Flight Test

Charging Ahead…

With no hydrocarbons in sight, flying with just electric power is a whole new experience. Oh, and get ready to learn some new acronyms too…

We tried to get our hands on Pipistrel’s Velis Electro after the Microlight Trade Fair at Popham earlier this year. The plan had been to fly the aircraft from Popham to Wadswick where, before flying, we’d take pictures of the all-electric Pipistrel while its batteries were being recharged using the three-phase power being generated from the handy solar farm next to the runway. It would have been all very green and virtuous. Admittedly, there was the small matter of needing to fly to Popham in the C182 to get the mobile charging unit (which weighs about the same as an adult), but the imagery would have been great…

Clearly that didn’t happen. Apart from anything else, we perhaps hadn’t fully appreciated the challenge of range/endurance that comes as standard with every electric aircraft, of which in the certified world there’s just one, the Velis Electro (CS-LSA). Pilot Deepak Mahajan, the UK’s Pipistrel dealer would have had his work cut out when returning from Wadswick to Damyns Hall, at least without some hefty tailwinds, not to mention the problem of transporting that mobile charger (which despite being mobile, obviously isn’t intended to follow the aeroplane around).

During one of FLYER’s weekly Livestreams (what do you mean you haven’t seen one yet? They’re every Thursday at 1930 on our YouTube channel), Claire Bartlett, one of Deepak’s instructors at Damyns Hall, brought the subject up again, so as soon as weather and availability holes aligned, Ed and I jumped in the avgas-burning C182 and headed east.

When we arrived, Deepak was busy taking the Velis out on a local demo. A great opportunity to hear how surprisingly quiet the aeroplane was on take-off, and how surprisingly noisy the process of recharging afterwards was (although we’re talking Dyson hand dryer loud, not IO-540 at max rpm, loud).

While Deepak was doing his thing, we sat down with Claire to talk through both the similarities and differences between traditional GA and the modern Velis, and of course between petrol-fuelled piston power and the all-electric models. I’m going to assume that we’re all far more interested in the electric powertrain side of things, so rather than any lengthy Velis vs Cessna/Piper comparisons, I’ll just say that the Electro is a sleek low drag composite two-seater with a T-tail and cantilever high-wing that’s equipped with pretty much full-span flaperons.

It’s a sweet flying aeroplane that only really requires the lightest of fingertips on the stick. The only thing that might catch out the unwary from a handling point of view is the adverse yaw created by the flaperons, particularly if you have two stages selected at low speeds. Unlike the stick, the rudder pedals need a decent push to move them and counteract the yaw. During the cruise when no flaps are set, the rudder pedals are more footrests.

New acronyms…

Converting onto the Velis not only ends up with you learning a bunch of added acronyms (you’ll get to know your SOC from your SOH, and your BMS from your RFT), but more importantly, you’ll end up understanding that energy management (by which I mean electrical energy) is front and centre to pretty much everything you do with the aircraft, with maybe the exception of washing it at the end of the day. And even that helps keep things as efficient as possible.

To put this energy management into some kind of context, you can fill the petrol version with 100 litres of fuel, which is roughly 40 times more energy than you’ll get in the batteries of the Electro, and even though converting all of that chemical energy into mechanical energy isn’t hugely efficient (most of it is lost to heat), there’s still a lot more energy in the fuel tanks than in the batteries.

Starting at the front of the aircraft you have a three blade fixed-pitch carbon fibre propeller that’s driven by the snappily named Pipistrel E-81-268MVLC motor and its dedicated controller. The job of the power controller/inverter is to convert the battery’s DC power to the AC required by the motor, and to modulate the frequency of that AC power, which basically changes the engine rpm when commanded by the fly-by-wire throttle, which of course is now a power lever. Given the aeroplane’s powertrain is basically controlled by software (some emergencies are dealt with by a reset rather than re-start), it would be possible to have the fixed-pitch propeller behave as either a traditional fixed-pitch prop or as if it had a constant speed unit. This is not within the gift of the pilot, and Pipistrel has chosen to make it behave as a standard fixed pitch propeller. If all that sounds a bit complicated, the engine still turns faster if you advance the power lever and slower if you pull it back. When you’re waiting at the hold, bringing the power lever all of the way back to zero stops the prop, which is a little strange the first time you do it. The motor provides a maximum 65kW of power for take-off (multiply kW by 1.34 to get hp – so this has just over 87hp).

