Alpha 2.8 TJIRC (AKA, TJIRC Great Wall 2.8m)

[Redbird]

And here is one showing the gears in the servo we are referring to, the JX PDI-HV0903MG
I don't know how true it is but I read somewhere that there are only a few servo manufacturers in China and JX is one of them.
A bad thing about the ones I've had is the absence of strain relief to the wires as they exit the case, so a blob of hot glue is needed there otherwise they could break at the soldering attachment to the little board inside .

 

[Konrad]

What do the screws bite into? I'm thinking of adding some cotton flocks to some epoxy and make a pad inside the tail cone to accept the screws. The flock will help keep the epoxy pad from splitting as the screw is installed. Do pre drill the screw hole.

 

[Redbird]

That would be a very good move Konrad because there is very little thickness for the screws to bite into after recessing the tab area slightly with an end mill in the dremel. I also shortened and filed the mounting tabs thinner attempting a flush mount so that the fins go 'home' properly. My screws did however firm up snugly without any backing added.

 

[Konrad]

With the KST X0-8 I was able to fully bury the servo in the stab without any distortion to the stab skins. The fact that I can get the mouting flanges into the stab allows me to mount the servo to the fuselage with the stab full seated.

I like your partially buried servo as it allows the use of a longer control arm without a drag penalty. I'm finding that most of my V-Tail installations are focused on getting as long a control horn as possible with the shortest servo arm a practical. I find that this gives me the greatest servo resolution which make a noticable improvement of how the ship responds and grooves.

I too use the 4 wire tail wiring loom (common +- bus bar).

 

[Redbird]

? The Kst 08 is lovely and thin.

 

[Wayne]

The servo business is an odd one. Traditionally radio manufacturers didn't always make their own servos, instead they contracted with servo manufacturers. In some cases you would have the same basic servo coming from the same factory, but with different quality. We would see this with Airtronics (Sanwa) servos. The Airtronics were really nice, but a Hyperion version of the same servo would have all sorts of issues. Clearly Airtronics had some VERY good quality control post manufacture, and maybe staff at the manufacturer when the products were produced. You did pay extra, but the servos were better and worth it.

These days there are many companies making servos. Some produce mostly for other companies, and others make their own. In all honesty I'm never 100% sure when I am buying direct or not. In some cases I have bought some brands from another brand. JX is one of those companies. They had a nice line of low priced servos that are nice units, not great, but nice. Good value for the money. When I would order from them, I might get servos with another brand name on them.. Not ideal for us.. At the time I was not buying them direct, but from a distributor, so this may have been the distributor doing the swap.. But I have seen the same basic servo sold under around 10 different brands now. In some cases, the same basic servo appears to be built by different manufacturers. I'm told that the parts are available on the market, so anyone can start assembling servos if they buy the parts and market them how they like. This is very much the case of the Hitec clones.

It can be tricky to figure out.. that is for sure. Take the Dymond D47 as an example. I have never figured out who the original manufacturer of this servo is. I have seen them sold under probably 12 different brands. Do they all come from the same place? Got me. I think it is interesting that Dymond recently told me they lost their manufacturer on these and were looking for a new one. I think the motors for the Dymond version come from Taiwan based on another report from Dymond..

 

[Konrad]

Marketing (branding) and QC programs aside. The gears are often what makes a great servo great. Many of the low cost servos have gear teeth that are little more than just "V" shaped. Higher end servos have gear teeth that have curved flanks and a lot of shaping at the tip and trough. KST also is constantly staying on top of their gears making improvements as the market changes (noise, strength, back lash and cost). In my review of the KST X0-8 H+ I try to show the material change between the v1 and v2 gears.

As a side note KST also uses a custom designed chip. This is great but can lead to supply issues. So if you want a KST servo get them whenever you see them in stock.

 

[Konrad]

Hit a slight snag.

Because of the geometry change in the fuselage V-Tail mounting boss, the upper case of the partially buried servo binds with the fuselage. This then distorts the V-tail stab as shown. This kink in the V-tail at the part line looks like a result of a change to bring the V angle up closer to 90°. While I like narrower V angles I'd have liked a straighter shot the bottom of the fuselage.

I'm going to live with this, as I think the longer control horn will have much greater benefit than any slight detrimental performance loss from the distortion.

 

[Redbird]

You could probably grind the the fuse internally for clearance also the servo case, lightly. I managed it but did go through the carbon at one spot. Ended up looking good apart from slight distortion of fin surface which doesn't bother me.

 

[Konrad]

I too have distortion of the mounting boss. I don't know if you can see the distortion in the mounting boss.

My V-tail skins measure 1.6mm thick and the V-tail mount skins measure 0.9mm thick. This allows for a 0.7mm step. This step does afford some room for the servo top to extend into the fuselage. I have thinned the very aft area of the boss where the servo makes contact to about 0.1mm thick. This still results in a strong bind.

I also see this mounting boss profile causing issues with the OEM control horns. Particularly if the horn is bound the the lower stab (control surface) skins. I'd like to see the OEM fill out the rear fuselage cross section to follow the V-tail and give us some internal room for the needed control systems.

