Joining the wing halves

I’ve spent my time on other priorities last three months, but now it’s time to pick up the build again!

When I finished the second wing half, I found two issues: the wings seemed to be twisted very slightly, introducing less washout than intended, and the retracts weren’t equally aligned, one sat a little deeper than the other one. I wanted to fix both issues before continuing with the build.

To start with the good news, when I turned over the wings, laying them flat on the table with the wing tips touching each other, I noticed the degree of washout was perfectly equal for both wing halfs! You can see the wing tips align perfectly well:

With the building tabs on the table, you can still see they hover about .5 mm above the table, but they are equal in that matter, so it won’t influence the flying characteristics that much. Maybe I can twist them back in position when putting the sheeting on, we’ll see, but anyhow I am satisfied with the result so far.

I then tested then wing halves clamped together with the right amount of dihedral, also a good fit:

For the struts, I had to alter one of the retract mounts, filing it down until the two struts had the same position, depth within the wing and alignment towards each other:

This is a close up of on of the retracts. They sit a little deeper than the plans suggest, but the fit is so tight that it doesn’t weaken the structure:

So with all those details sorted, it was time to join the wing halves. A very exciting task. It took me quite some time to get everything to fit properly, but when it did, gluing was a non-event actually:

Right wing progress

I’ve made a lot of progress on the right wing last week! The day before NYE I’ve pinned the main spar and the ribs on the building board:

And now, a little over a week later, I’m almost at the point that I can join the wing halves. I only need to add the sheeting of the control surfaces and the shear webs. Tonight I glued the retract rails in place:

Looks like the retract fits even better than the first one: it goes in almost completely when retracted! It’s only a matter of tenths of a millimeter, but this proves it’s possible to get it in completely. When everything still works out like this when the glue is cured, I’m going to adjust the other side again to get them both to fit like this.

Too bad I discoved one of the Robart wheels is almost unusable.. a bit of a disappointment for such a quality brand. It’s completely misshaped and so it doesn’t track well. It even touches the strut in two places. Think I need to order another set.

And today, all the way from Illinois, this arrived. It was well packed, delivered within two weeks (I ordered it on boxing day) and shipping was only $2!

I ordered this cowl because it’s from fiberglass, it’s finished and already painted. The cowl in the kit is from ABS plastic and consists of three parts. You need to cut those parts free and glue them together. And the checkered pattern is difficult to paint well, or at least time consuming. I think the quality of this ARF-version cowl is reasonable and it fits quite well, so I think I’m going to use this one. Although I will probably still try to build the one from the kit, so I can decide on the end result (and weight) of both cowls.

Left wing structure finished

Now I’ve resolved the retract installation issue I was able to finish up the structure of the left wing fairly quick. First, I’ve glued in the shear web for the retract (rails) which you can see here:

And this picture shows the completed retract mount and cutout rather well, you can see there’s not much left of the rib the retract’s sitting in, but nonetheless it’s still quite strong:

Time to prepare the control surfaces sheeting for the wing. I had to make four aileron sheets, two upper flap and two lower flap sheets. They are cut out from stock 1/16″ balsa using the patterns provided on the plans and then the edges are beveled so they could be joined at the trailing edges. The first line on each sheet indicates the bevel and the second one marks the end of the ribs:

Then I cut and added the shear webs to the back side of the main spars:

And sheeted the flap:

Sheeting the control surfaces isn’t difficult but you need to come up with a technique to hold the sheeting in place while the glue is curing (since I’m not using CA whenever I can). First, I’ll check how the sheeting should be fitted:

Then I use aliphatic resin (white wood glue) to glue on the upper aileron skin (the wing is upside down on the building board). I use packaging material which is quite spongy to hold the sheeting in place, when pressed under the wing, this will press up the sheeting against the ribs. Of course, doing so would lift the wing of the building board so I need the weight of a few batteries to hold the wing down:

Might not look very professional, but it sure works! The lower sheeting is glued with the same glue and held in place with pins and batteries while curing, here’s the end result:

Which means I’ve finished the structure of the left wing, so I’ve removed it from the building board:

After trimming off the tabs (except W1 and W11) and sanding both the spars and the sheeting flush with the root rib, I could turn it around and clean it up:

Time to clean off the building board and pin down the plans of the other half of the wing and repeat all steps, which should go faster as all prep work is done already. As we speak the ribs of the right wing are curing being glued to the upper spar already.

Retract fitting take two

And I’m back at the Jug again…! I received the new struts and wheels a few weeks ago, just didn’t have the time to work on the plane and I also got distracted by some side projects.

