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Electric Backpack Helicopter with Working Coaxial Rotor and Machinegun
Personal Aerial Vehicle with electric drive, 4 channel steerable coaxial rotor and spring-driven shooting M249 SAW light machinegun in scale 1:10
About this creation
*Visit my Lego helicopters blog also
**See model and building instructions in Lego Digital Designer (LDD): Here

Figure 1: PAV in landing approach above suburbs of Shanghai
See model in LDD: Here

1 Introduction and inspiration

It is a common misbelief that any realistic helicopter built from Lego is a large, expensive model with several thousand parts. For example, latest TLG 9396 Technic Rescue Helicopter set contains 1044 bricks, for which it contains main rotor with collective pitch control, driven fixed pitch tail rotor, and retractable landing gear, cargo ramp and rescue winch. But there is no electric drive (its only optional, sold separately as PF set), no cyclic pitch, no yaw control, no pilot figures. Myself already designed comparable sized Bad Guys Escape Helicopter MOC far exceeding the functionality of 9396 but at the cost of 1700 bricks.

Now I will show that just around the 500 bricks-range of an average sized Technic set, we can build electric driven helicopter with fully functional, foldable coaxial rotor with 4 channel controls, working rudder surfaces, retractable landing gear, shooting armament and fully poseable pilot figure.

To stay within such a limited materials requirement, we tried build the smallest possible Personal Aerial Vehicle (PAV) capable of lifting a man (not considering jet packs here because of their very limited 1-2 minutes flight duration). Building a compact, foldable PAV, capable of Vertical Take Off Landing (VTOL) and having reasonable range is enormous engineering challenge:

1.1 Multirotor PAVs

Recent developments in Li-Po battery technology, piezo-gyroscopes, GPS-navigation and control electronics made possible to build ever stronger and larger quadcopter drones and other multi-rotor crafts at reasonable price. Therefore, at the first sight, they seem to be very promising candidates to solve the 100-year old technical problem of creating everyday useable, affordable VTOL PAVs. The biggest advantage of multirotor craft is that complicated and expensive mechanics of variable pitch helicopter rotors are omitted, and all controls are achieved by changing performance of electric thrusters with fixed pitch propellers. Moreover quadcopters are easy to fold into very compact package. The most advanced prototype of manned multirotor crafts is E-Volo VC200 developed by Germans:

Figure 2: E-Volo VC200 multirotor helicopter

However multirotor crafts have serious disadvantages:
-Quadcopters are cheap, simple and smart, until you don’t have to trust your life on them: one engine from 4 is out and you are dead. (Just for comparison, WWII B-17 bombers could return to base 3 engines out 1 working). Designers of 4-thruster VTOL crafts usually propose ballistic parachute for that case. But deploying it from a destabilized, rolling craft, flying low, among buildings, air cables, lantern masts, etc. is just an invitation to your funeral.
-More redundancy can be achieved using more than 4 thrusters (e.g. VC-200 has 18 of them), but this means structural complexity and totally erodes foldability and compactness. Moreover small diameter thrusters streaming air at higher speed are less energy efficient than one large thruster with slow flowing air (drag and losses increase at second power of air speed!).
-However, bigger thruster is definitely not better: control of multirotor craft depends on quick changing rpm of fixed pitch propeller thrusters (while at helicopters, rotors have more or less constant rpm, and all controls are made changing blade pitch). But rotational inertia of big thrusters increases dramatically: doubling their size made from the same material means 8 times more mass x 2 times more velocity (squared) = 32 times more torque!
-Therefore maneuverability of multirotor crafts are more limited than helicopters. While best helicopters can fly aerobatics figures, hover upside down, etc., multirotors cannot recover if they are flipped by turbulence or wind shear (eg. VC-200 has separate pusher propeller to avoid the need of tilting it for high speed). Because of this, up to date no manned multirotor can safely leave 10-20m altitude, where “ground effect” helps to stabilize them.

