Kwik-Fli IV-40 (part I)

As usual in recent years, contemplation of the coming winter's build project was conditioned in no small part by the ever growing space and storage pressures in the hangar. Also flying opportunities during the Summer of 2017 seemed mainly to consist of days when the wind was a bit more than ideal and/or the threat of rain wasn't far away with the result that many models didn't get a lot of air time and I found myself falling back on those that are easy to manage at the field and can cope with a bit of turbulent air. So, the next build had to be something that would definitely earn its keep - and ideally something that would enable me to pension off one of the existing airframes to make room for it.


Taking that last consideration first, I found myself looking at the Crossfire. This 20 year old model still looks pretty good but, with over 200 flights in the book, it doesn't owe me anything and doesn't get flown too much nowadays so maybe that should be the one to be moved on? Looking around for a possible replacement/upgrade, I kept coming across classic F3A designs from the 1960s and 70s, and eventually found myself drawn towards the Kwik Fli. This is a design by Phil Kraft, who won the 1967 US Nationals with his Mk III version. The Mk IV differs only in having a slightly tapered wing and, although apparently Phil preferred the straight wing version in competition, I prefer the look of the Mk IV so opted for that version. As it happened, Martyn Kinder undertook a re-drawing exercise on both versions late in 2016 and produced a scaled down version intended for .40 i.c. power - hence the IV-40 designation.


Having downloaded and printed the plans, a wood order was dispatched and a survey of various bits in stock resulted in a pleasingly short shopping list. By the end of August everything was in place for a prompt start in September. Then I slipped a disc! A similarly excruciating episode 13 years ago landed me in hospital but mercifully this time I was hobbling about again within a few days and hopefully will avoid the operating table.


Anyway, the upshot was that this build got off to a fairly slow start, but cutting out a few wing ribs did prove to be quite a suitably therapeutic exercise.

September 2017
As you can see, the Kwik Fli has a fairly thick, symmetrical wing section. There was a time when, for electric power, one might have thinned this a bit to reduce drag and help with duration but with modern motors and batteries I didn't see any need to do this so stuck with the section as drawn. The plan does though show a leading edge formed from 1/4in square balsa set on the diagonal. I've never been a big fan of this method so I truncated the nose of the ribs so that a flat false l.e. could be fitted and the leading edge sheet glued over that before sanding the front flat again to fit the true leading edge. I feel that this method makes the sheeting job easier and gives a neater glue line as the l.e. is finally sanded to shape.

The Kwik Fli has a fixed tricycle undercarriage. However, this poses a serious problem if this model really is going to go into the storage space vacated by the hand-launched Crossfire. As it happened, I had a couple of electric retract units in stock and, offering them up to the plan, it looked like they could be accommodated without too much trouble so I ordered a nosewheel retract to go with them (fitting that in might be a rather greater challenge, but that's a problem for later!)

I did have a look around for a set of cheap oleo legs but without success. However another rummage through the appropriate bits box unearthed the wire legs supplied with the TopFlite DC3. Because I fitted air retracts to the DC3, these wire artworks had lain around for the last 17 years and so it seemed their time had come at last.

The original leg is shown on the right (upside down). Offering it up against the plan, I realised that, by inverting it, I could use an existing right angle bend for the axle and, most important, utilise the pre-formed shock-absorbing coil. As you can see, a bit of judicious bending was needed to the upper portion of the leg to allow the coil to clear the body of the retract when retracted and to ensure that the wheel cleared the main spar.

Rib W4 had to be moved a few millimetres outboard but, apart from fitting the mounting beams, no other modifications were necessary. These pictures show a dry assembly with temporary cardboard ribs in the W3 and W4 positions.

Once I was happy with the fit of the retracts, the dry fit was repeated with the real ribs, and it was at last time to get the glue out.

Martyn drew up an electric variant using liteply ribs. My personal preference is to use balsa and reinforce where necessary with laminations of 1/64 or 1/32in ply.

A similar technique was used for mounting the aileron servos.

I also departed from the script slightly for the dihedral brace. Rather than a single 1/16 brace to W2, I fitted 1/32in braces either side of the spar, extending the rear one out to W3. The shear web in the W3/W4 bay where the retract is fitted is also ply rather than balsa This does still leave a fairly nasty stress raiser in the spar at W3 but hopefully, with such a thick wing section, it will be okay.

So, in spite of the delayed start, I'm quite happy with the way this is coming together so far. More next month.

October 2017
With the wing panels joined, a start was made on the leading edge D box. I was keen to close the D box with the wing sitting on the 'feet' so the underside was done first. I've lost count of how many times I've sheeted in leading edges but holding the sheeting down still seems to come as a challenge every time.

Some random strips of spruce, old NiCd batteries and of course masking tape were used this time.

Once the D box was complete, the rest of the aileron structure was built onto the underside of the trailing edge sheeting. The 'feet' were then cut off the wing ribs and the underside sheeting completed.

The area around the retracts needed a bit of thought. Whereas I'm happy to seal servos in without an access hatch, retracts really do need to be removable so hatches had to be made.

The wheels do end up rather deep inside the wing. I'm not sure what the aerodynamic effects of such a big hole will be, but there is always the option of retro-fitting fairings if need be.

I don't know if it's just me but these pictures sometimes create an optical illusion of a thick-tyred wheel on top of the wing surface.

As a bit of an afterthought, I boxed in the retract unit from above to limit the extent to which mud can be spread about inside the wing.

Remaining tasks on the wing were to finish the ailerons, add ply facings to their ends and to the apertures, add the wing tip blocks and finally the leading edges. The whole thing still needs final sanding but it's basically complete so attention now turns to the fuselage.

The steerable nosewheel unit is quite compact but, offering it up against the plan showed that the firewall is going to have to move forwards to accommodate the retracted wheel in front of the wing. Happily the motor I have chosen is somewhat shorter than the one shown on the plan so the change in apparent nose length will be minimal. However, the retracted leg and wheel do occupy the space allocated to the ESC so it will have to move. The battery tray will also have to be raised a bit and of course there is a nosewheel steering servo and linkage to be fitted in there somewhere too.

To keep my options open, when tracing the fuselage sides onto 1/8in balsa, the nose section was extended forwards, upwards and downwards, with a view to trimming back later. Before cutting the slots for the tailplane, it seemed a good idea to build it first so I could see how thick it actually turned out to be, rather than adding up nominal balsa thicknesses.

The tailplane was built pretty much to plan except that, as for the wing, I opted to fit a separate leading edge cap strip rather than feather the 1/16in sheeting into the framework.

So, with the former positions marked and the tailplane slots cut into the sides, attention can turn to the formers. I think I'd better start with that re-positioned firewall. A first look indicates that the captive nuts for the motor mount will clash with the bearers for the nosewheel retract so a bit of thought is needed.

At this point, a spell of unseasonably good flying weather interrupted the build, then the month was gone and it was time to update this diary. Hopefully, I'll get back to the bench - or more precisely to those questions about the firewall - before too long.

Meanwhile, this page is getting a bit too long - click here for part II