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Hi. I've been working on it for a year or bit more. I think I'm on the finish line now as most problems I could think of are more or less solved. It started in that way - one of a rainy weekend we played with my son with some Lego and it turned to be a half-done airplane of mixed color. A few days after my son came up with 'Dad, will it fly?'. I answered "No, of course. It's too heavy and too weak for flying", but this idea literally stuck in my mind so I started to think about it from time to time. I started to evaluate things from different points of view. It was obvious that I should give up using Lego powertrain, there were no match for 0.8 - 1.2 KW required power. Other two main problems were weight and strength. Problem of weight could be moved away with enough power and enough speed. To cope with strength issue I decided to move on with glue. There was also no chance of controlling such a thing with IR stuff, so controls also should be implemented using standard RC 2.4 GHz equipment. This left me with only the airframe made of Lego, but nevertheless I'm pretty happy with it (Purists may stop reading here :))). However, the model could stand on a shelf (and this shelf have to be rather big :) without any glue, operate landing gear and may be even control surfaces using only Lego parts. This means there are no cheats like 'just glue this piece to that'. All connections are at first usual Lego connections, glue just added for stiffness. But to flying. First, I had to implement airfoil using Lego pieces. I did not choose Carsten Swendsen's way of making only wireframe with Technic axles and using some kind of monokote to cover it and instead decided to use System parts to make the wings and fuse. I managed to implement Kline-Fogelman airfoil using 93606 and some plates with stud inversion. May be this is not the best airfoil, but I think it weighs less than, for instance, flat bottom airfoil with some curved slopes on the trailing edge. Wings implemented that way have many space inside and could accomodate LiPos and motor controller. My first version of wing have following layout: Speed controller on the root part of wing, then nacelle with landing gear, doors and retraction mechanism, then batteries in configuration of 2 cells one above another and one another cell next to them. My chosen configuration was 2700 KV 590W motor and 3x2200 mAh batteries in each wing. (With covering removed) (Speed controller inside wing root) Landing gear was implemented using single 9.5L shock absorber with double springs and spacer made from modified 4624 wheel hub. Single 9.5L shock with extra hard spring compresses fully at 1.2kg and I had no chances making the plane lighter then 4 - 4.5 kg and any design with some kind of levers would weigh more and take more space. (And a bit boring video:) My evaluations showed that take-off speed with flaps deployed will be about 40 kph, so landing gear must be able to sustain this speed for at least 30 seconds. There were some doubts how much load could be handled by Technic axle rotating in shock socket at speed of 7000 RPM. I did some experiments with it. Now I know that axle will be damaged if used without grease handling load of 1 kg at ~ 5000 RPM, but could live with this load for 30-35 seconds if greased. This made me move away from that simple landing gear design, it was also rather weak and any non-ideal (or may be even ideal?) landing could break the strut or move it away from extended position - the locking was not good. (I'll post pictures of damaged axle and video of experiment a bit later) At the same time I was thinking of other ways to reduce weight and came up with idea to reduce weight by cutting flight time. I was targeting for 10 minutes before, but now I think that even 3-4 minutes will be enough to prove the idea of flying Lego airframe :) I found another motor/prop/battery configuration and combined it with new landing gear with 4 wheels per strut and more reliable locking. New layout is using 4x950 mAh batteries, 1200 KV 530 W motor and smaller speed controller and potentially could fit in root part of the wing at whole. However, root part have to be lengthened. This is going to enlarge wing area (which is good) but also probably will load root spars more (which is not so good). On the other hand, heavy batteries are moved from outer wing section into root section (and their weight is reduced; with lighter ESC saving is 90 g per wing) First version of wing also featured front spar for almost all wing length. In second version I decided to end front spar soon after engine nacelle. After gluing wing v1 it was obvious, that front spar of that length is not needed - wing is rigid enough. Also, when making wing v1 I had some mess with covering root section - it was 10st length and there is no 10x8 tile to cover it. Wing v2 have root part of 18st and could be covered easier. (Unfinished nacelle v2) To make control surfaces I used micro-motor directly connected to axle, holding the surface (except of flaps - flaps are connected by worm gear. On ailerons other side of surface is attached to potentiometer, connected to PCB of standard RC servo. Micro-motor is connected to PCB via H-bridge. RC servo operates at 5 V and this is very low voltage for Lego motor. With help of H-bridge, micro-motor is being fed by full battery voltage (11.1V or 14.4V) and moves with reasonable speed. For rudder, elevator and flaps I will have to add some gears to connect potentiometer. I hope the play in gears could be compared to play in servo-to-surface linkage of traditional RC setup. I started some mock-ups of fuselage about the same time as wing v1. Seems like I can fit all the required equipment (air compressor, front landing gear, flap mechanism, pneumo-switches) under the floor (slightly raised in nose section) and some other things (RC receiver, air tank) in aft section. This makes realistic look for cabin, it can be fitted with seats, crew and passengers :) Fuselage, however, is a quite ugly being made from System pieces. With one wing attached: Some amusing details :) Now I'm waiting for new powertrain and some missing pieces for nacelles. After that, if this layout proves doable and reasonable, I'm going to glue both wings, finish fuselage and rear control surfaces and attach wings so CG is in the right place. I think I'll be almost done at that point. However, there are some minor unsolved problems. Design of front landing gear and bay doors make it impossible to close doors when landing gear is down like in main struts. On the other hand, doors must open before extending gear and close after. And there is no place for third pneumo-switch. My idea is to make some kind of throttle to create delay in pneumo-cylinder operation. Also, there is no place for mechanical sequencer, so I'm going to make some simple microcontroller stuff for sequencing door-gear operations. I don't see anything hard here. And yes, there is still a bigger problem - will the whole thing work as expected? :) Well, at least it will be fun to watch how it crash. Sure I should have started with some simpler design - no retractable landing gears, some simple fuselage, something like WWI era airplanes, but my current design is so much fun to build, so I probably revert to simpler construction only after I done with current one. Hope you enjoyed this crazy thing.