1.2 Folding mini helicopter PAVs

An alternative (and older) way to create PAV is to miniaturize large helicopters. As they will cost almost as much as big ones, only military commando or law enforcement SWAT applications are viable, instead of civilian use. Here we focus on coaxial helicopters because they can eliminate the non-compact, hard-to fold, vulnerable tail rotor and its driving shaft. 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.

Figure 3: Kamov-type coaxial rotor

In 1971 the Kamov Design Bureau was ordered to produce new Kamov Ka-56 ultralight helicopter that should have been transported in a cylindrical container of 0.5m diameter and should have been assembled for flying in 10 minutes. The Soviet Navy stressed on developing SPECNAS demolition commandos to counterbalance much stronger western surface navies, so they required a PAV launchable from torpedo tube of submarines. Ka-56 was fully foldable, except the 4 rotor blades, which were easily detachable. It had extremely compact 40hp air-cooled rotary engine using simple car gasoline fuel. Unfortunately, it did not go beyond prototype phase as Soviet materials technology was unable to produce reliable edge seals for rotary engine. (It tends to stall without any warning signs because of eating edge seals, compared to piston engines which die slowly).

Figure 4a: Kamov Ka-56 deployed

Figure 4b: Kamov Ka-56 folded

2 Technical details of PAV

*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:

**In the forthcoming technical description, functional parts of PAV are referenced by numbers which can be found on technical drawings attached

***Parts of PAV are color-coded by their function:
- Yellow: Manual handles of working functions
- Gray/Black: static and dynamic parts
- Dark gray: Pilot’s torso
- Blue: Seat of pilot
- Orange: AA-batteries
- Green: Ammo belt
- Dark green: Weapons grip

Our PAV has two goals:

1.Build the first ever MOC with fully steerable 4 channel coaxial rotor. In 2010, Zblj already attempted working coaxial helicopter rotor with limited functionality: rotors can be tilted, but their blades cannot be pitched, which is practically unworkable. To build fully functional Kamov rotor is very serious modeling challenge, as there are no coaxial tube shafts in Technic parts, moreover they are too bulky for fine mechanic controls.

2.Build foldable PAV, which can be packed close to the size of Ka-56 folded, despite more bulky Technic parts. All parts (even rotor blades) of the PAV should be foldable.

Figure 5: Overview of mechanics
See model in LDD: Here

Building a small but realistic rotor was largely made possible by TLG introducing ‘Blade 16M with crossaxle’ in Bionicle. Originally it is intended as an oversized sword for some exotic Bionicle fighters, but apart from some decorative details, it is quite similar to Kamov-type coaxial rotor blades. Anyway TLG does not support us very well with aerodynamic SNOT rotor blades: putting a studded bar in the airflow is funny, but only until you play with Duplo. The first specialized heli rotor blade part was introduced in 2010, but - totally pointlessly - it has studs on the top, making all aircraft engineers really cry. Finally, in 2012 at 9396, a SNOT rotor blade was introduced, but its blade root is so awkward, that I prefer to build SNOT blades from curved fairing elements and Technic cross axles. But that solution is too bulky for a small rotor.

2.1 Drivetrain

Figure 6: Drivetrain
See model in LDD: Here

Creating drivetrain, the main task was to resolve the lack of coaxial tube axises in Technic, necessary to build coaxial rotor. Main rotor mast (2) is a simple cross axle with rotor hub of upper rotor (3) and driving disc for V-belt (14) fixed to its ends. ‘Technic hub’ of lower rotor (8) can spin and slide up/down on main rotor mast, while its groove accepts the V-belt driving it. The groove is more wide than the V-belt, therefore the hub can slide up/down, still driven by V-belt. Hub can moved up/down hooking a catch in its lower rim. Rotation of V-belts (9, 13) driving upper/lower rotor is reversed by pair of Z16 gears (11), driven by single PF M-Sized electric motor (15). As we found PF battery pack too bulky, we built custom, detachable battery pack, caging (18) 2 AA-batteries (17).

