Simon Chaddock

shepeiro Although quite light by the standards of the day the 110HP rotary was a big capacity engine of 16.3 litres and weighed 140kg to which you have to add another 19kg for the weight of the prop. The empty weight of the V25 was 383kg so you can see the engine and prop alone made up a significant proportion of it. The fuselage was a light weight structure made up of welded steel tube wire braced internally and fabric covered hence the rotary ended up close to the leading edge of the wing for CG reasons. For comparison the 9 cylinder Wright Whirlwind fixed radial engine also had a capacity of 16 litres was over twice the weight at 306kg but produced 4 times the power.

martin No but you are correct in part. It is a Fokker prototype numbered V25. It used the same fuselage and rotary as the DR1 but with a low mounted plywood covered monoplane wing. Faster with a better climb rate than even the Triplane and nearly as manoeuvrable. Fokker loved flying it but it was rejected at the mid 1918 comparative fighter trials as the low wing seriously restricted the downward visibility in the most favoured area (above with the target ahead) to spot potential combat. Fokker solve the issue by raising the wing on struts above the fuselage as the EV to be above the pilots eyeline. Not an ideal "engineering" solution but the layout was accepted into production as the DVIII in the last few weeks of the war. It is interesting that this "classic" fighter layout was not adopted by any of the winning powers for at least another 12 years as braced biplanes ruled.

The cowling I printed had to have a small nose radius to clear the rotary. To achieve a scale shape cowling the rotary had to be placed further back in the cowling which required quite a bit of reprinting as well as a new cowl. The other non scale issue is the conventional CCW 'E' prop goes the wrong way round for the rotary! Although Gem Fan make a 12x6 CW e prop they don't import them into the UK with a "not available in your country" message from their web site. A modest start on building the V25 fuselage in Depron. Quite some time since I built anything in this way.

As a boy I actually saw the "Flying Bedstead" at Farnborough and the HP115 a couple of years later. A bit about the HP115. As a good test pilot Buzz Aldrin asked if he could fly it (one for his log book?) but as he was slated for a moon mission NASA firmly said "No"! However when he got back from the moon he did apparently fly it. I suppose the first man to land on the moon has some "handling" credentials. After a few flights I discovered one calm evening that its extreme alpha and ground cushion effect could be used for a true low speed handling but you had to get everything just right! https://www.youtube.com/watch?v=7PFdq_mOQBE A true "flop" landing.

shepeiro My concern about the centre section is also on landing. Unless you really achieve a true "greaser" landing and on a smooth surface whatever the plane's sink rate is at the point of actual touch down the ground causes it to stop sinking instantly. This creates a high shock load far in excess (5+ times!) of anything due to flight. Of course some "give" in the undercarriage will spread the period at which the sink is arrested by the ground and will significantly reduce the instant nature of the shock load. I do not see the centre section spar as a box. To be one would require a rigid top and bottom surface between each of the sides. It would indeed then be very strong and stiff in both bending and torsion but would also be relatively heavy. Getting the right balance between strength and weight needs a good understanding of the loads involved and the best way to resist them. Sometimes when building to a strict weight target it is easier to reduce weight in areas where the loads are obviously minimal to allow a bigger weight margin where the loads are difficult to assess accurately or where any structural failure has serious consequences. It is not the wood itself that is the problem but where to best put it. As an example the full size Fokker Triplane used a single big and rather complex box spar to handle virtually all the loads. It needed no external wire bracing unlike conventional biplanes. The rest of the wing structure was very minimal to really just hold the fabric skin in place. Interesting that this drawing was done on behalf of the UK air ministry just after WWI as it was a design concept completely different to anything the allies were using at the time.

Spectacular! However from a purely structural point of view it does look to me like there is a lot of "wood" in areas that carry little load like the wing trailing edges and maybe too little at the high stress points like the wing centre section particularly on a twin with its battery in the fuselage and wide spaced undercarriage. Just an observation.

Also remember the nozzle itself can carbon up internally particularly when printing at higher temperatures. This reduces the filament "flow" below what is expected which gives print problems. Nozzles are cheap so I have a adequate spares of the sizes I use. I have just changed a 1 year old somewhat externally corroded 0.4 with an instant improvement in print quality.

The issue is not the voltage capability of the wiring but what electronics is "seeing" the 8.4V of the LiPo. A Futaba receiver itself is internally voltage protected but the servos may not be. However if you are using the BEC from the ESC to power the radio & servos then that provides a regulated 5V (or 6V) so no problem.

