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Feb 13, 2012, 11:54 PM
NorCal Electric Soaring (NES)
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Xplorer 4.0 ST Build Log

After flying 1 ½ years with an Xplorer 3.8 that was built by Claude Turner, I decided to build one myself. Got the model at Visalia in Oct 2011 from Soaring USA (SUSA) for a great price but didn't actually start the build until December. This was my first ever molded plane build.

There are other excellent Xplorer build logs like this one from Tony Kilwein. So I decided to do this log with the idea that I might be able to help other beginner molded plane builders. I'm not describing every single step I took but rather I'm highlighting those areas that cost me more time given my lack of experience. I'm including many pics of the linkage for all control surfaces so it's easier to replicate on your first build.

First, I do owe a big THANK YOU to several people:
1. Ed Stewart, who has built many Xplorers. SUSA gave me his number when I asked detailed build questions and Ed had great answers. If you decide to get your Xplorer built by someone, I heartily recommend Ed. He knows his stuff and his fees are more than reasonable.

2. Scott Meader, our local build expert who also has built a ton of Xplorers. Scott was very willing to share some extremely valuable tricks and suggestions. Could not have done it without him.

3. Many others, include WebbSolution/Dave, Kilwein, Bob @ SUSA, Target, Tom Watson, Tuan, and others. Much of what I am relating in this build log I learned from them.

One more point: I'm a long time advocate of honest feedback when I do projects like this. Since I started this build I have encountered other X builders who confirmed some of the problems I was having. It was surprising to me that several directly expressed concern over “consequences” if they publicly described the product issues they were having. I'll leave it at that but I do think our soaring community is missing something important when these kinds of issues are covered over. It's especially harmful to beginning builders like myself who are forced to deal with the same issues while armed with much less experience on how to handle them.


Right off the bat I found no build or setup instructions that came with the Xplorer 4.0 ST. None were posted on the NAN website nor on the SUSA website. Compare that to Samba Models or Kennedy Models who each have quite a bit of build and setup info either online or in a user guide. So the first thing I did was spend several days reading tons of great info on the RC Groups Xplorer thread. I condensed all that I found into a document, then I spoke with our local builder expert Scott Meader and others on RC Groups to get my first round questions out of the way.

I decided to build the plane for strength as opposed to extreme light weight but chose to do several weight-saving things:
1. replaced the rudder pushrod with a carbon fiber one (okay, it's a minor thing)
2. use Airtronics 94809 servos that are small, cost effective, very light, and have a great reputation.

Here are the servos and battery I decided on:

Elev and Rudd: MKS 6125H (0.89oz)
Flaps: MKS 6125 full size (1.01oz)
Ailerons: Airtronics 94809 (0.38oz)
Battery: LiFePO4 2S Hyperion 1450mAh pack

Since the MKS servos absolutely don't like voltages above 6.0v, I decided to use a Novak 5A Universal BEC #5465 linear regulator to regulate the voltage to 6.0v. I will end up trying a variety of different regulators and voltage reducers before too long. If you decide to run LiFe battery pack voltages direct to your MKS servos, MKS says specifically it can shorten the life of your servo motors. Beware.

Rudder and elevator pushrods and brass couplers

The rudder rod was replaced with CF one, .050” dia. I used JB Weld on both ends and Goop to anchor the rod plastic guide to the fuse. The rudder control throw goal was 30 degrees/36 mm each side per Tuan on RCG. Took some sanding and routing out where rod goes into fuse at tail but I get a full 30 degrees both sides. Had to drill out servo arm holes to 1.55mm/0.063" to fit the clevis that came with Xplorer. Control rod throw at servo is 0.5" for +- 30 deg deflection.

