Flight Test: Guimbal Cabri G2
A pur sang by virtue of its Eurocopter bloodline, the Cabri is not just the first new helicopter to come on the market in thirty years, but one of the best...
They’re making a UAV out of the Guimbal Cabri G2, which is good as it’s a super-capable helicopter, strong as an ox, brimming with sophisticated modern features and as near indestructible as an aircraft can be. It’s just a shame that in ordinary circumstances you have to put a pilot in it, because that’s where the trouble starts.
Now this applies to any helicopter, but it’s especially noticeable in the Cabri as it’s a particularly bulletproof machine with forgiving handling characteristics and no apparent vices. It will happily do things that would get you killed in a minute in other machines, low-G manoeuvres included. From a safety standpoint it’s hard to see how it could be improved by any design expedient save taking out the pilot’s seat. And that would kind of miss the point, don’t you think?
General aviation is a matter of man and machine in perfect harmony… as a pilot, you keep yourself trained to a pitch of perfection (don’t we?) and accept a calculated level of risk that adds piquancy to the pursuit. I have to say that with the Cabri, the level of risk is as near zero as it can get with a small helicopter, and it’s hard to see how it could conceivably have an accident if the pilot adhered to my wise old instructor Brian Balman’s most important piece of advice; “Above all, don’t do anything really stupid.”
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Unfortunately, this profound wisdom is lost on many. Three Cabris have crashed, out of 35 sold, all of them because of cack-footed handling or overtly stupid behaviour. One suffered an engine failure when the pilot was proving that the Cabri was not susceptible to mast-bumping in low-G manoeuvres by pushing the stick sharply forward, thus interrupting the fuel flow. To put a helicopter at risk of one type of accident by proving it to be immune to another gives ‘dumb’ a bad name.
The Cabri is the first helicopter to be certificated by EASA, and indeed the first new small helicopter to come on the market for thirty years. It’s the brainchild of a Frenchman, Bruno Guimbal, who set out to make something significantly safer than the training helicopters we know and love. Never mind the mantra of ‘simpler, lighter, cheaper’ ? Bruno’s philosophy was to design out those things that could lead to trouble. The most obvious improvement is the Cabri’s three-bladed main rotor, which eliminates the mast-bumping possibilities of two-bladed rotors. Piles of crash test dummies at Millbrook will attest to the fact that the Cabri’s composite monocoque construction trumps tubes and tin, and the EASA certification standard ? the most stringent ever applied ? has been surpassed in every respect.
It’s got a glass cockpit, too, and this does not mean taking the cheap road. There are forgings rather than castings where it matters, for added strength ? and you don’t get titanium firewalls at Lidl. In a training environment, where overheads are bloodcurdling and margins anorexic, this can raise issues ? but the Cabri can win back costs in other areas, so read on.
It’s pretty. Clean, flowing lines ? unusual for a helicopter, where aesthetics is often the last thing on the engineer’s mind. Moulded composites can do a lot for looks, and the enclosed fenestron tail makes the back end seem like part of the whole rather than a utilitarian afterthought. It appears small, being shorter and lower than its competitors, but it’s relatively wide ? pilot and passenger have as much elbow room as the occupants of, say, an R44, and ample headroom too.
I flew G-UIMB in the company of Oliver Heynes, Chief Pilot and Director of Cotswold Helicopter Centre at Kemble (alias Cotswold Airport, I’m told) and I was impressed to the point of shock at his willingness to throw it around. With 1,000 hours on two-bladed helicopters I’m circumspect about anything overtly aerobatic, but Oliver gave the Cabri a couple of minutes of fairground wingovers, high-G and low-G manoeuvres while I worried that the blades were going to chop off the tail. No chance… “That’s nothing, compared to what the factory test pilot showed me,” Oliver said. Glad I wasn’t there.
But let’s begin at the beginning. Bruno Guimbal was a Eurocopter engineer, a big proponent of the fenestron, who went out on his own to build the Cabri. He trousered a bit of French government development money ? they do that sort of thing in France ? and he’s had a lot of help, in cash and kind, from his old employers. Eurocopter’s motive for supporting him is only partly out of the kindness of its heart; now that Robinson has produced its turbine R66 the American company is eating Eurocopter’s lunch, so it needs something to fight back with. On top of that it’s looking at a trainer with Eurocopter characteristics, so you could start ab initio on the Cabri then step into an EC120 or 350. Whether Bruno could have pulled it off without Eurocopter is moot; you need credibility as well as money to get parts made, and being a semi-detached member of the Eurocopter household has done him no harm at all.
