RC Helicopters: March and April 2016

Written by Chris Mulcahy Low-speed 3-D setups Column As seen in the March and April 2016 issues of Model Aviation.

Last month [February 2016] I discussed my 6S 600-size Synergy E5s that I set up for low-speed 3-D flying. The inspiration for building the model came after I flew Gary Wright’s helicopter with a similar setup. I asked Gary to explain his setups and some of his thinking behind them. This is what Gary had to say:

Appropriate Configuration

Chris Mulcahy recently flew both of my Synergy helicopters at the Heli Extravaganza at Triple Tree Aerodrome, in Woodruff, South Carolina. During a texting marathon late one evening, he asked if I would write a brief composition about how I configure power systems for my helicopters. After giving it much thought, I finally relented to put “pen to paper.” I often fly lower rpm setups than most are accustomed to. I’m critical of efficiency and flight time, but not total power output. I like to have “appropriate” power and long flight times instead of extreme power and short flights. I often hear that people like the performance and convenience of electric power, but want the flight time of nitro power. They are associating electric power and flight time with the “nuclear” powered configurations that are often seen at events. It does not have to be that way. An engine is only capable of consuming fuel at a finite maximum rate, so peak power is limited to that rate. An electric motor will consume the energy in the battery as quickly as you desire, depending on the load you apply (gearing, pitch, weight, etc.). You can use the “fuel” in a 15-minute flight or a 3-minute flight. I like to configure my helicopters for the performance level that I need, plus some overhead. I don’t like to waste the fuel tank’s contents (battery capacity) by simply turning ballistic rpm. With my 700-size helicopter, I see data log peaks of more than 3.4 kilowatts. Although this is considered low for an electric (by choice because of my setup, the motor will handle a lot more), it still computes to roughly 4.5 hp, which is greater than the stated 3.7 hp of a popular nitro engine. Nitro at the stated horsepower is roughly 2,790 watts because 754 watts is roughly 1 hp. Most of my flights are 12 to 14 minutes in duration. Higher peak power and more runtime than nitro … what’s not to like? Although these setups aren’t for everyone, if you want the benefits of flying electric and flight times that are greater than nitro with acceptable performance levels, they work well. I’m currently flying two Synergy E5s. One is stretched to use 605 blades and the other is stretched to use 715 blades. They are basically 600- and 700-size helicopters. They have appropriate power for the way I like to fly (smooth 3-D, lower rpm ranges), but they won’t start and stop on a dime. A GPS logger in the 700 has registered 92 mph with the current setup, so although they’re not particularly “fast,” they aren’t slow helicopters. The 700 currently flies with three rpm setups: 1,250, 1,500, and 1,750 rpm. The 600 flies with modes for 1,500, 1,700, and 1,900 rpm. The 700 is a 12S setup using 4,350 mAh LiPo batteries, and the 600 uses 6S 5,500 mAh LiPo packs. The 700 helicopter averages 12-minute flights using a mix of the rpm. At 1,750 (high rpm), the helicopter will only fly for 7 to 9 minutes. If the entire flight is flown at 1,250 rpm, it will fly smooth, 3-D-style maneuvers for approximately 15 minutes. My 600 delivers 7 to 9 minutes, depending on how much time I spend in each flight mode.

Power System Configuration

I like to use low Kv motors and deep gearing. The lower Kv motor will have a higher torque constant (Kt) for the lower rpm I like to fly, plus it will govern better because it will require less throttle increase to maintain rotor rpm. These characteristics result in less need for overhead in the system “headroom.” The less overhead, the lower the current peaks will be. That means less load on the batteries, less load on the ESC, etc. Lower C-rated batteries may be used (which are generally lighter), and a smaller/lighter ESC can be utilized. With electrics, there are certain motor “constants” that can measure performance. The one we often hear of is Kv, which is a motor velocity characteristic. It expresses how many rpm the motor will turn per volt applied. There will be some variations depending on the speed controller used, but essentially, a motor of 500 Kv will spin 500 rpm for each volt applied. Io is the no-load current of a motor, and Rm is the terminal resistance. Kt is sometimes mentioned because it is a torque constant. Kt is inversely proportional to Kv, so if you know the Kv, you also know the Kt (Kt x Kv = 1,352). As you place a load on an electric motor, it does not react like a nitro motor. It will attempt to hold the same rpm (or close to it) and simply draw more current from the battery. If we are running the motor at 80% throttle, we add load with collective and/or cyclic pitch. There is some overhead, or headroom, available for the governor to increase throttle and maintain exact rpm. As this load is applied, the motor begins drawing more current to maintain rpm, the governor adds throttle for the ESC if it senses a drop in rpm, and even more current flows. Wouldn’t it be great if we had more torque available so the motor could maintain the rpm with less throttle increase?