Elevator control rod to clevis brass coupler: Experimented using silver solder to join these. Turned the soldering iron to near hottest temperature, then light sanded the rod, cleaned it w/ alcohol, and applied flux to both the rod and brass coupler. Silver solder flowed just fine. Using plenty of flux is a key. Solder used was from Fry's: Qualitek LF 217 No Clean Solder: .032, Sn95.5/Ag4.0/Cu0.5. Decided to add an outside layer of JB Weld over the solder joint which will protect against corrosion and add a secondary bond (see pic).
Elevator control rod throw = 0.5" for +- 16mm/+- 10 deg TE stab throw (in LAND it has 20mm/13 deg of down).
For neutral elevator:
(a) Stab LE should be centered at middle of fin's LE bump, or
(b) 94mm from stab TE to bottom of fuse at the rudder hingeline
(, or
(c) center the stab pin in slot and it should be very close (per Target and Dave Webb)
Using these guidelines, on the maiden flight the elevator was only 1 click off. Amazing.

Fuselage Servo Mounting

Since the fuse tray servo cutouts were too wide for the MKS, and since the pre-routed cutouts did not expose enough wood to allow for the servo screws to go into the tray with good wood all around, I had to improvise. Mounted the fuse servos using a small 3mm CF rod as a pin on the outside mounting ears of each servo and using thin copper sheet straps with 2 screws each (see fuse pic). The inside servo mounting ears are clamped down using a piece of PC board topped with a thin copper sheet, all screwed down with a single screw. Used silicon sealant on all screw threads and on inside mounting ears before tightening the center plate down.

After reviewing the elev coupling the servo is turning ~80 deg for full range of required motion. I measured ~10.5mm of servo arm radius to the control rod hole while my 3.8 Xplorer has 12mm. So the 4.0 elev servo has to move more for the same max range of elev motion. This is good since it yields more resolution (smaller servo steps) at the expense of slightly more deflection time/delay for full throws.

Wing Servo Mounting

Special thanks to Scott Meader for his suggestion on this. Decided to use servo trays for easy servicing. Added a 1/64" ply floor to the standard MKS servo trays. Used CA for tray-to-ply and a thin layer of 15-minute epoxy (no microballoons) for the tray to wing bond. Later I added a bead of 30min epoxy + microballoons around the rim of the tray (was concerned that the 15min epoxy may not have been strong enough due to inexact measuring of epoxy + hardener. Now I weigh both when mixing so I get exact amounts.). Lightly roughed up the inside wing surface and cleaned it with alcohol before bonding. Wired up both servos to receiver and turned the radio on with the flap trim set to zero and adjusted linkage for zero flap deflection. Then I applied the epoxy, pressed the trays down onto wing (no need for saran wrap under servos since the trays have a ply floor), taped them in with blue masking tape, and left them alone while the epoxy set--all the while leaving the radio and receiver on. This ensures no linkage binding when epoxy sets and cures. Note the pics for neutral arm positions.

Routed out the top flap wing linkage slots using a Dremel. Center a 1/4" wide slot on the brass flap horn and 24-25mm long. Make sure the slot is square with the wing TE. This is a much much longer iterative process than described here since you need to check your linkage rod at the servo arm limits and make sure you have enough clearance on the skins and flap material. I went slow on this step but ended up with a nice result.

Flap trays: Weight of both stock trays is 3g. After ply floors added and CA thread hole soak: 5g (0.18oz) for both.

Servo horn issues

Had trouble guessing how far the aileron and flap horns had to be screwed in for proper linkage geometry (no docs, no pictures). Turns out (after helpful calls to SUSA and Ed) the brass horn threads on the provided horns were too long for the molded-in threads. So I had to grind down the aileron horns so they fit all the way in without binding. Also had to buy and use a 3mm tap to carefully clean out the embedded horn threads that had some epoxy debris. After initially trying to connect the linkage clevis to the horn and being very tight, I mistakenly thought I had to drill out the flaps horn pin hole which ended up causing slop in that fitting. SUSA sent replacements for free which was appreciated but those horns needed even more grinding to work on this plane. Maybe you always have to grind your horns down, I wouldn't know.

Flap linkage took me awhile

First, before you try to stage your flap servo linkage, you will need to loosen up the flap hinges. Otherwise, the amount of servo force that will be required at max flap deflection is huge and will make your servos draw way more current than usual. I had the following procedure suggested and it worked: manually deflect the flap and use a flat bladed screwdriver to carefully scrape along the internal hinge line. Be very careful to not bang into the fragile wiper. After doing this for 20 minutes each side, it was much better. After doing lots more manual deflections after that, it was even better.

