First I would like to thank Bernhard (forum name – DrKite) for helping me in getting some pictures and information of the Flettner-282.
I started this project at mid last year after doing the huge scratch built CH-113 tandem helicopter.
I prefer doing scratch build projects where I can design and create most of the parts myself and in addition to take engineering challenges to satisfy my nature as an R&D engineer for many years. The R&D process is very rewarding when everything works but it takes many hundreds of hours (and money…) to get there and it's very easy to quit when things do not work as expected.
About 3 years ago I decided that all my projects will have 2 rotors. Personally I think the tail rotor is an historical mistake that we pay for the last 60 years or so. One other project in line (one that I design in my head for 3 years) is large VTOL which is in my mind the ultimate project that takes all the design, experience and effort into one big project (hi Larry, I know you love the V-22 but I have another project in mind…
).
A bit of history
The Flettner-282 was designed by Anton Flettner in Germany since 1941 and was active in trials and service till end of WWII. The FL282 is a dual intermeshing rotor helicopter where the two rotors are located at the center of the heli with 12 degrees tilting to each side allowing the rotor blades to pass over the other head in a synchronized motion without colliding.
The FL282 had about 23 versions, some of them where used only for tests. The goal was to create one of the first helicopters at that time, able to hover and land in a small area and carry one person (one version added another person behind the rotors) and some small hand carried bombs (5Kg each). The mission was scouting, monitoring ships and submarines and the first vehicle to land at ships far from a shore. The marine version of the FL282 was successfully tested with ships and it used to land at the back of the ship or sometimes over the top of the big gun array (after adding a plate over them). The method of landing at a ship was – the pilot threw a rope to the ship crew while hovering above and the crew attached the rope to a winch and pulled the helicopter down to the landing post.
The idea of the helicopter was new at that time and Flettner and his team made a lot of efforts to convince the Luftwaffe that it’s a good product. One of these efforts was to take a German housewife and teach her to fly the helicopter. They proved that even a normal housewife can fly it after only 3 hours of training as the vehicle was extremely stable and easy to fly.
The selected version
The first two documented version (V2 & V3) were round shape body and fully closed and glazed cockpit. The difference between V2 and V3 was the addition of a vertical fin at the elevator in V3. I have no details for version 4 & 5; I only know they were used for tests. Starting from V6, Flettner changed the body to the more known square shape (and ugly to my opinion…) and removed the closing of the cockpit. In later versions some had half closed (at the bottom) cockpit and some were fully opened like an ultra light. I have no idea how the poor pilot had to fly it and sit under the massive air flow of the two giant rotors without any roof…
I selected V2 as it looks like an airplane with rotors and to my opinion look much better than the later versions. I assume the change in later versions was to reduce cost.
The FL282 was made of metal tubes covered with external cloth. The profiles at the rudder and elevators were made of wood. The cover of the central section (bottom and top) was metal and the cockpit was made of metal and glass (in V2) or plexiglass in later versions.
View of the V3 version -
My first build attempt
When I started the project I put as a goal to try and create it as close as possible to the original even with the materials. The first attempt was using brass tubes. The selection of brass was because I thought I can solder the joints and it will hold the load and vibrations and soldering is a work that can easily be done at home with a large solder iron. At that time I didn't have any experience with welding and always thought that Aluminum welding is something for professional.
I have to admit that I had great concern that the soldered joints will not hold the vibrations (as actually happened) but at that time I didn't had other option that I could use my experience.
I designed the whole project as I always do using CAD software. Every tube, profile and part is designed, assembled and the main transmission and rotors is even simulated. The result was a work of art (see in the pictures and judge yourselves). Every tube and shape was cut and manually bent by the shape I designed in the CAD. All tubes were fitted to each other and then soldered to place. Even the rudder and elevator profiles are made of small diameter and light brass tubes, manually bent to shape and soldered.
A view of the tail with everithing made od brass tubes, including the rudder and elevator profiles -
A view of the complete body made of brass -
The first trial with the engine was a catastrophe. The vibrations tore many of the soldered joints. I almost gave up. After some considerations I decided that I can't stop the project and the solution is to learn how to weld brass with silver welding. I got a small Propan/Oxigen torch and practiced on silver welding brass tubes. After that I welded all of the 350 joints again (first melt the tin/led solder and then do a silver weld) for the whole project.
This came out much stronger. However the next engine trial was not successful under rotor load trying to hover. The rotor load and the engine vibrations broke the main joints and I stopped. The conclusion was that the silver welding did not hold because there was remaining of solder on the tubes. My conclusion was that I have to build everything again!
I stopped the project for several weeks and after gathering new energy and encouragement from my colleagues I decided that I will take the challenge as in a R&D project and start all over but this time will do it with Aluminum and face the extra challenge of it as welding Aluminum was considered as something I will never do myself.
