If the full size aircraft is an ultra-light then the model is an ultra featherweight. In a light head wind you can fly the model Skyfox along at walking pace!
As the costs of flying general aviation aircraft become prohibitive for many, ultralights are an attractive alternative. All kinds of ultralights are taking to the skies, from powered hang gliders to sporty racing machines. One of the most popular of all is the Skyfox.
Designed and manufactured in the United States as the Kitfox this ultralight combines the looks of a bygone era in aviation with modern high tech materials matched with the small but powerful Rotax engine. The classic radial cowl similar to the Monocoupe, and vintage look of the airframe gave the Kitfox plenty of character.
In Australia the aircraft is manufactured under license in Qld., and is known as the Skyfox. The most obvious difference is the nose area which replaces the radial cowl (it was only a dummy anyway) with a flatter and more streamlined shape.
This revised nose actually gave the Skyfox a few more knots on the clock compared to the original Kitfox.
The Skyfox is classed as a non-aerobatic aircraft. The seemingly feeble looking wings have no mainspar. The strength lies in the leading edge which is thick aluminium tubing from which the highly undercambered ribs extend to the trailing edge. The covering and supporting struts gives the wing rigidity it needs. The wings can be folded back for storage.
From reports, the earlier Skyfoxes experienced aileron flutter with their broad strip aileron hinges away from the trailing edge. This problem was overcome by addition of mass counter balancing at the ends of the ailerons. The ailerons also can droop down as flaperons.
Nonetheless the Skyfox has proved to be very popular in this country. One pilot hedgehopped his Skyfox from Queensland to Avalon, Victoria to attend the International Airshow Downunder, spreading the journey over several days.
The Skyfox can be classed as an ultralight and give the pilot a bit more freedom from red tape, or be registered as a GA aircraft. Some flying schools use Skyfoxes for initial flight training at very economical running costs before students make advancement to heavy metal aircraft.
Airborneıs Skyfox is a 1/5 scale model designed for a .35 - .40 motor. Outlines are shown for the Kitfox nose area for those who prefer the original version.
If the full sized aircraft is an ultralight then the model is an ultra feather weight!
Itıs nothing but sticks with a thin covering, plenty of wing area for a tiny engine to pull it along.
As with the full size Skyfox the model is not intended for aerobatics. The aerofoil used looks like something from an Old Timer model. Relaxed, slow flying or touch and goes is probably as much as you can do with this model.
Despite its impressive slow flying characteristics the model Skyfox should not be considered a trainer.
BUILDING THE MODEL
Unlike the full size Skyfox the model does have a decent sized spar for the wings.
Cut out all wing ribs, riblets, leading edge and trailing edge to size. Check size of servos you intend using for the ailerons. Mini servos are recommended but standard servos might just squeeze inside the inboard wing. Cut slots into the trailing edge for the ribs.
Prop up the spar over the plan by 5mm and the underside front edge of the shaped trailing edge by 1.5mm. Pin to the building board. Cut out a section from the rear of the leading edge at the inboard wing to allow for the brass tubing. Pin the leading edge down onto the board.
Glue all the ribs into place from W2 - Wing tip. Bind and epoxy brass tube, crimped at outer end to ply plate WP then glue into place together with W1. Glue in the rear brass tube also crimped at the outer end. Add riblets R1 over WP, then glue the remaining riblets R out to the wing tip. Allow to dry.
Add the balsa wing tips, carve to shape. Note the different depth of the downturned wing tip plates of the Skyfox compared to the Kitfox. Sheet over the inboard wing between W1 - W2. Add the plywood strut supports.
The strip ailerons/flaperons may appear to be rather flimsy and vulnerable to damage. The model proved them to be quite satisfactory though when test flown and were not mass balanced. Any damage is likely to occur if mishandled during transportation or rigging.
Cut out all the plywood aileron hinge brackets. These can be glued permanently onto the wing after covering the wing, but are used as an integral part of the aileron unit.
Cut a strip of hard 3mm balsa to the size of the aileron. Accurately cut and glue in the plywood inserts.
Fit a length of piano wire through hinge brackets and epoxy the wire onto the plywood inserts taking care not to allow glue to run into the hinge.
Cover the aileron with top sheeting. Sand to profile shape.
Note: Later Skyfoxes had a symmetrical profile. Add another lamination below the top two and make symmetrical if desired. It would make the ailerons stronger.
