Company History:
Quadruplex - By Carl Schwab
ORIGINAL DEE BEE SERVO—
This is a short description of the original orange box servo designed by the late Don Brown based upon the Mighty Midget, MM, motor. This servo is often referred to as the TT spring-centered or TTsc servo, but there is MORE to it---many subtleties. BTW, TT referred to Twin Tension spring design.
Don was very familiar with the MM motor from his experiences with Simpl-Simul a.k.a. Galloping Ghost. He knew it was a tough little motor, had good brush wear, and the brushes were easily replaced. Most modelers that were experimenting with direct-pulsed servo drive used the MM motor.
When Don started using Walt Good’s TTPW system, he experimented with adding an additional 7 to 1 gear reduction. This brought the overall ratio to 49 to 1. Beyond merely adding the second gear pass, Don added a VERY UNIQUE centering-spring and mechanical stop at each end of the stroke that was non-jamming and completely reliable. The output wheel rotated +- 51.4 degrees between the stops and as supplied, the output wheel had push-rod connection holes at 4 locations spaced at 90deg near the outer diameter. This made servo direction reversal possible by using the diametrically opposite hole. This simple arrangement made installation easy and quite flexible. BTW servo direction reversal could also be done by reversing the two leads to the servo. Indeed some modelers added a small 2-pin, non-polarized plug in to series to the servo. Reversing this plug reversed that servo direction. SIMPLICITY.
End-to-end of the servo throw causes the motor to rotate 14 (+-7) revolutions. The stalled power of the servo is comparable to any feedback servo using the MM motor-- no SURPRISE. Because of the constant pulsing (10 pps for the MKI, MKII, MK21), the servos had a dither that removed friction effects and allowed infinite resolution. Also this pulsing kept the drive relay points burnished and clean and provided a voltage to the servo motor that varied from -2.4vdc to +2.4vdc following the Tx stick motion. If you grasped the output wheel, the servo was FORCE-PROPORTIONAL as the transmitter stick is varied end-to-end and NO delay! Don’s TTsc design provided a centering action that now made the output wheel follow the Tx stick particularly over the central region. Blowback FORCE from the control surface provides additional centering action. In truth, the TTsc FORCE, + Blowback FORCE, oppose and balance the servo motor FORCE. Blowback FORCE depends upon control surface deflection angle and airspeed. So a particular Blowback FORCE value is produced by many different combinations of airspeed and deflection angle. Recall the servo inputs are servo motor FORCE, TTsc FORCE, and Blowback FORCE. Because airspeed affects the Blowback FORCE, as airspeed varied, the pilot simply adjusts the stick position to provide the desired FORCE from the servo motor. The servo tends to compensate in a beneficial manner as the airspeed changes. SIMPLICITY.
Walt Good, Howard McEntee as well as Don and myself were well aware of this phenomenon. Maynard Hill recognized this also and preferred the smoothness of the MKII with the original Dee Bee servos for his near 150 mph speed runs.
Unfortunately the “FORCE-PROPORTIONAL” label did not catch-on and never appeared in any of the Quadruplex ads so far as I know.
So the orange box servo needs no defense---modelers loved them once they flew them and they surely helped the MKII achieve great RELIABILITY by SIMPLICITY.
