Mysterious Power: The Story of a Miraculous Landing

Electrics By Terry Dunn | [email protected] As seen in the February 2023 issue of Model Aviation. WE ALL LOVE a good crash story. In this edition of the "Electrics" column, I am going share an incredible story about an airplane that should have crashed—but didn’t! I think you will be amazed. You might also learn a little bit about electric power systems at the same time. First, let’s wrap up some unfinished business from my last column.

Go/No-Go Ideas

In my October column, I wrote about using small squares of hook-and-loop tape (Velcro) to indicate the charge state of my batteries. It is a simple "life hack" that helps me to avoid taking off with a depleted battery. I closed the column by asking you to write in with alternate methods for managing batteries at the field. You responded in a big way! Thank you! I am impressed by the clever and simple things that many of you do to distinguish a charged battery from a dead battery with just a quick glance. I cannot share them all here, but I will provide a few examples. Art Pesch stated that he tucks the power wires between the balance plug wires after a flight to indicate that the battery is used. He then separates the leads just before charging. Doug Nasto wraps a #64 rubber band around his batteries after a flight. Several of you shared similar variations of the rubber band method. That makes a lot of sense, because #64 rubber bands are a staple of the RC hobby. Use what you have! Many modelers wrap a #64 rubber band around a battery after it has been flown. This prevents the battery from accidentally being reused without charging. Photo by Doug Nasto. Another common method is to clip something onto the battery wires to indicate when a battery is a no-go for flying. Bruce Rumrey uses plastic clips from bread bags (do those things have a name?). Bob Roska prefers mini clothespins. Bill Perry uses small binder clips from the office supply store. There are lots of common gadgets from which to choose. The key is to be consistent with your usage. Clipping an object onto the wires is another common way to indicate the charge status of a battery, as seen here with the binder clip on Bill Perry’s LiPo. Photo by Bill Perry. Will Casazza wrote that he doesn’t do anything at all to visually distinguish his charged and discharged batteries. He measures the voltage of each battery just before using them. That’s probably the most foolproof method of all.

Dude, Where’s My Battery?

Fitz Walker’s 80-inch wingspan Blohm & Voss BV 141 is a unique model that flies well. It was recently the subject of an extremely mysterious incident. My friend, Fitz Walker, shares my love of unusual airplanes. One of the warbirds in his fleet is an 80-inch wingspan model of the Blohm & Voss BV 141, an asymmetric observation airplane that was flown by the Luftwaffe in World War II. Fitz’s model is powered by a Turnigy RotoMax 1.20 brushless motor with a 16 × 12 three-blade propeller, Castle Creations Phoenix ICE 100 ESC, and an eight-cell 5,000 mAh LiPo battery (two four-cell 5,000 mAh Spektrum Smart batteries wired in series). It flies well and always gets a lot of well-deserved attention wherever he takes it. Fitz was flying his unique model at the Best Electric in South Texas (BEST) event when he noticed something fall out of the airplane during an inverted pass. The canopy had fallen off during a previous flight under similar circumstances, so Fitz assumed that was the case again. Just to be safe, he immediately rolled the airplane upright, lowered the landing gear, and brought the BV 141 in for an uneventful landing on the runway. As the model coasted to a stop after landing, Fitz realized that the motor was not responding to his throttle inputs. In fact, none of the controls were responding. He walked over to retrieve the airplane and discovered that the canopy was indeed gone—but so was the flight battery! The battery was what Fitz had seen fall out of the airplane. No wonder the throttle was unresponsive. This is how Fitz found his model after landing: no canopy and no battery! Photo by Fitz Walker. Here is where things became interesting. The model’s flight battery also powered the radio system through the BEC circuit of the ESC. There was no other battery aboard the airplane. To summarize (read with your best Rod Serling impression):This airplane lost its only power source in flight, but it was still somehow able to roll upright, lower the electric retracts, and fly a controlled approach to a smooth landing on the runway!

Mystery Solved … Mostly

The peanut gallery proposed several theories that ranged from sorcery to divine intervention; however, the strange scenario was eventually explained with science. It all boils down to the fact that electric motors and generators are really the same thing. If you provide it with electrical energy, you get mechanical power that spins the propeller. Conversely, spinning the propeller manually causes the motor to generate electrical power at the leads. Brushed motors generate DC power, while brushless motors generate AC power. When we apply that knowledge to the case of Fitz’s BV 141, things make a bit more sense. After the battery departed the airplane, the propeller began windmilling. This spun the brushless motor and caused it to generate AC electricity that was fed back to the ESC. The ESC rectified the AC electricity to DC electricity, which fed the onboard BEC. The power going into the BEC was sufficient to operate the landing gear and servos all the way to landing. Fitz later proved this explanation by spinning the airplane’s motor with an electric starter and measuring the voltage output from the BEC circuit. Although we can explain how the onboard radio gear was able to operate without a battery, this incident is still quite remarkable—and lucky. Things could have turned out much differently if Fitz had been flying at a lower altitude, using a lower-pitch propeller, or enabling the brake function of the ESC, just to name a few. Many things had to align just right for the BV 141 to keep flying. Let’s also not overlook the fact that the asymmetric BV 141 requires the hefty 8S 5,000 mAh battery to attain the correct longitudinal and lateral balance. How the model continued to fly in such an unbalanced state is still a mystery to me. Maybe Fitz is a sorcerer!

Wrapping Up