The rear section requires that the side frames be built up over the plan, then cross members added to make up a box section. Sheet the top and bottom areas for strength.
Accurately join up the front and rear fuselage sections over the plan. With the cabin floor or fuselage base flat on the board lift up the tail end by 84mm for correct alignment. Allow glue to set thoroughly.
Complete the sheeting under the tank area and planking between F1 and F2.
Mount the engine either on blocks or directly on firewall with radial mount. Trial fit fuel tank, fuel lines and throttle cable.
Build up engine cowl from balsa blocks and planking. Cut away air intakes and underside opening for flow through cooling. The top cowl half is removable for access to engine.
Make up the door frames, one side or both should be removable for access to radio gear. Complete the tubular cockpit framing, and build up top cover rear of the cabin.
These are built up flat over the building board. Round off edges, bevel the leading edge of control surfaces. Fit hinges and control horns to suit, as well as the elevator joiner.
Note there is a gap in the fin for the tailplane to pass through. Leave the top of this gap open when covering. Also pay attention to fabric covering from fuselage to tailplane section. See cross section on plan.
Bend the tailplane struts and check fitting. Fix permanently in place after covering.
The fuselage is built up in two section, front and rear, then combined. Decide the nose section you wish to make, either Skyfox or Kitfox, and cut out shapes accordingly.
Cut out all plywood formers F1 - F4, engine mounting blocks, fuselage doublers, and all cabin pieces. Build up front section by assembling and gluing cabin area components over the plan followed by the nose section up to F1. See the 3 - view diagram shown on plan which shows what is required.
Build up the struts for the wing. Trial fit the brass tubes for wing joiner rods across F2 and F3 with wing and struts in place. Make any adjustments necessary for correct alignment. The wings have about 1° dihedral each side.
Bend, cut and solder undercarriage to shape. Assemble together as shown on plan. Use lightweight 4² wheels which are rather big on a model this size.
Make up tailwheel unit and attach to rear of fuselage.
Install all radio gear and connecting rods, check movement of all controls. All up weight was a mere 2.2kg.
COVERING THE MODEL
The model lends itself to a variety of covering materials being mostly open structured, however something which resembles fabric covering is best to use rather than shiny plastic ones.
I chose Solartex and painted the model with spray enamels. The colour scheme was based on a Skyfox which used to be at Sunbury airfield and was flown by John Rogers of Airborne at one time.
I added a few more graphic stickers to enhance the colour scheme, after all itıs my aeroplane.
All the cockpit glazing was fitted and glued in place with silicone sealant. Make sure engine bay is thoroughly fuel proofed.
Install all the radio gear and engine back into the model and balance the aircraft to C.G shown on plan. Check the flying surfaces for any warps.
FLYING THE MODEL
The Skyfox is a high lift, lightweight model, therefore should only be flown in calm or light wind conditions only. Blustery winds will flip the model over on the ground or if you do manage to take to the air will see the model flying backwards.
Large control surfaces make the Skyfox respond very quickly. Combine this with its inherently stable configuration and you will soon understand that this model requires gentle fingers to fly it.
After completing the pretake off checks the engine, a brand new OS LA40 was brought to life, running as smooth as silk.
The controls were handed over to my personal test pilot Robert McDonald who had no hesitation to fly the Skyfox.
Full power and away she went. Next moment saw all hell break loose when the model climbed vertical, looped over, engine cut out and model out of control!
The Skyfox pulled out of the loop and flipped over in long grass beyond the runway undamaged. What happened? A quick inspection revealed the aerial had a kink with a break in the wire, limiting the radio range to only a few meters. I shouldıve done my radio checks thoroughly before trying out a new model!
Next time round with all things in order saw some ³real² flying. The Skyfox takes to the air almost instantly with only about quarter throttle. It cruises with just a tick over of the engine. The LA40 provided too much power for this model.
As previously mentioned the model is non-aerobatic, but who cares? I tried a loop, roll, stall turn very nicely executed, but take care not to overspeed the Skyfox or you may over stress the delicate frame work.
Time to land and the Skyfox is lined up on final approach. The flaperons when put down slow the model to a crawl. In a breeze you can walk alongside it. The Skyfox floats onto the grassy strip and stops in no time. I was rapt!
Skyfox plans are available from:
Airborne Plans Service
AU$38.50 plus AU$2.00 P&H2
Plan No. 582
Click here to order this plan
This page was last modified on the 21-May-02