 

[Doc James Hammond]

CAD Design problem - reminds me of several problems I have had like that but unfortunately not found until I had made CNC masters.

Expensive to machine new ones for such small mistakes but it has to be done..

Doc..

 

[Konrad]

I'm surprised at how many "Flaws" pass the final checks with a lot of CAD programs. A key feature of CAD (good ones) is the fit and clearance tests. Like you said it is a lot easier to move bits and bites rather than material after the fact!

The only CAD/CAM program I really used was Catia v5. This was a full blown CAD program. Ok, at $12K per license per year 15 years ago, it had better be! I assume that programs like Auto CAD have these fit check functions. The hard part is that all components need to be modeled for this to have any validity. I don't think anybody bothered to model the wire control horns.

To be clear the Alpha 2.8 will work fine as long as the wire control horns are placed high in the control surface. This is done to clear the fuselage mounting boss.

I like that the effective length of the horn is 16mm. Combine this with a servo arm of 4.5mm to 5mm and I should have real good servo resolution and power.

 

[Konrad]

Again I need to thank @Jonathan Wells for the threading of the push rod into the servo arm. This has allowed me to place the push rod much closer to the servo axis of rotation (Makes a much shorter servo arm as the thread is the retention feature.).

As it is, the 4mm servo arm looks about right for racing if I over drive the servo by 120%. I'm thinking I may actually need to go as large as a 6mm servo arm if I want some sport rudder inputs.

 

[Konrad]

Since I want to add some flocks and epoxy backing to the back side of servo mounting pads, I thought that I might put this on top of the servo mounting pads to make a 2mm to 3mm stand offs. This should allow me to slide the servo 2mm to 3mm deeper into the stab. It is hoped that this will alleviate some of the bind with the servo top and fuselage.

As a side note I measure the V-tail angle as 101°. I'm thinking this will result in a very nice tracking ship, large wetted tail volume, long tail boom, and nice narrow angle. Add this to the thinner wings and the Alpha 2.8 should be an all around Strega killer!

 

[Einar]

Excellent pictures and documentation! Would it be a possibility to install the X08 Plus servos in the same way as the wing aileron servos, with pockets cut into the underside of the V-tail fins and using a control horn and conventional clevis pin linkage to the control surface? It would probably not look as clean. On the plus side, this would allow easy assembly and disassembly of the tail.

 

[Konrad]

Yes, like this. I don't like all the drag those bits hanging out in the wind cause.
I don't see any added ease of disassembly. This has one electrical connection per stab side. What I'm showing has one mechanical connection per side.

Then there is the problem of all those bits gettting caught on the weeds and rocks durring landings.

 

[Konrad]

Excellent pictures and documentation! Would it be a possibility to install the X08 Plus servos in the same way as the wing aileron servos, with pockets cut into the underside of the V-tail fins and using a control horn and conventional clevis pin linkage to the control surface? It would probably not look as clean. On the plus side, this would allow easy assembly and disassembly of the tail.

Thank you. I try to give enough detail that anybody can duplicate my set up. Wether this is desirable or not, flight testing will show.

To be clear, I'm not building this to be a sport glider. Rather as my third tier back up for my F3F racing attempts. (Read dirt cheap racer).

 

[Einar]

Yes, like this. I don't like all the drag those bits hanging out in the wind cause.
I don't see any added ease of disassembly. This has one electrical connection per stab side. What I'm showing has one mechanical connection per side.

Then there is the problem of all those bits gettting caught on the weeds and rocks durring landings.


View attachment 7031

This is what I had in mind, but it should be possible to do it with a little less use of plastic and better color matching of the servo covers ;-)

 

[Konrad]

As I was going to add some epoxy/flocks to give the servo mounting screws something to purchase. I thought why put it inside the fuselage when I can use this mixture to make some servo standoff to move the servo deeper into the V-tail.

I sanded the area to remove all paint and give the epoxy some teeth to help with the bonding. I placed two blobs of epoxy and flock on the end of the servo cut out ends, and waited (about 2 hours) for the epoxy to set firm (green). I then shape these blobs to give me clearance to allow the servo top the clear. I waxed the servo (release agent) and then press the servo into the blobs, keeping the servo mounting flanges 3.5 mm above the fuselage. Waited for the epoxy to reach a hard set (not fully cured) and trimmed the epoxy flock mixture with a very sharp knife to allow the V-tail to slip over the servo and seat. I slid the V-tail over the servo to make sure everything is aligned as the epoxy fully cures overnight.

I hope the release agent works and that it isn't too difficult to free the servo in the morning. I also hope that the flock keeps the epoxy block from splitting when I install the servo mounting screws. This was done to get rid of the fuselage to servo bind I have on the right V-tail. As the left servo can be placed 5 mm forward I don't think I'll have this servo top to fuselage bind issue with the left side.

 

[Konrad]

This is what I had in mind, but it should be possible to do it with a little less use of plastic and better color matching of the servo covers ;-)

If you do this (and I recommend you don't, for the drag and landing problems) please document what you have done and post the results in this thread.

All the best,
Konrad

P.S.
Breaking the plastic parts might be better than tearing off the hinged surfaces.