Unholy Chalupa MKIII

Beware: unreasonably long post ahead. I probably should've posted this in chunks, but was hoping to fly the model first. That always seemed to be just around the corner, but thanks to British weather and seasonal man-flu that still hasn't happened.

The original Chalupa is a model aircraft I designed that was built over the Easter bank holiday last year. It was pretty simple but fun to fly, though not very pretty and the wing construction was rather flimsy. After it was built I continued tweaking the CAD drawing of the model. One of these tweaks was a lasercut version of the wing with an interlocking design that was a lot easier to build. Unfortunately, due to the hacks required to mount it to the old fuselage, it was still not brilliant structurally. When the second wing broke early this year I decided to do a full rebuild of the model, using the latest version of the drawing.

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The most major change to the model was the wing mounting system. The original and lasercut wings were built as a single panel held by a tab and a magnet under the fuselage. The intention of this system was to provide a quick release effect in the case of a crash or bad landing. Unfortunately, the wing tended to detach even in a good landing, and yank on the wing servo cables, in one case ripping off the housing from one servo and spilling tiny gears across the field. This also put a lot of stress on the center section of the wing in many cases. In the new design, the wing is built in two panels. A pair of carbon rods slide through holes in the fuselage. The rods continue through holes in the inner three sets of wing ribs. They should have enough flex in them to absorb some impact in the case of a bad landing.

As you can see from the comparison above, I also redesigned the fuselage to be much less boxy.

MKIII Chalupa Parts

I cut the parts for the MKIII Unholy Chalupa at the start of May, using the laser cutter at Nottingham Hackspace. The model was finished around the 21st of June. Since then it's mostly been cluttering up my desk, waiting for a good day for a test flight.

Fuselage sides

The first step was to build up the fuselage sides. The side panels could have been built as a single piece if they were hand cut, but using the laser meant I could rely on the accuracy of the wing mounting holes. This allowed me to use the holes to perfectly align the doublers. This is important on this model, as the positioning of these holes will ensure that the wing sits exactly perpendicular to the fuselage. Once the doublers were on I attached the fuselage formers to the sides, unfortunately there were some errors in the CAD drawing, so I had to do a bit of trimming at this stage to get them to fit correctly. You can also see the 1/8th square balsa strip running along the top of the fuselage side. This is left unglued at the tail section for the time being so that the sides can be bent inward later.

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Once the glue on the formers had set, I carefully aligned the two fuselage halves and glued them together. A variety of clamps and blocks was used to ensure that everything was aligned and that the fuselage sides were vertical. The grain on the rear former runs vertically. This prevents it snapping if it is pushed from the top, but leaves it weak laterally so it could break if the fuselage is squeezed hard. To solve this problem, a cross grain doubler was added to the top portion of this former. A 1/8" square strip was slotted in temporarily to keep it aligned. This doubler also provides extra surface area for gluing, which will come in handy later.

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Once the fuselage sides and formers were in, the bottom sheeting was fitted. In the straight section of the fuselage this was straightforward. In the tail section this was slightly more complex, made more so by the fact that I had to trim part of the tail bottom section due to a CAD error. This prompted a rethink of how I link dimensions in my drawings, which I may write about later. Once trimmed the tail sheeting was pinned to the work surface and pressed firmly onto the rear former to ensure it remained straight. Once glue was applied, the sides were pulled in, held vertical using blocks and set squares. I also glued the 1/8th strips to the top of the tail section at this point, to lock in the bend.

Desk fly

At this point it was possible to put the model together to see how it would look and provide a motivation boost. The wings were held together by friction.

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The next phase of the build was to skin to the top part of the fuselage. The first part to be skinned was the hatch. This section detaches to provide access to the batteries and radio gear. Since it is separate it could easily be rebuilt if something went wrong. A 1/8" square spine was fitted into the notches at the top of the formers to support the 1/32" sheet was used for the skin. This was a slow process as it was initially glued vertically to the side of the hatch, then glued and pinned in sections once the glue set. This took several days, but very little actual work. Most of the time was spent waiting for the glue to dry.

Spine

The spine for the rest of the fuselage was glued in sections, taking advantage of the doubler on the rear former to provide extra surface area. At this point some 1/16" strip was added to provide a nice area for the sheeting to seat as well as additional glueing area.

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The sheeting for the cockpit and turtle deck was a bit more complex than that of the hatch. It was held in place and traced from the underside of the model, then cut and test fitted. A few attempts and some trimming were required to get this right. I left the rear end of the turtle deck untrimmed until I knew how the tail surfaces would fit.

