RC Flying Today
George M. Myers 70 Froehlich Farm Road, Hicksville, NY 11801
ABSTRACT
- TREK electric power controller.
- Electric power calculations.
ELECTRICS
For the past two years I have been very enthusiastic about electric-powered airplanes. They are so quiet and peaceful. You should try one.
It's dead! My electric motor controller succumbed to crash damage. It was an STW, a high-rate (4 kHz), soft-start on/off power controller with BEC (battery eliminator circuit), designed to handle 10 cells and 25 amps for F3E (FAI) class electric-powered sailplane competition. It includes opto-isolation with low-voltage and high-temperature cutoff and is small and light. It worked very well, but repair is a problem because it is made in Germany.
I'm not trying to take over Bob Kopskis' world, but I do feel it's all right to talk about a new, "Made in the U.S.A." power controller that I bought for my WACO Electric sailplane (see my February 1993 column for more about this model).
TREK POWER CONTROLLER
I bought a TREK ESC-200-Optical speed controller to replace the dead STW. The TREK is a high-rate (2.5 kHz) linear proportional control with opto-isolator — but no BEC. The TREK draws 35 mA from a four- or five-cell receiver (RX) pack. It is designed to control up to 75 amps being drawn from six through 18 Ni-Cd cells. Its MOSFET ratings are 120 amps continuous and 300 amps surge. (MOSFETs are a special type of transistor: metal-oxide-semiconductor field-effect transistor.)
The TREK controller is the smoothest, most linear power controller I have ever used. It can turn the prop as slowly as about one revolution per second, or as fast as it will go. The change in rpm varies linearly from off to full on. The motor stops whenever the transmitter control signal is lost, so there are no flyaways under power and no crash-and-burn if the prop stalls the motor.
The TREK ESC-200-Optical weighs 40 grams (1.5 oz.) with connectors and measures 3/4 x 1 x 3 1/4 inches, with a 3/4 x 3/4 x 1-inch lump where the MOSFETs stand up on the circuit board. TREK Industries plans to produce a compact model (about 1/2 x 1 x 1/2 inches) in the near future. When comparing sizes and weights, remember that the TREK requires a separate RX battery. Including a four-cell, 150-mAh pack, the overall size is about 1 x 1 3/8 inches and the weight is 88 grams.
TREK Industries, 75 Oregon Ave., Medford, NY 11763 is the manufacturer. ESC-200-Optical speed controller is available from SR Batteries; Tel: (516) 286-0079. Price: $89.95.
(It is convenient having both the manufacturer and a retailer nearby.)
Okay, so I have a new electric toy and I'm playing with it. It seems likely that you will have the same questions that I had as you proceed up the learning curve.
ELECTRIC POWER ESTIMATES
The first thing that confuses people are the terms used to measure electricity. When you look at a bag of oranges, you can see how big the bag is and how many oranges are in it. When you look at a battery, you can see how big it is, but you can't see how much charge it contains. So you are forced to estimate the charge, based on something.
The charge wants to get out, so we can measure the pressure on the door. That's called voltage. Unfortunately, the pressure doesn't change much as the charge runs out, so voltage isn't a particularly good way to estimate charge. About all you can tell is that the battery is fully charged (over about 1.4 VDC per Ni-Cd cell) or discharged (under about 1.0 VDC per cell). An expanded-scale voltmeter (ESV) lets you guess whether the battery is half-charged, more or less.
When we let the charge run out, we can look through a window in the wire and count the electrons as they pass a point. Well, not really! But the flow of charge through a wire is called amperes (amps), and our "window" is called an ammeter. When the flow is very small, we measure it with very small units, called milliamps (mA — "milli" means 1/1000 of something).
If we let the charge run out and measure the time until something happens, then we can say that the charge that went past our window was "X" amps times "Y" minutes, or "XY" amp-minutes. Since most small batteries were created to power things that use very little power and run a long time, batteries that you can hold in one hand are usually rated for their total charge capacity in milliamp-hours (mAh).
Charge capacity — milliamp-hours (mAh). A convenient dimension people want to use is amp-hours. To get capacity in amp-hours, divide the mAh rating by 1000. To get capacity in amp-minutes, divide the mAh by 1000 and multiply the result by 60 (minutes per hour). It would be a lot easier if power cells were rated in amp-minutes, wouldn't it? Until that day, paste this in the field-kit case: divide by 1000 and multiply by 60. A quick shortcut is multiplying mAh by 0.06 to get amp-minutes.
