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  1. #51

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    RE: Piper Cub Sailplane

    Cool, you know what, I think it shows great promise. A few fixes and it would fly fine, especially if you went slope soaring.

    My personal observations.......nothing really new to you though

    #1-It really needs to go on a diet (Just for comparison, I have an enlarged Sagitta 900, wingspan of 111", and it weighs only 39oz) The overall weight and short coupled tail together won't give funny flying.....the CG however..................................

    #2-Perhaps a bit tail heavy. I do recall you mentioning that you went with only a 5% static margin or something. Anyhow, being short coupled with large control surfaces you cuold move the CGforward and I think it would work just fine.

    #3-Towhook too far aft....but you knew that.

    #4-The test launches were too slow. The wing loading is just a bit to high and it wanted more speed, and it was pretty much on the ragged edge of a stall the whole glide. That is why the controls, particularily elevator, didn't seem to do anything. I cringed when I heard you say you were switching to high rates, Iknew what was going to happen on an already stalled airplane with "extra control-ability". It did just what I thought.

    So, really, no surprises for you in my observations. I have no doubt that it could still fly just fine even being short coupled, just get that CG forward a bit more, and get more speed on it..........or reduce the weight.

    Now, all that being said, I showed my father-in-law this thread and he thought it was awesome. He said to me, "you know, I have a bunch of wing bits and other parts from wrecked airplanes, might have to copy the idea".

  2. #52
    Slope Pilot's Avatar
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    RE: Piper Cub Sailplane


    ORIGINAL: Nodd

    Its raining today :-(
    That's funny, it is raining here too.
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  3. #53
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    RE: Piper Cub Sailplane

    I appreciate the insites guys. Most of what you've pointed out has been echoed in my mirror threads on the other forums. So you're not alone in your observations. Before I get into what I've done to fix things, here's a few thoughts...

    I agree there's other materials but I have lots of scrap carbon fiber laying around my workshop from other projects. So when it comes to reinforcing high stress areas, CF is what I reach for. Its not as expensive as many think, its crazy strong & fairly easy to work with. I like it.

    Yes she's heavier than I'd have liked but a lot of that is due to the Cub's stock parts. Not sure what kind of foam its made of but the wings especially were heavy from the get-go. There's not much I can do about that, it is what it is. If I had this to do again & I was serious about making a "nice flying" sailplane I'd probably start with a lightweight built-up wing & work from there.

    I totally agree, the hand-launches weren't much use. Poor Roger didn't have enough "umph" to get her moving fast enough for a real test flight. That was one of the reasons why I chose to move on after just two tosses. A hill would have been better but the nearest slope soaring site is hours away.

    Anyway here's what I've done...



    After Further Analysis...

    Well I goofed up. I've no clue why I chose to just eye-ball the location for the high-start hook, especially as I'm well aware of the 30° ahead of the CG rule for tow-hooks. Look how far back I had my tow-hook positioned!..



    Its no wonder she behaved like she did, I had the tow-hook WAY TO FAR BACK...



    Aerotowing
    So lets simplify things. I lobbed off the nose-skid & "tundra sized" wheel & now we have a nice smooth fuselage to work with. The wing struts are history too...



    After re-balancing here's how she sits...



    Unfortunately due to the Guppy shaped fuselage the tail is still up in the air & it looks like she'll probably rock forward on her nose again during aerotow. Still though I think its an improvement. At the very least she looks cleaner...



    I'm going to concentrate on aerotowing for now. I'll worry about adding a new high-start hook later on.

    So anyway, weather permitting, we'll give her another try tomorrow.

