Three Meter Radian

Fig.1: 3m Radian, 2m (stock) wings, UMX Radian. Kaiju Corgi for scale.

Recently, my copilot and I were looking at buying a Multiplex Cularis. We wanted a top-notch thermalling plane, something with lower wing loading than our old Radian. Then we did some research. Turns out, the Radian actually has a better wing loading than the Cularis. In fact, it has a phenomenally low wing loading--2.33 kilos per square meter vs. the Cularis' 3.05. I'd gotten a Bird of Time not too long ago, (Requiscat in Pace), but even that has 2.55kg/m2.

Then my copilot suggested something totally crazy. Something that could never work.

Why don't we just glue two sets of wings together to make them longer? Since the Radian's wings are so simple, with no moving parts or any taper along much of their length, it would be relatively easy to do, and we could change the wings out for the normal ones without any modification of the fuselage.

For this build, you will need:

MATERIALS

2 sets of Parkzone Radian wings (≈$40 each)

1 additional Parkzone Radian spar (≈$6)

6 barbecue skewers

Some glue

Some packing tape

5/16" wooden dowel

TOOLS

Square

Drill

Measuring tape/stick/ruler

Saw/long bladed utility knife

Dremel/saw capable of cutting carbon fiber

Shears

PROCEDURE:

First, you'll need to make the outer portion of the wings. This is done simply by cutting off the "shoulder" portions of one pair of wings, the part that seats into the fuselage.The line between the wing and the shoulder isn't square, so use a square to make the cut square. Square square square. The easiest way is to press it flat against the underside of the wing and align it with the square aligned with the trailing edge.

There's a fiberglass strap that runs the length of the wing to stiffen it, so you'll need to cut that. I've found that kitchen shears are the best tool for the job, but a utility knife will work, it's just a little harder.

Fig. 2: The white tape in the middle conceals the fiberglass strap.

Next, cut the inner section of the wings. I cut them off at about 20" (50cm)to get a nominal 3 meter wing, but you could concieveably get an extra inch or two out of each wing if you really wanted, but at that point the wing starts to curve significantly. Cutting them a bit shorter would actually solve a lot of problems we'll get to in a moment.

Once you've decided on your length, mark your wing with the square and cut it, once again chopping through the fiberglass strap.

Now for the hardest part: The spar only extends about 14" (35cm) into each wing. Therefore, if you cut the wing further out than that, there's no contiguous channel for the spar between the two wing sections.You'll have to bore out the last 6" (15cm).

At first, we tried sliding the spar into the stock spar channel until it hit the end. We then tightened the chuck of a drill around the spar and used the drill to bore out. This worked very well--except that the slight curvature of the wing made the spar bust out the bottom. We had to clear out material above it to bend the wing down flat.

Fig. 3: A 1/2" capable drill will accept the spar, which can be used to bore a hole of the proper dimensions.

On the next one, we bored in from the other side, aligning the spar as best we could and then drilling to meet up with the stock channel. This worked much better.

Now we join the wings. After considering our options, we decided to leave 1 of the stock spars in each wing, connected by a third, removable spar section, rather than having a full-length solid spar. We made this decision since neither of the sections of either wing lined up perfectly around the spar, and it would probably be impossible to route the spar through the channel once the wings were contiguous.

We aligned the wings and pre-punched barbecue skewers into either half, parallel to the spar.These are to lend a bit more rigidity to the joint. We placed one ahead of the spar and two behind, with the last of these as close to the trailing edge as we could put it without endangering the integrity of the foam. We did a test-fit to check alignment, and found that the two wing halves lined up nicely. Then we placed the spar in the wing tip and slid it back into the root. the alignment was thrown off considerably, but after re-inserting the skewers into their channels, the main spar was pulled back into line fairly well.

We used two types of glue to join the halves. First, we wetted the foam to be joined fore and aft of the spar with a damp cloth, leaving a dry patch right around the spar itself. We administered foaming (Gorilla) glue to the wetted areas, again, leaving a bare patch right around the spar. Then we squirted some hot glue into our dry bare patch and forced the two wing halves together. We used two glues because the hot glue hardens very quickly, and we wanted the wing to hold itself in the proper shape while the foaming glue dried.

Laying a long strip of packing tape under the joint, we pressed the underside of the wing flat on a table and held it there, making sure the leading and trailing edges were perfectly aligned, until the hot glue hardened. Then we wrapped the joint tightly in packing tape, pulling so that the leading and trailing edges were bound together. Over the next half hour or so, the foaming glue expanded to fill in the gaps and cured, making a lightweight joint that won't get sloppy with heat.

Finally, we made the central spar. We measured the distance between the two pemanent spars by holding the wings up to the sun and observing the shadow through the wing. The distance between them was about 10" (25cm). We cut a section of our spare wing spar to the proper length and then inserted a 20" (50cm) piece of 5/16" (8mm) dowel into the tube so that it stuck out equidistantly on either side. After a test-fit that confirmed it fit perfectly between the permanent wing spars, we used foaming glue to secure the dowel inside the spar.

Fig. 4: the central spar and wing-lock hooks.

Once the glue had dried, we assembled the plane. Absolutely no modification needed to be made to the fuselage. We found that the wings were a bit flexy, but not terribly. We also glued some wall-hanging hooks on the top of the wing and used elastic cord to secure them, to prevent this sort of thing from happening again.

Fig. 5: The wing retainer system

RESULTS

The final wing weight increased by only 215 grams, while the wing area increased by 2280cm2. This means that the wing loading on a stock fuselage with a 2200mah battery would drop from 2.33kg/m2 to 1.82kg/m2--an astonishing 40% less than the Cularis. Additionally, since the Radian's wings flare up at the tips, not all that wing loading is really being taken advantage of--the angled portions of the wing don't lift as much as the flat portions do. With the new wings, nothing but flat area is being added.

BUT  HOW DOES IT FLY?

We took the plane out to maiden it and...it was beautiful. The best maiden we've ever had. We were skeptical if such a drastic change could yeild any benefit--or even not simply make the plane unflyable--but in calm air, at least, the aircraft flies way, way better than stock.

It climbs just as easily, and though rudder authority suffers a bit, judicious use of throttle to wash the surfaces in air makes it turn just fine. We were initially worried about landing area, but the bird is a fair bit slower and as a result may even be able to land in a shorter distance than the stock Radian. It's also a lot easier to catch.

We wanted it for a long-range fpv plane, and in that regard it seems very promising. The wings are much, much stabler than stock, possibly because the wingtip dihedral stabilizers have a longer lever-arm to act on. Via my OSD I can report that it maintains level flight at a little more than half the power it takes to do so with the stock wings.

It thermals gloriously. We're not exactly competition level pilots, but my copilot flew for half an hour off throttle on its first day, and could probably have kept it up until dusk without touching the prop. The biggest challenge was to keep it from specking out into the stratosphere.

CONCLUSION

This is a brilliant, brilliant modification, especially since it's totally modular. If you want the speed and agility of the stock Radian (words I never thought I'd say) then you can just pop the old wings back in to your fuselage and get in the air with no other adjustments. Plus, you have the materials on hand to make the glorious 1m Radian.*

*WARNING: 1m Radian has not yet been thorougly tested on account of not being remotely airworthy.