Rebuilding 8068 Rescue Helicopter into a realistic Light Assault Helicopter
Evaluation of 8068 Rescue Helicopter
When I first saw 8068 Rescue Helicopter, there was a definite „Whao effect”: finally a realistic looking LEGO chopper!
In the history of LEGO Technic helicopters, this is the 3rd-4th serious attempt to model a real helicopter. (You can find an excellent summary about their history at May 2011 archive of http://technic.lego.com/en-us/Designers/Blog/Default.aspx It has no pictures, but you can easily hunt down pictures from Internet):
- 8844 dated in 1981 resembled an early Bell Model 47 with tailwheel tricycle landing gear
- 8456 dated in 1996 had some slight
similarity to AH-1 Cobra
- 8444 dated in 1999 resembled Eurocopter
Tiger (without weaponry)
- 8068 in 2010: Eurocopter Dauphine/ Gazelle (2nd version)
The strong points of 8068:
It is the airframe, especially because of retractable tricycle landing gear, which has a fairly compact mechanism in the cabin floor.
Less successful but still acceptable features
are:
- The seats: they are too big for minifig
scale, too small for LEGO Technic or even Belleville figures. Correct scaling
to the latter ones would make the chopper too big requiring more parts and
resulting higher retail price. But moderate upscaling of seats and cockpit
doors would have made 8068 more usable.- The rescue winch: it has no crane arm as in the reality, so poor rescued guy will be shattered at the edge of cabin floor during lifting…
Weak points of 8068:
It is very clearly the dynamic system, which is rather primitive for a medium sized Technic model:
- Monobloc 3-blade main rotor: Ironically, the very first LEGO Technic helicopter 852 dated in 1977 had the most realistic working 2-blade, variable pitch Bell-rotor with collective controls connected into cockpit:
However it was big and clumsy because of the lack of specialized parts. Since that, LEGO never attempted variable pitch blades.
- Non-connected controls in cocpit:
- Main/tail rotor gearing ratio is 1:1: this is a common mistake of all LEGO helicopters, as this is usually around 1:10 in reality. Earlier posts showed that this can be raised to 1.6:1 at 8068 switching gears
- Gas turbines are not connected with drivetrain
- Hubcap?! instead of Fenestron tail rotor: Fenestron is a small diameter, multi-blade, variable pitch ducted fan tail rotor invented by Aerospatiale and used first on Gazelle. It gives the advantage of reduced noise, less drag, and less vulnerability (modern examples: Eurocopter EC-135, RAH Comanche). Replacing it even with a fixed fan propeller would be a strong simplification, but LEGO used here a hubcap of a large diameter wheel: if you see carefully, one blade has negative pitch, the next one has positive, and so on: thus air goes nowhere from this! This is very crude mistake, considering that quite a realistic looking fenestron (and Eurocopter Dauphine) was built from classic LEGO parts, however in somewhat bigger (1:36) scale (see: http://acidcow.com/pics/858-rotors-69-pics.html ). Moreover, none of the LEGO Technic helicopters has variable pitch tail rotor at all. In the reality, this little tail rotor hub means the difference between life and death.
Of course, it is very easy to criticize someone else’s design. Therefore, in my next post, I will show that all these problems adressed above can be solved using parts solely from 8068 Rescue Helicopter. And even we do not need the hubcap…
Building more realistic helicopter from
8068 Rescue Helicopter
Main rotor:
(Technical note: Flybar is a pendulum-like device fixed with cardan-hinges to main rotor axis transversal to rotor blades. It was invented by Robert Young (Buffalo, NY) in 1946 and used first at Bell Model 47. It acts as a stabilizing device: it tries to keep its original plane of rotation because of gyroscopic torque in case the helicopter is tilted by disturbances in airflow. Changing the cyclic pitch of rotor blades through control rods, it counterbalances tilting. In Hiller system, the pilot can shift only the plane of rotation of flybar with collective and cyclic controls through the swashplate, and only the flybar controls main rotor blade pitch. It has the advantage of increased stability over the Bell-rotor, where the pilot directly controls collective and cyclic pitch of rotor blades trough swashplate. Its disadvantage is less responsive control, so there is a mixed system called Hiller-Bell, which is stable AND responsive but mechanically more complex)
To create such a thing we need 3 critical specialized parts: 1 swashplate (a large diameter bearing with ball joints), 4 ball joints for control rods, 1 cardan hinge slideable on main rotor axis. Of course 8068 had none of these stuff, so the real challenge was further simplification of Hiller-rotor without loosing its functionality (see attached picture):
- Building a sliding cardan-hinge would be theoretically possible from 8068 parts, but its size would make the whole rotor ridiculusly bulky. So I omitted the whole thing, and flybar really „flies” around main rotor axis, wich has a single ring closely rounded by flybar hub components. So flybar can tilt in any direction around main rotor axis and can be lifted and lowered vertically. But how it will stay strictly transversal to rotor blades once it is not linked directly to main rotor axis?
