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Light Attack Compound Helicopter
With all working 8-blade foldable main rotor, 6-blade ducted fan tail rotor, ailerons, elevator, turboshafts, 8 channel cockpit controls, electric drive, RC-control, all shooting electric Gatlin-gun, Sidewinder, AMRAAM, TOW, Hellfire missiles, FFAR rockets, Bionicle pilot figure, working ejector seat, in scale 1:10
About this creation

Figure 1: Light Attack Compound Helicopter banking left up
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1 Introduction and inspiration

Our Light Attack Compound Helicopter (LACH) model is a piece of concept art, but strictly tied to real engineering principles, trying to modernize the idea of Compound Helicopters: combining rotors/ wings/propellers of helicopters and rigid-wing aircrafts to achieve higher level speed, avoiding the phenomena of “retreating blade stall” of classic helicopters. (See history, advantages and disadvantages of Compound Helicopters: here). Our model is also a modernized, low-cost remake of Hughes AH-56A Cheyenne Compound Helicopter of Vietnam era:


Figure 3: Hughes AH-56A Cheyenne Compound Helicopter

AH-56 was a highly complex and deadly expensive war machine well ahead of its time. It resulted in lot of teething problems and delays until most of its systems became obsolete. Then the whole project was cancelled in favor of more simple and cheap Bell AH-1 Cobra, but many of its elements were used in AH-64 Apache later.


Figure 3: LACH in tight combat turn
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In Light Attack Compound Helicopter (LACH), we experimented and share with MOCers community range of new Lego designing/building techniques. These innovations were inspired by work of two excellent MOCers:
- “Almost like real” jet aircraft airframes of Justin Davies
- Mind blasting mechanics of Sheepo’s car models
My purpose was to recreate both in single model in scale 1:10, following my “whatever you see, is working correctly” principle:

List of RC-controlled power functions:
1.Bell-type, semi-rigid, foldable, 8-blade main rotor with pitchable rotor blades (first ever from Lego) and 2-stage reduction gearing
2.6-blade, variable pitch, fenestron (ducted fan) tail rotor,
3.Turboshaft engines with moving 2 stage compressors, gas generator- and power turbines, with electric drive of 1 PF M-motor
4.Shooting, 6-barrel, 3.2mm (real caliber 32mm), belt-feed rotary (Gatling) gun with electric drive of 1 PF M-motor (first ever from Lego)

List of manual controlled power functions:
5.Motorized ammo drum containing 186 32mm shells in disintegrating elastic belt, with manual controlled direction shift winding drive (first ever from Lego)
6.R.J.McNamara-type laser rangefinder of rotary gun built in Mindstorms color sensor housing

List of manual functions:
7.Collective and cyclic blade pitch control of main rotor with swashplate, input mixing linkage, connected with yoke and collective lever in cockpit,
8.Yaw control of tail rotor connected to yaw control pedals in cockpit
9.Aileron controls connected to cockpit
10.Elevator control connected to cockpit
11.Shooting, spring driven, rail launched AIM-9L Sidewinder missiles with wingtip launcher
12.Shooting, spring driven, rail launched AIM-120 AMRAAM missiles with twin launcher,
13.Shooting, spring driven, rail launched AGM-114 Hellfire missiles with quadruple launcher,
14.Shooting, spring driven, tube launched BGM-71 TOW missiles with quadruple launcher,
15.Shooting, spring driven, FFAR unguided rockets with LAU-61 16-round launcher tube,
16.Spring driven ejector seat with canopy breaker

List of new building features:
17.Double SNOT, 1 stud thick, detachable, swept back wings with moving flaps and ailerons
18.Double SNOT, 1 stud thick, foldable rotor blades
19.Thin-walled “Technic spars + System wall elements as fairing” composite airframe
20.Bionicle-Technic pilot figure with 5-finger palms
21.Aircrew rescue parachute (auxiliary model) (first ever from Lego)
22.Single seat life raft (auxiliary model)

List of minor features:
23.Lifting/rotating cockpit canopy
24.Extendable refueling boom
25.Rotating dome for IR-camera
26.Moveable front and back radar antennas
27.Lubricant tank and cooling radiator
28.Retractable cockpit ladder
29.Pitot tube
30.Transparent head up display for rotary gun and TOW missiles
31.Helmet display and microphone of pilot
32.Rearview mirrors
33.VHF, UHF, IFF, GPS, SRS, ELT, VOR, LOC, TACAN aerials

2 Action screenshots of LACH


Figure 4: LACH in dogfight with MD-500 Defender
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LACH has the same rotor diameter as MD500 Defender, but has 6 times the firepower, and it can be twice as fast in short dashes, so the result of the encounter is quite predictable. Tracers of 32mm HE shells fired by 6-barrel rotary gun seem to be a continuous stream of fire because of the high rate of firing (6000rpm).


Figure 5: LACH ground attacks M-777 howitzer battery
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Main tactics of LACH is to loiter relatively silently among ground obstacles until target acquired, then jump on it in a high speed dash using its afterburners, breaking through AA defenses like a ground attack fighter.


Figure 6: LACH overview right
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LACH has disproportionally big stub wings and elevator surface for a helicopter, providing 80-90% of the lift in high speed mode. Then, main rotor blades are pitched to almost 0 degrees to avoid “retreating blade stall” phenomena. Of course, large stub wings take their toll from lifting force at hovering. Therefore I chose the more complex 8-blade main rotor, which is normally used at heaviest transport helicopters. An 8-blade rotor generates the same lift force as a 2-blade rotor with double diameter. Relatively short rotor blades have the advantage that trajectories of their tips have lower variance, which reduces induced drag of main rotor at high speed dash. Also this is the reason why blades have no flapping hinges, just yawing hinges, forming semi-rigid rotor.