The new gear, Robart 3/8″ offset RoboStruts with Robart diamond thread wheels (3-1/4″):

Unfortunately, after puzzling everything together I found out this setup also didn’t fully retract into the wing – the struts were protruding on the bottom instead of through the top of the wing. I spent a few nights measuring and tweaking and came to the conclusion I had to rotate the retract rails a bit to get the retracts and wheels to sit a little deeper into the wing. Lowering them wasn’t an option, because that would weaken the structure of the wing too much. Still, the struts stick out a tad but that’s something I can live with (or something I have to live with I guess). Also, because I’ve tilted the whole assembly by two degrees, I’ve created a positive camber in the wheel stance. It’s the best compromise I could come up with and in the end the wings just aren’t thick enough for retractable scale struts and wheels, certainly not with the wheels retracting before the main spar (the actual scale location would be further back where the wing is thicker, but that would require you to cut through the main spars and this would complicate the build even more). Anyhow, I’m happy with how it turned out. These struts and wheels are going to look great (even more if I can get the doors to fit as well later on). Here you can see the gear retracted in the wing:

And this is with the gear out. It’s hard to see but the struts should sit at 85 degrees because of the 5 degree dihedral, but due to my altered setup they are positioned at 83 degrees when lowered. Furthermore, alignment is spot on:

And here you can see how the struts still protrude a little (note that the balsa wing sheeting still has to go on):

I have made a little vid to show the retracts in action from different angles:

When I was satisfied with the setup, I glued the rails in place with 30-minute epoxy. Now I can finally continue with the rest of the wing (mostly sheeting), before starting with the other half. Final assembly of the retracts will be done when both wing halves are finished and joined, so I can align the struts (equal camber, toe-in / tracking and general aligment such as height and forward angle).

Left wing structure

In the past two weeks I spent a few evenings to finish up the wing structure of the left wing. Despite the complexity of the wing structure, it goes together quickly and almost effortless. You can see I’ve added a bottom main spar, leading edges, multiple aft spars (flaps, ailerons and inner spar), the outer wing tip structure and multiple servo hatch supports. By the way, I changed the way the ailerons are controlled: the plans suggest one central aileron servo with push rods and bell cranks, but this hard to adjust later on and also prone to result in a sloppy control mechanism, causing flutter far more easily than with a short rod and a direct connection between servo and control. So I added two servo hatch supports where the aileron servo will go and dropped the whole push rod and bell crank construction. A lot easier to build too. Here’s the wing structure almost finished:

And from another angle:

Then I cut out some rib material and placed the retract rails to test fit the retracts and struts. It looks like it fits in very well but there’s a problem:

As you can see in the following photo, there’s not enough clearance for the wheels, which are 3-1/2″ high and 1″ thick:

I tried to play with the tracking of the wheels and rotated the strut upward a bit, but I still cannot get enough room under the strut for the wheels to fit in. I find it strange, because these struts look a lot like the ones suggested in the manual – they even say you might be able to fit a 3-3/4″ wheel in there… – but long story short, this isn’t going to fit.

The original full scale struts have an offset, where the center of the wheel is aligned with the center of the strut. I initially bought these straight struts because they look like what the manual suggests, they are called ‘P-47 Struts’ and they fit the retracts I chose, being from the same manufacturer. Now I think I should look out for offset struts, because they give me more play and as a bonus, they resemble the full scale struts a lot better. So I returned the straight struts and ordered Offset RoboStruts instead (thanks for the great service!)…

Although I cannot continue with the current building step of installing the retract rails, I may be able to do some more things further down the list, like skinning the control surfaces and installing the shear webs. To be continued!

Starting with the wings

I had to wait for the retracts to arrive before I could start with the wings. Luckily this didn’t take long and last week I received my order. It’s a set of 85-degrees 60-120 sized retracts with a pair of matching oleo struts from E-flite. E-flite calls them P-47 struts, probably because they fit their Hangar 9 Jug, but they do not resemble the real ones all that much.. which isn’t a problem for my semi-scale build, even more because they look very nice and of course, a lot better than wire struts:

Before starting building up the wing structure, next to some general prep work, I had to modify a number of parts to fit these specific retracts. Most modifications are mentioned in the build instructions, but I had to pick only some of them and I also needed to modify the rib cutout for the retract mechanism, as I use servoless retracts that didn’t exist when these plans were drawn:

Once that was done, the structure of the left wing arose very quickly on the building board:

Nice to see how all those ribs line up, floating above the plans due to the building tabs:

I would have liked building both wings simultaneously, but I do not have enough room for that, so I have to build them one at a time, but I think this could go very fast from here nonetheless.

Finished the fuselage

Last week I finally got the time to finish up the fuselage!