Impressive numbers time. The monocoque fuselage weighs 20kg, and the tail boom 5kg ? that’s composites for you. The weight saving means you can tank 170 litres of avgas, which gives you five to six hours’ endurance, if your bladder and your bum can stand it. Vne is 130 knots. Power-off rotor RPM limits are 610 down to 450: enough for the most inattentive autorotator.
REMOTE CENTRAL LOCKING
I walked around the Cabri with Andy Moorhouse, Cotswold’s Sales Director. It’s not a long journey, but there are some interesting stops along the way. First off, it has remote central locking. (For the benefit of any thieves reading this; when it’s locked the starter is disabled.) Fuel filler on the left side, and just the one ? for a single Kevlar-lined tank. Under the lift-up cowling on the left side there’s a four-pot Lycoming O-360 J-series engine, normally aspirated, and the drive belt is at the front, just behind the firewall. The engine produces 145hp at a nominal 2,650 revs, although you can pull out to 180hp if you’re caught out by the height of the tree.
The clutch system is neat ? a hydraulic ram pivots the entire engine to tension the drive belt. It only moves a couple of mil, but that’s enough. It has dual ignition but only one magneto, mounted on the back of the engine ? the second spark comes from a plasma ignition system, providing smooth delivery at low revs. The oil filler is easily accessible for topping up, unlike some helicopters I could mention. The tailcone, made of a single composite moulding, is bolted to a tubular lattice with multiple load paths to obviate the consequences of a failure, and tail rotor pitch actuation is by an enclosed semi-rigid steel rod that looks like a Bowden cable but isn’t: Andy insists it’s a ball bearing-supported flexible linear shaft and it’s rather more expensive.
Except that we shouldn’t call it a tail rotor because it’s a fenestron and the pilot needs to be fully aware that a fenestron is not a tail rotor. More of this later; for now, note that the fenestron has seven blades enclosed in a duct that markedly reduces the chances of one of the most common fatal accidents helicopters suffer ? people walking into tail rotors. The blades feel flimsy and are loose on their bearings, but they’re remarkably efficient and damage tolerant ? they’ve sawn one through at 30 per cent span and run the tail at max revs for 100 hours without any further damage ? and even rotating nine times faster than the main rotor, it’s much quieter than a conventional tail rotor. The vertical fin is offset to aerodynamically unload the fenestron in cruise flight ? where hovering requires 80 per cent tail rotor thrust, cruise requires only 40 per cent. The stinger at the bottom is sacrificial, so if you ding the tail you can buy a new one for €50. The strobe sits on top of the vertical stabiliser, which is the highest point on the helicopter ? higher than the rotor hub.
On the right side of the fuselage there’s a big access hatch to a luggage compartment that can take two airline carry-on cases or a set of golf clubs ? 40kg is the weight limit. You can stick your hand into it through a small window behind the pilot’s seat, so it’s accessible in flight for charts, sweeties etc. You can get another 5kg of stuff in a second compartment under the binnacle ? handy for the removable left-hand controls ? and there’s room for more under the seats, although you don’t want to compromise their crashworthiness.
EASILY INSPECTED ROTOR HEAD
Inspecting the head is made easy by the fact that it’s relatively low, and you only have to stand on a step on the skid. Check the pitch links and the elastomeric lead-lag dampers. The skids are mounted on rubber bushes that allow the fuselage to move independently to reduce the chances of ground resonance. They also make sloping ground landings easier ? the fuselage stays more level while the skids tilt.
Hop in and the first revelation is that the doors fit and keep out the weather! There are cords to hook around the door handles so you can open the doors a few inches for ventilation, and the airflow at speed will push them closed against the pressure of the gas rod so it doesn’t get too windy. Smart.
The trim is leather and the binnacle is padded; no exposed metal here. It feels very upmarket. A quick tour of the cockpit; on the firewall between the seats there’s a panel of circuit breakers, the ELT switch, headphone sockets and the fuel selector. At your left hand, just beyond the collective, is the heater control; a single knob to turn. Moving up the binnacle, G-UIMB has a Mode-S transponder and a comms box, a row of switches, then the flying instruments: VSI, Alt and ASI, DI and, in the middle, a digital AH and your true friend; the slip ball. A screen at top right gives you airframe and engine information, and to the left of it are three lights: amber, green and amber. If the screen fails, the green light will tell you your rotor revs are good, the top amber light will tell you they’re high and the bottom amber will say they’re low, along with aural warnings. Simple and effective; you can fly on this alone.
