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Nov 27, 2019, 11:37 AM
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turbonut's Avatar
Super clean work Ken....I really need to try the mixed materials build some time
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Nov 27, 2019, 04:15 PM
Ken Stuhr
Thread OP

Lading edge inlets


OOOPS..........LEADing edge inlets---couldn't edit the title..........

Thanks for your comments guys............

I've had practice doing these inlets on 2 previous models, but they're still a task requiring a bit of interpretive art Its very hard to find historic photos that show shape details of these inlets very well, especially their blends into the parent wing shape. But I've had a bit of professional experience in similar areas to lean on, so hopefully the result comes close to faithful........
In this first pic, the leading edge has been cut away in the inlet area. The gap is a bit wider than the opening so that the block that will be placed can also incorporate the radii that fair the shape into the wing. Also, there has been 1 layer of 3/4 oz glass and epoxy placed on the wing skins behind the cut edge within the red scribble marks. The blue rectangle indicates about where sheet balsa will be glued to fair the inlet into the wing. The glass is there to prevent erosion of the parent wing skin when sanding this fairing sheet down to zero thickness where it meets the wing skin as well as on each end where the inlet block will be radiused and faired into the wing.
This next pic shows the fairing sheet being glued to the bottom skin. Same was done on the upper surface, but the fairing is larger on the bottom and extends further aft since the inlet itself is centered a bit low on the LE. I used Sigment for this as it sands well and can be blended with sheet balsa perfectly.
Next, the blank piece for the inlet can be seen on the bench. It has been beveled to fit the space in the wing as closely as possible. This piece is taller than the removed piece to achieve the proper inlet 'height', and next shows it glued in place.
Next, carving the rough shape of the inlet using a #26 blade. The idea here is just to get tangent with the upper and lower sheets just glued to the wing skins.
The next pic is just showing the shape after basically done. A bit of rounding also has been done at the very front edge.
Next, trying to fair in the ends properly. Some shape has been done to the balsa, but some filler was needed as well. First is the top surface, bottom next pic. Can also see how the added sheet has been tapered fair with the wing at its aft edge.
Next was the initial cut at the inlet opening. Very careful here as the balsa is very soft and easy to go awry.
Finally, here both inlets have been final-shaped. Next, a better look at the inlet.
Almost done! Here, glass has been applied over the entire area, including around the LE and into the inlet throat.
Last, a look at the job after priming. This detail added 1.5 ounces per each.
Last edited by flyboyken; Nov 27, 2019 at 04:16 PM. Reason: title fix
Nov 27, 2019, 09:36 PM
AndyKunz's Avatar
I appreciate the step-by-step photos.

Andy
Nov 29, 2019, 02:27 PM
Ken Stuhr
Thread OP

Elevons


The process for doing these now. I usually follow the step of building these or ailerons or flaps as an integral part of the wing, shaping them with the rest of the wing to ensure continuity. Usually works fine.
This first shot shows just the wing with the elevon cut away. The size and shape are scaled to match the prototype surface as closely as possible and has proved completely adequate for R/C flying.
Doublers have been glued to ribs where hinge points will go. These are just soft balsa. 4-40 set screws thru the bottom skin and into the doubler blocks will be used to hold the hinge points in place. Also shown is the shaping of the ribs and skins to accept the elevon bullnose.
Next shot shows a rolled up piece of 100 grit sandpaper which I use to carefully form the shape in the ribs and skins.
Next, a blurry shot of the elevon end that shows the bullnose. This was sanded into a piece of balsa that was glued to the front of the elevon after it was cut from the wing structure.
And finally, the desired fitup for the elevon in the wing.
The servo location is shown in the next shot. This is the bottom skin, glassed for durability as a hatch will later be cut here to access the servo. Primer/filler applied and sanded to blend into the skin smoothly. The servo was in place before the skinning was completed, and so the next shot shows the pushrod connected to the elevon using a circuit-board control horn and ply doubler. The cap screws are temporary and will be replaced with set screws.
Nov 30, 2019, 09:50 AM
Registered User
Ken, Your progress narrative and photos are really appreciated on your flight proven model! Al
Nov 30, 2019, 11:18 AM
Ken Stuhr
Thread OP
Thanks Al!