Figure 7: Front view
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One can see that cockpit of LACH is very similar in concept to the one of F-16 fighter aircraft: pilot sits 30 degrees bent backward, and the resulting low silhouette allows to place something under the cockpit (here the gun pack, instead of air intake of engine). While some extra space is acquired using angled side bulges of airframe to place extra controls/fuel there.


Figure 8: Right side view
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Combination of multi-blade main rotor and fenestron (ducted fan) tail rotor results in relatively silent hovering mode. Against the common belief, most of the noise of conventional helicopters is NOT generated by main rotor itself. Colliding and interfering shock waves of tail- and main rotors, result in the well-known, easily noticeable “flomp-flomp-flomp” noise. A multi-blade main rotor with fenestron has the noise similar to turboshaft engines.


Figure 9: Left side view
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LACH has nothing to do with silence when attacking at high speed with the help of afterburners. Turboshaft engines normally pump through much more air than necessary to combust their fuel. If extra fuel is injected in their jet exhaust, it can generate serious amount of thrust necessary for high speed. Afterburner is a simple, cheap, compact, low-drag device compared to pusher propellers normally used at compound helicopter designs. The biggest disadvantages are enormous fuel consumption and noise. Therefore, LACH is not capable of sustained high speed flight. It can only make short high speed dashes necessary to attack an acquired target and quickly get off. You can observe the interference lines caused by bouncing shock waves in the jet blast of afterburner.


Figure 10: Rear view
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Whatever is damn loud and generates lot of heat - like an afterburner - that is easy to detect and can be an excellent target itself. Therefore – besides Air Interceptor (AI) radar - LACH has rear looking Airborne Early Warning (AEW) radar…


Figure 11: Top view
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… and one more AEW radar at the top of rotor mast.


Figure 12: Bottom view
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While conventional helicopters usually try to reduce their IR-signature mixing hot exhaust gases of turboshaft with cold air, this is pretty hopeless for LACH when using afterburners. So it has 90 jetissonable IR-flares/decoys at its belly side to confuse IR-homing weapons – just like ground attack fighters.


Figure 13: Left belly view
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Besides the 32mm 6-barrel rotary gun in detachable underbelly gun pack, LACH has 6 external hardpoints to carry Anti-Aircraft and Anti-Armor missiles or Unguided rockets for ground attack.


Figure 14: Right belly view
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We present LACH with rather eclectic set of ordnance: it is not realistic to mix up Anti-Aircraft, Anti-Armor and ground attack weapons in single load…


Figure 15: AIM-9L Sidewinder Launch
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...this is just for the show of variety of weapons can be hanged there.


Figure 16: Power dive
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Besides AI radar in the nose radome, LACH has a small rotating turret at the right side of the nose for night vision cameras and laser target designators, covered by yellow-colored IR-transparent quartz-glass dome.


Figure 17: Over sea
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Just behind the cockpit canopy, one can see the air intake of lubricant cooling radiators of main rotor and turboshaft engines. I used the same trick as at WWII Mustang fighters: air warmed by radiators exits at the back of main rotor in small nozzles generating some extra thrust at high speed.

3 Technical details of LACH

*This part is technical and for helicopter 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 LACH are referenced by numbers which can be found on technical drawings attached

***Parts of LACH are color-coded by their function:
- Yellow: Manual handles of working functions, Rotor blade tips
- Gray/Black: Static parts
- White: Dynamic parts
- Blue: Rotor blade folding locks, Ejector seat of pilot
- Red: Ammunition, Triggers of weapons
- Orange: Explosive/Inflating parts, Lubricant


Figure 18: LACH Mechanics Overview
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3.1 Dynamic systems of LACH

3.1.1 Main rotor


Figure 19: Main rotor cutaway view
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The key to create 8-blade rotor hub is a pair of TLG part ‘Parabolic rings’ (see R13 in Figure 19), which have octagonal symmetry. Eight blade yawing pivot shafts (R12) made of ‘Connector peg 3 studs’ are fixed to the rings with 16 ‘Stick with holder’ parts. The latter are the weakest part of the rotor, as all centrifugal force and weight of blades stress them. ‘Beam 3 studs with fork’ (R15) connects blade yawing pivot (R12), blade pitch rod (R11) and blade pitch half axis (R16) into single unit, which can yaw relative to main rotor mast (R14) to compensate vibrations of blades generated by their downwash colliding with airframe and stub wings. The 8 yawing units just touch each other, therefore they loosely align themselves circularly in 45 degrees spacing. When main rotor is rotated, centrifugal force gives blades more exact 45 degrees alignment. Rotor hub is covered by a shield plate ‘Disc 80mm’ (not shown here). At the top of the shield, AEW radar radome is seated on ‘Turntable 2×2 studs’.


Figure 20: Main rotor folded view
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Designing folding blades for 8-blade rotor in Lego is not an easy story, as double SNOT, “spar + fairing” type blades are thicker than real blades, and there are lot of them. One thing that helps is that they are relatively short, so 4 of them can be folded backward and 4 of them forward, as they will not exceed the length of airframe. The basic idea of folding came from blade folding hinges of Sikorsky MH-53J, which I modeled in my earlier MOC Working 6-blade foldable main rotor. I redesigned this to increase rigidity of hinge and degrees of freedom blades can be folded. The core of the solution is a 90 degrees hinge (see R5 and R6) which can rotate 360 degrees freely BOTH on blade pitch half axis (R16) and blade root half axis (R17) it connects. This enables to rotate and tilt blade any degree relative to rotor hub, and even rotate blade around its own longitudinal axis in the meantime. The latter allows laying folded blade close the surface of airframe, saving considerable space. Hinge can be closed by double locking pins (R3). In the reality, both double locking pins and hinge pivots have explosive bolts (R4), to allow quickly detach blades before ejector seat is deployed, avoiding rotating blades colliding with that. The hinge can tolerate centrifugal force pretty well, but it is pretty weak against gravity/lifting force affecting the blade, moreover cannot transmit blade pitch force. Therefore, there are 90 degrees foldable double locking pins (R2) in blade root, which completely bridge the closed hinge, connecting blade root (R17) with blade pitch halfaxis (R16) directly. Blade root locking pins (R2) are secured before hinge locking pins (R3).


