SkyTank Heavy Battlefield Helicopter WIP Stage 2 . The first Lego Technic helicopter with fully steerable, electric driven coaxial rotor in scale 1:10 . *Visit my Lego helicopters blog also
SkyTank Heavy Battlefield Helicopter in scale 1:10 - Work in progress, Stage 2
We can see large mass of VTOL (Vertical Take Off Landing) aircraft MOCs inspired by fantasy games and movies raping very basic principles of physics: pizza-sized rotors lifting oversized crafts (e.g.: Helicarrier in the movie Avengers), highly unbalanced mass distribution of craft (Batman stuff, Halo), etc.
Unfortunately, realistic helicopters and tilt rotor aircrafts are rarely attempted in Lego with detailed mechanics and working controls (e.g. with both cyclic and collective control of rotor blades) because of their high complexity.
To go against these trends, hereby we represent SkyTank Heavy Battlefield Helicopter in scale 1:10 made of approx. 15800 bricks with the following features:
Figure 1: Left forward view (right half of cockpit is not finished yet). At the nose you can see electrically operated forward gun turret, rotating glass dome for IR cameras, extendable refueling probe (white), Pitot-tube, electrically operated windscreen wipers, rearview mirror. Underbelly, there is the left nose landing gear (retractable, electrically operated). Behind cockpit canopy, there is the cooling radiator of lubricant of main rotor reduction gearing. On the left stub wing, there are 24 FFAR rockets in LAU-61 cylindrical launch pod, 4 Hellfire-, 1 Sidewinder- and 1 AMRAAM missiles.
2. List of working functions:
Function 2.1. First ever Lego Technic heli with fully steerable, electric driven, 8-blade coaxial main rotor. It uses Sikorsky’s ABC (Advanced Blade Concept) type (see: Sikorsky X-2) high-speed rotor.
Figure 2: Overview of 8-blade, ABC-type coaxial main rotor assembly with engines. Decal covers of rotor blades are omitted to show their internal structure. At the top, there is a positionable laser rangefinder. On the static armoured collar between the two rotors, the white radome is for long range radar. Also you can see the 2 PF XL sized motors driving the rotor and the forward 3 PF M sized servo motors for rotor control.
Function 2.2. First ever Lego Technic heli with 6-channel fly-by-wire rotor controls + 4-channel active rotor suspension, controlled by Lego Mindstorms NXT computer, processing signals of dual controls at cockpit.
Figure 3: Internal view of cockpit (not finished yet). Lego Mindstorms NXT computer at the top. Below that are 2 PF batteries inside forward fuel tank. In the nose, above the gun turret, there are 4 horizontally aligned Mindstorms pushbuttons of fly-by-wire signal generator of dual flight controls. Pushbuttons between gun turret and magazine are triggers of electrically operated guns.
Function 2.3. First ever Lego Technic heli with electrically operated forward/rear gun turrets and electric/spring driven 50.8mm (2in) 6-barrel rotary guns. They shoot belt-feed projectiles of ‘Cross axle 3M’ parts.
Figure 4: Overview of electrically operated, 6-barrel 50.8mm (2in) rotary gun turret. At its right side, there is ammo belt inlet with projectiles (red) and short range gun aiming radar (white radome). Light gray transmission shaft at the back is for training control, dark gray one for laying control, the red pullrod releases the gun turret from its cradle for maintenance/ emergency. At the top of the barrels you can see 4 recoil springs from part ‘Shock absorber hard’ driving bolt forward. On the other side of the gun there is a PF M sized electric motor, which rotates a hook through chain and worm gear drive. The hook pulls backward the bolt against the force of recoil springs, then releases it to shoot. The dark gray box behind the barrels is the laser rangefinder. Barrels are rotated electrically.
Function 2.4. Pointblank gun aiming provided by R.J.McNamara-type laser rangefinder in Mindstorms color detector casing built into gun turrets
Function 2.5. Elastic, disintegrating ammo belts are made of 'Rubber damper 2×2×1M' parts, and extracted from underbelly armored rotary magazines electrically.
Figure 5: Underbelly magazine of 50.8mm (2in) projectiles from upward view. Elastic ammo belt is spooled on the rotating reel in the middle and pulled out by electrically driven belt extractor gears. The magazine is armored resisting hard external hits, but it can fall apart easily by internal magazine explosion, to prevent break it through the armor of cockpit floor. Magazine is locked to that by the 4 red clips, and it can be dropped releasing fixing hooks in cockpit in emergency or reloading. Magazine capacity: 64 rounds.
