F3X plane by Milan Demcišák & Norbert Habe
Milan Demcisak is a well know Slovakian manufacturer who produced the Demo series, and more recently the sucessfull Extreme F3F. Mid 2011, Milan released the successor of the Extreme F3F, called Rotmilan (The Red Kite, a medium-large bird of prey). The Rotmilan is a joined development between Norbert Habe (HN sections) and Milan Demcisak. This is a 3m F3F/B glider with the following specs:
Fuselage is made from Kevlar and Carbon. It is 2.4 friendly. The ballast tube is already installed, as the servo tray which is moulded and painted with the same color of the fuselage. At the rear of the ballast tube, there is a small hole to evacuate the air when putting the ballast in it. The canopy is in fiber glass. The routing of the wires coming from the wing connectors is along and under the ballast tube. It is easier to do compared to the Extreme but still tricky. Elevator control rods are already in place and are made from a light metal tube. You just need to install an glue the M2 Treaded couplers at each side. The servos tray is designed to receive elevator servos such as the MKS DS95/DS92 or DS6125e. Servos dimensions is about 13mm x 24mm. The fuselage is very thin, but well designed so you can install either 4 to 5 cells NiMh, but also Lipo or LiFe batteries.
The tailplanes are identical to the Extreme one, but a little bit bigger. They reuse the very simple and efficient aluminium control horns to be connected to a metal clevis. the tail joiners that was carbon and wood on the extreme (which had the bad habit to break during hard landing, acting as a fuse) have been replaced with carbon rod, so are much stronger. The wing are using spread tow carbon fabric: 'Spread tow' is a new development in carbon fibre reinforcement whereby a sophisticated production process spreads out each tow (bundle) of carbon fibres making them significantly flatter and wider than they would be in a conventional woven fabric. Spreading the tow in this way creates ultra-flat strips of unsupported unidirectional carbon fibre which is then carefully woven to create a biaxial carbon fabric with an unprecedented flatness and a unique appearance. The illustration below shows the structure of the fabric. The wing are strong and light as we could expect with the use of STF. The unpainted area show the nice matrix of the fabric, I like it.
The large and solid carbon joiner will allow to use brass ballast in it, to be added to the 900g of fuselage ballast provided with the kit. The rotmilan moulding quality is absolutely not notch in every detail. The fit and finish are just superb.
In addition, the Rotmilan is bringing a innovation for the ailerons and flaps linkages that is really a step forward on conventional linkage. I will come back later on it , but to make it short, the servos arm is replaced by a aluminium part that include an axis to move a carbon linkage at the exact dimension. On the ailerons/flaps side, there is another axis which is removable from the root of the ailerons/flaps thanks to a tool provided in the kit. The main avantage are that you have absolutely no slope on the linkage, and that the linkage is totally integrated in the wing.
Below are the different component weight on my Rotmilan:
I started the assembly of the Rotmilan by the servos mounting using the drive and exclusive drive system delivered with the kit by RTGmodel. I used the wood servoframes from MKS, as I decided to mount 4 x DS6125 glider in the wings. I modified a little bit the servoframe by suppressing the connecting part of the frame in front of the servo arm.
There are 2 control rod lengths. The shorter ones are for the ailerons, and the longer ones for the flap. The difficulty of the installation of the servos is that, because of the fixed rod length, you need to position rigth the frame, when you glue it, taking into account the neutral position of the servo, to maximize the travel down for the flap, and travel up for the ailerons.
I installed the carbon control rods on the ailerons and flaps, using the long brass tool provided, to extract and insert the metal axis. Then I glue the servoframes, with the servos in place, protected by a slim plastic film. I used 10 minutes epoxy from R&G. I'm pretty satisfied with it, and had never any problem with it.
Then the servos installation with the aluminium part is straight forward. The result is very neat, clean, and without any slop.
I used the DS6125-e (same case than the DS95) for the elevator. To terminate the elevator control tubes, I inserted and glued a small piano wire (5 cm long), then glued the M2 Treaded couplers with rapid epoxy, then pinched them carefully. On the elevator side, I used metal clevises, but plastic clevises from MpJet on the servos side.
