GeeBee R3 750mm TRILOGY. PART 2: The Build

...so how does this work?

The GeeBee is a nice-quality kit, with solid EPO fuselage making it a smarter-looking proposition than a Rarebear, for example. While having a simple screw-together construction, it does however its construction poses a few questions.

How does the battery setup work, and where's the best spot for the receiver?

The first question is a puzzler, for me anyway having only had planes with unfettered access to the battery. Obviously, unless you're happy remove the wing every time you fly it, you need the ESC lead to remain in the battery hatch so it's accessible through the small hatch opening.

This I found achievable by mounting the ESC against the side of the fuselage, preventing the lead from falling back into the fuselage. This setup seems quite reliable, although packing the battery in and then feeding in the lead and connectors is still squeezy and awkward.

Regarding the receiver, in this case a twin-antenna Orange R615x, I mounted it in the central fuselage cavity which corresponds to a cavity in the wing moulding above. And, connecting all the servos from the back and the ESC from the front gives a very neat installation.

Next challenge is the bracing wire; five pairs of different length wires needing to be clipped into tiny hooks. Fifteen tediously fiddly minutes later I was finished.

The last trick was spinner and propellor. The latter simply gets tightened with a nut, with the spinner base in behind it. Then the spinner is installed; it comes with double-sided tape but, really, that's never going to hold. So I superglued it in place, with cardboard slipped in behind the base to keep it spaced off the cowling.

With everything in place the little GeeBee was looking good...

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...

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...

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...

Float Plane Adjustment

...fine tuning the floats

Having done a water test with the Trainstar floatplane I've been researching techniques on launching floatplanes. In the process I came across more detailed sketches of floatplane configuration which caused some concern regarding how I'd installed the floats.

While I'd got the basic setup correct, with CoG in front of the step and the front of the floats ahead of the spinner, the proportions weren't right, with the CoG being too far ahead of the step (around 45mm) when it should be about 10mm, and the tip of the floats was about 25mm ahead of the spinner when ideally it should be greater than this.

The setup and installation of the floats is quite fiddly and awkward so I wasn't really keen to repeat the experience, and additionally moving the floats would mean a set of open holes in the floats which wasn't desirable.

However the rationale behind the setup criteria were quite compelling; having the CoG too far ahead of the step means rotation during takeoff is harder, and having the floats extending further forward reduces the risk of the plane capsizing which, despite having invested in a rescue craft, is a highly desirable feature. Finally, and perhaps the deciding reason, was that the Trainstar's CoG was in fact a little far back, which would be corrected by moving the floats forward without needing ballast, something I absolutely don't want to do.

So I bit the bullet and uninstalled the floats. I'm still in the process of attaching them about 30mm forward, however this position has adjusted the CoG perfectly in terms of the step and the floatplane as a whole.

So, I just need to finish off the screws and the seal around the mounts, and do a bit of sealing of the unused holes and a cosmetic touch-up. The weather has been terrible lately, so I might just pop down to the local pond again and do another water test, this time perhaps trying to get the Trainstar planing more and, perhaps, even airborne for a few feet to give myself a bit more confidence...