Function 2.6. Side-by-side cockpit with dual controls, spring driven ejector seats, lockable armored doors is loosely based on Russian Kamov Ka-52 helicopter.
Function 2.7. Four variable pitch, 6-blade turbofan/turboshaft combo engines with rotating particle separator, gas generator, power turbines, afterburner, variable cross section nozzles, and pitch controls connected into cockpit.
Figure 6: Top cutaway view of outer left turbofan/turboshaft combo engine. 6-blade variable pitch fan provides forward thrust if set to positive pitch/ or breaking if it is set to negative by pitch control arm (third axle at the top). At small pitch, engine can provide high power output towards main rotor by its transmission shaft (second axle at the top). The white disc in the middle is centrifugal particle separator preventing stones and dust to enter into compressor. The engine has afterburner and variable cross section nozzle to provide extra forward thrust in emergency.
Function 2.8. Pitchable elevator surface of T-tailplane connected into cockpit.
Figure 6: Pitchacle T-tail elevator surface above rear gun turret seen from the back side (partially completed).
Function 2.9. Electrically operated retractable dual nose- and main landing gears.
Figure 7: Left main landing gear lowered. The long “leg” of landing gear is necessary because cargo bay is modular, it is not deployed in skycrane mode. So landing gear cannot be fixed to that. Landing gear layout is very similar to the Russian Kamov Ka-26 coaxial cropduster/utility helicopter, except that here landing gears are fully retractable to achieve high speed.
Function 2.10. Positionable observation dome at the top of main rotor with R.J.McNamara-type laser rangefinder. Dome laying/training controls are connected into cockpit.
Function 2.11. Electrically operated windscreen wipers.
Function 2.12. Armored magazines and gun turrets are droppable by cockpit controls in emergency/ for faster reloading and service
Function 2.13. Mission Module Container hanged centrally underbelly including 2 electrically operated bomb bay doors, 2 small side doors and 2 detachable stub wings for externally hanged weapons with 8 hardpoints.
Figure 8: Overview of Mission Module Container with left detachable stub wing (right side unfinished yet). Gears at the roof are the clutch in the drive of bomb bay doors/ personnel rescue winches (the yellow lever can shift drive between them). The whole unit is droppable downward, when fixing hooks locking the 6 latches at the top are released from cockpit. It facilitates reloading bomb bay and stub wings weaponry with one move, deploying new fully loaded unit.
Function 2.14. Two personnel rescue winches at side doors of Mission Module Container, operated electrically, controlled from cockpit or cargo bay.
Function 2.15. Heavy duty skycrane winch located at the bottom of rotor mast, operated electrically, controlled from cockpit (it has a rear window to check hanged cargo during flight).
Function 2.16. Electrically operated rear Cargo Ramp Module, controlled from cockpit.
Figure 9: Cargo Ramp Module (right side unfinished yet). The sloped ramp can be lowered to -30 degrees by a parallelogram-type planar linkage driven by worm gear placed at the rear end. Its drive will be clutched to SkyTank through the hole in the roof.
Function 2.17. Mission Module Container and Cargo Ramp Module are droppable by cockpit control in emergency/ at reloading/ at skycrane duty.
Function 2.18. Eight folding seats in Mission Module Container.
Figure 10: Left wall of cargo bay/ bomb bay/ passenger deck in Mission Module Container. (right bomb bay door is not ready yet). Blue stuff are folding seats and tables for 8 passengers, just like at Russian Mil MI-24 Hindbattlefield helicopter. White stuff are radiators of cabin heating. Red lever is for opening side door. Radiators and seats when folded are perfectly in line with the 1 stud thick side wall. So the 19 studs wide airframe can accommodate cargo with 17 studs width and 16 studs height. Left/right bomb bay doors can be locked to each other to form load bearing cargo bay floor.
Function 2.19. Eight foldable rotor blades with elastic yaw dampers and aerodynamic spar-and-ribs structure covered with decals.
Function 2.20. The 6 main rotor controls, 4 main rotor suspension, 2 guns, 1 skycrane winch have 13 PF M-sized servo motors in total. As Lego PF M-sized and servo motors are quite sizeable (3×3×6 studs or more), there is not enough space in SkyTank to solve further power functions with separate servo motors. Therefore, there is a 14th PF M-sized servo motor under the floor of cockpit, driving a 12-fold distributor gearing, which provides forward/reverse/neutral clutching by levers reachable from pilots seats for further 12 power functions:
- Nose landing gear open/close
- Main landing gear open/close
- Forward turret laying left/right
- Forward turret training up/down
- Forward magazine extract/pull back
- Rear turret laying left/right
- Rear turret training up/down
- Rear magazine extract/pull back
- Bomb bay door/ personnel rescue winches up/down
- Cargo ramp up/down
- Windscreen wipers
- Fly-by-wire steering signal generator
Figure 11: 12-channel distributor gearing at cockpit floor (partially completed)
3. List of non-working features:
Feature 3.1. Externally hanged weapons at stub wings of Mission Module Container: Hellfire, TOW, Sidewinder, AMRAAM missiles and FFAR unguided rockets in LAU61 launcher attachable to 8 hardpoints.
