Keil Kraft Invader

Arnaldo Correia electrifies Keil Kraft’s vintage towline glider.

The Invader was one of six Keil Kraft kits recently chosen for re-release by Ripmax. During my (mis-spent) youth I flew free-flight gliders with long, slender fuselages and was always fascinated by the vintage, full fuselage gliders of yore.

So, I was more than a bit intrigued by the Invader’s (probably) Martin Mariner-inspired tailplane and P-35 / P-47 lookalike wing tips. So, when it became available once more, I took the plunge and decided to build it.

To me both the Invader’s name and design are reminiscent of WWII, and indeed it was designed for Keil Kraft during that period (1942 - 43) by Albert Hatfull, whose beautiful wing section-like logo was ever-present on his Keil Kraft plans and which can fortunately still be seen in this modern rendition.

THE KIT
Like its siblings the Invader is supplied in a bright coloured box, illustrated with several images of the model, both bare bones and covered.

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All the parts you need to build the kit are here, down to the paper clips used to make the tow hooks. The balsa sheet parts are laser-cut.

Inside you’re in no doubt that this is a modern kit with the presence of laser-cut balsa parts and a CAD-drawn plan alongside the more traditional content of balsa strips, white covering tissue, acetate sheet (for the windscreen) and an accessories bag containing two paper clips and a couple of rubber bands, the latter being meant to be converted into tow hooks.

Building instructions are detailed both on the plan, in English, and on a separate A4 sheet in German. The draughtsperson who converted the plan to CAD has provided dimensions both in Imperial and metric, and thoughtfully added a few isometric views not present on the original plan.

The parts sheets are of medium density balsa wood (between 8 and 9 lb /cu. ft.) and nicely laser-cut. Removing them from the carrier sheets is a doddle, not forgetting to remove the charred wood from the edges that will be in contact with the covering with a couple of gentle sandpaper strokes – any charring left visible will look unsightly through the covering.

After reading the building instructions and studying the plan I came to the conclusion the instructions had most likely been copied directly from the originals without checking the new plan or the kit contents, as the builder is instructed to make parts from strip or scrap balsa that are, in fact, already included on the laser-cut sheets.

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The denser the wood, the stronger it should be. Consequently, I started by grading the strips according to their weight; makes it easier to choose the ‘right one for the job’ later.

Construction is mostly conventional, with a couple of unusual steps thrown in to spice things up. So, having read the instructions from top to bottom and assimilated the important stuff, I then went my own way as I wanted to introduce a few changes, some of which were due to my wish to convert this lovely vintage glider into an electric-powered park flier, and others being due to my ingrained construction habits.

Rather than recounting the manufacturer’s instructions word-by-word I’ll focus on the points where I digressed.

FUSELAGE
The instructions kick off by building the two side frames, later adding the 1/16” balsa nose reinforcement inserts ‘N’ and joining the side frames using the built-up former ‘D’. I didn’t like the idea of adding the N parts later as any discrepancy between the drawing and pre-cut pieces could make fitting an issue. I therefore decided to build the side frames around them.

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Put a couple of temporary 1/8” balsa spacers between inserts ‘N’ and you can easily build both side frames, one above the other, while still incorporating the second ‘N’ during the construction. A little Sellotape keeps the spacers temporarily in place.

The first side isn’t a problem as the 1/16” N lays directly against the building board. To make the second side I put a couple of small 1/8” sq. balsa scraps as temporary spacers between the first-laid N part and the second before starting to build the second frame, ensuring a perfect fit.

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When building the second side over the first I use Sellotape rather than plastic sheet to keep them apart, so fewer pins require repositioning.

Using a single former is a technique sometimes seen on slab-sided models of the 40s, but I can’t really see the reason why. My preference is to use two main formers, which eases the build later on, so to that end I duly built two D formers.

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The fuselage can be built using just one former, as directed on the instructions, but from past experience using two formers makes for easier assembly.

If you follow this course do keep in mind that in order to fit into the new locations properly the rear former has to be shorter, and the front one taller, than the originals shown on the plan. This also means you should glue the last diagonal spacer in place on both sides only after the side frames and D formers have been assembled together.

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Using two formers means that the missing cross-brace diagonals are shorter so they are added after the sides are glued together.

After the side frames and D formers are glued together the rest of the fuselage is built in the traditional manner. I used Baptista Pereira’s tip of using temporary braces to keep everything in shape while adding the cross members and it worked like a charm.

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Temporary crossmembers keep the fuselage straight while adding the spacers. Be sure to add the crossmembers inside the side frames, though, as this makes for an easier clean-up afterwards.

