Flight Manual




 (with enhanced variometer zeroing section)





The Comstock Balloon Autopilot is designed to fly a Roziere balloon indefinitely at a constant pressure altitude.  It accomplishes this by opening and closing a user-provided fuel solenoid valve to control the flow of fuel to the burner.  This autopilot can also fly any hot air balloon.


The Model IV autopilot also allows the level flight altitude to be changed while the autopilot is flying the balloon.





1. About ten minutes before intended AUTOPILOT use switch on the autopilot to WARMUP or MANUAL.  This step is not mandatory but allows the temperature of the unit to stabilize before use and will prevent negligible altitude drift which might otherwise occur while the unit warms to its subsequently stable operating temperature.


2. If you wish only to use the unit's BLAST switch to manually operate the burner solenoid valve, switch the autopilot to MANUAL to enable the BLAST switch.


3. If you wish the autopilot to engage, assure that the ALTITUDE SHIFT feature is disabled and then switch the unit to AUTOPILOT.  The autopilot will engage only after you continue to fly the balloon for a short period of time.  UNTIL THE AUTOPILOT ENGAGES, YOU MUST CONTINUE TO FLY THE BALLOON MANUALLY USING ONLY THE BLAST SWITCH ON THE FRONT OF THE AUTOPILOT.  When the autopilot engages and begins to fly the balloon itself it will chirp and flash the red Malfunction LED indicator twice, and the green A/P in control LED will illuminate and remain illuminated as long as the autopilot remains in control.  At this signal, you can cease flying the balloon manually.  The autopilot will have locked onto the pressure altitude at which the balloon was when the autopilot engaged.


After you switch the unit to AUTOPILOT it will engage only after the following conditions are met:


(1) at least one minute must have passed since the unit was switched to AUTOPILOT, AND


(2) at least six seconds of heating using the blast switch on the autopilot must have occurred since the unit was switched to AUTOPILOT, AND


(3) the proportion of the time the burner has been on since the unit was switched to AUTOPILOT must have been at least 1% and not more than 50%.


Once every six seconds after the autopilot engages it considers heating.  This cycle can be altered to 15 seconds by holding the blast switch down during the instant that the selector switch is turned from MANUAL to AUTOPILOT.  If the autopilot is already engaged, only switching the unit back to manual and repeating the engagement process as described above can change the decision period.


The ALARM switch enables or disables the audible alarm.  While the audible alarm is disabled the yellow Alarm disabled LED indicator remains illuminated.


To disengage the autopilot switch the unit from AUTOPILOT to any other position (MANUAL, WARMUP, or OFF).




The level flight target altitude can be changed while the autopilot is flying the balloon.  This allows adjusting the level flight altitude without disengaging and re-engaging the autopilot.  This feature has no effect unless the autopilot is already engaged, and this feature should be disabled before the autopilot engagement process is begun.


If the autopilot is flying the balloon, you can shift the level flight altitude as follows.


1. Set the ALTITUDE SHIFT knob to the rate at which you wish the target altitude to be changed.  Select slower rates until you are familiar with how the balloon and autopilot combination you are flying reacts to this control.


2. Switch the ALTITUDE SHIFT switch to Enable to start the altitude shift.  This causes the target altitude to be incremented or decremented at the rate dialed in.


3. Exactly as the balloon reaches the new desired altitude, switch the ALTITUDE SHIFT feature to Disable.  The autopilot will lock onto the altitude at which the balloon is when the ALTITUDE SHIFT feature is disabled.


NOTE: At any time the autopilot is in control, switching the ALTITUDE SHIFT feature on for a couple seconds and then off again will lock onto whatever altitude the balloon is at when the feature is switched off.  This might sometimes be useful. 




A digital display of the raw variometer output is at the left of the autopilot front panel.  This display is provided for use in zeroing the variometer and is not intended as a flight instrument.  The display can be switched on or off by the switch below the display.


The data displayed here is uncorrected output from the variometer.  If the ambient temperature of the sensing unit within the autopilot is far from 70 degrees Fahrenheit, say 10 degrees Fahrenheit, this data could vary by more than 100 feet per minute from the actual rate of climb because of zero drift of the variometer due to temperature changes.  Using its separately sensed altitude data over time the autopilot computer periodically automatically corrects for this drift.  These corrections affect the operation of the autopilot but are not reflected in the raw data displayed by the digital display.




Before use, the variometer must be zeroed for the orientation in which the autopilot will be in use.  The autopilot is delivered with the variometer zeroed for whatever orientation the purchaser has specified.  The autopilot variometer can be zeroed in the field by adjusting the trim potentiometer on the topmost circuit board to cause the variometer raw data display to read zero.  It is normal for the rate of climb displayed to vary while the unit is being observed on the bench.  This is due to subtle air pressure changes resulting from heating/cooling system fans, wind, temperature changes, and minor inherent instability.  As noted in the above section, the autopilot regularly automatically corrects for drift in the zero point of the variometer.   


The consequence of the variometer zero adjustment having drifted off is that the autopilot may, upon engaging, immediately climb or descend.  After about five minutes of flight the autopilot will automatically correct its rate of climb input and the balloon will move towards the correct level flight altitude.  From then on the autopilot will fly the balloon close to the correct target altitude.  During the first five minutes of engagement for each foot per minute the variometer reads high the autopilot will fly about two feet below the level flight target altitude, and vice versa.  