- The solution came from a limitation: total lack of ball joints. Thus, both control rods have two simple hinges at their both ends tilted by 90 degrees, and 2 hinges can slide on flybar rod sideways. This allows all necessary movement of flybar but keeps it transversal to rotor blades.
- As there was no any large diameter bearing for swashplate, I built reels of 2 original landing gear wheels into the flybar hub as rollers on half-axises longitudinal to rotor blades. Swashplate here is really just a plate under rollers with a hole in the middle letting through the main rotor axis. Plate can be lifted and lowered for collective controls: it lifts flybar hub through rollers, which increases pitch of blades through control rods. The short red elastic rod on the top of the rotor (everybody believes that it is a clamp to lift the helicopter by crane…) acts as a torsion spring decreasing blade pitch gently pushing flybar hub rollers against swashplate. Also the plate can be tilted longitudinally or transversally for cyclic control: it changes the tilting of flybar hub, which will influence its plane of rotation, which provides cyclic blade pitch control through rods.
- Collective and cyclic levers go in the cabin roof from swashplate forward to cockpit, where vertical bars running behind the front seats connect them with collective and cyclic control arms placed between front seats
- I omitted flapping and dumping hinges ususally connecting rotor blades to main rotor hub in Hiller system, but their attachment looks like bearingless „elastomer” joints of more modern rotors. In turn they are foldable by 90 degrees making the helicopter storable in compact spaces.
Tail rotor:
- Tail rotor hub setting pitch of 2 tail rotor blades with short control arms can slide on tail rotor axis sideways
- A 90 degree joint acts as pitch control arm, pushing a roller (made from reel of 3rd original landing gear wheel) to tail rotor hub forcing it sliding forth and back on tail rotor axis
- Tail rotor blades are made from shorter type of fairing elements, which are quite aerodynamic: quickly turning tail rotor they make reasonable wind, and this is what we want…
- Tail rotor transmission shaft and pitch control rod run along tail boom under the longer type of fairing. I found good use of the thin long black flexible rod given to 8068 leading control rod through fairing, so it is fully covered giving more clean lines to the helicopter
- Jaw pedal controls are before the left
forward seat. Their reversed movement is ensured by triplet of gears. Unlike
real helicopters, jaw control rod runs backward in the cabin floor to avoid big
„crowding” around main rotor mast. Behind left rear seat, under the fairing,
there is a hinge transmitting jaw control to control rod running parallel with
tail boom
Drivetrain:
- Single engine is placed at the tail of cabin
under tail rotor transmission shaft covered by fairing. It is geared 1:1 to
main rotor axis and 1:1.6 with tail rotor shaft. Air intakes are at the top
side of tail boom
- The 3-part tail rotor transmission shaft is
very mildly curved enabling to match gearing
Airframe:
My intension was to creat very compact cabin like Hughes Defender but having 4 seats capable of accommodating 4 LEGO Technic or Belleville figures (or 4 Playmobil figures – oops, thats another company…).
- This was done maximally utilizing flexible bars and fairings (8068 is good source of these) shaping the cabin.
- Vertical part of seats are also griders of the airframe to save space and material.
- As forming the cabin tail cone of Hughes Defender proved troubleful from available material left and collided with the placement of the engine, moreover the 4-seat cabin became slightly bigger, the final airframe more resembles to Eurocopter Squirrel (earlier called Aerospatiale Ecureuil), except that it has the engine under tail rotor transmission shaft covered by fairing like in the Defender.
Other parts:
- Instrument panel: virtually unchanged from
8068- Weaponry: 2 side machine gun pods with variable elevation built from 4 left over fairings and 2 rods similar to Hughes Defender
Summary:
8068 was a good source of material for
building much more realistic, but somewhat smaller helicopter, even some
valuable stuff is left over (1 gear, 1 rotor blade, 1 fairing, etc.). What I
could not solve because lack of material:- Cyclic control arm works reversed compared to real helicopters and controls only forward-backward pitch, but not rolling: the swashplate can tilt transversally, but not enough rods, joints hinges are left for roll control
- Gearing ratio between main rotor and engine/tail rotor is not enough big
No comments:
Post a Comment