Freya - our experimental UAV


The first step one has to take when building a rotorcraft-based UAV is to obtain suitable flying platform. Hobby helicopters are often being used for this purpose, being convenient mainly because of their affordability. Due to the considerable weight of the avionics, powerful 60 or 90 sized machines are most commonly used (size of the RC helicopters is told by the engine displacement. xx size means engine displacement of 0.xx cubic inches). When choosing the right chopper the main concerns are load capacity, reliability and spare parts availability. Also, one has to bear in mind that setup, operation and maintenance of a hobby helicopter are difficult and responsible tasks, requiring a lot of knowledge and experience.

In our project we decided to incorporate the 90 sized Hirobo sst-eagle Freya EVO 90 RC helicopter. This decision was more or less given by the circumstances at the time (beginning of 2005), because a second-hand Freya, but in excellent shape, had passed my way and was offered for sale. A deal was made, and in the time being this proved to be a very fortunate decision. Reliable and robust, manufactured with proverbial Japanese precision, strong and stable in flight, and producing only mild vibrations (a very important asset), Freya turned out to be an outstanding working horse. In the meantime, I really tend to love this helicopter – the best RC machine I have ever flown in my life. Even with the additional 1.5 kg of the avionics weight, and an ugly GPS receiver on the tail boom, she maintains her excellent flight characteristics and undisputable elegance in her natural environment. In her case, it’s the nomen omen that passes thru my mind whenever I fly her, and reminds me of her celestial namesake :-).

From the technical point of view, Freya is a two-blade machine with Bell-Hiller flybar stabilizer, incorporating Heim H1 type of swash plate management. Powered by the Yamada ST91 two-stroke engine with Youngblood muffler and with KOK Composite carbon-fiber 680mm FAI Extreme symmetrical blades, she can bear about 2 kilograms of payload. The overall weight of our machine is approx. 7kg, main rotor diameter 1560mm, length 1375mm, width 200mm and height 543mm. Tail rotor diameter is 270mm, gear ratio 9.5:1:4.77. Two Futaba S9202 servos are driving the cyclic, Futaba S9350 digital servo drives the collective, and Futaba S9254 digital servo controls the tail rotor pitch. The throttle is controlled by another Futaba S9202. Futaba GY-401 gyro is used as a backup for the tail stabilization (in the manual flight mode – the RAMA system does not incorporate this gyro, it’s there just for the human control purposes). Spektrum DX-7 2.4GHz RC set is used for the manual control.

The RAMA control system is placed in an aluminum container, located between the landing gear skids. The container is attached to the helicopter frame using four cable ties and rubber dampers. It can be shifted forwards and backwards, to fine-tune the CG (Center of Gravity) position.


experimental_uav.txt · Last modified: 07/01/2010 09:30 by ondra
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