Duo-Mono
October 1950 Air Trails Hobbies for Young Men

October 1950 Air Trails

Table of Contents These pages from vintage modeling magazines like Flying Aces, Air Trails, American Modeler, American Aircraft Modeler, Young Men, Flying Models, Model Airplane News, R/C Modeler, captured the era. All copyrights acknowledged.

Cal Smith's semi-scale control-line model of the Duo-Mono bi/monoplane is certainly an unusual-looking airplane that might make a good subject for an electric power conversion. It is based on one of Maurice Henri Delanne's designs featuring a larger primary wing and an offset smaller secondary wing. The model shown here has a 31" span for the main wing and about 22½" of span for the secondary wing. The fuselage is around 25¾" from tip of the spinner to back of the rudder. A .30-size engine is used, yielding 70 mph flights at full bore. Construction is standard balsa and plywood, with fully sheeted wings. Burt Rutan (one of aviation's all-time greatest innovators), a couple decades later, was famous for his canard and dual-wing (not biplanes) designs - such as the very unique and popular Quickie. 

See also the "Delanne and His Duo-Monoplanes" full-scale-airplane article.

We model the Duo-Mono

By S. Calhoun Smith

Is it a tailless bipe or a mono with a misplaced empennage? The Delanne builds easy, performs like a dream

Experiments in various wing arrangements with full-scale aircraft have resulted in some pretty strange shapes through the years of aviation development. Successful results have been frequent; yet fallen along the wayside we find combinations such as the canard, tandem wing and circular wings. Today designers have pretty well settled for the conventional wing stabilizer or all wing set-up.

The original versions of Maurice Delanne's double-monoplane were singularly successful. They possessed a high degree of maneuverability, a high top speed, together with a very low landing speed. Air Trails has been fortunate in securing from Mr. Delanne drawings of his Sport Model 20 which was built and flown in France during 1938-1939.

The design lends itself admirably to sport control line flying, and although not a strictly scale version of the large Delanne 20, we hope the design liberties taken will not offend.

The 31-in. span model weighs in at 24 ounces. Considering both wings as effective area (according to Mr. Delanne) gives a total of 280 sq. in. wing area. Wing loading is 8.5 oz. per 100 sq. in. If only the front wing is considered effective area (184 sq. in.), wing loading comes to 13 oz. per 100 sq. in. The model has good stability in flight, but we would rather assume the loading to be higher than the 8.5 oz. figure. If you split the difference, loading is probably closer to 10-11 oz. per 100; at least that's the way the model feels out on the end of the wires. So don't attempt the full stunt pattern with this ship although you can have plenty of fun with big loops, horizontal eights and inverted flight. The K&B "29" in the nose furnishes plenty of power - speed is a little above 70 mph.

Construction follows standard practices and a strong light structure has been achieved without excess weight.

The fuselage can be built first. Cut out the two 1/8" sheet sides. These should be of medium hard or hard stock. Next cut out the two 1/16" plywood sides from the pattern shown on the plans. The plywood and the balsa sides should be glued together with hard glue such as Weldwood or Casamite. Clamp uniformly and set aside to dry thoroughly.

Cut out the fuselage formers from the stock indicated. Be careful that the formers are of equal width for the forward section so that the fuselage sides come out parallel.

When assembling the fuselage sides and formers, the top rounded portion of the formers can be cut off level with the top of the sides so that the sides can be placed upside down directly over the top view for building. Let the nose hang over the edge of the workboard so the firewall, former 2, can be located easily. Glue the firewall 2, plywood 3 and hardwood bellcrank mount with Weldwood. Small brads nailed through the sides into these pieces will insure a good joint. Add formers 4, 5, 6, 7, 8, 9 and 10 in that order. Check sides with triangle or square for squareness.

When this basic fuselage structure has dried thoroughly it can be taken up from the workboard. Further work can be done "in the hand." Add the top of the formers and the block balsa turtle deck. The section between former 3A and 7 is planked with 1/8" x 1/4" strips. This planking can be left rough on top until the front wing is fitted and joined.

Bend the landing gear from 3/32" dia. steel wire and attach to plywood former 3 with "eye" or "J" bolts. Next the 1/16" plywood floor can be added between plywood formers 2 and 3. This should be glued with Weldwood. Face the inside of the plywood sides ahead of the floor and firewall with 1/8" sheet balsa. This serves as a stiffener and additional surface for the nose block when the sides are shaped and rounded off. Add the 3/8" x 5/8" strips along the bottom of the lower cowling. Next add the 1/8" sheet bottom over this section with the grain running across the fuselage.

The engine should be fitted in place temporarily now so that proper clearance of the sides and bottom can be checked. The upper and front cowling can be carved from solid blocks and checked for fit around the engine. The cowling can be assembled in one piece and held with suitable bracket and screw through the top and guide pins or dowels from the front portion into the front face of the fuselage sides. A simple angle bracket can be attached to the firewall with the top engine mounting bolt. This bracket should have a nut soldered to its underside. A bolt is then passed through the top cowling to serve as a hold-down.

Got everything so far? O. K., take a short blow.

A couple degrees of right thrust should be used on the engine. This can be built into the firewall when joining to the sides or added later with washers under one side of the engine mount bolts. Make allowances for the offset position of the thrust washer and shaft when carving the nose block. Mount the spinner and carve the block accordingly.

The fuel tank can be installed now and filler lines and fuel line positioned to complete the power plant section. Remove the engine and give the inside of the entire nose section and cowling a coat of thinned Weldwood to fuel proof.

The fuselage bottom should not be added until the rear wing is built and installed. This is a simple job; 1/2" sheet is cemented to the bottom and carved to fair into the fuselage sides.