Figure 21: Double SNOT main rotor blade with folding hinge
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I built custom SNOT blades from curved fairing elements (R22, R24, R25) and Technic cross axles (R19) and System plates (R23). The price is the enormous weight of the blade: it something similar if you would try to build helicopter rotor blades from concrete blocks in the reality.


Figure 22: Preparing to stow into hangar of littoral gunboat
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To get the big picture, how blade folding does work, we put there Figure 22 above. At the landing deck of a smaller littoral gunboat, deck crew prepares to stow LACH into hangar:
-The yellow guy directs the movement of vehicles on the deck.
-Green guys are repairmen: one of them just removing hinge locking pins at the top of the main rotor, the other folds the blades to airframe and secures them.
-Blue guys move stuff on landing deck under the control of yellow guy: detached stub wings and elevator surface are placed on special carts. Moreover, 4 lifting pulleys are attached to landing skid (all in orange color) to make LACH moveable on deck. (All wheels of the cart and pulleys are steerable and have brakes).
Using this system, LACH with folded blades can be stored in a very compact space, allowing it to be stationed on littoral/landing ships with small hangars, tremendously extending their power projection capacity.

3.1.2 Drivetrain


Figure 23: LACH Drivetrain
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With its deadly heavy 8-blade main rotor, LACH is a seriously under-powered Lego model, just having one weak PF M-motor (E10) in the drivetrain. I could place two more PF XL-motors at the place of turboshaft engines (they could fit there pretty well) to change the situation. Instead of it, I chose to build turboshafts engine models with rotating internal parts (E2-E8), and let the rotors just spin slowly. Making the situation worse, there was no space left in airframe for a separate motor to drive ammunition drum of rotary gun (W12). Therefore a driveshaft goes forward from main rotor reduction gearing, and drives ammo drum through a manually controlled direction shift/clutch (E15) with worm gear (E21). Tail rotor transmission shaft is placed in an armored tunnel (E16), because by wide historic battle experiences, this is the most vulnerable part of a single rotor helicopter.
It is a frequent mistake in TLG or MOC Technic helicopter models that main rotor is geared to tail rotor 1:1 while in the reality it is usually 1:10. Although necessary reduction gearing eats lot of space, we included a compact 2-stage unit for main rotor, therefore gearing ratio of main rotor : tail rotor = 1:7.68 and gearing ratio of main rotor : PF M-motor or turboshaft engines = 1:4.63.


Figure 24: Turboshaft engine model with afterburner
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3.1.3 Collective blade pitch control


Figure 25: Collective main rotor blade pitch control
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Although TLG produces specialized swashplate part (a large diameter bearing with 2 sets of ball joints and Cardan-hinge in its center), it is rather bulky and it has erroneous design, as it cannot slide up/down freely on main rotor axis. Therefore the key of creating any compact working helicopter rotors from Technic is to avoid the need of conventional swashplate. This is done in the following way:
-All 8 rotor blades have individual rubber torsion springs (R7) made of TLG part ‘rubber dumper 2×1×1’ forcing gently their half axises (R16) through blade pitch arm (R8) and its linkage (R9) to zero degree pitch.
-Therefore, changing blade pitch requires simple blade pitch pushrods (R11) connected with hinge (R10) to blade pitch control arms (R8) instead of using ball joint-connected linkage.
-Blade pitch control pushrods can slide up/down aligned by leading slots of Blade pitch/yaw hinge housing (R15), so they are NOT aligned by swashplate.
-Therefore, swashplate (C2) can be just plain plate with a hole in the middle letting through main rotor mast, and being tiltable/ liftable around that. Lower ends of blade pitch pushrods (R11) have sliding shoes (C1) which slide on the plate’s surface.
-Swashplate is aligned by pin+fork assembly (C3), which allows it lift/tilt but prevents its rotation around the main rotor mast (R14).
Collective pitch control is done lifting/lowering swashplate (C2) with the help of collective sleeve (C10) sliding on main rotor mast, held by a long arm, which rotates around pivot (C9). Collective sleeve is lifted against the force of torsion springs (R7) by upper collective swingarm (C11). It is connected with collective lever (C52) in cockpit through upper collective trackrod (C71), middle collective swingarm (C72), sliding block (C73) and lower collective trackrod (C74). Locking arm (C53) helps the pilot locking collective lever in a given position.

3.1.4 Cyclic blade pitch control


Figure 26: Cyclic main rotor blade pitch control
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Cyclic pitch control of main rotor blades requires tilting swashplate (C2) forward/backward or left/right. Swashplate is supported against depressing force of torsion springs (R7) from downward in its center by a sliding ring around main rotor mast, which is at the top of collective sleeve (C10). Therefore, if vertical cyclic control trackrods (C5) lift/lower two of neighbored corners of swashplate (C2) through flexible ball joints (C4), it can be tilted in any direction. (Maintaining only 2 cyclic trackrods - instead of 3 or 4 usual in the reality - helps to save LOT OF space around main rotor mast, but the price is that left/right tilting of swashplate is mechanically less efficient and exact.) Two upper cyclic swingarms (C6) transmit cyclic control inputs coming from upper horizontal cyclic trackrods (C7) towards vertical cyclic trackrods (C5), while rotating mount of swingarms can move up/down with collective control sleeve (C10). This is how cyclic- and collective control is decoupled, forming collective-cyclic mixing linkage. Middle cyclic swingarms (C8), lower cyclic trackrods (C75), forward cyclic 90 degree swingarms (C76), and their coupling bridge (C77) connect yoke (C14) with collective-cyclic mixing linkage.