  4. #54
    Slope Pilot's Avatar
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    RE: Piper Cub Sailplane

    Good luck on tomorows aerotow, I am sorry to say this but I never thought the whole skid and wheel thing would never work.[]
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  5. #55
    Moderator da Rock's Avatar
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    RE: Piper Cub Sailplane

    It's kewl to use the materials lying about for sure. Sometimes there really is a good reason to use other materials even if it means checking the medicine cabinet or swinging by the drug store. CF is great stuff for sure too. But consider how much greater area a tongue depressor could have covered had it been used for the connector/spar/stiffener/wing joiner. Foam needs something stronger for support, but foam really doesn't need much support. In fact, if we could find a stiff and at the same time flexible material, we'd be even better off. Of course, then we'd also have to find a glue that'd flex yet hang onto the foam while it flexed and..... etc..... So.......

    When we're screwing around with foam, one problem it presents is how weak the stuff is. Heck, even if we got the perfect foam glue bonding things to it, we can often pull those things right out of the foam with our fingernails. The foam fails everywhere, immediately adjacent to the perfect glue. We wind up needing reinforcement that covers a whole lot of foam for reinforcements or connectors to really help the foam out.

    The advice wasn't really focused on the cost or overkill that CF brings to the job. It was more about the limitation of foam. It was also an attempt to push the 'add more lightness' mindset. A longsince deceased foamy I whacked together for fun got a strip of paper for a wing joiner/center stiffiner. The paper was porous and I had a foam 'white glue' that stuck like... well, stuck like glue to foam. The paper was cut out to be wider at it's 'wingtips' than at it's center. My thought was to provide as much area where the stress riser might show up that there just might not be a stress riser there that would be stronger than any stress inboard on the two wing panels. Turns out the crash autopsy showed one side had the most failure inboard. The plan probably worked better than not, in other words. I hadn't weighed the wing before and after the joiner/stiffener application, but it truly didn't feel like any weight had been added.

    I've since found some quality tongue depressors and use them for stirring epoxy. Break one down the middle lengthwise and you get two glue stirrers with different size applicator tips at the opposite ends. Find a flat one and it can be used for a joiner/center spar. They can easily be used vertically. With a touch more work, one can go on flat. If put on the top of a cambered wing, I've put it on flat with front and side edges sticking up. A sanding block evens out the ugly, unaerodynamic ridges front and rear. It also 'tapers' the strength of the support of the overstrong wood front and aft of that added wood. lol.... Yeah, I know that consideration might not be true, but it's comforting to pretend we're genius designers employing brilliant, clever ideas. And good thing about most of our work is we often get to autopsy the design to see how our clever construction stood up.

    Never admit that the crash disappointed you. Always grin and tell eveyone how you did it to see if some reinforcement helped.

    anyway.... enjoying all your threads and videos. It's not often we run into anyone on the internet with the self assurance to showcase the 'less than perfect results from the development models'.
    Good flying wit ya today

  6. #56
    Moderator da Rock's Avatar
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    RE: Piper Cub Sailplane

    BTW, something I did over the years of drawing up 'original' designs was to run all the planform measurements through the pitch stability formulas. Nowadays we've extremely lucky that those formulas no longer require our doing the figuring. They are all over the internet. All we need do is whip out a yardstick and pencil and do a couple of minutes of measuring. We can plug the measurements into an online app like geistware and hit Enter. bling... we got some really cool results. OK, we got two worthwhile bits of info. But it really is useful.

    One is the suggested CG. It's really good to have a dependable, safe location to start from. If we're making cute things like the Cub, that have a significantly out of the ordinary planform then in fact the CG really might not be best at the old rule of thumb. And in fact we also get a second bit of info that is useful over the long run for people who find themselves continually doing out of the ordinary stuff. and anyway... That little Cub has it's original horizontal tail moved right up behind a wing that just got a hunk more of area and span. Not exactly ordinary.

