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

[Konrad]

Love it when chemistry works! The epoxy cured, the release agent released. What more can a guy ask for first thing in the morning? I know a hot cup of tea!

12 hours into the full 24 hour cure cycle and I have a stable servo mount. I was a bit surprised that even with the cotton flock that the index holes in the servo flange were reproduced.

Now to install the servo for the left V-tail.

 

[Konrad]

On the odd chance that someone would want to do these mods to their ship here are some nuanced hints that might help. Now I don't know what manufacturing tolerance there are so with your ship check as you go. First the Ball links supplied are 5mm with a 2mm bore. This results in far too much clearance with the 1.6mm wire. I dropped down to 4mm Ball links to fit the supplied wire control horns. I silver soldered the balls to the wire. I used the threaded end of the wire to help the epoxy keep the horn in place in the control surface. As the hoops are the same size this did not gain me any clearance in the tail cone.

I placed the wire control horns 7+mm out from the V-Tail part line. I placed the spheres as close to the inside wall of the tail cone to avoid the ball hoops from colliding at the center line. I relieved the fuse wall for added clearance.

To keep the threaded push rod from binding the inner wall of the fuselage one should only leave one thread overhanging the bottom of the servo arm. I use a 1.6mm drill for the 2mm x 0.4mm threaded hole in the servo arm.

The Epoxy stand offs should be 3mm to 3.5mm tall. To keep from snapping off the KST supplied screws in the stand offs I drill a 1.1mm (1.2+mm would be prefered) pilot hole in the epoxy/flocks stand offs.

 

[Konrad]

More details

 

[Konrad]

Unlike the Alpha 2.8 with rectangle wing ballast, my Great Wall has round 11.9mm wing ballast tube. As 11.5mm rods are an odd size here in the USA I've decided to use centerline fuse mounted ballast tubes. This is to allow 19mm tungsten slugs and it is hoped still allow fast roll rates needed in F3F racing.

 

[Konrad]

Well, here it is, the new year and I literally have nothing to fly as an F3F racer or F3F trainer. So I'm looking at my Alpha 2.8 to build, as at least this ship doesn't have any crash damage I need to address. (Well other than some shipping damage and that has been repaired).

I put this aside last year as I saw some issues with the wing box. You Might recall that I tore out the wing box in my Redshift in an earlier crash. This Redshift crash has been in the back of my mind. The Alpha 2.8 has the spar shear web stop at the joiner wing box. It looks like the design then has the rear of the wing joiner box acting like the shear web. As both the rear wing box and spar shear web overlap this should be adequate when looking at the bending loads on the wing. My particular wings are showing a rather dry bond with the spar caps and the rear wing box. This is a concern for me as it is the wing box that holds the wing fore and aft during landings (read abrupt arrivals!). I will have to add some rather large glue fillets to try to enhance the bond that keeps the rear of the wing box in place. From an esthetic perspective I'd like to see some bonding between the shear web and the back of the wing box.

It might be a bit too much to ask at this price point, but I'd like the wing box wrapped in Kevlar. That joiner is placing a lot of leverage on all the wing box joints. Some fiber across the joints will go a long way towards adding a lot of durability to the airframe.

 

[Konrad]

Added Ballast tube and nose tray. Ballast tube is placed for 9 slugs to be centered on the 97mm to 98mm CG.

 

[Doc James Hammond]

Added Ballast tube and nose tray. Ballast tube is placed for 9 slugs to be centered on the 97mm to 98mm CG.
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With great respect to your excellent modificiation skills, Konrad - I'd have made that tray longer so as to try to reinforce the common breaking point just in front of the wing. Even with a 'dotted' bonding along the wing stub, it makes a lot of difference. Otherwise...dang!

Doc.

 

[Konrad]

I did, or so I think. I carried the plywood past the nose cone part line to the area you mentioned. I don’t have any added anti crush spreaders, (solid cross bar) to try to keep the leading edge of the wings apart. I guess I could have add one above the ballast tube. But the plywood fingers straddling the ballast tube going aft with glass roving should be serving much the same function. Also seeing how the wing joiner performed on my last Redshift crash I’m not too sure that the classic crushed fuse applies to ships with this large a wing joiner. I’m thinking the wing joiner box is now the weak point.

