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Old Oct 06, 2010, 10:03 PM
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United States, OR, Corvallis
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Question
Sig Riser Airfoil modification???????

OK, so, now that I have 2 planes under my belt, the humble House of Balsa "2x4" with a flat-bottomed, thick "aquila-ish" airfoil and the now discontinued Ace RC Easy Eagle with a nice slightly cambered SD 3-0-somthing-something airfoil. Both fly nicely, the Easy Eagle is REALLY nice and floaty and I REALLY REALLY like it.

I've been thinking of ordering a Sig 2M Riser from the local hobby shop and I recently looked at the plans for the Riser online and it's another flat-bottomed "aquila-ish" looking airfoil. I'd like to modify it with some camber.

HERE'S THE QUESTION(s): Since I feel confident I have the skill to line up all the ribs and do this modification fairly well, I would like to modify all the ribs into a slightly cambered shape to make it fly REALLY nicely, Does anyone have any opinion on this?? Has anyone done this sort of thing?? Has anyone done it specifically with a Sig Riser (or HOB 2x4?)?? Does anyone think it would be a waste of time because they think the Riser already is "the bomb"(-as the kids used to say)?? An inquiring mind wants to know!!

Thanks -Paul
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Old Oct 06, 2010, 10:19 PM
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You're using the terms incorrectly. Any asymmetrical airfoil has camber. Draw a line down the middle and you'll see how it curves. However, a mod I've heard off is to sand the front bottom part of the airfoil so it curves up a little bit. Said to work well on Paragons. Do a web search on, say, the Eppler 205, Clark Y, or one of the Drela foils to see what this curve might look like. It's just at the very front of the underside.

The 2 meter Riser, as I recall, has nice handling and is very floaty as is. This mod would make it faster, however, which can come in handy if it gets windy or you're in sink.

I suggest you might find that a 100 inch glider will fly better. Probably not that much more work to build, either, if it uses similar construction. As I recall, the Riser 100 has an airfoil that's got the kind of curve I'm talking about. People seem to like it, but I haven't flown one. Other options might be the Scepter 100 (flies ok, and faster than a straight flat bottom kind of wing, but the one I flew was overbuilt and heavy. Might be much lighter if built to the plans), Oly 2 (slow, handles very well) or Oly 2s (faster, haven't flown one). If you want to go even bigger, I've heard good things about the Mirage, which is also supposed to be pretty simple.
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Old Oct 07, 2010, 01:23 AM
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Easy Eagle-S7032 airfoil.
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Old Oct 07, 2010, 08:54 PM
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Take the view of the airfoil. Draw points along the chord that are half way between the top and bottom of the airfoil. This is the mean camber line. A completely symetrical airfoil would have a straight camber line and no camber. A flat bottom airfoil already has a lot of camber. That's what gives it the great slow flying ability.
What you are thinking of doing is actually making an undercamber airfoil. This would mean that the bottom surface (lower camber in the figure) is cambered in the same direction as the top surface of the airfoil - the leading and trailing edges are curved downward. This type of airfoil would be used for very slow flight but would tend to suffer in performance at higher speeds. Anything over calm winds would be difficult for the glider to penetrate into. That's why current gliders don't use undercambered airfoils.
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Old Oct 07, 2010, 09:17 PM
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Airfoils and stuff......

Thanks for all the info and review, I sorta forgot the specifics of everything mentioned so that was good usefule info.

Mr Prodjx - Yes, no kidding, talk about having the answer right under my nose!That plane just flies like butter. or butta, or whatever.

But in regard to the one of the original questions, Has anyone actually done an alteration of the ribs on a kit to achieve what they thought they wanted flight-characteristic-wise and have it come out doing what they wanted???

Thanks - Paul
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Old Oct 08, 2010, 07:35 PM
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Quote:
Originally Posted by lincoln View Post
You're using the terms incorrectly. Any asymmetrical airfoil has camber. Draw a line down the middle and you'll see how it curves. However, a mod I've heard off is to sand the front bottom part of the airfoil so it curves up a little bit. Said to work well on Paragons. Do a web search on, say, the Eppler 205, Clark Y, or one of the Drela foils to see what this curve might look like. It's just at the very front of the underside.

The 2 meter Riser, as I recall, has nice handling and is very floaty as is. This mod would make it faster, however, which can come in handy if it gets windy or you're in sink.

