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MARTIN MARS

BY IVAN PETTIGREW

CONSTRUCTION NOTES

For ease of transportation, the model may be built with the wing in three sections, and the fin and

rudder removable. All parts are held together with wire clips and nylon snap links. They can also

be kept in place with the small rare earth magnets now available. Assembly at the field does not

requite any tools, and set up time is only a few minutes. Assemble the wing upside down so that

the nylon clips under the wing, and aileron clevises are easily accessible. If using a typical seven

seat mini van, it is best to leave the middle and rear seats upright (without headrests,) and slide a

six foot long sheet of thin plywood in from the rear, resting on top of the seat backrests. The

centre panel of the wing will sit across the centre passenger seat, with the outer wing panels on

top of it. Because of the tip floats, put these panels on top of the wing “upside down.” The

fuselage sits on the plywood sheet, leaving room for other models and equipment.

FUSELAGE

The fuselage is a simple box with bulkheads added to the top and bottom. Build the two sides of

the fuselage first, and join together. Notice that the sides slope inwards slightly towards the top,

the width of the fuselage at the top being less than at the bottom. The sides of the fuselage aft of

bulkhead #10 are in two sections. The upper ones come together at the tail, while the lower ones

come together at the point of the second step.

Add bulkheads and sheeting to the curved upper surface of the fuselage. The lower semi circular

bulkheads from 11B to the tail should then be added to the bottom of the upper section of the

fuselage. In the case of bulkheads 11b, 12b and 13b, only the semi circular part of the bulkhead

should be fixed in place. The remaining part between the upper and lower sections of the

fuselage will be added after these semi circular bulkheads are covered. Before proceeding to

cover these, read through all of the instructions and get the complete picture in your mind. While

the shape of the outer skin on the section of the hull from bulkhead 11b to 13b is the most

challenging part of the fuselage construction, it is not as difficult as it might seem. My problem

is not in doing it, but in explaining it in words that make it easy to follow. The sheeting can then

be applied to the bottom semi circular bulkheads from #11b to the tail, but bear in mind that a

“V” shaped wedge from bulkheads 11b to 13b will later be covered by the skin that is applied to

the sides of the lower part of the secondary step. That sheeting will extend up to join the sheeting

over the semi circular bulkheads. Some of this “V” shaped wedge that will later be covered over

can thus be omitted to save weight. Or it can be removed later by cutting holes in it from inside

the fuselage.

The sides of the fuselage are just sheeted from the bottom up to a point an inch or two above the

water line. First do the sheeting for the forward section, going back as far as bulkhead 10b. To

sheet the lower section from bulkhead 10b to the end of the second step, including the 1/8” balsa

fillet attached to the keel just beyond 13b, use a single piece of 1/16” balsa 22” x 4”. It can be

glued to the side (lower section) of the secondary step. The top curls out to meet the sheeting that

was earlier applied to bulkheads 11b to 13b. It is much easier than it sounds, especially if this 4”

wide piece of sheet is dampened before applying. Most modellers attempting a project like this

Summary of content (7 pages)

PAGE 1
MARTIN MARS BY IVAN PETTIGREW CONSTRUCTION NOTES For ease of transportation, the model may be built with the wing in three sections, and the fin and rudder removable. All parts are held together with wire clips and nylon snap links. They can also be kept in place with the small rare earth magnets now available. Assembly at the field does not requite any tools, and set up time is only a few minutes.
PAGE 2
have built a low wing scale model like a Spitfire that has a gusset where the trailing edge meets the fuselage. One way to think this section through, is to lay the fuselage on its side, and think of the 1/8” gusset at the end of the secondary step as being the gusset where the trailing edge of a Spitfire meets the fuselage. If this is still not clear, ask for pictures on line of similar construction with the Albatross flying boat built much later.
PAGE 3
Cut the main spars from 1/8” sheet balsa and join together over the plan so that the dihedral angle is correct. At each end of this section of the main spar, build the box that will receive the plug in outer wing sections. At this point the spars for the outer wing sections should be made, and the hardwood tongues attached that plug into the box that was made on the main wing section. It is easier to make these fittings at this point than later in the construction.
PAGE 4
movement. A very slight amount of up elevator may be necessary at the point of lift off, especially if operating off glassy water. In the prototype, Mabuchi 550 motors were used. These are the basic Speed 600 type can motors that have been around for years. They are not the best of motors, but were used in this model mainly because there were four surplus motors with Master Airscrew gearboxes on hand from the days of powered gliders.
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the fuselage. Allow about an inch overlap where the film joins the top edge of sheeting on the lower part of the fuselage sides. It should not be continued to a point below the water line. . EMERGENCY FLOTATION With an electric powered model, because of the weight of the batteries and motors, there is not enough flotation to keep the plane on the surface of the water in the event of a crash, or the hull being punctured.
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For further details on flying boats, read the page about “Flying Boat Design’ on the web page. September 2005 update As of mid 2005, the Kyosho Magnetic Mayhem motors are no longer available in most countries. Other racing car or buggy motors of 23 turns give an almost similar performance. The Peak Racing 23 turn Jaguar is a possibility. It has ball bearings and adjustable timing, so can be timed for reverse rotation.
PAGE 7
should be used for each motor. Try to use the same size props as used in the model previously, and choose a motor/gearbox combination that will turn the props at about the same speed as when used with the brushed motors. Some of the cheaper brushless motors are not that much more efficient than brushed motors that are used with the right loading, so don’t expect wonders. My personal feeling is that for the dollar spent, the biggest improvement in performance is achieved by first going with LiPo batteries.