Al is one of my best flying buds and R/C pilot emeritus at his Port Angeles, WA club. If any member there needs a test pilot, they go to Al!! Everybody agrees, he can fly anything and probably even some of my planes
Dec 01, 2019, 01:11 PM
Ken Stuhr
Thread OP

Nacellles


OOPS--toooo many lllllllls in nacelles!!!

The decision to mount motors aft in the nacelles was a happy one, avoiding all kinds of complications with extension shafts, access hatches, couplers, etc., and possible because of the forward CG of the basic wing structure and those systems shown earlier.
The nacelles are a glass/epoxy layup, done over a positive form. Because the nacelles are open on the bottom, they released fairly easily from the form as they could be pried off if there were any hangups. So 4 were made without problems.
The wing got 2 lams of light cloth where the nacelles would be mounted to strengthen the balsa skin. Also, lams were added below the TE where the nacelle mount screws would penetrate.
The next pics show an untrimmed part with the motor. Its a very close fit and required special care for mounting the motor.
Here are the 4 nacelles and the 'firewalls' that will hold the motors, and then a shot of one in approximate position.
Next is how the nacelles will sit on the wing. Even tho the bottom edge of each was part of the mold form, still a lot of fiddling to get a nice clean final fit. I primed and faired the reinforcing glass to make a smooth finish when covered.
Next, here are the 4 nacelles with their mounting blocks in place. These will accept 2 screws put thru the bottom of the wing TE. The right part had to get some reinforcement as I didn't use enough glass........
Next, you can see the mount holes as well as an access hole in the skin. Motor leads come out here, and the tab you can see on the front of the nacelle goes in and engages the skin as a hold-down.
As to the motor mount, the next shot shows shims used to center the motor, which is shown in the next blurry shot. Next, the motor is bolted to the firewall and slid into position in the nacelle. The shims are actually in place from inside the nacelle, holding the motor centered and ready for gluing.
Finally, glued in place with a prop on the shaft.
The black glue used was a mistake as it turned out. This was the rubberized CYA that is really good stuff, very strong and tough and bonds almost anything. Well, with the vibration and flex of the nacelles, the joints failed. Instead, I reglued everything with Shoo-Goo.
Here's the final result, a nacelle in place. Also, a shot underneath showing the mount screws.
Dec 04, 2019, 01:02 PM
Ken Stuhr
Thread OP

Landing gear


One of the last items before joining the wing halves.

First, cutting the doors free from the lower wing skin. The door areas were glassed internally before much construction, and these areas also got external lams of light cloth after the skinning was complete. This process froze the shape of the doors to match the wing contour even long after cutting loose. The saw used to do this are model railroad things with very thin blades and come in handy for all kinds of jobs. I heated up and removed a blade from its handle for more flexibility when cutting. The retract mount blocks can be seen in this picture and are the ones put in place on the ribs before the ribs were glued in place.

Next shows the nose gear retract in place and the steering pushrod going to the servo. Much of the one rib had to be cut away and I now wish I had thought this thru a little better for a less difficult installation.

Next, the nose gear retracted. Yes, it goes aft instead of to the side as in the prototype--one of the liberties I took. The small replaced panel hopefully never gets removed again.

Here is a main gear well. One rib has been cut for retract clearance. The retract mounting plate goes aft and butts against the spar web.

A fuzzy pic of the ply retract mount that screws into the blocks seen on each rib. This idea failed in practice as this ply plate allowed too much flexing of the retract flanges, and they broke on several occasions after hard landings. I later installed a beam above the retracts, extending from one rib to the other, in contact with the upper skin and the retract housing, and epoxied in place.

Here's the retract in place. The scale tires are so large that some shimming had to be done to get the leg angle such that the tires fit inside the wing.

I have yet to finish the main gear doors. I initially tried to have the retracts pull them closed with a length of rubber between, but the retracts were too weak. I'm going to attack again over the winter with servo actuated doors. The nose gear door does actually work with the same rubber band closure, so that part is done.
Dec 14, 2019, 05:55 PM
Ken Stuhr
Thread OP

Wing join


With all these steps done, joining was next.