I suggest you might find that a 100 inch glider will fly better. Probably not that much more work to build, either, if it uses similar construction. As I recall, the Riser 100 has an airfoil that's got the kind of curve I'm talking about. People seem to like it, but I haven't flown one. Other options might be the Scepter 100 (flies ok, and faster than a straight flat bottom kind of wing, but the one I flew was overbuilt and heavy. Might be much lighter if built to the plans), Oly 2 (slow, handles very well) or Oly 2s (faster, haven't flown one). If you want to go even bigger, I've heard good things about the Mirage, which is also supposed to be pretty simple.
I believe this is referred to as a Phillips entry airfoil.

Rather than mess around with all that, why not just build a Riser 100? It has the type of airfoil you're looking for already, and will fly much better than the 2M Riser. And it's cheap! 70 bucks direct from Sig.
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Old Oct 08, 2010, 10:44 PM
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I haven't done it on a kit, but I did it on a scratch built wing. Worked fine. The wing was a "Hershey bar", i.e. untapered, 10 inch chord, 2 meter for my Oly 650 fuse. Airfoil from root rib of a crashed Antares. About 79 or 80 separate ribs on each side of the very heavy spar. No sheeting. Floated pretty well and was also MUCH faster than the Oly 650. It was heavier too, as the winch stalling spar caps were 1/2" X 1/4" maple.

I also scratch built a wing with a dowel leading edge and flat facets, just to see how bad it would be. Pretty bad, too, but not so bad that I couldn't win a contest now and then. Maybe because it was thin and faster than most totally (almost) flat bottomed airfoils.

I had a Gentle Lady fuse that I put an E205 wing on once. Much faster and usually more fun to fly.
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Old Oct 09, 2010, 02:26 PM
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More Good stuff!!!!!.......

Lincoln - Thanks, that tells me a LOAD in the right direction. Some of that goes with what I just read from old Dave Thornburg writings, something to the effect of "good flying comes less from low sink rate than it does from having more GO" or something like that stated in a very Dave Thornburgian sort of way.


LVsoaring - Yes, I did look into the Sig Riser 100 and yep, you're correct. Great price for a plane everyone seems to like who has one. The only drawback is I bike and hike to fly and load everything on/in my backpack and use 2 piece wings so at 50" per wing section, it's got a high chance of getting whacked and damaged in transit, so I've put myself on a 2m outer limit for now. Plus, it's illegal to do anything RC in the town where I live but the parks ground workers and general park users where I fly the most like watching my 2m Easy Eagle and HOB 2x4, and 2meters is probably about pushing the limit of what the park ground staff will turn a blind eye towards and what park users won't feel intimidated/endangered with. I want that Sig 100 but I really don't want to get shut down and possibly get other RC sailplanes shut down that use the same park. Some guy did show up recently with a gas engine(big engine,no muffler) control-line medium size plane(36" wingspan?)and decided not to fly it, which I'm glad for since again, That's just a little too big of a hotspot on the radar screen of things at the park.

All that said, I may just get a Riser 100 anyway for elsewhere outside of the park, to me it's just as classic as the Goldberg Gentle Lady but better design.

Thanks for the input- Paul
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Old Oct 09, 2010, 04:05 PM
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I saw your work on your Easy Eagle. You have skills. Don't limit your self to a 2M. Turn that Riser 100 wing into a three or four piece wing.

Ken
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Old Oct 09, 2010, 05:14 PM
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Hmmm, back to the Riser 100.....

Ken- Hey,thanks for the compliment, I must say I'm pretty pleased with the Easy Eagle, I wish I'd read Thornburg's idea on keeping wing-mount dowels completely INSIDE the fusalage first, DANG! I might re-do that later, I'll do it on my Great Planes Spirit 2m(still in the box and next in line)first and go from there.

I've wondered about the stability/amount-of-slop/play with more wing sections like 3 or 4 like you mentioned. It did just occur to me that with a 3 piece wing, it would be fairly easy to run 2 spoilers from 1 servo in the middle of the middle section or just off-center if needed for structural integrety to keep some weight down. However, I've found that I like the way both of my planes fly and they're both a little heavier than the weight stated on the box.

Thanks for the input - Paul
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Old Oct 10, 2010, 12:02 AM
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If I'm not mistaken, it's probably better to keep the weight down and use a thinner section, at least if everything is nice and smooth and the fuse isn't too big, has too many leaks, or is a bad shape. With a more traditional airfoil, a bit of weight is probably your friend, as, at least in 2M, is a fairly low aspect ratio.

3 wing sections with well fitted joiners doesn't seem to be a problem.