ESC cooling is why I add a finned heatsink to my ESCs, particularly on foamy EDFs, and is arranged so the fins poke out into the outside airflow. On a low power "chuck glider" power conversion where space for effective ducting is limited I have resorted to mounting the small 10A ESC on the outside wall of the cockpit. In general the motor and battery can live with some heat, an ESC is far more critical as a serious over heat can take out all control.

Fraggs To be kind to the motor bearings I would definitely cut the motor shaft down so the prop adapter was right up against the front of the motor. It also reduces the load on the fixing of the motor mount to the fuselage foam! My prop adapter is a bit different but I still got the prop as close as possible to the front of the motor. Note I also replaced those two big screws on the motor with grub screws to save weight. Every little helps!

Extra gyro effects are a possibility but the plastic rotary itself is quite light. In fact its lighter than the rather big prop. Most of the weight comes from the brushless motor buried in the crankcase unlike the full size whose cylinders, cylinder heads and even the pistons were machined from steel. We shall see!

The Uberursel running inside a scale LW-PLA cowling. https://www.youtube.com/shorts/9CWa-rKoraE Inside the cowling the "windage" created by the cylinders is reduced so the RPM is noticeably a bit higher. I have just noticed it is turning a CCW prop whereas like most rotaries they rotated CW! I shall have to get a 12x6 CW.

Henry No the rotary is rigidly fixed to the motor so the electric motor, the dummy rotary and the prop act as one! They are actually all connected by the prop driver so it can be disassembled if required. As the object is to portray true scale the prop is pretty big so it requires a small low kV motor and a low cell count. Not a lot of power but it will be incorporated into a light weight WW1 airframe. In today's video the rotary is using a scale diameter 12x6 prop. It is running at full power on the fully charged 2s and spinning at a about 6000 rpm. https://www.youtube.com/shorts/kOjTkcVH5x8 Only 51 Watts but with the large prop it still produces quite a bit of thrust. When the rotary is in a close fitting cowl that has a smooth inner surface the loss in RPM due to "rotary windage" is not that great. In full size a rotary was compact but relatively heavy and had to be close behind the prop which meant that such planes had really short noses so the weight of a dummy rotary is not too much of a problem. How many RC Fokker Triplanes or Sopwith Pups have non scale "extended" noses or have to add considerable nose weight actually in the cowling? A modern brushless motor is just not heavy enough.

In 1918 Fokker had 2 prototype monoplanes built basically using the Triplane fuselage and Oberursel 110hp rotary. One with a mid fuselage wing V17 and one a low wing V25. At the same time two similar were built using Mercedes DIII engines. They were entered into the ministry fighter competitions. The Mercedes versions although with an excellent performance were ruled out due to poor visibility as well as the limited Mercedes engine supply being reserved for the Fokker DVII. The two Oberursel prototypes were also declared unfit due to poor downward visibility. To overcome this objection Fokker built a prototype with a parasol wing that would be closer to the pilot's eye lane as the Fokker EV. It was accepted for production and was later renamed the DVIII. With its plywood skinned low wing the V25 was remarkably advanced for the period and predated what was to become the classic fighter layout by some 18 years! I have previously built a small 24" span sub 250g Depron RC V25. And it had a simplified Oberursel that went round with the prop! Flew well enough but was very twitchy and did not like landing on any sort of grass! https://www.youtube.com/watch?v=3Uv03i4kraU My intention is to build a bigger 40" version using the bigger scale Oberursel. Hopefully it will be a more manageable flier.

Just for fun a scale model of a 9 cylinder rotary that is fixed to the out runner so it goes round with the prop. https://www.youtube.com/shorts/roqPMTxWeM4 The 2212 1000kv on a 2s is driving an 11x3 printed prop and even that is not scale as it should be a 12"! Not yet run at full power either.

The LE 2204 has a 3 mm shaft. The folding prop comes from Ebay via China so you have to wait. The 5x3 I used with a 3s no longer seems available. This 6x3 folder & spinner for a 3mm shaft is about the cheapest at the moment. I use it with a 2s. May sound a bit expensive and ideally the motor cowl has to match the spinner but provided you "no power" land the prop will not get broken.

Of course on the Fokker Triplane the aileron actually extended beyond the wing tip.

