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May 04, 2015, 05:37 PM
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Build Log

Hmodel 6.6M Arcus Build Thread v2

Hi all,
While waiting for the other site (where my original build thread was located) to get restored, I'm going to do my best to post most of the original build thread pictures and then continue with the thread. As I posted in the original thread, there have been a number of extremely informative build threads on the Arcus of both sizes already, so I'll just post on what I may be doing a bit differently or just find interesting about the build. The Arcus is just about ready to go at this point!

Hmodel Arcus:

I went with the Arcus Competition and asked Radim to add extra carbon to the wings :-)
I ordered the following options: scale cockpit, water ballast, and fuselage seam repaint.

A little about me: I got my start in slope soaring and PSS as a teen, later moved to helicopters and powered aerobatics, and currently enjoy pattern but never got the scale sailing bug until now. Here are the two videos that sold me on large fast high performance scale gliders...

Arcus 6,67 Seiser Alm Mai 2012 (9 min 51 sec)

Air Burns Clip 2 (8 min 17 sec)

Last edited by rjtw; May 05, 2015 at 09:35 AM.
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May 04, 2015, 05:42 PM
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I decided to add wipers to the flaps and ailerons and keep all linkages internal on the wing to keep drag to minimum.

A great tutorial on how to do wipers is here (re: dynamic soaring):
May 04, 2015, 05:59 PM
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The wing has four moving surfaces on each half - two ailerons and two flaps, for a total of eight surfaces. I decided to use RDS on the outer two ailerons per wing and LDS on the inner two.

For LDS, I used the Aviogate servo arm found here, along with my own linkage and internal horns:

For the linkage, I did not use the supplied 2mm Aviogate stainless rod because I felt it was probably not up to the task for these huge flaps at 200 mph! The inner flaps are 4 feet long each! However, I discovered that the solid aluminum MP Jet clevises (from Esprit Models) with the 2mm pins were a perfect fit for the Aviogate arms, and have everything tied together with the 3mm threaded rod supplied by Radim. The combination is super solid.

For RDS on the inner and outer ailerons, I chose another Aviogate offering:

These feature 3mm rods and a very nice servo connector. The rods were a perfect fit into the pockets already built in to the inner ailerons. I had to install pockets on the outer ailerons. There is very, very little room there (the aileron is perhaps only a quarter inch thick at the leading edge) but managed to get them shoehorned in.

Edit (6/16): The other critical feature of the flap servo installation with the internal drive is that the servo be absolutely rock solid. Although the wing skins are very stiff, and could be further stiffened with further carbon or a plywood mounting plate etc., ultimately even with such reinforcements there is going to be some flex in the wing skin as the servo loads are transmitted through the mount and into the skin. Because the flex translates into a small movement of the pushrod, and because the resulting deflection of the control surface is inversely proportional to the length of the control horn on the flap, the very short control control arm with this internal drive system amplifies even the SMALLEST flex in the skin. I tried a few experiments initially and wasn't happy with anything involving attaching the servo to just one skin.

So instead, as shown in the picture below, I came up with a 1/4" plywood servo mount that attaches BOTH skins to each other (thus preventing any torsional flex whatsoever) and for good measure goes all the way from, and is bonded to, both the main spar and the trailing edge spar.
Last edited by rjtw; Jul 01, 2016 at 01:49 PM.
May 04, 2015, 06:18 PM
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Wing Wiring and Connectors

Adding the wiring and connectors to the wings. I decided to go with Robbe's PSS-2018 power box, which is similar to the other power box system out there with voltage regulation, redundant batteries and redundant receivers. The PSS-2018 is focused on Sbus and features four primary sbus cable outputs on two seperate voltage regulators, special heavy duty sbus cables which can support up to eight servos per cable, and heavy duty connectors. It also has a third voltagecregulator driving the receivers and a bank of eight standard PWM servo outputs. I ended up removing the standard cables already installed in the Arcus and installed Sbus throughout, with the exception of the elevator servo which is still connected into the Sbus system but for which I left in the preinstalled servo cable.