To power the motor there are two lithium-Ion battery packs, one in the nose and the other in the rear fuselage, each delivering 11kWh, so a total of 22kW/h. For an electric car comparison, the Tesla model 3 has 50kWh or 75kWh. If you are interested in the detail, inside each battery’s aluminium casing you’ll find four separate layers. Each layer contains 288 individual battery cells making 1,152 per battery. Each battery pack also has an integral Battery Management System (BMS) which monitors, manages and balances the individual cells, the associated cooling system and the calculation of the all important State of Charge (SOC) and State of Health (SOH), of which more later. Critical numbers like voltage, SOC and SOH are all displayed on the Electronic Powerplant System Interface (EPSI), which is basically the electric equivalent of a sophisticated engine information system in a piston aeroplane. It’s an instrument that gets a lot of pilot attention throughout any flight. To keep everything at the right temperature there are two liquid (50% automotive glycol, 50% water) cooling systems, one for the batteries and one for the motor/inverter, with the inverter being the piece of equipment most likely to get very hot very quickly should there be a cooling system malfunction.

Charge, versus health

Before talking about energy use it’s worth covering off SOC and SOH. The first of these, State of Charge, is fairly simple and can be thought of as the electric equivalent of a fuel gauge. With a bit of luck you’ll be starting off at 100%, but if it happens to read 50% or less, go directly to the charging point before going flying, and once you are flying, plan to land with at least 30% SOC. The second, State of Health, is not quite so straightforward. If we stick to the fuel analogy, it sort of means the fuel tank capacity, but, while you would expect the fuel tank to remain the same size through its life, a battery’s capacity will gradually diminish, meaning that while you may be departing with 100% showing on the SOC, if your SOH is only reading say 40%, you will have considerably less range or endurance. Your ‘fuel tank’ gets smaller over time, so range and endurance planning and management requires knowledge of both figures.

The EPSI will also give your Remaining Flight Time (RFT) value, but this is (obviously) only calculated on the current power setting (which is why that number looks scary when you are climbing out under full power), so has to be treated appropriately.

Point of no return…

To make it easier to grasp these concepts for an electric newbie, Claire ran through some numbers. Assuming your SOH is 100%, a 1,000ft climb at Vy will use 7% of your SOC, but if you are flying around with an older set of batteries that have an SOH of 40%, that same climb will use 10% of your 100% SOC. Once you are up there and in the cruise with power set to 25kW (which translates to just under 80kt) every 10 minutes of flight will use up another 19% of your state of charge (or 28% if your SOH is 40%). From these numbers it should be pretty clear that when flying the Velis you need to be on top of your planning, and very situationally aware when it comes to energy levels. If you need any more persuasion, the fact that the aircraft has no heater (too many magic pixies required) and a section in the POH discussing Point of No Return (PNR) calculations should be enough of a clue. In flight there are of course enough cockpit warnings too…

With such a precious resource it pays to understand the systems so that you don’t squander minutes in the air while on the ground. You know all of that faffing you do while the temperatures are coming up in an avgas aeroplane? You really want to get that done and out of the way before you touch any of the switches, although to be fair, the master and avionics switches only power on the small 12v battery, so up to that point at least you’re not depleting that essential SOC number. Turning on the master sets a self-test in motion, and in the space of a few seconds various temperature warnings and lights are tested while you monitor progress. The haptic stall warner is also tested and feels similar to a giant text message arriving through the top of the joystick. No giggling at the back. Assuming nothing’s failed (don’t fly if it has), you are good to move on to the next two switches which engage the main batteries and the powertrain. This is where things start getting serious as the cooling pumps are brought online. So check the area, power on, check the area again and advance the power lever. If, like me, the power lever had been left open before engaging batteries and power, nothing would happen. To get things going, you first need to bring the lever back to zero power (it’s not called idle as the prop’s not turning) and advance it from there. Needless to say you use as little power as you can during the taxi, so none of that inadvertently riding the brakes malarkey.

Power test

There’s a quick power test at the hold to ensure that you have more than 50kW available, and with one stage of flap deployed, it’s full power smoothly applied and away we go. Acceleration is brisk without being urgent, noise levels are low and everything is so very smooth. After about 300ft the flaps are brought up and power adjusted to Maximum Continuous Power (MCP) which is 48kW. Less power and a slower climb doesn’t deliver better endurance, and we level out at 1,000ft +/- a bit for my gash flying with the curved and visually misleading coaming. I fly around the local area enjoying the handling and experimenting with power settings and their effect on the RFT.