I got the Aluminum welding powder and rods and practice myself how to weld and started the project again. This time I used aluminum and a re-enforced structure to hold the engine and mechanics.
The current project progress
This time I designed a very strong structure to hold the engine and mechanics. It has much thicker rods and a better construction. The rest of the body looks the same but instead of using tubes for the body I used aluminum strips that have more welding area than tubes. For the cockpit I used aluminum tubes and welding them is a work of art as it is much more difficult to do without melting them and still get an accurate shape and fitting. This time I didn't do the rudder and elevator profiles from aluminum. I will do them from plywood as in the original helicopter.
There are over 350 welding joints for the body and many more at the cockpit. There were a few joints at the center construction that I couldn't do myself with a torch and I asked a professional to do it with argon welding. The result is a very strong body that holds very nicely under load and vibrations and the total reinforced structure weigh about 6Kg (2Kg less than the weaker brass body).
A view of the aluminum version just prior to liftoff -
The mechanics
I designed the mechanics myself and manufactured all parts manually. Some of the parts are made of aluminum and the main plates are made of 4mm glass-epoxy which is a very strong material and holds the load and vibrations very good.
The aluminum parts are milled and the main plates are cut very accurately with dremmel disk as they are huge (40cmX24cm each).
A lot of thinking and experience was put into this mechanics. It has to withstand the enormous load of two large rotors, be compact as much as possible, easy to assemble and handle, do the synchronization of the two rotors in a simple and rugged manner and be detached from the body as a whole in a simple and easy way for maintenance purposes.
The result looks very good. The benzin engine, transmission, servos, gyro etc. are all assembled into it and I can detach it from the body by opening 12 easy to reach bolts. The bolts are attaching the mechanics to the aluminum frame using rubber shock observers. Even the electrical connections are simple – all connections to the servos, engine, gyro etc. are located in one D25 secured connector attached to the mechanics from the back, so open one connector, take out the mechanics and that’s it.
The Rx, batteries, switch etc. are located as in the real helicopter at the cockpit.
A view of the mechanics from the front -
Control
The real helicopter had mechanical mixing of the two rotor's pitch controlled by the pilot. The control had tilting to front/aft, differential pitch for yaw (and additional large rudder that was affective at higher forward speed) and small tilting to the sides. The differential pitch (DCP) is very sensitive as it's located at the center (not like the tandem helicopters that are more stable). The problem with the DCP in the real helicopter was at autorotation. When the engine stopped the yaw control was reversed because of the torque elimination of the engine drive. The pilots doing an auto would be confused and probably crash. To overcome that problem, Flettner added an engine governor device that mechanically reversed the yaw control if detected a low engine drive RPM so the pilot was not aware of that problem.
When I first designed my control I thought that I have a modern electronic control and I will use a better control. I will use DCP for roll axis and one rotor forward while other rotor backward for the yaw and this will save me the problem of the reversed action in autorotation.
The first trials proved me wrong. The result was an unstable control and the helicopter always moved in diagonal way and almost flipped to the front in a diagonal direction. I made many analysis for the aerodynamics and figured out that it is wrong because the DCP causes more lift at the rotor with more pitch, hence should roll to the other direction but on the other hand there is a vertical vector causing a shift to the higher pitched rotor side causing sliding to the other direction and tendency to flip as the yaw control of reverse direction in combination with the above vectors caused it to be un controlled and flip diagonal.
The current state is that I change the control exactly as was in the real helicopter. I use DCP for yaw, tilt forward/aft for pitch axis and slight tilting to the sides (must be very careful not to over tilt and hit the other rotor head) for roll axis. I intend to solve the autorotation problem by flipping electronically the yaw control in case the engine stops.
For the electronic control I use the TH-2 product from tech-mp as I always use in my projects (I'm the first beta site user and helped defining its requirements) and I can do anything I want to change the mixing and configurations using my PC and a USB connection.
Another strange problem I detected is that the left (LH) and right (RH) blades profiles are not identical. I use SAB 810mm blades and maybe this is not true for other vendors. Usually nobody checks this because in a standard helicopter you either use LH or RH blades. Even in a tandem helicopter the problem is less affecting and can be solved easily by trimming. But in the FL case any slight change in lift of one rotor causes noticeable yaw reaction. It took me many takeoff trials to figure that out and now it works fine after I made a mechanical difference of 3 degrees in pitch between the left and right rotors.
The video in this link – My First liftoff trial video
shows my first takeoff trial after I corrected the control settings. I landed after a short trial as I was too excited to continue…
Next steps
The next process is to do stable test flights and finalize the setup and conclusions. After that I will finish the scale heads I designed and test them. Then I will start covering, painting etc. and do the scale items such as pilot (I started building a 1:3 scale pilot myself as I didn't find one), meters, chair etc.
That’s it for the first publication, I will update as I progress.
