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Thread: Cessna L19 Bird Dog

  1. #1
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    Cessna L19 Bird Dog

    Apparently my Bird Dog thread was eaten by the evil RCAT server hack of 2013. Major bummer as there was some great feedback from the RCAT community. Anyway all is not lost, I have a mirror thread over at RCGroups so I'm just going to copy-n-paste it from there. So here goes...
    Last edited by Nodd; 10-14-2013 at 02:51 AM.

  2. #2
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    It seems I'm the happy recipient of this beautiful 1/6 scale airframe. The previous owner/builder (Dr. Joe Makovich) wasn't able to complete her & so she sat for many years. When I heard she was just gathering dust I happily volunteered to take over the project. As you can see she's been masterfully crafted...



    All balsa/plywood construction (note the side windows haven't been cut out yet)...



    She's a nice size at 72" (1828mm)...



    Love the scale corrugation work on the control surfaces...



    Beautiful functional fowler flaps...



    Beneath the fiberglass cowl lurks a gas motor mount...



    ARF
    So technically this is a kit built airframe (someone built her from the plans up) but for my purposes, she's ARF. From what I've read about Jack Stafford's kits, this was no easy build. Even though I enjoy kit building, its nice to have the bulk of the work done, especially as it appears to have been done well.

    So what are my intentions for this Bird Dog?
    As an avid RC sailplane pilot I spent last Summer learning the finer arts of being towed aloft by an RC tow-plane. This year I'd like to return the favor & do my share by aerotowing with my own tug. So I'm entertaining the idea of outfitting this bird-puppy as an aerotowing workhorse.


    Ben Diss' L19 Bird Dog tow-plane

    Its high-wing layout should make for a nice stable platform. Its 72" wingspan, 1/6th scale should allow me to tow everything from an Easy Glider to a three meter glass-slipper. As a noob tug pilot I don't want to tow anything larger or should I say more expensive, until I have more experience. The big fowler-flaps should come in handy for diving back to the field after release. And lastly she's scale so should be a welcome addition at many of the scale aerotowing events I attend.



    Gas Vs Electric
    Yeap there's no doubt Gas rules when it comes to hauling sailplanes up, up & away. You can tow all day with just a couple gallons of the smelly stuff. With a line of eager glider-jockeys waiting to fly you don't want them waiting around while you're charging batteries.



    I'm not a gas kinda guy though. I have not-so-fond childhood memories of my father having to start my 0.49 cox engine for me. I was afraid to stick my fingers anywhere near that thing, hated everything about it, the prop, the noise, the smell, did I mention the prop? 35 years later I still feel uncomfortable around gas planes which makes no sense, as electrics are just as likely to lob off a few digits *shrug*

    Irrational phobias standing, this will be an electric aerotow tug
    Several of my club mates have electric tugs & as long as there's extra batteries, they work out just fine. We've been averaging around five or six tows per battery, at say five to ten minutes per tow, all together that's enough time to fast-charge a second pack while towing with the other. Theoretically an electric tug should be able to tow non-stop with only minimal downtime while swapping out batteries.

    Weight
    One of the concerns with this airframe is the weight. The fuselage is primarily 1/4" balsa planking. The wing is fully sheeted top & bottom. She seems pretty hefty so I ran the numbers...

    33 oz fuselage
    28 oz wing
    5 oz tail
    13 oz aprox .60 sized brushless motor
    15 oz 4S 4000mAh LiPo
    3 oz 100a ESC
    11 oz 8 standard servos
    16 oz odds & ends
    24 oz fiber-glass, paint, covering
    ____________________
    148 oz or 9.25 lbs total (4.19 kg total)

    With 714 sq/in wing area that equates too...

    30 oz/sq.ft (91 g/sq.dm)
    13 oz/cubic.ft
    27 mph stall speed (43 Km/h)

    Coming from a glider background those numbers scare the heck out of me. Then again if I run the full scale Bird Dog's numbers I get the same cubic wing loading.



    Do I need to find ways to lighten her up?
    I'm somewhat loathed to go hacking up the fuselage cutting lightening holes all over the place but maybe that'd what I need to do. I was also thinking about maybe removing some of the wing's sheeting, maybe cut lightening holes in the ribs. Leave the wing as a partial open structure & recover with Ultracote instead of sheeting. Basically put her on a diet. What do you guys think?

  3. #3
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    Edit: Lots of folks gave me grief about converting her to electric power. They also worried about her being too heavy.

    This project is on hold for the time being while I get some funds together but I did chat with a very accomplished pilot over on the RCAerotowing forums who has a 2M Porter tug. He had great success towing last season, here's how his tug stacks up against this one...

    2M Pilatus Porter
    wing area: 803 sq.in
    weight: 180 oz
    wing loading: 32 oz/sq ft

    2M Bird Dog
    wing area: 714 sq.in
    weight: 156 oz
    wing loading: 31 oz/sq ft

    So that's encouraging. I ran various setup options through RealFlight sim & found this Bird Dog should perform very nicely on a 6S setup using a .60 sized electric motor (possibly a little bigger). Yes she'll fly heavy but from what I've been told, a little weight is a good thing for towing anyway (as long as the power is there). You don't want the sailplane yanking the tug all over the place.

