
I guess I'll have to stiffen that horiz stab a bit more than just by taping it.
With only the top cover installed, the torsional strength of the fuse is very bad. I glued in two rectangular foam reinforcements, which helped a bit. I guess when the lower fuse cover is glued in, the strength will come.
You can also see here how I placed the push rods, nothing special.
NOW comes the interesting part...
Having only a hand-held cutter, I had to separate the main wing panel into 7 parts. Be very careful to have all sides square while doing so. Or even better, get yourself a decent hot wire cutter.
Some hours (and a lot of sweat) later, this is how the wing looked like. The nose section is cut straight and covered with a 6mm depron. This is where the wooden LE will attach to.
If you look closely, you'll see quite a few bad mistakes I made. It seems my CF tube got "pushed out" of the core by the Beli-Zell (PU glue that I used), at some places it's not completely flush with the top of the airfoil.
Even worse, I pulled the wire through EPS a bit too fast, the wire bent, and produced visible "beer stomach" on my segments. What you see here is actually already sanded down considerably! A view from behind shows the bend very good, in a place where EPS is too thin to be sanded down. I don't know yet what to do with this - if I leave it like that, I'll either have to push the balsa covering very hard so that it contacts the EPS at all places, or I'll have to go for a mix "UHU por on the tops, PU on the lows" to have the PU fill out the gaps.
Or just call it quits and build the wing again. I guess that would be a better way. That's what Chuck Norris would do!
I could kick myself for this!
The next problem is that the S6063 airfoil ends extremely thin! I see no other chance but to follow the airfoil up to a point where the EPS is some 3mm thick, cut off the rest and replace it with something which has *some* thickness and strength to it.
This is what I came up with for this "someting":
These ailerons are a sandwich of 3mm depron, sanded a bit down and thinned further towards the end, covered with 1mm balsa. This is the first time I did something with balsa, it was a very pleasant experience! This wood can be bent, it connects very good to UHU Por, and can be sanded in just any form without effort. While each part of the sandwich for itself is VERY weak, the completed construction is impressively stiff. I think it will work pretty good!
Stay tuned for more...
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