As the nose would take not only the 1/16” ply motor firewall but also the flight battery, I added a couple of 1/8” balsa pieces to the nose top and bottom, rather than relying on the 1/8” sq. spacers and covering to take the stresses.

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Lower nose sheeting was extended to provide a good-size tray for the LiPo.

To be able to access and change the battery part of the nose top was replaced with a hatch, made from a few scraps of balsa and secured using Neodymium magnets.

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I needed a hatch on the nose to ease the replacement of the battery, so I doubled nose former 3 with 1/8” balsa to make fitting the windscreen easier.

Using two D formers instead of one means part K has to be adapted, too. I trimmed it to fit in the first bay and replaced the back part with a length of 1/8” x 1/4” balsa strip.

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The instructions tell you to cover the bottom of the nose with card, but I used 1/64” ply instead - much stronger.

Instead of cutting patterns ‘X’ from card I used 0.4mm ply, thereby adding weight and strength where it’s most needed. I also changed the location of gussets G7, G9 and G10 slightly to provide a better fixing for the wing and tailplane.

WING
The outer panels of the polyhedral wing have an elliptical t.e., which I chose to laminate in one go against a 6mm Depron mould rather than follow the instructions.

To do this I first soaked the 1/8” sq. balsa strips in water for about half an hour, before coating the contact edges with PVA. The strips were then pressed tightly against the Depron mould, using pins to keep the laminations together, before letting them dry overnight.

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Outer wing panel t.e.’s can easily be laminated against a Depron former in one go, rather than adding one strip at a time as stated in the instructions.

The wing (and tailplane) ribs were slightly out in terms of accuracy, with the spar slots being about 1/32” too large. No big deal, as it’s easy to insert small 1/32” balsa scraps to get a good fit, but it’s annoying nevertheless. This also affected the wing tips, where the lower spar had to be filed slightly to get a good fit with the top spar. In a way that turned out to be useful as it makes the joint between both spars at the tip much stronger.

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Using a pre-laminated t.e. speeds up the construction. Note the small 1/32” balsa scraps used to compensate for the oversize spar slots on the wing ribs.

The wing structure is perfectly adequate for a free-flight model but as my R/C conversion would bring additional stress to the airframe I added 1/16” light balsa webs between the lower and upper spars on the centre panel.

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Lightweight 1/16” balsa webbing on the centre panel makes for a stronger wing for a negligible weight increase.

Last, but not least, I reinforced the outer panel to centre panel joints by gluing 1/32” ply dihedral braces to the lower spars, using the supplied balsa items for the upper spars.

TAIL TROUBLE
One of the things I like most on twin-tail models is to see the two rudders moving together. Alas, the Invader’s small size would make passing control snakes through the tailplane to actuate the rudders a bit tricky, while the V-tail precluded the use of a bellcrank system.

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0.8mm piano wire elevator pushrods run inside nylon inner pushrod tubes.

My first thought, then, was to use tailerons to control both pitch and roll/yaw as I’ve noticed old-timer models with just one elevator exhibiting unwanted roll and yaw, where up elevator induces a turn towards the elevator side and down elevator a turn to the opposite side.

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Rudders are controlled through pull-threads with small rubber bands pulling them on the outer sides; knots are used to adjust the rubber bands’ tension.

I unfortunately overlooked the fact that the Invader tailplane has a large dihedral, since ailerons and large dihedrals do not go well together. To nevertheless test the tailerons idea without wasting those nice laser-cut tailplane ribs, I built a tail mock-up out of 6mm and 3mm Depron and all test-flying so far has been done using that.

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No vintage model build is complete without a bare bones photo! The tail is the Depron mock-up used to check the taileron idea.

In the end I had to use to use both elevators and rudders, the elevators being trimmed on the outside to allow the rudders to move freely.

MOTOR & RADIO
My chosen powertrain for the Invader was a 10g ‘bell’ type HobbyKing 18-11 2000 KV micro outrunner, 10A ESC, 300mAh 2S LiPo (secured to the lower nose sheet with Velcro) turning a GWS 8 x 4.3” propeller cut down to 7” diameter; an ensemble that draws 4.4A at 4.700rpm.

The servos are 4.4g Turnigy units, controlled via a HobbyKing 6-channel Rx.

COVERING
The colour scheme is unashamedly copied from a wartime Aeromodeller cover, down to the Union Jack on the sides of the fuselage.

I elected to use Solarfilm Litespan for the covering on account of its puncture resistance. The model’s name was taken from the Ripmax website, edited and laser-printed on white decal sheet, as were the Union Jacks. (Note that I have them flying towards the tail on both sides, as they should). The rest of the decorations were made from Litespan, glued in place with Balsaloc.

