Sperry M1 Messenger

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A 1990's kit from Czechoslovakia, designed around the Modela CO2 motor

The Modela motor (a early version), 0,27 cc swept volume.

Wingspan 580 mm

Length 495 mm

Mass 100 gram

Modela motor 0,27 cc

Although marketed by Modela to promote their own product, the CO2-motor, the plan shows how to build the model for rubber power. Building instructions in the kit box were both Czech and German. Not aimed at beginners, the model was nonetheless simplified to make it suitable for outdoor free flying: interplane struts were discarded and the wings attached to the fuselage by rubber bands in order to reduce the risk of damage in the event of heavy landings.

This particular build is a joint effort that started off with the simple suggestion to build a fleet of the same type of model of which by chance 6 examples of the old kit were at hand. Within days, a project emerged that involved laser-cutting new parts sets and translating the instruction booklet into Dutch. The next development was enhancement of authentic details, in this case re-introduction of the Messenger's struts. What's next? 3D-printed CO2 motors?

Keeping things simple, my model will be rubber-powered and primarily aimed at indoor flying. The build commences with the fuselage sides.

It sometimes looks like the whole world resorts to the use of cyano adhesives. Superglue application with a tiny fork (modified sewing needle) facilitates quick and clean building but I still like white glues. Most need thinning with water.

Using laser cut parts provided by fellow modelers, the fin comes together.

The second fuselage side is built directly over the first one in order to make them identical. Transparent kitchen foil separates the two so that they do not adhere to each other.

As an afterthought the rear motor peg mount is installed – one bay nearer to the nose. The shorter rubber motor will be traded off for less nose ballast and easier trimming.

Before we proceed the two fuselage sides are cleaned up a bit. A sanding block is a must, as is a flat surface – in this case a glass plate. The sanding block's edges are angled in order to prevent snatching.

Below the two fuselage sides are joined together. Cardboard right angles keep things vertical while the glue sets.

The Zepto Creations.laser cut parts are a perfect fit: here the lower nose construction..

A rather solid plastic tailskid is included in the kit and the proposed attachment to the fuselage appears flimsy – so I go my own way with spring wire, needle and thread.

Classic box fuselage with bulkhead segments placed on the top cross pieces. The cockpit starts to take shape: two halves are first fixed to the longerons and when set moistened and bent over toward each other. As always it is vital to study and understand the plan + instructions before commencing work – in my case comprehension sometimes comes after the glue has dried.

Cockpit coaming and rear fuselage stringers are in place but some additional sheeting will be required behind the cockpit in order to attach the covering material there, I believe.

Until now the original kit contents have not been used and because I need to practice making cabane structures an afternoon was spent on an attempt. The choice is for spring wire, later to be enhanced with balsa fairings and perhaps some bamboo.

Because the kit's plastic parts can be used as a perfect example, the wire bending is not too difficult. Wire gauge is 1,0 mm.

After some deep thought it was decided not to solder the joints but resort to thread + cyano. What cannot be seen but is of vital importance: all joints must be scored with a file or sandpaper.

The cabane starts to take shape.

How to attach the cabane's legs to the fuselage is quite a brainer. The formidable English freeflighter Eric Coates described in detail how to go about this task but now this knowledge has to be applied in a somewhat different situation - a flimsy rubber powered model.instead of a diesel driven one with sturdy plywood bulkheads. Below I am preparing two brass tubes that will be solidly mounted accross the upper fuselage longerons. With luck, the cabane unit can be fitted without adhesives, so enabling later adjustment or repairs.

Below the cabane (and fuselage) are presented upside down. Within the fuselage the two brass tubes into which the cabane fits, are visible. Two additional struts run from the top of the cabane diagonally down to the top of the fuselage: these will be represented by the "coat hanger" that locates into short brass tubes that are mounted on a thin plywood bulkhead segment.

Fuselage still upside down, the ply bulkhead doubler with the vertical brass tubes is now glued in place. Two extra internal struts distribute loads to other crosspieces.

Fuselage now upright, the following are visible:

A1 and A2 are the short brass tubes for the front cabane struts

B is the starboard middle cabane leg attachment point (brass tube running through the fuselage)

C is the starboard rear cabane leg attachment point (brass tube running through the fuselage)

The nose block locates in a firm bulkhead that will also provide some area for fixing the tissue covering.

This is a modeler-friendly nose block as far as size is concerned. Getting the shape right takes patient sanding (outdoors).

This adjustable rear bearing was devised in order to change side- and downthrust selectively.

1,5 mm nuts and bolts, 2,0 mm ply and some tinplate from a 1950's cigarette tin (Mills).

In order to attach tissue covering behind the cockpit some sheet balsa scraps are fitted between the stringers.

Gussets holding the dowels for wing attachment also serve as mounting points for the rear undercarriage struts. In the original kit the strut mounts are plastic inserts but here the gussets have been doubled and brass mounting tubes are used instead.

Some 30 small discs are cut from plastic sheet and in each a 4 mm hole is made so that they can be piled together on a bolt. After securing with a washer and nut they are turned to shape in a electric drill..

The plastic discs are sorted into 3 groups and piled on a drinking straw with balsa separator disks in order to create dummy cylinders..

Fortunately only three are needed!

Undercarriage legs are inserted into the brass tubes in the fusellage and then connected to a wooden crossmember.

Rolled paper tubes are a convenient solution to many construction needs. In this instance they provide the body of the six "spark plugs" for the engine. A conventional pin is driven through the soft balsa that the tubes are rolled around, securing the "plug" to the rest of the engine while the pinhead serves to secure the "leads".

Each cylinder has two spark plugs. Probably more a safety neccesity than a performance-related feature.

For some reason one of the three cylinders has a longer exhaust stack. On this engine the stacks are made from drinking straws that provide some flexiibility.

The kit contains quite some molded plastic parts including the landing gear wheels. Mine are turned from a cross-grained balsa lamination. No lathe is required – a cordless drill will do the job.

The model on its legs.

Wings and tailplane I normally build first but this time they come last.

Ribs and trailing edges are all laser-cut by Zepto Creations.

Because old dogs don't do new tricks I stay with nitrate dope when tissue-covering and also use it for attaching. The framework perimeter receives a few coats of sealer (dope + talcum powder) with careful sanding in between. Finally, the tissue (cut slightly oversize) is laid over the frame and thinners run through the paper there where it meets the pre-treated wood. The thinners dissolve the dope which then adheres to the tissue. After the thinners have evaporated a light waterspray can be applied to tighten the covering. In order to prevent warps the assembly must be pinned down while the tissue dries.

As a last-minute resort to saving weight the entire tail was reconstructed from lightweight foam (not Depron). Some of the savings were lost in camouflaging the foam but in all 1 gram was shaved off the tail = some 5 grams up front.

The entire model, including everything except the rubber drive, comes to 83 grams and it is quite unlikely that nose ballast will be required as the detailed dummy engine already performs this role. Preliminary balancing confirms this.

About 10 grams of rubber will be needed to propel the Messenger and so in all it comes out 7 grams lighter than the 100 grams stated in the kit specifications.

Ultimately the original balsa/tissue fin and tailplane were fitted as they look better and the extra weight has no effect on flying characteristics. The picture below was taken by another contestant at the 2017 IIFI in Nijmegen (IIFI = International Indoor Fly In). Flight duration is around 20 seconds with ROG and acceptable landings. The green colour depicted below is a better representation than in my own pictures. Fitted with a pilot figure and more details (e.g. struts) this model is a good candidate for F4D competitions.