2.2 Collective blade pitch control

Figure 7: Collective blade pitch control
See model in LDD: Here

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 3 blades of upper rotor has individual rubber torsion springs (2) made of TLG part ‘rubber dumper 2×1×1’ forcing gently their half axises (1) to zero degree pitch. Correct pre-tension of torsion springs can be regulated by rotating (5) setting horns.
-Therefore changing blade pitch requires only pushrods (4, 9) connected to blade pitch control arms (3, 8) instead of using ball joint-connected linkage.
-Blade pitch control pushrods can slide up/down in the holes of driving disc at the upper rotor and hub of lower rotor, so they are NOT aligned by swashplates.
-Therefore, upper (6) and lower (10) swashplates can be just plain plates with tips of control rods sliding on their surface and a whole in their middle letting through main rotor mast. So they can be lifted up/down and tilted around main rotor mast, but they don’t have to be fixed to that with cardan-hinge.
Lower swashplate can be pushed upward against the force of upper torsion springs by a sleeve (11) sliding up/down on main rotor mast to achieve collective pitch control on all 6 blades. The sleeve is moved by a long collective control arm (16) rotating on a pivot (14) and connected to that with (13) half axis. Collective control arm provides reasonable arm of force for pilot to counteract force of torsion springs, but as it is pretty long, it can be folded backward with the help of 2 hinges (15) when not in use.

2.3 Cyclic blade pitch control

Figure 8: Cyclic blade pitch control
See model in LDD: Here

Cyclic blade pitch control requires tilting lower swashplate forward/back and left/right. At Ka-56, a long grip was attached to swashplate and pilot used that as an inverted yoke to tilt it. However, it obstructs forward vision and results in collective and cyclic control to be interlinked making piloting more difficult. Therefore I decided to build collective-cyclic mixing linkage just like at big helicopters, but in a very compact and foldable layout.
-Lower swashplate is tilted by up/down motion of vertical track rods (12) connected with ball joints (11)
-Vertical movement of vertical track rods is converted into horizontal movement of horizontal track rods (17, 19) with the help of two 90 degree swing arm units (14, 16), whose pivot axises (15) move up/down together with collective control.
-Horizontal track rods are connected to double jokes (20) rotating on pivots (21) fixed to collective and differential collective control arms. They form - together with horizontal track rods and 90 degree swing arms - parallelogram linkages, which compensate the movement of collective and differential collective arms, decoupling cyclic and collective controls.
-Double yokes are necessary, because at backpack helicopters, centrally placed yoke would make leaving the vehicle too difficult.
-Horizontal track rods can be folded backward 180 degrees when they are not used.

2.4 Differential collective yaw control

Figure 9: Differential collective yaw control
See model in LDD: Here

At single main rotor helicopters, yaw control is made by setting pitch of tail rotor with the help of yaw control pedals. None of them we have here, for the sake of compactness. Coaxial helicopters solve yaw control with differential collective blade pitch setting of upper/lower rotors. Whatever rotor has higher pitch than the other, it results in higher drag, rotating the craft in the opposite direction relative to the spin of given rotor. At real coaxial helicopters, main rotor mast is a tubular axis, and differential collective control rod moves inside that up/down, modifying collective pitch of upper rotor relative to general collective pitch. But in Technic, we do not have any thin, long tube axis with rod sliding inside that:
-Therefore, we will move lower rotor hub (6) up/down with the help of catch (7) sliding on its lower rim and vertical differential collective control rod (8). It results in modifying lower rotors collective pitch relative to general collective pitch setting.
-However, as the whole lower rotor loading force pulls lower rotor hub upward, setting its vertical position would require force far exceeding the manual force of pilot.
-But, we placed a steel counter spring (5) (from disassembling TLG part ‘shock absorber’) above lower rotor hub (6) on the main rotor mast fixed with adjustable double nuts (4) responsible for its pre-tension. The correctly adjusted counter spring alone serves as a regulating device for lower rotors disc loading: if general collective blade pitch increased, rotor loading will increase, then it lifts lower rotor hub against the spring, which decreases blade pitch and rotor disc loading, and so on. Differential collective input through control rod (8) will influence this equilibrium. So the actual position of lower rotor hub (and lower rotors pitch) is influenced by the mix of 4 factors: general collective pitch control input, rotor disc loading, counter spring tension, and differential collective pitch control input.
-Differential collective control is handled by pilot through a long lever (13) rotating around pivot (11) and connected with track rod (10). The lever can be folded backward 180 degrees with the help of two hinges (12) when not in use.

