Saturday, 7 July 2018

RTG Model 'Stribog' - review

Around the end of 2017, I started to look around for a new F3F model to accompany my much loved Needle 115. After hearing some good things about the Stribog I placed an order, and nine weeks later the box arrived - and it was a big one!

Fast forward to now... the Stribog has been assembled, tested, and flown in competition. Time to write a few words...

My Stribog at the North of England Open F3F

Components and unboxing

The Stribog is made by RTGmodel, perhaps better known as the manufacturers of the Rotmilan. Wing span is 2.9m which for me hits the sweet spot for F3F. Oh, and in case you're wondering, 'Stribog' is the old Slavic god of the winds - very appropriate!

The components were well protected against damage in transit. The fuselage and wings suspended in foam inserts, and the heavy steel joiner which I ordered as an extra was packed in foam and securely glued to the side. Some nice bags for the wings and V-tail are included in the basic price.

Fit and finish were well up to the standards expected, except for a few specific issues which I'll explain later in the review.

Fuselage

I'll just highlight the main features.

The fuselage has an integral servo tray, with cutouts to suit KST/MKS 12mm servos.

A ballast tube is already bonded in place. Ten steel slugs are included, for a total weight of 780g.

The V-tail surfaces are driven via the usual plastic rod in tube. Clevises are not provided (I used some metal ones from the spares box). Space is quite tight at the back, and it was necessary to grind a little material from the clevises to avoid any binding.

V-tail linkage (a moulded cover is provided)

Wings

The wings are very nicely moulded. Any traces of release agent were easily removed with isopropyl alcohol.

The flaps are notable for their reverse taper, narrow at the root becoming wider towards the tip. This minimises the projection of the flaps when full crow is deployed, thus reducing the chance of damage if you fail to retract them on landing. It also reduces the turbulent wake at the V-tail. Another side benefit is that the aileron and flap servo bays are next to each other (they share a single large cover).

Ply servo trays are provided. These are are CNC cut to suit both MKS and KST servos in both 'mini' and 'normal' versions.

Root area with steel joiner installed. Incidence holes needed rework.


LDS system

An LDS drive system is included, to suit either MKS or KST servos (specifed at the time of ordering).

The LDS links are pre-cut from 4mm glass sheet. At first glance, all four links appear identical, however one pair is slightly longer - these being for the ailerons. Some minor filing of the links was needed to clear the wing skin.

Servo tray and LDS assembly. The pencil mark at the left is my filing guide to clear the wing skin.

Output wheels. Note larger radius (left) for flaps

The links are pinned to the flaps and ailerons by means of 2mm steel rods which slide inside pockets under the hinge. The rods are threaded at one end, and extracted using a matching threaded tube (supplied). It's a fiddly job, so it's worth optimising the assembly sequence to minimise the number of extractions (just pretend they're your teeth!).

Flap and aileron servos are adjacent
All in all the LDS system required some care to assemble, however the result is a solid slop free linkage, and of course it leaves the wing free of protrusions.

For the wing/fuselage connection, I opted for a self-mating arrangement using Multiplex green connectors (not supplied).

Incidence pin issues

Most of the assembly went smoothly, however I did have some issues with the fit of the wings and V-tail. In particular, the incidence pins in the wing didn't match the holes in the fuse. I used a drill supported in a stand to mill out one side of the holes, with the fuselage supported at the correct dihedral angle.

A similar problem also affected one of the V-tail surfaces . In addition, the holes in the fuselage for the V-tail joiners were far too tight and had to be reworked. Lastly, the control surface hinges were too stiff. I was able to free them by relieving the inside of the Kevlar hinge with a combination of a jewellers screw driver and glass paper bent around 2mm steel rod.

I managed to overcome all these issues and now have a pretty well perfect setup, but it did add to the assembly time - and stress levels.

Some custom tools to aid assembly/fettling

Radio system

To control the Stribog, I am using my trusty Taranis X9D transmitter with F3F Template V5. The airborne gear comprises a FrSky X8R receiver, MKS servos  and a 2S 18650 LiIon battery.

With the battery in place, there's little space for the receiver and care was needed to prevent the antennae being kinked where they exit the case. The recently announced RX6R will fit more easily (though it will be necessary to make a Y-lead to connect a battery).

Radio installation. Ballast tube opening just visible at left.
A fair amount of noseweight was required, via a combination of a lead casting, and strips of lead around the battery.

Ballast

Fully assembled and balanced, the weight is 2420g for a loading of 39.8g/dm2. With all ten ballast slugs in place, the loading goes up to 52.6 g/dm2. There's neglible shift in CG ( < 0.1mm ) thanks to accurate positioning of the ballast tube.

The steel joiner adds an extra 1220g. In order to maintain the CG position, two slugs are required at the rear of the ballast tube. The steel joiner alone provides a loading of 62.4 g/dm2 rising to 70.1 g/dm2 with max fuselage ballast.

ASIDE: With all my F3F models, I take care to maintain the CG within +/- 0.5mm of the target position, irrespective of ballast. A ballast spreadsheet is invaluable for picking 'good' ballast combos. In addition, my OpenTx setups all have a special CAL mode. This ensures consistent centres and trims irrespective of ambient temperature, bent linkages etc. The result is a model which flies identically in all conditions.

Flight testing

The maiden session took place at Ivinghoe Beacon in a perfect 10 mph Westerly/ I managed two flights, 50 minutes in total. 

During first test session at Ivinghoe

Impressions:
  • The model is quick both in the straights and the turns.
  • The model is responds very well to camber changes
  • There was a slight tendency to drop a wing when flying slowly with camber dialled in. I have reduced the camber on the ailerons to alleviate this.
  • Recommended snapflap travel is large (8mm). I've dialled in some deadband to stop the flaps deflecting during minor pitch corrections.
  • The crow brakes are very effective. The recommended elevator compensation was spot on at 5mm. 
  • The recommended aileron and elevator rates were far too sensitive for my taste and I've reduced them

The second session took place in much stronger winds during which the sheer speed of the Stribog became apparent - during one glorious 10 minute spell, the model was flying faster than I can remember on this slope, blindingly fast on the straights and whipping around the turns. Very impressive!

I've since flown it at the North of England Open F3F, in very light conditions, and it managed to hold its own.

Overall

With a top 10 finish under its belt, my Stribog must be pleasing its wind god namesake, and I'm currently using it as my first choice model for F3F.

As for the airframe, the basic quality and finish are beyond reproach, however the assembly was marred by the incidence pin issues. The manufacturer has acknowledged the problem and a fix to the manufacturing process is in hand.

Since receiving my Stribog, a 'Plus' version has been released with a super slim fuselage and ballast in the wing.

Specification (as published on RTG website)

  • Wingspan 2867 mm
  • Fuselage length 1463 mm
  • Airfoil HN Straak
  • Elevator airfoil HN modif.  
  • Weight 2100 - 3900g/4500g 
  • Total surface: 60.9 dm2
Price as tested (with optional steel joiner): €1470

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