3.1.5 Yaw control


Figure 27: Tail rotor cutaway view
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One key point to create any fast helicopter is to hide or eliminate tail rotor, which is the most vulnerable, noisy and highest-drag part of any helicopters. But tail rotors like paramours: it takes lot of money and advanced technics to conceal them. I made quite a modeling effort to make compact fenestron (variable pitch ducted fan) tail rotor. TLG part ‘Wedge belt wheel’ has hexagonal symmetry enabling to build 6-blade fan. Pitching of its blades (T1) is solved cutting fan axis into 2 halves:
-The right hand side half axis carries the fan hub (T2) with 6 blade pitching shafts and it is driven by engines through transmission shafts (E16, E18) and several pairs of beveled gears (E17, E20).
-The left hand side half axis (C81) carries 6 blade pitching linkages (C82), and besides rotating with the fan, it can slide left/right, to set positive/negative blade pitch. Sliding is made by yaw control slide (C79) catching driving ring (C80) fixed on half axis (C81).
A variable pitch fan itself worth nothing without strong, compact and streamlined housing. TLG parts ‘Turbine 10 studs diameter’ serve as left/right stator vanes and bearings of tail rotor. Between them, there is a 32-sided polygon tube with 9 studs inner/ 11 studs outer diameter and 4 stud chord, created from ‘Technic lever 2 studs’ and ‘Light sword blade’ parts. Tail rotor house holds the tail plane and the T-type elevator surface at the top of it.


Figure 28: Yaw control
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Tail rotor (T2) is connected with yaw control pedals (C50) trough tail rotor pitch slide (C79), three trackrods (C19, C17, C16) and one direction reverser swingarm (C18) to get correct control layout. Yaw control linkage is a relatively simple mechanism, but it contains a strange bridge-shaped part in the middle. This is necessary to avoid the ammunition drum.

3.1.6 Aileron- and elevator controls


Figure 29: Aileron- and elevator controls
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In rigid wing aircrafts, aileron and elevator control linkages are connected to yoke. But in LACH, cyclic main rotor controls are already connected to that. In real compound helicopters, a highly complex computer software computes the distribution of steering signals from yoke among rigid wing- and helicopter controls considering dozens of flight parameters read from sensors (especially in transition between hovering and high speed flight). This is far beyond the current technical level of Lego Technic, Power Functions, and even Mindstorms, therefore we simply placed rigid wing controls with separate levers in the side of the cockpit, utilizing the cavity of side bulges of airframe. The problem is that it requires so many handed pilot as Indian godness Shiva. There is no rudder surface in the vertical tail plane and it is not connected with yaw control pedals any way. The reason is that tail plane is in the aerodynamic shadow of the main rotor hub, so simple rudder would be ineffective. Therefore yaw control in high speed flight is solved by left/right trimming of afterburner thrusts with the help of afterburner switch (C54) and afterburner trim ring (C55) placed on collective lever (C52).

3.1.7 Extendable refueling boom


Figure 30: Inflight refueling
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As afterburners have tremendous fuel consumption, LACH is equipped with extendable in-flight refueling boom. It is retractable in right airframe bulge, besides right aileron controls. In the reality, airframe side bulges contain main fuel tanks.

3.2 Structural parts of LACH

3.2.1 Thin-walled Technic-System composite airframe


Figure 31: Thin-walled Technic-System composite airframe
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To build high speed craft, we have to have sleek airframe. Building from System bricks purely, it is difficult to achieve, as the thinnest possible wall is 1 stud thick (in case of curved outer surface, it is even more). If we want to squeeze a Bionicle-sized pilot, the bulky PF Battery Pack, 8-stud diameter ammo drum into a 8-stud diameter closed airframe, we need really-really thin walls. Curved ‘Wall elements 4×4×6’ are thin, but they have really bad and weak connectivity. Therefore, we embed them into a Technic frame of longitudinal and cross spars. Another difficulty is that wall elements are multiples of 9.6mm System brick height, while Technic studs are 8mm, therefore they can be matched only in 12 studs (96mm) long repeating modules.

3.2.2 Wing structures


Figure 32: Wing structures
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I was always unsatisfied with TLG angled wing plates, and wanted to create a double SNOT wing structure, which is close to 1 stud thick, can be swept back in various angles, can have moveable flaps and ailerons with perfectly smooth, unfaceted surface, and can contain internal mechanics. It was not very easy story as I had no serious experience building rigid wing aircraft. LACH is a relatively simple compound helicopter, but contains surprisingly lot of rigid wing aircraft stuff: while creating helicopter parts went smoothly, rigid wing parts were pain. Finally I managed to solve the task scarifying wing ribs: there are only spars in the wing and the gap between them is filled with ‘Lego Technic bricks 1×1’. Individually, fillers are weak, but there are lot of them, so they can substitute missing ribs, moreover they can contain internal mechanics. Rotating them 90 degrees, excellent hardpoints for underwing pylons can be formed at various points of wing. One disadvantage of this solution is the bigger weight of wing structure.

