One issue discovered was that the servo arms on my JR577 servos are not long enough, and my battery pack is a bit heavier than the 600mAh unit specified since it is 1100mAh. This makes hitting the CG a bit difficult, being nose heavy.
With the short arms and the weight issue I started to think about alternatives. Spending money on a lighter battery pack is not possible. Finding longer arms in this small town is not possible, and I don't really want to be cutting more foam out of the wing in that area. Putting micro servos in as described here IS possible!
Some thought showed that if I take the 2 wing servos out of the Coyote I will still have 3 micro servos available for that, which is enough if I take the awful rudder off it. This thinking took overnight into Sunday....
Didn't like the mechanics of those linkages anyway. They bound on the edge of the servos, and the hook parts are really nasty and did not work smoothly. (Yes, I am a perfectionist and am probably being too fussy. So what, live with it)
Next up is fitting the elevons so I can check the control rod geometry before cutting the new servo holes.
The remote receiver part goes near the trailing edge with the wire in a channel. The rearward aerial went through the wing on the other side. oops, but not serious since that area gets at least 3 layers of tape over it. The main receiver aerials go into holes in the sides of the cutout. Laying out the AR6200 receiver for a wing like this took a bit of thought since there is no vertical space in which to place the secondary unit in the preferred orientation. This arrangement fits and appears to work. I did a rough range check in the house without problems, and will do further testing before flying (or even taping it shut).
One possible snag with this receiver (all the Spektrum receivers actually) is access to the receiver for binding. One needs to rebind if the tx gets reset in any way (like if you reset the model memory), or if the trainer switch is depressed during tx switch on. The manual refers to this happening negligently, and while I think I'll never do it, who can be sure? I'd like to avoid having to cut it open for access, so am thinking of sticking the battery lead into a spare channel and putting a servo extention into the battery socket. The female socket will then be taped to the outside making it possible to fit the bind plug there at any time. This will save a lot of time at the slope should binding become necessary.
A little note on carving the EPP foam. That stuff is TUFF. My usual craft knife was too blunt after being used on some other things and I had to grab a new blade. I then got out my new no.11 scalpel and it cut it well, but after a bit it also became blunt. Still scary sharp eh, I'm scared of it, but it doesn't cut quite as easily as when it was new. Carving the bottom of the cutouts needs a 'chisel' shaped tool, and I happened to have one that came in a very cheap set of 'Exacto' clones (which I really bought for the 4 handles int he box). I sharpened that chisel on my fine oilstone and it cuts very well, so I think I will be able to resharpen the no.11 blade for continued joyful cutting.
You cannot just push the chisel, it will eventually cut, but will push a lot of EPP out of the way first, possibly causing tearing where you don't want it. What I do is wobble it from side to side about .5 to 1mm, so the edge has a slicing action in the foam.
These images show the mounting of the micro servos. Soem creative carving lets them sit so that the arm is in line (or nearly so at least) with the control wire angle. Each is attached with double sided tape to the side of the cutout, and further braced with some polystyrene blocks.
A bit of thought and cutting some salvaged polystyrene and correx got the power/charge jack mounted. I am using the 'BlackJack ' idea since it is such a good one. I cut a hole through the top surface in one of the servo bays. A peice of correx was cut to fit and a hoelmade in it for the jack. I tried to superglue the jack to the correx, but that came loose in the instant I tried to push the jack in. Later research showed that correx is notoriously difficult to glue onto. Sigh, ok, plan B. I mixed up some epoxy and applied it to the socket threads and inside the hole in the correx and that appears to be holding ok.
That socket now acts as both the on/off switch and as the charge jack. Rather cunning I thought.
Here you can see how the servo bays are filled with polystyrene blocks to prevent the servos moving. The servos also have double sided tape (from my wifes craft supplies) holding them onto the outside sides of the cutouts. Over that lot go the kit supplied correx covers, which are taped in place.
Got the packing tape onto the wing. This smooths out the protruding servos a bit, and makes the wing much more solid feeling.
You can see the charging jack plugged in, since the battery is now plugged in and taped over.
Strangely, the CG has now moved rearward, requiring that I now insert some noseweight, somehow. Must be the tape, because before I taped it the CG was forward of the required point. What a fun learning process.
I had hoped to complete the build on Friday night, but I was way too tired and went to bed early. And then the weekend got absorbed by 'other stuff like building another fence panel, mowing the lawn, visiting friends and so on. The weather was a bit bleak as well, so I decided not to push and make mistakes in the control arm mounting process.
Spent some time in the workshop trying to come up with a neat way to install nose weight. The goal is to have it internal so it doesn't create drag, the problem is to get it in there in a neat and easy way.
The solution I came up with is to use thinwall tube (large one in the pic is 16mm, small one is 9mm) with the edge sharpened to cut the foam. After the cut, the core is pulled out leaving a hole precisely sized to take the precisely sized lead rods. In the pic can be seen several rods made from 5 oz fishing sinkers. I used my lathe to turn them to size, though I guess they could be cast to size.
A test hole has been made in the polystyrene block and the 16mm 'rod' inserted. The hexagonal chunks are bits cut from the end of a sinker, for use as temporary weghts.
The hole made with the 16mm cutter pipe. It is impossible to get the core out in one piece unless you go all the way through. I didn't do that because I wanted the hole to bottom out on the piece of correx that covers the battery hole on the underside. The blue color inside the hole is the covering on the battery pack.
The 16mm weight is fitted, as is one of the 9mm rods. The left hand 9mm rod is left proud for illustration. Each weight is marked and cut to match the slope of the surface at each hole. I didn't bother to try and cut them all exactly tangential to the surface.
Final balance was achieved with 3x9mm rods in place as well as the 16mm rod. I would not have guessed so much was needed as the CG only needed to move about 7mm forwards. I cannot weigh such small amounts of lead (yet), but my guess is about 3oz was inserted. A piece of the fibre reinforced tape was placed over this lot, and wrapped around the leading edge, retaining the weights and giving a little more pointy end strength.
At last I can give the flight pack a top up charge before waiting for the right wind for a maiden flight. A few tosses in the back yard show it needs a bit of speed, but nothing frightening, and it bounces off brickwork pretty well (-:
The charger is a constant current circuit I built myself, currently set to 70mA rate (the variable resistor cannot dissipate much more than this). It is in a box that housed a very early TV remote, I think it was ultrasonic. It now has a relay driven output that is only connected when the power is on, so when the countdown timer expires, the relay disconnects the battery to avoid discharging it through the charger circuit.I do hope to purchase a good automatic charger for NiMH packs soon. I believe this will be a good investment that will make my battery packs last longer.
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