Saturday, 7 July 2018

RTGmodel Stribog review

Around the end of 2017, I started to look around for a new F3F model. After hearing some good things about the Stribog I placed an order, and nine weeks later a huge box arrived at the door.

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

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 excellently packaged - all components were suspended in foam inserts securely bonded to the box. 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 has an integral servo tray, cut to suit KST/MKS 12mm servos. A ballast tube is already bonded in place. Ten steel slugs are included, weighing 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 prevent any binding.

V-tail linkage (a moulded cover is provided)

Wings

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

The flaps are notable for being 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 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 links are pre-cut from 4mm glass sheet. At first glance, all four links appear identical, however the ones for the ailerons are slightly longer. Some minor filing of the links was needed to prevent contact with 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 free of protrusions.

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

Incidence pin issues

I was pleased that both the carbon and steel joiners were a perfect sliding fit.

However I experienced 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 very small (~0.5mm), but such are the tolerances that it caused an issue. After some umming and aaring, I decided to use a drill stand to mill out one side of the holes, with the fuselage supported at the correct dihedral angle. A good fit was achieved after a few iterations. (The fit of the joiner was excellent, however the incidence pins to help spread torsion loads).

A similar problem affected one half of the V-tail, though in this case I simply elongated the holes in the fuselage using a needle file. Care was needed to match the incidences of the two V-tail halves. I also found that the V-tail joiners would not fit in the fuselage, because the tubes in the fuse were too narrow. Fortunately I had a round file of the correct diameter and managed to achieve a good sliding fit.

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

I managed to overcome all these issues, 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 precious little space for the X8R 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 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 (a ballast spreadsheet is invaluable for this). In addition, all my OpenTx setups incorporate a special CAL flight mode. This ensures consistent centres and trims, irrespective of ambient temperature. 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.

I've since flown it at the North of England Open F3F, in very light conditions, and it managed to hold its own. [23/8/18: I have since won the English Open flying this model.]

Overall

In terms of performance, the Stribog has more than lived up to expectations - it's fast in a straight line yet turns well. Handling is good too. My model is already proving a good 'un in competition, and I intend to use it as my number one F3F machine with the Needle as backup.

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

On the proviso that the incidence pin issues are fixed (as the manufacturer claims), I can certainly recommend the model as a top-notch F3F competition model.

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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