3.3 Armament of LACH


Figure 33: Armaments overview
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3.3.1 32mm, 6-barrel, belt-feed, electric driven rotary gun

This gun was inspired by Rochester Air Show in 2002, where I pretended to be a stupid foreign tourist not speaking any English, and climbed through the fence to touch and photograph 30mm 7 barrel rotary cannon of A-10 put on static display in live:


Figure 34: 7-barrel rotary gun of A-10 (Yes, that is my finger there)

In any rotary barrel (Gatling)-system guns, there are separate bolts for each barrel moved by a helical groove in gun's rotor, and they are locked to individual breeches during firing. You can see an excellent animation about its working here:


Figure 35: Rotary gun animation

(Note: rotary gun is erroneously referenced as "chain gun" in most FPS computer games. The real chain gun has one barrel and one bolt controlled by an electrically driven chain running rectangular cycles on 4 sprockets. Rotary gun is also often confused with revolver gun, which has one barrel and bolt, but several rotating chambers. If you do not understand technical details of operating systems of automatic weapons, see the excellent and easy-to understand article of Wikipedia about it.)
It would be very hard to build rotary gun with exact mechanics from Lego Technic in reasonably small scale, as weak point of Technic is lack of rods/tubes/barrels with multiple diameters, which can be built together coaxially, as Gatling-system heavily relies on these types of components.
Moreover, in Lego Technic, we cannot use chemical propellant to launch projectiles, as ABS material would not tolerate high pressure and temperature. We have to simulate firing with mechanical launching projectiles. We can get propellant springs for shooting mechanisms disassembling TLG parts ‘Shock absorber extra hard’. They have 1 stud (8mm) diameter and 2 studs (16mm) length, which can be compressed to 0.5 studs (4mm). LDD cannot draw steel springs, so in the LDD models we replace them with TLG part ‘Corrugated pipe 2 studs’ which has similar shape and size.


Figure 36: Firing cycle of rotary gun
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Operating principle of my rotary gun is very strong simplification of the original one:
-There are six 3.2mm bore barrels built from overlapping ‘Connector pegs 2 studs’ and ‘Technic tubes 2 studs’. Connector pegs are slightly bored up in diameter to allow projectiles ‘3.2mm shaft 3 studs’ run inside without serious friction. This is illegal operation, but connector pegs are not destroyed and they do not lose their original function. Barrels are held by 6-holed ‘Wedge belt wheels’ on a long main shaft made from ‘Technic cross axle 32 studs’. Barrels can be rotated with main shaft.
-There is single bolt driven by 6 consecutive propellant springs placed on bolt leading rod made of ‘Technic cross axle 16 studs’. Driven by force of springs, bolt can press ‘Antenna 8 studs’ through ‘Rubber damper 2×1×1’ ammo belt links, pushing projectile into top barrel and launch it. Bolt has a catch at its rear end.
-An electric driven, rotating hub has 2 hooks placed 180 degrees. Hooks cyclically grab bolt catch, pull back bolt against force of propellant springs, then release it, letting springs to move it forward. Bolt travels back/forward cyclically 8 studs (64mm) during this. One rotation of double hook unit results in 2 cycling of the bolt.
-Rotation of barrels is not really necessary for the firing cycle described above. It also does not make any sense, as there are no cartridge cases to eject, and barrels do not get hot and don’t have to be cooled during firing. Therefore, barrel rotation is purely just for the show.


Figure 37: Basic mechanics of rotary gun
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Most of the gearing necessary to serve the firing cycle are placed at the back of the gun, in well protected housing. Shaft (W614) is driven by PF M-motor. From this, drive goes in 3 different directions:
-Through Z16 gears (W613, W612, W605) and pair of Z8 gears (W604), a worm gear (W602) drives Z24 gear (W603) of rotating hooks with 1:24 gearing ratio. Then rotating hook alternates bolt with 1:12 gearing ratio compared to PF M-motor.
-Through Z16 gears (W613, W612, W605), contra-rotating pair of Z8 belt receiver gears are driven with 1:1 gearing ratio.
-Through Z24 gear (W615) and pair of beveled gears (W616), a worm gear (W617) drives Z24 gear (W618) of barrels main shaft (W7) with gearing ratio 1:72.


Figure 38: Breakup of dynamic parts of rotary gun
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This way gearing ratio of bolt : barrels main shaft = 1:12 / 1:72 = 6:1. As we have 6 barrels, this gearing ratio just gives the correct timing of barrel rotation to the movement of bolt. Barrels try to rotate continuously even when bolt is in the top barrel, and locks its position. But main shaft is enough long for that its torsional flexibility compensates the temporal locking of barrels.
Belt receiver gears (W9) spin much more (1:1) than necessary in average for continuous movement of the belt (1:48). This is because belt has to suddenly “jump fast” when bolt fully retreats, quickly advancing next projectile in line with bolt. There is an aligner pin, which stops next projectile in correct position, and when belt is stopped, belt receiver gears (W9) continuously slide on rubber belt links (W10) with moderate friction. This holds next projectile in its place until it is launched.


Figure 39: Recoiling chassis of rotary gun
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Recoil of a 32mm rotary gun firing 6000 rounds/ min in the reality would rip apart such a light craft as LACH without recoil dumper mechanism. Therefore the gun – altogether with its mechanics enveloped by the strongest possible chassis – can slide back 1 stud (8mm) in a cradle, which is fixed to the belly of airframe. Recoil is dumped by 2 recoil springs (W601) disassembled from TLG part ‘Shock absorber extra hard’.


Figure 40: Rotary gun overview
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The gun with its cradle looks like an underbelly portable gun pack at the first sight. However, because of lack of space, landing skids are also mounted to the cradle, and it plays important role in the longitudinal rigidity of airframe. Airframe’s internal longitudinal spars cannot go through the huge ammo drum, which is bridged structurally by the help of gun cradle. Although the gun pack can be detached by 4 locking ears when LACH is lifted by crane, it cannot fly without that.
We can see a frequent mistake in concept art VTOL/ future helicopter MOCs, that an overly heavy gun is placed at the nose of the vehicle. This moves Center of Gravity (COG) well forward of lifting engines, which makes physically impossible to hover. Against it, we did take care about that ammo drum and gun pack is exactly in the COG of LACH, therefore consumption of ammo and recoil of heavy gun should not seriously influence stability of the craft.