Given my servos I did not have installation dimensions to duplicate for the linkage so I had to do some discovery. Once the servo trays are in, you still can adjust the horn height, linkage rod length, and neutral servo arm position. Rather than describe days of experimentation, radio settings, linkage changes, etc I would rather tell you to use my wing horn and servo pictures as a guide and use the following info to set yours up:

Flap : rods 78.0mm clevis pin to pin, servo arm 10mm radius (using 2nd hole on std MKS plastic arm)
top wing surface rod slot: 7.5mm wide, 25mm long
Ailerons: rods 76mm clevis pin to pin, servo arm 6.6mm radius (cut down the 94809 standard circular plastic arm)
top wing surface rod slot: 6.5mm wide, 22mm long

I strongly suggest this $10 piece of software to help you understand how to set up servo linkage the right way the first time before you epoxy your trays in.

Aileron Servo mounting and linkage

Epoxying in the servos: Roughed up inner wing surface and bottom of servo trays. Then used x-acto blade and gently scored both surfaces in 90 degree cross hatch pattern, then washed clean with alcohol. Placed servos in trays over a layer of Saran wrap, then screwed them in. Then turned on TX, servos connected to RX, and left in neutral while 15min epoxy cured for an hour. Saran wrap worked great, servos popped right out the next day.

With the plastic servo trays that I had selected, the stock servo arms and clevis would bang into one side of the tray plastic. Either I had to find another tray, file the tray down (which would have weakened it considerably), or find another work-around. The solution from Scott Meader was to use 1.84mm steel rod with a z-bend at the servo and a smaller Dubro clevis on the other end. Worked like a champ with plenty of tray clearance to boot. Another benefit: the smaller clevis required a smaller top wing slot.

Used ThreadLocker

After getting the linkage dialed in, I used Threadlocker Green on threads on both sides of each linkage rod. When it dries you will have no slop at the clevis threads but you will have to heat them up with a soldering iron if you want to unscrew a clevis later. Per another RCG suggestion I mixed up some 5min epoxy+ microballoons and carefully applied some around the tops of the flap linkage horns. You may not want to do this but I did it to further stiffen the linkage and prevent the horns from rocking back and forth from deflection forces and hard landings. IMPORTANT: do not apply epoxy to the threads, only to the top (per the pic) after the horn has already been screwed in. The idea is that you can easily route and chip the epoxy out if you ever need to get at the horns. I had to do it once so far and it was very easy.

Joining the upper and lower wing skins

Not everyone does this but it will help to transfer some of the servo's twisting forces to the bottom skin. This will reduce the tendency for control surface flutter during aggressive launches and high G maneuvers. I decided to join the upper and lower wing skins around the flap servo boxes. Used 1/4” x 2” vertical grain balsa to create sub-ribs on the two non-reinforced sides. After rough-sanding and trial fitting it to “close” I used gap-filling Gorrilla Poly glue to lock them in. Don't over-apply the glue, it expands quite a bit. Makes a very solid upper/lower wing skin connection. I did not do any upper/lower joining on the aileron servo boxes. I may add that later but it takes very aggressive launches now with the tight linkage I have in use.

MKS Servo chattering

To make a week-long story short, the installed MKS flap servos initially were chattering to the point where the servo current was excessive and potentially damaging. I was told others had experienced this same problem too. This usually happened to me when they were deflected to their midpoint throws. After running through a checklist of possible causes and getting help from MKS and RCG folks, I was able to eliminate the problem by making sure that:

1. The servo linkage rod was as short as possible,

2. The shortest servo arm possible was used (initially used 12mm, ended up with 10mm),

3. The flap horns were locked in place and not rocking in any direction—at all, and

4. Applying thin CA to the flap horn's servo clevis pin to fill in any remaining micro gaps.

Main wing panel bolt hole misalignment

When I first tried to bolt down the main wing panel onto the fuselage I could not get the forward bolt to screw in past the point where the bolt head reaches the top wing surface. Turns out the front bolt hole that was molded into the wing was manufactured at a slightly incorrect angle. This was confirmed by one of my Xplorer builder contacts who said that NAN indeed had a bad batch with that problem.

