Towing Electric Planes To Increase Range? I’m Not Convinced, But It’s Interesting

Sign up for daily news updates from CleanTechnica on email. Or follow us on Google News!

---

One of the great pleasures of my global decarbonization efforts across multiple domains is that I get to speak to absurdly bright people who come up with very lateral solutions to common problems. One of this week’s conversations, with Damon Vander Lind, founder and CEO of startup Magpie Aviation, is worth writing a bit about.

Full disclosure: I wrote explicitly about one of Vander Lind’s previous efforts, the airborne wind energy effort Makani, which was acquired by Google (now Alphabet) first in 2014 when I was deeply unconvinced that it could overcome the many obstacles that stood between it and viability, in my engineering options assessment for all of airborne wind energy the same year when I concluded that the entire space wouldn’t be viable, and again in 2020 when Alphabet finally pulled the plug on the firm. Vander Lind was lead engineer and ran Makani after its founder, Corwin Hardham, passed away prematurely.

And I’ve written extensively about the category of Vander Lind’s second aerospace effort, electric vertical take-off and landing aircraft (EVTOL) that transition to horizontal flight to serve the almost non-existent and not-going-to-grow urban air mobility (UAM) space. In his case, he was lead engineer on the Kitty Hawk Heaviside, also now defunct. That too was funded by Google, although less directly. Google co-founder Larry Page put the money up for it, and Vander Lind got the prototype Heaviside to flight testing before the company shut that program down and focused on Wisk Aero, its joint venture in the space with Boeing, which is throwing away $450 million of its shareholders’ money on an autonomous EVTOL. Vander Lind didn’t head off to Wisk Aero.

Having been a vocal critic of two entire categories of Vander Lind’s career, I was somewhat surprised when he reached out to me late in 2021 over my assessments of EVTOLs and UAM. Our initial conversation was excellent, deep, and broad. And at the time, I expressed a common refrain of mine, that I really wish talented engineers wouldn’t let themselves get seduced by easy money to do fun engineering projects that are dead ends and won’t move the climate needle, something Paul Martin and I have discussed several times as well. Vander Lind agreed, and admitted that he’d had challenges with the usefulness and market viability of both Makani and Kitty Hawk. He shared that he seems to have no problems finding people to fund his efforts, but his next one would be focused on a more viable solution that would have bigger benefits.

He and I kept in touch over the intervening months, without making time for another call. He did get more funding, established a stealth company to do something unstated, and then, last week, unveiled the focus of the firm. He reached out to see if I’d be up for another chat, and of course I was.

First, a bit more on Makani. The development path for the startup included an initial 20 kW prototype of its tethered carbon-fiber wing, followed by a 600 kW full-scale solution which it flew in Hawaii in 2018, leading to the usual bunch of emails and messages from airborne wind types who think I’m wrong despite their continuing failure to be viable or sell anything. Publicly, I’d only found evidence of a single test flight, so this time I asked how many times they’d flown the beast. Turns out it was dozens, but less than 100. More than I’d expected, and interesting.

One of the topics Vander Lind and I had discussed in our first call, something I also discussed with Heart Aerospace co-founder and CEO Anders Forslund around the same time, was the rule of thumb I apply for physical prototypes, which is that they have to be quarter-scale by volume and mass or non-linear scaling effects would show up late in development. Makani’s 20 kW device wasn’t quarter-scale, and in this conversation Vander Lind talked a bit about the results, which was that they found out about some elasticity concerns which they might have been able to foresee but hadn’t, and hadn’t discovered with the smaller device.

Makani was trying to fly a 28-meter (92 feet) wide, 1050 kilogram (2314 lb) device, traveling in a wind-driven circle at perhaps 130-140 kph (80-85 mph) when operating as designed at distances of 440 meters (1443 feet) from its mooring point with an up to kilometer long carbon fiber and aluminum electrified tether. That’s a big engineering challenge, and they found things that they didn’t expect and that the too-small prototype didn’t reveal. Heart doesn’t have that challenge as they are designing a bog standard suspended box high-wing passenger airplane where the physics and design points are incredibly well understood by the way, something Forslund and I agreed on after talking about it a bit. Their bench test engine, small RC version, and integrated copper/iron bird were fit for purpose. Something to note for others engaging in novel and complex engineering spaces.

Enough about Vander Lind’s past. What is he doing with Magpie, is it viable, and will it move the climate needle? I’m not sure, but I’m not not sure either. The jury is out, but the trial is underway.