Figure 10: Controls summary
See model in LDD: Here

On the figure above, we can see summary of working all controls together. During dead-engine autorotation crash-landing, collective pitch of blades can be changed only in a very narrow window around +8..10 degrees to preserve autorotation and rotor inertia which are vital for survival. This makes differential collective yaw control ineffective. Therefore coaxial helicopters use airplane-like rudders for yaw control during autorotation. Rudders also help to stabilize high speed level flight. Therefore we placed two simple manually moveable rudder surfaces on control arms. They are designed to fold backward 180 degrees when PAV is not in use.

2.5 Retractable landing gear

Figure 11: Retractable landing gear
See model in LDD: Here

In Hollywood movies, pretty female FBI superagents wear backpack helicopter, very-very BIG GUN and sexy high heel booties together with ease. In the reality, weight of fully loaded PAV is around 100kgs (+pilot, +weapons), totaling up to 220kgs. It should be stopped and carried by human legs, even at bumpier autorotation crash landing… One can see that it is physically impossible. Therefore we need some simple, compact retractable landing gear carrying bulk of the weight besides pilots legs. I designed a monocycle landing gear with twin narrow wheels (3). It opens down between the legs of pilot and secured by swingarm (6). It is retractable manually by lever (4) and held retracted by knob (8). When PAV is folded, wheels of retracted landing gear are still usable to move folded rotor unit on the ground as a wheelbarrow-like something.

2.6 Detachable battery pack

Figure 12: Detachable battery pack
See model in LDD: Here

Battery unit is detachable because of more compact folding, and enabling quick replacement with a charged unit instead of lengthy charging. Battery pack – together with electric motor - has important role maintaining correct Center Of Gravity (COG) of the craft, offsetting the weight of pilot.

2.7 M249 SAW light machinegun

PAV by default is not designed for dogfight, because of the high drag and vulnerability of a standing human body in the airflow. The primary reason it carries armament can be really understood when you are in urban combat with insurgents at suburb of Fallujah: “If we could quickly place a machine gun on that roof over there and strafe their back…”. Besides providing fast airlift of a machine gunner at confined spaces, PAV can fire machine gun fixed on retractable side console during flight as self-defense, just it is not very steady firing platform because of its small weight. Therefore, helmet of pilot is equipped with helmet display, linked to targeting telescope of the machine gun. Machine gun is fixed that way that it counterbalance weight of engine, which is placed 1 studs left from centerline, and ammunition belt container is the closest to main rotor mast, to achieve firing should have neutral effect on COG of the PAV.

There was a big modeling challenge to put actually shooting machine gun there. I opted modeling M249 Squad Automatic Weapon. It was a deadly expensive engineering marvel at the time of its introduction in 1984, summarizing all the experiences of materials technology, air cooling, recoil- and gas operated systems. Its gravely reduced weight enables continuous walking covering fire barrage without the need of shifting barrel. The dual feed (belt, box magazine) without any modification increases operational flexibility.

