Sopwith Baby (oz16752)

About this Plan

Sopwith Baby. Radio control scale model for indoor flying. Wingspan 27-1/4 in, for electric power.

Quote: "Scale on floats for Indoor RC. This lightweight Sopwith has a target weight of just 150g. Sopwith Baby, by Ken Sheppard.

Having struggled over the last two seasons to achieve realisticlly slow scale flight with a true scale structure airframe, I've never been able to get down to the ideal wing loading of 2.5 oz/ sq ft, that seems to be the ideal loading would allow that elusive low flight speed. Both the AW FK3 (MF May 2002) and the Albatros CIII (MF Jan 2003) are super little fliers and look superb in the air, but the weight of the 8 x 50mAh Nicad flight pack always meant that the loading would err on the high side of the aforementioned optimum.

With the Sopwith Baby, I tried another approach, using the big breakthrough in power sources - Lithium cells. These little Lithium Polymer beauties are revolutionising indoor electric flight, giving huge increases in duration and startling weight saving, compared to their nicad equivalent cells. There are drawbacks with Lithium, however - you need a dedicated charger, they take quite a long time to charge up and they will be damaged irrevocably if the voltage level in each cell falls below a certain level - all these drawbacks do not, however, detract from the improvements to practical indoor RC scale flight that these little cells offer.

This time, I started out with an all up target weight of 150g and worked back to a wing area that would give me a loading of 2.5oz/ sq ft. The next step was to find a suitable subject that was a fairly simple design, of suitable proportions - tail/wing areas, fuselage section - based around my chosen motor/prop combination (the same as fitted to the CIII).

The Sopwith Baby was an obvious choice, as it has fairly short span, broad chord, single bay wings, a rectangular section fuselage and a simple rotary cowl. The only drawback was a very short nose moment. That's why I opted for the floatplane version - I figured that the floats could be built very light, but with the advantage that as the float tips extended way in front of the prop, they afforded maximum effect from minimal nose weight (added to the front ends of the floats) to balance that short nose.

To offset this nose moment even further, I positioned the servos, Rx and batteries within the cowl. The ESC fits behind the firewall, but in front of the CG. The servos that I used were the Saturn S54 micro servos (distributed by Irvine Ltd) that are very tiny and relatively powerful.

Build Notes: The construction follows the same format as both the FK3 and the CIII - balsa ribs, strip leading edge and spars, with carbon rod trailing edges on the wing, thin sheet tips and laminated balsa tail outlines. I called on the services of Falcon kits again to CNC cut the wing ribs for me, to my specified wing section. The accuracy of the ribs and the quality of the wood stock is superb. The service doesn't cost a lot and the saving in time is excellent! (contact details at the end of the article). The adjustable ailerons that I built into the Albatros were very effective, but as the Sopwith has four ailerons, rather than just the two on the CIII so, bearing in mind the tight weight target, I omitted them on the Baby, relying on rigging-in wing twist to keep the port wing up in LH turns..."

Sopwith Baby from R/C Model Flyer January 2004.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.

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