Flight test: the pioneering Pipistrel Velis Electro

The Pipistrel Velis Electro in flight

the pioneering Pipistrel Velis Electro - Credit: Keith Wilson

A full flight test, plus operating and engineering evaluation, of the World’s first certified electric aircraft - and it came from GA, never forget!

Words: Dave Unwin | Photos: Keith Wilson 

The Pipistrel Velis Electro, grounded at a flight school

Showroom appeal – the Velis Electro is a very sexy looking training craft that will attract attention at any club or school - Credit: Keith Wilson

I ease the power lever forward from the ‘idle’ stop, the three-blade propeller whirrs into life and the Velis trundles out onto the runway. I’ve been waiting several years for this moment, and the moment is now−I’m going to fly an electric aircraft! Line up with the centre of the runway and push the power lever smoothly forward to the stop. A quick glance inside confirms ‘airspeed alive’ and the needles of both power gauges (propeller rpm and power in kW) at the top of the green arc. I don’t really take in the actual numbers−but this is a very ‘green’ aircraft (even its registration is G-OGRN) and green is good!  

The acceleration is more than adequate, albeit with less urgency than the same manufacturer’s piston-engine Alpha or Virus. Rotate at about 45 knots and the Velis lifts off at fifty. Climbing comfortably over the wires at the end of Runway 03 at Damyns Hall I retract the flaps, let the Velis accelerate towards 75 and aim to intercept the Cessna cameraship with plenty of geometric cut-off. Another scan of the panel and I can see that power is steady at MTOP (Maximum Take Off Power, 65kW) the increasing speed is causing the rpm of the fixed-pitch prop to creep into the yellow arc and the colour of the digital rpm display has also changed to yellow. Time remaining is… Wow!

When we were taxying out I’m sure it said 55 minutes, but now it says 22! As I start to ease the power lever back it occurs to me that 22 minutes isn’t very long to fly an air-to-air formation shoot and a full flight test, and that I’d better get into formation quickly! But then another thought occurs, and having brought the power back from MTOP and through MCP (Maximum Continuous Power) to around 25kW I study the ‘Time Remaining’ readout again, and see that it has more than doubled, to 45 minutes. Phew! 

The underside of the Velis Electro aircraft

Battery cooling air enters through the scoop on the side of the aircraft and exits from the hole underneath - Credit: Keith Wilson

The first electric aircraft of many?
The Velis Electro is the first electric aircraft to attain EASA certification, and consequently already has its own place in aviation history, yet from a distance this diminutive machine is quite unprepossessing. Indeed, as our C172 taxis across Damyns Hall’s rather uneven turf towards FlyAbout Aviation’s office, I can see several parked Pipistrels, and they all look essentially the same. Regular readers may well have already read my impressions of the Alpha and Virus, and if you have, you can skip the next paragraph as−aft of the firewall−the Velis’s design and structure are essentially the same. What you’ll be wanting to read about is the innovative power train and the complex arrangement of batteries, chargers, controllers, inverters and cooling systems that make electric flight possible. 

And if you haven’t, well−it’s a small two-seater fitted with a tricycle undercarriage and a high, cantilever wing which is mostly constructed of composites (primarily carbon fibre and Kevlar over a carbon/aramid sandwich) with magnesium, steel and aluminium used for the metal components. The undercarriage features a steerable nosewheel oleo leg and mainwheels carried by a composite bow. Toe-operated hydraulic Beringer disc brakes are standard. G-OGRN’s wheels are faired by large, snug-fitting optional spats. 
The wings of the Electro are subtly different from its non-electric twins. There are no airbrakes (unlike both the Alpha and Virus) and the flaperons’ maximum deflection is the same as the Virus; ‘Flap 2’ −only 19°. I would prefer the 25° available on the LSA version of the Alpha. 