The rest of the fuselage can be sanded smooth and given a coat of filler or dope. Later when wings are assembled to the fuselage, final filling and doping can be completed.

The rear wing is next on the list. Cut out all ribs, leading edges and. spars. Assemble the frame and ribs over the plan. Block the leading edge and spar up 1/16" off the bench at the tip. At the center rib block up the leading edge 1/4" and the spar 3/16". (Now go back and read the last two sentences again so you won't forget.) Bevel the leading edge so that the wing sheet will curve easily off the rib onto it. Plank one half of the wing with 1/16" sheet 6" wide. If this is unobtainable, join two 3" wide sheets. Cover other half in similar manner. This must be done by halves because of the slight break in taper angle at the center.

When dry, remove from the workboard and cover the other side with 1/16" sheet. Check wing against any twist by sighting spanwise while you are cementing top sheet down.

The elevators can be made at this point. If a Veco horn is used make a sandwich of two layers of 3/16" sheet. Groove out inside of the sheets for the horn wire, then cement the clamp until dry. Carve elevators to proper cross section and assemble to wing with fabric hinges or your favorite type system which works out well.

Assemble the rear wing to the fuselage, installing the pushrod and bell crank therein. Add the line leads and you are ready to button up the bottom of the fuselage. Check control system for complete ease of movement. No binding or stiffness should be evident.

The twin rudders can be carved from hard 1/4" sheet and cemented to the tip face of the rear wing. Note offset angle to the right shown on the plans; both rudders should be the same.

Construction of the front wing follows the pattern of the old favorite Fireball. Make top and bottom wing half sheets to width needed, out of 1/16" sheet. Cement ribs in place on right sheet half. Omit rib F1 until later. Bevel the 1/4 in. sq. leading edge and cement in place along the sheet and ribs. Now assemble the left sheet, ribs and leading edge. Join these two assemblies with the 1/8" sheet spar joiner and two F1 ribs at center seam. This forms the bottom wing surface. Bevel the leading edge strips and trailing edges of the sheet on this lower surface and on the top surface sheets.

The plywood line guide strut can be cemented in place against the side of rib F6 on the left side before the top sheet is put down. Cut a slot in the skin beside the rib to allow plywood to pass through.

Add the top wing sheets on first one wing half, then the other. Check constantly by sighting span wise to prevent any twist. The wing should not have any warps in either panel. Cover the center seam with a 2" wide strip of silk or aircraft fabric to further strengthen it. Now add the 1/2" sheet tip blocks and the 1/4" dowel line guide to the strut to complete the wing construction.

Join the front wing to the fuselage now. Cut the top planking to form a tight-fitting saddle. Check for parallel relation with the rear wing, viewing model from nose and tail. When everything is snug and aligned, cement wing down and use plenty of goo at all points of contact with the fuselage structure. A shallow plastic wood or balsa fillet can be added to both front and rear wings to further strengthen and clean up the wing-fuselage junction.

If a good finish is desired, the entire model should be sanded and filled, then covered with lightweight tissue for a good undersurface. Then put on several coats of filler or primer and as many coats of dope as desired.

The original model was finished with a minimum of dope in order to hold the weight down. Two coats of talc-dope filler and two coats of dope were used. Fuel proofer was added over this.

It is recommended that a fuel-proof finish be applied from the wood outward, using Sta or Aero-Gloss for the entire job.

Check balance as indicated on the plan side view before flying. The original model turned out a bit nose heavy, and although on first flights it went like a streak, up control was at a minimum. About one ounce of lead was added to the tail and immediately results were better. The landing gear was moved a little ahead to insure better ground running characteristics.

The Delanne wing arrangement allows for a more rearward C. G. location than is used on both full-scale and model aircraft. With, C. G. and bellcrank as shown, performance is fine. Even better maneuverability would probably result if the C. G. and bellcrank were moved back another half inch.

If any modeler wishes either to enlarge or reduce the size of this ship for flying with other engines, it is important that the same relationship of wing stagger and gap be retained. This is the essence of the Delanne design upon which the excellent performance of this rather unconventional ship depends.

Duo-Mono Plans

Bill of Materials - Duo-Mono

(Balsa unless otherwise specified)

3 pcs. 1/8" x 3" x 36" fuselage, formers and wing ribs. 5 pcs. 1/16" x 6" x 36" wing sheets. 2 pcs. 1/4" x 1/4" x 36" wing leading edges. 4 pcs. 1/8" x 1/4" x 36" fuselage planking. 1 pc. 1 1/8 x 2 1/4" x 12 1/2 fuselage top blocks. 1 pc. 1/2" x 3" x 24" fuselage bottom, wing tips. 1 pc. 1/4" x 3" x 12 rudders. 1 pc. 1" x 3" x 3" nose block. 1 pc. 3/16" x 3" x 12" elevators. 1 pc. 1/4" x 3/8" x 24 rear wing spar.

1/4" hardwood plywood 2" x 2 1/2" firewall. 1/8" hardwood plywood 2 1/2" x 3" former 3. 1/16" hardwood plywood 5" x 9" fuselage sides. 3/8" x 1/2" hardwood bellcrank mount. Scrap 1/16" sheet for bottom.

1 ft. 0.093" dia. steel wire, landing gear. 15 in. 0.062" dia. steel wire, pushrod and tail skid. 30 in. 0.031" dia. steel wire, line leads. Veco 3" bellcrank. Veco elevator horn. 2" dia. wheels. 1 5/8" dia. Scamper Plastic Spinner. Berkeley "B" stunt tank. "Eye" or "J" bolts for landing gear. Cement, Weldwood, dope, tissue, Trim-Film as required.

Posted May 8, 2021