Figure 13: Firing cycle of M249 SAW light machinegun
See model in LDD: Here

I developed M249 for my Handbook of Building Working Guns for Bionicle Figures MOC earlier:
-Propellant force for shooting is provided by 2 steel springs from disassembled ‘Shock absorber extra hard’ TLG parts (13).
-Rotating bolt is made from TLG part ‘Mini rapier’ and accelerates projectiles in 3 stud long track.
-Barrel is made from ‘Connector peg 2 studs’, which have 3.2mm inner bore
-3×15mm projectiles are the only non-TLG parts, as TLG does not produce reasonably small parts because of children safety, moreover ABS material is too easy for projectiles. So they are made from 3mm copper or aluminum wire can be found as electric wiring in any hardware store.
-Projectiles are sticked with tiny pieces of use chewing gum into cavities of a non-disintegrating ammo belt made of TLG parts ‘Technic beam 2×1×1’ and ‘Connector peg 2 studs’. Belt is advanced by an autoloader when bolt is moved.

Firing cycle of M249 is the following:
-PHASE 1: Cocked bolt (11) is released by rotating charging handle/trigger (14) 180 degrees outward
-PHASE2: Springs (13) press bolt forward. Tip of the bolt locks the belt in its actual position, and projectile (1) is pressed from belt into barrel (2) and launched.
-In the meantime, bolt (11) hits knob (8) of loading arm (9), and forces rotate it down 11 degrees, against the force of torsion spring (10) made from ‘Cross axle 2 studs’.
-Therefore, belt catch (7) placed at the end of loading arm clicks into new position at the belt.
-PHASE3: When bolt (11) is pulled back again at re-cocking, it relives pressure on loading arm knob (8), then retreating tip of bolt unlocks belt. So, the loading arm – forced by torsion spring – lifts 11 degrees and advances belt one position further through bolt catch, and new the projectile moves in line with cocked bolt.
Of course, M249 is equipped with folding bipod, targeting telescope, pistol grip and stock to enable it to be carried manually by charging pilot:

Figure 14: Action screenshot of M249 SAW light machinegun
See in LDD: Here

3 Action screenshots of PAV

Figure 15: Strafing ground forces
See model in LDD: Here

Figure 16: Surprise attack on battery
See model in LDD: Here

Figure 17: Top view
See model in LDD: Here

Figure 18: Right side view
See model in LDD: Here

Figure 19: Left side view with landing gear deployed
See model in LDD: Here

Figure 20: Bottom view
See model in LDD: Here

Figure 21: Front view
See model in LDD: Here

Figure 22: Back view
See model in LDD: Here

4 Dimensions of PAV

Figure 23: Folded view
See model in LDD: Here

Kamov Ka-56 can be folded into a cylinder with 0.5m diameter, but one has to disassemble 4 rotor blades for that. My PAV can be folded into a cylinder with 0.88m diameter (without battery pack and armament), and all of its parts are foldable: 6 rotor blades are folded downward 90 degrees, 2 control levers are folded backward 180 degrees, and landing gear is retracted. It results in a cylindrical package can be rolled on landing gear wheels like a wheelbarrow. It cannot be loaded into a torpedo, but certainly can fit in crew compartment of an APC or MRAP. Considering that I had to work with rather bulky Technic parts, it is not a bad result. Its exact dimensions are:

Main rotor diameter: 43.00 studs / 344.00 mm / 13.54 in, Real size: 3.44 m / 11 ft 3.35 in
Main rotor disc area: 2 × 1452.20 sqstuds / 929.41 sqcm / 144.06 sqinch, Real size: 9.29 sqm / 99.91 sqfeet
Separation of rotors: 4.50 studs / 36.00 mm / 1.42 in, Real size: 0.36 m / 1 ft 2.17 in
Height with pilot standing: 31.50 studs / 252.00 mm / 9.92 in, Real size: 2.52 m / 8 ft 3.15 in
Folded height: 21.50 studs / 172.00 mm / 6.77 in, Real size: 1.72 m / 5 ft 7.68 in
Folded diameter (without armament and battery pack): 11.00 studs / 88.00 mm / 3.46 in, Real size: 0.88 m / 2 ft 10.63 in

