Saturday, 7 July 2018

RTGmodel Stribog review

My Needle 115 has served me well in F3F competition, however at the end of 2017 I decided to look around for a new machine. After hearing good things about the Stribog I placed an order, and nine weeks later a huge box arrived at the door.

Fast forward... the Stribog has been assembled, tested, and flown in competition. Has it replaced the Needle as my number one model? Read on!

My Stribog at the North of England Open F3F

Components and unboxing

I bet you're wondering about that name, well apparently 'Stribog' is the old Slavic god of the winds! The model is made by RTGmodel, perhaps better known as the manufacturers of the Rotmilan. Wing span is 2.9m, for me this hits the sweet spot for F3F.

The model came well packed - all components were suspended in custom foam inserts. Wingbags were included in the price.

Fit and finish were well up to the standards expected, except for some specific issues which I'll describe later.

Fuselage

The fuselage is beautifully moulded, and features an integral servo tray with holes pre-cut for KST/MKS 12mm servos. An intricately moulded tail cone is provided, to cover the aft linkages. An integral ballast tube receives up to 10 steel slugs (supplied), for a total of 900 g. The optional steel joiner adds an extra 1.2 kg.

The V-tail surfaces are driven via the usual plastic rod in tube. No clevises are not provided, so I used some spare metal ones. Space is quite tight at the back, and it was necessary to grind a little material from the clevises to avoid 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 design is unusual, in that the flaps are are narrower at the root than the tip. Because of the wing dihedral, this minimises the downward projection of the flaps when full crow is deployed, thus reducing the chance of damage on landing. It also reduces the turbulent wake due to crow. Another side benefit is that the aileron and flap servo bays are next to each other (they share a single large cover).

The servo trays are CNC cut from plywood, and suit MKS or KST wing servos in both 'mini' and 'normal' form factors.

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 (you specify which type when ordering).

The LDS links are pre-cut from 4mm glass sheet. At first glance, all four links appear identical, however the ones for the ailerons are very slightly longer.

External servo bearings are provided - these fit in glass holders which you have to glue to the servo mounts - it's important that the holders are accurately positioned so the bearing axis is parallel with the servo output shaft.

Some filing of the links was needed to prevent them rubbing against 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 at the control surfaces by means of 2mm steel rods which slide along pockets under the hinges. The rods are threaded at one end, and extracted using an internally 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 and planning, however the result is a solid slop free linkage, and of course it leaves the wing completely clean.

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

Incidence pin issues

While the general quality of the mouldings was excellent, I did experience some issues with the fit of the wings and V-tail. First, the wings refused to slide on to the fuselage, because two of the incidence pins didn't quite match the holes - the discrepancy was small (~0.5mm), but sufficient to be an issue. In the end I used a drill and vertical stand to mill out one side of each hole, with the fuselage supported at the correct dihedral angle. Fortunately I was able to achieve a good fit without the need to back fill and redrill the holes.

A similar problem affected the incidence pegs on one V-tail surface. This time, both holes in the fuselage were too high by around 1 mm, and I elongated the holes using a needle file being careful to maintain the correct incidence. I also found that the holes in the fuselage for the V-tail joiner were too narrow. Fortunately I had a round file of the correct diameter and managed to achieve a good fit.

Finally, the control surface hinges were far too stiff as received. I was able to free them by relieving the inside of the hinge, using some glass paper bent around 2mm steel rod. It's worth spending time on this to prevent premature wear on the servos.

The first three issues could potentially affect alignment of the flying surfaces, and I therefore regard these as critical. In the end I managed to overcome all of them, 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. In the model are a FrSky X8R receiver, MKS servos  and a 2S 18650 LiIon battery.

With the battery in place, there's precious little space left for the receiver, and care was needed to avoid kinking the antennae. 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 54.5 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 63.3 g/dm2 rising to 70.3 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 (a ballast spreadsheet is invaluable for this). In addition, all my OpenTx setups incorporate a special CAL flight mode. This allows me to compensate for any drift due, for example, to changes in ambient temperature or altered linkages. The result is a model which flies more or less identically in all conditions.

Flight testing

The maiden session took place at Ivinghoe Beacon in a perfect 10 mph Westerly. Two flights were flown of 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 very sensitive to camber and reflex settings. It's worth getting your presets correctly dialed in.
  • There was a slight tendency to drop a wing when flying slowly with positive camber in the climbout. I have reduced the camber on the ailerons to alleviate this.
  • Recommended snapflap travel is pretty 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 suggested 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 pinging around the turns.

Overall

In terms of sheer performance, the Stribog has more than lived up to expectations - it's fast in a straight line yet turns well. Handling is good, although the model feels slightly less agile than the Needle.  Bottom line: it's now my number one F3F model, with the Needle as backup. [23/8/18: I have since won the English Open flying the Stribog!]

As for the airframe, the basic quality and finish are right up there, however assembly was somewhat marred by the incidence pin issues.

I can recommend the model, with the proviso that the incidence pin issues are fixed - as the manufacturer has since advised. Since receiving my Stribog, a 'Plus' version has been released with an ultra-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 (including optional steel joiner): €1470

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