 

[Doc James Hammond]

I did, or so I think. I carried the plywood past the nose cone part line to the area you mentioned. I don’t have any added anti crush spreaders, (solid cross bar) to try to keep the leading edge of the wings apart. I guess I could have add one above the ballast tube. But the plywood fingers straddling the ballast tube going aft with glass roving should be serving much the same function. Also seeing how the wing joiner performed on my last Redshift crash I’m not too sure that the classic crushed fuse applies to ships with this large a wing joiner. I’m thinking the wing joiner box is now the weak point.

Gotcha - couldn't see from the pics. Hope you can get the pushrods through or are you going with rear-end servos?

On the Spada, it will be tailplane mounted 6mm servos. You can have "Tailerons" and "Flaperators"

Doc.

 

[Konrad]

Yes, there is still space below the wing joiner and below the “servo tray” for a stiff carbon push rod.
It was a real and genuine concern and should have been aired, so thank you. It took some ship in the bottle kind of work to get the glass roving down the side.

This Alpha 2.8 does have KST XO8 plus in the V tail.

Wow, HSO6 in the tail. I’m wondering how the gears will hold up to the shock loads of many an F3F landing (site and pilot concern).

 

[Doc James Hammond]

6 mm servos on test now Konrad.

Doc.

 

[Konrad]

Don’t think I’m not noticing that you aren’t mentioning servo brands.

You said Spada will be tail plane mounted servo. Is this going to be an optional feature or is the design going to be limited to using tail plane mounted servo?

 

[Doc James Hammond]

Don’t think I’m not noticing that you aren’t mentioning servo brands.

You said Spada will be tail plane mounted servo. Is this going to be an optional feature or is the design going to be limited to using tail plane mounted servo?

Hi Konrad - the servos I have on test are 4 types of Kingmax servos with two types being 6mm and two types of 10mm

The tail servos will be an option on the Spada. I'll provide for them and mark the space for them but not root out the servo pockets in the tails.

Cheers,

Doc.

 

[Konrad]

Well, It looks like I may have mis-spoke.

With where I placed the nose tray I can't get smooth action with 4mm carbon push rods. I think if I had placed the tray 3mm further up the nose even 5mm carbon push rods might run smooth. But I'd loose the ballast plug stop feature I currently have with the tray partially spanning the ballast tube end.

In my case this isn't a concern not being able to run carbon tubes as I didn't place any mid-boom push rod support bulkheads in the boom. If one was to use a KST-115 style servo one could place the tray deeper and run the carbon rods aft. With the way I have all the support structured tied together this Great Wall fuselage will never see carbon V-tail push rods.

The KingMax servos are known as substandard servos around here. I do hear that some folks like them, but I fear it may just be price bias. I do like the KST as the X-10 servo is as good if not better than the MKS of the same configuration. On my Alpha 2.8 tail servo installation I was surprised that the fit issue was with the upper part of the servo case hitting to bottom of the V-tail cone. A fleshed out tail bulb would help with servo installation and look a lot better regardless of flipper actuation.

 

[Doc James Hammond]

Yes, the original ones from Kingmax were a bit naff. The servos I'm testing are supposed to be the "new and improved" versions = hence the test.

The UK GPS guys are using them in their large models and swear by them - but again they are using the new type.

Of course, the 6mm types are new kids n the block.

let's see.

So far they look fine but time will tell.

Doc.

 

[Konrad]

Let us know the new series and part numbers when and if they pass the mustard!

 

[Wayne]

Simple Test Doc - This is the first one we do for any servo that claims great things. We attach a 12" pointer on the servo arm (Like the second hand on a clock) and we see if it can recenter from both directions. Most fail this simple test, even $50 servos fail this! If it passes this test, then put a 1 yard (1 meter) pointer on there and repeat. If it centers perfectly, then it is probably an MKS or a KST.

Now using a transmitter or a good servo tester, move the servo slowly back and forth, does the pointer move smoothly or does it tend to be twitchy? If smooth, probably an MKS, if twitchy, probably not an MKS.

Most servos can not pass the 1 meter pointer test, but their manufacturers will tell you how great their servos are, etc. and they will be charging KST or higher prices.

Lets face it, you could use $20 servos in a high end glider and it will still fly pretty darn well. Will it fly better with $40 servos? How about $60 servos? $120?

If a company is going to charge $40 or more for a servo I think it needs to be at the level of other $40 servos. There are a lot of reasons that MKS and KST are so popular in the high end servo category, they deserve to be there!

I fly a wide range of models. The servos range in price from about $5 to about $60. A lot of this is based on the performance I'm targeting and budget for the plane. If I'm flying a larger/faster plane you better bet I'm going to invest in high quality servos for it. I just want to make sure the quality is actually there!