I’ve mounted the firewall and the motor mount. Went together very well. Because of the right and down thrust angles, you cannot just center the motor mount on the firewall and my mounting points did not match those of the plans. So I aligned the motor mount using CAD drawings I’ve made. Seemed like a daunting task, but it wasn’t:

Then I recessed the rudder for the elevator joiner wire. Not mentioned in the instructions or on the plans, but a very obvious step needed to make the rudder and elevator fit together:

So I finally could test fit and assemble the tail and check the controls throws of the elevator. Looking good:

Then I buit and installed the fuselage antenna, just behind the cockpit, from 1/8″ birch ply. It’s remarkable what such a small detail does to the overall appearance of the plane:

… installed the servo tray:

And finished the battery hatch! This was a small little puzzle on its own, but I managed to get everything sorted and I’m very pleased with the result. First I’ve installed a canopy latch on the front side of the hatch:

Then, I glued some wooden dowels into the other side of the hatch:

After drilling the corresponding holes in the fuse itself, the hatch fits snugly into the fuselage opening and there’s absolutely no play at all. A nice and clean installation:

And last but not least: the pin of the canopy latch is in the right location to represent the front antenna on the fuse of the Jug! So it’s scale too :-)

By the way, I’m not sure if I can do the checkered pattern on the cowl, but if so the Snafu absolutely is my first choice to replicate visually.

Motor mount

After cutting a prototype by hand I’ve changed the design of the motor mount slightly. The sides are cut out differently to save some weight and I’ve removed the cooling holes in the front for more strength. Also, the offset turned out to be too large and was reduced from 80 to 74.5 mm. Then I found that hand cutting didn’t deliver the results I’d hoped for and the plywood from the DIY-store wasn’t good enough (too much voids and hard to get clean cuts). So I decided to order my parts laser cut, which was easy because the design was made in CAD in the first place. I’ve added the motor brand and type engraved in the front, not only because it looks cool, but mostly because the front plates are the only parts where it’s difficult to tell which side is up.

Yesterday I received the parts cut by Laserbeest, only one day after placing my order! Looks good doesn’t it?

Because my design is interlocking, it doesn’t need glue to stay together in the direction of the pull of the prop, which lessens the change of ripping off the motor on a sudden throttle increase when a glue joints fails. All parts only fit in one way too, to make it fool proof to assemble. The exact offset for this application and the added one degree downthrust (I know this motor has way more torque than the adviced two- or four stroke engine as designed by Top Flite) make it a breeze to install. I hope the tapered design adds to supporting the relatively heavy and large motor.

Here’s a dry fit of all parts, which went together like a dream:

I really enjoyed the process of designing my own parts, maybe I’ll design a complete airframe in the (near) future… this tastes like more! Now find some time to assemble and install this mount and continue building the Jug :-).

Maiden flight

As mentioned in my previous post, the maiden flight of the Christen was great! It tracks like it flies on rails and it didn’t need a single click of trim! Also, the CG seemed to be spot on right away with the batteries all the way back on the tray. Had a slightly bumpy landing on the first try, but I couldn’t be happier with how this maiden went! I have a few pictures of the maiden flight I’d like to share, thanks to my fellow club mate Frans.

Waiting in the pit, ready for takeoff:

Coming out of a looping:

Some more in-flight shots:

It has a beautiful presence in the air:

Bringing her back in:

And a happy post-maiden photo to conclude this build:

I really could recommend Hangar 9, as the plane was easy to build with only some minor issues, but more importantly it flies superb. Having advised control throws and expo settings in both low and high rates is an amazing help too. Of course, you could fine tune anything to your liking, but it’s reassuring to have a well flying basic setup to start with. Also, in retrospect, having chosen for an electric setup paid off as well, as it’s easy to install, start and fly. But it turned out to be very powerful too. So it’s clean, simple and with this size of motor and prop it doesn’t even sound bad either.

Looks like I have a new favorite plane in my hangar!

A successful maiden!

Just a quick post to tell it flies superb! Today the Christen Eagle flew its maiden and it’s so smooth, docile and powerful at the same time! The plane tracks beautifully and is easier to fly than I thought. Only flew in low rate though. I did four flights today and already got to some basic aerobatics like loops, rolls, Hammerheads and a half Cuban Eight. The Power 90 really is sufficient, you can fly just over half throttle and on full throttle it seems to climb forever. Great for very large loops and Hammerheads. The only challenge is landing… the Eagle comes in very well and steady like a rock, but you should be aware of the Multiple Landing Syndrome. My first landing was good for a first, but ended up bouncy nonetheless. Second one was spot on, third had a small bounce as well and the fourth was okay.

I will post some pictures of the maiden once I’ll receive them from a club mate.