5 Summary

In realistic modeling of a coaxial helicopter PAV in Lego, we bounced back on the same problems as real engineers did: Whatever can fly one person VTOL safely, is just too heavy to be easily carried by one person, cannot fly serious amount of extra cargo, has short flight duration (15-20 minutes), and deadly expensive, moreover difficult to pilot manually. For example, a 4 seat light helicopter can carry 4 persons and cargo, enough fuel for 2 hours flight duration, is more fast and more steady gun platform, can be equipped with state-of-the-art autopilot, and may cost less than 4 PAVs together. PAVs have the only advantages of portability and ability to fly in very confined spaces. Summarizing all these points: PAVs are technically feasible, just not worth to build them, at the current level of technology. But they are excellent play with engineering and modeling challenge.

6 References

The battery attacked at action screenshots is my earlier M777 Light Towed Howitzer (Working) MOC based on the real BAE Systems M777 155m L39 ultralight towed field howitzer

Figure 24: M777 light towed howitzer
See model in LDD: Here

Building instructions
Download building instructions (LEGO Digital Designer)


 I made it 
  July 22, 2014
Quoting Nerds forprez Dr. Gabor, Your stuff is amazing. I need to sometime see if I can make one of your models. I have always loved helicopters; as a medical professional within the aeromedical community it is important for me to see what goes on in the mechanics of the machines my patients operate. Thanks for sharing and keep up the great work!
Thanks. Let me know if you managed to build something, or if you need any advice. You really have a cool job. I teach SAP at university, that's definitely not an excitement-factor job, so my Lego designs compensate that a little bit. My wife is oncologist, so I have the picture about how much determination and energy medical profession requires.
Gabor Pauler
 I like it 
Matt Bace
  July 21, 2014
Wow! Yet another extremely impressive feat of engineering. Your solutions for the collective and cyclic controls are top notch. I'd love to see LEGO work something like this into their next Technic helicopter set, but that could be another 10 years from now :-(.
 I like it 
  July 21, 2014
Dr. Gabor, Your stuff is amazing. I need to sometime see if I can make one of your models. I have always loved helicopters; as a medical professional within the aeromedical community it is important for me to see what goes on in the mechanics of the machines my patients operate. Thanks for sharing and keep up the great work!
 I made it 
  July 21, 2014
Quoting Nick Barrett Fascinating. Your helicopter designs always inform and inspire. Thanks for sharing.
 I made it 
  July 21, 2014
Quoting Matt Bace Wow! Yet another extremely impressive feat of engineering. Your solutions for the collective and cyclic controls are top notch. I'd love to see LEGO work something like this into their next Technic helicopter set, but that could be another 10 years from now :-(.
Thanks. You are perfectly right. Until the advent of TLG's 9396 Rescue heli set I believed that a realistic helicopter set would not be marketable: requires too much materials, and too difficult to build. But then they come up with 9396, which has enough material, specially developed rotor blades, but the internal mechanics (except retractable landing gear) looks like somebody designed it in the kindergarten... I think that it is contraselection of designers within TLG that prevents them to make better things, not the market.
 I like it 
  July 21, 2014
Fascinating. Your helicopter designs always inform and inspire. Thanks for sharing.
 I made it 
  July 20, 2014
Quoting Centurion Cone How do you make this stuff? it's crazy! you're by far the best lego designer I've ever seen!
Thanks. It took 10 days to design it. But I am working on different types of helicopter rotors for 3 years.
 I like it 
  July 20, 2014
How do you make this stuff? it's crazy! you're by far the best lego designer I've ever seen!
By Gabor Pauler
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LEGO models my own creation MOCpages toys shop Electric Backpack Helicopter with Working Coaxial Rotor and MachinegunTechnic

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