    It would be nice to have an idea of how much tail moment that Cub wound up with. Will it tell us anything we can use? Unfortunately, unless we've been running the numbers for awhile, probably not. But we can see what we got for anything in the future. We remember the results we saw in the flying of one model and the results in another. I know that works for a couple of stunters developed over the years (many years ago when the TX was a handle and the transmission was not wireless like today) and a number of sailplanes developed fewer years ago. In fact, I wrote down all the numbers and kept a notebook over those years. The tail volume and tail moment and a couple other valuable details were often highlighted in yellow. It really helped a lot to keep from developing the next masterpiece into less master and more pieces. Now if I could just find the damn notebook, I would offer a suggestion as to how small a tail volume might be too small and other sage suggestioxxxxx .... uh...... sage BS.

    anyway.... don't stop now, you got everyone's attention
    Good flying wit ya today

  7. #57
    Nodd's Avatar
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    RE: Piper Cub Sailplane

    YouTube Video, click to watch...
    Cub Sailplane Part 2

    The video pretty much speaks for its self. We'll try again soon.

  8. #58
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    RE: Piper Cub Sailplane

    The guppy shape of the fuselage is a major problem. She's rocking forward while under tow. This places the wing at a negative AOA, which maybe okay once she gets moving enough for the elevator to kick in.

    When we were towing, I noticed the Fun-Cub was getting airborne first. I've seen this happen before & ordinarily its not ideal but that maybe exactly what's needed here. Once the tow-plane is up she'll be pulling up on the Cub's nose. That should in theory help get her un-glued from the ground. That's assuming the tow-plane doesn't stall out first.

    I'm hoping the dolly will fix all this though. It should keep her at a nice neutral AOA & also reduce the rolling friction. I hate to rely on a dolly though. If I was to build a new fuselage (which I may consider but we're not at that point yet) here's what I'd do different...



    A more forward position for the wheel should keep her from nosing over & help reduce friction. It looks like we'd also need a small tail-skid to keep the AOA from getting too extreme. Additionally, placing the tow-release up under the nose would reduce the tendency for her to rock forward. I have the release setup this way on one of my other aerotow gliders & it works well. Anyway that'd require major surgery, probably a whole new fuselage. I'm not giving up on the current setup just yet.

    A guy over at RC Groups said they've been hand launching their gliders while aerotowing. My flying buddy Bernd suggested we try that but everyone at the field shot down the idea. I found this video showing that it is possible...

    YouTube video, click to watch...
    Hand Launch Aerotow

    The sailplane in the video looks like a floater though, my Cub maybe a little too clunky. Maybe we'll give it a try though *shrug*.

    So tomorrow we'll try the dolly & possibly a larger tow-plane this weekend. Onwards & upwards!

  9. #59

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    RE: Piper Cub Sailplane

    I would change the incidence of the wing in comparison to the fuse. It looks like the wings chord line is 0* to the fuse. I doubt that the wing at 0* will generate enough lift to support the glider at practical airspeeds. It also looks the longitudinal dihedral is 0*. This will require the stab to carry quite a bit of uptrim at cruise. How about shimming the wing to +2*?

  10. #60

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    RE: Piper Cub Sailplane

    I'd say you're on the right track: tow-release lower than C/G and wheel axle below wing's leading edge (old rule of thumb).

    And you'd need an unusually fast towplane (due to high wing loading of the Cub). After hand launch the plane seems to be just lowering it's nose to gain speed (means enough pitch stability) but there's simply not enough height (potential energy) to get it and establish on a stable glide slope.

    Thanks for this thread!
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  11. #61
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    RE: Piper Cub Sailplane

    Yeah the more I think about it, the more I realize if I was to make a new fuselage, I'd probably want to build a nice lightweight wing to go with it. If I'm going to do that I'd probably scale everything up some. So basically we're talking about a whole new aircraft. So for this airframe I'm stuck with what I have. I think she will aerotow, with the help of a dolly &/or a bigger tug but if not then we'll look at sticking a winch-hook on there again or maybe just shelf her until I go slope soaring. This project was only ever meant as a way to have some fun with my old Cub's parts before I toss then out & also to learn about working with foam. If she flies, great, if not, no biggy. We're not done yet though *grin*

    I could play around with the wing's incidence but I'd like to see her fly before I'd consider making any changes to the flying surfaces.