Figure 12: Hellfire missiles
Figure 13: TOW missiles
Figure 14: Sidewinder missile
Figure 15: AMRAAM missile
Figure 16: FFAR unguided rockets and LAU61 launcher
Figure 17: And last but not least, the good old 3-ton Bunker Buster Bombshell, together with a 100-pound Credit Card Buster Bombshell.
Feature 3.2. Internally hanged weapons in bomb bay of Mission Module Container: two 3-ton GBU bunker blaster and two 2-ton incidentary bombs.
Feature 3.3. Extendable refueling probe at nose.
Figure 18: Extendable refueling probe in extended position.
Feature 3.4. Double rotating quartz glass dome for IR-camera system at nose.
Figure 19: IR-camera dome.
Feature 3.5. Two short range radars for gun aiming at gun turrets and two long range radars at rotor mast armored collar.
Feature 3.6. Gyro-stabilized targeting periscope for TOW missiles on cockpit canopy.
Feature 3.7. Pitot-tube, rearview mirrors, UHF/GPS/TACAN/IFF aerials.
Feature 3.8. Main-/side-/overhead instrument panels, and dual Head Up Displays (HUD)
Feature 3.9. Two fully poseable pilot figures in scale 1:10 from Bionicle elements with safety belt and helmet display.
Figure 20: Pilot figure with ejector seat and folding paraglider plus working spring-driven handgun. At pilot figure, we did not use the standard Bionicle ‘Hand 2×3M’ part, as we wanted hands with 5 moveable fingers to fit better to crowded controls in cockpit. Safety belt is made from ‘Motorcycle chain’ parts. The big box behind the head is for the paraglider/parachute. Two cylinder, two-stroke, vertically opposed Rotax-style paraglider-motor is placed at the back of ejector seat, between ejector rockets (two compressed 'motorcycle shock absorber' parts). The red cylinder beneath seat is fuel tank for paraglider engine, the orange cylinder is inflatable life raft, the blue pressure bottle is the emergency oxygen supply.
Figure 20b: Motorized paraglider deployed. Air chambers of paraglider wing are modelled with 'pallisade brick 2×1×1studs, transparent' parts.
Figure 20c: Life raft deployed. It is stored in a cylindrical cannister beneath the ejector seat and inflated from compressed air bottle upon contact with water. It is equipped with radio- and flashlight beacons, ropes, climb-up ladder, working spring-driven signal handgun. Besides main tubes, there are tent tubes to facilitate covering the raft with hood to increase crew survivability at high seas.
Feature 3.10. Armored glass panels with emergency open levers and breakers for ejector seats.
Feature 3.11. Rotor blade detonator charges and cables to clear flight path of ejector seats.
4. Detailed technical description of SkyTank
*This part is technical and for heli builders with at least some experience. If you do not understand how do helicopter controls work, you can find an excellent summary at: www.aviastar.org
**In the forthcoming technical description, functional parts of SkyTank are referenced by numbers which can be found on technical drawings attached
***Parts of SkyTank are color-coded by their function:
- Yellow: Manual handles of working functions
- Gray/Black: static and dynamic parts
- Red: sliding/friction part, weapons trigger, projectiles
- Dark blue: seats
- White: plastic covers or domes
4.1. 8-blade, ABC-type, foldable coaxial main rotor
Coaxial main rotor a of helicopter is one of the most complicated mechanical devices working at high speed, high load, in adverse environment. So their design means enormous engineering challenge. In Lego Technic, it is further complicated with almost perfect lack of coaxial tube-axises and running gears. Also, lack of very small mechanic parts (because of children safety issues) means additional problems creating working AND compact rotor hubs.
4.1.1. Designs, which gave inspiration to SkyTank’s main rotor
The world’s most widely used coaxial helicopter rotor was invented by the excellent Russian engineer Nikolay Ilyich Kamov in 1947. It was used first at his small Ka-8 personal helicopter, and then continuously refined into the highly sophisticated coaxial rotor of Kamov Ka-52 Alligator combat helicopter. Kamov’s rotor is a complex mechanical device performing many control- and automatic stabilization functions, which are performed by electronic tools at other helicopters. You can see an excellent animation about its working here. It proved reliable in the harsh environment of Russian Navy’s North Sea Fleet shipboard use, relatively maintenance free and highly effective in crop-dusting role. But vulnerability of its small mechanic parts were never tested in real air combat until this point.
