Written by Don Slusarczyk jets
As seen in the Spring 2019 issue of Park Pilot
Back when I sold EDF (electric ducted-fan) motor systems, one of the most common requests was for an EDF system that worked well on 3S and 4S LiPo batteries, with no modification other than swapping out the battery. The reason for wanting such an EDF system was easy enough to understand, but unfortunately, it is more complicated than that. Without getting into the detailed physics behind it, I used to tell people that a system optimized for 3S will not be optimized for 4S and vice versa. They would have to choose a 3S or 4S system and stay with it if they wanted an optimized setup. Why can’t you have an EDF system that is optimized for 3S and 4S at the same time? The reason is a combination of factors, but one of the biggest is that the rotor is the same for both setups. On a propeller-driven airplane, many times you will find that the recommended 3S propeller and 4S propeller are not the same. The 4S propeller will always be smaller in diameter to help reduce the load on the motor. This is not the case on an EDF aircraft because the rotor remains the same. A multiplier factor can be used to help estimate the expected increase in wattage: Wf (Wattage factor): Wf = desired LiPo voltage ÷ current LiPo voltage2 How does the wattage factor work? It is a multiplier that you use to estimate the expected wattage if the flight battery pack increases or decreases in cell count. As an example, when going from a 3S to 4S LiPo, you can expect the wattage to be at least 14.8 ÷ 11.12, or 1.78 times more than what it measured on 3S. A 300-watt 3S system will measure 300 x 1.78, which equals 534 watts (or more) on 4S. The higher wattage also means higher amps. The amps will increase in direct proportion to the increase in voltage. Going from 3S (11.1 volts) to 4S (14.8 volts) means 14.8 ÷ 11.1, or 1.33 times more amps. If you are at 30 amps on 3S, you can expect at least 40 amps on 4S. This wattage factor also works in reverse when removing a cell from the flight pack. Going from 4S to 3S on a 4S EDF system will lower the total system watts because the wattage factor becomes 11.1 ÷ 14.82, or 0.56. The new estimated system watts will be 600 x 0.56, or 336 watts. Your aircraft might have flown well on 600 watts with a 4S LiPo battery, but it will probably not fly so well on 3S with only 336 watts. This is the inherent difficulty in trying to make a 4S/3S EDF system. The power is almost cut in half when you drop that one cell, or the power doubles when you add one cell.
The wattage factors listed in this chart can be used to estimate expected power levels when changing the cell count of the flight battery. Refer to the text for details about how to use these factors.
What is wrong with having the extra power produced from the 4S LiPo? In general, nothing, but we must be fully aware of what the extra power means physically for the EDF power system. I have seen people toss a 4S LiPo into a 3S airplane then fly with no consideration for what is happening to the power system because of the higher pack voltage. The first thing to consider is whether the stock ESC is still adequate. Many of the 64 mm 3S park flyer jets come with a 30-amp ESC. The stock power system measures approximately 27 to 28 amps at full throttle. Going up to 4S means the amps will be approximately 36 to 38, so the possibility of burning out the stock ESC becomes a concern. The next potential issue is burning out the motor because of the higher watts. The calculated watt factor is 1.78, but small EDF motors will heat up and lose efficiency. The wattage factor becomes closer to 2.0. A 300-watt 3S system will now be pushing 600 watts on 4S. It is possible that the motor cannot sustain the extra heat generated from the higher watts. This excess heat can lead to the motor magnets becoming demagnetized or, more typically, a breakdown of the enamel wire coating in the internal windings, causing a short that burns out the motor. If you really want to fly a 3S EDF power system on 4S, take a few precautionary steps before you go out to fly. Take a couple of wattmeter readings on your 3S system with a fully charged LiPo battery to get an idea of what the typical output is. Let the EDF run at full throttle for approximately 10 to 15 seconds to settle, then take your reading of the watts and amps. Do the same on your 4S LiPo and see what the new watts and amps are. If your new amp reading exceeds the stock ESC rating, you will have to install a new ESC rated for the higher amp load. If the new watt reading is more than double what it was before, there is a good chance that the motor will overheat if you fly. Pilots sometimes use a small-capacity 4S LiPo, which sags in voltage under the load, reducing the amps and watts down to acceptable levels. This practice is harsh on the battery and can lead to puffing and overdischarging because the low-voltage cutoff typically has to be set low or disabled to prevent the ESC from cutting off in flight. In this case, I believe you are better off getting a dedicated 4S EDF system. FMS (fmsmodel.com) makes a nice-sounding, 11-blade, 64 mm EDF that can be retrofitted in many of the 3S park flyer EDFs that are on the market. For more information, see my Winter 2019 “Jets” column to read about how I upgraded a Freewing Lippisch P.15 (freewing-model.com) with an FMS EDF unit. Finally, I have created a chart covering a range of typical EDF flight battery packs and the associated wattage factors. I hope this chart will give you a better understanding of how adding or removing a cell can change your total system wattage.
The FMS 11-blade, 64 mm EDF system comes in a 3S or 4S version. The KV3150 on the label of this motor indicates that it is for 4S operation. The 3S is labeled KV3900.
Swapping the smaller stock ESC (bottom) for a higher-capacity ESC (top) is almost always necessary when going from a 3S to a 4S power system.
A wattmeter is required to accurately assess the increased amps and watts when going from a 3S to a 4S LiPo battery.