Magpie’s concept is that an electric passenger aircraft, a big turboprop like a DeHavilland Dash-8 Q400 carrying 90 passengers but with an electric drive train, would take off from San Francisco’s SFO heading for perhaps Los Angeles’ LAX. Commercially available battery energy densities with sufficient volumes and the right characteristics for aircraft as recently as this month would have made that distance impossible. I mean, you could probably retrofit a Q400 and fly it that far empty, but not with any reserve or divert, and you wouldn’t have any juice left in the batteries when you landed. In other words, it couldn’t be certified and flown.

Chip in a few dollars a month to help support independent cleantech coverage that helps to accelerate the cleantech revolution!

I’m on record as saying that this is a timing problem, not a fundamental limitation, and that we’ll be seeing fully electric passenger aircraft capable of crossing the Pacific Ocean around 2070, but aviation has to decarbonize much more rapidly than that. (I’m sure that the person who demands that I be explicit about the specific battery chemistry and all technologies will chime in again as I re-assert this, as they can’t seem to help themselves.)

And CATL’s announcement of a 500 Wh per kilogram battery in mid-April 2023, almost double Tesla’s batteries, might make this trip viable regardless, but I haven’t done the math. That battery is targeted initially at the aviation market, but the firm indicates that it will be releasing a ground transportation version later this year, and as it owns about 34% of the market and has a track record of delivering what it promises when it promises it, I tend to believe the company. But still, battery energy density is going to be a constraint for aviation for decades. CATL’s new batteries won’t get a Q400 scale aircraft over the mountains and 940 miles to Denver’s DIA and its massive demonic horse sculpture, as an obvious example.

But Vander Lind had an insight. Among other things, he’s a glider pilot. One thing that glider pilots get is using tows from other aircraft to get up to altitude and over to the thermals and ridgelines that are so much fun. It’s a common solution, although some places have winches instead. This was even a World War II technology, where 13-28 troops and their gear were towed up to altitude and then released to fly silently and undetected behind enemy lines.

Look back at that map from Google above. Count the number of small airports between SFO and LAX. Imagine that when the Q400 gets up to altitude and out of SFO controlled airspace, perhaps 20 or 30 miles south, a smaller plane with a lot of batteries flies up from one of those airports, unwinds a long tow cable, the Q400 noses a connector into the latching mechanism, then lets the small plane pull it along for a hundred miles before unlatching. The small plane with lots of batteries flies down to another of the airports to recharge. Perhaps another small plane pops up to give another tow for another hundred miles.

The Q400 uses its batteries most for ascent and reserve and divert range. Operators can optimize use for battery life. Smaller regional airports get activated with tow aircraft and regional air mobility (RAM) along with aircraft charging, solar panels, and storage on their copious free ground. Electricity requirements at major airport wouldn’t climb as much as quickly. Longer flights become close to zero carbon in operation, not to mention a lot quieter. And don’t think the number of airports between SFO and LAX is unusual in the developed world. There are 5,030 airports in the USA and about 2,800 or so in Europe.

So, is this technically viable? It should be, but there is one outstanding question I had for Vander Lind. Let’s start with the basics, though. Towing gliders works and there are disconnects and the like for the tow aircraft that are already well iterated. In the military, fighter jets going longer distances nose up to airborne fuel tankers and refuel at speeds of 180 knots to 325 knots, and have been for decades. The combination means that it’s at least technically feasible to do a mid-air tow connection and release. And the strain on the cable and connectors isn’t nearly as great if all you are doing is hooking up at altitude instead of being towed to altitude.

It’s actually not as hard as it sounds to thread the needle of the connector either. Put vanes and actuators on it and the end of the cable will come to the boom out of the nose of an aircraft. This has been common technology on military ordnance for decades, and isn’t rocket science. Some physical intelligence like the cone currently at the end of refueling booms will do a bunch more. This is where my scale of prototype concern arises. Vander Lind and team have done this in mid-air with a proof of concept already with existing powered aircraft, but at light aircraft scale, well below Q400 scale. Will there be concerns that arise? Vander Lind and his team think it will actually work out better at larger scale, but time will tell.

Why not just mid-air refueling with electricity instead of towing? After all, Makani’s carbon fiber-wrapped aluminum tether would work just fine in this case. Well, jets gulp down fuel in 5-7 minutes, but electricity takes longer, and they are pretty sure towing is more efficient in this use case.

What about stresses on the airframes? That’s obviously a concern. My instinct is that carbon fiber and aluminum retrofit air frame strengthening on the towed aircraft would be viable. And the tow aircraft would be engineered for the strain.