Figure 41: Rotary gun and its ammunition drum

Rotary ammo drum is used when large amount of ammo has to be stored in a relatively narrow airframe, loaded very quickly, and spent cartridge cases cannot be ejected outside airframe because of the danger of jet engines sucking them. Real ammo drum has a stator cylinder unit with radially aligned shallow lamellas, and an electric/ or oil turbine driven rotor unit with helically formed plate. The rotating helix presses forward cartridges among lamellas.
It is not possible to model this structure directly from Lego in reasonably small scale, as we have no strong and thin plates there, especially helically formed ones. Therefore, I simplified ammo drum as a large, 8 studs (64mm) diameter, 14 studs (112mm) long motorized spool (W12), which can wind up elastic ammo belt. Tip of projectiles in drum points towards its 2 studs (16mm) thick central shaft. Although the elastic belt has the tendency to unwind itself from spool when left unattended, but it is not enough fast for the rotary gun. Therefore engines can drive the spool through a direction reverser/clutch (E15) manually controlled by lever (C33). Belt is released from spool among 4 aligner rollers, then forms a loop before entering belt receiver gears of gun. The loop in belt between ammo drum and the gun is necessary to compensate that gun advances belt in jumps instead of continuous move, and gun has a recoil movement.


Figure 42: Rotary gun emplacement in LACH
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Aiming of the gun in the reality happens with Head Up Display (HUD) or helmet display. We could model them in scale 1:10 only as non-working features. To enable some limited targeting of gun at pointblank range, we fixed a laser target designator on the gun: R.J.McNamara manufactures laser range finder built in Lego Mindstorms ‘Light Sensor’ part.
PF M-motor of the gun and laser rangefinder are remote controlled by “blue” channel of PF ‘IR Receiver’ (E12). Moreover, we placed a Mindstorms ‘Pushbutton’ (C39) beneath instrument panel in cockpit to enable pilot manually fire the gun by hand with trigger lever (C38) or by any leg directly pressing the pushbutton.
Besides the rotary gun, I designed shooting models of several US airborne missiles and rockets for LACH. Missile shells are generally modeled with ‘Rounded brick 2×2×1 studs’. In scale 1:10 it gives the diameter of 160mm (6.3”). Considering 125mm (5”) diameter of Sidewinder, 152mm (6”) of TOW, 178mm (7”) of Hellfire and AMRAAM, it seems to be a good compromise.

3.3.2 AIM-9L Sidewinder Missiles


Figure 43: AIM-9L Sidewinder
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AIM-9 Sidewinder, is the most successful, but still cheapest Infrared-Homing, Short-Range, Air-to-Air missile used by 27 nations, with more than 110,000 units produced and 270 kills up to date.
Our shooting AIM-9L model is propelled by 4 propellant spring from TLG part ‘Shock absorber extra hard’ and launched from a wingtip-mounted launcher rail (W115) made from ‘Cross axle 16 studs’. Missile is fixed with sliding ears (W109) on the rail, which has trigger arm (W116) locking missile and propellant springs compressed. Also it has aligner pin (W117) to align stabilizer surfaces of missile to prevent them colliding with launcher during launch. Missile has a “spinal cord” (W114) made from ‘Outer cable 144mm’ to prevent it falling apart by the shock of launch.

3.3.3 AIM120 AMRAAM Missiles


Figure 44: AIM120 AMRAAM
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One thing that AIM-120 AMRAAM semi-active/active radar-guided Advanced Medium-Range Air-to-Air Missile can destroy with 100% efficiency is taxpayers' money. One unit of AIM-120D costs the friendly amount of $1,470,000. It is more than most of the possible targets in third world countries air forces will cost... This missile is too good compared to potential enemies.
Our shooting AMRAAM model is pretty similar to Sidewinder in its structure, except that it has twin launcher mounted on underwing pylon.

3.3.4 AGM-114 Hellfire Missiles


Figure 45: AGM-114 Hellfire
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AGM-114 Hellfire is a Fire-and-Forget Air-to-Surface Missile with semi-active laser homing produced by Lockheed Martin. Its original purpose was Anti-Armor, but in recent War of Drones in Iraq and Afghanistan they are mostly used against commanders of rebel forces. We model them with M229 quadruple launcher pod. The shooting mechanism is pretty much the same as at Sidewinder. The difference is that short steering/stabilizer surfaces of Hellfire are modeled with TLG parts ‘Wall element 1×2×1’.

3.3.5 BGM-71 TOW Missiles


Figure 46: BGM-71 TOW
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TOW stands for Tube-Launched, Optical-Targeted, Wire-Guided Anti-Armor Missile developed by Hughes Aircraft in 1971. The basic TOW is rather outdated weapon, but building that in shooting version was a big modeling challenge. Its launch tube requires 3 studs (24mm) diameter tube in scale 1:10. TLG has 2 similar parts ‘Engine 3×3×6’ and ‘Tube end 4.85’ but both tubes are closed to prevent kids to make shooting weapons from that. Therefore, we rolled up 8 ‘Lamella for garage door’ parts circularly to form a 3 stud diameter launch tube (W402). As this type of tube is pretty weak, we built a strong girder (W401) outside of it to provide necessary structural rigidity as part of quadruple launcher. 2 twin breaches (W404) of launch tubes are fixed to rear end of girder, and they are flappable up/down 45 degrees. When they are flipped up, a self-contained propellant unit (W405-W409) can be loaded from the back, the missile (W411-W415) with tube cover hood (W410) - fixed gently at is nose tip - can be loaded from front, and missile + propellant assembly can be gently pressed backward into flipped down breach, locking that. Breaking up trigger pin (W408) it disengages flying spigot type bolt (W409) made from ‘Standard 53mm’, releasing 4 compressed propellant springs (W406) made from ’Shock absorber extra hard’. Spigot flies out together with the missile. (Spigot type shooting mechanism is used in the reality in some special types of mortar shells, e.g. WWII ASW mortar “Hedgehog”). The missile has side nozzle rocket motors in its middle, while at its back there is a chamber storing wound-up control wire (modeled with some thin threads). End of the wire is fixed to launcher, so when missile flies out, it releases control wire, just like in the reality. As the front hood (W410) is fixed only gently at the missiles nose tip, it is broken off by hood breaker horns (W403) at launch. Targeting of TOWs happens through Head Up Display (HUD) (N4) and controlled by a small joystick (C60) placed on the top of yoke (C14).

