Dynam DC-3 And The Battery At The Back

...it's finally finished its migration

This afternoon I finally moved the DC-3's battery right back against the stop and gave it a fly. The only other change was that I flipped the battery around so that the leads were facing forward, so that I can connect the plug without having to remove the battery.

With the leads now facing forward the final move hasn't made much difference, with the centre of gravity at 75mm. The main bonus is that no care is required installing the battery; just slide it right back and you're done.

At 75mm CoG the DC-3 takes off very easily; it basically floats off the ground within a few metres, and it will cruise steadily at a fraction over half throttle. It's glide is also improved, although with power off you need significant elevator input to offset the effect of the wheels, but this would even be the case if you were coming in to land with retracts.

With the elevator now on on 100% travel, the Dak is fully controllable on landing, the final mass shift now allowing it to be floated onto the ground in a most pleasing manner, and provides a margin of safety for correction if you misjudge the sink rate a little.

The only drawback is that the Dak becomes light in the nose at low speeds, mainly evident when cruising into the wind. Here the extra airspeed gets it climbing and, without correction, the climb gets steeper as the negative moment from the undercarriage diminishes with reducing speed.

Left unchecked, the Dak will go into a spectacular vertical stall and then wing-over into a dive. Apart from that it is completely stable; all the aerobatic and stall maneuvers I've tried have not managed to induce any sort of spin or other unstable characteristic.

This seems a small price to pay for much improved behaviour of the Dak by having the battery at the back...

DC-3 at 70mm CoG

...and flying well!

Yesterday, I again nudged the DC-3's battery a little rearwards, with the front of the battery now a distant 70mm from the front of the tray, equating to a CoG of 70mm from the leading edge, and found the DC-3 still flew happily.

The take-off was really good, you never really know how well it's going to turn out, and I enjoyed buzzing it around the field, enjoying the clouds starting to catch the light of the rising sun, first those in the east followed by those in the west.

The Dak flies very easily now, although I will do some more experimentation with turns, where it can tend to go into a stall-type turn, perhaps just needing extra rudder to boot it around at lower speeds. Also, I'll also do some sore stalls and spins to see whether the the light nose leads to any instability at low speed...

Dynam DC-3 and The Case of The Receding Battery

...and where it stops nobody knows

Having established that the DC-3 was very nose-heavy on its first flight, I have been gradually moving the battery rearwards and assessing the change in flight characteristics with each change.

My ultimate aim was to get the DC-3 to glide reasonably well power-off, however with the amount that I've shifted the battery and with it still being nose-down, although much less than before, I'm wondering whether I'll have to settle for "most improved".

The battery has so far receded a substantial 70mm from its initial position hard up against the front of the recess, and the CoG is now somewhere south of the maximum 65mm suggested by Dynam. Makes me wonder whether they actually ever flew it or whether they just stuck with the theoretical figure and let you figure it out the hard way.

While I understand that CoG is not something made up, the moment-effect of the undercarriage is obviously substantial, exacerbated perhaps by the low-wing configuration of the Dak. And, despite having overstepped the CoG limit by a mile, the Dak is still completely stable; this morning I put it through a series of loops, rolls and stalls and not once did it suggest that it was going unstable or enter into any sort of spin that would have indicated a too-light nose.

And the Dak has been flying much better for it; now happily cruising around at about a third power, and turning much cleaner than the stall-turns evident it its first flight. So I think I'll continue inching the battery backwards, although in much smaller increments, until I've got it flying level without elevator trim, and perhaps even gliding a little better...

Hobbyking Servo Tester

...with centering function

I recently received the Hobbking servo tester which I've just tried out, a little unit I'd been meaning to get for some time. Diminutive in size and cost, the little tester is highly regarded for the facility it provides, primarily in being able to centre servos.

Servo centering is being able to put the servo at the neutral, or centre, position, where it is at the centre-point of its travel range. This is useful to be able to pre-install the servo arm without having to wait for the model to be assembled with ESC & battery installed.

Additionally, using a little trick, you don't even need an ESC to power the servo tester; by using a 2S battery, connected using its balance lead, you can get the tester up and running easily and conveniently...

DC-3 Centre of gravity

...getting there slowly

This morning I had the DC-3 out again at the field, this time with a revised centre of gravity. The instruction manual indicates 60-65mm and I've been flying at around 60 which clearly doesn't work.

To  correct things I moved the battery 30mm forward which put the cog at pretty much 65mm. Not exactly sure how much difference this was going to make, I left the flap setting and elevator trim unchanged just to be on the safe side.

The takeoff was good, the Dak easing off much more easily now, then started to climb and didn't stop until it was pretty much trying to go vertical. So, clearly, the mass shift makes quite a bit of difference.

After a little panic. I managed to the Dak straight and landed, then I reduced flaps and the elevator trim before trying again.

This time things were much better, wing the Dak cruising around happily. It's still too nose-down to glide very well, making me think I'll ease the battery back a centimetre or two and see how that goes...

Stinger Assembled

...but servo centering still a pain

I finished off the assembly of the Stinger 64 EDF today; a very nice quality kit that goes together well. The only piece that doesn't really fit is the pink trim bit that fits between the fuselage spine and the tail.

Something I wasn't sure about was the installation of the EDF, as I suspected the fan, or at least the cowling, would vibrate. So, to be on the safe side, I hot-glued the cowling to the fan housing in a few places, and then glued the cowling into the fuselage before I installed the wing.

The only thing that was a pain, although not unique to the Stinger, was the servo centring. In the past I've had to reinstall the servo arms so that they're centred once the plane has been powered up. I thought I was missing something, but checking a few videos of this subject seemed to confirm this.

The problem is that, in the case of the Stinger and the Walrus, the servos are installed in recesses without access to the servo arm, so I ended having to cut a section of foam out from one tail servo. In the case of the tail there's no option as, like flaps, they both move together and there's no trimming option.

I have read about a servo tester which includes a facility to set your servo to the centre position. I think this lets you set the centre position so as to install servo arms prior to installing the servo on the plane, however it doesn't help with servos already installed on a plane. 

Perhaps the plane manufacturers should use this instead of just randomly installing the servo arms...