The single pitot tube on the starboard wing incorporates an alpha-sensing port for the haptic stall warning system. The tail unit is like that of the Virus−a tailplane and separate elevator mounted on top of a mildly swept-back fin, a broad-chord rudder and a metal tail skid. Rudder and elevator feature small moulded tabs on their trailing edges: the rudder has a non-adjustable trim tab, but the pair on the elevator provide compensation for trim forces at high speeds, the elevator being cambered to provide enhanced responsiveness in slow flight and improved stall recovery by producing more downforce at low speed. The elevator and full-span flaperons are pushrod operated and an electric servo motor drives a spring-bias system mounted on the elevator push-pull tube for pitch trim. The rudder is cable actuated. 

Obviously, the design of the air scoop in the lower cowl is very different from piston-powered Pipistrels, and there’s also a very large scoop on the port side of the fuselage and a small outlet in the belly. More on these later... 

So far so familiar, but it’s the powertrain and its controls that really intrigue me, and unusually−but also unsurprisingly−they’re all made by Pipistrel. The fixed-pitch prop features three carbon-fibre blades set in an aluminium hub and is driven by an axial-flux electric motor which can produce 65kW (88hp) at takeoff. A dedicated high-voltage H300C power controller manages the three-phase AC supply to the motor. Maximum continuous current is an impressive 300 amps, and that’s a lot of power. Both the electric motor and power controller are liquid-cooled by the same system, which consists of an electrically driven pump, and a radiator filled with a mixture of 50% distilled water and 50% automotive glycol.  

The main energy source is two Li-Ion battery packs−again built by Pipistrel−located fore and aft of the cockpit and accessed via removable fuselage panels. The batteries are also liquid-cooled (by a separate system) with the same 50% water/50% glycol mixture as the motor and controller. Propelled by dual electric pumps, coolant flows through a large radiator located behind the aft battery pack compartment. The aft battery access panel on the port side incorporates a big scoop to feed air to the radiator, hot air being ejected through the small hole in the belly. Electric fans behind the radiator provide additional cooling when the batteries are being charged, and the external charger, batteries, pumps and fans are all continually monitored by the Battery Management System (BMS). The BMS performs myriad functions, including calculating the battery State Of Charge (SOC) and State Of Health (SOH). SOC is self-explanatory, while SOH is essentially the ‘age’ of the battery, which affects how much energy it can store and how much power it can deliver. 
There’s a lot of sophisticated equipment crammed into the compartment aft of the cockpit, so it comes as no surprise that there’s neither a recovery parachute nor baggage bay. 

The charging socket on the Pipstrel Velis Electro

'Fuel filler’ flap at the nose opens to reveal an inner cap and behind that the charging socket, and even the charging plug is styled like a fuel filler nozzle - Credit: Keith Wilson

The charger touchscreen monitor on a Pipstrel Velis Electro aircraft

The charger’s touchscreen offers easy control and monitoring of battery charging - Credit: Keith Wilson

Charging precautions 
Deepak Mahajan, who through Fly About Aviation is the UK distributor of Pipistrel, had ensured that both batteries were fully charged before we arrived. However, I was still able to watch with considerable interest as the big, three-phase charger was connected to the aircraft via a large socket on the starboard side of the cowl and the charger turned on. The charger and BMS ‘talk’ to each other, and as it was quite a warm day not only did the BMS activate the coolant pumps but also the radiator fans. As you may have already concluded, the greatest risk of a ‘thermal runaway’ is when the batteries are being charged, and this is a situation that should not be taken lightly, as lithium battery fires are self-sustaining and almost impossible to extinguish. This produces a problem: obviously, you don’t want to recharge the aircraft inside the hangar, but the batteries are a little fussy as to the temperature at which it’s done. If they’re below 20° or above 45°C, the charger auto-compensates by reducing the power, and while temperatures above 45° are rare, less than 20° is quite common (in fact across the UK annual temperatures average a daily high of only 14°C). Hmm... 

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In fact, even this simple process threw up quite a few interesting questions, such as how much recharge times vary with temperature, and even the electrical power available at the airfield. Deepak explained you need a 415V three-phase AC supply rated at least 32 amps. The charger supplied with the aircraft can recharge at rates from three to 32 amps (the lower current helps keep the batteries cool−important on a hot day−but charging will take considerably longer.) 