3.3.6 FFAR Mk4 Unguided Rockets with LAU-61 Launcher


Figure 47: FFAR Mk4 Unguided Rockets with LAU-61 Launcher
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FFAR stands for Folding-Fin Aerial Rocket, a Spin-Stabilized, Solid-Fuel device with AP or HE warhead launched in volleys from cluster of launch tubes. These rockets have 70mm diameter (2.75”), while 1 stud in scale 1:10 is 80mm, which is pretty close.
My shooting FFARs use diminutive of the flying spigot type mechanism we used at TOW. The LAU-61 tubular launcher pod itself was more modeling challenge. It has backward sliding breach, holding the 16 firing pins of FFAR rockets, which hold rear end of their spigots.

3.4 Instruments and avionics of LACH


Figure 48: Cockpit left side view
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I created a highly poseable Bionicle-Technic composite pilot figure with 5 finger palms and the following dimensions:
-Height: 24.00 studs / 192.00 mm / 7.56 in, Real size: 1.92 m / 6 ft 3.54 in
-Shoulder: 7.00 studs / 56.00 mm / 2.20 in, Real size: 0.56 m / 1 ft 10.04 in
-Hip: 5.00 studs / 40.00 mm / 1.57 in, Real size: 0.40 m / 1 ft 3.74 in
-Thickness at waist: 2.50 studs / 20.00 mm / 0.79 in, Real size: 0.20 m / 0 ft 7.87 in


Figure 49: Cockpit canopy view
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My purpose was to create the most realistic cockpit possible with eight and half working control channels:


Figure 50: Cockpit interior view
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-Collective lever (C52)
-Yoke with cyclic pitch left/right + forward/back (C14)
-Yaw control pedal (C50)
-Left/ right ailerons (C51, C27)
-Elevator (C22)
-Ammo drum wind/unwind (C23)
-Trigger of rotary gun (C38)

They had to be squeezed in 8 studs (64mm) outer diameter cockpit where there are only 0.5-0.8mm gaps between cockpit walls and shoulders/ hips/ helmet of pilot. The key for the success was thin walled airframe technique used and usage of side bulges of airframe to place rigid wing aircraft controls. The instrument panel has the most important flight instruments, 2 multi-function display, radar display, HUD (C54-C70) as non-working features. There are 3 rearview mirrors (N5) at the frame of windshield.

3.5 Accrual Systems of LACH

3.5.1 Working ejector seat


Figure 51: Cockpit cutaway view
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All military aircraft worth as much as it can save its crew from emergency, as new helicopter can be manufactured in couple of weeks, but it takes 21-23 years to manufacture new pilot. LACH has spring driven ejector seat by TLG part ‘Assembly shock absorber’ (H5). It sits compressed on belly spar (S3). At the top of it, there is a catch (H7) restrained by pair of ‘Technic lever 7×1’ (H6), which have rotating mount (H9) at their bottom part. If restrains are pushed left/right aside by pilot with pair of ejector levers (H8), they release propellant spring, and seat (H2) lifts upward on a central launching rail made from ‘Cross axle 5 studs’ (H10). As there was no space left for safety belts because of the numerous controls, back of pilot’s torso is fixed to belt with locking pins (H11). Canopy breaker horns (H3) placed on head rest (H12) break off canopy to clear launching path. There are parachute and life raft packs (H4) attached to ejector seat.
Canopy at normal opening can be rotated some degrees upward/back by hinge (S24), just to release its locking studs on windscreen frame. Then canopy can be lifted 4 studs (32mm) by a vertical shaft (S27) sliding in elastic mount (S26) made from ‘Rubber damper 2×1×1’. Then lifted canopy can be rotated around shaft (S27) 90 degrees left or right to clear opening of cockpit. This solution was necessary because we have very limited clearance between canopy and rotor blades.


Figure 52: Ejector seat deployed
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All helicopters equipped by ejector seat require quickly detachable main rotor blades. We have 4 explosive bolts (R4) in each blade folding hinge as non-working features, to clear flight pathway of ejector seat. At Figure 52, we can see LACH hit by a Surface-Air-Missile (SAM) and its tail all but annihilated. Breakup of tail rotor transmission shaft auto-initiates ejector seat deployment, before LACH starts to rotate uncontrollably and burning jet fuel reaches 32mm shells in ammo drum. 8 rotor blades are exploded first, letting centrifugal force to clear them. Backward flying blades “decapitate” T-tail plane and elevator surface to prevent it hitting the pilot later. Then canopy is broken off by canopy breakers, and ejector seat can fly off with pilot on clear path.

3.5.2 Auxiliary model: Aircrew Rescue Parachute


Figure 53: Aircrew Rescue Parachute
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3.5.3 Auxiliary model: Aircrew Single Seat Life Raft


Figure 54: Aircrew Single Seat Life Raft
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3.6 Maintenance of LACH


Figure 55: Reloading and refueling
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Figure 55 shows refueling and reloading LACH on the landing deck of a small littoral gunboat:
-Purple guy refills fuel through extendable refueling boom.
-White guy checks life support and accrual systems of pilot. He stands on a retractable cockpit climbing ladder fixed under the left hand side airframe bulge. One can observe the opened position of cockpit canopy: it is rotated upward a little bit in a leveled position, then lifted, then rotated 90 degrees right. When it is opened, it almost touches main rotor blades.
-Red guys reload ordnance: ammo belt is simply wound up into ammo drum with the help of external handle of ammo drum drive clutch (C35). Two ordnance guys just lift an AIM-9L Sidewinder onto right wingtip mounted launching rail. Two AMRAAMs and their propellant springs are still on the orange colored ordnance cart.


