Nice job on the build -- looks great

Mike

Now that the retract motor is installed I thought the nose could get some attention. This is the one area that I expected to have trouble getting the foam board to take on more double curvature than it would be happy with - and sure enough, the card outer surface buckled in many places requiring more light-weight filler to cover it over..
The nose ring is cut from a bit of liteply that I had lying about (much of the model is built of stuff from the spares box) with some of the card stripped from the foam board cut into strips to provide more gluing surface.
Under the nose some curved surfaces are required to redirect intake air to flow over battery and ESC that need it for cooling.

Generally skinning the airframe is a bit of a non-event so I'm not taking a lot of shots to show it. The tail end has been fully skinned at this stage but still needs fairing to the tail surfaces.

As I make progress on the airframe I am well aware that I am putting off the rather important matter of the switching system for the main gear retracts. I am going to have to face it soon though.

Robin

Originally Posted by limodune

Nice job on the build -- looks great

Mike

Thanks Mike - looks big too! That is, it takes up a lot of space in my living room. (I don't have space in the workshop to accommodate anything remotely close to this size.)

Here's some progress on some of the smaller bits... the ailerons. Very little structure in them compared to a balsa build as I'm using the stiffness of the foam board . I hope the short sequence below makes the process self-evident.
Meantime I am applying lightweight filler in all sorts of places to hide the most unsightly joins and distortions. Foam board is very prone to compress with pressure so depressions caused by slap-dash work or abrasion are frequent.
I've just ordered some more filler and some wood hardener - the latter of which I will have to test to see if it attacks the foam core or the filler. I'm pretty sure it'll harden card in just the same way as it hardens raw wood.

Robin

And a couple more bits and pieces... The underside airscoop provides the opportunity to pull cooling air out from the fuselage . Air from outside is compressed by a flap (or two flaps actually) making it speed up and provide suction to draw the air from inside.
Some later marks had this structure moved further aft under the cockpit.

Speaking of flaps - the under-wing flaps are inset in the wing and, since the structure of them will be visible from outside, I thought I would revert to some more fussy construction, using balsa for the ribs. The basic flaps are foam board with both card faces left on and the balsa stiffening is simply glued on with PVA.

Robin

More details. I toyed with the idea of making a plastic molding for the more rounded chin radiator but, as can be seen, decided against it, making it also from foam board. I still have the cockpit to make a mold for - which will be quite enough of a challenge. Probably a day's work on its own.

Still to do:
Connecting controls.
Forward tail fixings.
Wheel covers.
Cockpit, pilot and canopy.
Electrics for gear retract.
Spinner
Preparation for and Painting.

Robin

I have been busy doing bits and pieces over the last week. Getting the airframe pretty much complete has taken time - mainly by periodic applications of lightweight filler left to dry overnight or other times when it won't get disturbed part way through its drying time. But the most taxing thing has been getting the retract/flap mixing system working right and giving the amounts of movement I want. This is now almost complete so I thought the system I devised might be of interest.

As you will have seen I am using a pair of mechanical retract units for the main gear which works off a straight push-pull unit in the fuselage which, since I detest the blink and you've missed it retract times offered even by dedicated so-called 'retract servos', is a motor/gearbox combination with an advertised speed of 10 rpm. By my maths (admittedly not my best subject at school) that should give end to end times (180 degree rotation) of 20 consecutive operations per minute. That is three seconds or better than twice the time offered by the slowest retract servo available. This it does, but it still seems too fast due in part to some of the time taken being used in end locking which is invisible from outside the model.
In retrospect I should have searched out a unit giving 5 or 6 rpm.

Fettling the mechanical mixing has taken much of the last three days spent taking it apart, adjusting and reassembling - all in a virtually complete airframe. Pretty frustrating at times - causing me to desert my post so as to do something completely different until ready to get back into the fray.

The main reason for all the adjustments has been my desire (fuelled by my Futaba 6EX radio's limitations) to have flaps go down about a third of total movement automatically when the gear goes down leaving the rest of the movement to the flap servo operated by the simple up or down switch of the Tx.
To achieve this my basic design (see diagram below) was to install a simple plywood arm attached to the retract unit at its moving end and a permanent pivot attached to the main spar. Half way (approx) down this is the central pivot point of a triangular plywood cam linking the flap to its servo but with the usual fixed position pivot point moved by the main arm to which it is fixed.

Pretty nifty I thought - and now it seems to be working well I agree with my designer self... though the engineer self has some misgivings!

Robin

Very nice work

Hi Robin! it's a very clever and nice idea!
I like mechanism, although I've never been so good at designing something similar!

Great ideas. Very interesting what you did here. I nowadays solve every problem with some kind of software. Either through programming my wonderful 24ch Jeti DS-24 transmitter or by designing in Fusion 360 and then 3D print it (either FDM or Resin). But sometimes it seems that with new technology also other things disappear. Like your amazing mechanical feat.

