OK guys, safety comes first so here goes:
In my RCJI review of the BobCat I questioned the servo specification on rudders and elevators, see page 54 of the April/May 2002 issue. The BVM spec. of the 3421 mini servo on the rudder I believed to be quite inadequate for a gyro controlled rudder and I recommended this servo be upgraded to a 9411 as was fitted to my review BobCat. Time was to prove that assessment was correct, the 3421s could not cope and the 9411 is now recommended by BVM and JR recommends that the 3421 is not used for a gyro application.

I also mentioned in that article that the 3301 servo for elevators was only adequate with 500 sized engines and that my next BobCat with a bigger engine (now almost complete) will have more powerful servos (at least 9411s) for a greater RESERVE of power. The reason is this: The tailplane and elevators of the BobCat are, like most model aircraft control surfaces, basically crude lacking both aerodynamic and mass balancing. The BobCat elevators are heavy, having ply skins and mine weigh a little over 2 ounces. The center of mass of these elevators is about 1 3/4 inches fom the hinge line so that the torque resisting the servo power is 2x 1 3/4 or 3.5 ounce inches when the model is static.

I know from flight tests with my JetCat GPS equipped F15 that the F15 pulls a peak of 10.9 g in moderate aerobatics and upto 12.9 G in hard turns. ( I spoke to Horst Lennerz of JetTronics about this at the JWM in South Africa and he agrees that these values are realistic). I am sure that the big, thick wing of the BobCat can easily attain load factors of 10 and in a 10 g manoevre the torque generated by the mass of the unbalanced stab. is now 35 oz/ounces, so more than half of the servo power is used just to hold the elevator against G forces. The BobCat elevators have considerable travel so we must add blowback forces to the torque required to hold the elevator position and my guess is that the servo is having a very hard time indeed and in some instances is inadequate to provide the required pitch response.

It should not be forgotten that in a high G situation the heavy ailerons will also cause the aileron servos to pull a high current to just hold position which will do nothing to help battery voltage. If the long leads to the elevator servos are not heavy duty and a 4.8 volt battery is in use, the actual voltage at the elevator stabs. may be insufficient for the servo to deliver its rated 60oz/inches further reducing pitch control.

The solution is easy, (certainly easier than mass balancing the elevator as is done on full size machines) just use a bigger servo, at least a 9411 or a 8411. Sure, the servo will protrude slightly below the stab. skin. and when I emailed BVM recently to order some BobCat parts I mentioned I was using bigger servos and suggested BVM make some little ABS fairings as they do for the wheels on the top skin, no reply to that issue ! Many fullsize aircraft with thin wings have their servos (PCUs) mounted externally in fairings and there are no flutter or buffet problems even on Concorde at Mach 2. Make sure ther is NO play in the servo mounting, there is little or no vibration protection requiredand I screw mine down HARD , again see the picture on page 54 of RCJI.

The slightly heavier servos may require some noseweight, no problem use a bigger battery and if its a nicad. make sure it is an SCR type which can deliver high currents without significant voltage drop.

I use twin Duralite batteries with 5.2 volt regulators with heavy duty leads THROUGHOUT the BobCat and have experienced no problems whatever.

The BobCat is a delight to fly. (I fly mine with the excellent PST 600 R which someone in the US malicioulsy tried to discredit by writing to the AMA claiming he had seen one explode) and my XL will soon be airborne with a JetCat 160, Jetcat GPS, JetCat speed limited and Data link system to obtain in flight performance data, but with JR 8411 servos for pitch control.

There may be other contribitory factors to pitch control degradation such as flow seperation on the diamond shaped airfoil (was it tested at all angles of incidence in a wind tunnel ?) but my guess is that a decently powerful servo with a reserve of strength will see the problem disappear. If that does not fix the problem then it has to be aerodynamic, something I very much doubt.

In the meantime I must get back to writing the report on the JWM in South Africa where I, and the other competitors, received the warmest of welcomes and was loaned a model with which to compete in the open class. ( Thanks James and Mark). and where the South Africans proved they have fixed the Fiber Clkassics Mig 29 hydraulic problems, even offering me the chance to fly it for them after the contest.

I hope this helps to prevent any further uneccessary loss of any more BobCats.

Regards,
David Gladwin