Figure 21: Kamov’s coaxial rotor hub.
In Lego Technic “helicopter industry”, Zblj attempted first creating working coaxial helicopter rotor in his Ka-50 lookalike design in 2010:
Figure 22: Zblj’s Lego Technic heli with tiltable coaxial rotor, 2010
He efficiently resolved the lack of coaxial tube axis in Lego Technic using series of driving rings and running gears from Lego Technic gear shift:
Figure 23: Zblj’s coaxial rotor tilting mechanism
However, in his solution rotor blades cannot be pitched, and there are no cyclic-, collective- and yaw controls, flapping hinges and yaw dampers. Rotor hubs can be tilted as one monolith unit, which is still very far from real working of coaxial rotors. Moreover, he used not very aerodynamic rotor blades.
4.1.2. Structure of SkyTank’s main rotor hub
Our purpose designing SkyTank was to model coaxial helicopter rotor strictly adhering to real engineering principles and include all working cyclic-, collective-, and yaw controls, yaw dampers: “Whatever you see there is working correctly”. However, we found impossible to build Kamov’s rotor in reasonable compact size because of the many small mechanic parts. Therefore we turned towards modeling Sikorsky’s Advanced Blade Concept (ABC)-type coaxial rotor (see: Sikorsky X-2). It is more simple and robust mechanically than Kamov-rotor, but much more complex in terms of electronic servo controls, and can achieve much higher speed. In ABC-rotor, blades are more rigid and more close to each other than in Kamov-rotor. Ratio of rotor separation:blade length is 1:8 instead of 1:4. This results in more compact rotor hub with less drag. To prevent collision of more close blades, there are no flapping hinges for them, and strong vibrations of main rotor is damped by a complicated active rotor suspension system of computer-controlled servo actuators.
Figure 24: Cutaway overview of main rotor and outer engines
Rotor control is also different: there are no vulnerable conventional swashplates and external control rods. Instead of them, main rotor mast is a tube with an internal cavity formed from four ‘Turntable 7 studs diameter’ parts with 3-studs inner hole stacked on each other (see Figure 24). In the central cavity of rotor mast, there are 2 stator (non-rotating) swashplates, and 6 stator control rods. 3 of them are attached to the upper swashplate in 120 degrees spacing, 3 of them to lower swashplate in the same way. Both swashplates can be tilted and lifted independently from each other in rotor mast cavity by lifting/lowering their attached control rods by servos. All 8 blades can be pitched, and their pitch control arms intrude into rotor mast cavity, catching swashplates, which drive them. So all rotor controls are armored being inside rotor mast cavity. With 2×3 servos, they can perform all steering functions of the Kamov-rotor: Collective, Cyclic roll, Cyclic pitch, Differential collective for yaw control, Differential cyclic for high speed level flight. The price of these advantages are the 1 extra steering servo (6 instead of 5) and 4 extra active rotor suspension servo, and much-much more difficult control software.
Figure 25: Upper hub of main rotor. The 4 “arms” are the yaw damping hinges of rotor blades. Rotor blades are held in place by exploding bolts, which can be triggered by 4 black detonator cables to clear flight pathway of ejector seats in emergency. The red parts are pitch control arms of rotor blades. From these, thin ‘knife’ parts intrude into the gap between double discs of upper swashplate in the middle. The swashplate does not rotate, but can be lifted/tilted by 3 control rods not shown here. The other 3 control rods attached to lower swashplate can slide freely in the holes of upper swashplate, as the two swashplates can be controlled independently by 3-3 servos.
Figure 26: Armored middle collar of main rotor connecting the upper- and lower rotor hubs by its upper- and lower turntable. The collar itself does not rotate, that’s why forward/rear long range target acquisition radars (white domes) can be placed on that. At its left/right sides, you can see the direction reverser transmission gearing, transmitting drive from lower rotor hub to upper rotor hub. The 6 control rods can slide freely up/down in the holes of central driving disc of the middle collar, and they prevent the collar to rotate together with rotor hubs, to asses correct transmission.
Figure 27: Lower rotor hub works exactly the same way as the upper rotor hub, just it rotates in reversed direction.
4.1.3. Servo controls of main rotor
Figure 28: Internal mechanics of main rotor.
