Antonov AN 124 Rusian

After all the fury of motor testing the 5g servos have arrived so work can resume on the tail plane.

I even remembered to set both servos in the same physical orientation to ensure both elevator halves move the same!
Completion of the tail feathers is now critical to establish how 'tail heavy' the fuselage will be which is needed to counter the nose heaviness of the wing when the motors are attached. This in turn will determine where the batteries need to be paced which in turn sets where the ESCs must go.

It all a bit chicken and egg but the only thing I can be certain of is that whatever choice I make once all is complete it will probably need to be altered.

As mentioned I mounted the elevator servos in the same orientation so both elevator halves move together but when I tested them the servos moved in opposition - like ailerons.
To quote Victor Meldrew "I simply don't believe it!" the new servo works in the opposite sense!
Fortunately I had not glued on the top skin so it was not too much of a job to extract the servo and turn it over and rotate the servo arm 180 degrees.

So now the pair of servos are mounted as if for ailerons but the surfaces will move like elevators and I have checked they really will - twice!

The final bit of nonsense is the 1.4 m elevator servo extension lead. Despite being only 26AWG it weighs 14 g - the same weight as the pair of elevator servos!

Klippy
Good point but don't worry the answer is basically yes.
I am not too bothered if there is a slight displacement discrepancy at full deflection as any resulting net torque from the elevators in such circumstances when in flight is likely to be the least of my worries!
More important is they do both repeatably 'centre' at exactly the same position.


Just out of curiosity I have printed a new nacelle/motor mount that allows a bigger pear shaped tail cone to be used. This takes the exhaust nozzle area down to 87% of the FSA rather than the 95% previously tested.

With a bit of careful redesign of the motor mount, hence the need to reprint it, the cone angles and the external length of the cone has been kept the same.
Testing to follow but unless the thrust benefit proves to be spectacular I will definitely not be replacing all the existing motor mounts.

Are you leaving them orange?

Out of interest, how does the extra 1.2 ounces of thrust look with your predicted power to weight ratio ?

crash
If I do really end up with four times 8.3 oz (33.2 oz or 941 g) the thrust will be close to 80% of the 'guestimated' weight. Hopefully it will be 'adequate' for basic flying.
paul
There is a huge area of 'skin' so any form of 'decoration' will just have to wait. Even a few grams of paint could be significant.

At this stage the reason why the motor mounts and pylon are that colour (its actually red) is with that colour filament the individual print layers bond well together with the very thin walls that I am using so improving the structural integrity of a higher stressed component. The better? looking silver does not bond as well and thus parts printed with it are significantly weaker.

Well that did not go well!
Under test the motor on the 87% FSA
nozzle nacelle proved to be badly out of balance even with no prop! In fact only the first of the 4 motors has proved to be really smooth. Each further one run so far has been steadily worse!

Imbalance wasn't the end of the story. As it was run up to full power - it just reached 8 oz - but then suddenly stopped dead and restarted. The huge negative torque resulting from the sudden 'full brake?' stop seriously strained the motor mount and pylon. As soon as full power was reached it did the same again this time irreparably damaging the pylon and mount.
It ran perfectly on a 3s but not on 4s so it is most likely is an ESC fault so a duff motor and ESC. I have no spares.

2 more Little Bee 20A ESCs are on order but the Emax 2205 drone motors are quite expensive if bought singly in the UK indeed I can buy 4 (2CW and 2 CCW) from Hong Kong for virtually the same price!

Next job is to couple up the 4th motor and ESC to see how well that runs.

Wouldn't it have been a good idea to bench test it all as far as practical before installing it?
still looks like orange to me

The fourth motor and ESC are the best so far!
Silky smooth and delivers no less than 8.9 oz thrust on a freshly charged 4s.
I turned the camera on its side to get all the test stand in shot but then discovered I have no way to 'rotate' the video but the sound track does indicate the smooth running.

The stand is not fixed down in any way, just resting on a set of kitchen scales.

I have balanced the 'duff' 3rd motor as accurately as I can (the magnets are not particularly evenly spaced!) but it is still not up to the standard of the others.

That sounds really nice ! Which cone were you running with Simon ?

The last test was using the 95% cone. the same one that gave 8.3 oz with the other motor but to be fair the 8.9 oz used a better battery and it was fully charged.

Although I only have 3 workable motors I decided I might as well install the last good one on the RH wing and join the wing halves together as creating the mounts for the 4 nylon retaining bolts for wing centre section is the next big task. Once the wing is mounted the wing root and upper fuselage fairings can be completed.

For my own satisfaction the full '3 motor' wing mounted on the fuselage.

It seems that the more that gets completed the more remaining there is still to do!

Ooops!
Big embarrassment I got my maths wrong!
What I thought was a 95% FSA pear shaped cone actually turns out to restrict the nozzle to 80%.
I found this out when I calculated the 'pear' shaped cone required for the recommended 85% FSA and found it was actually smaller than those I had already printed!
Did the sums several times, both backwards and forwards, to confirm the result.
The correct smaller 85% on the left with the original (not 95% but 80) on the right. They really are red not orange.

It is no great problem as the cone is only glued on to the back of the motor mount and can easily be replaced so the AN 124 build itself is not effected.

As soon as I get another 'good' motor and ESC I will use the test stand to check the thrust with a range of cones - 100, 95, 90 & 85% FSA!