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Setting Up Your BobCat
By
Joe Rafalowski
With the proliferation of BVM Bobcats in our club I would like to share my experiences setting up the plane. Hopefully these suggestions will decrease the amount of experimentation necessary to make a
Bobcat truly enjoyable to fly.
The first area of concern should be the mechanical systems. Thoroughly plan your installation taking into consideration where servo leads should connect, air lines should route, and engine
components be placed. The Bobcat is surprisingly complex if it has to be disassembled for maintenance. The retract system used is very reliable but the smooth stop braking system can be sensitive to mis-alignment causing the
air system to dump pressure. The brakes need periodic maintenance, as they are also prone to leakage. For that reason the landing gear air and the brake air systems should be separated. In case the brake system should
malfunction the landing gear system will still operate.
Do not use the originally specified JR 3421 servos for rudder control. The JR 9411 digital midsize servo easily fits the fin cutout. Several incidents of servo
malfunction attributed to the gyro control gain settings have been documented. Rudder authority on the Bobcat is very positive with excessive rudder displacement causing a tendency to snap roll. The larger servo should
alleviate the problem.
Fellow Georgia Jets member James Moody is building a Bobcat where the elevators will also have JR 9411 servos. This will require custom servo cover plates as the rear portion of the servo breaches
the top skin by .2”. I am currently making molds for fiberglass plates. The reason for the larger servos is that I noticed as speed increased in dives, the elevator response became somewhat sluggish. I suspect that a
forward C G contributed to the condition however, the extra torque and larger gear train would help in more positive control.
Balance the Bobcat as accurately as possible on the balance holes (see plan
sheet). Use a mechanical balance, not your fingers, to level the plane. You probably will need nose weight if you are using the Jetcat p-120 or ram 750 for propulsion. A five cell 1400 mah battery pack with a regulator is the
minimum recommended. You could probably fit an 1800 mah pack in the nose in place of balancing lead.
I have the Budd Engineering laser system and will consider lending it to fellow Georgia Jets members for your control
system setups.
I highly recommend the following set up for aileron reflex in the landing configuration.
1 Reflex ailerons up 1/4 inch measured at the outboard tips.
2 Set elevator to trim down about 3/16
inch with aileron reflex selected.
That's it. The elevator travel is approximate as some fine-tuning is necessary for individual airplanes. You will notice no unusual control tendencies with reflex selected.
The airplane will slow as if a speed brake was deployed. A little extra throttle is required on approach and landing. The plane will set up in a slightly nose high attitude if you fly the correct landing speed. No ballooning
tendencies were noted as throttle more accurately controls descent rate. The plane was very easy to land with this modification.
I’ll be happy to help with any setup questions that you may have as you get closer to
flying the Bobcat.
Joe
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