The solution was to carefully route out the top bolt head mating area and file the underlying defective bolt hole to the angle that mated properly with the fuselage bolt threads. I was unsure of how much wood material there was underneath the outer wing glass so I called Ed Stewart who confirmed there is plenty of wood in that area to support the repair job I had to perform. Let's hope NAN does a better job of QA from now on.

Wiring harness problems

The “ready to use” wire harness from for the 4.0X I bought was not quite so ready to be used. Two major problems had to be dealt with. I spent several days diagnosing and dealing with the issues, which were:

1. The DB-9 connector wires on the male wing connector were directed straight down through stiff shrink insulation and bumped into the wing's inner upper surface. No way they would fit without cutting off the shrink tubing from each pin with an Xacto blade, carefully bending the wires to go left and right instead of straight down, then coating the bare solder connections and pins with liquid tape (I use PlastiDip available at Harbor Freight).

2. The fuselage DB-9 female connector's pins were not making reliable connections. I'm very lucky I didn't blow the issue off and try to fly it because the wing servos would have surely failed. Turns out that 4 of the female pins (pins 1,5,6,7) had very weak grip forces on the male pins. All it took was a slight rocking of the bolted down wing and the servos on the RH side would stop working. Very dangerous.

Since I had already epoxied in the harness servo connectors into the main wing panel I was not prepared to rip the wing side of the harness out and return the whole thing which is what I was told to do if I wanted a refund. So I just replaced the defective DB-9 at my own expense and time. The bigger loss of time was troubleshooting the cause. Would have been nice for the seller or vendor to offer apologies for the poor workmanship or offer some form of compensation (which I asked for) but sadly I got neither. Caveat emptor.

Final Fuselage Wiring Details

I decided to take the extra time and cut out the excess servo wiring lengths and re-crimp new connectors on those wires that were too long. This eliminates the usual rat-ball of wiring against the receiver and makes the front of the fuselage much less cluttered. I also took extra time to do the wiring between the battery, switch, and receiver connector using 20awg silicone wire. This wire takes very high currents and heat without danger of melting. If you need to get your own supply of wiring, servo connectors, pins, and a crimp tool I suggest Hansen Hobbies. They have a great selection at good prices.

Tuning and Trimming

I assembled a good set of starting points for plane setup from the Xplorer thread on RC Groups as well as some pointers from Ed Stewart. Here is what I started with:

Ail UP/DOWN: 25mm UP/ 15mm DWN
Ail-flap coupling: flap 4mm UP / 3mm DWN
Elev UP/DWN: as much as poss.
launch TE: flap-13mm DWN, ail-9mm DWN
Thermal Camber: 2mm, Superfloat camber: 4mm
Reflex: 2mm, zoom reflex: 3.5-4mm
VR6 slider camber control: 4mm max (for super float setting, set in SD-10G's Trim Auth menu)
Expo: I use some but not lots. Add to suit your preferences.
(I have not yet added snap flaps but will do so soon)

Be sure to do a good radio range check and set your failsafe control surface positions before your first flight.

Final towhook setting that gives very aggressive stable launches: 9.0mm in front of the 120mm CG, or 23.25” behind tip of skeg nosecone.

First flying impressions

Final AUW at CG 120mm: 75oz. This plane flies really well, lots slower than my 3.8X, and on landings I can slow it down to a crawl before dumping the nose in. Launches are steep and very stable. Reflex zoom of +3.5mm seems to work really well. Catches and indicates thermals exceedingly well. Also, even tho' the flap throws are only ~75 degr, they are more than enough to slow the plane down. Normal reflex mode is much more important for the X4.0 as compared to the 3.8. Even with the elev unchanged, as soon as reflex is switched in the plane scoots along much faster. Will be especially valuable in windy conditions. High start launches are plenty aggressive without any launch TE switched in.

I have not yet tried any ballast but I will probably start flying soon with an 8oz rod and see how it responds in breezy conditions.

I'll update this post with more flight performance data as it rolls in. Any questions don't hesitate to ask! Hope this will be/was some help to another beginner or two out there.

Chris B.
Last edited by SoaringDude; Feb 20, 2012 at 10:42 AM.
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