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I originally intended to use the tail from the previous version of the model, but changed my mind after some consideration. On the original the elevator was a single piece with a slot in the vertical fin to allow it to move. This led to a lack of surface area on the fin and some yaw issues in flight. Because of this, I modified the plan to use a slot in the elevator with a just a small notch in the rudder for it to pass through. At this point power supply issues had taken the laser cutter out of action, so I had to make the extra parts by hand. I printed and traced the plans then cut them out. Since the notches on the cross grain parts of the surfaces were quite intricate I built the main sections first, then traced the notches on the cross grain parts from the real cut. After a few attempts I managed to get a nice tight fit.

It's important that the horizontal stabiliser and fin remain perpendicular, so blocks and clamps were used to hold them in place while the glue cured. A tiny sub fin was attached below the stabiliser, to give a place to hinge the lower part of the rudder. On the original model this was a big problem and lead to a very sloppy rudder response.

Complete fuselage

At this point the fuselage was mostly complete, and I moved onto the wings. You can see here that I've shortened the model a bit since the last picture of the fuselage. The original intention was to hide the motor away, but the problems accessing the mount made me change my mind. I think the model still looks good with the motor exposed.

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The wings are quite simple, but it's important that they are built straight. Because of this they were glued in several phases, with blocks and clamps ensuring that everything is straight and true. Unfortunately, for reasons I no longer recall, my original parts had tiny difference in the rib spacing in the outer panel. Despite taking great care to make sure the leading and trailing edges matched, I still managed to mess this up on one wing. This left the wing ribs slightly skewed, but in the end it doesn't cause a big problem once the wings were attached. Even so, I fixed this up on the plans so it won't be an issue should I build another.

You can also see that while the glue was setting the wings were placed back to back. This was to make any misalignment visible, so I could correct it before the glue set. I also measured the height of the trailing edge at each end of each panel to try and eliminate any twist. If I build another, I intend to make a jig that will slot over each rib of the wing, keeping them all at the same angle to the work surface.

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Whilst the glue was setting on the wings I installed the radio gear into the fuselage. On this model it's a tight fit, and the receiver will be difficult to get at once the model is covered. Fortunately this shouldn't be necessary except in rare cases, in which case the covering can be cut off to gain access.

Wingtip installation.

The wingtips were installed with the wings attached and the model standing on its nose. I used some 1/8" strip packed around the tips to to keep them horizontal, as the slight bevel left by the laser cutter would create problems if they were just glued on directly.

Servo hatch

Sub ribs were added to the wings for servo mounting. They slot onto the wing spars and the carbon rods that attach the wings to the fuselage. The servo is screwed into a slot the thicker of these sub ribs. The thinner two sub ribs provide a mounting surface for the sheeting around the servo horn, giving a place to attach the covering around the gap.

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To hold the wings in place, I settled on nylon bolts that fit into T-nuts in the inner rib. Since there were no holes for the bolts in the plans I had to manually drill these. I fitted the wing in place and pressed on the T-nut to leave in impression on the fuselage side, marking where the holes should be drilled. Although this holds the wing well, it is difficult to insert and remove them, so it's not the best method if the wing needs to be removed regularly. I need to put a bit more thought into a nicer mechanism at some point.

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Another benefit of the carbon rod wing attachment is that they provide a good hardpoint against which the landing gear can be be mounted. This was my first attempt at wing mounted landing gear and I'm fairly happy with how it turned out. It should transfer a lot of the landing loads to the rods instead of the wing.

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The leading edge of the wing was built up from a stack of 1/8" and 1/16" strip. I used parcel tape to protect the wing ribs while sanding these to match the desired profile.

Tail install

After covering the tail and fuselage, I installed the tail. I took a lot of care to make sure it sat correctly and that the stabiliser was horizontal, before adding glue and clamping it in place.

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I waited until after the model was covered to fit the control linkages. The tail surfaces required relatively long pushrods, so I used adjustable EZ-link style fittings to make them easily adjustable, rather than risk messing up fixed length pushrods. This also keeps them removable as they are only bent at one end. For the aileron surfaces I used fixed length linkages since they are short and would be easy to replace if the length came out wrong.

Once a few more minor tasks were out of the way, the model was mostly finished and ready to go.

Unfortunately, thanks to the weather I haven't had chance to fly it yet, so it's been cluttering my desk. Since I was out of hooks to wall mount it in the usual way, I ended up bending a large wire hook that clips to the landing gear and could be screwed to the wall.

Wall mounting

Wall mounted

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