    Well that's the plan for now, I just need to scrape some funds together. I'd really like to have her flying this Summer.

  4. #4
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    It's been a while but I'm back working on this project, woohoo!

    Motor Selection
    I did some chatting with my buddies over at RCAerotowing.com & after considerable deliberation, I ended up acquiring this bloody great hunk of copper & aluminum...



    This L19 is not going to be lacking for power *evil grin*

    I check out the real thing
    Here's a real Bird Dog aerotowing at the Wurtsboro 1-26 Fun Fly last weekend...



    More goodies arrived...



    Started work on the wing saddle area. The stock setup seemed pretty wimpy, especially as I'll be removing much of the side planking for the windows. So I'm reinforcing the area some...



    Bellcranks? Umm no
    The wing's ailerons & flaps are hooked up via old school bellcrank linkages connected to a couple of servos mounted in the center of the wing. I'm going to modernize things by installing four servos, one for each control surface. First order of business is to go digging for bellcranks...



    How quaint...



    Anyone need bellcranks?..



    The old linkage system is out. Holes are cut ready to install the four servos...



    Corrugation
    Looking at the reference photos I took at Wurtsboro the other day, you can clearly see corrugations in the control surfaces...



    My L19 has corrugations too...



    Having recently spent a LOAD of time glassing, finishing & painting another project I'm not in the mood to paint this one too. Instead I'm simply going to cover her with Ultracote. Those cool corrugations I figured would be a problem so I decided to get rid of them...



    What was I doing? The corrugations are cool, leave them be! After hacking up just one aileron thankfully I came to my senses.

    I decided to do a quick test & see how they'd look covered with Ultracote...



    Okay so that doesn't look exactly like the real corrugations but it kinda works. It's certainly more interesting than a without them. I'm sold, lets keep the corrugations. Only problem is I'd already hacked them off one aileron.

    Two hours later I had a new found respect for Joe, the original builder. Wow corrugations are a LOT of work, cutting, sticking, trimming, sanding...



    On the plus side I got to use my nifty strip cutting tool...



    Remind me, never to go hacking up other people's hard work before putting some serious thought into it...



    Servo Hatches
    With the aileron restored to its former corrugated glory I turned my attention to the wing's servos. Triangle pieces of hardwood were added to each servo bay...



    I created servo hatch covers & drilled holes for the mounting hardware...



    I was hoping to get the servos installed but spent most of my time messing with the corrugation issue instead. Hope to get more accomplished tomorrow...
    Last edited by Nodd; 10-14-2013 at 04:09 PM.

  5. #5
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    Servos
    Attaching the servos to their hatches. The kitchen wrap is to keep them from sticking to the epoxy while it dries...



    Servos mounted...



    I was hoping to setup the linkages next but the receiver I'm planning to use crapped out so with will have to wait until I can get a replacement...



    Besides I need to take care of this spaghetti before I can drive the servos anyway...



    There that looks somewhat neater...



    Wing Mount
    The stock wing mounts (nylon bolts) are fine but there's not a whole lot of meat holding those in there...



    Nothing a little carbon fiber can't fix...



    Stuck that to the trailing edge...



    I'll let that setup over night then drill out the bolt holes. I may fiber-glass over the area also, we'll see...



    The front of the wing is held in place with a hardwood pin. I may swap that out for a metal or CF rod but I wanted to reinforce the area too...



    Then again the wing pin & nylon rods are supposed to fail in a crash. I don't want to make those too strong but the holes they mount through should be bullet proof in my opinion.

    For whatever reason the wingtips where never installed. Looks like I have some carving to do, fun fun...



    Well that's all for today...


  6. #6
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    Here's some interesting Bird Dog trivia I just heard. The L19's tail is actually off the Cessna 170. Check it out, they're exactly the same...


  7. #7
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    I love working with carbon fiber... I hate working with carbon fiber
    I know better than to use large drill-bits on CF ribbon, stupid mistake...



    I'll deal with that later. On a more positive note, we have a Rx...



    Remember the other day when I said something about not messing up someone else's hard work? Well I'm at it again.

    Today I started ripping the wing apart...



    I removed the bottom sheeting ahead of the spar...



    Why? WHY!
    Well I noticed there's no sheer-webbing between the two spruce spars (scarey). I'm not into wings that snap (have some experience with that I do). Now maybe no sheer-webbing is fine on a fully sheeted wing, the skin carries the load/stress blah blah blah. Only trouble is I've chopped four bloody great servo access hatches in the wing's skin. So I'm thinking sheer-webbing would be good. So off comes the bottom sheeting so I have access.

    Unfortunately while pulling the sheeting off, part of the ribs came off with it...



    Oops. Well I have a fix for that...