The charger was then turned off and disconnected, and we were good to go. Not pulling or pushing on any propeller is always good practice, but pushing on the Velis’s is strictly forbidden, because it has a direct-drive electric motor, and pushing would place an ‘abnormal’ load on the bearing. 

The typical Pipistrel top-hinged doors provide easy cockpit access and I like the elegant pin-and-clip devices which hold them open and the very positive system (three pins actuated by a single handle) that locks them closed. I’m ambivalent about the tinted Lexan windows and find the DV panel noticeable by its absence. The controls are very familiar: the rudder pedals adjust, the sticks are nicely sited, the flap lever is slightly awkwardly positioned and the trim indicator difficult to see, although I very much approve of the fact that the trim switch is correctly colour-coded green. The inflatable lumbar support is a nice touch. 

Mounted on the floor and between the seats is a quadrant that carries the T-handled power lever, while the pedestal that braces the instrument binnacle carries numerous circuit breakers and four silver toggle switches for the Master, Avionics, Battery and Power. 
The instruments are particularly interesting. The top row consists of a large analogue ASI and altimeter, with a multi-function Horis unit between them. Made in Slovenia by a company called Kanardia, this very sophisticated electronic device is fed from various sensors via a CAN (Controller Area Network) bus. It offers several display options, but the primary page is a very comprehensive AHRS (Attitude Heading Reference System) display, which shows attitude in roll and pitch, airspeed−both IAS and TAS, vertical speed, altitude, outside temperature, wind speed and direction, heading and the barometric setting.  

A Pipistrel Velis Electro aircraft in flight

The Velis has the same crisp handling as its piston-engine partners - Credit: Keith Wilson

It’s an amazing bit of kit, but I already knew it, and was also familiar with the Kanardia electronic tachometer and VSI. Instead, the unit I really needed a briefing on was the EPSI 570C (Electric Propulsion System Indicator). This has multiple pages: for example, when connected to the charger, it shows the state of charge in percentage and the temperatures of the batteries and inverter. Now that we’re going flying, the ‘flight’ page shows rpm and power in both digital and analogue formats, State of Charge (SOC) in percentage, time remaining, voltage in the main and auxiliary avionics battery, temperatures of the front and rear propulsion system batteries and coolant temperatures for the engine and battery systems.  

There’s also an annunciator panel above the Horis, which has ‘Master Warning’ and ‘Master Caution’ captions to inform the pilot about malfunctions and failures in the propulsion system. These are reinforced by aural warnings. There’s also a second, small warning panel which is specifically designed to warn about battery over-temperature. It is analogue and consists of battery temperature sensors and two warning LED lights (one for each battery pack) which illuminate if the battery temperature exceeds 58°C. At the base of the panel−and looking somewhat dated amongst all the digital displays−is a classic slip ball. Regular readers may have noticed that currently and shockingly I have resisted any electrical puns (except for the three in this sentence) and that’s because this is serious stuff.  

My brain was so full of information on the new systems that I told myself to start concentrating on actually flying the thing, while continuing to acquaint myself with the panel. There is a plethora of placards and the one which caught my eye warned that the limiting speed for Flap 2 is only 65kt. Deepak had already told me that sixty was a sensible approach speed, so that didn’t leave much room for error, particularly during a go-around! 

Time to go, so under Deepak’s watchful eye, working from left to right, I select Master on and the aircraft self-tests the ‘Batt Overtemp’ warning lights and haptic stall warning system. ‘Avionics’ is next and we check the propulsion system’s battery SOC and SOH, and the auxiliary battery voltage. Radio and transponder on, and then one checks that the power lever is at ‘cut off’, select ‘BATT EN’ and then ‘PWR EN’. Incidentally, if you do select PWR EN (power engaged) when the lever is off the stop nothing happens, until you pull it back to ‘cut off’. Behind our seats, coolant pumps and radiator fans stir into life. I have enjoyed the privilege of operating the throttles of literally hundreds of different types in performing flight tests for Pilot, and yes, the visceral rumble of a big radial or a reverberating V12 cannot be denied. Yet there was something strangely satisfying about inching the power lever forward and watching the three blades whirr almost silently into life. Indeed, the sound is similar to a very large electric model aircraft. Taxying across grass at a sensible pace takes about 3kW, but the most interesting aspect is that because the motor is so smooth and quiet you notice every creak and groan of the airframe−sounds you normally do not hear.  