  12. #62

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    RE: Piper Cub Sailplane

    I'm not convinced that different incidences alone would do any good. It's quite common practice that such flat-bottom-wing/slab-stab models have the flat bottom and the stab parallel. That's about 2 degrees geometric decalage (longitudinal dihedral) and even 4 degrees aerodynamic. But these models are balanced at 36% to 40% MAC while your Cub seems to be balanced considerably more forward. So it has to fly quite fast already due to wing loading and in addition it's trimmed/balanced to fly even faster (not at best-glide or slowest-sink speed). Of course you could increase decalage but I would rather set the balance point back because that would unburden the hard-working wing from making up for the stab's down force.

    By the way, I don't think that the short moment arm requires more decalage. I think it is compensated by more wing downwash on the horizontal stab. The usual aft balance point ist mainly due to big airfoil pitching moment (coefficient is -0.08 or even -0.1). And consider stall and wingtip drop even on such a square Cub wing. As far as I know, the real thing has a bit washout!

    Edit: Seems you didn't use flaps for launching (neither for aerotow). I think that would be the simplest/cheapest way of trying to get that kite airborne. The flaps won't make for much lift but for a decent decalage adequate to the now fore balance point. That should make the model slower and hence towable, and it might come off ground without further ado. At least I think so, and it's worth a try.

    Do you actually know the C/G's vertical position? (to know where the tow release should be)

  13. #63
    Nodd's Avatar
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    RE: Piper Cub Sailplane



    I'll have a flight report & video tomorrow... woohoo she flies!

  14. #64
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    RE: Piper Cub Sailplane

    Here's the video you've all been waiting for...

    YouTube Video, click to watch...
    Piper Cub Sailplane Part 3

  15. #65
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    RE: Piper Cub Sailplane

    It looks like a superbly fun glider. Like you mentioned the landing approach did sort of resemble a greased manhole cover But it tows and glides well enough for sure. And it was obviously worth the time invested.
    Witty saying to be plagarized shortly.....

  16. #66
    Slope Pilot's Avatar
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    RE: Piper Cub Sailplane

    Great job!
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  17. #67
    Nodd's Avatar
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    RE: Piper Cub Sailplane

    Thanks guys. Yeah I was pretty sure she'd fly if only I could get it off the ground. What I didn't expect was how nicely she flies.



    Flight Report
    Well the video pretty much tells the story but here's a few thoughts...

    The ground handling isn't good.
    The guppy shaped fuselage with a big bulge right at the CG makes her rock back-n-forth causing a lot of friction. Although the shape looks good its does cause issues for sure. A wheel placed at the leading edge of the wing would have helped. I may fashion a custom dolly for her.

    Tow speed.
    With the moderate wing-loading I expected her to fly pretty fast & she does get up & go when she wants too. I was afraid she might tend to overtake the tow plane like many slick modern scale sailplanes will do. The flaps were effective air-brakes though, pretty easy to control the speed on tow.

    Handling
    She flies like she's on rails, this is one of the most stable sailplanes I've flown. That's surprising considering the absurdly dinky fuselage & short-coupled tail. I expected her to be really pitch sensitive but she's not at all. The 40% expo on the elevator should be helping but it seems there's sufficient h-stab back there to make her natively stable. Turns are nice too. I have a fair amount of aileron differential programed & also mixed in some rudder (except when the aerotow release is closed) so she stays nice & coordinated. With the flaps also acting as ailerons the roll rate is pretty snappy for a sailplane. Again with the moderate wing-loading she carries a fair amount of energy which is great for aerobatics. Makes for some serious fun tossing her about the sky.

    Landing
    The big barn-door flaps were very effective. Although I've not tried a full flap, 45° nose down landing yet, I'm thinking she'll do that without picking up much speed. I have a 1/4" of crow programmed into the outboard ailerons to further slow things down but mainly to keep the wing-tips flying at low speeds. After all it was a tip-stall-o-doom that destroyed the original stock fuselage, I don't want a repeat performance. The inboard ailerons remain functional even when acting as flaps, so roll control remains crisp all the way down to stall speed. One of the easiest planes to land I've flown.

