Mercura (oz11803)

About this Plan

Mercura. RC patterm plane. Wingspan 56 in, for 45 - 61 power.

Quote: "BEFORE DIVING into the construction side of this article, I would like to give the philosophy behind the design.

With the FAI noise limit at 105 dB at one metre, I was having problems getting my Titan (oz8567) design model down to this low figure, therefore I had to think of the requirements of a new design. These were:

(1) an enclosed engine
(2) enclosed exhaust system, doing away with the tuned pipe as this increased the revs, and therefore the noise
(3) slightly smaller lighter model to keep the power to Weight ratio the same
(4) keep the exhaust heat away from the receceiver and servos, etc.

Mercura fulfilled all of these requirements. The inverted rear exhaust motor is cowled with the air running along the duct under the wing to cool the exhaust system, the wing insulates the radio gear from the heat of the exhaust and no over-heating of the engine, exhaust or the radio has been experienced.

The construction is very straightforward, but it should be remembered that this is not a beginner's model and as such I will not give a cut and glue building instruction.

Fuselage: The fuselage sides and formers are assembled and if possible put in a simple jig for the top and bottom planking to be done. The remainder of the construction is with block, especially round the engine, where as much solid balsa as possible is used to dampen out vibration and therefore noise. The tank floor may need changing slightly to accommodate other rear exhaust motors and their manifold pipes.

One of the major differences with this design is the position of the wing dowels which are on the side of the fuselage, the advantage of this is the readily accessible tank bay. These dowels are fitted very securely with their top edge on the centre line of the wing. The leading edge root fairing is fitted on completion of the wing fitting.

The engine cowling is constructed using sheet and block and on the original models the method of fixing was pegs and a glider type canopy catch. Particular interest should be paid to the internal shape of the cowling so as to obtain maximum cooling.

Tail, fin and rudder: The tail plane and fin are of conventional construction using the sandwich method to make the ribs. Once assembled they are sheeted and the elevators and rudder tack glued on for sanding. Foam tail surfaces may be used if required,

Wing: A conventional construction is used on the wing and the basic structure should be checked for straightness prior to fitting the inter-spar webbing and skinning. Undercarriage can either be fixed or retractable. (The flight performance of the model is not affected wheels up or down). The ply plate method of installing retracts has worked well for me in built-up or foam wings, but the builder may prefer his own method.

Mounting the wing on the fuselage is something that requires care. The sides of the cut-out in the leading edge of the wing are faced with thin ply, these faces should be an exact fit across the 2 dowels mounted at the front of the fuselage, all that remains are to slide in and carefully glue the 2 hardwood plates to the ply wing facings, which should rest on top of the dowels to hold the wing up in its seat. Once these plates are correctly fitted they should be doweled to the ply. Hardwood blocks or ply plates are fitted to the rear of the wing cut-out to accept the wing retaining bolts.

The wing should be a tight fit and is removed by lifting the back of the wing up (when the model is invented) and sliding back. At this point in the wing construction the centre section of the wing, out to a distance of 2 in beyond the undercarriage mounts, should be covered with lightweight glass cloth or doped nylon. This is an import-ant part of the construction and does not relate to whichever method of finish you use.

With the wing fitted to the fuselage the underwing duct is now planked with tubes being fitted for the wing retaining bolts, the size of the opening at the rear of the duct should be at least 4 times that of the intake area..."

Mercura, Radio Modeller, August 1982.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.

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