The multi-function electronic Horis unit on the Pipistrel Velis Electro

Looking like an AI, the multi-function electronic Horis unit sits between analogue ASI and altimeter - Credit: Keith Wilson

Charge, current and battery temperature display on a Pipistrel Velis Electro

State of charge, current and battery temperature are all indicated on one screen - Credit: Keith Wilson

Simple power checks 
As expected, the pre-takeoff checks are simple: there’s no fuel pump, mixture, carb heat, mags and so forth. The power checks are simply: set full power to ensure the system is producing at least 50kW, then lever back to cut off, check motor and battery temperatures, both batteries ‘Active’ and that’s it. With the camera aircraft we had briefed a stream takeoff with a five-second interval, and I was amused again when I pulled the power lever back and the prop stopped. A quick check of the ambient conditions (well above ISA, for although the airfield’s elevation is only 60ft it is a warm 26°C), with about 8kt blowing across the grass runway. 

Off we go and, as full takeoff power is restricted to a maximum of ninety seconds, I soon ease the power back. If ever there was a flight when I needed to make the power lever my intention and not my reaction while flying formation, this was it. If an aeroplane has a constant speed prop, I always set it to fully fine while moving into position, as this not only gives prompt acceleration but also a braking effect. Pipistrels have a well-deserved reputation for being ‘slippery’, which is why all the other ones I’d flown were fitted with airbrakes. However, the Velis not only lacks airbrakes, but there won’t even be any drag produced by the prop turning the motor on the ‘over-run’, because during the walkaround I’d discovered I could rotate it freely with my little finger! 

We have too much ‘overtake’ so while we are still a fair way back I test my theory about the ineffectiveness of the prop as an airbrake. As I’d suspected, there is almost no braking when the power is reduced. I pull the power lever right back, glance at the EPIS and am amused to see that the ‘Time Remaining’ indicator has increased again, and we now have 48 minutes of playtime. Of course! I should’ve realised that at max takeoff or even max cruising power that the batteries are discharging at a prodigious rate, but as soon as you reduce the power supplied by half you double the endurance! 

I soon slot into formation, and even though Mark Hadley is flying a super-smooth lead it is not easy. The high wing never helps the field of view and the lack of propeller braking in a very low drag airframe is making station keeping difficult. I am also very aware that every time I add power the ‘time remaining’ reduces. 

A Pipistrel Velis Electro in flight

The Velis shows its distinctive, sailplane-like lines - Credit: Keith Wilson

With the pictures finally in the can it is time to break away from the camera ship and I turn my attention to the flight test card. The formation detail has demonstrated that the Velis has the same crisp handling as its piston-engine partners, so having checked controllability I move quickly onto an examination of the stability. Again, just like its fossil-fuelled brothers the stick-free stability around all three axes is strongly positive directionally, positive longitudinally and neutral laterally.  

Moving straight onto stalls and slow flight confirms that the Velis is slow to decelerate, but the stalls themselves are perfectly straightforward, either flaps up or flaps down, or with the power at either idle or maximum. A particularly interesting facet is that the pitch trim changes with flap are almost indiscernible. Adequate aileron authority is available deep into the stall, while a very curious anomaly (and something I’ve never encountered outside of a turbine powered aircraft) is the stick shaker. As the stall is approached it really does vibrate the stick quite vigorously. 

Although the Velis has been designed as a purpose-built circuit-flying trainer, I would be remiss in my duties if I didn’t at least briefly examine the cruise performance. With the power set at 25kW it cruises comfortably at 70kt indicated and 1,900rpm, and the relatively high wing loading of 63kg/sq m gives a pleasantly firm ride, but the time remaining meter is clicking down remorselessly, and with less and less playtime remaining it is becoming increasingly apparent that we need to head back to Damyns Hall.  