    Thermalling
    Before you laugh, if I can resist the urge to point the nose towards the ground & fly aerobatics, I think this daft little glider might actually thermal. Granted it'd probably need medium to strong lift but if I keep the airspeed up, the sink-rate isn't actually all that terrible. I know from flying some of my other heavier sailplanes, a high wing-loading doesn't necessarily mean she drops like a brick. Those do have nice long, high aspect wings with fancy airfoils though. We'll have to see.

    Slope Soaring
    We're talking an hour or two road trip at least if I want to hit the slopes. I think it'd be worth it though as there's little doubt where this silly Cub belongs. Fairly aerobatic, tough foam construction, big flaps for small landing areas, good wind penetration. She belongs on the slope.

    Conclusion
    So what I believe I have here is a "flat-land slope soarer". She's docile enough for some lazy thermaling (assuming there's enough lift) yet aerobatic enough to tare up the sky when she wants to. The aerotow capability means I don't have to shelf her until we hit the slope. I love the attention she gets too, definitely something you don't see every day. I wish she wasn't quite as beat up looking but at least I don't have to worry about scratching the paint. I could see myself building another one, perhaps a lot bigger, probably a lot lighter construction. I think that'd be something fun to take to the various scale aerotow events I attend. That's assuming they're okay with a "fantasy scale" glider in their midst. *grin*

    A big thanks to the New Canaan Remote Control Society for putting up with my silliness. You guys are the best. And also for those that contributed advice here on the forums. Well time to move on to the next project. Stay tuned for more "adventures in radio control"...

  18. #68
    Moderator da Rock's Avatar
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    RE: Piper Cub Sailplane

    Excellent outcome....

    Design development is an artform. And a bit of, "hey, just try it." thrown in.

    Amazing to see the inboard ailerons in action during that roll. We certainly have some trick stuff in our tiny little radios, don't we. Clever of you to utilize it.

    [thumbsup]
    Good flying wit ya today

  19. #69
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    RE: Piper Cub Sailplane

    Very nice.

    And, now in the current December 2012 RCSD

    http://www.rcsoaringdigest.com/


    Jared
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  20. #70
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    RE: Piper Cub Sailplane

    UPDATE
    Well its been a couple of months now since the maiden (or should I say first successful) flight. The Cub sailplane is still flying great, I have loads of flights on her now. I even took her to a few giant scale aerotowing meets, where she raised more than just a few eyebrows. Here's a few photos shot by RC Aerotowing sailplane guru Len Buffington...



    I wonder if this tug is up to the task of towing my mighty Cub...



    LOL yeap I'm pretty sure she has enough horse power...



    Off the ground in two feet...



    Up up & away!..



    Some day I'll build a bigger one. Anyway thought you might enjoy that.

  21. #71

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    RE: Piper Cub Sailplane

    Neat project!

  22. #72
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    RE: Piper Cub Sailplane

    Well done Nodd, a fitting outcome to a great little project.
    You can see with your low turns how how it behaves so well.

    Good luck with her,

    Brad
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  23. #73
    Nodd's Avatar
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    RE: Piper Cub Sailplane

    UPDATE
    We're deep into the Winter now & I'm still playing with my Cub sailplane...

    YouTube Video, click to watch...
    Piper Cub Sailplane Snow Tow

  24. #74
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    RE: Piper Cub Sailplane

    I can't believe that you had the nerve to hook it up to the line on that big Pawnee. It looks like a Great Dane with a mouse in its jaws....
    Witty saying to be plagarized shortly.....

  25. #75
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    RE: Piper Cub Sailplane

    LOL, yeap it looked totally ridiculous. On one tow we went vertical with the little Cub rolling as we climbed. I mostly flew my 4m Discus that day, took the Cub along for comic relief. The guys with the big ships got a kick out of it.


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