Nodd's Great Planes Spectra
 

http://www.scipie.com/rc/spectra/gre...tra_splash.jpg


Back in the '80s I used to fly the heck out of my wooden rudder/elevator Gentle Lady. Wonderful little glider, would launch her from a high-start. Most flights were only a couple of minutes but once in a while I'd hook a thermal & it was game on. Since then I've flown all manner of sailplanes & I still enjoy thermal duration. Recently I've had a yearning to return to my roots & fly an old wooden two channel. My buddy Roger had an old beat up Spectra collecting dust so offered me the airframe. Okay so its not a Gentle Lady & it is electric powered (no high-start) but close enough.


So anyway after looking the airframe over & doing some surfing online I've decided I'd like to make a few modifications before slapping some electronics in her. The following is a record of my adventures with this fun little glider.


And before you guys start in with the comments, which I welcome by the way... I should mention that I'm not at all worried about maximizing performance or optimising handeling. This project is purely for the fun of tinkering with an old airframe. While I welcome advice you should know I'm a stubborn SoB & will invariably end up doing things my way anyway. I have a pretty clear idea of what I'd like to do here & I think you'll like what I have planned. And if not, then you can tell me "told you so" when she falls out of the sky. Enjoy...

The Wing
First thing is to remove the patchwork of old covering. There had to be four or five different brands of covering on there, this wing has been repaired that many times. A heat-gun helps release the covering, comes right off...


http://www.scipie.com/rc/spectra/IMG_0280.jpg


Now I've heard lots about how the Spectra is prone to tip-stalling. Folks have had success taming this beast by simply adding sheeting to the outer panels.


Mine are in pretty bad shape. The trailing edge is all sorts of warped-n-chewed up & sheeting her is not going to be as simple as slapping some 1/16" on there. So I've decided to rebuild them. First order of business is to cut off the old outer wing panels...


http://www.scipie.com/rc/spectra/IMG_0282.jpg


And seeing as I'm rebuilding these wing panels I figure I may as well take this opportunity to have some fun & do a little modding. Using the stock airfoil shape I drafted a new longer set of wing panels...


http://www.scipie.com/rc/spectra/IMG_0284.jpg


The new design will increase the wing-span from six feet to eight. I also decided to try using half-ribs instead of sheeting. Like sheeting these little riblets help maintain the airfoil shape between each main rib but at a fraction of the weight. No they're not as effective as sheeting but good enough for this old gal. So here's how the new wing panels might look...


http://www.scipie.com/rc/spectra/spectra_cad_003.jpg


And here's my template for cutting the ribs...


http://www.scipie.com/rc/spectra/spectra_cad_004.jpg


After printing my rip templates on regular 8½" x 11 paper I use contact cement to glue those to my balsa. Then after rough cutting the ribs on a scroll-saw I use a belt sander to finalize the shape. Not as slick as laser-cut but a lot faster than hand cutting with a knife...


http://www.scipie.com/rc/spectra/IMG_0329.jpg


I'll cut the notches for the spar tomorrow & then we can start assembling these...


http://www.scipie.com/rc/spectra/IMG_0330.jpg

You might want to throw some tabulator spars like the early CraftAir or Paragons, they work, but they look really nice too! I'd stick with the 2 meter wing since you ask, the longer wing will wallow on a 2M fuse and stab.

Because I'd like to keep the tip panels light I'll not be using sheeting so we need some way to keep the covering from sagging between the ribs. Saggy covering is no good for the airfoil, no doubt. You're not the first to suggest turbulator strips...

Turbulator Strips
http://www.scipie.com/rc/spectra/turbulators.jpg

Sheeting or turbulator strips are not the only solutions however. There's also the half-ribs option. Much like turbulator strips these also help maintain the airfoil by supporting the covering between the main ribs. For this sailplane I'll be using half-ribs, one between each full-rib...

Half Ribs
http://www.scipie.com/rc/spectra/half_ribs.jpg


You're quite correct, slapping an eight foot wing on an airframe made for six will affect its handling. However I've run the numbers through my sailplane calculator & the horizontal stabilizer, while on the low end, does has sufficient area. The vertical stabilizer on the other hand will need to be enlarged a little. So with a slightly bigger rudder she should not wallow too badly.

Good points, both.