 

[Doc James Hammond]

Simple Test Doc - This is the first one we do for any servo that claims great things. We attach a 12" pointer on the servo arm (Like the second hand on a clock) and we see if it can recenter from both directions. Most fail this simple test, even $50 servos fail this! If it passes this test, then put a 1 yard (1 meter) pointer on there and repeat. If it centers perfectly, then it is probably an MKS or a KST.

Now using a transmitter or a good servo tester, move the servo slowly back and forth, does the pointer move smoothly or does it tend to be twitchy? If smooth, probably an MKS, if twitchy, probably not an MKS.

Most servos can not pass the 1 meter pointer test, but their manufacturers will tell you how great their servos are, etc. and they will be charging KST or higher prices.

Lets face it, you could use $20 servos in a high end glider and it will still fly pretty darn well. Will it fly better with $40 servos? How about $60 servos? $120?

If a company is going to charge $40 or more for a servo I think it needs to be at the level of other $40 servos. There are a lot of reasons that MKS and KST are so popular in the high end servo category, they deserve to be there!

I fly a wide range of models. The servos range in price from about $5 to about $60. A lot of this is based on the performance I'm targeting and budget for the plane. If I'm flying a larger/faster plane you better bet I'm going to invest in high quality servos for it. I just want to make sure the quality is actually there!

Excellent test procedure and excellent advice - thanks Wayne.

I'm doing (or rather plannng - not doing it yet) something similar, except that I will also include a 24 hour backwards/forwards non-stop run for each and then re-do the centering tests to see what deterioration I get. I'm a little worried that the servos might show up OK in the beginning, then rapidly decline.

Totally agree on the servo selection advice. Basically you don't put Chinese tyres on a Ferrari.

Cheers,

Doc.

 

[Konrad]

Good discussion on servos. I do a 5 min burn in on all my servo to try to weed out as many infant mortality issues. This alone has saved me thousands on Freewing ARFs!

Also just looking at how the servo is assembled can give one clues as to the longevity of the servo. Here I'm showing the Corona SB-2038 I had a 50% (3 out of 6) fail in the first 5 minutes. I think this is due to the IC Chips being over heated with the soldering iron as there are no pads to attach the motor, pot, and leads. These wires are soldered to the component directly. This poor planning in the PCB results in failed components. I wrote this up in a review but didn't see it published.

On this Alpha 2.8 I'm planning to use some of my dated KST 135. These are 6v servos but I'll be using 2, 18650 cell (8.4v) to drive the X08 servos in the tail and receiver. So I added a voltage regulator into the wing's wiring looms. These are MKS SBEC. This is a great misnomer as these aren't BEC's nor are they switching BEC. The "S" in the MKS SBEC is for the nomenclature Servo. SBEC is by convention a switching BEC. Again these are not switching voltage regulator but rather simple linear voltage regulators. This are rated at 2 amps. and should be fine driving 2 KST 135. Now I do have some concerns I don't have any cooling to the voltage regulator. But with (8.4v - 5.1v)x 2a = 3.29 watts of wasted energy I think I'll be fine.

Some details about the wiring loom. I'm using a common bus architecture this means the the servos share the positive and negative voltage lines. So I only need 4 wires to cross the wing to fuse part line. As I'm using the great multiplex 6 pin green connector with gives me 2 spare pins to use as redundancy pins for the plus and minus wires. You might see the jumper. (Pins are the number one failure point in aviation electrical system)

Also note that the wire splices used to pig tail off the common bus bar are staggered, so as to minimize any wiring bulges in the loom making it easier to thread down the wing.

You might recall me saying I was going to re-bond the wing joiner box with some epoxy and Cab-O-Sil. Cab-O-Sil is just what you don't want if you want your epoxy to flow into spaces to aid in bonding. So I'm using straight low viscosity epoxy to help bond the joiner box to the inside of the wing skins.

 

[Wayne]

Ah, Corona. (Is that from a song?) They were one of the first brands we offered, then we killed them due to quality issues. Jump forward a bunch of years and some folks asked us to stock them again. I think the prices and I like the little 843 servo, but the quality is not much better. They seem to remain decently popular though and don't get many complaints so we do offer them, but not very excited about them.
(I really don't think they make the 843, it is just a bit too nice for them. LOL)

Requests for improvement or changes to the products go nowhere. I have a couple of planes with the 843 servos, and some folks like the cheap Sbus options and others like the cheap wing servos. So they stay in our offerings.