What about clear air turbulence with significant foot-per-second drops and raises? Automatic decoupling with handshakes at either end would turn the linked planes back into two aircraft, and electric motors on the towed aircraft aren’t gas turbines that need to be warmed up and spun up.

What about pilots? Another couple of pilots employed for the same trip, and pilots at both ends of the cable have to be certified in operations. Pilot availability is a huge sticking point of the global aviation industry right now for a variety of reasons, one of which was airlines retiring older pilots at the beginning of COVID-19, leaving far too few pilots around as we’ve been returning to normal. But the industry is responding with great pay packages for pilots again, and this solution would align with the growth of regional air mobility and electric aviation, so it would be viable.

An airline CEO acquaintance of Vander Lind’s has apparently bought 100 Pipistrel electric aircraft and is getting hundreds of pilots their hundreds of hours of required flight time very rapidly and cheaply. And autonomous flight is coming, although passenger autonomous flight and required digital air traffic control are going to be available and somewhat common around 2040 in my regional air mobility maturity model. And as I discussed with Kevin Antcliff of autonomous flight company XWing and lead of NASA’s RAM study, autonomy is required for full flowering of the space, but there can easily be enough pilots for early business models. Vander Lind’s perspective is that pilot shortages are cyclical and the right time to do a start that requires more pilots is in the trough, so that they are available when the solution hits the market.

What about the cable getting into the props of the towed plane? Not a concern on a lot of turboprops like the Q400 or the ATR-72, as they are dozens of feet back from the nose of the plane, but there are some smaller airframes like the Beechcraft King Air B200 where the props are in front of the cockpit. Maybe they aren’t suitable for this model and maybe a longer nose boom could work, but they are also the scale of aircraft that could be replaced with a new battery-specific airframe with the CATL batteries for amazing range. The Eviation Alice is designed for the same number of passengers as the Beechcraft.

What are the alternatives? Well, Heart Aerospace, EOS Aero, and Ampaire are going down the hybrid path. In that model, an onboard generator running on SAF biofuel kicks in if divert or reserve are required, and maybe used a little in flight for range extension or battery life preservation, but 90%+ of flight is fully electric. That model has proven trajectory to full electrification with cars, but would start with plug-in hybrids obviously rather than going through the temporary non-plug-in pathway. While Heart was aiming at purely battery-electric for its original 19-seater when I spoke to Forslund, it has shifted to hybrid for its current 30-seater. EOS Aero and Ampaire founders and CEOs started with hybrid, as they told me when I spoke to them at various points, and Ampaire’s Cory Combs is very bullish on hybrids for existing airframes, including replacing the auxiliary power units (gas turbines) on passenger jets with batteries instead, something that also has the benefit of working above 10,000 feet where the air is too thin for conventional APUs, requiring rapid dives if there are engine failures.

So where does this leave us? Well, battery energy density keeps bouncing upward. CATL basically doubling range last week is big news. Also out of China last week was the announcement that researchers had hit 711 Wh per kilogram with pouch-based batteries. This is research, but they are claiming practicality, and commercialization is rapid in China.

Current Tesla battery energy densities are completely fit for purpose for 4-10 passengers airframes designed for them for up to 200 miles and with hybrid models for up to 90-100 passenger turboprops with 200-300 mile range. CATL’s new battery extends that substantially, but small planes are already range limited more by passenger and pilot bladder sizes than battery capacity in any event.

This is another use case for regional air mobility, and autonomous cargo flights with flight paths avoiding schoolyards and the like will be assisting in activating those airports too.

And as I pointed out a while ago, sustainable aviation biofuels are fit for purpose, there is enough biomass in one pathway — stalk cellulosic ethanol to biokerosene and biodiesel — to fulfill the total requirement I project for the two modes of transportation which will require it, longer haul aviation and shipping, and there are about seven more biofuel pathways, including animal dung and municipal food waste.

In other words, Magpie’s solution is one of what is now a toolkit of solutions for decarbonizing aviation. Will it prove viable in the market? Maybe, but it is an interesting addition, and unlike Makani and EVTOLs, not fatally flawed from the outset. I suspect the complexity, requirement for careful scheduling for mid-air interceptions, and commercial aviation passenger safety concerns will weigh heavily on it. It’s more complex than hybrid drivetrains, but they have different challenges. In other words, I’m bullish on aviation decarbonizing, and am pleased there’s another solution being tried out. And I’m pleased that Vander Lind’s talents are focused on something with both more basic utility and viability that would actually move the needle on climate change.