Cessna L19 Bird Dog

[Nodd]

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?

[CHELLIE]

Do I need to find ways to lighten her up?

Nope, Leave her as she is, you need the weight for momentum and stability to pull the gliders with, if your tow plane is to lite, it will just get tossed and yanked around.

[BBCorvette18]

If it was designed to fly like that why wouldn't it fly with the proper sized electric motor? That thing looks to good to start cutting holes in it.

[Nodd]

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...

Nodd RC 049 - Wurtsboro Full Scale Aerotow (12 min 1 sec)


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...

[pd1]

Nice project. Glad you're going with the larger motor.

I used to fly full sized Bird Dogs many years ago, I liked them.
The wing is basically the same on most late model Cessnas. The tail was off the 170.
The 170 had a 320 cubic inch motor and the L-19 has a 470 cubic inch motor.
Not a lot of difference in top speed, but the L-19 had a short take off roll and would sure climb.

Yours should be pretty scale like in performance.

[Nodd]

That's really cool to hear from an actual L19 pilot. From what I've heard, yeah they're real work-horses. As I'll be towing some pretty sizable sailplanes with this, a big motor was a must have.

That's really interesting about the tail. Yeah look at that, they're the same all right...



Cool stuff. Okay on with the build...

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...

[maxflyer]

Why not consider airbrushed corrugations? If done well one would have to get right up next to it to tell the difference.

[Nodd]

Faux corrugations is a thought. If the Ultracote doesn't work over the actual corrugations I may give that a try. It looked pretty good in my test covering though. Thanks for the idea.

[fhhuber]

I sometimes replace 1/4 inch dowels for wing mounting with bicycle pennant fiberglass rod. Its the same size and much stronger.

The problem with replacing that dowel in your wing now is drilling the old one out. I'd consider drilling and inserting 2 more before replacing that one. Looks like good access available for this... If you are paranoid.
A pair of 1/4 inch dowels holds the wing just fine in my 18 lb 50 CC Corsair.

[Nodd]

That's an idea, using fiber glass rod as a wing-pin. I still like the idea of them failing during a crash though. I guess the trick is to use something strong enough that the wing doesn't fall off in flight, yet weak enough to let go when things go bad.

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.

[fhhuber]

Well... a lot of it was creating work for yourself.

I agree it needed the shear web. for at least the center half of each wing panel.

Better to be a bit paranoid...
I'm not as paranoid about wing strength as you. I wouldn't mess with the upper sheeting.

[poppawang]

Very interested in this build.
I have a set of the plans from RCM as well as the cowl and window cutouts from fibreglass specialties.
I'll be starting my build soon.
Nodd, would it be ok to ask for a copy of the instruction book? I think that may be very helpful during my build.

thanks
Andy

[Nodd]

Yeah I've seen all too many wings fail recently. It's not all that much work to beef things up a little. I'm just going to be a lot happier flying this knowing what's under that sheeting. Doing these mods is as much about calming my nerves as it is about improving airworthiness.

Hey Andy. Yeah it looks like the RCM plans are for the same model. It has the same stats & both were designed by Rob Rich, gotta be the same plane. I scanned the manual for you although I should warn you its old-school, all text with no pictures (the horror!). You can download that here...

L19 Manual (PDF)

If you need photos of anything let me know. It'll be nice to compare notes. Stay in touch as your build goes.

[gramps2161]

Nodd always nice to see you post up a build and follow along. I don't think there has been one that I haven't learned something new. Looking forward to your progress, and watching the changes you make to this plane.

By the way peace of mind is priceless.

[poppawang]

Many thanks Nodd!
I read through this and already it clarified a few things.
Funny no pictures though.
I should have all my wood within a couple weeks and be ready to start cutting parts.
Here is the scheme I will be doing. I'm going to try out flite metal over a glassed finish.
I'll post a build blog here as well if anyone is interested.

Andy

[gramps2161]

Andy be great to have ya do a build and welcome to the site.

[Nodd]

Originally Posted by gramps2161

By the way peace of mind is priceless.

AMEN to that *smile*

That's a sweet scheme there Andy. I've found chrome to be a bit of a challenge, shows up every imperfection. But when done well, oh boy does it look great. Can't wait to see how yours turns out. Best of luck.

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.

[pd1]

Great idea with the dishes for weight.

[Nodd]

Yeah I've used plates a few times now for stuff like this. I figure I have a bunch of them & they're easy to stack for adjusting the weight. Magazines, socks filled with coins, floor tiles etc, all sorts of stuff around the house to use.

No more progress on the Bird Dog for a little while
I'm headed to a big aerotow this weekend & then doing some fishing, will get back to this next week.