Last edited by rjtw; May 13, 2015 at 12:25 AM.
May 04, 2015, 06:24 PM
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Installing the retract servo. The retract unit weighs 1003 grams and features functional gas struts and even a built in brake and brake servo mount, even though I did not order the brake option -- it is all set up and ready except for a flex cable! I haven't set up the brake at this point, but may in the future after I fly it and decide if I need it.
May 04, 2015, 06:39 PM
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The Arcus has a very nice molded in mount for the elevator servo in the fiberglass rudder post. Radim, the designer and builder, has thoughtfully thickened the glass around the mounting areas to nearly a quarter inch, which I felt was enough to accept machine screws. After enlarging the cutout for my servo and drilling and tapping holes, I installed the screws with a drop of epoxy on each which should hold them in but still allow for removal if ever required. I made my own carbon pushrod with 3mm hardware on each end, and a carbon fiber horn embedded into the center of the elevator. Super tight.

On the rudder, I ended up going with DuBro's heavy duty steel pull pull cables but used Radim's supplied 3mm hardware on the rudder end. The rudder servo is mounted on the supplied aluminum mount bolted onto the aft bulkhead.
Last edited by rjtw; May 04, 2015 at 07:57 PM.
May 04, 2015, 06:50 PM
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Vario pickup

I fed the vario tube through the forward fin and down through the existing hole for the elevator servo cable. The vario tube is a perfect fit inside the installed hollow plastic tube in the tail boom that leads all the way up to the cockpit area.
May 04, 2015, 06:58 PM
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I think that brings me up to date on the old build log.

Edit (5/16): Here are the servos I used in the build. So far I've been happy with these choices, they were all money well spent. My biggest concern were the huge inner and outer flaps with the internal linkage, stresses are high and there can't be any slop at the risk of having flutter. The HV747s have been perfect in that role and in fact I was very impressed with all the MKS servos I used here. All servos are brushless and metal geared:

Outer (tip) and inner ailerons: MKS HBL6625, rated at 9.6 kg-cm and .12s @ 7.4V. BTW the max thickness servo that fits is 10mm in those outer ailerons.
Outer and inner flaps: MKS HV747, 13.8 kg-cm and .14s @ 7.4V.
Rudder, tow release, spoilers: Futaba S3071 HV, 10.5 kg-cm and .17s @ 7.4V
Elevator: Futaba BLS173 HV, 7.6 kg-cm and .10s @ 7.4V
Retract: MKS HBL960 (replacing a Futaba S3071HV 6/2018, see posts 77/78)

The HV747s are small and lightweight but crank 192 oz-in of torque and that's at 7.4V, they can be run even higher, and the tiny 6625s have about the same torque as the full size S3071s at 133 oz in! And just for comparison the 3071 yanks the huge retract up and down like nothing. In fact, I ended up slowing the retract servo down (via the transmitter) out of fear it would break something!
Last edited by rjtw; Jun 04, 2018 at 06:13 PM.
May 04, 2015, 07:22 PM
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Battery, Battery Backer, Cockpit Wiring

I decided to add a special battery shelf in the nose to keep the batteries up high and as far forward as possible. The lead shot will also be installed/poured in on top of this shelf in front of the batteries. I installed the retract servo on the bottom of the shelf. This does mean that I'll have to remove the forward instrument panel to install or remove the batteries, but I'm ok with that

I installed the PSS 2018 at the very rear of the cockpit floor on a custom mount to hide it under the rear seat, and used flexible tubing I found at Pep Boys to route all the wires. That last picture of the PSS2018 shows it with everything connected - and with sbus, it isn't much. I have the first Rx mounted directly in front of the PSS on the cockpit floor, while the second Rx is on the wall just beneath the forward edge of the front seat. I'll orient the antennas at the proper angles and in different geometric planes later. Getting close!
Last edited by rjtw; May 05, 2015 at 09:41 AM.
May 04, 2015, 07:37 PM
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PSS 2018 control display