Wing Assembly
I printed my CAD work on 8½" x 11" paper, taped the sheets together & we have plans...

http://www.scipie.com/rc/spectra/IMG_0333.jpg

Started gluing the structure together...

http://www.scipie.com/rc/spectra/IMG_0335.jpg

This will have to do for the day...

http://www.scipie.com/rc/spectra/IMG_0338.jpg

Looks good, your typical well thought out clean work. Yesterday I was just thinking of what you going to be building this time.I looked you up I couldn't find any new projects. Love your Gullwing electric sailplane.

What airfoil did you chose? You say stock, but do you know what they used? Did you know that the AG-34 to AG38 where design specifically to counter the sag between ribs?

I saw some speed props that kind-of went the other way though, they use nobs on the leading edge to copy Gray whale flukes which have them for lower drag. I had been thinking of building a wing, then rapping each rib with some CF tow I have before covering to get a rise at each rib, the thing that is against doing that is it will look as though I'm a worst builder than I am.

The Great Planes Spectra uses the Selig 3010 airfoil. Here's the website on the kit:

http://www.electrifly.com/largeelectrics/gpma0540.html

Once you get it done, we know you'll love it!

I took this photograph of the center wing panels & imported that into my 2D CAD software (CorelDRAW). I then digitally traced the airfoil shape & used that to create the ribs for the outer panels...

http://www.scipie.com/rc/spectra/IMG_0284.jpg

Thanks Bill, good to know. Yeah I have high hopes for this little glider, should be a nice lazy floater.

Chris,
Neat winter building project. As a suggestion, you might want to calculate the horizontal tail (stab) area with respect to the original wing area and determine its percentage. Then consider enlarging the stab area to the same percentage as the elongated wing. The vertical (fin) area should also be increased but you can probably achieve this with a simple dorsal addition to the existing fin. The aerodyam experts can probably chime in on these mods also. You'll enjoy flying this modded Spectra once winter abates here in the northeast.
Rgds,
Art ARRO

Thanks Art. As I mentioned earlier (post #4) I've run my modifications through a tried & trusted sailplane calculator spreadsheet & the stock horizontal stabilizer is of sufficient area to cope with the larger wing. The software did recommend enlarging the vertical stabilizer however which I am planning to do.

That's a very sensible thing to do for sure. It is worth considering beyond the spreadsheet's answer if that is just an "it'll work" result. When the tail moment is reduced, the effort required to control pitch is increased. That might mean the horizontal tail is not only having to work harder, but that work results in more drag than necessary. When you enlarge the wing without matching the tail to the increase, you can wind up with a glider with less performance. The unchanged tail winds up generating excess drag to do the extra work.

Increasing the size of a tail by increasing the span often gives a more efficient tail. Same idea as higher aspect ratio wings..........
Just a thought...

True Mr Rock. There's not much point to a larger wing if any benefits are lost because the tail is working too hard. I do not believe that's the case here however. These old two channel gliders have bloody huge tails to begin with. The horizontal tail is within the comfort range specified by the spreadsheet. If it was outside of that I'd definitely be making changes.

Although I'm modifying the wing to some extent, I'm trying very hard to resist the urge to rebuild the entire airframe which is something I could easily get carried away with doing. Building something completely new by replacing parts one by one is not my goal. Better to just scratch build something in that case (which I've done on occasion). I'd like to keep this mostly Spectra. I'm confident she'll fly fine as outlined & as I stated in post #1, this project is not about maximizing performance. I just want to have fun personalizing this old gal & get her in the air.

If it's not too late you might consider the half ribs extending a little past the spar. The biggest source of problems with airflow separation on many of these wings is the sudden straight line dip at mid rib right at the point of maximum thickness occurs. Namely the spar. This is the reason for Mark Drela's Allegro Lite having the top sheeting extend back to around 50 to 55% of the chord. Doing the same with the half ribs on your wing would produce some of the same gain.

I have to agree that the wing will now be a little too long to go with the fuselage. You'll likely find that some of the Spirit's good manners have left for Florida. It's not only a case of enough damping. There's other issues with the tail controls not having a long enough lever arm to work with on the new longer wings. Sure, it'll still fly in a stable manner and all that. But I suspect you're creating enough "features" in the handling that the graceful balance of the controls found on that old Gentle Lady might not be there any longer.