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Baptista Pereira ready to launch the Invader for the first flight after the control surfaces mods were introduced.

Including 0.18oz (5g) of ballast up front to get the C of G in the right place, the Invader weighed in at 5.1oz (144g), ready to fly. There is room for improvement on this figure, however it compares pretty favourably with the quoted AUW of 3.9oz (110g) (3.9oz) for the free-flight version.

During flight tests I also fitted a 2S 500mAh LiPo, which increased the weight by 0.4oz (11g).

FLYING
As mentioned above there were some initial control issues due to the tailerons not working; in fact the model simply refused to turn. During those short test flights, I realised that I’d seriously underestimated both the down and side thrust needed, as the Invader turned strongly left under power but would glide nicely in a straight line with the motor off. 8º of down-thrust and 5º right side thrust corrected this issue.

It was clear that the model really needed rudders. Luckily Baptista came to the rescue, suggesting I use the same system used on free-flight A1 gliders; a pull-string with a rubber band on the opposite side to counteract it. Simple, light and easy to do, even on a completed model. And the light tension provided by the rubber bands didn’t adversely affect the model’s lightweight servos.

Installing this new control system was very easy, but then the wind came; much too strong for the lightweight Invader. Fed up with the weather, I grabbed an opportunity to fly it indoors. My flying friends thought I was a bit crazy. A new, comparatively large, untested outdoor model being tested indoors on a handball court? I feared they might be right…

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The Invader proved to be quite capable of flying comfortably indoors. An added advantage was that it was much easier to check the model’s trim in the gym’s still air.

Fortunately, they were wrong. The model flew steadily and beautifully, although the first turn at the far end of the gym was negotiated with some trepidation on my part, fearing that it wouldn’t complete the manoeuvre inside the gym’s width. But It did, and with room to spare.

After a few circuits I cut the motor just after the turn at the far end of the gym, letting the model glide in for landing; a little flare and the Invader touched down softly onto its front skid. Some more flights followed using both 300mAh and 500mAh LiPos.

With my confidence growing with each successive flight, hour-glass eights and proper, well-rounded, ‘S’ figures were soon being interspersed between the easier oval circuits and I ended up doing one-and-a-half ‘S’ (three turns) along the length of the gym, the Invader withstanding the tight turns with ease.

FUTURE THOUGHTS
I’ll probably make more use of the 2S 500mAh LiPo due to the extended duration it offers and I’ve ordered a couple of folding propellers, which I’m very much looking forward to testing to see if I can improve on what I feel is already a good, vintage gliding performance.

Also, the Depron mock-up tailplane will be replaced with the appropriate balsa item, using the same control surfaces and deflections.

SHORT FINALS
Invader isn’t really suitable for beginners, who should seek out an easier model to start with - the Ace and Gipsy from the reissued Keil Kraft range immediately spring to mind. If you’re not a raw beginner the Invader is definitely worth your money; for a small investment you’ll get a very distinctive model that will give a lot of fun, both when building and flying.

My only issue so far relates to the weather, not the model. The Invader is small and lightweight, so don’t tempt fate by trying to fly it in strong winds. Believe me, I tried, and I was lucky to get it down in one piece. To fly it inside a gym was the only way to test it under the prevalent weather conditions.

Rest assured that the Invader flies beautifully, as might be expected from a free-flight model, and if you have access to a gym the size of a handball court you can confidently fly in there - you just need to juggle the throttle stick a bit!.

I’m longing to try it outdoors, looking up for thermals and seeing the sun shining through that covering... I guess I’ll have to be patient!

DATAFILE
Name: Invader
Model type: Free-flight vintage towline glider
Manufactured by: Keil Kraft
UK distributor: Ripmax
wwww.ripmax.com
Wingspan: 40” (1016mm)
Fuselage length: 24.8" (630mm) R/C; 24.5" (622mm) F/F
All-up weight: 5.5oz (0.15kg) R/C (w/500mAh LiPo); 3.9oz (110g) F/F
Wing area: 1.1sq. ft. (0.1sq. m)
Wing loading: 5oz / sq. ft. (1.5kg / sq. m) R/C (w/500mAh LiPo)
Powertrain used: 10g ‘bell’ type HobbyKing 18-11 2000 KV micro outrunner, 10A ESC, 300mAh 2S LiPo, GWS 8 x 4.3” propeller, cut down to 7” dia.
LiPo used: 2S 300mAh and 2S 500mAh
Functions (servos): Elevator(1); rudder(1); throttle (via ESC)
Control deflections: Elevator ± 0.27” (7mm); rudder ±0.31” (8mm)

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