    Maybe if I don't say anything, nobody will notice...



    Anyway back to the sheer-webbing. As you can see there isn't any. So lets add some...



    Ideally the webbing should go between the two spars but as they're already assembled, getting a good fit would be nearly impossible. Next best thing, mount em to the side...



    Despite being a not so ideal C shaped spar, that should still add a lot of strength. It's better than no webbing at all that's for sure...



    Yes I could create a box spar by adding webbing to both sides but I really don't feel like removing the rear sheeting as well. I think this'll do just fine.

    While I'm at it, lets spruce-up these spruce spars...



    Adding a strip of carbon fiber ribbon will turn my lowly spruce spar into a bar-o-kryptonite...



    Of course I'll need to add CF to the top spar as well.

    Sun Roof
    The real L19 sports an array of spiffy windows in the top of the wing...



    As you can see, on my Bird Dog I have two major wing-spars running right through where the windows should go...



    So what to do? I could go re-engineer the center section of the wing & move the spars but that seems a little extreme (I'd like to be flying this thing sometime this Summer). I could change the shape & location of the windows so they don't interfere with the spars. But then every L19 guru would be giving me grief. What I think I'm going to do instead is simply make fake "painted on" windows. They'll look scale & won't compromise the structure. We'll just pretend they're limo tinted glass right?

    Anyway I'm not sure if I've made progress today or just created more work for myself. I think I'll be a lot happier knowing the wing has at least some reinforcing when I'm nose down in a dive, returning to the field after a tow. Can't wait.

  8. #8
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    Edit: Some guy insisted that the shear-webs were unnecessary.

    I believe this airframe was designed before the benefits of sheer-webbing were fully understood. It really does make a huge difference in a wing's strength. For this type of wing I'm absolutely in favor of adding them. Then again it's one of those things we'll never know... did it need webbing or not? No way to tell really.

    As far as the weight, there's really no practical way of making her significantly lighter. Cutting holes in the wing's sheeting won't net me much. Same applies to the fuselage. The large motor & battery will contribute negatively but if I'm flying something that's a little heavy I do not want it underpowered. There's also some very good arguments for a heavy aerotow tug. In most cases you want to the tow-plane to be fairly heavy so that the sailplane doesn't tug it all over the sky.

    Would I have preferred a lighter airframe, you bet. But as I was gifted this one, I'm going to give it a try. Worst case, if it flies like a brick, I simply drop all this hardware into some other airframe. So far however the numbers aren't all that bad. She'll have a wing loading comparable to your average war-bird.

    Poor Man's Wing Bagging
    Okay so I have this carbon fiber ribbon I want to stick to my spruce spar. Gluing 72" all at once & applying some pressure to keep it down is a bit of a challenge...



    I could use a vacuum bag to apply pressure while it dries. I don't have one of those though. I do have an idea however. First I need a pillow...



    A length of foam insulation noodle wrapped in kitchen-wrap...



    Left over pieces of wood from Ikea furniture (always wondered why it wobbles). Some scrap wood screwed to those at 90°...



    The wife's best china...



    And of course some glue...



    Set the wing atop the pillow. Apply the glue & tape the CF in place...



    Place the foam noodle on top of that...



    The Ikea T things go on next. Note the use of a pair of HobbyKing goody boxes as stands...



    Plates go on top to add weight...



    So I think you get the idea here. The wooden Ts distribute the weight evenly to the foam noodle that in turn applies nice even pressure to the CF/spar...



    Let's see if that worked...



    That looks promising. Yeap worked like a charm...



    Sweet! Okay I have people coming over tonight so that's all for today.

  9. #9
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    1/16" Vs 3/32"
    Well I picked up some balsa to re-sheet the wing. Turns out I needed 1/16", brought 3/32". So that's on hold until I visit the hobby shop later this week.

    Motor Mount



    I looked at lots of motor mount options & decided to go with a bolt & shims system...



    Figuring out the geometry is always a bit of a challenge. First thing was to set the fuselage level on the bench. Kindle Fire bubble level app to the rescue...



    Looks like the firewall is 90° to the datum line, that makes life easy...



    I want to setup thrust angles, offset & hopefully end up with the prop somewhat centered in my cowl. Time for some CAD...



    These drawings provided the exact length to cut the nylon shims so I'd end up with 4° down, 2.5° right thrust. Gotta love computers...



    Next I drilled & installed a fresh set of blind nuts...



    Yeap that's a BIG motor although it doesn't look crazy big...



    4° down, confirmed...



    2.5° right, confirmed...



    And will the cowl fit?...



    A little trimming...



    That looks promising...



    Happy with that, nice to have the motor in...



    She's starting to look like an airplane, sweet!

  10. #10
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    Today was the first nice day we've had in a while, spent most of it at the flying field. I did manage to spend an hour in the workshop though.

    Firewall
    Added some ventilation holes to cool the battery...



    Installed the ESC...



    I also did some fiberglass work on the cowl but nothing worth photographing.

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