I fly from a reasonably flat area but it is only cut irregularly by a council tractor gang mower hence the field is never a bowling green. ☹️ As a result virtually all my planes are hand launch/belly land and have where possible design features incorporated such as rear high mounted pusher props, folding tractor props or are EDFs. It does also mean you have to get used to glide "no power" approaches and landings. 😉 When the grass is short(ish) and with the appropriate plane a ground "slide" take off is possible, even with a scale EDF, but it does require it to have a pretty high thrust to weight ratio. https://www.youtube.com/watch?v=gPX54hVu1Xg

Fraggs Yes, I understand the marking is intended to be a battery hatch for a power conversion. However I did not use it on either of my FX707 conversions as they were aimed specifically at retaining a good glide performance using a relatively small motor and battery so a good CG could be achieved with the battery simply placed in the cockpit. Like you I was also concerned at the impact on strength and rigidity of the fuselage from such a big cut out placed directly under the already substantial opening for the wing mounting. I suspect if a bigger heavier motor sufficient to drive say an 7x6 prop rather than a 6x3 then the resulting bigger battery would likely have to be placed and accessed via the indicated underside markings. I note that many of the FX707 conversions posted on You Tube end up at 400+g so are "sport" planes rather than the 320g of my light weight power glider. It rather depends on what you are after.

4 strokes are hard on prop grip. The classic solution is a serrated prop driver like this. In this case a DIY job but it needs a lathe to do it.

It always amazes me how effective and light a pull/pull can be particularly if you use monofilament line. I used them for the elevator on both my FX707 chuck glider conversions. I feared a servo in the tail would be difficult to counter with the intended motor & battery in the nose. The servo was "let into" foam close to the rear of the cockpit with just its arm exposed. In this position the supplied servo lead easily reached the radio. The monofilament has been painted black so the camera can see it! The top and bottom elevator horn is positioned so the line has a clear run. I made use of the stretch in the monofilament line (it has a 10lb breaking strain or about 16 times the weight of the plane!) to put in a modest pretension so as to not put too much side load on the servo bearing or the elevator hinge and to avoid bending the foam fuselage! Note the forward position of the servo arm to give more down than up elevator travel. The XF707 has flaps so it requires quite a bit of a "down elevator" mix when the flaps are fully lowered. I wanted to ensure still more down would be available if required.😉 Note the line pretension is still there after a year and the XF707 still flies nicely. In fact I flew it this morning. Having said that I would not recommend a monofilament pull/pull line for anything but a real light weight.

Yes indeed about the weather, or for my planes the wind, so I will be there.

What is the likely weight of the elevator servo? I am always concerned at the weight of the servo linkage becomes significant in a really light plane. As soon as I started using micro 3.7g servos in my lightweights it quickly became apparent that a long snake and the servo mounting soon doubled the bare servo weight. I quickly began to favour mounting servos adjacent to the control surface, either within the tail plane itself or the fin with the shortest linkage possible. As a by product the resulting improvement in the linkage mechanical efficiency tends to compensate for the low power of the micro servo. As the servo become smaller the benefit of positioning close to the control surface is ever more apparent. I have 3 twin boom planes where remote servo elevator linkage would be complex so in all these cases the servo is in the tail plane. The only tedious bit is running the servo wire through the tail boom. As an example a 2.5g elevator servo buried in the tail plane of my sub 250g DH Venom. Even a 2.5g servo is thicker than the tail plane so it is obvious but it allows the linkage to be direct and simple. The Venom has three such servos (aileron x 2 & elevator) and they all run happily from a 1A BEC in the ESC. The full "stall" amps of a 2.5g servo is only 0.5A! I work on the principle that if the aerodynamic loads stall all three servos at the same time its going to crash anyway. 😉

Jon H I suspect it is the old fuel that is the problem. The ether in it evaporates very quickly. The smaller the engine capacity and the more exaggerated the porting to get the power the more important the ether becomes as an "ignition promoter". It does not surprise me you had no trouble starting a Mills 1.3. With its modest port timing and long stroke it will run on almost any diesel mix!

Just a point but servo action is measured in a fraction of a second however a differential thrust response is significantly slower due to the inertia of the motor & props. The danger is too much differential may be applied before the expected yaw reaction is noticed from the ground. The same will apply when removing the differential thrust. As a well practised "bank and yank" pilot adverse yaw can normally be kept well under control with differential aileron movement in conjunction with an adequate fixed fin area except of course for the notorious Super Cub!