Servo control of the 6 main rotor control rods are solved with 6 PF M-sized motor. Between the motors and control rods there are 6 units of 1:4096 reduction gearings made of 4 consecutive pairs of 'worm gear' and 'Z8 gear'. It means that 11.37 rotations of PF M motor lifts the grey arm attached to rotor control rod one degree, enabling highly precise rotor control.
Figure 29: Compact, quadruple worm gear set of main rotor servos with 1:4096 reduction ratio
4.1.4. Rotor blades
Figure 30: Internal structure of a rotor blade.
Rotor blades have spar-and-ribs structure with 67 studs length and 7 studs width. Black parts at the blade root are elastic yaw dampers made of 'Rubber damper 2×2×1M' part. Blades are foldable 45 degrees manually disengaging yaw dampers (4 blades forward to rotor mast 4 blades backward). This enables compact shipboard storage of SkyTank. Blades are covered with decals to create light but aerodynamic surface for them. Plastic foil decals are parts of many Lego sets so they can be considered as regular Lego components. Blades have symmetric cross-section, as ABC-rotor air may flow reversed at retreating side blades at high level speeds.
4.1.5. Main rotor’s power transmission
Figure 31: Upper cutaway view of outer left turbofan/turboshaft combo engine
When 2 PF XL electric motors drive the main rotor, 4 engines are driven back through their power output shaft by main rotor, so their internal parts are moving, creating quite a realistic effect. The 2 outer engines combine features of large turbofan engines of commercial airliners (mainly providing thrust by turbine-driven fan) and the high rate shaft output of helicopter engines. That’s why they have 6-blade variable pitch fan (conventional turbofans usually have fixed pitch blades). When SkyTank is hovering, fan pitch is set zero or very low positive, and most of the power output is sent to main rotor through output shaft. Main rotor pitch is high positive in this case. Conversely, at very high level speed, fan pitch is set high positive, providing thrust. Only small output is sent to main rotor, just keep it rotating, to let centrifugal force to stretch rotor blades, which are set to minimal positive pitch. Moreover, if level speed exceeds angular speed of blade tips, at retreating side-blades negative pitch is set to generate lift. The two variable pitch fans also help to eliminate the usual vertical control surface necessary for coaxial helicopters: when they are in autorotation crash-landing mode with dead engines, the conventional method of their yaw control (setting different collective pitch on upper- and lower rotors) is not working very well, and pretty dangerous eating up vital rotor inertia. Therefore, their yaw control is solved by airplane-like vertical fins. But they are the most vulnerable parts of coaxial helicopters. Here we have left and right variable pitch fans under protective cover, providing differential thrust yaw control. It is used not only at autorotation crash-landing, but during dogfighting, when sharp vectored thrust turns, and reverse thrust braking (both fans are set to negative pitch) are vital. Because of similar reasons, the 2 outer engines have afterburners and variable cross-section nozzles also, just like at jet fighters.
4.1.5. Main rotor’s active suspension system
Figure 32: Active suspension servos of main rotor (partially completed).
As ABC-type rotor uses more rigid blades than Kamov’s because of the smaller clearance between the two rotors. Therefore shock waves and vibrations generated by airframe and blades disturbing each other’s downwash cannot be absorbed by flapping hinges. Strong vibrations would quickly destroy rotor and airframe structure. This issue is resolved by 4 servos in main rotors active suspension. They generate vibrations in just the opposite phase than ones generated by main rotor, to mutually extinguish each other. In each unit a PF M-sized motor drives a crankshaft and pushrod connected to a hinge, which transforms rotating motion to alternating motion. Arm of force of the driving hinge is 3 times longer than the hinge connected to main rotor frame, so the servo units lift/depress main rotor frame cyclically up/down 1/6 studs. Correct phase control to kill main rotors vibrations is achieved by Lego Mindstorms NXT board computer controling 4 suspension servos via IR connection.
4.1.6. Main rotor’s observation dome
Figure 33: Actuator of observation dome of main rotor.
In most battlefield helicopters, there is a layable/trainable observation dome at the top of main rotor, enabling helicopter to track targets when hiding behind some cover. In the reality its positioning is solved by small servos, but at SkyTank, we clearly had no space for servos at the top of the rotor. Instead of that we will use the central hole of swashplate/ control rod driving discs, where an ‘Outer cable 16M’ part runs from the bottom of the rotor to the top. In the observation dome, it is fixed to the training/ laying hinge. At the bottom of main rotor, there is an actuator device, which can rotate the cable for laying of the dome and lift/lower it for training of the dome. As swashplate- and driving discs do not rotate, friction of the cable is not meaningful.