All the universal/JR plugs has been crimped with the wire. this allow to prepare the exact cable length.
In the fuselage, the MPX green plug is glue with cyano. then I protected the root of the wing and the fuselage with tape, and glued the green plug on the wing side with the wing in place. this gives a perfect alignment but can be a bit risky. In the past, it happened once where everything stayed glued together, too bad !
The battery is made from 4 cells Eneloop Power XX delivering 2500 mAh (2 + 2 configuration), with low self discharge. The receiver is the Jeti Duplex R8. I needed 156g of lead to achieve the recommanded CG of 104 mm.
The final weight per component is:
This gives a total flying weight of 2207 gr
Flying the Rotmilan:
I maidened the Rotmilan in light wind (~ 10 km/h) on my club slope. I started from the settings provided by the manufacturer (i.e CG at 104mm), and started to play with the CG, the snapflaps, etc ... The Rotmilan flies very well. It has a good natural straight speed. You see it when diving, or when doing some DS. At 104 mm, I had to trim up a little bit. Then I increased the snapflaps but was not obtaining the grip I was expecting in the turns.
Later in the afternoon, I moved the CG backward, and reduced the snapflaps, and the Rotmilan started to rock much better. It became reactive, grippy, and was accelerating much better. Even in the light wind, I managed to get some EM turns. Quickly I went to the dark side of the slope, still with 10 km/h of wind. The Rotmilan showed very good capabilities in light DS, with immediate acceleration, and seemed stiff enough, thanks to the spreadtow fabric at 45°.
The servo drive system works perfectly on the ailerons and flap. The control is accurate, and allow the right travel, without any slop. Using the neutral position shift on the futaba T12FG transmitter, with 35% up / 65% down of travel on the MKS DS6125 servos, the crows are really good ! I forgot: the color scheme is perfect in the air. I'm sure some of you will find it a bit simple and square, but I can tell you that the plane is extremely visible in the air whatever the position. There is always a color contrast with te background and the upper side and underside are very different.
Then I participated to a competition, while continuing to discover the Rotmilan. After the maiden flight in light wind, I finished at 105.5 mm. During the competition, I felt than I could move the CG backward again. I ended at 107 mm. The plane is still very gentle and forgiving with this CG. I also use some ballast, starting at 2 slugs, then moving to 4, then to 6 slugs over 10. This means I was flying with 500 to 600g maximum. The Rotmilan is carrying ballast very well, and I easily imagine that in strong condition, I will need more than the fuselage ballast. Hopefully, the joiner can receive something like 400 or 500 gr of additional ballast, possibly more if using tungsten.
Exchanging with my friend CM Cheng in Hong Kong, we shared the same conclusion. The Rotmilan, is a fast plane, stable, forgiving, that is to say you won't pay cach a mistake during a turn. Moreover, CM is performing very well with the Rotmilan with a win last week-end in a F3F competition. The exclusive servo drive created for the Rotmilan works perfectly, and I'm very satisfied. This is simply the best of conventional linkage.
I propose few videos: First one is my 35.56s flight at Font d'Urles. You will see how stable and smooth is the plane, which is very good for EM turns.
The second one is showing the Rotmilan doing some DS at the local slope. I found the DS to be excellent exercice to test a F3F plane, see how the airfoil behaves, and what speed and how fast you can get it depending on the wind.
Third video has been recorded during a F3F training session with good conditions.
the last video has been take recently during a short afternoon after we changed from summer time to winter time ! As you can see on the video, it is autumn now ! unfortunately, the light was poor, so the video is a bit dark, sorry for that. Anyway, with a wind about 15 to 20 km/h on a vertical slope, the Rotmilan was really nice to fly. I had only 550g of ballast in the fuselage. With such wind, I obtained the best compromise without any camber or reflex position. I think this video shows very well the straight speed easily achievable, without diving from 300m height. The snapflap setting is now satisfying.
For information, Milan just opened a web site with the information to contact him. RTGmodel website.
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