Vintage RC Helicopters - Hirobo Enstrom Shark
Hirobo Enstrom Shark 280C - 40 (1977)
Original model for 40 size engines, flatbed mechanics and incorporating the Hirobo original head. The flat bed mechanics achieved collective pitch operation by raising the whole swashplate and linkage assembly with a pivoting arm. Whilst simple in execution it did have a drawback in that as the collective arm rotated to move the linkages up or down from the central position, it also moved them forward; because the servo's and their rod length's were fixed this was in effect the same action as putting in slight 'back' and right stick movements at the same time. Thus changes in collective pitch resulted in unwanted cyclic pitch interaction and due to the design of the rotor head also resulted in a 'pitch' change to the flybar paddles. This head was very stable but the movement of the flybar paddles made it very inefficient outside of a small range of setting around the Zero flybar paddle position. Pilots generally set this position to suit their flying style; for beginners it would have been at the 'hover' position so as to give good control whilst learning; for more advanced pilots it was changed to give zero paddle setting at the 'circuit' position so as to give good flight performance.
The control system was four channel, requiring only four servo's as one servo did the duty for collective and throttle control plus cross connected by a linkage to the tail rotor servo arm also provided coupling to achieve mechanical collective-tail rotor pitch mixing. An additional feature was the inclusion of a 'pull start' system thereby removing the need for heavy starting equipment i.e. Battery and external starter motor.
Hirobo General parts list - 1977 listing including the Enstrom 280-C Shark
In order to remove the 'unwanted' effect of the paddles moving with collective pitch, the Collective de-coupler rotor head was fitted and was also available as a conversion kit for retro-fitting to a original rotor head.
Although originally designed for 40 size engines, Hirobo had commissioned a specific helicopter engine, the HGK 45; this would directly replace a 'standard' 40 and thus could be used in the Enstrom Shark.
For 1979 as per the 60 size Huey, the model was update to included the scale rotor head, the revised collective 'pitch up' linkages which removed the collective-cyclic mixing of the original design, option of hand pull or belt start systems and optional 'free-wheel' adaptor to allow autorotation.
This manual is suitable for setting up the Shark's flatbed mechanics post 79 - Enstrom Shark(60) Build manual
In 1980 the model was updated because whilst forty size engines were suitable for hovering and general circuit work they lacked the urge needed for more spirited flying. Unlike the Huey and the JetRanger the body incorporated a 'lattice' tail boom and similarly to the Lama, updating for the new 50 size engines that were appearing on the market was achieved by simply extending this 'tail boom' in order to allow longer rotor blades to be fitted.
For the ultimate though adding an extra 0.5 Kg or so in weight, the pull start could be replaced with an onboard electric starter; cost £120 which was the equivalent to the cost of an engine and gyro together so did not really sell to the 'average' helicopter modeller.
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