While the autopilot is engaged (flying the balloon), it continuously monitors altitude error, rate of climb, and acceleration.  If any of these exceeds a maximum preset value, the autopilot sounds the audible alarm and lights the red Malfunction indicator.  THE AUTOPILOT WILL CONTINUE TO ATTEMPT TO FLY THE BALLOON DESPITE THE ERROR CONDITION.  The limits are: altitude more than 500 feet above or below the selected altitude, rate of climb or descent greater than 300 feet per minute, or acceleration more than 1000 feet per minute per minute up or down.  When the error condition is corrected, the audible alarm and red Malfunction LED indicator turn off.


The Comstock autopilot has proved to be completely reliable in service.  However, if correct operation of the autopilot is critical to the success of the flight, prudence would dictate carrying a spare. Thus, in the unexpected case of the autopilot for whatever reason becoming inoperable, the autopilot could be replaced with the spare.  This would require only actual physical replacement of the unit and disconnection and reconnection of four wires and the ambient pressure source tube.





The unit was originally designed to fit on the back of the desk, beneath the avionics stack, in the original standard Cameron Roziere capsule.  However, it can be mounted in any position and location that allows convenient access to the front panel during flight.  On the back of the unit is a RF-suppressing connection block for power and burner-control relay contacts, with voltage-spike absorbing varisters installed on it.  Also on the back of the unit is a ¼” tubing barb fitting for connection to the outside ambient pressure


The 12 VDC power should be connected through a 1-ampere circuit breaker or fuse (not provided) to the barrier-strip terminal connector block on the back of the unit.  Be careful to respect the polarity marked.


The two solenoid valve wires connect to the barrier-strip terminal connector block.  These terminals connect to the relay contacts within the autopilot. The relay contacts should be connected by external wiring and electric power to open and close an electric solenoid valve that feeds fuel to the burner.  Either the autopilot relay contacts or the blast switch mounted on the unit’s front panel closes the circuit between the two relay contact terminals.  These contacts are rated for a maximum of 3 amperes.  If a larger current load is possible, an additional external relay should be provided.


The autopilot is delivered with the variometer zeroed for operation with the variometer face vertical.  On request, the unit can be delivered with variometer zeroed for any orientation desired.  Operation in an orientation other than that for which the variometer is zeroed might cause a slight altitude error, but only during the first five minutes of engagement.





As with any electronic device, keep the autopilot dry.  Avoid dropping the unit or otherwise subjecting it to severe physical shock.







Although the variometer incorporated in the autopilot is itself sensitive to radio interference, particularly in the VHF band, the autopilot incorporates appropriate shielding and RF-suppression circuitry to prevent any interference with its operation from electromagnetic radiation.  These features render the autopilot immune to RF-interference.  Regardless, prudence would dictate testing the unit after installation to confirm that none of the possible onboard electromagnetic radiation sources cause any deflection of the variometer needle. 





The autopilot is designed to operate reliably from about -20 degrees C (-4 F) to more than +60 degrees C (140 F).  It will probably work fine considerably above or below these temperatures.


The unit requires 9-16 volts DC and draws an average of about 0.4 amperes.  This figure does NOT include the current drawn by the solenoid valve(s), which might typically be about 1.25 ampere per valve for the time the valve is open.  If the burner needs to be on about 10% of the time, the total average current draw of the autopilot and one typical solenoid valve together would be about a half-ampere.


The autopilot will function correctly down to less than 9 volts DC.  Most other electrical devices in the capsule rated for 12 volts DC would likely already have ceased to function at a voltage this low.





Re-read the rules of engagement above.  If the balloon needs the burner to be on less than 1% of the time, perform the engagement procedure as described using the 15-second control interval.  After at least six seconds total of heating have occurred during the engagement procedure, depress the blast switch and hold it down until the autopilot accepts control.  The balloon will ascend some, but the unit will most likely gradually settle down to correct operation at the desired altitude.


The autopilot will do a poorer job of flying the balloon if the duty cycle of the burner is less than 1%.  This will occur near mid-day, either shortly before the balloon begins to solar or just after the balloon ceases solaring.  Regardless, the autopilot will under almost all conditions fly the balloon more closely to level than any human pilot could.


If the balloon requires heating more than 50% of the time, perform the engagement procedure as described above using the 5-second control interval.  After at least one minute has passed, stop heating.  The autopilot will take control as soon as the average duty cycle of the burner has dropped to 50%.  The balloon will descend some, but the unit will most likely gradually settle down to correct operation at the desired altitude.





The autopilot will typically fly the balloon almost perfectly if the duty cycle of the burner is close to 50%.  In large rtw Roziere balloons at about 30,000 feet altitude over a wide range of burner duty cycles the autopilot will typically maintain the balloon within plus or minus 100 feet of the desired altitude -– for as long as fuel is available.  At lower altitudes the altitude range is much less, typically within plus or minus 25 feet at 10,000 feet.  Strong vertical motion of the atmosphere may cause the autopilot to be unable to control the balloon.



Good Luck!