Blohm Voss BV.215
After I saw my friend’s MIG 15 ducted fan RC model, I started to think about the model of similar concept. I like unusual shape planes, not commonly seen on airfields and one such model I found on this web. It was Blohm Voss BV.215, rapier powered version built by Ruja. And when I read that it flies, the decision was made. I thought that it’s a bit daring to build such model as my first ducted fan model, the original plane never made it to the air, … but the beauty caught me. I usually prefer classical style, but this time I decided to use foam, mainly due to possibility to sand the inner channel for the air intake. With classical balsa design I would have to make the channel from fiberglass and I was too lazy to do that. Plus I did not want to spent too much time building the plane, as I was not sure whether it will fly.
I started with the fuselage, as the most complex and time consuming part. I used pink foam for the fuselage. The templates were made of 3mm thick plywood and I used hot wire to cut the semiproducts. I glued the halves of each part together and sanded and filled the inner channel to be as smooth as possible. Then I glued the parts together and again sanded, filled and lacquered the connections as later it will be difficult to reach it. I glued in the plywood former including parts for the wing fixation and another former to hold the ducted fan. The best would be to make the back part from the fan removable, so the channel is uninterrupted, but I had no idea how to technically made it, so I decided to make the upper part of the fuselage removable and fan is put into the fuselage from above. The fuselage is modified a bit compared to the original, to make the inner cross-section as large as possible. I did not make any special calculations regarding the channel areas, only very briefly, so I hope that the ratios will be functional. I also did not follow precisely the nose of the fuselage, the intake is as large as possible plus there is a hole for cooling the electronics. Just at the beginning of the channel there are 4 more holes for cooling air intake. It goes off at the back part, where there are holes leading into the channel behind the fan. The cover goes tightly into the fuselage, however it is still secured with the pins at the back and magnets in front.
The wing is made of ordinary foam, with balsa leading trailing edges, on the tips there are balsa blocks used to hold balsa rudder (fixed) and elevons (controllable). At the back part of the fuselage, where there is removable cover of the fan the wing is not connected to the fuselage. Then I made the balsa wing tips, enforced by glassfiber, the elevons themselves have reinforcing carbon strip in the trailing edge. The final part is strong and weights 12 g. The servos are in the ends of wings in covered box. Balsa rudders and elevons are covered with vlies, foam parts with paper.
Next step included lacquering, cockpit cover and painting. The jet nozzle from the original plane is not made to avoid further reduction of output channel cross-section. Finally I made the removable protection steel wire arcs for better landing and the model was finished. The final weight reached 480 grams, I find that acceptable.
The first flight totally corresponded to the concept of the model and my fear. Luckily a friend hand launched it, so I had time to change the controls fast, trim the model and somehow enjoy the first flight. The fan is pretty strong (it eats about 25 A on the ground). Half throttle is sufficient for horizontal flight. Used airfoil is very fast, I have never controlled anything that fast. The model glides well, but the speed in powered and glide modes are almost the same. The landing is really adrenalin and kind of hard due to the speed, no matter how well we prepare the descend. I hope to learn it better, if the model survives. After flight experiences I would use slower airfoil, flaps and increase the overall dimensions for about 10-20%. However, the expectations were met, it flies great, a bit faster than I thought, plus there is no such model seen around.
Wingspan 920 mm, flight weight 480 g, motor TURNIGY HXT 2435 inrunner - 4400 kv, fan diameter 55 mm, controller Mystery 30A, servos 2x HXT 500, receiver MZK sexta mini, battery ZIPPY Lipol 3S 1500 mAh, 20 C (Rhino 1550 mAh, 30C). Model was built from July to October 2009.
[Josef Baco]
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