I had a GL back when as well. What a graceful and yet playful model it was. As you say the controls were excellently harmonized and there wasn't a bad trait to be found other than not wanting to fly fast. On calm to mild days it was a delight to fly. And you know.... You do realize that with all the work you're doing you could easily BUILD a GL from SCRATCH, right? :D

At any rate we're not going to sway the sort of man that can build a J3 glider and still retain secure in his manliness..... :D So I'll just wish you "Bonne Charpente"

I like the sheeting to 50% chord idea, that makes good sense. Interesting idea, perhaps for another project.

Well I don't agree. Increasing the size of the vertical stabilizer should compensate for the larger span. Is this optimal? No but for this project it'll be fine. There are plenty of short coupled designs that fly beautifully. Case in point, here's me back in the '80s (note the Ghost Busters tee-shirt)...

http://www.scipie.com/rc/spectra/flying_wing_1980s.jpg

Now granted this is a flying-wing design but it does kinda illustrate that a tail doesn't have to be four feet behind the wing to be effective. The rudder on this was bloody huge though. No ailerons, this was a polyhedral rudder/elevator design. Sure wish I could remember its name, awesome sailplane though.

Hogwash. Spending a couple of days slapping together a pair of wing panels is NOT anywhere near as much work as scratch building an entire airframe, even a kit.

Anyway I don't want to spend a whole lot of time defending what I'm doing. As I said in my opening post, I have a pretty clear idea of what I want to do here.

Warning: The following may not be what you would have done.

Windlord, but the tips are different than the plans. You just couldn't help yourself even then. : )

I have a set of plans that I blew up to 200%. I built the fuse, but never finished the wing.

You're spot on about the wing build. I used to build one a week just to compare airfoils back in the salad days when new airfoils showed up every couple of months.

As for going on the defensive... Please don't consider unrequested advice as anything more than what most of it is: musings and such dredged up by your very entertaining projects Very often much of the advice is intended for the modelers attracted by your threads. Consider it more as incidental detail intended to enlarge your presentation.

BTW, when I was churning out all those wings I made up a couple with much greater than original wingspan, just to see what there was to learn. Near as I could tell, the test mule never showed symptoms of anything suggesting too small a vertical tail when the longer wingspan wings were strapped on. And there was no indication the horizontal tail might have insufficient power. What did show up was a suggestion of hunting when cruising. And there never was a clear indication of enhanced climb in thermals. I expected to create a better "soarer" and saw something akin to the opposite. In fact, on marginal days I was pretty sure the longer wings were a penalty. I never did build a larger tailed version of the models to see what they would show.

Your unique threads are some of the most enjoyable on the internet, and most certainly are going to kick a lot of glider guys awake and that's going to generate thoughts beyond yours. Don't spend any time defending. Heck, everybody learns from your successes and mistakes, and you obviously have no problem describing both.

BIGGER WING IS BETTER! *grunts like a caveman* more space for the lift-fairies to sprinkle their magic pixie dust.

Windlord aye? That sorta sounds familiar. I think mine may have been a variant of that as there are some subtle differences. It was an impressive airframe for sure with that enormous eight foot wing. I recall having issues balancing it though. I ended up having to put an obscene amount of lead in the nose, made her a lot heavier than I would have liked. I had thoughts about making a new fuselage with a slightly longer nose in hopes that'd fix the issue but I moved on to other projects instead.

A 200 percent version? !!! That would have been something. WOW

I think the early '80s sailplanes had too much lift, this caused lots of drag which decreased range. Anything will go up in a thermal, the way to find a thermal is to move around and hunt for it.


Ditto that!

Yeah I hear ya but you get enough folks speculating all this stuff about your design (mostly negative stuff I should add) you kinda feel obligated to explain your reasoning. It's fine, it's just off-putting is all. Anyway here's some more wacky Nodd thinking for you all to chew on...

Sheer-Webs
I swear I'm not doing this just to make you guys crazy. Yes the grain in my sheer-webs is NOT vertical *shudder*. I've been over this in my other build logs so not really looking forward to revisiting it here. In a nutshell though, I've discovered grain direction is not as critical as one might think (please feel free to read up on the subject & form your own opinion, fascinating stuff). I do it this way simply because it's easier.

