The following is an evaluation of three timers I have used. They have been discussed some on this forum in the past.  They are:  The Kim Doherty timer (KDT), the Z-Tron V.4 and the FM-9 timer that Will Hubin just released. The evaluation considers the following attributes: User friendly operation, RPM adjustment, time set capability and changes, ramp up in power at start, ramp up of RPM in flight, start delay, ease of changes, cost, and safety provisions. Note that in this evaluation, only the Shultze 18.46 F2B ESC was used.

User friendly operation:
The KDT is the most user friendly timer of the three. It is truly “plug and play”. Simply connect it to power (BEC) and the ESC, and go through the start process. It ramps up to ~7500 RPM for ~20 seconds, and then to the final power setting. No programming is necessary. This timer does require the use of the Shultze 18.46 F2B ESC. To operate the system, the primary power switch is turned on. This powers up the timer, but does not start the motor. To start the motor, a magnet is passed in front of a Hall sensor which then activates the start sequence. It runs the defined time, and stops. It can not be re-started without passing the starting magnet in front of the Hall sensor.
The Z-Tron requires going through a sequence of dip switch changes to set numerous parameters. It took several attempts to get it right before flight. It can be used with various ESC’s. To start this system, the power is switched on and the system then engages immediately.
The FM-9 timer requires programming before use as well. The sequence of settings is very straight forward, and it does accommodate a number of ESC’s.  There is a single push button switch that is activated to start the system.

RPM Adjustment:
The KDT has a rotary position encoder that allows 20 RPM adjustments from ~8000 RPM up to ~12,000 RPM. This is used real time when the motor is at ramped up power. This feature is the digital analog of the internal combustion engines needle valve. It is the single best option on this timer!
The Z-Tron has a pot that has several “detents’ that adjust the rpm by ~20. However, there IS a limit to how much it can be adjusted.  With the ESC mentioned above, the RPM had to be set at ~12% of maximum to get the desired 8650 RPM.  With that, there were bands of RPM not achievable due to the limits of the pot. This was discussed with Mike Palko, and he has the same issue using the Shultze F2B ESC. If the RPM needed were in this band, the prop would have to be re-pitched to get the desired flight speed. The speed is changed by way of the dip switches. There is a sequence to go through, and it takes a little time to become familiar with it. You must record the settings yourself, as there is no display to visualize them.
The FM-9 timer has a programming box that is attached to the timer to set adjustments. It adjusts in 40 RPM increments, which is adequate. To do the adjustment, the programming box must be connected to the timer board. The routine to change the speed is simple, and fast. When changing, the exact RPM is shown on the screen, so there is minimal chance for error. Changing the speed with the FM-9 is slower than the KDT, but not unreasonably so.

Time set capability:
The KDT can adjust the flight time incrementally by moving the position encoder while the starting wand is against the Hall sensor. It is measured by the number of “detents” in the rotary encoder. The timer is encoded with a flight time of 6 minutes and 16 seconds. It will default to this if the encoder is not touched during the time the magnet is in front of the Hall sensor.
The Z-Tron flight time is controlled by the dip switch position at time of power up. It can be adjusted in 10 second increments. This is one strong point of this timer.
The FM-9 flight time can be adjusted in 1 second increments. It is shown on the programming board, so there is little chance for error in this system.

Ramp up in power at start:
The KDT starts by gradually ramping to 7500 RPM. This allows time to walk to the handle without having the system at flight power. At 20 seconds, it ramps up to flight power in about one to two seconds.  
The Z-Tron starts by running up to full system power (after the scheduled delay) for a few seconds, and then backs down to flight power level. This is known, but every time it happens, it causes concern that something has gone wrong.
The FM-9 starts by pushing the button, and ramping up to flight power, after the scheduled delay.

Ramp up of RPM in flight:
The Z-Tron is the only one of the three that can vary the RPM throughout the flight. This can be programmed in so that it: Stays the same throughout the flight, increases speed throughout the flight. Or decreases speed throughout the flight. This is one advantage this timer has over the other two.

Start delay:
The KDT does not have a start delay. It does however come up to ~7500 RPM and hold there for ~ 20 seconds to allow time to reach the handle. It then ramps up to flight power. There are no adjustable features for this time frame.
The Z-Tron has a start delay that is programmable.
The FM-9 also has a programmable start delay.

Ease of changes:
For the KDT, the changes discussed previously that are adjustable are easy the change.
The Z-Tron is the more difficult timer to change. All programmed functions need to be written down; otherwise there is no way to tell where they are set prior to flight. The flight time is easy to read though, as it is a function of the dip switch position at time of start.
The FM-9 is easy to change any function. One just needs to connect the programming box to the timer, and run through a quick sequence.

Weight:
KDT  17 grams
Z-Tron  13 grams
FM-9  8 grams

Cost:
The KDT was selling for about $35.00 (I have to remember, because it has been a while).
The Z-Tron timer is $49.00
The FM-9 timer is $10.00; however, it requires the programmer which is $75.00

Safety provisions:
The KDT is the safest timer of the three. To start, the power needs to be connected, the arming switch on, and a magnet passed in front of the Hall sensor.
The Z-Tron and the FM-9 are started simply by moving a switch once the power is on.

I personally use separate arming switch on my planes now. It provides the ABSOLUTE safest way to disarm the system. The switch goes between the battery and the ESC. Without the plug in, it simply can’t run.

This is simply a summary of the functions available from these three suppliers. Which one is better?  That’s not the purpose of this discussion. Every user is going to have their own criteria for what they want in a system. Which one am I using?  I am currently using the KDT. I have used it since early 2007, and am satisfied with its functionality. I have used the FM-9 and the Z-Tron as well. They all work as advertised.

It’s a great time to be flying Stunt, with all this reliable and capable equipment on the market now!  

Suppliers: If I have something wrong, please speak up. I have recorded what I have experienced.

Paul Walker