Simon Chaddock

Pretty calm this evening (8pm) so I flew my flapped FX707 Albatross conversion. A flat site and by then no thermals so it was a minimum power "cruise" flight with the flaps set at 15 degrees to give the lowest sink rate. With power on all the time except for the actual landing the flight lasted 35 minutes 5 seconds before the LVC first cut in. Not bad as it uses a 1000mAh 2s! The cells were both showing 3.5V at rest after I landed so probably did no favors to the LiPo but very relaxing nevertheless.

Yes it is very light being made of 3mm Depron sheet with some LW-PLA panels. The light weight construction is more to do with the low power being used to keep the rpm of the scale rotary within reason than about the tail weight. To get an idea I put two micro 3.7g servos for rudder and elevator right at the extreme tail. A bit of a worst case scenario as I doubt they would ever be that far back. With motor mounted on the fire wall to see roughly where it would balance. This was just a "ball park" set up and it looks possible. No rudder and elevators yet but they are Depron. Of course actually mounting the servos and connecting them up will add a bit more tail weight but the scale rotary and prop will hopefully adequately compensate for it. The Rx and the ESC are small and light enough to be positioned as required. This leaves the relatively heavy 2200mAh 2s battery as the main "balancing" item. There should be more than enough room somewhere between the cockpit and the motor firewall to get a workable CG when the airframe is fully complete. Well that's the plan.

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.