Model Avionics RPM Limiter “The Rev-Max”
Model Avionics now has available a new device called an “RPM Limiter”. Another name for the RPM limiter is an over speed governor. Power extraction from the engine is higher and the need for a fast servo or a fast super servo has been removed. For most modelers the normal governor has suitable performance, but for someone wanting maximum power supplied to a rotor system and with absolutely no over-speeding the RPM Limiter is the ticket. The electronic set-up features of the Revmax are pretty simple.
The limiter target rpm can be set from the radio or onboard the unit using a small resistor pot. The device has a setup mode to program your throttle mechanical stops and to tune the device to your radio system. Remember to enter the setup in the normal flight mode and if using an AUX channel for rpm adjustment to have it set at 100%. The status light will flash green for setup and turn yellow for SS mode by raising the throttle stick fully and exiting the mode at that time. Lowering the throttle stick will return to normal servo mode, at which time you’d then exit. The nice thing about this light (LED) is it also tells you when the Rev-Max is potentially active or not. This way you may ensure that it switches on and off at your command so you may easily see where clipping occurs during flight-testing.
The unit when activated allows normal throttle signals from the receiver to pass directly to the throttle servo under most conditions. This is carried out anytime the engine rpm is the same as or lower than the target or limit rpm. By doing so removes governor-think time related to under-speed computations thus offering a direct link in the same manner as a normal pitch-throttle curve relationship would. There is a hitch, you need a fair, but certainly not an excellent throttle/pitch curve relationship. With a normal governor in a general sense, a throttle curve matters much less since the governor simply adjusts the throttle servo up or down for both under and over speed. The limiter however only “clips” at the top when an over speed begins to occur. In other words it will not compensate for under-speeding. So if your throttle curve does not produce enough throttle opening, the rotor rpm will bog down or the rpm will be lower than what you desire. However, if you adjust the throttle curve to give the rpm you desire or even a tad bit more, the limiter will remove any possible over speed condition. The limiter needs to think only in one direction and only at specific times. At all other times the throttle system is direct linked to the receiver with no lag or governor dead band.
Dead band is needed to prevent a governor from hunting. The reason is because if the corrections at the throttle servo are made too soon for both the over and under speed conditions, the system will become confused just like a gyro does with too much gain. Under speeding decisions are not made by the Rev-Max, they are made directly by the throttle curve, so when the limiter stops clipping rpm, ‘wham’, the required throttle servo position is there right now being un-damped by the typical governor firmware. The unit references the throttle curve and not governor delay or underspeeding damped deadband.
This limiting cooperates when using cyclic to throttle mixing since you cannot mix too much, as the limiter will clip any excess rpm by closing the throttle as needed. The better your curves the less the unit has to do, so there is a technical incentive to be basically precise, although in a practical sense excessive clipping does not seem to hurt performance. It actually seems to give decent punch out with no lag or overshoot. In this situation there is still no underspeed calculation as would be the case with a governor so it’s faster, extracting more engine potential to the rotor. Alan Szabo one of the top US flyers runs his rpm limiter in a wide-open throttle curve configuration at all points on his throttle curve. Just remember if the unit turns off in this flight mode the rotor will show a massive over-speeding at lower collective values.
If we ask ourselves where typically the worst condition for over speeding occurs, everyone will answer near half collective/zero collective pitch or when the rotor is aerodynamically driven thus off-loading the engine. Since the engine is not tasked, clipping rpm does not matter for extracting maximum power output. With the unit made active for say 3-D rpm limiting, and then say you switch into a normal low rpm flight mode the rotor rpm simply drops to what the curves demand. The caution with the system is to make sure enough throttle is available to maintain a desired rpm. If you want to fly inverted on a normal rpm curve simply raise the throttle points to an over speed condition relative to a lower rpm clipping limit you will now have set into the unit for that specific flight mode. With modern radios this can be automated into the flight mode switch for a seamless operation.
Here is how we setup our helicopter. First we flew with the limiter off using a 3-D pitch curve and adjusted the V-throttle curve to maintain equal climb upright and inverted by trimming the pitch curve endpoints to the available engine power. We then added in a 30% cyclic to throttle mix and flew again with the unit off. Next we turned the unit on and adjusted the limiter setting to drop the rpm ever so slightly. Finally we hammered the machine around the sky looking for any under speeding noting the collective stick position where it might happen. Any under speeding is now adjusted with the respective throttle curve point(s). This process takes about one flight to perfect. The result is a compromise between a flat line wide open throttle curve and the conventional V-curve with throttle/cyclic mix.
Finally the frosting on the cake is carried out using the throttle to tail rotor mix for those demanding power hungry pirouetting tricks. We found with the 50-sized helicopter that the mix is not really needed, but for the R90SE it certainly boosted performance. You can take your basic 3-D setup and easily make quick use of this device with minimal disturbance to the existing radio parameters.
While we may have not gone too deep into how the unit works it can be stated that performance results are notably better when compared to most any other governor on the market today. We found a midrange power enhancement when weighed against a regular governor compares favourably to going from 15% to 25% nitro content in the fuel, or to that of using a tuned pipe. While the attention to set-up is slightly more detailed than with a typical engine governor the results are far superior. There is no need for a fast servo to make up for governor lag and peak engine power is always on tap. If as a beginner or intermediate you do not want the extra effort or thinking involved with the limiter setup then the normal governor is always an alternative.
Competition pilots have always wanted a direct link between the throttle and rotor but had to put up with annoying over speeding. This is because different maneuvers load the rotor differently at similar collective settings. Some complained of a slight lag between power demand and engine supply. Many flying the bigger helicopters presently employ a governor as the lesser of evils, while hard-core stubborn perfectionists refuse to have any part of a governor. Today we have the cure for both.
We have spent more than two years evaluating the Rev-Max in four different machines and during this time period we were constantly comparing it to the conventional governor and the conventional non-governed configurations. We initially had two additional “full governor type” pre-programmed units (GV-1 and TJ-Pro) that were ‘plug in and go’ at the flying site when questioning any new manoeuvres or situations. All you need do is flip the sensor so the fan magnet polarity works with both brands. Even though the extra performance of the rpm limiter is obvious the first time out, our intent was to find any in-flight performance handicaps. The limiter has none based upon this extensive product use. For any seasoned flyer this product should be the first choice and would be considered the “Cat’s Meow”.