If it really must be a 2 meter, you might consider the Mountain Models Jester. While I haven't flown it, it's supposed to be similar to the Sovereign and that one flies ok except that the tail could be a bit longer. The airfoil is supposed to be a 3021, not as thin as some of the later airfoils, but much faster than the one on the Riser 2M. Certainly the Sovereign is faster than a Riser.

Depending on how ambitious you are, you might consider their short kit for the Allegro Light. I had a chance to fly Mark's original example (electric version), and it's (of course) a great flyer. At least if you can deal with a stall which is somewhat sharper than some, though not unreasonable.
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Old Oct 10, 2010, 05:19 PM
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airfoil wing mods

Hi,
I recently built a Gentle Lady but changed the airfoil to Drela alegro lite foils.
You can use Profili and print about any airfoil you can imagine. We also did a top and bottom spar and shear webs with CF .007 top and bottom.
But I agree with the previous advise go for a bigger glider. I just this last week got 32 minute on my 3 meter bird.
For the money the big Sig Riser sounds great. As with all gliders build the tail LIGHT as possible. I have never built a Sig kit I didn't like!
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Old Oct 10, 2010, 11:46 PM
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It is good advice to move up to at least a 100" plane, but you will always come back to your 2m planes. Just that all the cheaper/more popular 2m planes are entry level designs, so they fly slow and are a bit too draggy. For more range from same launch height you need more speed (distance with time), and its the wing that causes all that drag. Some of the wing-tips on these woodys are really chunky. Drag reduction is more important than reducing weight (to a point). I modded a Daydream for as much drag reduction as possible...30% thinner airfoil, full d-tube. 3/16 balsa tail sharpened and bevelled to knife edge and bolt on wing. The result was very sucessful, but had to be more carefully flown because the stall was a lot sharper and the plane was not so floaty anymore. Climb rate was a bit slower too, had to keep the speed up and make more circles. But I was finding more lift! Due to the longer range I was able to head out for those distant buzzards and cover more sky otherwise. Raising the leading edge will not give that much advantage, you need a thinner wing, without compromising strength. Thats the big advantage of mouldies over woodies, to the point where you need trailing edge control to enhance low speed flying. Still keep the leading edge fairly round for stall control.
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Old Oct 11, 2010, 11:07 PM
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If you make it like an Allegro Light, you can have the float and still have some speed, and if it gets windy, the airframe is strong enough to handle some ballast. But you have to have better eyes than if you were flying, say, a Duck, or whatever they call that thing.

Or maybe make it like a scaled up DLG (which isn't really all that different). I'm surprised at the speed range these can have and still be reasonably forgiving.
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Old Oct 13, 2010, 09:51 PM
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More good stuff.......

Thanks for the additional input, more good ideas to think deep thoughts about for future planes.

I did take the Easy Eagle out yesterday to some fields at an open-space area hi-starting in a moderate breeze but a little too late in the day to really find thermals(I'm sure they're out there, I'm still not the best at getting into them and staying there).

Anyhow, that plane seems (by my limited experience) to have some really good flight characteristics such as it penetrates into breezes really nicely as it comes off the hi-start, I can swing it around quickly or slowly to fly with the wind(downwind) and it does'nt lose much altitude before it speeds up, starts leveling out and travels with plenty of speed to when I turn again into the wind pretty far downwind. THIS IS WHERE IT REALLY GETS COOL -as it comes back, from quite a distance and not really with much height- it sailed all the way back and overshot over and beyond me upwind again most of the time when I intended to bring it back to catch it. I did manage to catch it once yesterday (and twice in one session at the park where I usually fly). Every time I've caught it flying into the wind, it still had enough speed to where I had to convince myself I could really make a whole upper body catch and not get clobbered by it as it came in on it's own flight path(using rudder and no elevator-it wasn't aimed in a dive toward me).

Since I fully acknowledge my sub-optimal piloting skills, I did'nt use the bolt-on wing and modified it for a rubber-band/dowel wing mount. As well as it flies with that aerodynamic "messiness" I bet it would be a small but noticible improvement if I did a Thornburg style internal dowel with cover panels and tape. Someday, I see that alteration coming. At this point, a bolt-on wing for me would result in a LOT of downtime fixing torn-up airplanes from the occassional bad landing.

I'm definately looking forward to seeing what the S-3021(???) airfoil does on the GP "Spirit" 2m when I get it off the shelf and get it built.

Thanks again for the input - Paul
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