Back in the circuit I pull the power right back and once again the time remaining increases considerably. Deepak recommends a slightly flatter approach than I prefer at 60kt and full flap – which in the Velis Electro means 19°. In calm conditions you could safely shave 5kt off the approach speed but there is quite an unpleasant burble of turbulent air caused by the trees of Warwick Wood on short final. Through the gap in the hedge with the speed nailed to sixty, I pull the power lever back to the stop but the Velis still floats a fair way into the field, reinforcing my initial suspicion that it’s under-flapped. Nevertheless, we stop comfortably about two thirds of the way along the 650m runway with only minor braking.  

I would have liked to have flown another couple of circuits, but the SOC indication was now well below 50% and this means that takeoffs are not allowed. As an indicator of how much energy is consumed at MTOP, if the SOC indication is below 15% an amber caution message stating ‘NO GO-AROUND AVAILABLE’ appears on the annunciator panel, which must concentrate the mind−although I suspect that you could still safely fly one more circuit if you really needed to. 

The central handbrake-style lever and seats in a Pipistrel Velis Electro

Rather awkwardly positioned, the central handbrake-style lever operates the flaps but at least you can tell where they are in an instant by touch - Credit: Keith Wilson

The aft battery and complex liquid-cooling system on a Pipistrel Velis Electro

Behind the seats, where you'd normally expect to find the fuel tank, sits the aft battery and its complex liquid-cooling system - Credit: Keith Wilson

Well, the current (groan) limitations of battery technology mean the electric aircraft is not yet a practical club machine, and neither Deepak nor Pipistrel claim that it is. It’s also full of contradictions. It’s very expensive to buy, and very cheap to run. When you’re waiting to takeoff at a busy airport you’re consuming no ‘fuel’, but if you need to increase takeoff performance you can’t off-load any. Another downside (and unlike a conventional aircraft) is that the aircraft’s performance does not increase as the energy reserve (‘fuel remaining’) reduces. 

The Electro is of course excellent for circuit training but I think there’s more to it than that. The people at Pipistrel are very smart, and I wonder if the electric powertrain will eventually be paired with a hydrogen fuel cell? Alternatively (and almost certainly) battery technology will improve. It is undeniable that the age of the electric car is almost upon us, and practically every major manufacturer now produces at least one model. The big issues for all electric vehicles are energy density and recharge times, but will these be resolved as electric cars gain traction? Possibly. Think about the early jets−the Jumo 004 had a TBO of around twenty hours. Who could have foreseen that within ten years a jet engine’s TBO would increase a hundredfold, while today engines like the R-R Trent may remain ‘on the wing’ for over 20,000 hours! 

Nevertheless, if you are still thinking “well it can’t do this and it can’t do that,” you’re probably the sort of person who, standing on the dunes at Kill Devil Hills in December 1903, would’ve asked “What use is that thing? It can’t carry a passenger, and even more irritatingly, while the Wrights are messing about with their useless flying machine they’re not fixing my bicycle!”

The peddles on a Pipistrel Velis Electro

Spidery rudder pedals incorporate toe brakes – very much a bonus when it comes to ground handling, we’d say - Credit: Keith Wilson

Length    6.47m 
Height    1.90m
Wing span    10.71m      
Wing area    9.51 sq m     

Weights and loadings 
Empty weight    428kg
MTOW     600kg
Useful load    172kg
Wing loading     63.09kg/sq M (12.91lb/Sq Ft) 
Power loading (MCP)    10.41kg/Kw (17.17lb/hp) 

Vne (IAS)    108kt
Cruise (TAS)@6000ft    102kt
Stall    45kt
Takeoff over 50ft     409m
Land over 50ft        450m
Climb rate    650fpm 

Pipistrel E-811 axial-flux electric motor, producing 88hp (65kW) at 2,500rpm, powered by two Pipistrel PB345V124E-L Lithium-Ion batteries and driving a Pipistrel composite three blade fixed pitch propeller 

W/ mobile charger, ex-VAT     €185,000