Rotor Tachs

The rotor tach has been a standard on full size helicopters since the early days due to the critical nature of maintaining exacting rpm (rotations per minute). The main reason is because structural limitations might be exceeded either by an excessively low or high rotor rpm (+/-15%). For the most part models have the advantage of totally ignoring this parameter. There are issues we share with our bigger brethren one of which is efficiency and another of cyclic response. A faster turning rotor will produce a more aggressive cyclic control and at the same time store more energy. The latter may be used to fly the machine through temporary situations demanding a momentary spike in unavailable engine power. This is sometimes the case with the smaller models when pushed to the limits.
With any model helicopter rotor there is a sweet rpm which offers the best lifting averaged over all collective settings, however this may not be the ideal for a desired cyclic rate. It also may or may not be an optimum for a selected gear ratio needed to extract maximum engine power or efficiency which of course would be the most prominent consideration. We may tweak the rotor and engine setup to meet a best compromise but in the final analysis we are often guessing. The methods of rpm estimation at our disposal are audio/visual in nature. We can sample climb rates and listen to the engine or rotor during various aerobatic maneuvers. The further the model is away from us the more limited this guesstimating becomes. A finely honed modeler has little issue in making a model fly well but when it comes to perfection he or she is still relying on their eyes and ears by means of estimation.

Manufacturers have for many years recognized the need for a tachometer as a means to sample rotor speed with respect for achieving extra performance. They have also marketed engine governors to meet this end. Governors must be told what rpm to manage and do their best based upon rotor load extremes. Many governors have no rpm display. However, a governor will let the target rpm slow down when the rotor load exceeds the available engine power. Since the engine power curve peak relates to the gear ratio and in the final analysis to rotor rpm it becomes an important point of interest. Further more if the engine is set too lean the governor often may not be able to prevent over-speeding. The same can be said of the non-governed setup. Engine governors with rpm displays can offer a single sample record but only at a specific point. This makes for quite a limitation. The remote tach will help accurately verify matters.

The onboard type tach must be within visual range and carefully viewed away from direct sunlight. This component attaches to the helicopter and is read by means of the integral LCD (liquid crystal display). Flying close to view the onboard display may cause one to become nervous and along with better safety standards these days careful consideration is a certainty. This type is quite limited if you ask me, unless of course you spend your flying time hovering about. You simply cannot quantify the rotor variance under changing loads during aerobatics. I have seen bicycle speedometers economically modified for on-board helicopter application but they still suffer these limitations. In my mind this method of sampling is best left to the fixed wing lot or the beginner going it on his own.

As far as I know only two companies supply a fully functioning remote tachometer, one is Miniature Aircraft and another Model Avionics. The latter is newest kid on the block. Since I have both products I can tell you either do a very good job at measuring rotor rpm in all flight situations. They do however need a second set of hands, thus relying on the buddy system.

The remote optical tach operates by means of an adjustable shutter speed matched to its LCD. A neat thing is that the full size helicopter industry use a similar method to track and balance rotors. The shutter speed functions like a strobe light might by visually freezing movement in space. If the light flashes at the same frequency as the rpm then the rotor will be frozen to the eye. When a shutter opens and closes at the same frequency as a spinning rotor then the rotor will appear stopped. Since we have a technical means to correctly correlate shutter speed to rpm by way of a display then we now know the rpm. The shutter speed is nothing more than a spinning disk with two equally spaced openings passing over a viewing window. Because the DC powered motor has brushes it is a simple matter to count the number electrical spikes occurring here and electronically convert it to an accurate display value. The rpm of the spinning shutter/disk is carefully controlled and maintained by the tach’s internal electronics. The tach is tuned to the rotor rpm by spooling the internal motor up and down until a stopped rotor image is seen. At this point the the tach “spooling” switch is released and the internal rpm locked along with the display reading.

 

The rotating disk shutter from Miniature Aircraft and the Rotating drum shutter from Model Avionics.

With this device rotor over and under speed can be quickly detected then measured by adjusting the shutter speed for a stopped image, noting and adjusting against the direction of rotor rotation in the viewer. At the time the tach rpm is adjusted for a stopped image is the time to read the display. Where this interpretation becomes most handy is during full power climbs and other high power demands. It also helps during maximum rotor off loading during negative pitch descents and zero collective 3-D positioning. Pitch curves, cyclic rates and mixture settings may be safely and accurately peaked by this method. This tool shines when evaluating governors and conventional pitch/throttle relationships. It is my recommendation that every club have one to be shared by all. At the very least it takes all the guess work out of rotor rpm evaluation and doubles as an educational tool.