These outer wing panels are tapered so the sheer-webs are not all the same height. So to simplify production I cut a tapered strip of balsa based on the height of the spars...

http://www.scipie.com/rc/spectra/IMG_0345.jpg

I then cut each sheer-web to length...

http://www.scipie.com/rc/spectra/IMG_0346.jpg

Glue it in place & ta-da! it's the correct height...

http://www.scipie.com/rc/spectra/IMG_0350.jpg

I then took a closer look at the sheer-webbing in the wing center section & was horrified to find the webbing missing from these four bays. Now granted this wing has been repaired a gazillion times but as far as I can tell, there's no sign that sheer-webs were ever installed in these four bays. Very odd, I wonder what the story behind that was? Ran out of wood maybe? Geesh!...

http://www.scipie.com/rc/spectra/IMG_0352.jpg

Anyway I added sheer-webs to these missing bays (used vertical grain this time to match the existing webbing).

Half-Ribs
With the sheer-webs installed on both sides of the spar it was time to install my nifty little ribblets. I've always wanted to give these a try, liking the look of this...

http://www.scipie.com/rc/spectra/IMG_0357.jpg

Windlord? Close but not quite. It's a WindFREAK. My fuzzy memory says it was a Roger Simpson design. It appeared in RCM way, way back in the mid 80's as I recall. A flying buddy of mine built one.

A case in point example of flying traits that goes with DaRock's story. About the same time as my buddy built the Windfreak I did an RO-8. It flew well but it had some odd features. Such as when in a thermal if I slowed it down a hair too much it would try to level the wings and fly out of the thermal. I just chalked it up to being how it was. Fast forward about 3 or 4 years and when I wanted to do my first electric powered sailplane I simply built a new fuselage for the wings and tail from the RO-8. By that time I realized that these traits of the model's handling were due to a lack of stabilizing authority. Not that it was unstable. Just that the size and leverage of the tail was not able to properly handle the forces at all flying speeds between stalling and lawn darting. So I made the tail about 2.5 inches longer and even reduced the size of the vertical slightly to make up for the added length. The new electric version with the longer tail was and still is a total dream to fly. The slow speed thermaling issues and sometimes sluggish reaction to rudder inputs went away totally. The electric version flies with the same snappiness and positive reactions to inputs as the Gentle Lady. Just while flying a little faster.

Since it appears that you're rebuilding the wing on this model as well as making a new wing you'll be in a good position to see if this occurs in this case.

And that's where the problem was for the most part.. A number of things suggested the extra wing was a penalty when hunting. It also seemed to be no faster climbing in the thermals than before or with others of it's like that didn't have the extra area and more efficient aspect ratio wing. The hurdle us glider guys will never cross is that we have to subjectively judge so many of our "tests". When the test fails badly that ain't so hard to do. But most times the improvements and failures are low order. And almost always there are a multitude of other things going on. For example, Nodd's got a handful of things going that should improve the new wing over the old one beyond the extra area and span (and AR).

But by golly, it's fun so far, ain't it.

Shear web grain direction works great either direction. Early magazine articles often argued the two directions. Bottom line is both beat none all hollow (pun intended).

Nodd, your pictures are quite a stimulus this time of year for some of us to get off our...... etc. Something exciting about a structure pinned out with the glue drying. Jeez, can almost smell the balsa.

Yeap that'd be my flying wing, WindFREAK. Cool thanks for joggin the old noggin. I was going to say I got the plans from the AMA but RCM is a more likely source. I might have to revisit that plane, she sure was something. I can't say I experienced stability issues but looking at it now I would say the rudder, as huge as it was, probably could have been larger still. Not sure the weird -45° wingtips were much help either. Now that I know what it is I wouldn't mind building another WindFREAK. It's always bothered me that I wasn't able to enjoy her due to the weight issues. She was built crazy strong like a brick-poop-house. Not sure why, could probably stand to go on a diet. If I were to build another I'd also flatten the wing some, install ailerons, set it up for aerotow or e-power. Here I go again... can't leave well enough alone.

Anyway getting way off topic... back to the Spectra, back to the workbench Chris...

FWIW, here's what I did with my Spectra.
Sheeted outer wings with 1/32 balsa. Enlarged rudder by 1/3
Changed to 2217 low kv outrunner and 3s 2200 lipo.
Need that bigger lipo for balance anyway.
The motor & lipo change cut 20 ounces from weight making it one of the best flying 2 m woodys IMO.

Bo

PS: I really believe shear webs should have vertical grain to maximize strength.