Appropriate Power System Configuration

Using a lower Kv motor provides a higher torque constant (Kt). They are, after all, inversely proportional. At the lower rpm that I enjoy, the average current is drastically reduced, so Io comes into play. Io will go slightly higher, along with Kv, so if we want a low Io (unloaded current), we want a low Kv. A motor requires 5 amps of current just to run, for example. Let’s also say that you want ballistic power and have the helicopter configured to deplete the battery in three minutes and it’s a standard 700-size helicopter with 12S 5,000 mAh LiPo batteries. Three minutes would be 80 amps average current to use 4,000 mAh (it’s common to only use 80% of the pack). Five amps are used to turn the motor, so you have 75 amps to do work. Six percent of the energy consumed is not used to fly the helicopter. Now let’s say that you want a 12-minute flight time, so you gear differently and drop the rpm to allow the motor to only draw a 20-amp average. Five amps are used to spin the motor and 15 are used to do the work for you, so you’re only utilizing 75% of the consumed energy to spin the head. In the higher-rpm, 3-minute setup, you had 94% of the energy doing the work. Io often goes up and down proportionally with Kv, so the lower Kv motor also helps in that regard. When selecting gearing, the traditional manner is to compute the Kv by the nominal voltage to provide the rpm that the motor will turn, and apply a gear ratio that will attain your rpm plus some overhead. The point where I have the greatest disagreement with this is the amount of overhead that many think is “required.” For example, many pilots and online calculators will say that you need at least 20% overhead. I disagree with this being an across-the-board amount. Different flying styles need more overhead (for stick banging, high-energy styles), or less overhead (smooth, “old-style” 3-D needs almost none). When I compute for a gear ratio, I use the Kv times nominal voltage (cell count x 3.7), and apply the available gear ratios for a particular model. I want to select a pinion that provides barely the rpm I want to fly in my highest rpm flight mode. In other words, minimal or zero calculated overhead. Most batteries in use today will maintain higher than 3.7 volts per cell, so there actually is a little overhead in the calculation. I will fly the machine, work on tuning the flybarless system, the governor, etc., and if the rpm I’m using in the highest rpm flight mode is adequate, and I don’t notice any bogging or lack of power, I’ll drop a tooth in the pinion (assuming I’m not already at the smallest pinion). If there is still no bogging or lack of power (indicating I still have more overhead than needed), I’ll drop another tooth. If I’m at the smallest pinion, I will start dropping timing in the ESC. This lowers the Kv slightly, so it’s like a fine adjustment to the Kv of the motor. The commutation algorithms on various speed controllers are not the same, therefore the same motor with five different ESCs can, and usually will, vary the actual Kv you use. Raising and lowering timing also raises and lowers the Kv slightly. A side effect is that it will also slightly raise and lower the temperature of the motor. It can also increase and lower the temperature of the ESC as a by-product of the motor, drawing more or less current. When I am setting up a new model, the initial flights are only a minute or two long before I stop to check temperatures. The manufacturer might state all sorts of numbers on the maximum continuous and burst watts, the maximum continuous and burst amps, etc., but it primarily boils down to temperature. If the temperatures are okay, the system will live. The only real caveat to that is an extreme current spike such as what would be seen in an overgeared, ballistic rpm setup. When looking at performance logs for the ESC, the amperage will be jagged with up and down spikes. My goal is to have the lowest peak spikes, while still having adequate performance for my flying style. If it’s flying great, but I see high current spikes, I want to eliminate them. Eliminating high current spikes lowers the temperature and makes the components happier. Achieve this by changing to a smaller pinion, lower timing, and possibly a lower Kv motor. Temperature is the prime concern and current spikes are a close second. I see logs from some of the smack-style 3-D pilots that show average currents in the 70- to 100-amp range and spikes of more than 200 amps. My 700-size helicopter averages less than 20 amps and rarely spikes above 70. A helicopter that I currently fly has a 400 Kv motor and the gears are 12/121. This flies on 12S, so 44.4 (voltage) x 400 (Kv) x pinion (12) ÷ main gear (121) results in 1,761 possible rpm. I wanted 1,750 rpm. It would actually turn slightly more, so I had to tweak it if I wanted to optimize. I started with a 13-tooth pinion that never loaded down, so I went to a 12 (the smallest available). There was still no loading, so I began dropping timing. I used a Castle Creations Talon HV 120 ESC. Default timing is 5° and I had started at a 15° advance. I continued to drop the timing until I arrived at 0. I still have enough overhead because it does not load and bog down, and I’m using the smallest pinion and lowest possible timing. I would have preferred to use a slightly lower Kv motor so that I could find some loading down and raise timing a few degrees to compensate—knowing that it’s optimal—but I don’t have a lower Kv motor. I’ll have to make do with 12- to 14-minute flights. Because the overhead is not extreme, the gearing allows lower throttle settings without overheating the ESC, so the normal range of 200 or 300 rpm from flight condition 1 to flight condition 3 can be increased. I’m down to 1,250 rpm in my lowest rpm mode, so the actual range I use is 1,250 to 1,750. I’ve had it lower—1,100—but the ESC starts getting warm and the collective feel is softer than I prefer. Yes, the overall power is lower than the “standard smack” setup, but it’s appropriate for the way I want to fly. I can execute any 3-D maneuver, although slower and smoother (which I prefer), and it flies a long time on a charge. Is it for everyone? Definitely not, but I would bet it’s appropriate for many pilots. SOURCES: International Radio Controlled Helicopter Association (IRCHA) www.ircha.org Synergy R/C Helicopters www.synergyrchelicopters.com Hacker Motor USA (913) 214-6995 www.hackermotorusa.com Castle Creations (913) 390-6939 www.castlecreations.com