To the right of the screen, brightness controls, a button to cycle through fuel state data, and a stopwatch button. The fuel state thingy seems a bit over-engineered. You can display instant fuel flow, average fuel flow since you started up, or contents and time remaining to a disconcerting silence. I’d never move it off the latter. Above the screen, some warning lights: starter, governor, rotor brake, oil pressure, main gearbox temperature, low fuel and alternator. E2B compass on top of the binnacle, and yawstring beyond it, together with a chinagraph marker to show when it’s vertical ? the Perspex is one-piece, with no divider. The seats are not adjustable, but the pedals can be flicked between two positions. In the roof you’ve got the mixture control, the rotor brake and the magneto and plasma ignition switches.
The collective has a twist throttle at the end then, moving back towards the pivot, a metal grip for the hand and a twist-on friction. You can hold the collective by the metal grip and keep one finger on the throttle to see what the governor is doing. Governor on-off is a toggle on the end of the lever, and the start button is next to it. The cyclic has a PTT falling to your index finger and a frequency flip-flop on top, next to the coolie hat; this latter puts two electric trim motors under your thumb.
Master on, and the TV springs to life, presenting first a self-test page that points up anything that’s suspect. Then it shifts to the instrument display. The Multi-Limit Indicator (MLI) is a half clock face display with a digital pointer running from zero to 120 per cent, and replacing an MAP gauge and the associated mental calculation. The air data computer calculates your power limits on the basis of air temperature and density and presents it either as PWR, which displays a red line coming down the gauge as you pull towards the limit, or FLO, which is when you can’t get full power and you’re in overpitching territory. In ISA, FLO mode kicks in above 3,800ft. In the centre is a full clock face, dual-needle engine and rotor RPM presentation with a green arc for the rotor ? 515 to 540. On the right is a bar graph for fuel, with the aforementioned readout in litres and in time remaining to dry tank. Along the bottom of the screen you’ve got little clocks for carb temperature, cylinder head temperature, oil temperature, and oil and fuel pressures. The carb heat toggle switch allows you to revert to manual and choose hot and cold. Attention-getting alerts involve clocks and readouts turning from green and yellow to red.
After the master switch, flick on the switches from left to right ? strobe, electric fuel pump, and alternator. Rotor brake on, mixture rich while you’re up there, magneto and plasma ignitions on. Set ten per cent throttle, then push the button and the Lycoming purrs into life ? that plasma ignition does good work. Throttle to idle for warm-up, then set it to 1,000ERPM when there’s a bit of CHT movement, release the rotor brake and flick the clutch switch. Run through your normal checks when the time comes and you’re ready to fly.
Andy Moorhouse had given me an hour-long briefing and had impressed on me a couple of things. First, start with three inches of right pedal and go on from there: the main rotor is powerful and obviously it goes French-way-round. Second, if you’re used to American helicopters you’re not going to like it to begin with, but as you get conditioned to it you’re not going to want to go back. He was absolutely right ? I didn’t like it at first. The controls felt stiff, but super-sensitive. Even though I thought I had far too much pedal in, the nose yawed left on take-off. The collective felt twitchy and the right-skid-low attitude unnatural.
So I’m sitting there, patting head and rubbing tummy but - as the French will have it ? upside down in a mirror. After a thousand hours on American helicopters my left arm is wire-locked to my left foot: if I raise the collective, my left foot goes in without me thinking about it. Lower it, and the left foot comes out. In theory, your feet should respond to the direction in which the nose yaws. In practice, muscle memory often causes you to screw up. It’s a bit like driving a car on which the pedals have been switched around ? okay in theory.
Off I trundled down the taxiway like a Channel ferry leaving Dover in a November storm. I realised later I should have been flying it on the coolie hat and making less use of the stick, but half my problems arose from the fact that just breathing on the collective called for substantial footwork to keep straight. If you’re ab initio you’ll have no problem. If you’re coming from Bell, Robinson, Schweizer, Enstrom or MD, you’ll need to practice when nobody’s looking.
