Author: Gordon Buckland
Edition: Model Aviation - 2013/03
Page Numbers: 93,94,95
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RC Soaring — 2013/03

What makes the Xplorer such a popular F3J model?

by Gordon Buckland [email protected]

Background

The Xplorer F3J model, produced by NAN Models in Bulgaria, has been among the most popular molded F3J/Thermal Duration models worldwide since its introduction to the US market in mid-2008. The Xplorer enjoyed success in Europe in 2007, and Benedikt Feigl won the F3J World Championship in 2008 flying a V-tail version.

The original D-box iterations proved slightly fragile under the heavy winch feet used by some US pilots. NAN redesigned the wing structure with a full-carbon version in 2009, and in 2010 introduced a lighter Spread Tow (ST) version averaging 70 ounces. The ST edition has been successful in three spans — 3.5 m, 3.8 m, and 4.0 m — and is regarded as reliable and robust.

Design characteristics

With wing loading between 7.2 and 8.5 ounces per square foot, the Xplorer’s dead-air performance is exceptional among similar F3J designs. The model’s floating characteristics make it docile and easy to fly, providing a solid platform for pilots to outlast opponents in light lift. The exaggerated “in-section” undercamber of the NAN F3J airfoil contributes significantly to its excellent floating and low-speed performance.

That undercamber is also the Xplorer’s Achilles’ heel: it makes getting the model moving into a headwind on the long run home more difficult. Many pilots have misjudged returns and landed out, often because the relatively high drag at moderate and higher airspeeds penalized the final approach. Nevertheless, the Xplorer has won many contests in the US while the popularity of some other legendary models has waned.

The X2 evolution

It seemed clear any improvement would need a more slippery airfoil. In April 2012, Bob Breaux of SoaringUSA announced a batch of new X2s featuring an airfoil promising better penetration (“legs”) than the original. Bob also promised the X2 would be significantly lighter while retaining full strength.

I ordered a 3.5-meter X2 because I prefer the lighter, more nimble 3.5 m span over the larger variants. The model arrived well packaged in a timber-reinforced box. I weighed each component: including all supplied hardware, the entire airframe came in at slightly more than 50.5 ounces. That suggested a finished weight in the low 60-ounce range using quality electronics — roughly a 7-ounce improvement over my previous Xplorer (67.5 oz). At 1,176 square inches, this should produce a wing loading under 7.5 ounces per square foot, similar to the lightest 4.0 ST Xplorers.

Build and setup

I selected electronics for reliability, precision, and light weight:

  • Ailerons: MKS DS6125 Minis
  • Flaps: MKS DS6125s
  • Elevator: MKS DS6125H
  • Rudder: MKS DS6188 (lighter)
  • Accessories: MKS wooden wing trays, standard SoaringUSA wiring harness, Spektrum Carbon AR9310 receiver, four-cell 2/3A battery, and clear servo covers from Merrill Brady

The total weight of components was 61.43 ounces. After the build (details in the online column), I added only slightly more than 1/2 ounce for adhesives, shrink wrap, antenna tubes, and tape. With the model initially balanced at 120 mm from the leading edge (LE), it required 3/4 ounce taped to the four-cell pack, producing a final ready-to-fly weight just under 63 ounces.

Flight impressions

The new foil has improved an already contest-winning sailplane. The X2 is so light it’s easily hand-thrown into lift and returns from downwind with authority. It accelerates quickly on the toss; handling is crisp and predictable. The X2 penetrates wind much better than the original Xplorer and carries speed without a heavy-drag penalty. The flaps provide good landing authority for precise spot landings when properly mixed. Unballasted, it exhibits greater predictability than before.

The model gives excellent feedback to an attentive pilot: it responds to the slightest active air yet remains gentle and very predictable when turning in thermals. The lighter weight contributes to quicker acceleration in the zoom, and the streamlined section appears to help the vertical climb after the zoom go on seemingly forever compared with the X1.

Conclusion

The X2 is forgiving and does not require special skills to extract great performance. It has no bad habits and does everything a Thermal Duration pilot needs with style. Lighter, faster, and still superb in light lift, the 3.5-meter Xplorer X2 is a significant step forward at an affordable price. I love mine.

Fly downwind and soar.

SOURCES

Transcribed from original scans by AI. Minor OCR errors may remain.