This reminds me of my work on the DC-6 where I read how they did things in the 1940ies and 1950ies. Such as the the spring loaded rudder mechanism that drives the flying tab on the rudder to allow the pilots to move the huge rudder and by using springs they still had the "feeling" of the force applied in their feet.

Thanks gents.
I get a lot of satisfaction from designing things like this. It often leads me into problems in that my actual engineering is done more by eye than by measurement, The flap mixing has one excellent effect and one drawback - both being part of the one thing... I have the retracts working at different times, that is, one leg goes up before the other - but that means that the flaps operate in that part of their working similarly out of step with each other. I am hoping that, since I am using a 3-axis stabiliser on aileron (and rudder) that will take care of the asymmetry in practice.

Alexander,
Welcome!
We have been talking on here long enough to know each others' strengths, but for others' information I have never felt comfortable paying out big money for what is after all just a hobby and consequently I have perhaps made a point of taking the cheapest route and making it work. I get a sort of perverse pleasure from it.
I have also championed the building of light models so as to achieve scale flight speeds - which also means I pay less in building materials.
I'm not quite sure how my epitaph should read in light of this but perhaps a well-known speech by Winston Churchill could be paraphrased as.... 'Never... in the field of human endeavour... has so much enjoyment been achieved from the investment... of so little...' etc.

But I hope to live long enough for you to have flown your DC-6!

Robin

Very much enjoying this build and can't wait to see her fly.

You trust a gyro to dampen out the effect of asymmetrical flaps?

As the asymmetry is not the result of any direct electronic input from the Tx I am hoping the stabiliser will read any resulting roll as if it were turbulence. It remains to be seen if my trust is well founded!

Robin

Sorry - I've been silent for a couple of weeks now. Some work has happened on the Yak though.
Having installed all the power train - although I still have the battery pack stowage to finalise - I did run the motor with the intention of checking that the 70A ESC is adequate... and although I only got the throttle to about 3/4 and an indicated consumption of 38A at that point the indications are that I should have no problem reaching (and even exceeding) scale speed. The reason I only got to 3/4 throttle was that I was holding the model with one hand and began to wonder if it might tear itself out of my grasp and do some serious damage in the living room (and to itself). Discretion asserted itself.

I also completed the forward pin and loop fixing system, which I did in the simplest way possible - so the end of the pin is visible on the left (port) side of the fuselage.
(I did take some photos this evening but while the camera does very well in low light situations the main living room light blew and a replacement is on order but not yet arrived - so I thought it advisable to try again when light is better and I can get much better definition)

I have also painted the model (very rough and ready using emulsion tester pots) and have installed exhaust stacks and cockpit canopy - but more of those when I can show you photos.

Robin

So with the benefit of a reasonable amount of light here are the photos to prove what has happened in the last couple of weeks and that I won't be entering the model in any scale competitions. I really can't be faffed to spend a lot of time on finishes when my main purpose is to bet models flying slow enough to look scale in the air.

The exhausts on the Yak-9 are set inside a projecting housing and the stacks themselves are regularly spaced (which they weren't on the original Russian engine). I modeled them loosely on a re-engined example simply because I could then use the same former for pulling all 12. Unless fellow club members spend time on this forum and have happened across this thread I don't expect anyone to comment on the errancy!

The cockpit was not straight forward either. At this scale I had to start from 13.5" of 4" square (actually 3.8") fence post. That was a strenuous day of sawing, gluing on side extensions, carving etc. and next day doing the first pulls. I did the canopy in two pieces, starting with the rear section aft of the sliding part (which is fixed on my model - I'll leave it to the real scale experts to add all the fiddly bits) using some sheet acetate I must have bought a long time ago when I had found my stock getting low. I then found that there is acetate and there is acetate. The rear section is smaller that the front though so when I tried the front part and had to apply a bit more heat I met a problem. The wretched material bubbled! I tried another pull and got the same annoying problem so I cut and fitted the better of the two to use as a pattern for the final one - which was produced without bubbling when I used part of my last sheet of original clear material. Framework was cut from the stripped card from scrap foam board, painted grey both sides and stuck on using Uhu Power glue.

The single heavy machine gun took me less than an hour. Some scrap balsa for the blister, slot cut in the foam board and gun muzzle from an inch of dowel. The slot was lined with a bit more scrap card. Filler round the blister and paint.
The Cannon fires through the middle of the spinner .... which reminds me of the need to make the spinner, which I have been putting off for weeks.

The use of felt pen for the red stars was also less than ideal. Clogged on the emulsion paint and gave a patchy finish, but I hope it might help if I give the whole model a spray of satin clear sealer eventually. Cost £3.50 too! Ah well...

Robin

Okay, now THAT is gorgeous. I have a soft spot for Russian Great Patriotic War birds. And it seems your build was fast - at least compared to mine, which can in some cases drag on for years, and I rarely build from scratch. What's your wingspan and your AUW?

Have you tried PETG for canopies? It molds well and doesn't seem to have the issues you describe.