Into the climb at 50 knots, and we’re well into the speed range that lets the vertical fin offload the fenestron. With 70 litres of fuel and two hefty chaps on board we saw 900fpm at 90 per cent, but only momentarily because I was all over the place. Levelling off requires a more than usually nose-down attitude. I applied some collective friction, and that produced an improvement in my flying. It doesn’t take long to get the new technique. Any power change needs to be led with pedal ? it’s like flying a Tiger Moth. Concentrate, look out the window, and it’ll come together. Eventually.
CAREFUL ROTOR BALANCING REQUIRED
At 90 per cent power I pushed the nose down and as the speed built, so did the vibration levels. Having a powerful three-bladed main rotor calls for careful balancing with the blades and the dampers, and their characteristics can change with the weather.
Oliver says an improvement in vibration at higher speeds is project number one at the factory. He showed me how we could trim the aircraft out then let go of all the controls, and she’d carry on happily in a straight line. We leaned left, she turned left, we leaned back, the nose came up and she started to climb. I try a few steep turns, and the RRPM could almost be painted on the dial. After 15 minutes I’m starting to feel comfortable, but I’m still hampered by habits ingrained from years of R22 flying, and Oliver takes over to fling her around a little. If you can subdue the notion that low-G is apostasy, you’d quite enjoy it. A powerful head on a 1,500lb body makes for an exciting ride, but you wouldn’t want to do it on your first flight.
Deceleration and spot turns are where you need to understand your fenestron. Unlike a conventional tail rotor, where the thrust to power relationship is linear, a fenestron is more effective at low pitch, but less effective between about 40 and 80 per cent pitch. From 80 to 100 per cent it is significantly stronger. This means that the last inch of pedal travel packs a mighty punch, but the pilot who thinks it’s a tail rotor is unlikely to stamp it all the way to the floor. So coming to the hover was the next thing I made a mess of. The tail starts wagging below 40 knots when you lose the aerodynamic effectiveness of the vertical fin, and as you begin to bring the lever in you need lots of right pedal or you’ll arrive sideways, as I did.
Turning left, you need to understand that non-linear fenestron response. If you overdo the pedal, the helicopter will start spinning on the spot. If you counter with the amount of pedal input you’ve been used to in other helicopters, it’s not going to stop. And if, god forbid, you pull up the collective, it’s going to spin even faster. The Cabri is the only piston-engined helicopter with a fenestron, and the instant availability of torque without turbine lag speeds the yaw. The cure is to get your right foot all the way to the floor and sit there until it stops ? and don’t touch the lever. A pilot trained on the Cabri will start a spot turn left with a twitch of left pedal, and then advance the right foot to control the rate.
Recovering from autorotation, there’s an added factor if you let the RRPM get a little low. As the governor comes back in at 2,000ERPM it will want to boost the rotor revs, and there will be a noticeable yaw kick as it does so. If at the same time you’re decelerating through the 40-knot loss of aerodynamic anti-torque zone, it might be a bigger kick than you’re prepared for.
I don’t want to overstate this ? if you were coming to it new, you’d hardly think about it. But a lot of pilots have habits that are hard to break, and they can’t afford to lose concentration. The accidents that have befallen the Cabri have all been because the helicopter was better than the pilot. G-UIMB had to be bashed straight after an instructor got crossed up and went into an uncontrolled yaw on arrival. Turned out he’d not flown the Cabri for two weeks, and had spent that whole time flying other-way-round tail rotor helicopters.
By the end of our sortie I’d begun to tame the little tyke, and a few hours on board could have inculcated in me the techniques that make flying it easy. As we sat waiting for the temperatures to drop I reflected on what I liked about the Cabri ? the feeling of relative luxury, the quietness, the glass cockpit’s sophistication and, yes, the knowledge that in the right hands, it must be the safest small helicopter there is.
Who’ll buy? If you’re a private owner with an R44 and you usually fly two up, you need to look at the Cabri. The R44 has a rebuild life of 2,200 hours or 12 years, which means that for maximum cost-effectiveness it needs to fly 183 hours a year. If you’re flying 100 hours a year, you’re pouring money into a black hole. With the Cabri, the only 2,200 hour component is the engine; everything else is on condition.
British schools teaching foreign military pilots could present the Cabri as a better stepping stone to Eurocopter products than the American equivalent. Cotswold Helicopter Centre is charging £295 an hour plus VAT for training, and therein lies the key. A well-trained pilot who doesn’t do anything stupid could fly the thing until doomsday and never miss a heartbeat. I’d be surprised if the UAV version has an accident, ever.