Here's the first secret formula:
- 1 amp-minute = 1/60 amp-hour = 16.67 mAh
To estimate how long an electric motor should run on various battery packs, put the motor on a convenient test mount (some kind of legal-use plane) and do this:
- Install the prop and/or gearbox you will use.
- Connect your power pack via your power controller.
- Measure the amps drawn in the early part of the run. (If you don't have an ammeter, see below for estimating the amps.)
- Measure the run time — the length of time your power unit runs on one battery charge.
- Write down everything.
Example calculations:
You start with a six-cell, 1,200-mAh battery. You measure the current drawn by the motor/propeller combination and find that it is 20 amps. How long should the motor run? Here's the next secret formula:
- Run time (minutes) = amp-minutes ÷ amps
Calculate amp-minutes from mAh: 1,200 mAh × 0.06 = 72 amp-minutes.
So, run time = 72 amp-minutes ÷ 20 amps = 3.6 minutes from a single battery pack.
If you measured the run time with a stopwatch, then you don't need an ammeter. Calculate the average current drawn like this: 72 amp-minutes ÷ 3.6 minutes = 20 amps.
More examples:
- If a different prop causes the motor to draw 12 amps: 72 ÷ 12 = 6 minutes.
- If you switch to a bigger, 1,800-mAh pack: 1,800 × 0.06 = 108 amp-minutes; with 12 amps draw: 108 ÷ 12 = 9 minutes.
- If you increase from six to seven cells (small change), current scales roughly by cell count: new current ≈ 12 × (7/6) = 14 amps; run time ≈ 108 ÷ 14 = 7.7 minutes.
- If you reduce to five cells: new current ≈ 12 × (5/6) = 10 amps; run time ≈ 108 ÷ 10 = 10.8 minutes.
Making big changes in cell count (e.g., seven to 14 cells) will not result in linear changes, because much of the increased current will go to heat instead of torque. You're better measuring the current if you make such a switch.
In flight, the propeller will unload a little, decreasing the amps drawn and increasing the run time slightly. If you run the motor a short time, then let the batteries rest, they will recover a little; that also can increase total run time slightly. The current drawn from the battery will not be constant but will diminish steadily at a rate dependent upon the load and the condition of the battery. What I'm telling you is that you may get a little more run time in flight than the ground-test calculations show. Be alert to a big increase in run time — when that happens it may mean your battery pack is getting old and may have a shorted cell.
You'll find a lot of black powder coming out of the motor when you draw too much current. You may also see sparking at the commutator, which can interfere with your radio. Expect the brushes inside the motor to wear out rapidly if you exceed the manufacturer's recommended cell count.
Ni-Cd Battery Factors Chart
- Battery capacities (mAh): 250, 500, 800, 900, 1,000, 1,200, 1,800
- Amp-minutes: 15, 30, 48, 54, 60, 72, 108
- "10C" amps: 2.5, 5, 8, 9, 10, 12, 18
- 20-minute amps: 0.75, 1.5, 2.4, 2.7, 3.0, 3.6, 5.4
These numbers are correct no matter how many cells are in the pack, so long as all the cells are connected in series. Parallel connection of cells is bad.
Battery design and condition are big variables, particularly when you exceed a "10C" discharge rate. ("10C" means dumping charge at a rate [amps] that is 10 times the one-hour capacity marked on the cell.) As an example: 10C for a 1,200-mAh pack (1.2 amp-hours) is 12 amps.
My electric helicopter dumps charge at about 20C. It's high but not intolerable. On the other hand, my WACO setup dumps at about 70C! That eats batteries, but on-board battery weight is the criterion in both cases. The difference is that the F3E has climbed out of sight in 30 seconds, so I'm not going to be using that excessively high current for very long during any one run. However, I prefer to see the helicopter hover for at least five minutes.
Sport fliers of stiff-wing airplanes use propellers that drain the on-board power pack at a rate of 20C to 30C and expect a battery pack to last about five minutes. Some batteries tolerate high discharge rates better than others. A battery that has a diameter similar to its length usually tolerates high discharge rates better than one that is slender, like a pencil or coin.
The opposite of discharging is charging. You force the battery's chemistry to reverse itself. That takes patience. In general, you want to charge power packs at a rate less than 3C, which will get the job done in about 20 minutes. One-hour rates are better, and a C/10 charge is necessary after a long run, or for live flights, to rebalance the cell capacities.
Transcribed from original scans by AI. Minor OCR errors may remain.