Building instructions
Download building instructions (LEGO Digital Designer)

Comments

 I made it 
  January 27, 2018
Quoting hydra boss this is great. I´d like to build it but I´m not sure if some pieces will fell of and won´t stay at the places I´ll put them. Can you please say me, if there are any falling pieces?
Never tested in reality. However in by later models there are much more developed versions, which have slightly better chance to work in reality.
 I made it 
  January 27, 2018
Quoting hydra boss this is great. I´d like to build it but I´m not sure if some pieces will fell of and won´t stay at the places I´ll put them. Can you please say me, if there are any falling pieces?
There are 3 reasons of falling pieces: 1. LDD cannot connect the brick, which physically can be connected (eg. cannot distort rubber connectors of ammo belt, so they are emulated with placeholder bricks) 2. Sevearal brics are thrown by LDD from the model, because it is a pretty old one - LDD refrehments can alter the size of the bricks, and even small changes will cause collisions. 3. Rotor blades and landing skid are beyond physical tolerance of lego bricks, they will brak off anyway.
 I like it 
  January 20, 2018
this is great. I´d like to build it but I´m not sure if some pieces will fell of and won´t stay at the places I´ll put them. Can you please say me, if there are any falling pieces?
  December 7, 2016
This looks BEAST! one question: would the machine gun actually work? i was thinking of building the same and putting it in/on a tank(chassis).
 I made it 
  August 5, 2016
Quoting Raymond van de Waarde instructions wont download
I just tried, and it is downloading. After opening the file in LDD, one have to wait 15-20 seconds besides normal opening time, as LDD will drop 22 bricks. This is a result of updating LDD brick sizes among different LDD versions, which causes collision of former OK. bricks. As the MOC has 9600 bricks and many imported parts, it may not open on machines with memory problems or lot of memory resident programs/garbage in memory. If you still experinece problems, try to open partial models at the links inside the text, or reply back.
  August 5, 2016
instructions wont download
 I like it 
  November 25, 2015
hi instructions wont download can i have them email to me thanks at ado_myers72@hotmail.com
 I made it 
  February 12, 2015
Quoting Damon Corso Very cool, this thing looks evil!
Thanks.
 I like it 
  February 12, 2015
Very cool, this thing looks evil!
 I made it 
  January 8, 2015
Quoting Jeremy McCreary Gabor, an astounding magnum opus of LEGO engineering packed with great tricks to digest. Superb technical write-up that could only have come from an engineer. Have you had a chance to build any of the working components? If so, I'd love to see some photos. I only wish that the annotated LDD images were sharper on my screens -- especially the ones with labeled parts. Are the resolutions limited by LDD?
Dear Jeremy, I haven't had a chance to try the design in real, its purpose is rather showing interesting building ideas. LDD screenshots resolution is limited by the maximal resolution of the actual display, which is a silly feature in LDD. Pow-raying takes quite an effort at MOCs consisting several 10000 bricks. Thats why I added all LDD models, so you can see them in whatever size. Currently I design a really large heli MOC coming in the spring 2015. Compared to that one, this is dead simple!
 I like it 
  January 7, 2015
Gabor, an astounding magnum opus of LEGO engineering packed with great tricks to digest. Superb technical write-up that could only have come from an engineer. Have you had a chance to build any of the working components? If so, I'd love to see some photos. I only wish that the annotated LDD images were sharper on my screens -- especially the ones with labeled parts. Are the resolutions limited by LDD?
 I made it 
  October 8, 2014
Quoting Nick Barrett There's nothing else quite like this; awesome design with your usual attention to detail. Love the fact that it all works!
Thanks. I just hope that it would work. At some critical points, it is questionable that TLG parts would tolerate the stress. Another 3-4 development iterations are required to make really playable model from it. My intension here was create an educational model representing some new building techniques.
 I like it 
  October 8, 2014
There's nothing else quite like this; awesome design with your usual attention to detail. Love the fact that it all works!
 I made it 
  October 6, 2014
Quoting Henrik Jensen Another fantastic helicopter, made with your usual thorough review of all systems. For the moment I havn`t carefully studied the entire build, but I will be back and look again.
Thanks, I am looking forward to that.
 I like it 
  October 6, 2014
Another fantastic helicopter, made with your usual thorough review of all systems. For the moment I havn`t carefully studied the entire build, but I will be back and look again.
 I made it 
  October 5, 2014
Quoting Kurt's MOCs Astounding!! My mind cannot comprehend how much detail you put into your projects and this is no different. Spectacular work and presentation. I'll pour myself another scotch and spend the evening enjoying your descriptions and the amazing mechanisms.
Thanks. I would recommend apricot brandy instead of scotch, you should try once.
 I like it 
  October 4, 2014
Astounding!! My mind cannot comprehend how much detail you put into your projects and this is no different. Spectacular work and presentation. I'll pour myself another scotch and spend the evening enjoying your descriptions and the amazing mechanisms.
 I made it 
  October 4, 2014
Quoting killswitch95 [DEMON] @ Centurion, probably one of two things, either it would work, and be an epic lifelike model, ooooor it would blow itself up, either way it'd be cool!
We can close out blowing up, PF M-motors are very weak and Lego is too heavy for that. In real life building, I would replace turboshaft models with 2 PF XL-motor for better playability, they could fit in the same space (5 studs diameter). Here the primary purpose was demonstrate building ideas and techniques.
 I like it 
  October 4, 2014
@ Centurion, probably one of two things, either it would work, and be an epic lifelike model, ooooor it would blow itself up, either way it'd be cool!
 I like it 
  October 4, 2014
So what would happen if I built this and turned it on?
 I made it 
  October 4, 2014
Quoting Centurion Cone Fantastic as always!
Thanks!
 I like it 
  October 4, 2014
Fantastic as always!
 
By Gabor Pauler
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