Bigger engines are more critical to rotor over-speed damage and bigger helicopter rotors are under substantially more stress. By adhering to correct rotor rpm through the use of a governor and/or tach will offer engine longevity and ensure the safest setup. Certain large rotor scale ships are running highly stressed 60 rotor heads. Running normal 60 rotor speeds is not recommended nor advised due to the massive blade weight and size. Care is needed to stay under the strain threshold and so the tach becomes an ideal tool. The beginner has no idea what is normal other than by what the local expert states as a general value, something like 1500-1600NR. He has no past basis for how the helicopter should sound or feel. This tool takes some of the work load off the local expert by allowing the not so expert to help themselves. This is a very good thing!

Switch placement and electrical energy consumption test of the Miniature Aircraft Tach.

The first unit to look at is the older tried and true Miniature Aircraft P/N #0524 remote optical tach. It has been around for a very long time and is quite reliable. It uses a single rocker switch for rpm up and down along with an on/off switch. The unit is powered by a 9 volt battery and tends to use battery energy rather quickly. One should carry a spare battery with it, depending on usage. Measured stabilized current consumption is 189 ma. Servicing requires a screwdriver to separate the metal casing halves to be granted battery access. Spool up time with the MA tach is measured at about 5 seconds defaulting to 1500 rpm. Bandwidth is 1000-1990 rotor speed with a 10 rpm resolution utilizing a 4 digit display. The last digit is not used and remains at zero all the time. A good overall figure even though some run higher speeds these days. The large plastic spinning disk uses two viewing slots spaced at 180 degrees. This gives two optical frames per rotation. The one I have includes a nice leather pouch with a Velcro closing flap for storage. It also includes a three page instruction manual. It has an auto shut off feature for the motor after 3 minutes of inactivity but retains the last set rpm. I’d recommend that it be turned off to save battery energy rather than use this function (so does Miniature AC). As expected it works with both left or right hand rotor systems and multi blade rotors. The difference being in the somewhat slow moving number of frozen blade images. A 3 blade rotor will show a six blade stopped image while a 4 blade machine blade displays four. A two blade rotor as can be reasoned shows a stopped flybar and rotor image. Bumping the up/down rpm switch is ideal when the target speed is approached for a quick and accurate sample. Speed is controlled by a precision circuit monitoring the motor frequency. What can I say but it works well and lasts a long time. Pricing and bandwidth might be a consideration for some.

Model Avionics Tach showing the results of electrical energy consumption tests.
The next unit is made by Model Avionics a small but highly focused company dedicated to general model helicopter support in the electronics department. Due to the small but attentive product line, amendments or enhancements happen rather quickly. One popular item is the Throttle Jockey engine governor which has undergone several evolutionary changes over a relatively short period. These changes were implemented from the direct input of various facets of model helicopter pilots. What started as a basic unit with a variable in-flight ability now exploits a simple ground adjustable single rpm format, an extreme rpm version is now available and the newest type has super servo support. To my understanding a newer and unique dynamic mixture adjusting feature is under development consideration.

Back to the Model Avionics extreme RPM optical tach. The range of operation is 1000-2400 rpm with a 10 rpm display resolution. The smaller display is 3 digits wide. The higher rpm specification allows one to easily explore the magnitudes of over speeding. Some of the smaller electrics run at very high rpm so this may be a consideration. Sampling accuracy is claimed and found to be to be better than 1%. The smaller plastic cased unit uses a 9 volt battery and spool up is about 3 seconds. Battery removal is by means of a sliding plastic door. Sleep or off mode is automatically activated after 30 seconds of switch inactivity but the last rpm is maintained. Sleep mode uses about what a wrist watch might in the way of energy consumption. Pushing any button turns the device on. The shutter uses 1 viewing slot through a rotating drum which equates to two optical frames per rotation. The drum holds a magnet for the pickup to track rpm. You could almost call it a governor and this is why its circuitry is less complicated. The smaller viewing window works fine but it does mean you need to have your eye very close to the tach. The operation is the same as the Miniature Aircraft unit but instead of a single rocker switch with an additional on/off switch it uses two, one for up and one for down. There are minor cosmetic differences between each unit but that’s about it.

Twin switch configuration of the Model Avionics Tach and the results from a proper rotor speed and setup.
In use both prove to have the same “real world” accuracy and ease of handling although I initially prefer the single 0524 rocker switch operation because that is what I was familiar. I also have a preference to the slightly larger viewing window of the 0524. I really like the Model Avionics earlier sleep mode, modern design, higher rpm sampling, the added battery longevity due to the exceptionally low 32ma stabilized current drain (measured) and the very attractive pricing. There you have it the new with the old and either will do the trick.