The endurance numbers in the POH for an A to A flight (which only requires a 10 minute reserve) range between 52 minutes with standard cruise power (25kW) and batteries that are fully charged with a 100% SOH to 13 minutes for the worse possible combination of a high power setting and a 0% SOH.

Our first circuit works out fine, speed control is critical, with such a slippery airframe it would be easy to be too fast, and that will obviously make for a long landing. It is the landing performance that is critical with the Velis, so if there’s enough room to land there’s enough to take-off too (all things being equal).

Downwind for the last landing the RFT was reading 13 minutes, we were the only aircraft in the circuit so there wasn’t a great deal of pressure and the landing and taxi back were relaxed. I can see how that might not be the case at a busy airfield, where a high stress situation might well lead to an approach that’s sufficiently fast or ragged enough to require a go-around, the dwindling RFT and SOC only adding to the pressure during the second approach. According to the training manual, if your SOC is down at 30% there’s enough battery for another circuit, and in extremis one more short one. While not the kindest thing for the battery, it is certainly better than re-arranging the undercarriage or running into the proverbial hedge…

As the training literature points out, you will be happiest when flying an electric aeroplane if you do not directly compare it to an avgas one. It’s different and it’s the start of a new class rather than the end. The Velis Electro is amazingly quiet, very smooth, comfortable and it should be very reliable while requiring minimal maintenance.

Less than £5 per hour

The cost of ‘fuel’ is less than £5/hr, against which you have to balance the capital investment (think €180,000) and the extra care that you’ll want to lavish on the aeroplane. It can be operated in temperatures as low as -20C, but batteries should be stored between 0°C and 30°C meaning you are going to need some kind of plan for winter.

Realising that the high capital cost is a barrier for UK schools, Green Airside, a new aviation venture has been launched. It plans to buy up to 50 Velis Electro aircraft (the first 10 have been ordered) and to make them available on lease to flying schools. I think there is a niche for electric training aircraft, and integrating them will surely bring some operational challenges to overcome, but if the progress of electric aviation is anything like the progress I’ve seen in the automotive world (I’ve had three different electric cars over the last eight years), the future’s very exciting.

Electric learning

Converting onto the Velis Electro takes more than a bit of a briefing and a couple of times around the circuit. There are a few relatively small handling differences that you’ll notice – speed control can be a challenge if you are used to a draggier airframe and there’s a bunch of adverse yaw at slow speeds with both stages of flap deployed, but the main area is obviously the powertrain, its associated systems and the various new emergencies that might conspire to spoil your day.

Pipistrel-online.com is the portal that’s been set up to offer various (paid) courses covering Pipistrel aeroplanes. I worked my way through the Velis Electro course before writing this and found it superb. There’s lots of material (plan for five decent length sessions over perhaps five days) which explain, test and deliver course completion certificates should you reach the required pass mark. Top marks to Pipistrel and its course developers. You can buy permanent access for €300 (which will also cover all future updates), or two weeks of access for €150.

Tech Spec

Performance

Max speed (Vne) 108kt
Cruise speed @ 35kW 90kt
Stall speed 45kt
Take-off distance 409m
Landing distance 450m
Rate of climb 647fpm
Endurance Up to 50 mins
Glide Ratio 15:1

Weights & loadings

Seats Two
Max take-off 600kg
Empty 428kg
Payload 172kg

Dimensions

Wingspan 35ft 1in (10.71m)
Wing area 102.4sq ft (9.51sqm)
Length 21ft 3in (6.47m)
Height 6ft 3in (1.9m)

Spec

Airframe Composite
Engine Pipistrel E-811
Max power 57.6kW
Propeller Three-blade, fixed-pitch Three-blade, fixed-pitch
Avionics 5.7 inch LCD EPSI 570C
Propeller Fixed, tricycle

Manufacturer

Pipistrel Aircraft www.pipistrel-aircraft.com

Contact

Fly About Aviation, Damyns Hall Aerodrome, Essex T: 07540 899 690, www.flyaboutaviation.com

Price

Aircraft 175,000 euro +VAT (includes M-20 charging box)
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