For the longest time I was debating on where to put the PSS display... under a seat? Replacing the rear instrument panel? There are compromises in every potential location. I finally decided on convenience but without altering any existing scale features in the cockpit, and in a way that is easily removed (reversible) should I ever want to enhance the scale features by placing it on a 1/8 ply 'wall' at the rear of the cockpit. I started with a cardboard template. The asymmetrical outline of the wall was made to clear some of the carbon tow reinforcement protruding around the inside upper rear of the cockpit area.
May 04, 2015, 07:58 PM
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Next up?
May 05, 2015, 12:57 AM
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I think I need a pilot...!
May 05, 2015, 07:20 PM
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Looks like balance will take about 1170 g of lead (2 lbs 9 oz) plus the 530 g battery pack (1 lb 2 oz). I'll leave out maybe half a pound for fine balance and potential future pilot and build a small weight box with removable weight for fine tuning.
Last edited by rjtw; May 06, 2015 at 12:27 AM.
May 06, 2015, 12:11 AM
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I have three conditions set up: Normal, Termic (that's Radim's term), and Speed.

In Normal, all wing surfaces are set neutral and there is snapflap with up elevator (only). In Termic, same but all surfaces are slightly cambered, and in addition the ailerons are programmed slightly differently (more differential, less throw, the inner ailerons do more of thecwork compared to the outers). In Speed, I've reflexed all surfaces just slightly, and in addition added snapflap in both directions. Radim only recommends Normal and Termic and I've set up everything in those two modes to his spec. Radim also recommended landing in Termic mode.

In all modes, aileron control moves the inner and outer ailerons and outer flaps, while camber/reflex operates on all surfaces.

I asked Radim why he doesn't have a Speed mode and reflex the surfaces for high speed or inverted flight. He stated that according to both his airfoil calculations and experiments, reflexing does nothing for speed (the airfoil has so little camber to begin with). However, he suggested it may be useful for inverted flight.

In all modes, I also have additional camber control on the left slider. This gives me up to full down flaps, up to the mechanical limit if I choose, and up to about 1 cm of reflex measured at the inner flaps (though I have limited this to just a few mm). This amount of throw leaves me the option of mixing aileron control to the inner flaps in the future if I ever want to.

Spoilers are on throttle, and I'm experimenting with disabling them when the retracts are up so that I can keep the throttle stick in a middle position for more comfort. I've also -- for now at least -- slaved the retract and tow release together (up wheel, tow releases). I hate dealing with switches, so with this setup I only have two to deal with - one three-position switch for the modes, and the retract/tow two position switch.

I do not have any aileron-> rudder mix set up as I prefer it that way, but let's see how it flies and go from there.

My Futaba 14SG has worked out beautifully, but its built in wing types max out at 4 ailerons and 2 flaps, whereas the Arcus has 4 ailerons and 4 flaps. I ended up adding several manual mixes to deal with that. Easy enough but Futaba provides a 4 ail 4 flap wing type on the 18mz and should provide it here too. PM me for any desired details on my setup.

About the only real issue I encountered was that in my initial setup I could just not get enough up throw out of the outer flaps in response to aileron input. It turns out that Futaba assumes/demands that certain channels be on the same wing... but this is totally undocumented. For the record, Ail1, Ail3, and Flap1 must all be on one wing (with Ail2, Ail4, and Flap2 the other). I had my flaps flip-flopped and after adjusting outer ail differential just could not get any 'up' throw on those flaps, but gobs of down throw. I got a hunch and flipped the flap channel assignments, which immediately fixed the problem!
Last edited by rjtw; May 06, 2015 at 12:31 AM.
May 06, 2015, 08:23 PM
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Great thread

I feel like no one is encouraging you on this one rijtw but I'm sure lots are watching. It is always special to see a big expensive model like this being prepared for flight. They may be AFR but there is still lots to do and it really has to be done right with an investment like this.
Great thread, keep up the good work.


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