King Rat Project

King Rat Project

Indoor Electric Radio "King Rat"

Genesis

[Image] Some time ago at the AEFA indoor, I watched as a small stick and tissue, rubber powered model was launched. Circling upwards, it collided with the backboard of the basketball hoop, losing all forward momentum. Then, from a complete stall, it dropped neatly under the board, recovered headway and flew on in level flight. "There lies salvation for the RC challenged", I thought. Further enquires revealed the model to be a "Hanger Rat", sourced from "Fly-In Models" at Murray Bridge SA. Among the thousands of Hangar Rats sold, some had been successfully converted to Micro RC, usually retaining the benign flight characteristics. These micro conversions were quite expensive as they utilised specialist micro motor, radio and actuator gear. The arrival of mass produced "pico" and "naro" R/C equipment prompted Fly-In Models to develop a larger (double) size "King Rat" as an affordable electric RC indoor slow flyer. Retaining the geometry of the original Rat with wing loadings of around 9g per dm2, attributes of slowness, extreme "cornering" ability and the mildest of stalls were assured. The "Fly-In Models - Radio King Rat" is the basis of this project.

Reworking a design always teaches the modeller something and I hope what follows is of general interest as I attempt to relate modifications made before and after flight testing, the ideas behind them and their implementation. The design effort is measured by "better", rather than optimum. The emphasis is process and structure, and I have a definite "thing" about jigging. Perhaps there is a perception that the accurately built, repeatable and interchangeable assembly of components might lead to production! I still don't know what a Cuban Eight is exactly, not that excited by pattern ships and warbirds, but remain impressed by an application of basic technology to produce a model whose performance belies its angular, agricultural appearance.

Design Considerations - Points of Departure

The wing / tailplane / vertical stabilizer structures as shown in the kit drawings appeared barely adequate, with no evidence of redundancy. Good! No need for change there. The sliced rib construction with no notches is quick and the transition from decidedly wobbly to suitably rigid after covering and applying the dope is magical. (My latest indoor wing is, however, slightly larger and carbon fiber reinforced - intended to fly at even lower speeds). The narrow, balsa sheeted fuselage as drawn, was less satisfactory. Also, the "half in - half out" component mounting, would always look like an after-thought. The wing bracing as proposed was relatively bulky and drag inducing. Similarly, the UC could be improved. I decided to re-configure the fuselage and produce a cleaner, more efficient structure. Access to a 3D CAD modelling package was available, so any proposed new components were fully geometrically resolved and plotted at full scale. The CAD package allows addition, subtraction, intersection etc. of solids, planes and lines, hence the virtual model is useful for producing assembly jigs and mould profiles. Three dimensional jigging gives control when fitting off struts and hangers, restrains the model while the dope sets and enables precise building in of trim items such as washout, down and side thrust. Stored and transported in the jig, the model is protected from warps and hangar rash.

Radio King Rat at DAC

Pre-flight Modifications

An assembly may become both lighter and stronger if the cross sections are reduced in area, and those lighter members are then re-positioned to reduce the moments acting through the frame. The new design for the fuse consisted of twin warren trusses, with curved chords top and bottom, spaced laterally with balsa struts. The battery bay, access hatch, receiver bay, undercarriage fixings, cabane structure, and main wing bracing connections are more or less contiguous within a short, sheet reinforced section of the fuselage. Bracing became 1mm Carbon fiber, bound and cyano'd to wire hooks, in turn fitted into aluminum tubes bound to the balsa frame. Unlike the wing, the rigidity of the fuse was not critically dependent on the covering, but the integrity of the structure is certainly enhanced by the doped Jap Tissue. The UC was triangulated and bent from 1mm Wire and angles developed on CAD. A cowl was developed and vacuum moulded from 0.18mm clear PVC sheet. Motors and servos were mounted on beams built into the frame. Lead-outs were then required in connecting the kevlar pull / pull cables to the rudder and elevator.

Post-Flight Modifications

When flying the model outdoors the open framed wing flexed constantly with changing load. The extreme under-camber delivered some up pitching especially when throttled up into the breeze. Penetration with this wing was unconvincing, read non-existent. The original wing shown in the image above has been reframed with an additional bottom chord and partial web infill (.4 mm Ply). It has also been recovered top and bottom to create a stronger and more stable structure for flying in gentle outdoor conditions. A new indoor wing has now been built. It remains open frame, as in the image, but some stiffening changes have been made such as fillets and 10 mm flats at the tips. The diagonal balsa brace is now coincident with the carbon fiber hangar and the "spar strut" was slotted on a table router to accommodate a 2 x 0.4 mm carbon fiber flat inserted along the entire length of the span. Wash out was built into the frame and not primarily the outcome of restrained doping which was used on the original wing.

Construction

Foam Core Jig

Cowl

Foam Core Jig

The jig is constructed of 5mm foam core, cut with a scalpel and straight edge. This model cannot be constructed in free space. A 200g model with 1100 mm span is not likely, on its own, to define a satisfactory form in construction. Careful jigging ensures geometric alignment through stages of frame assembly and covering. I fixed the Jap Tissue to the frame with "UHU" glue stick "soft" adhesion so there are no wrinkles or tension pulls. Doping and the inevitable repairs utilise 50% Acetone and dope by volume.

Cowl

Vacuum Moulder

The cowl image, rendered from the CAD working file, shows the GWS B motor in position. The surface modelling software facilitates close tolerancing of the moulded form. Profiles were extracted, then layered up to make a male mould. The resultant vacuum moulded PVC form was stiffened with balsa flats at the screw points. The balsa was fixed to the plastic with turpentine thinned "Liquid Nails". The finish is Tamiya AS12.

Vacuum Molding

A plastic planter box is the basis of the Vacuum Moulder shown in the cut away CAD image. Clear 0.18mm PVC sheet is taped to the platen, placed in the oven at 200 C until plasticity is apparent. Then the domestic vacuum cleaner, connected at the base of the box, is switched on and the platen quickly placed over the mould. The process is instantaneous. If there is a miscue, return the platen to the oven and the PVC will reform as a flat sheet!

In Flight

Control Detail

Flight Performance

Indoor

The Radio King Rat flies well. It is stable, flies slowly, remains level under throttle, has a forgiving stall, and can turn very sharply. On 7 cell x 200 mAH NiMH expect a number of indoor flights per charge.

Outdoor

Expect ten minutes with the above $14.00 battery in calm conditions. With Lithium batteries the duration could be measured in hours. Radio King Rats have been looped, perhaps someone has performed a Cuban Eight!

Next Project

Conversion of an old Flatbed Plotter to NC Depron Cutter. Interchangeable parts and jigs unlimited!

John Morrison

*** 10 January, 2010 10:26 PM +1000 ***

Last updated 10 January, 2010