First shot just shows getting all the wiring lined up and situated before the halves go together. Also doing final trim of any skin overhang of the root ribs that might not allow a close fitup.
Next, the 2 halves are actually bonded here. I was very happy how close the halve's contours matched. Any gap between the skins was filled with an epoxy slurry of resin and talc and the scraped flush with the surface.
I applied several stepped layers of light glass over the joint line as seen in the next 2 pics. These were lightly sanded and then filled with a hi-build lacquer primer, brushed on, to fair the reinforcing to the balsa skins.
In the final shot you can see the hatch cut for batteries and radio. Had to cut a rib away to allow a battery floor to be installed. This was simply 1/8th lite ply.
Dec 14, 2019, 10:32 PM
Ken Stuhr
Thread OP

Finally up on its legs


Here's a few more of the gear and battery hatch, one with the batts in place, and then on its gear. This is all the farther I took the build before a test flight.
Dec 15, 2019, 03:16 AM
Slip the surly bonds...
Sopwith Mike's Avatar
What a project! Very nicely done Ken.
Dec 15, 2019, 11:20 AM
Ken Stuhr
Thread OP
Thanks Mike!!
Dec 15, 2019, 12:50 PM
Ken Stuhr
Thread OP

Tailcone


.........or whatever its called.

This was a built-up balsa thing, with half-formers to start as in the first pic. This is a fairly normal way to do a round or almost round shapes but it also gave me a clear horizontal center plane on which to base the wing cutout. So it was an upper and lower half as opposed to left/right for that reason. The shape of this body is very nuanced and getting it wrong really sticks out. I had a copy of a large-scale inboard profile straight from Northrop that showed the outline very clearly, so my loft was based on that.

The next jumps ahead a bit, with planking done and halves joined as well as the cutout for the wing made. I use Sigment or Ambroid glue for planking as it can be sanded to invisibility with the balsa.

The very tip of this body was made from balsa blocks laminated and turned to profile on a wood lathe, then joined to the built up body. Once done, I could do a final sand on the planking to fair it into the turned blocks. It was at this stage that it was glassed with a couple lams of light cloth and epoxy.

Final shot shows it in position, having been primed/filled and final sanded. It would eventually be glued in place.
Dec 15, 2019, 11:11 PM
Ken Stuhr
Thread OP

The start of the finish


I found this film covering at HK a while ago. Its an iron-on and very very light. Despite its terrific rolled/burnished aluminum look, you can see light thru it if you hold to a source. Don't know HOW they do this, but its very welcome, yet I think undiscovered. Here's a link:

https://hobbyking.com/en_us/covering...-5mtr-406.html

Thot to begin with the tailcone. Here's a couple shots showing the panelization, which I will admit, is not strictly prototypical! The film itself has a 'grain', and this looks really good if you put adjacent panels at 90 deg to each other. The hole you see is for the aft crew station, which was used for gun sighting using the then-new remote directors. My brilliant plan was to paint the aft cone with a newly-found auto 'chrome' rattle can, which I did, but it was the case that 1) masking the film ahead pulled the film up when done and 2) heat from the iron affected the paint if even slightly touched. This led to a comedy of dos and redos bordering on hysteria But I eventually got there as in one of the pics.

As to the film pulling up---I had to keep the iron heat low as it did affect the epoxy/glass underneath, making the weave pattern show or possibly even bubbling if not careful. It turns out the manufacturer changed their formula from the before time from a thermoset to a thermoplastic type resin, and then lied about it. So if I was careful and went slowly, things worked OK. I still chase pulled-up corners, etc.

One shot then shows the whole tailcone in place on the wing before the paint episode.
Dec 17, 2019, 07:54 PM
Ken Stuhr
Thread OP

More finish stuff


First pic is of the approach on the inlets. Used the same spray paint on these, top and bottom, as they would be impossible to do with film. On the first model, I covered the inlets with Flitemetal, which worked nicely, but by now I seem to have an aversion to the stuff.

This next pic is rather poor, just shows some of the panel lines in blue Sharpie. Did the whole upper surface in this way, cut and fit each 'panel' of film to fit these marks. The cockpit opening has also been cut here.

Next, the bottom covering. I had decided to model my idea of an operational plane, hence 'B-35', not XB-35. In SAC markings of the day, the bottom surfaces were in 'anti-flash' white, presupposing the plane would be near a nuclear detonation at some point. I used Ultracote gloss white, which went down well. Did not attempt to do panels on the bottom. Again, when ironing close to the silver painted inlets, the chance of heat smudging the paint had to be avoided. PITB!!

Here are my 2 staff, always on hand to help. Could be the space heater running just below the bench.........

The next 4 shots show top surface covering in progress and done. Most of the panels have the film's 'grain' alternated, which I think turned out pretty well.


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